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Design, Fabrication and Working of a Wall Climbing Robot 1 Design, Fabrication and Working of a Wall Climbing Robot DESIGN, FABRICATION AND WORKING OF A WALL CLIMBING ROBOT Session 2006-08 Project Advisor Mr. Yasir Mehmood Submitted By Muhammad Abdul Qadir MCS-06-07 Muhammad Luqman Ali MCS-06-23 The University of Azad Jammu & Kashmir Department of Computer Science & Information Technology Mirpur Campus 2 Design, Fabrication and Working of a Wall Climbing Robot 3 CERTIFICATE It is to certify that the following students have completed their project on DESIGN, FABRICATION AND WORKING OF A WALL CLIMBING ROBOT M. Abdul Qadir M. Luqman Ali MCS_06_07 MCS_06_23 It is to certify that following students have successfully completed their final year project as prescribed by the CS&IT Department of AJK University for the course of Artificial Intelligence Project Advisor _______________________ External Examiner _______________________ Chairman Department of CS&IT Mirpur (A.K) _______________________ Project Coordinator _______________________ Design, Fabrication and Working of a Wall Climbing Robot 4 DEDICATION To our respected and affectionate parents, teachers and friends who sacrificed all the comforts of their lives for our future and whose love and affections have always guided us to face the challenges of life with patience and courage and their guidance inspired us throughout our life to get our destination. Design, Fabrication and Working of a Wall Climbing Robot 5 ACKNOWLEDGEMENT All praises to ALLAH, the source of all knowledge, wisdom within and beyond our comprehension. In fact, we cannot repay even a single blessing conferred upon us by ALLAH. So first of all we offer our thanks to “ALMIGHTY ALLAH”. Who gave us courage and interest to complete our project, and without whose help and will, it was not possible for us to finish it. We pay our sincere gratitude to our kind parents whose love affection and priceless prays for us, let us do our work with devotion, which finally let us complete our project. We are highly privileged in taking the opportunity to record a deep sense of gratitude and indebtedness to our project coordinator “Mr. Yasir Mehmood”, Chairman CS&IT Department, who kindly assigned us with such an interesting project and under whose sincere guidance and scholarly approach, this important piece of work finally, achieved its goal. His consistent advice and supervision generated the vigor for excellence in its pursuits, without which this would never have been materialized. Last, we consecrate our sincere thanks to all our friends and colleagues for their helps and valuable suggestion during our project. Team Members Design, Fabrication and Working of a Wall Climbing Robot 6 Abstract Almost in all developed countries of the new age robotics is flourishing and help establish technology and can be said as the back bone of industries. In Pakistan robotics is just on the verge of global robotic competition and it is the need of new age to introduce different and efficient working principles, phenomina and economical fabrication techniques to enhance and optimize basic robot concept. Wall climboing robots is basically based upon optimized fabrication techniques and working phenomina that can solve tipical industrial problems, relating to inspection methods, live observations and security system. Different jobs and operations can also upgrade the purpose of WCR. As for as research on WCR is concerned, this study will be helpful for robot making industries. The material selection problems can be solved through this fresh and latest research. Uses and ideas of material selection in any type of structural design in robotics and simplicity that has been introduced here is brighter step towards robotics development. Design, Fabrication and Working of a Wall Climbing Robot 7 TABLE OF CONTENT 1. INTRODUCTION ………………………………………………..……..9 1.1 Background ……………………………………………………………………..…9 1.2 Purpose and scope ………………………………………………………………..10 1.3 Project time line ……………………………………………………………….....10 2. LITERATURE REVIEW ……………………………………………..12 2.1 Development of semi-self contained wall climbing robot with scanning Type suction cups ………………………………………………’’……………....12 2.2 Walking and running of the Quadruped Wall Climbing robot …………………..12 2.3 A wall climbing robot using propulsive force of propeller ……………………....13 2.4 Machine that can walk and climb on floors, walls and ceiling …………………..14 2.5 Development of small size window cleaning robot by wall Climbing mechanism …………………………………………………………...…14 2.6 Vortex’s wall climbing robot ……………………………………………….…….15 2.7 Gecko inspired surface climbing robots …………………………………….…....15 2.8 Stanford’s skickybot wall climbing robot lizard …………………………...…….15 2.9 The BIGALLO wall climbing robot …………………………………………...…16 . 3. MECHANICAL SYSTEM ……………………………………………17 3.1 Overview …………………………………………………………………….......17 3.2 Different techniques to stick robot onto wall ……………………………………18 3.3 Mathematical calculations ……………………………………………………….19 3.4 Mechanical structure …………………………………………………………….24 3.4.1 Main column …………………………………………………………………….24 3.4.2 Legs ………………………………………………………………………….…..25 3.4.3 Slider housing ……………………………………………………………….….26 3.4.4 Slider ………………………………………………………………………….…27 3.4.5 Eccentric ………………………………………………………….….….……..28 3.4.6 Washer …………………………………………………………….…..……….28 3.4.7 Tee …………………………………………………………………….………...29 3.4.8 Screws …………………………………………………………….….……..….29 3.5 Suction cups ……………………………………………………….….……......31 3.5.1 Selecting a vacuum cup………………………………………….….…………32 3.5.2 Forces holding the cup on the wall …………………………….….………..35 Design, Fabrication and Working of a Wall Climbing Robot 3.5.3 Material used in vacuum cup…………………………………….….……….36 3.6 Vacuum pump……………………………………………………….….………37 3.6.1 Types ……………………………………………………………….…………..37 3.6.2 Performance measures …………………………………………..…………..38 3.6.3 Positive displacement…………………………………………….…….…….38 3.6.4 Techniques ……………………………………………………….…….……..40 3.6.5 Vacuum pump verses venture effect……………………………...........41 3.6.6 Vacuum pump used ……………………………………………………...42 3.6.7 Syringe …………………………………………………………….………42 3.7 Tubing ……………………………………………………………..………43 3.8 Sealant …………………………………………………………..………...45 3.8.1 Types of sealants ………………………………………………..……….45 3.8.2 Silicon sealants ………………………………………………..…………45 3.9 Gait of robot ……………………………………………………..………..49 4. ELECTRO MECHANICAL SYSTEM ……………….……….51 4.1 DC motor ……………………………………………………….………...51 4.2 Power supply …………………………………………………….…… ...57 5. PROBLEMS AND RECOMMENDATIONS …………….…...63 5.1 Suction cups ……………………………………………………….……..63 5.2 Vacuum pump ……………………………………………………….…...63 5.3 Materials …………………………………………………………….……64 5.4 Size of DC motor…………………………………………………………64 5.5 Limited direction of motion ………………………………………...……64 6. APPANDICES ………………………………………………..…65 6.1 Aluminium casting alloy LM6 ……………………………………….….65 6.2 Mild steel …………………………………………………………….…..67 6.3 Brass ……………………………………………………………………..69 6.4 Sand casting ……………………………………………………………...72 6.5 Cad diagrams ………………………………………………………...…..80 7. IMPLEMENTATIONS ……………………………………...…91 7.1 Future planes………………………………………………………..……91 7.2 Conclusions ………………………………………………….….……….93 7.3 References …………………………………………………….…………93 8 Design, Fabrication and Working of a Wall Climbing Robot 9 CHAPTER # 1 INTRODUCTION In our introduction to WALL CLIMBING ROBOT we have some background history of all kinds of WALL CLIMBING ROBOTS, secondly the purpose and scope of the project, then lastly the project timeline. 1.1 BACKGROUND There are a variety of potential applications of robotics to wall climbing operations that can increase efficiency and safety. These include inspection of concrete walls, to access of under side of bridges, reactor pressure vessel inspection and cleaning tall buildings which are usually performed by humans. Wall climbing robots have the potential to provide a revolutionary step in doing dangerous tasks that are usually performed by humans. Thus increasing the rate of human life .Recently, there have been many demands for automatic cleaning system on outside surface of buildings such as window glass by increasing of modern architectures. Some customized window cleaning machines have already been installed into the practical use in the field of building maintenance. However, almost of them are mounted on the building from the beginning and they needs very expensive costs. Therefore, requirements for small, lightweight and portable window cleaning robot are also growing in the field of building maintenance. As the results of surveying the requirements for the window cleaning robot, the following points are necessary for providing the window cleaning robot for practical use: It should be small size and lightweight for portability. Design, Fabrication and Working of a Wall Climbing Robot 10 Automatic operation during moving. 1.2 PURPOSE AND SCOPE The purpose of the project is to develop a robot that can climb wall. Keeping this in view, our purpose was to build a machine that can stick on walls and climb upwards. After studying the literature written by the previous searchers, suction cups technique was used to stick the robot to the wall as all the required materials were available in the local market. In suction robot that are used for wall climbing robot, they used onboard pump that created a pressure drop inside the vacuum cups that are pressed against the wall or ceiling. As the vacuum cup are pressed against the wall, the on board pump will start to pump the air in the vacuum cup out to the surrounding. After some time, the pressure inside the vacuum cup is lower than that outside the vacuum cup thus it is able to stick onto the wall. The suction adhesion robots are normally used on valley bridges and the temperature could be around 0 to 50 degree Celsius. As for the weight, this type of robot should not be more than 35 kg. The centre of gravity of the robot must be kept as close as to the vertical surface as possible, as the force required to hold the whole weight of the robot will be reduced. Although this kind of robot is normally used to do the cleaning for high buildings and under high way bridges, this will in turn replace human beings thus reducing the accident rate. This will further increase the human being rate of living. 1.3 PROJECT TIMELINE The time management was one of the crucial factors while undertaking this project. So, the time had to be managed properly in order to meet the task in time. Following is an outline of our program showing our targets achieved in last semester. Design, Fabrication and Working of a Wall Climbing Robot 11 Design, Fabrication and Working of a Wall Climbing Robot 12 CHAPTER # 2 LITERATURE REVIEW Many techniques were studied for sticking the robot to the wall. The list of the papers studied with there abstract is given below. 2.1 Development of a Semi Self-contained Wall Climbing Robot with Scanning Type Suction Cups Tomoaki yano, tomohiro suwa, masato muraxami, takuji yamamotq A semi self-contained Wall Climbing Robot with Scanning Type SucMon Cups is developed and tested. This robot has two vacuum pumps on it. The robot is connected to the equipment on the ground only through the electric power cables and information cables. From the experimental results, the robot can walk wound on walls, clear steps, and stick on cracks and gaps. I; he walking speed attains I38[cm/min] which is 59% faster than the wall climbing robot I with tubes connected to the external vacuum pump on the ground. These experimental results show higher possibility towards the development of a complete self-contained wall climbing robot by putting a battery and a CPU on the robot. 2.2 Walking and Running Of the Quadruped Wall-Climbing Robot Akihiko Nagakubo and Shigeo Hirose The development of a wall climbing robot which is able to move over the surface of "3Dimensional terrain", a terrain including floor, wall, and ceiling of any kinds of structures with agility and terrain adaptability is strongly demanded in many industries. The conventional wall climbing machines were far from fulfilling the demand, authors thus have been developing U Design, Fabrication and Working of a Wall Climbing Robot 13 wall-climbing robot with four-terrain-adaptive legs and basic mobility were already demonstrated by the manufactured prototype model NINJA-I. However, as there are almost no examples of wall-climbing quadruped animals and conventional machines, the gait control method specific for the quadruped wall-climbing is not at all known by now. As the first step to consider a general gait problem of a quadruped wall-climbing robot, this paper investigates a gait of the robot on a vertical and flat wall. The gait is analyzed with the criterion to maximize the locomotion speed under the constraints of predetermined conditions of the supporting-legs position, order and phases of swing legs to prevent turn over motion. As a result of the analysis, the optimal standard gait, named "Wall Gait", is shown to maintain foot posture of A shape and moves the leg in the order of leg1 - leg2 - leg4 - leg3 in static walk and the order of "pace" in dynamic walk. 2.3 A Wall Climbing Robot Using Propulsive Force of a Propeller Akira Nishi A robot capable of moving on a vertical wall of high rise building s has been expected for a long time to utilize it for rescuing, wall inspection, fire-fighting, etc. those hazardous tasks are suitable missions for the robot. A wall climbing robot using thrust force of propellers has been developed. The thrust force is inclined a little to the wall side to produce the frictional force between the wheels and wall surface. As the strong wind is predicted on the wall surface of buildings, the direction of thrust force is controlled to compensate the wind force acting on the robot. A frictional force augmenter is also considered, which is an air foil to produce the lift force directed to the wall side by the cross-wind. Its effect is tested in the wind tunnel. The overall performance of the robot is examined by computer simulation and a model was constructed and tested on the wall. Design, Fabrication and Working of a Wall Climbing Robot 14 2.4 Machine That Can Walk and Climb on Floors, Walls and Ceiling Shigeo Hirose, Akihiko Nagakubo and Roysei Toyama The configuration design for a wall-climbing robot which is capable of moving on diversified surfaces of wall and has high payload capability is discussed and a developed quadruped wall climbing robot, NINJA-1 is introduced. NINJA-1 is composed of 1. Legs based on a 3D parallel link mechanism capable of producing a powerful driving force for moving on the surface of the wall. 2. 2. A CP (conduit-wire-driven parallelogram) mechanism to adjust the posture of the ankle 3. A VM (Valve-regulated Multiple) sucker which can provide suction even if there are grooves and small differences in level of the wall. Finally the data of the trial manufactured NINJA-1 and the up-to-date status of the walking motion is shown. 2.5 Development of Small-Size Window Cleaning Robot by Wall Climbing Mechanism Tohru Miyake, Hidenori Ishihara, Ryu Shoji and Shunichi Yoshida The objective of this research is to develop the small-size and light weight window cleaning robot. The prototype of window cleaning robot has been developed. The dimensions of prototyped robot are approximately 300mm x 300mm x 100mm and its weight is approximately 3 kg. The prototyped robot consists of two independently driven wheels and an active suction cup. The control system which includes traveling direction controller using accelerometer and traveling distance controller using rotary encoder and edge sensors ware installed for autonomous operation. This paper includes background and objectives of this research, prototyped mechanical systems, moving control system, experimental result of basic traveling control and window wiping motion by comparing to with or without of motioned control system, some discussions in each experiment and a conclusion. Design, Fabrication and Working of a Wall Climbing Robot 15 2.6 Vortex's Wall Climbing Robot It utilizes a vortex vacuum to suction itself to vertical surfaces, and then relies on the wheels to get it moving. The vortex climber is based on a so-called "tornado in a cup" technology. Figure in Appendix D. 2.7 Gecko Inspired Surface Climbing Robots Carlo Menon, Michael Murphy, and Metin Sitti, Member, IEEE Many applications call for robots to perform tasks in workspaces where traditional vehicles cannot reach. Using robots to perform these tasks can afford better human safety as well as lower cost operations. This paper focuses on the development of gecko inspired synthetic dry adhesives for wall climbing robots which can scale vertical walls. Many applications are of great interest for this kind of robot such as inspection, repair, cleaning, and exploration. The fabrication of synthetic dry adhesives inspired by nature is discussed as well as the design of prototype wall climbing robots. Results are presented and discussed to show the feasibility of novel Gecko inspired robots. The gecko’s ability to stick to surfaces lies in its feet, specifically the very fine hairs on its toes. There are billions of these tiny fibers which make contact with the surface and create a significant collective surface area of contact. The hairs have physical properties which let them bend and conform to a wide variety of surface roughness, meaning that the adhesion arises from the structure of these hairs themselves. 2.8 Stanford's Stickybot Wall-Climbing Robot Lizard The robot gecko has feet coated with a polymer designed to mimic the properties of setae, the tiny hairs on gecko feet that enable the lizards to climb walls. That allows the bot to clamber freely without the surface in question having to be doused with slime, as required by MIT's bot. Figure in appendix D Design, Fabrication and Working of a Wall Climbing Robot 16 2.9 The BIGGALO Wall Climbing Robot The BIGGALO Wall Climbing Robot was an ELEC/CENG 499A Design Project Course project at the University of Victoria. Figure in appendix D. BIGGALO (BIG uGly And LOud robot) is a pneumatics based robot that is designed to climb relatively smooth vertical surfaces. It was designed and built during the summer semester (May-August) in 2003 by a team of engineering students at the University of Victoria. The project itself was an exercise in mechanical design, control, and microcontroller programming. It worked to combine the mechanical, electrical and computer engineering fields together. It uses externally compressed air to generate suction at each of its suction cups, and solenoids to move pneumatic cylinders. All of this is controlled by a central microcontroller. The robot can climb walls through a series of motions, by turning off the suction to a few suction cups, forcing the cylinders to extend, placing the suction cups down and reengaging suction, it can move across vertical surfaces very easily. (BIGALLO WALL CLIMBING ROBOT) Design, Fabrication and Working of a Wall Climbing Robot 17 CHAPTER # 3 MECHANICAL SYSTEM 3.1 OVERVIEW The robot to its present form came through lots of changes. We started designing our robot keeping in mind BIGGALO wall climbing robot. Many limitations due to the nonavailability or financial restraints came our way and we kept on changing our design so that we could make a model from local resources. Our wall climbing robot is two legged having two suction cups on each leg and uses suction technique to stick on to the wall. Two legs of the robot are always in contact with the wall while the other two are moving. It is designed to keeping in mind the lesser weight. A single actuator moves the robot as well as provides necessary vacuum to keep it on to the wall. Design, Fabrication and Working of a Wall Climbing Robot 18 3.2 DIFFERENT TECHNIQUES TO STICK ROBOT ONTO WALL One of the most challenging tasks in climbing robot design is to develop a proper adhesion mechanism to ensure that the robot sticks to wall surfaces reliably without sacrificing mobility. After review of the above mentioned literature and there developed prototypes we concluded that So far, four types of adhesion techniques have been investigated Magnetic devices for climbing ferrous surfaces Attraction force generators based on aerodynamic principles; Bio-mimetic approaches inspired by climbing animals Vacuum suction techniques for smooth and nonporous surfaces. Magnetic adhesion devices are most promising for robots moving around on steel structures. Robots using permanent magnets or electromagnets can be found for climbing large steel structures for internal inspection of iron pipes. However, their applications are limited to steel walls due to the nature of magnets. Choosing to create attraction force based on aerodynamic principles including the use of propeller is a complex task. Robots which create attraction force based on aerodynamic principles have demonstrated the capability moving on brick and concrete walls with considerable success. However, the power consumption and noise are two issues need to be addressed for some surveillance tasks. Keeping in view that all the materials are available in market, using the technique of the propulsive force was not possible as there are no vendors to make an exact propeller of required specification Dry adhesive called geckos are synthetic polymers designed to mimic the properties of setae. Setae are the naturally occurring adhesive in the feet of lizards which enable them to climb almost all kind of surfaces. The ability of geckos to climb on sheer surfaces has been attributed to van-der-Waals force. Van-der-Waals force refers to the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules. It is also sometimes used loosely as a synonym for the totality of intermolecular forces. Van-der-Waals Design, Fabrication and Working of a Wall Climbing Robot 19 forces are relatively weak compared to normal chemical bonds, but play a fundamental role in many fields. A recent study suggests that water molecules of roughly monolayer thickness (present on all surfaces) also play a role. Nevertheless, a gecko can hang on a glass surface using only one toe. Efforts continue to create a synthetic "gecko tape" that exploits this knowledge. So far, research has produced some promising results - early research yielded an adhesive tape product, which only obtains a fraction of the forces measured from the natural material, and new research are being developed with the goal of featuring 200 times the adhesive forces of the natural material. In applications for non-ferromagnetic wall surfaces, climbing robots most generally use vacuum suctions to produce the adhesion force. We choose suction cups as the technique to stick our robot to the wall as they were available in the local market. 3.3 MATHEMATICAL CALCULATIONS The robot is supported on the wall with a vacuum cup .Their is a pressure difference between inside of the cup and outside. This pressure difference produces the force which holds the cup and as a result robot on the wall. The force is given as Where Pat Atmospheric Pressure Pin Pressure inside Vacuum Cup A Area of Vacuum Cup Design, Fabrication and Working of a Wall Climbing Robot 20 The force which does not let the cup slide on wall is frictional force. Where F R µ Reaction Force Frictional Force Coefficient of Friction between Pad and Wall, it depends on the materials of Wall and pad The forces acting on the robot are shown in a free body diagram below Design, Fabrication and Working of a Wall Climbing Robot Where W 21 Weight of Robot Angle of Inclination The change in volume to create required pressure can be calculated from the following calculations Where Pat Pin Atmospheric Pressure Pressure inside Vacuum Cup V1 Volume at t=max V2 Volume at t=0 Now the distance of centre of gravity from the cups varies .The cups below the centre of gravity requires less force than the cups above the centre of gravity to keep the robot attached to the wall. Where vacuum cups. Are the distance from centre of gravity to the centre of the respective Design, Fabrication and Working of a Wall Climbing Robot 22 Suppose that the distance of cups from centre of gravity is same then the above equation becomes the following equation. The cups above the centre of gravity are at same height from centre of gravity so the forces acting on these cups can be added. Similarly forces acting on the cups below the centre of gravity can be added. Hence As it can be seen from diagram that forces the vacuum in the vacuum cup while and and act opposite to the force created by act along the force created by the vacuum in the Design, Fabrication and Working of a Wall Climbing Robot 23 vacuum cups. For the selection of the cups of equal diameter the force acting on the cups above the centre of gravity must be considered. DC Motor Speed Control without Feedback (Open Loop Control): It is desirable to have a variable speed electric motor that can provide an adjustable speed to the process which is then maintained constant. This can be accomplished with the DC Electric motor with the voltage controller Torque of the motor is exactly proportional to the current and voltage is directly proportional to the angular speed. As shown in following two equations. Where K constant of proportionality Real amplifiers lose voltage as the output current increases due to output resistance assuming an overall amplifier gain We can state the equation of the amplifier as Since the torque driving the load is the motor torque and since the inertia, friction and the disturbance torque oppose the motion the torque balance can be stated as From this above equation we can eliminate which gives us the speed of the motor. Design, Fabrication and Working of a Wall Climbing Robot 24 Where A area B linear viscous damping coefficient G amplifier gain J Tl Tm motor torque T V control volume Rotational speed d input speed (set-point speed) n natural frequency m motor speed Output position 3.4 MECHANICAL STRUCTURE 3.4.1 Main Column It is the chassis of the whole robot. It keeps all the parts together. It is made of aluminum6061. It is also called LM6. The chemical composition of this alloy mainly consists of aluminum and silicon. As it can been seen from the detailed drawing of the part, it was difficult to carve it out from a block of metal. Hence this part was made by sand casting process. LM6 is widely used for castings as it possesses exceptional casting characteristics and gave us the required Design, Fabrication and Working of a Wall Climbing Robot 25 strength of 176.87 N/mm2. LM6 exhibits excellent resistance to corrosion under both ordinary atmospheric and marine conditions. For the severest conditions this property can be further enhanced by anodic treatment. LM6 can be anodized by any of the common processes, the resulting protective film. Main Column Due to high silicon content this alloy is difficult to machine and causes rapid tool wear. Hence after sand casting, to give finish to the part carbide tipped tools with large rake angles, relatively low cutting speed, a cutting lubricant and coolant was employed. It cannot be heat treated as its strength falls rapidly at elevated temperatures. A detailed drawing of the part is given in Appendix D 3.4.2 Legs The legs will hold the chassis with the suction cups. The leg is pivoted with the chassis or main body. This part is also made by process of sand casting. Hence LM6 aluminum alloy is Design, Fabrication and Working of a Wall Climbing Robot 26 used due to its exceptional casting characteristics. After casting the part was machined for finishing. . A detailed drawing of the part is given in Appendix D legs 3.4.3 Slider Housing This part is fixed in the center of the chassis as shown in the fig. this part is made up of mild steel. As it is evident by the name given to this part, it will hold the slider which has to slide in it. Hence the part has to be stiff and hard. That is why mild steel is chosen as its material. As mild steel is easily machine able and the part is not of difficult make, hence part is machined out of the mild steel block. It is also cheap but it has poor corrosion resistance i.e. it rusts. It will also hold the syringes that create the vacuum. . A detailed drawing of the part is given in Appendix D Design, Fabrication and Working of a Wall Climbing Robot 27 Slider Housing 3.4.4 Slider As it is evident from the name given to the part, it will slide. Sliding motion will take place inside the slider housing. As low friction is required between the slider and its housing hence slider is made of brass. Brass has chemical combination of copper and zinc. . A detailed drawing of the part is given in Appendix D slider Design, Fabrication and Working of a Wall Climbing Robot 28 3.4.5 Eccentric This part connects the motor with the slider. This part is made of mild steel as it will deliver the power from motor to the slider. A detailed drawing of the part is given in Appendix D eccentric 3.4.6 Washer The main column and the legs of the structure are separated by washers. Washers are used to reduce friction between the moving leg and the main column. As washer is employed to reduce friction so the material used for washer is brass. Similarly washers are used between slider and legs. . A detailed drawing of the part is given in Appendix D washer Design, Fabrication and Working of a Wall Climbing Robot 29 3.4.7 Tee The tee is made of mild steel. It is connected at the mouth of pump assembly. . A detailed drawing of the part is given in Appendix D tee 3.4.8 Screws Screws of different specifications are used as required. The details of the screws are mentioned in the fig. A screw used as a threaded fastener consists of a shaft, which are usually cylindrical and in many cases tapering to a point at one end and with a helical ridge or thread formed on it, and a head at one end which can be rotated by some means. The thread is essentially an inclined plane wrapped around the shaft. The thread mates with a complementary Design, Fabrication and Working of a Wall Climbing Robot 30 helix in the material. The material may be manufactured with the mating helix using a tap, or the screw may create it when first driven in (a self-tapping screw). The head is specially shaped to allow a screwdriver or wrench (British English: spanner) to rotate the screw, driving it in or releasing it. The head is of larger diameter than the body of the screw and has no thread so that the screw can not be driven deeper than the length of the shaft, and to provide compression. Screws can normally be removed and reinserted without reducing their effectiveness. They have greater holding power than nails and permit disassembly and reuse. The vast majority of screws are tightened by clockwise rotation; we speak of a right-hand thread. Screws with left-hand threads are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread. Left-hand screws are used on rotating items such as the left-hand grinding wheel on a bench grinder or the left hand pedal on a bicycle (both looking towards the equipment) or hub nuts on the left side of some automobiles. Threaded fasteners were made by a cutting action such as dies provide, but recent advances in tooling allow them to be made by rolling an unthreaded rod (the blank) between two specially machined dies which squeeze the blank into the shape of the required fastener, including the thread. This method has the advantages of work hardening the thread and saving material. A rolled thread can be distinguished from a thread formed by a die as the outside diameter of the thread is greater than the diameter of the unthreaded portion of the shaft. Bicycle spokes, which are just bolts with long thin unthreaded portions, always use rolled threads for strength. Hexagonal (hex) socket head has a hexagonal hole and is driven by a Hex Wrench, sometimes called an Allen key or Hex key, or by a power tool with a hexagonal bit. Tamperresistant versions with a pin in the recess are available. Hex sockets are increasingly used for Design, Fabrication and Working of a Wall Climbing Robot 31 modern bicycle parts because hex wrenches are very light and easily carried tools. They are also frequently used for self-assembled furniture The screws used are of ISO metric standard. The details of this standard are given in appendix. Screws used are recessed pan head screws. In figure shown below screw ‘a’ is a pan head screw. The list of part number used is 1. ISO 7045 – M5 x 12 – 4.8 – Z 2. ISO 7089 – 5 – 140 – HV 3. ISO 7045 – M3 x 8 – 4.8 – Z 4. ISO 7045 – M5 x 10 – 4.8 – Z For nomenclature of screws consult Appendix B. 3.5 SUCTION CUPS A suction cup is a device, usually made of rubber or plastic, that sticks to smooth, nonporous surfaces. They are usually used to attach objects together with the use of suction. Upon pressing the suction cup to a surface, the air pressure inside is drastically reduced. The relatively higher atmospheric pressure outside prevents the cup from lifting off the surface; friction does the rest of the work. Suction cups are believed to have first been used in the 3rd century, B.C., and were made out of gourds. They were used to suction "bad blood" from internal organs to the surface. Hippocrates is believed to have invented this procedure. Suction cups are used on nerf darts and can also be found on plungers. The modern suction cup was Design, Fabrication and Working of a Wall Climbing Robot 32 patented in 1882, and is based on the suction cup-like features on octopus arms. The patent has expired. 3.5.1 Selecting a Vacuum Cup a) The Load Or Lifting Capacity Of A Vacuum Cup The load or lifting of capacity of a vacuum cup is determined by the formula below F=PxA F = Weight of the object in pounds, multiplied by a safety factor P = the expected vacuum level in bar. A = the area of the vacuum cup in square millimeters. b) Safety Factors For Vacuum Lifting: Always include safety factors when calculating lifting capabilities. Horizontal Lift = 2 Vertical Lift = 4 Safety factor of 2 is recommended Safety factor of 4 is recommended when cup face is in horizontal position. when cup face is in a vertical position. Design, Fabrication and Working of a Wall Climbing Robot 33 Example: 0.68 bar is maximum holding force. The goal is to lift a 5'x5' cardboard box weighing 10 kg. Load lifted in horizontal position (safety factor=2) Using the formula, the area of the vacuum cups required can be calculated in the following manner: • F=20 kg. x 9.8 (weight x safety factor or 10 kg. x 2) • P=0.68 bar • A=2880mm2 (A=F/P) Therefore, 2880mm2 is the total area of the cup or cups needed to lift this load horizontally. Load lifted in vertical position (safety factor=4) • F=40 kg. X 9.8 (weight x safety factor or 10 kg. x 4) • P=.68 bar • A=5764mm2 (A=F/P) Therefore, 5764mm2 is the total area of the cup or cups needed to lift this load vertically. Again, for stability, we recommend using more than one cup. Therefore, for an application to be lifted: If you were to use 5 cups, you would look for cups with an Approx. Area in.2 of 4.… (20/5 = 4) If you were to use 4 cups, you would look for cups with an Approx. Area in. 2 of 5…. (20/4 = 5) c) Relationship between Cups Dia And Lifting Force The chart below indicates the relationship between the cup diameter and lifting force at various vacuum levels. Please note a vacuum cup adheres to a surface as Design, Fabrication and Working of a Wall Climbing Robot 34 the surrounding or atmospheric pressure is greater than the pressure within the vacuum cup. While selecting the cups consider a safety factor of 3 to 1. Vacuum 0.17 Level (bar) Cup Diameter (mm) 25 0.5 50 3.5 75 7.5 100 14.0 125 21.5 150 31.0 0.34 0.51 0.68 0.75 0.82 0.88 0.93 1.00 1.5 6.5 15.5 28.0 43.5 63.0 2.5 10.0 23.5 42.0 65.5 94.0 3.5 14.0 31.0 55.5 87.5 126.0 3.5 15.0 34.0 61.5 96.0 138.5 4.0 16.5 37.5 67.0 104.5 151.0 4.5 18.0 40.5 72.5 113.5 163.5 4.5 19.0 42.5 75.5 117.5 170.0 4.5 19.5 43.5 78.0 122.0 176.0 d) Three steps for selection of cup From above we conclude that selection of cups is done in three steps Step 1 Select the size by determining the best seal or picking area - ideally you are seeking a smooth surface to achieve a good seal. The lifting capacities of the different cups of varying sizes are given in the table above that is calculated. The larger the cup the more lifting force will be available. If however, a single cup cannot be used consider multiple mountings of smaller cups on a manifold block. Step 2 Select preferred vacuum cup style. For example, if the sealing area is not smooth, featheredge cups are used and if a pulling motion or immediate lift is required a bellow style cup is used. Design, Fabrication and Working of a Wall Climbing Robot 35 Step 3 Find the matching or corresponding diameter. The area of the cup determines it's bonding capability and force between the cup and the picking area surface. So larger the diameter, larger is its bonding capability. 3.5.2 Forces Holding the Cup on the Wall There are two physics principles involved i.e. friction and air pressure. Friction keeps the suction cup from sliding down the surface and air pressure keeps it against the surface. When the suction cup is on the wall, gravity tries to pull it down. An upward force must oppose gravity or the suction cup will fall. Friction opposes motion between two surfaces. The cup would slide along the wall, so a frictional force opposes the fall. In order to have a frictional force, a perpendicular force is required or the force should be at right angles to the wall. This force is provided by air pressure. The suction cup is designed so that when it is pressed against a smooth surface air is squeezed out of the cup. The suction cup is smooth and can fill any microscopic holes with its squishy rubber. It prevents air from coming back into the cup. Design, Fabrication and Working of a Wall Climbing Robot 36 The cup is also springy. Once it has been pushed, it wants to return to its original shape. Because air has been squeezed out of the cup, the cup makes a partial vacuum. The pressure in the cup is less then the pressure of the air out side. Because the pressure is greater outside the cup then inside, a force pushes on the cup to keep it against the wall. This is the force at right angles to the wall, which is needed for friction. As the pressure difference inside and outside the cup increases, the perpendicular force on the cup increases. After a time, most suction cups fall of because air gets into them. This increases the pressure inside. Eventually, the cup isn’t forced against the wall hard enough by the outside air, so friction cannot hold it up. Better quality rubber in the suction cups tends to keep the air out longer so the suction cups stick longer. 3.5.3 Materials Used In Vacuum Cup Vacuum cups are manufactured from two main materials: PVC Vinyl: This is primary material as it is found to be the softest yet hardest wearing material - the soft, pliable nature of the product offers excellent sealing capacity with wear resistance which is greater than rubber. Silicone: This material is also soft but provides a greater temperature range (see chart below). However it does not have the same wear resistant properties of vinyl. Although we carry some silicone cups in stock most are available on a special order basis Material Working temp Wear resistance Oil resistance Vinyl 0°c to + 50°c Excellent Fair Silicone -45°c to + 200°c Good Good For more details about the vacuum cup and its materials, consult Appendix C. Design, Fabrication and Working of a Wall Climbing Robot 37 3.6 VACUUM PUMP A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The vacuum pump was invented in 1650 by Thomas Savery. 3.6.1 Types Pumps can be broadly categorized according to three techniques: Positive displacement pumps use a mechanism to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere. Momentum transfer pumps, also called molecular pumps, use high speed jets of dense fluid or high speed rotating blades to knock gaseous molecules out of the chamber. Entrapment pumps capture gases in a solid or absorbed state. This includes cryopumps, getters, and ion pumps. Positive displacement pumps are the most effective for low vacuums. Momentum transfer pumps in conjunction with one or two positive displacement pumps are the most common configuration used to achieve high vacuums. In this configuration the positive displacement pump serves two purposes. First it obtains a rough vacuum in the vessel being evacuated before the momentum transfer pump can be used to obtain the high vacuum, as momentum transfer pumps cannot start pumping at atmospheric pressures. Second the positive displacement pump backs up the momentum transfer pump by evacuating to low vacuum the accumulation of displaced molecules in the high vacuum pump. Entrapment pumps can be added to reach ultrahigh vacuums, but they require periodic regeneration of the surfaces that trap air molecules or ions. Due to this requirement their available operational time can be unacceptably short in low and high vacuums, thus limiting their use to ultrahigh vacuums. Pumps also differ in details like manufacturing tolerances, sealing material, pressure, flow, admission or no admission of oil vapor, service intervals, reliability, tolerance to dust, tolerance to chemicals, tolerance to liquids and vibration. Design, Fabrication and Working of a Wall Climbing Robot 38 3.6.2 Performance Measures Pumping speed refers to the volume flow rate of a pump at its inlet, often measured in volume per unit of time. Momentum transfer and entrapment pumps are more effective on some gases than others, so the pumping rate can be different for each of the gases being pumped, and the average volume flow rate of the pump will vary depending on the chemical composition of the gases remaining in the chamber. Throughput refers to the pumping speed multiplied by the gas pressure at the inlet, and is measured in units of pressure-volume/unit time. At a constant temperature, throughput is proportional to the number of molecules being pumped per unit time, and therefore to the mass flow rate of the pump. When discussing a leak in the system or back streaming through the pump, throughput refers to the volume leak rate multiplied by the pressure at the vacuum side of the leak, so the leak throughput can be compared to the pump throughput. Positive displacement and momentum transfer pumps have a constant volume flow rate, (pumping speed,) but as the chamber's pressure drops, this volume contains less and less mass. So although the pumping speed remains constant, the throughput and mass flow rate drop exponentially. Meanwhile, the leakage, evaporation, sublimation and back streaming rates continue to produce a constant throughput into the system. 3.6.3 Positive displacement The manual water pump draws water up from a well by creating a vacuum that water rushes in to fill. In a sense, it acts to evacuate the well, although the high leakage rate of dirt prevents a high quality vacuum from being maintained for any length of time. Design, Fabrication and Working of a Wall Climbing Robot 39 Fluids cannot be pulled, so it is technically impossible to create a vacuum by suction. Suction is the movement of fluids into a vacuum under the effect of a higher external pressure, but the vacuum has to be created first. The easiest way to create an artificial vacuum is to expand the volume of a container. For example, the diaphragm muscle expands the chest cavity, which causes the volume of the lungs to increase. This expansion reduces the pressure and creates a partial vacuum, which is soon filled by air pushed in by atmospheric pressure To continue evacuating a chamber indefinitely without requiring infinite growth, a compartment of the vacuum can be repeatedly closed off, exhausted, and expanded again. This is the principle behind positive displacement pumps, like the manual water pump for example. Inside the pump, a mechanism expands a small sealed cavity to create a deep vacuum. Because of the pressure differential, some fluid from the chamber (or the well, in our example) is pushed into the pump's small cavity. The pump's cavity is then sealed from the chamber, opened to the atmosphere, and squeezed back to a minute size. More sophisticated systems are used for most industrial applications, but the basic principle of cyclic volume removal is the same Rotary vane pump, the most common Diaphragm pump, zero oil contamination Liquid ring pump Piston pump, cheapest Scroll pump, highest speed dry pump Screw pump (10 Pa) Wankel pump External vane pump Roots blower, also called a booster pump, has highest pumping speeds but low compression ratio Design, Fabrication and Working of a Wall Climbing Robot 40 Multistage Roots pump that combine several stages providing high pumping speed with better compression ratio Toepler pump The base pressure of a rubber- and plastic-sealed piston pump system is typically 1 to 50 kPa, while a scroll pump might reach 10 Pa (when new) and a rotary vane oil pump with a clean and empty metallic chamber can easily achieve 0.1 Pa. A positive displacement vacuum pump moves the same volume of gas with each cycle, so its pumping speed is constant unless it is overcome by back streaming. 3.6.4 Techniques Vacuum pumps are combined with chambers and operational procedures into a wide variety of vacuum systems. Sometimes more than one pump will be used (in series or in parallel) in a single application. A partial vacuum, or rough vacuum, can be created using a positive displacement pump that transports a gas load from an inlet port to an outlet (exhaust) port. Because of their mechanical limitations, such pumps can only achieve a low vacuum. To achieve a higher vacuum, other techniques must then be used, typically in series (usually following an initial fast pump down with a positive displacement pump). Some examples might be use of an oil sealed rotary vane pump (the most common positive displacement pump) backing a diffusion pump, or a dry scroll pump backing a turbomolecular pump. There are other combinations depending on the level of vacuum being sought. Achieving high vacuum is difficult because all of the materials exposed to the vacuum must be carefully evaluated for their out gassing and vapor pressure properties. For example, oils, and greases, and rubber, or plastic gaskets used as seals for the vacuum chamber must not boil off when exposed to the vacuum, or the gases they produce would prevent the creation of the desired degree of vacuum. Often, all of the surfaces exposed to the vacuum must be baked at high temperature to drive off adsorbed gases. Design, Fabrication and Working of a Wall Climbing Robot 41 3.6.5 Vacuum Pump versus Venturi Effect Air or steam ejectors seem to offer a cheap and simple alternative to a vacuum pump. What isn't appreciated is the substantial compressed air consumption required by vacuum ejectors. A medium-capacity air ejector needs 4000 liter/min of compressed air at 7bar: an output that requires 28.6kW of motive power. A rotary vane vacuum pump of similar capacity only requires a 7.5kW motor. In other words, it will use about one quarter of the power required by the vacuum air ejector. Vacuum air or steam ejectors are simple venturi devices with no moving parts. Essentially, they comprise a jet nozzle that is aligned with a diffuser. Compressed air is forced through the nozzle into the diffuser, thus entraining the gas in the inlet chamber and creating a vacuum. The sensible option from a power viewpoint, would be to opt for the vacuum pump in the first place, Arguably, venturi devices are simple to maintain (compared with a rotary vane design), but they are not maintenance-free, nor are the compressors that power them. However, any marginal benefits in maintenance costs for vacuum ejectors are heavily outweighed by their additional running costs. System Pros Cons Venturi Pump Strong Vacuum without large hoses, precise control of vacuum on/off, blowoff function an option, compressed air hoses are more flexible and can be bent without sacrificing vacuum strength because of positive pressure inside the hose Expensive component, compressed air needs to be vented somehow out of the clean environment Vacuum pump Slightly less expensive component, fewer tubes, no venting concerns, simpler component, large volume provided, Less precise control with no guaranteed precision on time lag, less flexible hoses for moving assemblies Design, Fabrication and Working of a Wall Climbing Robot 42 3.6.6 Vacuum Pump Used In order to minimize the size of vacuum pump we used a technique. As we required low vacuum so positive displacement pump was applied. We used a medical syringe to create vacuum. The syringe works on positive displacement principle to create vacuum. The syringe is driven by a four-bar mechanism. Two syringes are used, each provides vacuum to two vacuum cups. Syringe is connected to the vacuum cups through a plastic pipe. The pipe used is same as the pipe used to connect the medical drip to cannula in hospitals. 3.6.7 Syringe A syringe nowadays nearly always means a medical syringe, but it can mean any of these: A simple hand-powered piston pump consisting of a plunger that can be pulled and pushed along inside a cylindrical tube (the barrel), which has a small hole on one end, so it can suck liquid in and then squirt it out by the same hole. The word "syringe" came from the Greek word meaning "tube" via extracting a new singular from its Greek-type plural "syringes" In former times the word "syringe" also meant big two-handed pumps of this type used e.g. as early firefighting water pumps. Nowadays the word "syringe" is restricted to smaller devices, used to transfer small amounts of liquids or gases to or from otherwise inaccessible areas, including particularly hypodermic syringes used with a needle for injection. As a result, jet injectors are sometimes called syringes. One medical survival of the word "syringe" for uses other than injection, is in "syringing an ear out", i.e. washing unwanted matter out of the external ear canal. Design, Fabrication and Working of a Wall Climbing Robot 43 3.7 TUBING When using mechanical vacuum pumps or vacuum generators that transport the vacuum flow through tubing from the source to the suction cup. It is suggested that a few rules of thumb be followed. First, do not let the total cross sectional area of the vacuum lines be smaller than the vacuum port. ‘If [the pump] has a 3/4-inch vacuum port, don’t reduce that down to a 1/8-inch or three 1/16-inch lines. The total area should equal the vacuum port. It’s not scientific, but it’s a good rule of thumb,’ also, turn on the system with the vacuum cups unencumbered. If the pump shows any vacuum level at all then the lines are restricted. Flow is directly related to evacuation speed and also allows you to compensate for leakage caused by porous materials like corrugated materials. Design, Fabrication and Working of a Wall Climbing Robot 44 Design, Fabrication and Working of a Wall Climbing Robot 45 3.8 SEALANT A sealant is a viscous material that changes state to become solid, once applied, and is used to prevent the penetration of air, gas, noise, dust, fire, smoke or liquid from one location through a barrier into another. Typically, sealants are used to close small openings that are difficult to shut with other materials, such as concrete, drywall, etc. Desirable properties of sealants include insolubility, corrosion resistance, and adhesion. Uses of sealants vary widely and sealants are used in many industries, for example, construction automotive and aerospace industries. An example of a sealant is silicone. 3.8.1 Types of Sealants Acryl sealants Polysulfide sealants Polyurethane sealants Silicone sealants WTK sealants Firestop 3.8.2 Silicone Sealants Silicones are largely inert compounds with a wide variety of forms and uses. Typically heat-resistant, nonstick and rubber-like, they are frequently used in cookware, medical applications, sealants, lubricants and insulation. Properties: Some of the most useful properties of silicone include: Design, Fabrication and Working of a Wall Climbing Robot 46 Thermal stability (constancy of properties over a wide operating range of −100 to 250 °C). Though not lipophilic, the ability to repel water and form watertight seals. Excellent resistance to oxygen, ozone and sunlight. Flexibility. Good electrical insulation. Nonstick. Low chemical reactivity. Low toxicity. High gas permeability: at room temperature (25 °C) the permeability of silicone rubber for gases like oxygen is approximately 400 times that of butyl rubber, making silicone useful for medical applications (though precluding it from applications where gas-tight seals are necessary). a) Technical Details More precisely called polymerized siloxanes or polysiloxanes, silicones are mixed inorganic-organic polymers with the chemical formula [R2SiO]n, where R = organic groups such as methyl, ethyl, and phenyl. These materials consist of an inorganic silicon-oxygen backbone (…-Si-O-Si-O-Si-O-…) with organic side groups attached to the silicon atoms, which are four-coordinate. In some cases organic side groups can be used to link two or more of these -Si-O- backbones together. By varying the -Si-O- chain lengths, side groups, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions. They can vary in consistency from liquid to gel to rubber to hard plastic. The most common siloxane is linear polydimethylsiloxane (PDMS), a silicone oil. The second largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes. Design, Fabrication and Working of a Wall Climbing Robot 47 Polydimethylsiloxane (PDMS) b) Synthesis Silicones are synthesized from chlorosilanes, tetraethoxysilane, and related compounds. In the case of PDMS, the starting material is dimethylchlorosilane, which reacts with water as follows: n [Si(CH3)2Cl2] + n [H2O] → [Si(CH3)2O]n + 2n HCl During polymerization, this reaction evolves potentially hazardous hydrogen chloride gas. For medical uses, a process was developed where the chlorine atoms in the silane precursor were replaced with acetate groups, so that the reaction product of the final curing process is nontoxic acetic acid (vinegar). As a side effect, the curing process is also much slower in this case. This is the chemistry used in many consumer applications, such as silicone caulk and adhesives. Silane precursors with more acid-forming groups and fewer methyl groups, such as methyltrichlorosilane, can be used to introduce branches or crosslinks in the polymer chain. Ideally, each molecule of such a compound becomes a branch point. This can be used to produce hard silicone resins. Similarly, precursors with three methyl groups can be used to limit molecular weight, since each such molecule has only one reactive site and so forms the end of a siloxane chain. Design, Fabrication and Working of a Wall Climbing Robot 48 Modern silicone resins are made with tetraethoxysilane, which reacts in a more mild and controllable manner than chlorosilanes. c) Chemical Terminology Silicone is often mistakenly referred to as "silicon." Although silicones contain silicon atoms, they are not made up exclusively of silicon, and have completely different physical characteristics from elemental silicon. The word "silicone" is derived from ketone. Dimethylsilicone and dimethyl ketone (a.k.a. acetone) have analogous formulas, thus it was surmised (incorrectly) that they have analogous structures. The same terminology is used for compounds such as silane (an analogue of methane). A true silicone group with a double bond between oxygen and silicon (see figure) does not exist in nature; chemists find that the silicon atom forms a single bond with each of two oxygen atoms, rather than a double bond to a single atom. Polysiloxanes are called "silicone" due to early mistaken assumptions about their structure. d) Uses: Aquarium Joints Aquarium manufacturers have used silicone sealant exclusively from its inception in order to join glass plates, making aquariums of every size and shape. Glass joints made with silicone sealant can withstand hundreds of metric tons of pressure, making obsolete the original aquarium construction method using angleiron and putty. Design, Fabrication and Working of a Wall Climbing Robot 49 3.9 GAIT OF ROBOT The gait of this wall climbing robot is similar on both horizontal and vertical surfaces. Assume that the suction pad is a vacuum sucker. A vacuum sucker requires a little bit of time in order to pump out the air by a vacuum pump with a limited capacity (however, because the return to atmospheric pressure is done much faster, the time for this phase is disregarded in the primary considerations). When the vacuum pump is near its end position i.e. it is near its maximum vacuum it slows down. Speed of the vacuum pump is a function of sine. The robot is always held to the wall by two cups at a time. While the other to move forward or backward. As shown from figure the cups in diagonal configuration are connected to the one vacuum pump. So the diagonal cups alternatively hold the robot on the wall. The vacuum generated by the both the pumps remains same throughout the robot operation i.e. the amount of vacuum generated by both the pumps remains same. Design, Fabrication and Working of a Wall Climbing Robot 50 The gait of the robot can further be explained with help of above mentioned figures. Let us suppose that the robot is held on the wall by vacuum cups a & c in figure 1. The slider (shown in brown color in the figures) moves from its left most position to the right most position. With the movement of the slider, vacuum cups b & d moves ahead. The vacuum cups a & c remain at the same position and robot moves ahead. Now the posture of robot is as shown in figure 2. Now the slider is at its right most position and robot is held to the wall by vacuum cups b & d. this cycle continues and robot moves forward. Design, Fabrication and Working of a Wall Climbing Robot 51 CHAPTER # 4 ELECTROMECHANICAL SYSTEM 4.1 DC MOTOR PRINCIPLES OF OPERATION In any electric motor, operation is based on simple electromagnetism. A currentcarrying conductor generates a magnetic field; when this is then placed in an external magnetic field, it will experience a force proportional to the current in the conductor, and to the strength of the external magnetic field. As you are well aware of from playing with magnets as a kid, opposite (North and South) polarities attract, while like polarities (North and North, South and South) repel. The internal configuration of a DC motor is designed to harness the magnetic interaction between a current-carrying conductor and an external magnetic field to generate rotational motion. Let's start by looking at a simple 2-pole DC electric motor (here red represents a magnet or winding with a "North" polarization, while green represents a magnet or winding with a "South" polarization). Design, Fabrication and Working of a Wall Climbing Robot 52 The geometry of the brushes, commutator contacts, and rotor windings are such that when power is applied, the polarities of the energized winding and the stator magnet(s) are misaligned, and the rotor will rotate until it is almost aligned with the stator's field magnets. As the rotor reaches alignment, the brushes move to the next commutator contacts, and energize the next winding. Given our example two-pole motor, the rotation reverses the direction of current through the rotor winding, leading to a "flip" of the rotor's magnetic field, driving it to continue rotating. In real life, though, DC motors will always have more than two poles (three is a very common number). In particular, this avoids "dead spots" in the commutator. You can imagine how with our example two-pole motor, if the rotor is exactly at the middle of its rotation (perfectly aligned with the field magnets), it will get "stuck" there. Meanwhile, with a two-pole motor, there is a moment where the commutator shorts out the power supply (i.e., both brushes touch both commutator contacts simultaneously). This would be bad for the power supply, waste energy, and damage motor components as well. Yet another disadvantage of such a simple motor is that it would exhibit a high amount of torque "ripple" (the amount of torque it could produce is cyclic with the position of the rotor. DC MOTOR BEHAVIOR At a simplistic level, using DC motors is pretty straightforward -- you put power in, and get rotary motion out. Life, of course, is never this simple -- there are a number of subtleties of DC motor behavior that should be accounted for in BEAMbot design. HIGH-SPEED OUTPUT This is the simplest trait to understand and treat -- most DC motors run at very high output speeds (generally thousands or tens of thousands of RPM). While this is fine for some BEAMbots (say, photopoppers or solarrollers), many BEAMbots (walkers, heads) require lower speeds -- you must put gears on your DC motor's output for these applications. Design, Fabrication and Working of a Wall Climbing Robot 53 BACK EMF Just as putting voltage across a wire in a magnetic field can generate motion, moving a wire through a magnetic field can generate voltage. This means that as a DC motor's rotor spins, it generates voltage -- the output voltage is known as back EMF. Because of back EMF, a spark is created at the commutator as a motor's brushes switch from contact to contact. Meanwhile, back EMF can damage sensitive circuits when a motor is stopped suddenly. NOISE (RIPPLE) ON POWER LINES A number of things will cause a DC motor to put noise on its power lines: commutation noise (a function of brush / commutator design & construction), roughness in bearings (via back EMF), and gearing roughness (via back EMF, if the motor is part of a gearmotor) are three big contributors. Even without these avoidable factors, any electric motor will put noise on its power lines by virtue of the fact that its current draw is not constant throughout its motion. Going back to our example two-pole motor, its current draw will be a function of the angle between its rotor coil and field magnets: Every DC motor has six basic parts -- axle, rotor (a.k.a., armature), stator, commutator, field magnet(s), and brushes. In most common DC motors (and all that beamers will see), the external magnetic field is produced by high-strength permanent magnets1. The stator is the stationary part of the motor -- this includes the motor casing, as well as two or more permanent magnet pole pieces. The rotor (together with the axle and attached commutator) rotates with respect to the stator. The rotor consists of windings (generally on a core), the windings being electrically connected to the commutator. The above diagram shows a common motor layout -- with the rotor inside the stator (field) magnets. Design, Fabrication and Working of a Wall Climbing Robot 54 Since most small DC motors have 3 coils, the coils' current curves will overlay each other: Added together, this ideal motor's current will then look something like this: In this case, the peak-to-peak current ripple is approximately 0.29 mA, while the average motor current is just under 31 mA. So under these conditions, the motor puts about less than 1% of current ripple onto its power lines (and as you can see from the "clean" traces, it outputs essentially no high-frequency current noise). Note that since this is a 3-pole motor, and each coil is energized in both directions over the course of a rotor rotation, one revolution of the rotor will correspond to six of the above curves (here, 6 x 2.4 ms = 0.0144 sec, corresponding to a motor rotation rate of just under 4200 RPM). Motor power ripple can wreak havoc in Nv nets by destabilizing them inadvertently. Fortunately, this can be mitigated by putting a small capacitor across the motor's power lines (you'll only be able to filter out "spikey" transients this way, though -- you'll always see curves like the ones above being imposed on your power). On the flip side of this coin, motor power ripple can be put to good use -- as was shown above, ripple frequency can be used to measure motor speed, and its destabilizing tendencies can be used to reverse a motor without the need for discrete "back-up" sensors. Design, Fabrication and Working of a Wall Climbing Robot 55 PARAMETERIZING DC MOTOR PERFORMANCE As you tinker around with DC motors, you'll start to run across some interesting relationship. Namely you'll discover that torque and current are linearly proportional to each other, as are speed and voltage. Under a fixed load (torque), voltage and current will also be proportional to each other. Digging into the math (and I'll spare you this), it turns out that the current a motor draws is ultimately determined by the torque the motor produces. The generated torque is dependent upon the current I, and factors determined by the materials and internal geometry of the motor. Since the construction of a finished motor will not (!) change during operation, a constant of proportionality between the motor current and the materials / geometry dependent factors can be calculated for a given motor. This constant, the torque constant Kt, describes the torque generated by the motor for a specific motor current: Kt = T / I Or to put it another way, Current through motor = torque produced / torque constant I (Amps) = Torque (oz-in) / Kt (oz-in/A) in imperial units I (Amps) = Torque (N-m) / Kt (N-m/A) in SI units Because of the interrelationship of torque, speed, current, and voltage, the constant current operation of a DC motor produces constant output torque regardless of speed. Given a constant load (i.e. torque) the speed of a motor is solely dependent on the voltage applied to the motor. For DC motors operated at a constant voltage, the speed and torque produced are inversely related (the higher the torque, the lower the speed of the motor). Design, Fabrication and Working of a Wall Climbing Robot 56 We earlier saw that an EMF will be developed across a motor's brushes when its coil is rotated by an external torque -- the magnitude of this EMF is dependent upon materials / geometry factors, and upon the speed at which the coil is rotated. Once again, there is a constant of proportionality which describes the relationship between coil rotational speed and materials / geometry factors, commonly known as the back EMF constant (Ke). The back EMF constant is typically given in volts per unit of rotational speed (which in turn is generally expressed either in RPM or radians / second). If one takes the reciprocal of the back EMF constant, the result is a proportionality constant which relates the voltage applied to the motor terminals to the rotational speed of the coil. This version of the motor constant is commonly known as the velocity constant, Kv. The velocity constant is given in units of rotational speed (again, either RPM or radians / second) per volt. Since the motor construction does not change, regardless of what we're measuring, it turns out that these three constants (Kt, Ke, Kv) are all essentially the same number. The differences between the torque constant and the back EMF constant are simply a matter of the units used, while the velocity constant is simply a useful form of the back EMF constant. If the torque constant is specified in N-m / A and the back EMF constant in V-sec / rad, then: Kt = Ke = 1 / Kv Those of us who live in the U.S., though, are stuck with using more colorful units. Commonly used units for small motors are oz-in for torque and RPM for rotational speed. Using these units of measure, torque constants are often given in oz-in / A, back EMF constants in mV / RPM, and velocity constants in RPM / V. In imperial units, the relationships between motor constants are then as follows: Kt in units of oz-in / A Ke in units of mV / RPM Kv in units of RPM / volt Design, Fabrication and Working of a Wall Climbing Robot 57 Then, Kt = 1352.4 / Kv Ke= Kt / 1.3524 Ke = 1000 / Kv Kv = 1000 / Ke So what good is all this? It means that given a source of known rotational speed (an electric drill, or drill press if you have one), you can compute Ke for a given motor (clamp the motor shaft in the drill's jaws, measure the resulting open-circuit voltage, then do the math). Ke, along with the above information will then give you Kt (so you can compute your motor's theoretical torque at any given current), and Kv (so you can compute your motor's maximum speed at any given voltage). If you can measure stall torque, you can then compute motor efficiency (measured torque expressed as a percentage of the theoretical torque). 4.2 POWER SUPPLY In order to select the required power supply following term must be understood VOLTAGE Voltage is an electrical measure which describes the potential to do work. Higher the voltage, more its risk to you and your health. Systems that use voltages below 50V are considered low-voltage and are not governed by an as strict set of rules as high-voltage systems. Design, Fabrication and Working of a Wall Climbing Robot 58 CURRENT Current is a measure of how many electrons are flowing through a conductor. Current is usually measured in amperes (A). Current flow over time is defined as amperehours (a.k.a. amp-hours or Ah), a product of the average current and the amount of time it flowed. POWER Power is the product of voltage and current and is measured in Watts. Power over time is usually defined in Watt-hours (Wh), the product of the average number of watts and time. Your energy utility usually bills you per kilowatt-hour (kWh), which is 1,000 watt-hours. BATTERY LIFE Battery manufacturers define the end-of-life of a battery when it can no longer hold a proper charge (for example, a cell has shorted) or when the available battery capacity is 80% or less than what the battery was rated for. DEPTH OF DISCHARGE (DOD) The Depth of Discharge (DOD) is a measure of how deeply a battery is discharged. When a battery is 100% full, then the DOD is 0%. Conversely, when a battery is 100% empty, the DOD is 100%. The deeper batteries are discharged on average, the shorter their so-called cycle life. BATTERY STORAGE CAPACITY The Amp-hour (Ah) Capacity of a battery tries to quantify the amount of usable energy it can store at a nominal voltage. All things equal, the greater the physical volume of a battery, the larger its total storage capacity. Storage capacity is additive when batteries are wired in parallel but not if they are wired in series. Design, Fabrication and Working of a Wall Climbing Robot 59 AVAILABLE CAPACITY VERSUS TOTAL CAPACITY Since batteries depend on a chemical reaction to produce electricity, their Available Capacity depends in part on how quickly you attempt to charge or discharge them relative to their Total Capacity. The Total Capacity is frequently abbreviated to C and is a measure of how much energy the battery can store. Available Capacity is always less than Total Capacity. Typically, the amp-hour capacity of a battery is measured at a rate of discharge that will leave it empty in 20 hours (a.k.a. the C/20 rate). If you attempt to discharge a battery faster than the C/20 rate, you will have less available capacity and vice-versa. The more extreme the deviation from the C/20 rate, the greater the available (as opposed to total) capacity difference. However, as you will discover in the next section, this effect is non-linear. The available capacity at the C/100 rate (i.e. 100 hours to discharge) is typically only 10% more than at the C/20 rate. Conversely, a 10% reduction in available capacity is achieved just by going to a C/8 rate (on average). Thus, you are most likely to notice this effect with engine starts and other high-current applications like inverters, windlasses, desalination, or air conditioning systems. CONVERSION EFFICIENCY The conversion efficiency denotes how well a battery converts an electrical charge into chemical energy and back again. The higher this factor, the less energy is converted into heat and the faster a battery can be charged without overheating (all other things being equal). The lower the internal resistance of a battery, the better is its conversion efficiency. One of the main reasons why lead-acid batteries dominate the energy storage markets is that the conversion efficiency of lead-acid cells at 85%-95% is much higher than Nickel-Cadmium (a.k.a. NiCad) at 65%, Alkaline (a.k.a. NiFe) at 60%, or other inexpensive battery technologies. Design, Fabrication and Working of a Wall Climbing Robot 60 SELF-DISCHARGE The self-discharge rate is a measure of how much batteries discharge on their own. The Self-Discharge rate is governed by the construction of the battery and the metallurgy of the lead used inside. For instance, flooded cells typically use lead alloyed with Antimony to increase their mechanical strength. However, the Antimony also increases the self-discharge rate to 8-40% per month. This is why flooded lead-acid batteries should be in use often or left on a trickle-charger. As our motor draws 1.5 Amp current at no load, therefore a battery with a current of 4.5 Ah is selected. The battery should be sealed and rechargeable. Hence a lead acid battery is selected. A lead-acid battery is an electrical storage device that uses a reversible chemical reaction to store energy. It uses a combination of lead plates or grids and an electrolyte consisting of a diluted sulphuric acid to convert electrical energy into potential chemical energy and back again. The electrolyte of lead-acid batteries is hazardous to your health and may produce burns and other permanent damage if you come into contact with it. Gel batteries are used in back-up power supplies for alarm and smaller computer systems (particularly in uninterruptible power supplies) and for electric scooters, electrified bicycles and marine applications. Unlike wet cells, gel cells are sealed, with pressure relief valves in case of overcharging. In normal use they cannot spill liquid electrolyte The life of Lead Acid batteries is usually limited by several factors: Cycle Life is a measure of how many charge and discharge cycles a battery can take before its lead-plate grids/plates are expected to collapse and short out. The greater the average depth-of-discharge, the shorter the cycle life. Age also affects batteries as the chemistry inside them attacks the lead plates. The healthier the "living conditions" of the batteries, the longer they will serve you. Lead-Acid batteries like to be kept at a full charge in a cool place. Only buy recently manufactured batteries, so learn to decipher the date code stamped on Design, Fabrication and Working of a Wall Climbing Robot 61 every battery... (Inquire w/manufacturer). The longer the battery has sat in a store, the less time it will serve you! Since lead-acid batteries will not freeze if fully charged, you can store them in the cold during winter to maximize their life. Construction has a big role in battery life too, some designs are better at preserving batteries than others and the suitability of a design for a given application plays a role also. For example, flooded lead-acid cells will typically fare worse than their VRLA cousins in operations that involve a lot of jerky motion - the immobilized plates in VRLA cells will be stressed less than suspended plates in cheap flooded cells. Plate Thickness helps - the thicker the plates, the more abuse, charge and discharge cycles they can take. Thicker plates will also survive any equalization treatments for sulphation better. The heavier the battery for a given group size, the thicker the plates are, so you can use weight as one guide to buying lead-acid batteries. Sulphation is a constant threat to batteries that are not fully re-charged. A layer of lead sulphate can form in these cells and inhibit the electro-chemical reaction that allows you to charge/discharge batteries. Many batteries can be saved from the recycling heap if they are Equalized In closing, the design life of a battery depends in part on its construction, its type, the thickness of the plates, its charging profiles, etc. All these factors come together to determine just how long your battery may ultimately serve you. LEAD-ACID BATTERY Batteries use a chemical reaction to do work on charge and produce a voltage between their output terminals. Design, Fabrication and Working of a Wall Climbing Robot CHARGING THE LEAD-ACID BATTERY The discharge reaction can be reversed by applying a voltage from a charging source. 62 Design, Fabrication and Working of a Wall Climbing Robot 63 CHAPTER # 5 PROBLEMS AND RECOMMENDATIONS With aspect to mechanical system, there were certainly quite a lot of fabrications involved in the system. It took a lot of time to complete the whole mechanical structure. During this tenure there certainly have been quite a number of problems. Some rose due to ignored aspects of the project and other due to lack of resources. 5.1 SUCTION CUPS The proper calculations are very crucial for appropriate selection of vacuum pads. The range of vacuum cups is very limited in the local market. The vacuum cups available in market were either large or too small from our requirements. The vacuum cups available in market were manual operated i.e. the vacuum cups available were operated by hand. In order to make the vacuum cups connect with the vacuum pumps we have to drill holes in the cups. Nut bolt configuration was applied to the cups to join them with the structure. Again bolts were extruded from inside to give smooth passage to air. The vacuum pads available in market are of different qualities so while purchasing cups it should be kept in mind that cups are of same material and quality. 5.2 VACUUM PUMPS Vacuum pumps available in market were of huge size and weight. So we have to think out-of-box. To get the pump on board, no miniature size pump was available. As described earlier we used syringes to create vacuum. Syringes used were off-the-shelf products, to increase the vacuum level it is recommended to use custom made syringe with a larger dia. as the vacuum pump used doesn’t create vacuum continuously so the pipes and joints leading the vacuum to the cups must be sealed properly. Similarly two vacuum cups are connected to the same pump therefore if a leak occurs in one both the cups loose there hold on the wall. Design, Fabrication and Working of a Wall Climbing Robot 64 5.3 MATERIALS To reduce weight we used aluminum to make most of the parts unless it was necessary otherwise. It is recommended that to reduce weight the whole structure be made of plastics, synthetic polymers. This will decrease the weight of the robot manifold hence motor could be used of low torque and small size. 5.4 SIZE OF DC MOTOR In order to keep the center of gravity as close to wall as possible we have to place the motor under the robot i.e. its belly. Therefore we could not find a motor with such mechanical dimensions that it could be fitted under its belly and still the center of gravity is close to the wall. Hence we have to use such a heavy motor. This problem can be removed by using such a motor that could fit the required dimensions. The size of motor will be further decreased if weight of robot is decreased. 5.5 LIMITED DIRECTION OF MOTION Our robot is limited in its motion. It can only move forward or backward. This is because one vacuum pump is providing vacuum for two vacuum pads .Therefore, two vacuum pads have to stick to the wall at any given time. To change the direction of motion instead of two only one vacuum pad has to stick to the wall, as the motion continues the robot will turn in the required direction. Design, Fabrication and Working of a Wall Climbing Robot 65 CHAPTER # 6 APPENDICES APPENDIX A: (i) Alumnium casting alloy LM6 Mechanical Properties of aluminium (sand & die-casting) alloy LM6 Mechanical properties Sand Casting Die Chill Cast casting 0.2% Proof Stress (N/mm2) 60-70 120 70-80 Tensile Strength (N/mm2) 160-190 280 190-230 Elongation (%) 5 2-5 7 Impact resistance Load (Nm) 6.0 - 9.0 Brinell Hardness 50-55 55-60 55-60 71 71 - - Modulus of Elasticity (x103 71 N/mm2) Shear Strength (N/mm2) 120 Strength at Elevated Temperatures: Tensile strength and hardness of this aluminium casting alloy decreases fairly regularly with increasing temperature and become relatively poor at 250°C. Design, Fabrication and Working of a Wall Climbing Robot 66 Physical Properties of Aluminium (Sand & Die casting) Alloy LM6: Physical properties Coefficient of Thermal Expansion (per °C @ 0.000020 20-100°C) Thermal conductivity (cal/cm2/cm/°C @ 0.34 25°C) Electrical conductivity (% copper standard @ 37 20°C) Density (g/cm3) 2.65 Freezing range (°C) approx. 575-565 Machineability: Aluminium alloys of this and similar compositions are rather difficult to machine, due to their tendency to drag and to the rapid tool wear caused by their high Silicon content. Carbidetipped tools with large rake angles and low cutting speeds give comparatively good results when cutting lubricant and coolant are employed. Corrosion resistance: Aluminium LM6 exhibits excellent resistance to corrosion under both ordinary atmospheric and marine conditions. For the severest conditions, this property can be further enhanced by anodic treatment. Anodizing: LM6 can be anodized by any of the common processes, the resulting film ranging in colour from grey to dark brown. Application and general notes: Having high resistance to corrosion and excellent castability, aluminium LM6 is suitable for most marine 'on deck' castings, water-cooled manifolds and jackets, motor car and road transport fittings, thin section and intricate castings such as motor housings, meter cases and Design, Fabrication and Working of a Wall Climbing Robot 67 switch boxes, for a very large aluminium casting, e.g. cast doors and panels where ease of casting is essential, for chemical and dye industry castings, e.g. pump parts, and for paint industry and food and domestic castings. It is especially suitable for castings that are to be welded. The ductility of LM6 enables castings to be rectified easily or even modified in shape, e.g. simple components may be cast straight and later bent to the required contour. LM6 is equally adaptable for sand casting and aluminium diecasting (gravity diecasting and pressure die casting). It has excellent resistance to corrosion in marine environments, possesses excellent ductility, but is of medium strength and is not heat treated. Its strength falls off rapidly at high temperatures. Its elastic limit is low and there is difficulty with machining. (ii) Mild steel Carbon steel is sometimes referred to as 'mild steel' or 'plain carbon steel'. The American Iron and Steel Institute defines a carbon steel as having no more than 2 % carbon and no other appreciable alloying element. Carbon steel makes up the largest part of steel production and is used in a vast range of applications. Typically carbon steels are stiff and strong. They also exhibit ferromagnetism (i.e. they are magnetic). This means they are extensively used in motors and electrical appliances. Welding carbon steels with carbon content greater than 0.3 % requires that special precautions be taken. However, welding carbon steel presents far fewer problems than welding stainless steels. The corrosion resistance of carbon steels is poor (i.e. they rust) and so they should not be used in a corrosive environment unless some form of protective coating is used. Design, Fabrication and Working of a Wall Climbing Robot CHEMICAL COMPOSITION: Composition Units wt% Aluminum, Al 0.17 Chromium, Cr <0.12 Silicon, Si <0.17 Manganese, Mn 0.5 Nickel, Ni <0.19 Iron, Fe Balance PROPERTIES: Physical Properties Metric English Density 7.80 - 8.00 g/cc 0.282 - 0.289 lb/in³ Hardness, Brinell 121 121 Hardness, Knoop 140 140 Hardness, Vickers 126 126 Tensile Strength, Ultimate 420 MPa 60900 psi Tensile Strength, Yield 350 MPa 50800 psi Elongation at Break 15.0 % 15.0 % Modulus of Elasticity 200 GPa 29000 kpsi Bulk Modulus 140 GPa 20300 ksi Poisson’s Ratio 0.250 0.250 Machinability 65.0 % 65.0 % Mechanical Properties 68 Design, Fabrication and Working of a Wall Climbing Robot Shear Modulus 69 80.0 GPa 11600 ksi 0.0000170 ohm-cm 0.0000170 ohm-cm CTE, linear 20°C 9.50 - 12.6 µm/m-°C 5.28 - 7.00 µin/in-°F CTE, linear 250°C 11.7 µm/m-°C 6.50 µin/in-°F CTE, linear 500°C 12.8 µm/m-°C 7.11 µin/in-°F CTE, linear 1000°C 13.9 µm/m-°C 7.72 µin/in-°F Specific Heat Capacity 0.470 J/g-°C 0.112 BTU/lb-°F Thermal Conductivity 44.0 - 52.0 W/m-K 305 - 361 BTU-in/hr-ft²-°F Electrical Properties Electrical Resistivity Thermal Properties ADVANTAGES: Cheap Wide variety available with different properties High stiffness Magnetic Most carbon steels are easy machine and weld DISADVANTAGES: Poor corrosion resistance (i.e. rusts) (iii) BRASS Brass is any alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties. In comparison, bronze is principally an alloy of copper and tin. Despite this distinction, some types of brasses are called bronzes. Brass is a substitutional alloy. It is used for decoration for its bright gold-like appearance; for applications where low friction is required such as locks, gears, bearings, ammunition, and valves; for Design, Fabrication and Working of a Wall Climbing Robot 70 plumbing and electrical applications; and extensively in musical instruments such as horns and bells for its acoustic properties. Brass has a muted yellow color, somewhat similar to gold. It is relatively resistant to tarnishing, and is often used as decoration and for coins. Brass has likely been known to humans since prehistoric times, even before zinc itself was discovered. It was produced by melting copper together with calamine, a zinc ore. In the German village of Breinigerberg an ancient Roman settlement was discovered where a calamine ore mine existed. During the melting process, the zinc is extracted from the calamine and mixes with the copper. Pure zinc, on the other hand, has too low a boiling point to have been produced by ancient metalworking techniques. The many references to ‘brass’ appearing throughout the King James Bible are thought to signify another bronze alloy, or copper, rather than the strict modern definition of ‘brass’. PROPERTIES: The malleability and acoustic properties of brass have made it the metal of choice for brass musical instruments such as the trombone, tuba, trumpet, euphonium, and the French horn. Even though the saxophone is classified as a woodwind instrument and the harmonica is a free reed aero phone, both are also often made from brass. In organ pipes designed as "reed" pipes, brass strips are used as the "reeds". Brass has higher malleability than copper or zinc. The relatively low melting point of brass (900 to 940°C, depending on composition) and its flow characteristics make it a relatively easy material to cast. By varying the proportions of copper and zinc, the properties of the brass can be changed, allowing hard and soft brasses. Today almost 90% of all brass alloys are recycled. Because most brass is nonmagnetic, it can be separated from ferrous scrap by passing the scrap near a powerful magnet. Brass scrap is collected and transported to the foundry where it is melted and recast into billets. Billets are later heated and extruded into the desired form and size. Design, Fabrication and Working of a Wall Climbing Robot 71 Aluminum makes brass stronger and more corrosion resistant. Aluminum also causes a highly beneficial hard layer of aluminium oxide (Al2O3) to be formed on the surface that is thin, transparent and self healing. Tin has a similar effect and finds its use especially in sea water applications (naval brasses). Combinations of iron, aluminum, silicon and manganese make brass wear and tear resistant. A well known alloy used in the automotive industry is 'LDM C673', where the combination of manganese and silicon leads to a strong and resistant brass. The so called dezincification resistant (DZR) brasses, like alloy 'LDM G563' (known for its brand name 'Enkotal'), are used where there is a large corrosion risk and where normal brasses do not meet the standards. Applications with high water temperatures, chlorides present or deviating water qualities (soft water) play a role. DZR-brass is excellent in water boiler systems. This brass alloy must be produced with great care, with special attention placed on a balanced composition and proper production temperatures and parameters to avoid long-term failures. Drunen, Netherlands, has the only active production facility which makes these high grade brass alloys. The copper in brass makes brass germicidal, via the oligodynamic effect. For example, brass doorknobs disinfect themselves of many bacteria within eight hours. This effect is important in hospitals, but useful in many contexts. Brass door hardware is generally lacquered when new, which prevents tarnishing of the metal for a few years when located outside (and indefinitely when located indoors). After this most manufacturers recommend that the lacquer is removed (e.g. with paint stripper) and the items regularly polished to maintain a bright finish. Unlacquered brass weathers more attractively than brass with deteriorated lacquer, even if polishing is not carried out. Freshly polished brass is similar to gold in appearance, but becomes more reddish within days of exposure to the elements. A traditional polish is Brasso. Brass was used to make fan blades, fan cages and motor bearings in many antique fans that date before the 1930s. Brass can be used for fixings for use in cryogenic systems; however its use is not limited to this. Design, Fabrication and Working of a Wall Climbing Robot 72 The density of brass is approximately 8.4 g/cm3. (iv) Sand Casting INTRODUCTION Sand casting is used to make large parts (typically Iron, but also Bronze, Brass, Aluminum). Molten metal is poured into a mold cavity formed out of sand (natural or synthetic). The processes of sand casting are discussed in this section, include patterns, sprues and runners, design considerations, and casting allowance. PATTERNS The cavity in the sand is formed by using a pattern (an approximate duplicate of the real part), which are typically made out of wood, sometimes metal. The cavity is contained in an aggregate housed in a box called the flask. Core is a sand shape inserted into the mold to produce the internal features of the part such as holes or internal passages. Cores are placed in the cavity to form holes of the desired shapes. Core print is the region added to the pattern, core, or mold that is used to locate and support the core within the mold. A riser is an extra void created in the mold to contain excessive molten material. Typical Components of a Two-Part Sand Casting Mold. Design, Fabrication and Working of a Wall Climbing Robot 73 In a two-part mold, which is typical of sand castings, the upper half, including the top half of the pattern, flask, and core is called cope and the lower half is called drag. The parting line or the parting surface is line or surface that separates the cope and drag. The drag is first filled partially with sand, and the core print, the cores, and the gating system are placed near the parting line. The cope is then assembled to the drag, and the sand is poured on the cope half, covering the pattern, core and the gating system. The sand is compacted by vibration and mechanical means. Next, the cope is removed from the drag, and the pattern is carefully removed. The object is to remove the pattern without breaking the mold cavity. This is facilitated by designing a draft, a slight angular offset from the vertical to the vertical surfaces of the pattern. This is usually a minimum of 1° or 1.5 mm (0.060 in), whichever is greater. The rougher the surface of the pattern, the more the draft to be provided. SPRUES AND RUNNERS: The molten material is poured in the pouring cup, which is part of the gating system that supplies the molten material to the mold cavity. The vertical part of the gating system connected to the pouring cup is the sprue, and the horizontal portion is called the runners and finally to the multiple points where it is introduced to the mold cavity called the gates. Additionally there are extensions to the gating system called vents that provide the path for the built up gases and the displaced air to vent to the atmosphere. The cavity is usually made oversize to allow for the metal contraction as it cools down to room temperature. This is achieved by making the pattern oversize. To account for shrinking, the pattern must be made oversize by these factors, on the average. These are linear factors and apply in each direction. These shrinkage allowance are only approximate, because the exact allowance is determined the shape and size of the casting. In addition, different parts of the casting might require a different shrinkage allowance. See the casting allowance table for the approximate shrinkage allowance expressed as the Pattern Oversize Factor. Design, Fabrication and Working of a Wall Climbing Robot 74 Sand castings generally have a rough surface sometimes with surface impurities, and surface variations. A machining (finish) allowance is made for this type of defect. See casting allowance table for the finish allowance. APPENDIX B: ISO metric screw thread The basic principles of the ISO metric screw thread are defined in international standard ISO 68-1 and preferred combinations of diameter and pitch are listed in ISO 261. The smaller subset of diameter and pitch combinations commonly used in screws, nuts and bolts is given in ISO 262. The most commonly used pitch value for each diameter is known as the "coarse pitch". For some diameters, one or two additional "fine pitch" variants are also specified, for special applications such as threads in thin-walled pipes. ISO metric screw threads are designated by the letter M followed by the major diameter of the thread in millimeters, e.g. "M8". If the thread does not use the normal "coarse pitch" (e.g., 1.25 mm in the case of M8), then the pitch in millimeters is also appended with a multiplication sign, e.g. "M8×1" if the screw thread has an outer diameter of 8 mm and advances by 1 mm per 360° rotation. The nominal diameter of a metric screw is the outer diameter of the thread. The tapped hole (or nut) into which the screw fits has an internal diameter which is the size of the screw minus the pitch of the thread. Thus, an M6 screw, which has a pitch of 1 mm, is made by threading a 6 mm shaft and the nut or threaded hole is made by tapping threads in a 5 mm hole. Metric hexagon bolts, screws and nuts are specified, for example, in British Standard BS 4190 (general purpose screws) and BS 3692 (precision screws). The following table lists the relationship given in these standards between the thread size and the maximal width across the hexagonal flats (wrench size): Design, Fabrication and Working of a Wall Climbing Robot ISO metric thread 75 M M M M M M M M M M M M M M M M M M M 1.6 2 2.5 3 wrench size 3.2 4 (mm) 4 5 5.5 7 5 6 8 10 12 16 20 24 30 36 42 48 56 64 8 10 13 17 19 24 30 36 46 55 65 75 85 95 In addition, the following non-preferred intermediate sizes are specified: ISO metric thread wrench size (mm) M14 M18 M22 M27 M33 M39 M45 M52 M60 M68 22 27 32 41 50 60 70 80 90 100 APPENDIX C: Vacuum cups Bellows Cups with Convolutions Bellows cups have a pliable outer rim that will conform to curved or uneven surfaces while the bellows sections compensate for inconsistent stack heights. Under vacuum the accordion-style bellows cup will collapse on contact. The collapsing action simulates a short cylinder stroke lifting the product for a short distance, possibly saving the need for a separate lifting mechanism. Design, Fabrication and Working of a Wall Climbing Robot 76 Flat Cups with and without Cleats Flat cups without cleats are flexible and work well in applications that do not require lifting heavy loads. Flat cups only achieve the holding force of the area of the thru-hole. In food packaging for example, a flat cup can be used to apply a label to an uneven surface such as a package of chicken. Flat cups with cleats are strong with a rigid, low profile that will lift heavy loads. The low profile allows heavy loads to be lifted vertically without the cup “peeling” away from the product surface or deforming the object being lifted. These cups perform well when gripping smooth, flat heavy objects such as steel sheet, glass (television picture tubes) and coated corrugated. Oval Cups Like cleated cups, oval cups have heavy load handling capabilities due to their rigid design and large vacuum work area. Oval cups have the largest lifting force because they provide the most surface area for a given cup footprint. Deep Cups Deep cups are used for curved and irregular surfaces, not for flat surfaces. They can lift over corners and edges. Universal Cups Universal cups can handle flat or slightly curved surfaces. Severe Cups The severe duty cup line was originally developed to satisfy the harsh physical demands that a cup experiences in the high volume, automotive, sheet metal stamping processes. These cups are available in several styles, and have proven to be very reliable. Their design evolved from the knowledge gained by supplying vacuum cups to the Automotive OEMs for more than15 years. 77 Design, Fabrication and Working of a Wall Climbing Robot VACUUM CUPS - MATERIAL SPECIFICATIONS Cups are available in various durometers, colors and materials. Below is a general description of the various materials available and their characteristics. Material Working Temperature Wear Resistance Oil Resistance Durometer Application Fair A20-A75 Range general purpose material for most applications excellent for resistant applications good for resistance *Vinyl +32 to +125˚F [0°C to +52°C] Oil Resistant Vinyl +32 to +125˚F [0°C to +52°C] Good Excellent A40-A60 Range Polyurethan e +32 to +150˚F [0°C to +66°C] Good Good A20-A70 Range Chloroprene Nitrile SiliconeGrey SiliconeTranslucent Excellent -40 to +230˚F [-20°C to Excellent 110°C] +32 to +194˚F [0°C to +90°C] Good -50 to +400˚F [-46°C to Good 204°C] -94 to +392˚F [-70°C to Good 200°C] Good Good Good Good A50-A60 Range oil chemical general purpose material with good oil resistance and low temperature performance A50-A60 Range general purpose material with good oil and abrasion resistance A30-A60 Range good for applications involving high temperatures, food or non-marking situation A30-A60 Range Good for applications involving high temperatures, food or non-marking situation *Standard durometer for vinyl cups is A50 ±5 points — may vary with color. Other Materials Available - please consult factory: FDA Vinyl, Anti-Static Vinyl, FDA Silicone Design, Fabrication and Working of a Wall Climbing Robot 78 Vacuum Terms and Definitions: Bellows: The fold or collapsible area that allows the cup to compress like an accordion. Convolution: The folded area of a bellows cup that makes up 1 external “V”. Cleats: Bottom protrusions used for maintaining a larger vacuum area. Durometer: Method by which the hardness of a material is gauged. Insert/Fitting: Metal piece bonded or inserted into the material to allow fastening by threads or bolts. Vacuum Cup: Cup that requires the use of an external vacuum source to adhere to a surface. Vacuum Level: The magnitude of suction created by a vacuum pump typically measured in inches of mercury (”Hg) or mm Hg Vacuum Flow: The volume of free air induced by the vacuum pump per unit of time, typically measured in SCFM Permeability: Cardboard vs. steel — sometimes referred to as porosity Standard Atmospheric Pressure Measured at Sea Level: 1 ATM = 14.7 psi = 29.92”Hg = 760mm Hg = 1 Bar Conversion Chart - Vacuum vs. Pressure %vacuum In. Hg mm Hg bar PSI 10 3 76.92 -0.1 1.47 20 6 153.85 -0.2 2.94 30 9 230.77 -0.3 4.41 40 12 307.69 -0.4 5.88 50 15 384.62 -0.5 7.35 60 18 461.54 -0.6 8.82 Design, Fabrication and Working of a Wall Climbing Robot 70 21 538.46 -0.7 10.29 80 24 615.38 -0.8 11.76 90 27 692.31 -0.9 13.23 100 30 769.23 -1.0 14.70 Facts to Remember: 50 mm Hg = 1 PSI 1mm Hg = 1 torr (vacuum) 1”Hg = 25.4mm Hg 2”Hg = 1 PSI 29.92”Hg = 100 Kpa 14.7 PSI = 100 Kpa 14.7 PSI = 29.92” Hg 14.7 PSI = 760mm Hg 79 Design, Fabrication and Working of a Wall Climbing Robot APPENDIX D: (v) Cad diagrams MAIN COLUMN 80 Design, Fabrication and Working of a Wall Climbing Robot LEG 81 Design, Fabrication and Working of a Wall Climbing Robot SLIDER HOUSING 82 Design, Fabrication and Working of a Wall Climbing Robot Slider 83 Design, Fabrication and Working of a Wall Climbing Robot ACCENTRIC 84 Design, Fabrication and Working of a Wall Climbing Robot TEE 85 Design, Fabrication and Working of a Wall Climbing Robot WASHER 86 Design, Fabrication and Working of a Wall Climbing Robot SYRINGE 87 Design, Fabrication and Working of a Wall Climbing Robot BIGALO WALL CLIMBING ROBOT 88 Design, Fabrication and Working of a Wall Climbing Robot VORTEX'S WALL CLIMBING ROBOT 89 Design, Fabrication and Working of a Wall Climbing Robot GECKO INSPIRED SURFACE CLIMBING ROBOTS 90 Design, Fabrication and Working of a Wall Climbing Robot 91 CHAPTER # 7 IMPLEMENTATIONS 7.1 FUTURE PLANS Place the power supply on the robot. Add sensors onto the robot to look for obstacles. Create logic for the robot to be able to wander and not fall of or crash into things. Pressure sensors on the suction cups to determine if a cup is attached or not. Adding application devices (such as window washing apparatus). Modify the robot to move faster. Acquire better control over the exact position of the legs. Find ways to arrange equipment to allow a greater range of motion. DC motor position control can be applied as follows The motor equations are same as used in section 3.3 The amplifier equation is given as We have placed the position sensor and h is the gain of the sensor, θ is the angular position of the output shaft The motor speed is reduced to a lower speed of the output shaft with a speed ratio . Attendant upon the reduction in speed is an increase in torque from the motor torque to the output shaft by a gear train Design, Fabrication and Working of a Wall Climbing Robot As we know angular speed is a derivative of angular position. Torque balance equation is given as under From above equation we can eliminate θ .Now we can see that position is a second order function of input and disturbance. 92 Design, Fabrication and Working of a Wall Climbing Robot 93 7.2 CONCLUSION This compact and lightweight robotic platform provides a safe and effective means to deal with hazardous duty operations. With in the mechanical area our robust platform weighing approximately 3 Kgs is developed to climb on relatively smooth surfaces .From test runs, the vacuum cups are enough to stick the root on the wall but the vacuum pump is of less capacity. Although the vacuum pump is of less capacity but we have achieved a wall climbing of 40 degrees. 7.3 REFERENCES 4QD Motor Current Calculator, 4QD Information section ,2004,www.4qd.co.uk S.Hirose, A.Nagakubo,: "Legged Wall-Climbing Robot", Journal of the Robotics Society of Japan, Vol.10, No.5, pp.575-580, 1992 K.Ikeda, T.Yano,: "Development of a Wall Climbing Robot with Scanning Type Suction Cups", Proceedings of International Mechanical Engineering Congress Sydney, T.Yano, T.Suwa, K.Sakurada, M.Murakami,: "Development of a Wall Climbing Robot II with Scanning Type Suction Cups", Proceedings of the 2nd ECPD International Conference on Advanced Robotics, Intelligent Automation and Active Systems, pp.368373,1996 Luk,B.L., Collie,A.A. and Billingsley,J., "ROBUG 11: AN INTELLIGENT WALL CLIMBING ROBOT", In Proceedings of the I991 IEEE International Conference on Robotics and Automation, pp.2342-2347, I99 I . McGhee,R.B., "Vehicular Legged Locomotion", Advances in Automation and Robotics (Editer : G.N.Saridis), V01.1, Greenwhich, CT : Jai Press, pp.259-284, 1985. S. W. Ryu, et al.; Self-contained Wall-climbing Robot with Closed Link Mechanism. Proc. of the 2001 IEEE/RSJ Int’l Conf. on Intelligent Robots and Systems, pp. 839-844, 2001. Design, Fabrication and Working of a Wall Climbing Robot 94 Y WANG, et al., The study and application of wall-climbing robot for cleaning, Third Int’l conf. On Climbing and Walking Robots, pp. 789-794, 2000. R.D. Schraft, et al.; “Mechanical Design of an Autonomous, Lightweight Robot for window cleaning”, Proc. of the 33rd Int’l Symp. on Robotics (ISR), 2002 R. Siegwart and Illah R. Nourbakhsh; Introduction to Autonomous Mobile Robots, MIT Press, 2004 MRT CASTING LIMITED Founders, Machinists & Finishers Of Quality Non-Ferrous Cast Components Engineering Designer, V 30, N 3, May-June 2004 Machinery Handbook, Industrial Press Inc, New York, Edition 24 EFUNDA ENGINEERING FUNDAMENTALS , sand casting ,www.efunda .com www.dart-vacuum.com Design, Fabrication and Working of a Wall Climbing Robot 95