Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Final Report Table of contents Abstract………………………………………………… Introduction…………………………………………………… 3 Table 1: Existing Wheelchair Scale Products [3-5]……………… Purpose……………………………………………………………... Method……………………………………………………………… Outline of Report…………………………………………………… Design 1……………………………………………………………… Figure 1: Side view of scale unit……………………………………… Figure 2: Another side view of scale unit - entrance ramps visible……. Equipment……………………………………………………………… Figure 3: Strain gage array and Wheatstone bridge circuit……… Figure 4: Beam load cell, model LCEC – 1k………………………… Specification…………………………………………………………… Static Analysis…………………………………………………………. Figure 5: FW = 800 lbs at maximum loading…………………………. Block Diagram of System…………………………………………… Design 2………………………………………………………………... Figure 6: Front view of scale with four load cells…………………… Figure 7: Side view of scale with four load cells………………………. Equipment……………………………………………………………… Specifications………………………………………………………….. Static Force Analysis…………………………………………………... Assumptions……………………………………………………………. Figure 8: Free body diagram of four-load-cell design………………….. Circuit Analysis………………………………………………………… Block Diagram of System………………………………………………. Flow chart of the Microprocessor………………………………………. Figure 9: Flowchart of processing and display unit………………….…. User interface flowchart………………………………………………… Figure 10: User interface flowchart…………………………………….. Conclusion……………………………………………………………… Design 3………………………………………………………………… Figure 11: Side view of weighing platform with FTUs near corners….. Figure 12: Top view of weigh platform………………………………... Figure 13: FTU diagram showing bending material, strain gage, and leg pieces causing 3-point bending………………………………... Static Analysis………………………………………………………….. Tentative part List………………………………………………………. Wheatstone Bridge Circuit……………………………………………… Circuit Analysis…………………………………………………………. Block Diagram of System…………………………………………….… Flow chart of the Microprocessor………………………………………. Figure 14: Flowchart of processing and display unit…………………… User interface flow chart……………………………………………….. Figure 15: User interface flowchart…………………………………….. Conclusion……………………………………………………………… Choosing the Optimal Design…………………………………………. Optimal Design…………………………………………………………. Objective…………………………………………………………… Figure 16: Front view of scale with four load cells…………………. Figure 17: Side view of scale with four load cells………………… Figure 18: Top view of optimal design with four load cells……… Equipment…………………………………………………………… Static Force Analysis………………………………………………… Figure 18: Beam view simulating platform bending behavior…….. Figure 19: Beam view simulating distributed 500 lb load………… Analysis…………………………………………………………… Circuit Analysis…………………………………………………… Block Diagram of System…………………………………………… Flow chart of the Microprocessor………………………………… Figure 20: Flowchart of processing and display unit……………… User interface chart………………………………………………… Figure 21: User interface flowchart……………………………… Final Design Product Circuit Analysis ................................................................................ Mechanical Design ........................................................................... Scale Design-Finished Product........................................................... figures 22-35 Interface Design................................................................................. Engineering Standards and Environmental Considerations………….. IEEE Code of Ethics [11]…………………………………………….. Parts List……………………………………………………………… Timeline………………………………………………………………. Conclusion……………………………………………………………. References……………………………………………………………. Acknowledgments……………………………………………………. Abstract: A major problem with the weighing of the wheelchair-bound patient is convenience and ease of use of a wheelchair scale. It is necessary for patients having lung disorders such as COPD (Chronic Obstructive Pulmonary Disorder) or emphysema to watch their weight closely, and as a result, a wheelchair scale is needed that can weigh users once or twice a day and store a history of weights for tracking progress. Since it is inconvenient to exit the wheelchair and be weighed conventionally, most wheelchair scales weigh both user and wheelchair, then subtract the weight of the latter. Wheelchair scales are typically large, bulky, and not designed for home use. The wheelchair-bound population is one of the largest minorities in the nation and the prohibitive price-tag on current wheelchair scales create a significant market for a wheelchair scale that can be set up in one’s home and used as easily as a television. The typical wheelchair scale consists of a weighing platform, access ramps to the platform and a display with an interface. Most wheelchair scales have a price between $1400 and $3200 and exhibit a complex interface that does not log previous weights. The wheelchair scale outlined in this report will exhibit this unique feature, allowing the user to assess improvement. Furthermore, the total cost will be below the average wheelchair scale price, making it affordable and increasing the market potential. Three potential designs for the wheelchair scale are discussed, and an optimized version of one is presented in-depth. Introduction: Background: COPD is a disease in which the lungs have difficulty keeping their shape and the patient to inhale and exhale. COPD is irreversible, and as a result many patients are permanently wheelchair-bound and must also carry oxygen tanks. Weight gain is common with COPD sufferers and being underweight or overweight can adversely affect the patient’s health [1-2]. The device to be designed is intended to help wheelchair-bound COPD patients assess their relative health through their weight, as physical exercise is absolutely essential to keep COPD patients healthy. Another goal is to further increase the convenience encountered by wheelchair-bound users of weighing one’s self at home. The device to be designed is intended to help wheelchair-bound COPD patients assess their relative health through their weight, as physical exercise is absolutely essential to keep COPD patients healthy. Wheelchair scales have been designed and brought to market: A search on the U.S. Patent and Trademarks Office website resulted in numerous examples of devices intended to adapt a standard weighing platform or scale to wheelchair use [6]. The purpose of most scale adapting methods is to check the forward motion of the wheelchair by its wheels; many contain supports for contact with the wheelchair chassis itself to keep the platform surface from contacting either the back set or front set of wheels. Rishel (U.S. Pat. No. 5,448,022) describes a tip-on wheelchair scale adaptor consisting of a support on which the front of the wheelchair chassis is tipped and a support holding the back of the wheelchair so that both wheels are off the platform. Disadvantages of platforms adapting scales to wheelchair-bound individuals include significant wobble [6], which may cause discomfort to older persons. Also, this arrangement, as do many of the patented wheelchair scale platforms, requires “only one assistant” for each weighing operation. An improvement on the design is found in U.S. Pat. No. 6,268,572, where Wilson outlines a design for a platform with an access ramp to be placed on a standard weighing device, so that the individual can access the scale independently. However, the weight may need to be read by an assistant, as it is visible through a slot cut out in the platform (placed over the scale) and can be under the wheelchair chassis [6]. Based on the prior art of patented wheelchair scale adaptors patented, as well as the prior art status of any weighing device using a strain gage or similar measuring method, it can be concluded that the device cannot be patented based on its ability to weigh a wheelchair-bound individual. Many products exist on the market for the weighing of individuals in wheelchairs; most of these designs are lacking compatibility with standard computer systems and an interface that is easily accessible to individuals with hearing or vision impairments. The ability of the product to be patented is limited; however, it can offer options that cannot be found in the market yet. There is a large variety of products on the market of wheelchair scales incorporating various features. Wheelchair scales can be mechanical (typically balancebeam scales) or electronic (transducer-based), and have features added such as wheels and folding parts for easy storage and setup and smart-card functionality and/or memory for storing previous weights and weight of the wheelchair for taring. The wheelchair scale model by Seca model 664 [4] is a state of the art wheelchair scale that is lightweight, portable, and has the option of being able to store the weights of wheelchairs belonging to multiple users on smart cards. The weighing platform is large enough to accommodate any wheelchair user and can be entered using low-grade ramps from either the front or back. Shallow protective railings on either side of the weighing platform opposite the access ramps on either side are alternatively used as handles and the entire scale folds into a compact unit for side storage. The unit costs $1875 dollars plus $65 for shipping/handling [4]. Detecto, a division of Cardinal Scale Manufacturing, offers several models, mechanical and digital [3], with large rubberized-mat weighing platforms (minimum 26” by 30”) and a capacity and resolution standard in the industry (500lb and 0.2 lb, or 1000lb and 0.5 lb res. respectively). The FHD133II/FD144II Geriatric/Bariatric digital wheelchair scale incorporates a keypad with the display unit for typing in the wheelchair weight for taring. An ABLEDATA database search for wheelchair scales turned up 12 devices, even including a “do it yourself” entry on how to build a plywood platform to adapt a bathroom scale to a wheelchair [5]. A typical wheelchair scale solution costs within the range of $1400 to $3200 and most scales have load limits around 500 lb and 800 lb with resolutions of 0.2 lb. Most wheelchair scales are accessed by ramps leading to the platform, usually with safety barriers on either side of both ramp and platform. Table 1: Existing Wheelchair Scale Products [3-5] Found Using Abledata Model Fold and Roll Weigh 6550 Manufacturer Detecto Capacity 800 lb Resolution 0.2 lb Horizontal Dimensions Features Limitations Cost 28" * 32" Option of preset or keyed-in tare weight, wheels for easy moving, 0.7" LCD screen battery powered $2,315 Abledata Google search Model 6495 Stationary Wheelchair Scale Seca Model 664 Detecto Precision Weighing Balances 400 lb 600 lb 0.2 lb 0.2 lb 30" * 26" Optional AC adapter, padded handrail Immobile, only one entrance ramp $2,272 36" * 40.5" Smart Card compatibility option, HOLD function for keeping weight on screen after descending, extremely flat, batteries or AC adapter, folding design hardly any; one of the best models found $1,875 plus $65 S&H Purpose: The device to be designed is a smart weighing scale that wheelchair-bound persons can use to weigh themselves with daily frequency while inside a wheelchair and that will store previous weights along with other information. The need for this device is exhibited in patients with COPD (Chronic Obstructive Pulmonary Disorder) and other lung disorders such as emphysema. The individuals suffering from such diseases may have to monitor their weight frequently to avoid medical complications. Since long term monitoring of a patient’s weight may have to occur at home, it should not be assumed that assistance is always available and the device must be designed for a home environment. The device will meet market standards such as an 800-900 lb weighing capacity and a resolution of 0.2, and will go above and beyond the current market standards for the user’s ability to store previous weights, use previous wheelchair weights for taring, and accessibility to hearing impaired or vision impaired users. To be competitive in the wheelchair scale market, the device must have a cost below the average cost of $2000 for a wheelchair scale (see Table 1). The scale must be durable enough to provide long-term accurate weighing of the user and should keep the user as stable during the weighing operation as he or she is on solid ground. No assistance should be required for any level of operation for the device except installation. Taring (the process of weighing the individual by subtracting the wheelchair weight from the weight of wheelchair plus occupant) must be a process the user can execute unassisted. The system should have a simple, easy to use interface that can remember previous weighings of a user and also the weight of the wheelchair for taring. Interface components such as buttons and displays should be large and accessible. The cost of the device should be affordable compared to other marketed models filling the same need, and should create a pleasant weighing experience to help motivate individuals with respect to weight control. Method: Since the device consists of a supporting surface for a wheelchair and an access path to bring the wheelchair to it, wheelchair engineering standards must be applied to determine the size, height, and static and dynamic requirements of both. The tipping angle, the sideways angle at which a static wheelchair will tip over (measured for side, front and back) is of primary importance. The device must be designed so that the maximum wheelchair angle possible, with different sides of a wheelchair on the ground and highest point on the device respectively, must be lower than the smallest tipping angle. The wheelchair may be manual or powered; access ramps must be designed so that the effort for a manual wheelchair user to mount the platform is less than that required to ascend typical wheelchair-accessibility ramps to most buildings. If the wheelchair mounts the platform at certain high speeds, lateral stability tests for common wheelchairs should dictate under what limits the device should respond [7]. A budget of $2000 is provided for the design and implementation of the wheelchair scale. The total amount of money including the cost of parts, fabrication, and testing cannot exceed $2000. To be manufactured industrially, the device must be made out of parts, materials or assemblies that are commercially available from reliable suppliers. Examples of these are standard compression load cells, plywood boards, sheet metal, rubberized mat sections, wooden beams, etc… It must be possible to assemble all the parts into the device using conventional manufacturing methods. A set of tests must be employed (e.g. voltage/load, max. load, beam deflection) to determine beyond a doubt that the device performs according to specifications without destroying the functionality of the device. The operation of the device by wheelchairbound individuals and perhaps by walking individuals cannot endanger the health of either party. Safety measures such as smooth edges, barriers to block the slipping of wheelchairs and surfaces with traction should be implemented to prevent injury. Repeated use should not adversely affect the health of the user. The most common method of load sensing involves a compression load cell. The selection of this component is by far the most critical in all aspects of the design of the scale. A single load cell with a 1000 lb capacity will cost between $300-$500, and four load cells with a 250 lb capacity would each cost no less than $150, effecting a total cost of over $600. A four load cell design would enable more distributed placement of the platform on the cells and would perhaps enhance accuracy. Electronics equipment is normally inexpensive – a printed PCB board and microprocessor for the display and computation unit would cost around $150 including cords, display and housing. Building materials such as plywood, sheet metal and fittings may cost around $200 total. Together, the estimate of the total cost of the device to manufacture is $1800. This device can be built with the given budget of $2000. Outline of Report: Three different designs for the wheelchair scale are presented in sequence, denoted as Design 1, Design 2, and Design 3. Each design is introduced with a short description of the system and specific design issues. Labeled diagrams of the top, side and front views of the system are then provided with specific measurements. An equipment section describes the necessary materials and parts and their specifications. Analysis of the system follows in a section divided into static analysis and electronic analysis. The benefits and disadvantages of the system are then summarized in a conclusion section. Design 1 : Single Load Cell: Description: A single load cell will be used to determine the weight resting on the platform, since this component is particularly expensive, although a design featuring four load cells at the corners would be more stable and accurate. The corners of the platform will be supported by springs or another elastic element such that the platform must be depressed by some threshold weight before contacting the top of the cell under it. Access ramps on opposite sides of the scale with ramp angle no greater than 10 degrees will be level with the platform, which will have height no greater than 2 inches. The springs, load cell, and ramp will rest on or be supported by a crosspiece-assembly under the platform. The signal from the load cell will travel through a cable to a processing and display unit. The processing and display unit will have a memory capable of storing 11 numerical values with a precision of four decimal places (10 historical weights, 1 wheelchair weight for taring, accuracy of 0.2 lb) and will be connected to an LCD screen sufficiently large enough for low-vision accessibility. Large buttons and audible cues through a speaker will also make the device more accessible to handicapped individuals. The processor will hold a written software program containing: an interface through which users can weigh themselves, store weight, key in wheelchair weight and cycle through previous weighings an algorithm to calculate actual weight based on stored or inputted wheelchair weight and signal from load cell This processor and other components mentioned will be in a circuit powered by an AC adapter or standard batteries and will turn on and off by a switch or by a threshold load placed on the platform. 34" 30" Platform 2" Load Cell Spring 36" Note: Vertical scale different from horizontal scale for higher visibility Figure SEQ Figure \* ARABIC 4: Side view of scale unit 34" 30" Side Panel Platform 2" Load Cell 12" 36" 12" Note: Vertical scale different from horizontal scale for higher visibility Figure SEQ Figure \* ARABIC 5: Another side view of scale unit - entrance ramps visible 1.4" Equipment Load cells are the primary measurement instruments in weighing devices. Load cells consist of multiple strain gages (see figure 1), which measure resistance when it is put under a compression or tension force. Strain gages are made up of thin metallic foils called a “Gage Patch.” The multiple gages patches are connected tighter and attached inside of the load cell. When the surface of the cell experiences a force the metallic wires experience the same force. The multiple strain gages that make up a load cell are wired together to form the legs of a Wheatstone bridge (see figure 1). The Wheatstone bridge creates an electric current that results from the response of a load that is applied to the cell in the form of an analog signal. A microprocessor is then used to convert the analog signal to a digital signal, so the output from the load cell can be determined and read on an electronic LCD display [8]. Figure 3: Strain gage array and Wheatstone bridge circuit Figure 4: Beam load cell, model LCEC – 1k “Model LCEC load cells are designed to operate in all weather or washdown environments. Their low profile and high side load capability simplify mechanical installation considerations. Their weather sealing, high precision and repeatability make them ideally suited for rugged industrial applications such as testing, batching, weigh pits and other applications exposed to the elements…” SPECIFICATIONS Excitation: 10 Vdc (15 V max) Output: 3 mV/V nominal Calibration: NIST Traceable Linearity: ±0.03% FS Hysteresis: ±0.02% FS Repeatability: ±0.01% FS Creep (after 20 minutes): ±0.03% Zero Balance: ±1% FS Operating Temp Range: –55 to 90°C (–65 to 200°F) Compensated Temp Range: –15 to 65°C (0 to 150°F) Thermal Effects: Zero: ±0.0015% Rdg/°F max Span: ±0.0008% FS/°F max Safe Overload: ±150% of Capacity Ultimate Overload: ±300% of Capacity Input Resistance: 350 +50/–3.5 Allowable Side Load at Rated Load: 50% Rated Capacity Output Resistance: 350 ±3.5 Construction: High Carbon Steel Electrical: 5 ft. (1.5 m) insulated 4-conductor shielded color coded cable Metric Ranges Available - Consult Engineering *See Section D For Compatible Meters Ordering Examples: 1) LCEC-250 is a 250 lb capacity load cell, $265. 2) LCEC-1K is a 1000 lb capacity load cell, $285. ( *All information taken from Omega Engineering, 2004 [8]) Static Analysis: A statics analysis is a valid method of calculating the forces acting on the weighing platform because we can assume there is no significant dynamic behavior. The scale unit is a passive weighing device, and it is assumed the wheelchair user moves onto the platform relatively slowly. Two statics properties are used here – the sum of moments around a point and the sum of forces acting on the platform, both equaling zero. Assumptions: 1) platform of equal length and width greater than 30 in. 2) platform is supported at five points – at four corners by springs, and at center by load cell 3) axial centers of springs make 30 in. * 30 in. square representing platform modeled here 4) load cell is contacted by platform only if load is greater than some threshold, i.e. springs must undergo some displacement before load cell begins supporting FW F4 F3 FL F1 F2 30" 60 B A Platform Load Cell Spring 30" Figure 5: FW = 800 lbs at maximum loading A) Sum of moments about one corner, F1 : M F1 30 * ( F 4 F 2) 30 2 * ( F 3) 15 2 * ( FL) A2 B2 * ( FW ) 0 B) Sum of vertical forces F Y F1 F 2 F 3 F 4 FL Fw 0 * Load cell must be able to measure maximum weight of 800 lbs, as per design specifications. * Assume when midpoint between wheels is over center of load cell, most accurate reading is obtained. Since the platform is suspended in part by elastic elements and only supported occasionally by a stable fulcrum (the load cell), the tilting of the platform must be accounted for. C) Spring Constant: Assume maximum acceptable platform tilt angle = 5 deg. Side view of platform: worst case scenario is 800 lb load applied to middle of one edge of platform. At this point, platform must have angle of no more than 5 deg. Assume that two-spring system with springs supporting one-dimensional lever with load cell in middle as fulcrum approximates actual scenario – from symmetry, load reduces to 400 lbs Assume that lever with one end as fulcrum, spring on other end, load applied at spring is an approximation of the one-dimensional lever. Assume Hooke’s law applies to any displacement of spring: Max angle = 5 deg. Tan(5 deg) = 0.08749 = (Spring displacement) / (Lever length of 30 in.) = ( 400 lbs / k) / (Lever length of 30 in) Solving for k, the spring constant: k = 150 lbs/in. , approximately This constant provides a good indication of what springs will be suitable for supporting the platform. Any elastic element that is short and wide enough to allow movement only in a vertical direction (with tall springs, torsional and shear forces may cause significant movement to the sides) and will furnish approximately 150 lbs of force per inch of compression or tension from equilibrium is suitable to install at the four corners of the platform and scale base support so that an 800 lb force at the middle of one edge of the platform will not tilt the platform more than five degrees. If the platform is allowed to tilt, the wheelchair may roll to the side or off the scale, or the resulting gap between the platform and ramp could damage the scale or tip the wheelchair. Block Diagram of System: Load Power supply Force Transducer Signal Filter Signal Amplifier User Input MIcroprocessor LCD Unit Design 2 : Four Load Cells: A set of four load cells will be used to determine the weight resting on the platform. The load cells will be positioned at the four corners of the platform, supporting the platform statically and inhibiting movement. Since all force exerted on the platform must be sensed by the cells, they must be the only support for the platform; however, there cannot be a significant horizontal component to the forces exerted on the cells. Access ramps on opposite sides of the scale with ramp angle no greater than 10 degrees will be level with the platform, which will have height no greater than 2 inches. The load cells and ramp will rest on or be supported by a lower platform or crosspiece assembly under the platform. The signal from the load cell will travel through a cable to a processing and display unit. The processing and display unit will have a memory capable of storing 11 numerical values with a precision of four decimal places (10 historical weights, 1 wheelchair weight for taring, accuracy of 0.2 lb) and will be connected to an LCD screen sufficiently large enough for low-vision accessibility. Large buttons and audible cues through a speaker will also make the device more accessible to handicapped individuals. The processor will hold a written software program containing: an interface through which users can weigh themselves, store weight, key in wheelchair weight and cycle through previous weightings an algorithm to calculate actual weight based on stored or inputted wheelchair weight and signals from load cells This processor and other components mentioned will be in a circuit powered by an AC adapter or standard batteries and will turn on and off by a switch or by a threshold load placed on the platform. 34" 30" Platform Load Cell Load Cell 2" 36" Note: Vertical scale different from horizontal scale for higher visibility Figure 6: Front view of scale with four load cells 34" 30" Side Panel Platform Load Cell 12" Load Cell 36" Note: Vertical scale different from horizontal scale for higher visibility Figure 7: Side view of scale with four load cells 2" 12" 1.4" Equipment Load cells are the primary measurement instruments in weighing devices. Load cells consist of multiple strain gages (see figure 1), which measure resistance when it is put under a compression or tension force. Strain gages are made up of thin metallic foils called a “Gage Patch.” The multiple gages patches are connected tighter and attached inside of the load cell. When the surface of the cell experiences a force the metallic wires experience the same force. The multiple strain gages that make up a load cell are wired together to form the legs of a Wheatstone bridge (see figure 1). The Wheatstone bridge creates an electric current that results from the response of a load that is applied to the cell in the form of an analog signal. A microprocessor is then used to convert the analog signal to a digital signal, so the output from the load cell can be determined and read on an electronic LCD display [8]. Series 350 Shear beam load cells for Industrial weighing Capacities 300 kg to 5,000 kg 5 year warranty Simple low cost installation Alloy version fully potted and sealed to IP 66 Alloy (350a) or stainless steel (350i) load sensor Optional load feet, mounting plates, load buttons And supports Industry standard configuration Corrosion resistant Stainless steel version fully welded and Hermetically sealed to IP68 Optional anti-vibration loading assembly available 3000 divisions OIML R60 Class C Specifications [8] Load Cell Capacity Units Standard Load Ranges 300, 500, 750, 1000, kg 1500, 2000, 3000, 5000 kg Rated Output 2 mV/V±0.1% Accuracy Class 3000 n.OIML Combined Error <±0.017 %* Non-repeatability <±0.015 %* Creep (30 minutes) <±0.016 %* Temperature Effect on Zero Balance <±0.01 %*/°C Temperature Effect on Span <±0.006 %*/°C Compensated Temperature Range -10 to +40 °C Operating Temperature Range -20 to +70 °C Safe Overload 150 %* Ultimate Overload 200 %* Zero Balance <±2 %* Input Resistance 400 ±30 Output Resistance 350 ±1.5 Insulation Resistance >5000 M@ 100V Recommended Supply Voltage 10 V Maximum Supply Voltage 15 V Environmental Protection 350i=IP68 350a=IP66 – Load Foot Accessories for Series 350 Shear Beam [9] The cells could be excited (via a junction box) by a cost effective instrument such as our XT1200 unit. JB4T JUNCTION BOX Pricing from Thames Side-Maywood Hole size 4mm European X350A-300 US SHEAR BEAM LOAD CELL 108 COST EACH £68.00 TOTAL QUANTITY 4 X35913-FOOT $125.12 £272.00 $500.48 £9.00 $16.56 80 50 CAPACITY 300KG (660lbs) LEVELLING FOOT COST EACH QUANTITY 4 120 TOTAL £36.00 $66.42 ALL DIMENSIONS IN MILLIMETRES (Not to Scale) JB4T JUNCTION BOX (IP65) 4 WAY SUMMING 55 COST EACH £40.00 $73.60 The resolution required would easily be achieved (and much better). The load cell is approved to OIML R60 C3 which is the European Weights and Measures accuracy requirement. It would be important that the wheel chair platform structure that the cells would support is very rigid in nature because any bending or flexing would produce accuracy errors. Data and quotes provided by Thames Side-Maywood (logo below) [9]: Static Force Analysis: Since weighing a moving vehicle in the form of a two-wheeled or four-wheeled wheelchair is assumed to occur at low speeds, a dynamic analysis of the four-load-cell system is not considered necessary. The static analysis is based on the zero-sum totals of the moments about any point and the forces in any direction. The weight of the wheelchair and user is considered here as one force vector in position (A, B) from one corner, and the dimensional measurement of 30 inches square is specified from the center of each load cell to form a square. The part of the platform not inside the square formed by the supporting points of each load cell is considered unloaded, or free. The platform, load cells, and supporting structure for the cells are assumed to be relatively level. Since a maximum operating weight of 800 lbs (center loading) is specified, this value is used to calculate the worstcase scenario for static loading. Each load cell is fastened to the platform and supporting structure, making the entire structure immobile. Because of this, each loading point should have six degrees of freedom (3 translational forces, 3 rotational moments); however, only 1 translational force and 3 moments are considered in this analysis. Assumptions: 1) Dynamic effects are negligible 2) Wheelchair weight can be approximated as single downward vector 3) All relevant forces are vertical 4) The platform is fastened securely to each load cell FW F4 F3 F1 F2 30" 60 B A Platform 30" Load Cell Figure 8: Free body diagram of four-load-cell design Sum of vertical forces: Equation SEQ Equation \* ARABIC 1 F Y F1 F 2 F 3 F 4 FW 0 Sum of moments about point at F1: Equation SEQ Equation \* ARABIC 2 M F1 30 * ( F 4 F 2) 30 2 * ( F 3) A2 B 2 * ( FW ) 0 FW = 800 lbs maximum: * Assuming platform weight is negligible, worst case scenario can be assumed to be entrance loading. The wheelchair must at some point during the process rest entirely on the outer edge, presumably between the two front load cells. In this case, the weight is distributed almost entirely between the two load cells, making the necessary maximum loading weight 800 / 2, or 400 lbs. However, this is different from the maximum load that can be measured. If it is assumed that the scale system can detect when the average loading of the wheelchair is in the approximate center of the scale, an accurate average reading can be taken and the total reaction force on each load cell is approximately 200 lbs. It is this maximum force that must be measurable. The load cells described in the Equipment section have a maximum measurable load of 660 lbs. * The sum of all the load cell signals from the platform equals the total weight of the platform, assuming only vertical forces. This quadruples the possible accuracy of the scale unit and also makes the unit more stable than a four-spring, single load cell design. The readings from the load cells enable the processor to ascertain the position of the user on the scale as well, which should ideally be in the middle of the platform for the most accurate reading. Circuit Analysis: -5V Load Cell +5V R1 Input Output +5V R1 R2 R3 R4 In1 In2 In3 In4 Output +5V -5V Microprocessor out1 out2 out3 out4 out5 out1 out2 out3 out4 out5 LCD Display vcc Input vcc R2 ground Load Cell +5V ground -5V Load Cell +5V R3 Input +5V +5V Output +5V -5V Load Cell +5V R4 Output vcc 1 2 3 4 +5V +5V Input User Interface ground The above circuit is a generic view of a real circuit that is going to be mounted on our design. The design might have changes in terms of resistors values and the type of MosFets we are going to use in our optimal design. Also the real design might have a less inputs in order to make the design more accurate and affordable. The basic approach to the circuit is very simple, the four main outputs from the load cells are going through filters so the voltage values entering the microprocessor are without any noise and interference, also these voltage values go through some resistance in order to make them suitable for the requirements of the microprocessor, different microprocessors have varying input voltage acceptance values. The microprocessor also gets input values from the User Interface, all these inputs of the microprocessor go through calculations and memory storage sequences and then the result is displayed on the LCD Display. Block Diagram of System: The above block diagram is the electrical part of the design; the power supply is shown supplying a voltage to the load cell (force transducer) and at the same time, the load cell’s resistance changes in proportion to the load applied axially. A Wheatstone bridge circuit is used to detect the extremely small changes in resistance, and this constitutes the output signal from the cell. Once the load cell signal reaches the linear or steady state, then the analog signal is sent to the Signal filter where it is transformed from a noisy signal to a continuous one and then this output is sent to the Signal Amplifier for possibly voltage or current adjustments before the signal goes to the Microprocessor. The Microprocessor gets two inputs one from the user and the other from the Signal Amplifier. Once the Microprocessor is done with its arithmetic and all the memory storing and sorting, then the final weight is displayed onto the LCD display. The scale turns on as the user moves onto the platform with some minimum weight or when the power switch is toggled. Flow chart of the Microprocessor: The computation and user interface for the weighing system is accomplished by a microprocessor with user input switches and an LCD monitor, as well as possibly a piezoelectric speaker. The following is a microprocessor flow chart, which shows the different functions performed by the user and the microprocessor while computing the weight of the person (see Fig. 4). Analog output from the Load Cell The analog signal coming from the Load Cell is being converted to digital signal using the A/D component of the Microprocessor The A/D input of the Microprocessor The user enters the value of the wheel chair and then presses enter. The digital signal coming from the microprocessor is being sent to the input of the ALU component of the Microprocessor where the subtraction of the wheel chair weight from the total weight takes place. Wheel Chair input from the User The digital value of the weight being converted from the Load Cell is stored into some register ‘x’ and then the user value entered is being stored in some register ‘y’. The ALU component of the microprocessor does the calculations and stores the final value in the register ‘z’, which is also stored in RAM, so it could be accessed again. ALU functions of the Microprocessor Up to 10 different weights of the person are registered in this memory, for the records of the user. Random Access Memory (RAM) of the Microprocessor Digital to Ib/ kg converter The output of the Microprocessor is displayed on the LCD display. The LCD display Figure 9: Flowchart of processing and display unit User interface flowchart: The user pushes the button on the panel which brings up the previous measurements of the patient. Accessing the previous measuremtns . Scale Process Menu: 1-Power 2-Weigh 3-History 4-Change wheelchair weight Switch on Switch off Filtered weight If time > 5 min Access wheelchair weight Yes display No display Hold till scale is stable Display: “No wheelchair weight stored.” “Store wheelchair weight?” No wheelchair weight stored Yes No Input wheel chair weight Go to Menu If time > 2 min Filter weight “Error too much movement” Store Wheelchair weight as 1-10 Display “Your weight is__.” Go to Menu “Store to memory?” Yes Store in next available space, bump out oldest entry if full No Go to Menu Figure 10: User interface flowchart Figure 5 above shows the user interface flowchart enabling the user to navigate through four options – turn the scale on/off, weigh, check weight history, and change wheelchair weight. There are few menu levels, making the interface easy to use for most people. The memory is accessed by the processor whenever the user selects the option to actually store their weight, and the oldest entry for weight is deleted and all entries are moved back to make room for the new one. The user must know their wheelchair weight and type it on only once as it is stored in the memory. The machine memory must be able to handle repeated and constant use, and must not fail in a power outage. A battery can be provided to ensure that stored data is not lost. Conclusion: This design offers many beneficial features that make this product highly accurate. This design includes a higher cost aspect but involves a more accurate approach to weight measurements with the addition of four load cells. The combination of four load cells provides an in-depth average of the individual’s weight, because it will not matter where exactly the individual is placed on the scale providing easier access. This design also does not include springs to act as supports; instead the four load cells will act as supports for the platform with their placement in load feet. The display unit will be large enough for an individual with moderately low vision to see and operate. The operation of the scale and display unit will also be user friendly and allow enough memory to store collected weightings for up to 10 times. The circuit analysis in this design is more accurate and affordable to build. The usage of fewer resistors and capacitors is taken into consideration. Design 3 : Force Transducer Units: A set of four force transducer units (FTUs) will be used to determine the weight resting on the platform. The units will be positioned near the four corners of the platform, supporting the platform statically and inhibiting movement. Since all downward force exerted on the platform must be sensed by the FTUs, they must be the only support for the platform; however, there cannot be a significant horizontal component to the forces exerted on the cells. Access ramps on opposite sides of the scale with ramp angle no greater than 10 degrees will be level with the platform, which will have height no greater than 2 inches. The FTUs and ramp will rest on or be supported by a lower platform or crosspiece assembly under the platform. The signal from the FTUs will travel through wiring to processing circuitry and a computing / display unit. The processing and display unit will have a memory capable of storing 11 numerical values with a precision of four decimal places (10 historical weights, 1 wheelchair weight for taring, accuracy of 0.2 lb) and will be connected to an LCD screen sufficiently large enough for low-vision accessibility. Large buttons and audible cues through a speaker will also make the device more accessible to handicapped individuals. The processor will hold a written software program containing: an interface through which users can weigh themselves, store weight, key in wheelchair weight and cycle through previous weightings an algorithm to calculate actual weight based on stored or inputted wheelchair weight and signals from force transducers This processor and other components mentioned will be in a circuit powered by an AC adapter or standard batteries and will turn on and off by a switch or by a threshold load placed on the platform. Diagrams: Figure 11: Side view of weighing platform with FTUs near corners Figure 12: Top view of weigh platform Figure 13: FTU diagram showing bending material, strain gage, and leg pieces causing 3-point bending Static Analysis: At each corner of the weighing platform, a beam of steel with assumed uniform properties undergoes 3-point bending. The bending moment of the bottom of the beam (tension) directly under the loading sleeve is proportional to the transverse load on the beam, and the bending strain produced on the outer surface of the beam is in turn proportional to this moment. A strain gage is attached to this area of the bending beam and records the strain produced. Four strain readings are each read using a quarter-bridge Wheatstone bridge circuit, filtered, converted into load readings, and summed to calculate the total downward force on the platform. Bending bar material in Force Transducer Unit (FTU): A36 hot-rolled (structural) steel Given (to 3 significant digits): Length of each side of weighing platform: 0.889 m (35.0 in.) Total length of bending bar in FTU: 0.180 m (7.09 in.) Length of bar experiencing free bending moment: 0.150 m (5.91 in.) Width of bar: 0.02 m (0.787 in.) Thickness of bar: 0.003 m (0.118 in) Maximum measuring load on center of bar: 200 lbs (889 N) Maximum load on center of bar: 400 lbs (1780 N) Yield strength of A36 hot-rolled structural steel: 150 Map compressions 250 Map tension Elastic modulus of A36 hot-rolled structural steel: 200 Gpa Calculations: M 0 F1* R1 F 2 * R2 F 3 * R3 F 4 * R4 Fw1* Rw1 Fw2 * Rw2 F y 0 F1 F 2 F 3 F 4 Fw1 Fw2 1) Calculate forces F1, F2, F3, and F4 for wheelchair entrance to ramp (loading on edge of platform, see Diagram 3 in notes): Edge-loading, 800 lbs - each wheel assumed to transfer 400 lbs: Fw1 = Fw2 = 400 lbs Center-edge loading assumed: A = (0.889 m – C) / 2, B = (0.889m + C) / 2 Assume wheelchair wheel width of 0.7 m, A = .095 m, B = 0 m Symmetry: F1 = F2, F3 = F4 Sum of moments about axis through F1, F2 = (400 + 400lbs)*(0.0635m) + (F3 + F4)*(0.762m) = 0 F3 = F4 = -67 lbs, downward force (tension) Sum of vertical forces = -400 - 400 – 67 – 67 + F1 + F2 = 0 F1 = F2 = 467 lbs, upward force 2) Calculate forces F1, F2, F3, and F4 for wheelchair in center of ramp: Fw1 and Fw2 are centered: A = 0.095, B = 0.381 m Four-way symmetry: F1, F2, F3, F4 all equal Sum of vertical forces = -400 – 400 + F1 + F2 + F3 + F4 = 0 F1 = F2 = F3 = F4 = 200 lbs 3) Calculate maximum shearing stress in bending beam: Factor of safety (Nfs) = 1.3: 467 * 1.3 = 600 lbs Reactions at either end of beam: R1 = R2 = 600 / 2 = 300 lbs Max. shear in beam, Vmax = 300 lbs / cross sectional area Convert to Newton’s: 300 lbs * 0.4536 kg/lb * 9.8 m/s2 = 1334 N Shear: 1334 N / (0.02*0.003m) = 22.2 Map 4) Calculate maximum bending strain on underside of beam: Factor of safety (Nfs) of 1.3: 467 * 1.3 = 600 lbs Convert to Newton’s: m Mc I 600 lbs * 0.4536 kg/lb * 9.8 m/s2 = 2670 N I bh 3 M LF 8 m E Here, b = width of beam, h = thickness, c = h / 2, L is length of beam, F is the vertical load in the center of the beam, E is the elastic modulus, and M is the bending moment in the center of the beam. The bending moment equation is a generalized equation assuming ideal beam behavior, and yields the maximum bending moment in the beam, happening in the center. The maximum strain as produced under a 600 lb load is given as: max Mc LF IE 16Eaw 2 where L is the bending length of the beam, a is the width of the beam, and w is the thickness. For a load of 600 lbs (2670 N), a free-bending length of 0.150 m, a width of 0.020 m, and a thickness of 0.003 m (see given specifications above), Maximum bending strain = (2670 N) (0.150) / (16*200e9*0.02*0.003^2) = 6.953e-4 = 695 micro strain. = (delta L / L) Analysis: I. None of the stresses calculated exceeded the minimum yield strength for A36 structural steel, 150 MPa in compression. It is assumed that any stress exceeding this value will permanently deform the FTU, leading to failure or inaccurate readings. II. Strain gages typically measure 1-1000 or 1-1500 micro strain. The amount of micro strain calculated for maximum-loading conditions with a safety factor of 1.3 was 695 micro strains. Since the maximum loading for a centered wheelchair would be one-third this value, the maximum strain to be measured is 232 micro strains. This is very much within the operating conditions of a strain gage. Tentative Parts List: Part Qty Fasteners, 1/4" bolts with nuts 36 beams 4 support squares 8 sleeves 4 weigh platform 1 support platform 1 ramps 2 LCD display 1 Strain gages 5 A/D converter 4 Filters 4 Wheatstone bridge Circuit: The strain gages are measured using the Wheatstone bridge circuit analysis, where four resistors are placed of equal value, if the fourth resistor which is the strain gage itself doesn’t experience any changes then the circuit will generate no voltage, on the other hand if the circuit experience a slight resistance change in the strain gage resistor then the circuit will generate a voltage in millivolts. Since we are using five strain gages all their outputs are being added together and then send to the A/D converter of the microprocessor [10]. The fifth strain gage is placed in the circuit so the issue of temperature affecting the circuit is dealt with, the fifth strain gage is placed on the piece of metal similar to the one used for the other four strain gages, and one the temperature changes the resistance of the strain gage then we know how much calibration value we should add to the main circuit. The would be circuit of the strain gage is displayed below using only one strain gage [10]; W heatstone bridge circuit using one strain gage Vout in mvolts R3 Strain gage resistance R1 +5V R2 The above circuit shows only one part of the Wheatstone bridge analysis, the way it is designed, resistors R1, R2, R3, and strain gage resistor are all equal in value, which means no voltage generated to the output but if there is a slight resistance change in strain gage resistor due to strain then the output terminal gets a voltage value. Circuit Analysis: -5V Load Cell +5V R1 Input Output +5V R1 R2 R3 R4 In1 In2 In3 In4 Output +5V -5V Microprocessor out1 out2 out3 out4 out5 out1 out2 out3 out4 out5 LCD Display vcc Input ground R2 vcc +5V ground -5V Load Cell Load Cell +5V R3 Input +5V +5V Output +5V -5V Load Cell +5V R4 Input Output vcc 1 2 3 4 +5V +5V ground User Interface The above circuit is a generic view of a real circuit that is going to be mounted on our design. The design might have changes in terms of resistors values and the type of MosFets we are going to use in our optimal design. Also the real design might have a less inputs in order to make the design more accurate and affordable. The basic approach to the circuit is very simple, the four main outputs from the load cells are going through filters so the voltage values entering the microprocessor are without any noise and interference, also these voltage values go through some resistance in order to make them suitable for the requirements of the microprocessor, different microprocessors have varying input voltage acceptance values. The microprocessor also gets input values from the User Interface, all these inputs of the microprocessor go through calculations and memory storage sequences and then the result is displayed on the LCD Display. Block Diagram of System: The above block diagram is the electrical part of the design; the power supply is shown supplying a voltage to the load cell (force transducer) and at the same time, the load cell’s resistance changes in proportion to the load applied axially. A Wheatstone bridge circuit is used to detect the extremely small changes in resistance, and this constitutes the output signal from the cell. Once the load cell signal reaches the linear or steady state, then the analog signal is sent to the Signal filter where it is transformed from a noisy signal to a continuous one and then this output is sent to the Signal Amplifier for possibly voltage or current adjustments before the signal goes to the Microprocessor. The Microprocessor gets two inputs one from the user and the other from the Signal Amplifier. Once the Microprocessor is done with its arithmetic and all the memory storing and sorting, then the final weight is displayed onto the LCD display. The scale turns on as the user moves onto the platform with some minimum weight or when the power switch is toggled. Flow chart of the Microprocessor: The computation and user interface for the weighing system is accomplished by a microprocessor with user input switches and an LCD monitor, as well as possibly a piezoelectric speaker. The following is a microprocessor flow chart, which shows the different functions performed by the user and the microprocessor while computing the weight of the person (see Fig. 4). Analog output from the Load Cell The analog signal coming from the Load Cell is being converted to digital signal using the A/D component of the Microprocessor The A/D input of the Microprocessor The user enters the value of the wheel chair and then presses enter. Wheel Chair input from the User The digital signal coming from the microprocessor is being sent to the input of the ALU component of the Microprocessor where the subtraction of the wheel chair weight from the total weight takes place. The digital value of the weight being converted from the Load Cell is stored into some register ‘x’ and then the user value entered is being stored in some register ‘y’. The ALU component of the microprocessor does the calculations and stores the final value in the register ‘z’, which is also stored in RAM, so it could be accessed again. ALU functions of the Microprocessor Up to 10 different weights of the person are registered in this memory, for the records of the user. Digital to Ib/ kg converter The output of the Microprocessor is displayed on the LCD display. Random Access Memory (RAM) of the Microprocessor The LCD display Figure 14: Flowchart of processing and display unit The user pushes the button on the panel which brings up the previous measurements of the patient. Accessing the previous measuremtns . User interface flowchart: Scale Process Menu: 1-Power 2-Weigh 3-History 4-Change wheelchair weight Switch on Switch off Filtered weight If time > 5 min Access wheelchair weight Yes display No display Hold till scale is stable Display: “No wheelchair weight stored.” “Store wheelchair weight?” No wheelchair weight stored Yes No Input wheel chair weight Go to Menu If time > 2 min Filter weight “Error too much movement” Store Wheelchair weight as 1-10 Display “Your weight is__.” Go to Menu “Store to memory?” Yes Store in next available space, bump out oldest entry if full No Go to Menu Figure 15: User interface flowchart Figure 5 above shows the user interface flowchart enabling the user to navigate through four options – turn the scale on/off, weigh, check weight history, and change wheelchair weight. There are few menu levels, making the interface easy to use for most people. The memory is accessed by the processor whenever the user selects the option to actually store their weight, and the oldest entry for weight is deleted and all entries are moved back to make room for the new one. The user must know their wheelchair weight and type it on only once as it is stored in the memory. The machine memory must be able to handle repeated and constant use, and must not fail in a power outage. A battery can be provided to ensure that stored data is not lost. Conclusion: This design offers many beneficial features that make this product highly accurate. This design includes a higher cost aspect but involves a more accurate approach to weight measurements with the addition of the strain gage analysis. The combination of five strain gages provides an in-depth average of the individual’s weight, because it will not matter where exactly the individual is placed on the scale providing easier access the weight of the individual will be calculated very accurately. This design also does not include springs to act as supports; instead the five strain gages will act as supports for the platform with their placement in different part of the platform. The display unit will be large enough for an individual with moderately low vision to see and operate. The operation of the scale and display unit will also be user friendly and allow enough memory to store collected weightings for up to 10 times. The circuit analysis in this design is more accurate and affordable to build. The usage of fewer resistors and capacitors is taken into consideration. Choosing the optimal design: Optimal design is chosen because it provides the best solution and at the same time it is more cost efficient compare to the other projects designed. The first design lacks static stability, while the third design would cause measurement error in terms of calculating weights because of the strain gages used in the design. The strain gages are very delicate and the amount of bending they go through could affect the correct measurement of the weight of the person and at the same time the strain gages go through abrupt temperature changes that would cause instability in the process of weight measurement. For these reasons, the second project was our best choice, in terms of selecting the optimal design even though we still made some changes to the static analysis part of it, especially the amount of stress the platform is going to go through. Optimal Design: Objective: A set of four load cells will be used to determine the weight resting on the platform. The load cells will be positioned at the four corners of the platform, supporting the platform statically and inhibiting movement. Since all force exerted on the platform must be sensed by the cells, they must be the only support for the platform; however, there cannot be a significant horizontal component to the forces exerted on the cells. Access ramps on opposite sides of the scale with ramp angle no greater than 10 degrees will be level with the platform, which will have height no greater than 2 inches. The load cells and ramp will rest on or be supported by a lower platform or crosspiece assembly under the platform. The signal from the load cell will travel through a cable to a processing and display unit. The processing and display unit will have a memory capable of storing 11 numerical values with a precision of four decimal places (10 historical weights, 1 wheelchair weight for taring, accuracy of 0.2 lb) and will be connected to an LCD screen sufficiently large enough for low-vision accessibility. Large buttons and audible cues through a speaker will also make the device more accessible to handicapped individuals. The processor will hold a written software program containing: an interface through which users can weigh themselves, store weight, key in wheelchair weight and cycle through previous weightings an algorithm to calculate actual weight based on stored or inputted wheelchair weight and signals from load cells This processor and other components mentioned will be in a circuit powered by an AC adapter or standard batteries and will turn on and off by a switch or by a threshold load placed on the platform. Rest Bar ¼ in 5.1 in 1.24 in 18.3 in 2.5 in 3.2 in 27.5 in 18 in 36 in Ramp Load Foot Load Cell Figure 16: Side view of optimal design with four load cells 36 in 0.25 in 1.5 in Rest Bar 2.5 in 1.24 in 3.2 in 2 in Weigh Platform Load Cell 1.24 in Load Foot Figure 17: Front view of optimal design with four load cells Figure 18: Top view of optimal design with four load cells Equipment Load cells are the primary measurement instruments in weighing devices. Load cells consist of multiple strain gages (see figure 1), which measure resistance when it is put under a compression or tension force. Strain gages are made up of thin metallic foils called a “Gage Patch.” The multiple gages patches are connected tighter and attached inside of the load cell. When the surface of the cell experiences a force the metallic wires experience the same force. The multiple strain gages that make up a load cell are wired together to form the legs of a Wheatstone bridge (see figure 1). The Wheatstone bridge creates an electric current that results from the response of a load that is applied to the cell in the form of an analog signal. A microprocessor is then used to convert the analog signal to a digital signal, so the output from the load cell can be determined and read on an electronic LCD display. Series 350 Shear beam load cells for Industrial weighing Capacities 300 kg to 5,000 kg 5 year warranty Simple low cost installation Alloy version fully potted and sealed to IP 66 Alloy (350a) or stainless steel (350i) load sensor Optional load feet, mounting plates, load buttons And supports Industry standard configuration Corrosion resistant Stainless steel version fully welded and Hermetically sealed to IP68 Optional anti-vibration loading assembly available 3000 divisions OIML R60 Class C Specifications Load Cell Capacity Units Standard Load Ranges 300, 500, 750, 1000, kg 1500, 2000, 3000, 5000 kg Rated Output 2 mV/V±0.1% Accuracy Class 3000 n.OIML Combined Error <±0.017 %* Non-repeatability <±0.015 %* Creep (30 minutes) <±0.016 %* Temperature Effect on Zero Balance <±0.01 %*/°C Temperature Effect on Span <±0.006 %*/°C Compensated Temperature Range -10 to +40 °C Operating Temperature Range -20 to +70 °C Safe Overload 150 %* Ultimate Overload 200 %* Zero Balance <±2 %* Input Resistance 400 ±30 Output Resistance 350 ±1.5 Insulation Resistance >5000 M@ 100V Recommended Supply Voltage 10 V Maximum Supply Voltage 15 V Environmental Protection 350i=IP68 350a=IP66 – Load Foot Accessories for 350 The cells could be excited (via a junction box) by a cost effective instrument such as our XT1200 unit. JB4T JUNCTION BOX Pricing from Thames Side-Maywood Hole size 4mm European X350A-300 US SHEAR BEAM LOAD CELL 108 COST EACH £68.00 TOTAL QUANTITY 4 X35913-FOOT $125.12 £272.00 $500.48 £9.00 $16.56 80 50 CAPACITY 300KG (660lbs) LEVELLING FOOT COST EACH QUANTITY 4 120 TOTAL £36.00 $66.42 ALL DIMENSIONS IN MILLIMETRES (Not to Scale) JB4T JUNCTION BOX (IP65) 4 WAY SUMMING COST EACH £40.00 $73.60 The resolution required would easily be achieved (and much better). The load cell is approved to OIML R60 C3 which is the European Weights and Measures accuracy 55 requirement. It would be important that the wheel chair platform structure that the cells would support is very rigid in nature because any bending or flexing would produce accuracy errors. Data and quotes provided by Thames Side-Maywood (logo below): Static Force Analysis: Static Analysis: Optimized Design (4 Load Cells) Calculations (assuming central and edge two-point symmetrical loading from wheelchair wheels, at loading of 1000 lbs): 1) Maximum (surface) flexural stress in weigh platform 2) Shearing stress on bolts fastening platform to rest bar 3) Flexural stress in rest bar 4) Horizontal force on load cell Yield strength of A36 hot-rolled structural steel: 150 Mpa compression 21.8 ksi 250 Mpa tension 36.3 ksi Elastic modulus of A36 hot-rolled structural steel: 200 Gpa Platform Dimensions 29000 ksi 36” x 36” x 0.25” Rest Bar Dimensions (see diagram): Top: Side: 1.5” x 36” 2.5” x 36” Bottom: 2” x 36” Thickness: 0.25” Useful Equations: m Eq. 1) Mc I Eq. 2) I bh 3 M Eq. 3) Eq. 4) Eq. 5) Eq. 6) LF 8 m E I Ic ax 2 I rec tan gle bh 3 1) Assume platform is fastened to rest bars such that no moment can be developed about the joints (low bolt tension) and is supported along the joints evenly in the ramp direction. Wheelchair wheels are modeled with two 500 lb forces (factor of safety, F.S. = 0.8) spaced 18 in. apart, positioned symmetrically in center of weigh platform. This creates pure-bending (constant bending moment) between the downward forces. Used BeamView Java Applet (Virginia Tech, freely available online) to simulate platform bending behavior: Figure 19: Beam view simulating platform bending behavior Maximum Bending Moment: 4500 lb*in By Eq. 1, maximum bending stress = (4500lb*in)(0.125 in) / (0.5625 in^4) = 1000 psi = 1 ksi 2) Assume shearing force on bolts attaching rest bars to platform is flexural stress of 1000 psi (above) multiplied by bolt projection area Flexural stress distributed through 4 bolts: 250 psi Shearing force on 0.25” thick bolt holding 0.25” platform: = (250 psi)(0.25 in)(0.25 in) = 15.6 psi 3) Assume the cumulative 500 lb load on the bottom portion of the rest bar is loaded evenly across the 36” length: Moment of inertia of rest bar: Split into 3 rectangles of 1.25”x0.25”, 0.25”x2.5”, and 1.75”x0.25”: I[1] = 1.25*0.25^3+1.25*.25*(1+0.125)^2 I[2] = 0.25*2.5^3 I[3] = 1.75*0.25^3+1.75*0.25*(1+0.125)^2 Total I = I[1] + I[2] + I[3] = 8.07 in^4 BeamView Java Applet used to simulate distributed 500 lb load: Figure 20: Beam view simulating distributed 500 lb load Maximum flexural stress = (1500 lb*in)(0.125 in) / (8.07 in^4) = 23.2 psi 4) Assume flexural stress multiplied by thickness is horizontal force on bolts attaching load cells to rest bar Fx = (23.2 psi)(36 in)(0.25in) / 2 = 100.4 lbs Analysis: I. The flexural stress in the platform developed from two 500 lb concentrated loads spaced 18 inches apart and symmetrically in the center of the platform is 1 ksi, and the yield stress is 21.8 ksi. The stress developed is less than 5% of the yield strength, indicating the platform is able to support over 20 times the maximum loading for the scale. II. The shearing force of 15.6 psi developed on the bolts securing the platform to the rest bars is negligible and indicates that 4 bolts is enough to secure the platform for maximum loading III. The flexural stress in the rest bar is extremely small compared to yield strength, since the moment of inertia I is very large and the load is assume to be equally distributed. This indicates the rest bar will support the platform without yielding during maximum loading IV. The total horizontal force acting on the part of the load cell attached to the rest bar is 100.4 lbs. Typical shear-beam load cells will not tolerate sideloading (defined as loads perpendicular to the axis of the load measured) that is greater than 100% of the rated maximum axial load. The maximum load for the load cell is 660 lbs (300 kg), meaning that the sideload percentage is 100/660 or 15%, which is safely below the limit. This indicates that at a maximum loading of 1000 lbs, the horizontal forces acting on the load cells in a direction toward the center of the platform are within safe limits of operation for the load cell. This force also introduces a moment about the axis parallel to the load cell’s largest dimension. Although not permanently damaging to the load cell if inside safe limits (such as in this case), moments about this axis and sideloads will affect the measurement of the load cell, making the creation of a calibration curve necessary along with extensive testing of the scale accuracy. Circuit Analysis: -5V Load Cell +5V R1 Input Output +5V Output +5V -5V Microprocessor out1 out2 out3 out4 out5 out1 out2 out3 out4 out5 LCD Display vcc Input R1 R2 R3 R4 In1 In2 In3 In4 vcc R2 ground Load Cell +5V ground -5V Load Cell +5V R3 Input +5V +5V Output +5V -5V Load Cell +5V R4 Input Output vcc 1 2 3 4 +5V +5V ground User Interface The above circuit is a generic view of a real circuit that is going to be mounted on our design. The basic approach to the circuit is very simple, the four main outputs from the load cells are going through filters so the voltage values entering the microprocessor are without any noise and interference, also these voltage values go through some resistance in order to make them suitable for the requirements of the microprocessor, different microprocessors have varying input voltage acceptance values. The microprocessor also gets input values from the User Interface, all these inputs of the microprocessor go through calculations and memory storage sequences and then the result is displayed on the LCD Display. Block Diagram of System: The above block diagram is the electrical part of the design; the power supply is shown supplying a voltage to the load cell (force transducer) and at the same time, the load cell’s resistance changes in proportion to the load applied axially. A Wheatstone bridge circuit is used to detect the extremely small changes in resistance, and this constitutes the output signal from the cell. Once the load cell signal reaches the linear or steady state, then the analog signal is sent to the Signal filter where it is transformed from a noisy signal to a continuous one and then this output is sent to the Signal Amplifier for possibly voltage or current adjustments before the signal goes to the Microprocessor. The Microprocessor gets two inputs one from the user and the other from the Signal Amplifier. Once the Microprocessor is done with its arithmetic and all the memory storing and sorting, then the final weight is displayed onto the LCD display. The scale turns on as the user moves onto the platform with some minimum weight or when the power switch is toggled. Flow chart of the Microprocessor: The computation and user interface for the weighing system is accomplished by a microprocessor with user input switches and an LCD monitor. The following is a microprocessor flow chart, which shows the different functions performed by the user and the microprocessor while computing the weight of the person (see Fig. 21). Analog output from the Load Cell The analog signal coming from the Load Cell is being converted to digital signal using the A/D component of the Microprocessor The A/D input of the Microprocessor The user enters the value of the wheel chair and then presses enter. Wheel Chair input from the User The digital signal coming from the microprocessor is being sent to the input of the ALU component of the Microprocessor where the subtraction of the wheel chair weight from the total weight takes place. The digital value of the weight being converted from the Load Cell is stored into some register ‘x’ and then the user value entered is being stored in some register ‘y’. The ALU component of the microprocessor does the calculations and stores the final value in the register ‘z’, which is also stored in RAM, so it could be accessed again. ALU functions of the Microprocessor Up to 10 different weights of the person are registered in this memory, for the records of the user. Digital to Ib/ kg converter The output of the Microprocessor is displayed on the LCD display. Random Access Memory (RAM) of the Microprocessor The LCD display The user pushes the button on the panel which brings up the previous measurements of the patient. Accessing the previous measuremtns . Figure 21: Flowchart of processing and display unit User interface flowchart: Scale Process Menu: 1-Power 2-Weigh 3-History 4-Change wheelchair weight Switch on Switch off Filtered weight If time > 5 min Access wheelchair weight Yes display No display Hold till scale is stable Display: “No wheelchair weight stored.” “Store wheelchair weight?” No wheelchair weight stored Yes No Input wheel chair weight Go to Menu If time > 2 min Filter weight “Error too much movement” Store Wheelchair weight as 1-10 Display “Your weight is__.” Go to Menu “Store to memory?” Yes Store in next available space, bump out oldest entry if full No Go to Menu Figure 22: User interface flowchart Figure 22 above shows the user interface flowchart enabling the user to navigate through four options – turn the scale on/off, weigh, check weight history, and change wheelchair weight. There are few menu levels, making the interface easy to use for most people. The memory is accessed by the processor whenever the user selects the option to actually store their weight, and the oldest entry for weight is deleted and all entries are moved back to make room for the new one. The user must know their wheelchair weight and type it on only once, as it is stored in the memory. The machine memory must be able to handle repeated and constant use, and must not fail in a power outage. A battery can be provided to ensure that stored data is not lost. Scale Design-Finished Product This chronicles the designing and building process we went through to deliver the finished product, overcoming small and large obstacles and making the changes that were necessary to arrive at the end result. Our load cells arrived the first day we met as a group to build the project, giving us the chance to immediately test them and inspect them. We used a clamp, table, and labeled weights as well as a multimeter and voltage source to make a plot of voltage vs. load. Our project parts were obtained as needed instead of all at once, throughout the semester. The second step was ordering the voltage source and writing code to use the LCD. I decided to use freely available code written by professional developers after failed attempts to write it myself. The basic building blocks of the program were written one by one until the main code could be simplified to calling the functions as needed and hence easily changed. During this development, we decided to simplify the design by splitting the circuit into two: an amplifying circuit to acquire, amplify and filter the analog signal and the interface circuit to read the signal and communicate with the user. There were a total of six conductors – power, ground, and four load cell signals – needed between the two, enabling us to use any standard wiring cable with more than 6 conductors. We made heavy use of oscilloscopes to determine the signal-to-noise ratio and noise difficulties in the amplifier circuit caused us a huge delay. If the power supply had generated less noise or the filters able to keep the peak-to-peak range of the signal below +/- 2.5 mV, the 10bit A/D converter on the microcontroller would not be affected by it because it reads in 4.9mV increments. This would enable us to weigh in microseconds. However, the signalto-noise ratio was large enough to necessitate the use of an algorithm which waits for 5 equal weighings in a row (sample frequency 2.865 kHz) to use the value as a legitimate weight. This process ended up taking close to 30 seconds in the worst cases in the final unit; however, we a large object such as a wheelchair actually statically stabilized the scale to the degree that the weighing time was reduced to less than a second. Creating the PCB designs were straightforward for the amplifier circuit, and this circuit was assembled without any major problems. However, the protoboard of the interface circuit would not receive a steady keypad input, and it took an inordinately long amount of time to find out that this was due to floating voltages. This was easily solved by changing the PCB schematic to include 10K pull-down resistors on four pins. We were all surprised when the interface circuit, connected to the tested amplifier circuit with scale, worked correctly the first time. Testing the scale with known weights revealed that the scale took very long in certain situations to register a value, and that sometimes this value was not consistent. We found steps to improve consistency and accuracy, such as making sure the floor is a level surface, adjusting the load cells to be collinear with their bolts, and dampening the vibrations from the podium bar. When the cord to the interface unit is dangling, it creates a moment on the bar that is increased through leverage down to the adjacent load cell, offsetting the result by as much as 4 lbs. We also found that the load cells actually take upwards of 10 seconds to equilibrate to the correct weight value, and included this in the Owner’s manual. In conclusion, the scale actually works best when loaded with a wheelchair, and meets every single one of our Project Specifications. The design of the scale changed significantly from the original design from everything to the setup and the placement of the load cells. The most significant change occurred with the actual placement of the load cells. The original design called for the load cells to be attached under a Z shaped piece of metal that the main platform would be attached to. After learning more about load cells, it was determined that the load cell would need to actually be placed directly to the main platform. In order for the load cells to measure the tension produced by the load on the platform, the Z shaped piece of metal would have greatly reduced the overall load that would be distributed to the load cells and then measured, thus resulting in inaccurate readings of the person and their wheelchair. So the new design had all four load cells attach to the corners of the platforms. Side view of scale-figure 22 Top view of scale-figure 23 Block Diagram of System: FigureA1: Block Diagram of the system The above block diagram is the electrical part of the design; the power supply shown is supplying a voltage to the load cell (force transducer) and at the same time, the load cell’s resistance changes in proportion to the load applied axially. A Wheatstone bridge circuit is used to detect the extremely small changes in resistance, and this constitutes the output signal from the cell. Once the load cell signal reaches the linear or steady state, then the analog signal is sent to the Signal Amplifier where the signal is being amplified. After the signal is being amplified it is sent to the Signal filter where it is transformed from a noisy signal to a continuous one and then this output is sent to the Microprocessor. The Microprocessor gets two inputs, one from the user and the other from the Signal Amplifier. Once the Microprocessor is done with its arithmetic and all the memory storing and sorting, then the final weight is displayed onto the LCD display. The scale turns on as the user moves onto the platform with some minimum weight or when the power switch is toggled. Circuit Analysis: The circuit analysis is based on the designs of the Power Supply and general circuit which contain four Load Cells, Amplifiers, filters, resistors, capacitors, and voltage regulator. The Power Supply generates +12 volts and -12 volts of voltage, which feeds the whole circuit. The +12v and -12v voltages needs to be filtered before they are drawn to the rest of the circuit because the amount of noise generated by them could alter the results of the design as a whole. In order to do that Power Supply filter analysis is used below which describes the design process of linear filters for the output of the Power supply. a) Power Supply filter analysis: By hooking capacitors to ground shunts all the high frequencies to ground, while passing all the low frequencies so basically this works as a passive low-pass filter. The impedance is dependent on the frequency as the frequency increases the impedance decreases. The capacitors acts like a bypass capacitor, where the AC component of the signal is directed towards ground while the DC component of the signal is passed through the circuit. Vin is +12v and four capacitors in parallel are added together .1+.1+.1+.1 = 0.4µF for C. I = Cdv/dt V (t) = 1/C ∫tt0 i dt + v(0) since to is usually zero Xc = 1/2ΠfC With certain Xc values the frequency (f) goes to zero which is the DC value. Following is the signal shown before the power supply is being filtered with a noise value of approximately 215mV peak to peak. Figure A2: +12 volts signal before it is being filtered: Below is the signal when the power supply is being filtered, with a noise value of approximately 36mV from peak to peak. Figure A3: +12 volts signal after it is being filtered: b) Load Cell signal analysis: The Load Cell uses a Wheatstone circuit analysis in order to compute the voltage desired by the user. The circuit for the Wheatstone analysis is shown below; Figure A4: Wheatstone Bridge By looking at the circuit it is certain that there are two output voltages which are in microvolt range, therefore computing the difference between those two voltages requires a Differential OP-Amp which computes the difference and then amplifies the difference with a gain of 702. The gain was found by finding the voltage to pound ratio which is shown below; Weight (Ib) 0 15 25 Voltage (volts) 0.11 0.22 0.30 By taking the average we find out that 31µV of voltage is generated by a 1Ib force. c) Filter Design: The voltage coming out of the Load Cell is in microvolt range, which impounds the possibility of noise mixed with the signal. If the signal is mixed with noise that will dramatically decrease the effectiveness of the design. So in order to control the noise a Low-pass Active filter must be designed. Active filter because in order to obtain a sharp response with a passive filter, we would need to cascade several passive stages. Each cascaded stage, however, loads the previous stages. This loading attenuates the desired part of the signal, i.e., frequencies within the filter pass-band, as well as unwanted frequency content within signal. This problem is commonly known as insertion loss. Active filtering practically eliminates insertion loss due to the high input impedance and the low output impedance of an Op-Amp. Filter Analysis: The following First-Order Unity-Gain Low-Pass Active filter was used to filter the noise coming out of the differential Op-Amp. The Vout of this filter is the signal going into to the Microprocessor. The following filter is being used for our design where all the filter analysis was acquired from the following useful website: http://users.telenet.be/educypedia/electronics/analogfil.htm We want frequencies of less than 5 Hz passed through the circuit therefore the signal is being filtered in order to cutoff frequencies higher than 5 Hz. For our design the following parameters were used; F0 = 0.22 Hz C = 22µF R = 1/ (2Π*C*f0 ) R = 1/ (2Π*22µF*0.22) = 33KΩ The following is the signal before the output of the differential OpAmp is being filtered with a noise of approximately 30mV from peak to peak. Figure A5: Noise level before the filter: Below is the signal after the output of the differential Op-Amp is being filtered with a noise of approximately 20mV from peak to peak. The 20mV noise value is not exactly 20 mV its much lesser than that, by looking at the waveform of the signal attained from the oscilloscope, and taking the standard deviation of all the spikes, the voltage value of the noise seems to be more like around 2mV. Figure A6: Noise level after the filter: By taking the standard deviation of the different voltages of the waveform supplied by the oscilloscope we see that the noise is brought down tremendously; Record Length Sample Interval Trigger Point 2.50E+03 1.00E-04 1.25E+03 Source CH1 Vertical Units V Vertical Scale 5.00E-02 Vertical Offset 2.40E-02 Horizontal Units s Horizontal Scale 2.50E-02 Pt Fmt Y Yzero 0.00E+00 Probe Atten 1.00E+00 Firmware Version FV:v6.08 -0.125 -0.1249 -0.1248 -0.1247 -0.1246 -0.1245 -0.1244 -0.1243 -0.1242 -0.1241 -0.124 -0.1239 -0.1238 -0.1237 -0.1236 -0.1235 -0.1234 -0.1233 -0.1232 -0.1231 -0.123 -0.1229 -0.1228 -0.1227 -0.1226 -0.1225 -0.1224 -0.1223 -0.1222 -0.1221 -0.122 -0.1219 -0.1218 -0.1217 -0.1216 -0.1215 -0.1214 -0.1213 -0.1212 -0.002 -0.01 0.004 0.002 0.004 0.002 0.004 0.002 -0.01 0.002 0.002 0.002 0.004 0.004 0 -0.002 0 0.006 0.002 0 -0.002 0 0.002 0.002 -0.002 0.002 0 0 0 0 0 0.004 0 0.002 0 0 0 0.002 0.002 Standard Mean Deviation 0.000714 0.002413691 The standard deviation of the output signal shows how much noise is being generated by the differential Op-Amp which is about 2mV. The 2mV noise would not affect the measurement of our design and this was proved by testing the design. After the circuit analysis part of the design was complete then the circuit as a whole was designed on the Express schematic as shown below. Figure A7: Schematic Circuit After completing the schematic of the circuit, the circuit was linked to the PCB part of the Express software where the PCB board of the circuit was created as shown below; Figure A8: PCB board The PCB part of the design is being done so that our PCB board could be ordered and finally placed on the final design. Mechanical Design Ideally we want the scale that we designed to be very user friendly so that individuals in a wheelchair will be able to appropriately monitor their weight on a daily basis to keep track on the weight losses and gains. When designing this scale a requirement was that we would like to see the scale be able to be transported by another person for an easy monitoring use. The transportation of this device is vital whether it is used in a clinical setting or used in an individual’s home. Another requirement was that this scale would be very durable and withhold a large amount of weight. The scale will be very sturdy and resilient through out its lifespan. The scale is made out of low carbon steel. Steel is a very strong, robust, long lasting material that can handle many situations. However with the decision to use such a material that can endure physical abuse, the weight of the scale had to be increased. Steel is also a very heavy material hence it’s durability. Design changes were definitely constructed so that such a solid material could be used. The new design eliminated almost one half of the original designs material and thus it’s weight. Therefore with the addition of a resilient scale the requirement of having the scale be transportable was still maintained. The new design actually allows the scale to break down into 5 major components plus the nuts and bolts that allow the scale to be transported. The 5 major component produce the restriction on the individual transporting the scale, so that they won’t carry more than one component at the time and possibly cause harm to themselves by carry more things than they realistically can. Additionally eliminating half of the overall raw materials also reduces the cost significantly. The design process involved many stages of machining. Many holes were drilled in the angle iron and platforms through the use of the Miller and drill press. Sheets of metal were also cut and bended using the vertical band saw and the Betenbender. A small amount of welding was also needed for bolts and podium attachments. This process produced obstacle along the way but ones that were always overcome. Here is a look of different stages in the design development: This is how it all began with the cutting of the angle iron-figure 24 Next the Miller was used to drill into the angle iron-figure 25 The angle iron with all the completed holes-figure 26 The raw metal sheets that have yet to be machined-figure 27 The metal sheet cut down, and then with bended sides to increase the strength of the platform. Also all holes in the platforms have been drilled, and the back guardrail has been attached. Figure 28 A close-up of the welded bolts to the platform that go into the load cells, which allow for less pieces during disassembly. Figure 29 The full scale view, showing all welded bolts-figure 30 And then after a lot of hard work and machining the main base of the scale was ready for assembly. Figure 31 Close up of the underside of the platform, showing load cell and attachment nuts. Figure 32 The swivel, adjustable podium for the user interface and the LCD-figure 33 The LCD holder and the user interface-figure 34 The objective of this design is to create a scale that is easily accessible to wheelchair users. We want a handicapped wheelchair bound individual to have easy access to an affordable scale that will not require much assistance from others. The scale will have a clear easy readout display, to allow for the individual to keep track of their weight. This objective has been fulfilled. The scale appearance is not outlandish or eccentric. It is a soft dark gray that will fit nicely in a commercial or home environment, having an exterior with the same appeal of an appliance. The overall scale is made out of a very durable and strong metal steel, that will enhances the scale’s longevity and resilience. Interface and Program Design Measuring Techniques: Two different software approaches were tried in order to obtain a consistent weight value. The first approach was to employ a signal average, and the second was to close the weighing algorithm in a loop until an identical weight value was obtained a number of times in a row. Both algorithms summed all four 10-bit values corresponding to analog load cell outputs to obtain a total platform weight. The first algorithm measured a platform weight, then added this to a variable space 16 times. The number of weights that could be added was limited because of the maximum value that two variables can accommodate. Later, this number was increased to 256*16 or 4096 in an attempt to increase the consistency of the result. In both cases, the weights obtained by the A/D converter was not consistent enough for our needs, although the computing time was very fast and was the same each time. Each time a weighing cycle was performed, the weight would vary by as much as +/- 2 divisions (here, a division is defined as a single bit of accuracy from the A/D converter; the 10bit onboard A/D converter of the PIC16F874 thus yields 2^10 or 1024 different divisions). This translated to as much as +/- 3 lbs, giving a maximum difference between adjacent measurements of 6 lbs. This was wholly unacceptable. One possible reason that the signal averaging technique was not working was the nature of the signal noise. Instead of white noise, the signal was composed of brief, equally spaced pulses of noise in between which there was an area of much less noise. Although the RMS value of the amplified and filtered load cell signal was equal to the DC component of the signal, the peak-to-peak maximum values for the signal were the most important aspect. Because this value was almost +/- 100mV in the final PCB circuit installed in the scale, averaging would have had to sample many values to cover one period of the signal, and we were not able to do this. The error range doubled when the circuit was tested on PCB, presumably because of the large number of parallel traces carrying noisy +12V and -12V of power very close to traces carrying the weak load cell signal (see Filter Analysis). The second approach was to use a continuous loop that returned a value only when it had been exactly repeated 5 times in a row. This algorithm differed in that it could theoretically go on forever, if this condition is not met. A future software version should incorporate a break option for the user, if he/she feels too much time has been spent in a weighing cycle. The algorithm works as follows: first, a counter is set to zero and the platform weight is obtained twice and stored in two separate variables. If the values are exactly equal, the counter is incremented and the least recent value is deleted with the most recent value occupying that variable. Then, another weighing cycle is performed and compared to the current value. If they are equal, the counter is incremented again and this repeats until the counter is equal to a previously defined “tolerance” value, declared in the microprocessor code as “Tol” (see Microprocessor Code, under label “Weighloop”). We found that we could trade off speed for consistency, and settled on a tolerance value of 4 in the final code. This translated to the requirement of 5 equal weights in a row before the loop could return the value. In a vibrationfree, stable, EMF-noise-free environment, the loop could return a stable weight from the scale as quickly as 5 seconds, or in the worst cases, as long as 5 minutes. However, it is more important that the right weight is obtained each weighing cycle than the speed with which this occurs. If the user feels otherwise, it is a simple matter to reprogram the chip with a smaller tolerance value. A tolerance value of 3 was found to finish weighing almost 10 times faster. The same weighing algorithm is used for all parts of the program that need a weight, and this value is stored as a 16-bit value, between two 8-bit registers. All the weights are stored as a binary value corresponding to the value obtained by the A/D converter minus the taring weight and the stored wheelchair weight. These values are only converted to pounds when displayed on screen. This ensures that the original bit values are a standard in the calculations performed by the program. Using Microcontroller Memory Two values are subtracted from the weight when it is to be displayed on the screen after a weighing cycle (in menu option 1) and when it is to be stored. These are the tare weight (“Tareh”, “Tarel” variable names) and the wheelchair weight (“Wheelh”, “Wheell” variable names). Both are set to zero by default upon startup. The wheelchair weight can be specified by the user in menu option 2, done by manually typing in the weight value in pounds. This decimal value is stored as five decimal digit values in five variables designated as incoming decimal digits from the keypad. These are converted to binary in a subroutine and divided by a conversion factor to convert to the same scale as the A/D converter. The tare weight is set with menu option 4, which calls the weighing loop and stores the resulting weight as a tare weight. It is important to note that the weighing loop always returns the absolute weight, consisting of the platform itself, any objects on it, and any electrical offset in the amplifier circuit. This simplifies taring operations. The binary value stored in memory is actually the absolute weight minus both tare and wheelchair weights. Since the binary values obtained from the converter are not in units of pounds, a conversion factor was needed. A subroutine was written to multiply the digital value between 0 and 1023 divisions by a number less than 1 to obtain a number between 0 and 800 lbs. This conversion factor has four significant decimal digits (9999 divisions) and is sufficient in accuracy for this application. The inverse of the conversion factor is stored as well, to convert from a binary representation of an input in pounds to the appropriate decimal value. The conversion factor was successively approximated by testing the scale with known weights, and the final version contains a conversion factor accurate to the nearest 0.2 lbs The microprocessor’s data memory can hold 82 individual bytes without interfering with declared variables, translating to 41 different weight values. If EEPROM were to be implemented in a future software version for the same chip, the capacity would be 64 different weight values. Also, these values would stay in storage even during a power outage; currently, the memory is dependent upon a power supply. The block of memory is initialized to zero upon startup, and a new value is stored as follows: the values are all shifted downward in the block, as in a stack, with the first value moved to the second space, and so on until the last value is eliminated. Then, the weight to be stored is stored as the first value. In this manner, the memory acts like a shifting stack, keeping the most recent values as the first and maintaining itself by eliminating the oldest values. With a capacity of 41 weight values, a user can monitor his/her weight for well over a month if used every day. Testing and calibration techniques were made as rigorous as possible with the precision of available weights. The scale was considered to be in working condition only when three weighing cycles in a row returned the same value. Testing for hysteresis was performed by loading and unloading the scale with a known weight several times in a row and verifying the equality of the alternating loaded and unloaded values. The tools used to develop the firmware for the microcontroller as well as downloading into the chip were MPLAB IDE and the Qik Start Education Board, from Microchip and Diversified Engineering Inc., respectively. Engineering Standards and Environmental Considerations: It is important that as engineers we take into account our own ethical standards. In creating this design we made sure that we have and will continue to uphold the IEEE code of ethics and that as engineers our current and future designs will not be harmful to any individuals or the environment itself. In continuation of our design and our future work as engineers we will maintain that when something ethically wrong is occurring we will, however possible stop any harm from occurring. IEEE Code of Ethics [11] 1. to accept responsibility in making engineering decisions consistent with the safety, health and welfare of the public, and to disclose promptly factors that might endanger the public or the environment; 2. to avoid real or perceived conflicts of interest whenever possible, and to disclose them to affected parties when they do exist; 3. to be honest and realistic in stating claims or estimates based on available data; 4. to reject bribery in all its forms; 5. to improve the understanding of technology, its appropriate application, and potential consequences; 6. to maintain and improve our technical competence and to undertake technological tasks for others only if qualified by training or experience, or after full disclosure of pertinent limitations; 7. to seek, accept, and offer honest criticism of technical work, to acknowledge and correct errors, and to credit properly the contributions of others; 8. to treat fairly all persons regardless of such factors as race, religion, gender, disability, age, or national origin; 9. to avoid injuring others, their property, reputation, or employment by false or malicious action; 10. to assist colleagues and co-workers in their professional development and to support them in following this code of ethics Ideally this design should not produce anything that is harmful to the environment during the production phase, while it is in use, and after the life of the device has ended, therefore minimizing the devices short and long term effects on the environment. This design needs to take into account all past disasters and problems with other engineering designs and make sure that these previous problems are not recreated through this design or that this device does not create any new exploitations to the environment and when possible minimize the resources that the environment provides. The optimal design for the smart wheelchair scale does not have a short lifespan and will ideally be able to be used to the extent of an individuals entire life to monitor there weight. With the use of load cells, a LCD and a sturdy, durable material for the scale minimal operating malfunctions and deterioration of this product will exist. Disposal of the smart wheelchair scale and the end of its life will still contain materials that maybe stripped and recycled for use in other electronic products. For the remaining product, it will be able to be disposed of at you local landfill and it will not produced harmful substances into the environment because of the use of non-toxic materials. The smart wheelchair scale will not be a burden on the environments energy consumption. A small amount of electrical energy will be needed to operate the device and it will not require any fuel to operate. The ADA Standards for Accessible Design pamphlet specifies regulations for streets, walkways, buildings, etc. for the safe day-to-day accessibility of wheelchairbound persons. In addition to minimum widths and lengths for wheelchair-accessible areas, the document describes maximum angles for access ramps. The design presented here is within the bounds of these regulations. The American Disabilities Act specifies a 32” minimum length or width for a wheelchair accessible area and a maximum 1:12 ramp slope, both of which encompass the optimized design described in this report [12]. Parts List: Part Load Cell Load Foot Junction Box Platform Bolts Nuts Hinges Microprocessor LCD display Assorted electronic parts Timeline: Name X350A-300 X35913-FOOT JB4t A36 Structural Steel Carriage Bolt, Plain 18-8 1/4-20 Stainless Finished Hex Nut, Plain 18-8 1/4-20 Stainless Manufacturer Thames Side-Maywood Thames Side-Maywood Thames Side-Maywood Machine Shop Wink Fasteners, Inc. Qty. 4 4 1 1 16 Total Cost $500.48 $66.24 $73.60 $100.00 $1.53 16 $0.80 A217 16-gauge 1-1/16 TBD TBD Wink Fasteners, Inc. Austin Supply and Hardware, Inc. TBD TBD 4 TBD TBD $13.16 TBD TBD TBD TBD TBD TBD Conclusion: This design offers many beneficial features that make this product highly accurate. This design includes a lower cost aspect but involves a more accurate approach to weight measurements with the addition of four load cells. The combination of four load cells provides an in-depth average of the individual’s weight, because it will not matter where exactly the individual is placed on the scale providing easier access. This design also does not include springs to act as supports; instead the four load cells will act as supports for the platform with their placement in load feet. The display unit will be large enough for an individual with moderately low vision to see and operate. The operation of the scale and display unit will also be user friendly and allow enough memory to store collected weightings for up to 10 times. The circuit analysis in this design is more accurate and affordable to build. The usage of fewer resistors and capacitors is taken into consideration. References: 1) Diseases and Conditions Index. “What is Chronic Obstructive Pulmonary Disease?” National Heart, Lung, and Blood Institute. Medline Plus Copyright © 2004, http://www.nhlbi.nih.gov/health/dci/Diseases/Copd/Copd_WhatIs.html 2) National Lung Health Education Program. “Lung Treatment of COPD and Asthma”. Copyright © 2002-2004, http://www.nlhep.org/lung_trtmnt.html 3) Detecto, division of Cardinal Scales Mfg. Co. “Wheelchair Scales” (product inventory). Webb City, MO: http://www.detectoscale.com/wheelchairs.htm 4) “Seca 664C Smart Card…” (product inventory). Copyright © 2003 Precision Weighing Balances, http://www.balances.com/seca/medical-wheelchairscale.html 5) ABLEDATA keyword search, “wheelchair scale”. ABLEDATA, Silver Spring MD. 6) USPTO Quick Search, keyword “wheelchair scale”. United States Patent and Trademark Office, Last Modified: 2004 7) Douglas A. Hobson, PhD. “Development and Application of Wheelchair Standards” Powerpoint File, Sept. 1999, NIDRR, http://www.wheelchairnet.org/WCN_WCU/SlideLectures/DAH/WCStds1099.pdf 8) Product Search: LCEC Minibeam Load Cells. © 2001 Omega Technologies, http://www.omega.com/ppt/pptsc.asp?ref=LCEC&Nav=pref08 9) Series 350 Shear Beam Load Cell. © 2004 Thames Side-Maywood Ltd., http://www.thames-side-maywood.com 10) “The Strain Gage”. Transactions in Measurement and Control. Vol. 3, Omega Technologies, http://www.omega.com/literature/transactions/volume3/strain.html 11) IEEE Code of Ethics. IEEE Board of Directors, © 2004 Institute of Electrical and Electronic Engineers, http://www.ieee.org/portal/index.jsp?pageID=corp_level1&path=about/whatis&fi le=code.xml&xsl=generic.xsl 12) Department of Justice. “ADA Standards For Accessible Design”. 28 CFR Part 36, Revised July 1994, http://www.usdoj.gov/crt/ada/adastd94.pdf Acknowledgements: We would like to thank the people in the UConn machine shop, which include Tom, Serge and Rich for their advice, suggestions, and extreme motivation and support towards this project. Also the Rehabilitation Engineering Research Center (RERC) has made an outstanding contribution in order to fund this project. Many other professors on campus whose extremely important advice and guidance were appreciated include John Ayers who provided us with important circuit analysis guidance, Jeff Meunier who provided important programming advice, our advisor Dr. John Enderle, Bi Zhang, and Dr. Christian Davis. We would also not have been able to complete this project without the help of Christopher Liebler and Franscisco Rodriquez for all of the support and advice given throughout the development of the final product. Thanks so much, we greatly appreciate all of the help!