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ELECTROMECHANICAL SYSTEMS VERSION 1 The Engineering Council 10 Maltravers Street LONDON WC2R 3ER © The Engineering Council First published by The Engineering Council 1995 ISBN 1 898126 41 0 All rights reserved. This book is copyright material but permission is granted to make photocopies of pages for classroom use provided that the copies are used exclusively within a purchasing institution. No other reproduction, storage in a retrieval system or transmission in any form or by any means may be made without prior permission from The Engineering Council. ACKNOWLEDGEMENTS Text by John Cave Layout by Peter Stensel Line illustrations by James Wilkinson Series Editor John Cave Middlesex University CONTENTS Introduction 1 Section 1 Motors and gearboxes 3 Section 2 DC generator 14 Section 3 Shape memory alloy actuator 27 Section 4 Linear actuator 45 Study file 1 “Bit by bit” controller 49 Study file 2 Generator applications 60 Study file 3 Solenoids 63 Resources 67 ELECTROMECHANICAL SYSTEMS ELECTROMECHANICAL SYSTEMS INTRODUCTION As the name suggests, electromechanical systems or devices convert electrical energy into mechanical movement - and sometimes vice versa. Most of the common electromechanical components, such as electric motors and solenoids are used in combination with mechanical parts to provide actuation or movement. Solenoid Motor Solenoids are used, for example, as actuators in vending machines, cash registers and photocopiers. Electric motors, for example, are used in linear actuators (providing straight line movement), in electric window systems, operating tables, and robotic arms. ELECTROMECHANICAL SYSTEMS - VERSION 1 1 ELECTROMECHANICAL SYSTEMS A relatively new branch of engineering design called mechatronics involves integrating the three areas of sensing, electronic control and mechanical actuation . A modern camera is a good example of a mechatronic product. One of the goals of mechatronics is to reduce the number of mechanical components to an absolute minimum; nevertheless in any system where movement is needed, there remains a basic need for some kind of controlled mechanical output. This book examines four basic electromechanical systems and the components needed to design and construct them. The examples anticipate the general requirements of pupils and students for practical prototyping work and focus on the use of basic inexpensive components. Because some of these are unique to TEP, additional product information is given on page 67. ELECTROMECHANICAL SYSTEMS - VERSION 1 2 ELECTROMECHANICAL SYSTEMS SECTION 1 ELECTRIC MOTORS AND GEARBOXES A combination of electric motor and gearbox providing rotary actuation is one of the most common electromechanical products. A gearbox is really a method of matching the primary power input from a motor (high speed, low torque) to the required output (normally low speed, high torque). (Torque can be thought of as “turning power”.) Very often a gearbox is built in as an integral part of a motor unit, and this may also contains sensors to feed back positional information to a control circuit. A good example is a type of DC motor used in photocopiers. Because its rotor consists only of a coil winding with very low inertia, it can be accelerated or stopped very rapidly. ELECTROMECHANICAL SYSTEMS - VERSION 1 3 ELECTROMECHANICAL SYSTEMS For most practical prototyping work you are likely to be using cheaper motors with gearboxes that you can design and make to give required output characteristics. Electric motors Many types and sizes of electric motor are available - with three main categories operating in the lower voltage range (6-24volts). These are: • DC brush motors • brushless motors • stepper motors DC brush motor Stepper motor Brushless motor A DC brush motor uses a commutator to cause the magnetic field in the armature coils to change so that the coils will rotate between permanent magnets. Brushes Case Magnets Commutator Armature ELECTROMECHANICAL SYSTEMS - VERSION 1 End plate 4 ELECTROMECHANICAL SYSTEMS A brushless DC motor has a permanent magnet rotor and fixed stator coils - but no commutator. As the rotor turns, one or more sensors close to its edge send a signal to a control circuit that energises the stator coils in the correct sequence. Rotor Stator A stepper motor - sometimes called a stepping motor - has a permanent magnet rotor that revolves within fixed stator coils. Unlike a brushless motor, however, there are no sensors. The rotor is driven round by switching the coils on and off in a special sequence using a driver circuit. Stator Rotor ELECTROMECHANICAL SYSTEMS - VERSION 1 5 ELECTROMECHANICAL SYSTEMS Gearboxes A gearbox is an assembly of gears inside a frame or casing. A gearbox has a rotary input and a rotary output. The gears inside mesh together to give a required output torque and speed. The gearbox of a cordless electric drill is often larger than the electric motor in the drill! However, it enables the drill’s very small motor running at very high speed to be turned into a very powerful drilling action. A simple gearbox may contain just two gears meshing; a more complex one might contain more than 100. Small gearboxes are used in toys and many domestic appliances. They are sometimes used in unusual applications - for example, professional model makers use them to create the special effects for programmes such as 'Spitting Image'. The illustrations show a range of general purpose gearboxes that can be obtained ready made, or as kits. In one of these examples, the gears are arranged within a small plastic case. In the other two, the gears are mounted around the motor itself. All these gearboxes can be adapted to give different output speeds and torque. ELECTROMECHANICAL SYSTEMS - VERSION 1 6 ELECTROMECHANICAL SYSTEMS There are several methods for making gearboxes; this section examines two of them. 1. TEP gearbox using pre-punched sideplates. This gear box has two plates held together using bolts and spacing pillars - e.g. plastic mouldings, small lengths of tubing or a series of nuts. The driving motor is attached to the outside of one plate, and a gear train set out between the two plates. The motor can be either a solar motor or an MM28 type. The solar motor is more expensive but is quieter in operation, will operate from a smaller current (e.g. solar cells) and produces less electrical noise. Technical note: MM28 and similar cheap motors use graphite brushes on the commutator; the solar motor uses very fine precious metal brushes which make better contact. The difference between the two shows up clearly when their rapidly changing current consumption, due to the commutator action, is shown on an oscilloscope. For the solar motor, the changing current is seen as a relatively smooth signal; for the cheaper motor, it is very “noisy”. Precious metal brushes Gear ratios The gearbox side plates are pre-punched to give a choice of three different gear ratios. The gears supplied are a 16 tooth pinion for the motor, two compound gears with 10 and 42 teeth and a larger (output) gear with 60 teeth. The illustrations show how the gears are set out to obtain the different ratios. ELECTROMECHANICAL SYSTEMS - VERSION 1 Graphite brushes 7 ELECTROMECHANICAL SYSTEMS Constructing a gearbox Step 1. Secure either an MM28 or a solar motor to the side plate with screw-fastening holes. If a solar motor is selected, packing washers must be used to prevent the screws touching the motor’s armature. Push fit the pinion on the motor spindle. Step 2. Push fit the selected gears on the 2mm diameter axles and position these approximately between the plates to ascertain what spacers need to be added to keep the gears floating (moving) across the gearbox. You can use washers as spacers or small lengths of plastic sleeving. ELECTROMECHANICAL SYSTEMS - VERSION 1 8 ELECTROMECHANICAL SYSTEMS Step 3. Assemble the side plates by putting in the gears and the fastening bolts and spacers. Any excess axle material can be snipped off. Step 4. OPTIONAL Because the output shaft will be subjected to a load, the gear that drives it might slip. Also, the shaft is running, effectively, in an aluminium bearing. A better arrangement is to ream out the punched holes that form the output bearing and insert nylon bearing bushes. ELECTROMECHANICAL SYSTEMS - VERSION 1 9 ELECTROMECHANICAL SYSTEMS The output gear is then drilled out to take a 3mm diameter shaft. If the shaft is steel, it should be rolled against a file as shown to produce a rough 'spline' to lock onto the gear. This is done by placing the shaft on a hard wooden surface (not metal) and rolling back and forth with the edge of a file - pressing down hard. An excellent alternative shaft material is 3mm diameter fibrereinforced pultruded rod. The gear should be drilled to 2.9mm diameter to make a secure interference fit. The rod itself is cut either with a hacksaw or the special TEP guillotine. TEP guillotine 2. Gearbox using punch-tool method A two-plate gearbox can be made using a combination of TEP’s larger compound gears. In order to mesh properly the gears need to be positioned at a distance of 26mm between centres. (26mm is also the distance between the motor pinion and the first driven gear centres.) 40 teeth 12 teeth ELECTROMECHANICAL SYSTEMS - VERSION 1 26 mm 10 ELECTROMECHANICAL SYSTEMS If correctly spaced bearing holes can be punched along a pair of side plates, you can create gearboxes with different ratios just by using the correct number of gears. Any two gears meshing give a ratio of 3.3 to 1; three gears give approximately 10:1 - and so on. The bearing holes for the gears can be accurately punched out of aluminium plate up to 1mm thick using the special TEP punch tool. After one hole is punched , the aluminium plate is simply moved along the graduated straight edge by 23mm and another hole punched. ELECTROMECHANICAL SYSTEMS - VERSION 1 11 ELECTROMECHANICAL SYSTEMS A simple gearbox with a ratio of 3.3 to 1 can be constructed as follows: Step 1. Cut out the gearbox side plates - allowing for the distances between gear centres and material at both ends for spacing pillars. An example hole layout for the motor side of the gearbox is shown. As well as bearing holes, two holes are punched for the pillars and two for the motor fastening screws. 150 40 25 7 7 26 26 Step 2. Locate the motor side plate against a convenient point on the punch’s graduated scale and punch the first hole. Move the plate along by the required distance and punch the second hole. Do this for all the holes on the diagram. Repeat for the second plate but leaving out the two holes for the motor screws. ELECTROMECHANICAL SYSTEMS - VERSION 1 12 40 ELECTROMECHANICAL SYSTEMS Step 3. Using a hand reamer, open out the punched hole for the motor boss (6.2mm) and secure either an MM28 or a solar motor with the self-tapping screws. If a solar motor is selected, packing washers must be used to prevent the screws touching the motor’s armature. Push fit a pinion onto the motor spindle. Push fit bearing bushes into all the remaining punched holes (see page 9). Step 4. Push fit the selected gears on 3mm diameter axles and position roughly between the plates to ascertain what spacers need to be added to keep the gears floating (moving) across between the two plates. You can use washers as spacers or small lengths of plastic sleeving. Step 5. Assemble the side plates by putting in the gears and the chosen fastening bolts and spacing pillars. The spacers can be mouldings, short lengths of tubing or a series of nuts. Any excess shaft material is cut off prior to assembly. Note: 3mm diameter fibre-reinforced pultruded rod is an ideal shaft material for this gearbox. It can be cut using a small hacksaw or TEP’s guillotine. ELECTROMECHANICAL SYSTEMS - VERSION 1 13 ELECTROMECHANICAL SYSTEMS SECTION 2 DC GENERATOR The TEP generator is an electromechanical system consisting of a motorised reduction gearbox working in reverse; i.e. a low speed, high torque input is converted through gearing into a high speed, low torque drive for the generator. Most miniature electric motors work as either motors or generators but some are more efficient than others. The TEP generator is a solar motor. The generator principle When a conductor such as copper wire is moved within a magnetic field and cuts across the lines of force, an electric current flows. The direction of current flow can be determined using Fleming’s right hand rule. Flux rr Cu en Motion Lines of force t Current If the wire is made into a coil and moved in the field, a larger current flows. A current can also be made to flow by moving the magnet rather than the coil. ELECTROMECHANICAL SYSTEMS - VERSION 1 14 ELECTROMECHANICAL SYSTEMS The TEP generator has coils rotating between fixed magnets. It was in fact designed as an electric motor but like most miniature motors it works as a generator when the spindle is turned rapidly. The coils are wound around an armature and the current generated in them passes to a pair of terminals by means of a Case commutator and brushes. The ends of the coils are connected to commutator segments from which a direct current (DC) is drawn by means of brushes in contact with the segments. The simplified diagram of a single coil rotating in Magnets a magnetic field shows how the commutator works. The segments are mounted on the shaft and rotate with the coil. The current in the coil flows towards the top segment and away from the bottom segment. Half a turn later the current in the coil has reversed but it is still flowing towards the top segment and away from the bottom segment. So although the coil is generating an alternating current, the Armature commutator acts as a mechanical rectifier and supplies a direct current from the brushes. The direct current from this generator always flows the same way, unless you reverse the direction of rotation of the shaft. Brushes Commutator End plate Ideally, the output of a DC generator should be smooth like that supplied by a battery. This appears on a time graph simply as a straight line. Current Flux + Brush 0 Brush Commutator Time Current The TEP generator, however, provides pulsating DC. This is because the current flow rises to a maximum when the coils cut directly across the lines of force and then falls to a minimum when they move along the lines of force. Maximum N S Minimum 0 N Time ELECTROMECHANICAL SYSTEMS - VERSION 1 15 S ELECTROMECHANICAL SYSTEMS Improving the DC Output If the TEP generator is connected to a loudspeaker, the pulsations are heard as a loud sound. The pitch of this sound rises as the generator’s speed is increased. If the generator is used to power a radio, these pulsations may seriously interfere with the music or speech from the radio itself. For this type of application, a way has to be found to make the output as smooth as possible. The most common method of smoothing a pulsating supply, is to connect a capacitor in parallel with it. A capacitor is a device that stores electrical charge and is sometimes thought of as a rechargeable battery with very rapid charge and discharge times. + Generator G 0 The unit of capacitance is the Farad which is a very large value. A microfarad (µF) is one millionth of a Farad. If you place a 1000µF capacitor across a 4.5 V battery, it charges up almost instantly. If it is then connected - say - to an LED, this will light up, but only for a short period. ELECTROMECHANICAL SYSTEMS - VERSION 1 16 ELECTROMECHANICAL SYSTEMS If we need to convert the output from an AC generator to DC, it can be done with diodes which allow current to flow in one direction only. This process is called rectification. The simplest circuit uses just one diode and gives half wave rectification. As the time graph shows, the diode stops current flowing one way and produces pulsations with a gap between each. This output can be smoothed with a capacitor to fill in the gaps. Discharging Smoothed output Voltage + Charging When a capacitor is connected in parallel with the generator, it charges up during each pulsation and discharges to 'fill in' the gaps between pulsations as the diagram shows. A capacitor by itself does not give perfect DC but can turn a pulsating supply into one with only a small “ripple”. - Generator output Time Circuit symbol Diode (In order to produce a perfect DC supply, we need also to add a device called a voltage regulator which senses the ripples and corrects them to give almost perfect DC. Voltage regulators are available in a small compact package and are now quite low in cost.) G + Generator Voltage 0 0 Time A better form of rectification uses four diodes often supplied as single component. This is called full wave rectification. Voltage regulator Diode bridge + G Current 0 0 Time ELECTROMECHANICAL SYSTEMS - VERSION 1 17 ELECTROMECHANICAL SYSTEMS The TEP Mini-Generator The TEP generator was actually designed as a special motor to operate from very small sources of current such as solar cells. It is larger in diameter than other miniature motors because it has a bigger armature and slightly larger permanent magnets. It also has better spindle bearings and brushes that contact the commutator with very little friction. To generate useful current, the armature has to be turned rapidly. For example, to generate the output needed to energise a standard LED, the spindle has to be turned at a minimum speed of 300 revolutions per minute (r.p.m.). You can do this for just a moment by spinning it between finger and thumb. To generate a continuous useful current, the generator spindle has to be rotated at a speed of at least 1500 r.p.m. ELECTROMECHANICAL SYSTEMS - VERSION 1 18 ELECTROMECHANICAL SYSTEMS A Practical Mini-Generator With Gearbox The TEP generator cannot be driven directly by hand (or from several other power sources) because the speed of rotation provided is too low. A way has to be found to increase this speed to at least 1500 r.p.m. Connecting a power source to a generator to get the best performance is called matching. A pulley system can be used but unless stepped belts and pulleys are used, the belts are likely to slip. A step-up gearbox is the most common method employed. Stepped belt A gearbox for a the TEP generator is assembled very quickly using the two-plate method. The generator is supplied with two predrilled side plates and a selection of gears giving three different ratios. This gearbox is assembled by the following steps: • choose the gears to be used and position each of these on a length of 2 mm shafting (If you know the input speed and the required output speed of a gearbox, a suitable combination of gears can be worked out. The easiest method is to multiply input speed by the gear ratio. For example, if a 60 tooth driver gear meshes with a 10 tooth pinion, the gear ratio is 6:1. The driven gear will rotate 6 times for each revolution of the driver. In other words, it will rotate six times faster. If the driven gear is turned by hand at 50 r.p.m, the driven gear will rotate at 6 x 50 r.p.m. = 300 r.p.m. ELECTROMECHANICAL SYSTEMS - VERSION 1 19 ELECTROMECHANICAL SYSTEMS If the outside of the second gear has 50 teeth and meshes with a pinion of 10 teeth, this ratio is 5:1 and the speed of 300 r.p.m. now becomes 5 x 300 r.p.m = 1,500 r.p.m.) • fasten the generator to one side plate using self-tapping screws and the packing washers (the washers are important to prevent the screws touching the generator armature). • attach spacing bolts on the same plate and add enough nuts to provide spacing between the two plates. • place the shafts through the bearing holes and fix the second plate. To test the gearbox and generator, a length of 2 mm shaft can be bent as a crank. Where it passes through the driving gear, it should be flattened slightly or upset to give a form of “spline” to prevent the gear slipping round. ELECTROMECHANICAL SYSTEMS - VERSION 1 20 ELECTROMECHANICAL SYSTEMS The TEP generator gearbox side plates are predrilled to allow several different combinations of gears. If you want to drill holes for a different gear train, use the following method: Example: The measured distance is 32 mm and the shaft diameter 3 mm. Getting gears to mesh properly The most important aspect of making a gear box is making the gears mesh together properly. If they press together too tightly, there is a lot of friction and they may not turn at all. If they are too far apart, the teeth may jump over one another. You need to mark the bearing holes in the side plates that support the shafts very accurately to make sure the teeth mesh with just enough clearance to turn freely. Calculate it using the simple method below: Adding allowance of 1 mm for free running gives 29 mm + 1 mm = 30 mm. 32 mm - 3 mm = 29 mm between centres. The positions of the bearing holes for the shaft are marked out on just one plate since the two plates are clamped together for drilling at the same time. Mark out the position for the first shaft and make a small centre punch dot. (The position of this first shaft is important because the gear mounted on it has to mesh with one on the motor.) 1. Place the two gears on a flat surface with a short length of metal shaft forced into the centre of each. 2. Push the two gears together between finger and thumb and then measure the distance between the two shafts. Open a pair of metalworking compasses to the correct distance between centres that you have worked out for the meshing gear and scribe an arc. 3. Subtract the diameter of one shaft to give the distance between centres. 4. Add a small allowance to the distance to enable the gears to run freely. As a rule of thumb (a rough rule) add 1.0 mm to the distance for large gears and 0.5 mm for small gears. ELECTROMECHANICAL SYSTEMS - VERSION 1 21 ELECTROMECHANICAL SYSTEMS You can make a centre punch dot anywhere along this arc for the second gear shaft. Its position depends on how you have decided to set out the gear train. If a bearing hole for a third shaft is needed, the same procedure is repeated with the compass opened to the correct distance. The centre punch dots can now be made larger before drilling. The two side plates are finally clamped together with toolmaker’s clamps and carefully drilled. It is also a good idea at this stage to drill holes for the spacers. Driving the Mini-DC Generator The generator can be driven directly only if the means of rotation is very fast (e.g. a small CO2 motor used to power model aircraft). If the step-up gearbox is used to increase speed - i.e., matching power source to generator - a variety of power sources are available. These include: • • • • • • coiled spring or elastic falling mass hand turning air propeller or rotor water wheel connection to moving object (e.g. bicycle) Kg kg ELECTROMECHANICAL SYSTEMS - VERSION 1 22 ELECTROMECHANICAL SYSTEMS Making an Input Crank There are a number of methods for making cranks. These include: The first two of these power sources use stored energy and may need some form of speed regulation before attachment to the gearbox input. A spring or elastic band will tend to have high torque (‘turning power’) when fully wound up and less when it is run down. A falling mass will accelerate as it falls, but connection to the generator and gearbox causes drag and the mass - providing it is not too large - will fall at a more uniform rate. • Bending a long shaft. Although the gearbox can be powered through a 2 mm diameter input shaft, it is preferable to use one of larger diameter if possible. This provides more strength and makes it less likely that the first gear will slip on the shaft as it is turned. If a 3 mm diameter shaft is used, for example, the two side plate bearing holes also need to be drilled out to 3 mm. If the gear centre is drilled out to 2.9 mm, this will provide a tight interference fit. Nevertheless, it is an advantage to roughen the shaft so that it locks more tightly onto the gear. One way of doing this is to press the edge of a file against the shaft and slowly roll it back and forth over a smooth surface. ELECTROMECHANICAL SYSTEMS - VERSION 1 • Fitting the end of the shaft and a crank handle to a third piece of material as shown. The joints must be carefully considered. There are many options. A very easy one is to use metal studding for both the shaft and the handle. 23 ELECTROMECHANICAL SYSTEMS Performance of the TEP Mini-DC Generator The technical term for something connected to and driven by a generator is called the load. This could be a light bulb for example, or a radio. The size of the load is measured by the amount of current consumed or drawn and not the physical size. A small torch bulb drawing a current of 0.5 A represents a larger load than a radio drawing 0.3 A even though the radio itself is much bigger. To examine the performance of the TEP generator, a load has to be connected. The current flowing through the circuit and the voltage across it are both measured. For accurate measurements, load resistors are used rather than bulbs etc. A low value resistor represents a greater load than a high value one because more current passes. If the load is increased by reducing the value of the load resistor, the voltage will drop, because of the increased voltage drop in the internal resistance of the generator. A G Generator Load resistor V The load connected to a generator has to be correctly matched to the generator output. A large or heavy load reduces the output voltage and will also make the generator very difficult to turn. Remember that the generator is converting mechanical energy into electrical energy. If you power the generator without a load connected, it will turn easily. If you then connect a load such as a bulb, the generator becomes noticeably harder to turn. This resistance to turning is the work you have to do to produce electrical energy. ELECTROMECHANICAL SYSTEMS - VERSION 1 24 ELECTROMECHANICAL SYSTEMS To give a ‘feel’ for the effect of different loads, try connecting the TEP generator to the following: The TEP generator can be used in the same way because it is also an electric motor. It can also be used simply as a brake for other devices. The generator offers little resistance to turning with no load. However, as soon as you close a switch to connect - say -a load resistor, it offers considerable resistance to turning. When the generator is acting as a brake, the work it does is eventually converted into heat in the load (and in the coils of the generator). LED Small motor Small buzzer Connecting a load acts as a brake on the generator. This effect is used by electric vehicles to save wear on normal brakes and to conserve batteries. An electric vehicle going up hill is driven by a motor supplied from batteries. When it is going down hill, the motor is switched over so that it acts as a generator to recharge the batteries. In so doing, the motor offers resistance to turning and has a braking effect on the vehicle. + Generator G Smoothed output Voltage + Discharging 0 Charging Grain of wheat bulb Improving the Mini-Generator’s Output The pulsating DC output from the TEP generator can be improved by connecting a capacitor in parallel across the output terminals. This should be as large as possible but for most purposes a 2000 µF capacitor will suffice. Remember, though, to connect the capacitor to the generator the correct way round in relation to the polarity of the generator. - Generator output Time You can test for positive and negative by offering an LED to the generator terminals; it will light up only when the cathode leg is connected to the negative terminal. ELECTROMECHANICAL SYSTEMS - VERSION 1 25 ELECTROMECHANICAL SYSTEMS For an even smoother DC output from the generator, you should select a suitable voltage regulator from a supply catalogue and make up the recommended circuit. This usually involves adding just one or two external components; an example is shown. It is important to note that the regulator always needs an input voltage higher than the regulated output voltage. Voltage regulator I/P +7 to 25V 0.22µF O/P 7805 +5V Com 4K7 0.47µF 0V 0V With an AC supply, a transformer can be used to step output voltages either up or down. This is not possible with a DC supply. However, there is now a range of electronic circuits called DC to DC converters that will either increase or decrease the voltage from a DC supply. You should consult a supply catalogue under the heading of “DC to DC converter” to select a suitable device. Input Output DC to DC converter ELECTROMECHANICAL SYSTEMS - VERSION 1 26 ELECTROMECHANICAL SYSTEMS SECTION 3 SHAPE MEMORY ALLOY (OR “SMART WIRE”) ACTUATORS A relatively new type of electromechanical actuator uses shape memory alloy (SMA). Smart Materials Most materials that we use in products have properties which remain more or less constant in use. 'Smart' materials are different; they respond to external factors such as differences in light or temperature levels and change in some way. They are described as 'smart' because they seem to be intelligent or have a mind of their own. Smart materials are now being applied in everyday products. Examples include sunglass lenses (and spectacle lenses) which darken as light intensity increases and stick-on thermometers whose colour changes to indicate temperature. Smart materials are now even used in clothing! Reactolight glasses Stick-on thermometer Shape Memory Alloy (SMA) SMA is a smart material which, as its name suggests, has a memory. The most common SMA is an alloy (mixture of metals) of nickel and titanium - called nitinol. By means of special heat treatment, a piece of SMA can be made to 'remember' a shape. For example, a length of wire can be made to remember that it should be straight at temperatures above 70°C. If you bend this wire at normal room temperature into the shape of a paper clip, it stays bent and will continue acting as a paper clip. However, if you place it in a glass of water whose temperature is above 70°C, it immediately straightens out! When cool, it remains straight until it is bent again. ELECTROMECHANICAL SYSTEMS - VERSION 1 27 ELECTROMECHANICAL SYSTEMS This cycle of bending and then straightening when heated can be continued millions of times. The temperature at which SMA 'remembers' its original form is called the transition temperature and when this point is reached, it changes shape. SMA has a relatively high electrical resistance and can be heated to its transition temperature by passing an electrical current through it. Applications of SMA SMA can be used to give a mechanical movement when a set temperature is reached. For example, current applications include: • seals for hydraulic tubing (which shrink into position) • electrical connectors • fire alarm systems - to trigger a sprinkler • waste bins - to trigger a falling lid if fire occurs • coffee machines - to open a valve so that hot water falls on the coffee • air conditioning units - to move louvres or flaps to direct air movement • shower units - to control hot water control valves The advantage of SMA in these and many other applications is the fact that it provides large forces and movement at a precise temperature. It is also possible to pre-shape the SMA in different ways - for example as a spring or a flat plate. ELECTROMECHANICAL SYSTEMS - VERSION 1 28 ELECTROMECHANICAL SYSTEMS Smart wire has a relatively high electrical resistance and can be heated to its transition temperature by passing an electrical current through it. Before SMA was available, bi-metallic strips were really the only simple way of causing mechanical movement by change of temperature. A bi-metallic strip consists of two metal ribbons bonded together. One metal has a high rate of expansion when heated; the other has a low rate of expansion. When the strip is heated, it curls because one side expands more rapidly than the other. Bi-metallic strip Bi-metallic strips are commonly used to control thermostats in central heating systems and electric kettles. Unlike SMA, bi-metallic strips change shape gradually when heated - not all at once. Also, in practice, they cannot be made to change shape when current is passed through them. Smart Wire A common form of SMA is wire available in different diameters. This ranges, for example, from 5 mm diameter down to 50 microns (1 micron = 1/1000 millimetre). The SMA wire sample provided with this book is Nitol with a diameter of 100 microns. It is heat treated to 'remember' that it has a shorter length when heated above its transition temperature (70°-80°C) than below it. If the sample length of wire is held between two points it has a length of approximately 10 cm. When heated to between 70° and 80°C, it shortens by about 5% or 1/20 and exerts a useful pulling force. (The wire becomes shorter and it gets slightly fatter.) When the wire cools down, it relaxes to its longer length of 10 cm. ELECTROMECHANICAL SYSTEMS - VERSION 1 29 ELECTROMECHANICAL SYSTEMS Shortened Relaxed 10 20 The 5% change in length is constant for any length or diameter of SMA wire. This results in quite small movements for shorter lengths of wire. However, the movement can be increased by increasing the length of wire. To work out the amount of movement for a given piece of wire, you simply multiply its length by 5%. For example, for a wire 150 mm in length, the shortening is: 150/1 × 1/20 = 150/20 = 7.5 mm The 5% shortening of SMA can also be turned into a much larger movement using simple lever systems. ELECTROMECHANICAL SYSTEMS - VERSION 1 30 ELECTROMECHANICAL SYSTEMS SMA wire has to be stretched or biased to return to its longer length. The force required to do this is much smaller than the pulling force that the wire exerts when it shortens. There are two main ways of biasing: • Using a weight • Using a spring Using a weight Using a spring Because SMA has a relatively high electrical resistance, it can be heated to its transition temperature simply by passing current through it. This opens up many possibilities for providing mechanical actuation (movement) without any moving parts other than those the SMA is attached to! Also, for smaller diameter wires, the currents needed are quite small and can be provided from smaller batteries. +V Shape memory alloy wire symbol used for the purposes of this book. -V In a practical design using SMA wire, you need to know what force to use to bias it, and what force it will exert when it shortens. If you are heating it with electric current, you also need to know how much current to pass without overheating and damaging it. ELECTROMECHANICAL SYSTEMS - VERSION 1 31 ELECTROMECHANICAL SYSTEMS All these figures (for 100 micron wire) are provided in the table below: Bias force Pulling force 0.3 N 1.5 N Resistance Max. current Max. power 150 ohms per metre 180 milliamps 5 Watts per metre Shortening time Relaxation time 0.1 second 1.0 second Recommended extension Minimum bend radius 5% 5 mm Effective transition temperature 70°Centigrade Pulling starts at Pulling finishes at Relaxation starts at Relaxation finishes at 68°C. 78°C. 52°C. 42°C. Explanation of the Table The table tells us that at normal room temperature the wire needs to be stretched with a bias force of 0.3 newtons - which is roughly equivalent to hanging a weight of approximately 30 grams on the end. When heated to the transition temperature of between 70° to 80°C, the wire shortens about 5% in length and will exert a pulling force of 1.5 newtons - roughly equivalent to lifting a weight of 150 grams. The speed at which the wire shortens when it reaches the transition temperature is about 0.1 seconds. It takes longer to relax or stretch back to its longer length - about 1 second. The table also tells us that when heated, the wire actually starts changing length at 68°C and finishes at 78°C. When it cools, however, the stretching or relaxation does not take place until it has reached 52°C. The figures given in the table are the recommended ones for 100 micron nitol; if they are exceeded, the useful life of the wire will be reduced. ELECTROMECHANICAL SYSTEMS - VERSION 1 32 ELECTROMECHANICAL SYSTEMS The supply needed to heat the wire can be determined using Ohm’s Law. This states the relationship between voltage (V), current (I) and resistance (R), as follows: V=I×R I = V/R R = V/I The table gives us the resistance of the wire and also states the maximum current. Using Ohm’s Law, we can therefore work out the voltage needed. For example, what is the voltage needed to pass the maximum safe current through the 10 cm length of 100 micron sample wire provided with this book? Step 1 The resistance of the wire is 150Ω per metre. Divide by 100 = 1.5Ω per cm. The resistance of 10 cm of wire = 1.5Ω × 10 = 15Ω. Step 2 The maximum current is 180 mA or 0.18 A. (1 milliamp = 1/1000 Amp.) Step 3 V = I × R. Substituting the figures above gives: V = 0.18 A. × 15Ω = 2.7 volts. A 3 volt battery (two AA cells in series) can be used to power this length of wire because as current is drawn, its voltage will reduce slightly. To check that the power rating (the rate of doing work) is not exceeded, we can use the power equation W = I × V. If we substitute the above figures W = 0.18 × 2.7 = 0.49 Watts for a 10 cm length of wire and 10 × 0.49 = 4.9 for a metre length. This is the maximum figure given in the table. What voltage would be needed to supply a 15 cm length of 100 micron SMA wire? How many times per minute could a length of 100 micron SMA wire go through a complete shortening and relaxation cycle? ELECTROMECHANICAL SYSTEMS - VERSION 1 33 ELECTROMECHANICAL SYSTEMS Using SMA Wire It is important to make good electrical and mechanical connections to the ends of SMA wire. The wire cannot be soldered and must be joined to other conductors by mechanical means. It is also important to remember that where the wire is in contact with a metal component or surface, some heat will be conducted away and that the whole length of wire may not exhibit the memory effect. The response times given in the table are for SMA in a normal room environment. If the wire is enclosed in an insulated sleeve, for example, it will take longer to cool down and relax to its longer length. If air is blown over it, it will cool more rapidly. Insulated sleeve Wire Increasing Pulling Force The pulling force of SMA wire cannot be increased by supplying current beyond the recommended limit; this will damage it. However, two or more wires can be run in parallel. Two wires will give double the pulling force and so on. You must remember, though, that if the wires are connected in parallel, you also double the current needed to heat them up. ELECTROMECHANICAL SYSTEMS - VERSION 1 34 ELECTROMECHANICAL SYSTEMS Power Supplies Current supplied to the SMA wire must be within the recommended limit to avoid any damage. There are several ways of doing this including: HITACHI LONG LIFE HP7 HITACHI LONG LIFE HP7 • use of an appropriate number of 1.5 V batteries connected in series. I CH A T LIFE HI LONG HP7 • use of an adjustable power supply unit (PSU). • use of a series resistor to regulate the supply. It may not be possible to 'fine tune' a number of batteries accurately enough or you may have an unsuitable supply. In either case, current can be regulated by using a series resistor in the circuit. Ohm’s law can be used to work out the value of this resistor. [Note: the resistor should be a higher wattage type. The power in the circuit can be worked out using W (watts) = I (current) × V (volts). If a variable resistor is used, it should be a wire-wound higher wattage type.] ELECTROMECHANICAL SYSTEMS - VERSION 1 +V High wattage resistor SMA -V 35 ELECTROMECHANICAL SYSTEMS LM317T • use of a voltage regulator Case also Vout Vin Vout Adj Control Circuits 1. Open loop control In open loop control, there is no feedback. The supply is simply switched on or off - for example, using a press switch or a timer circuit. Switches that can be used include: reed switches operated by a magnet, micro switches, membrane panels. Reed switch and magnet Micro switch Membrane panel ELECTROMECHANICAL SYSTEMS - VERSION 1 36 ELECTROMECHANICAL SYSTEMS Supply current can be 'switched' by a thyristor, bipolar transistor or FET (field effect transistor). The example circuits show how sensors can control the supply switching. +V +V Touch pads SMA SMA 106 Piezo transducer IR530 100 K 1mΩ 0V 0V Thyristor triggered by shock FET switched on by placing finger across touch pads +V SMA Transistor switched on by water bridging across probes 2 kΩ BFY51 Probes 0V Bipolar transistors and FETs can also be used as the output stage of microelectronic control circuits - e.g. a 555 timer. Set input Time period input Process timer Output FET +V SMA 4.7 kΩ VR1 8 4 6 7 2 Set 555 680 Ω IR 530 3 1000 µF 0V ELECTROMECHANICAL SYSTEMS - VERSION 1 37 ELECTROMECHANICAL SYSTEMS 2. Closed loop control Closed loop control involves something feeding back (feedback) from the output to the input of a system. A central heating system turns on and off at a temperature set by a thermostat. A bi-metallic strip in the thermostat heats up and moves to switch off the heating boiler when an appropriate temperature has been reached. Contacts Bimetallic strip Temperature setting Thermostat Because SMA wire changes length when it is heated, the movement can be used as feedback - for example, to switch the supply on and off. A very simple example involves connecting a length of SMA to a microswitch. When the wire is relaxed the switch is 'on' and current flows through the wire. The wire then shortens, depresses the switch contact and turns off the supply. The wire then relaxes and the whole cycle begins again. Micro switch SMA Power supply ELECTROMECHANICAL SYSTEMS - VERSION 1 38 ELECTROMECHANICAL SYSTEMS [Note: Ingenious heat engines have been built from SMA materials using a closed loop system. In one example, a wire relaxes and dips into hot water. This causes it to change shape and move out of the water to cool down and relax again. The same cycle repeats over and over again and turns a small flywheel.] Experiments With SMA Wire • Lifting weights This experiment simply involves attaching a length of SMA wire to a weight (e.g. ball bearings in a bag) and observing the contraction when the wire is heated by current. The bias force is automatically supplied by the weight. Bearing balls • Amplifying movement with levers A simple two dimensional lever system can be assembled on a baseboard using polystyrene or card strip for the lever and a drawing pin pivot. The 'load' on the lever can be supplied by weights or a spring (e.g. elastic band). ELECTROMECHANICAL SYSTEMS - VERSION 1 39 ELECTROMECHANICAL SYSTEMS The distances from the pivot to (a) the wire attachment and (b) the weights can be expressed as a ratio. In the example shown the ratio is 5:1. For every millimetre moved by the wire end the weighted end will move through 5 millimetres. 10 cm 2 cm Ball bearings • Amplifying movement using geometry A weight is attached to the centre of a length of SMA wire so that it forms two sides of an inverted triangle. Over a range of angles the vertical movement of the weight will be greater than the linear movement of the wire. This effect increases as the angle at x increases (i.e. as the wire becomes closer to horizontal). However, the forces required also increase. Try experimenting with SMA wire at an angle at x of 140° and plot the movements of the weight on a piece of paper. x ELECTROMECHANICAL SYSTEMS - VERSION 1 40 ELECTROMECHANICAL SYSTEMS Practical Applications of SMA Wire Linear actuation SMA wire is most easily used to provide linear or straight line movement. The example shown uses SMA wire to pull a bolt in a simple lock. In this application very little linear movement is needed. If its length can be accommodated, SMA wire can often be used in place of a more expensive solenoid. A free-standing actuator can be made by containing the wire in a plastic tube. Door bolt SMA wire used in an electric door lock Compression spring keeps SMA wire stretched and bolt in 'locked' position • Angular actuation In many practical applications of SMA wire, a mechanical system is used to amplify movement. The barrier prototype model illustrated uses the lever principle to move and lift up the arm. The same principle can be used to provide the movements of a robot arm. SMA wire ELECTROMECHANICAL SYSTEMS - VERSION 1 41 ELECTROMECHANICAL SYSTEMS Rotary actuation SMA wire (or a cord extension from it) can be wound around a shaft, drum, pulley, or cam to produce rotary movement. For a given length of wire, the larger the diameter of the shaft etc., the smaller the rotation - and vice versa. If the shaft etc., is very small and expected to rotate through several revolutions, particular attention has to be given to biasing - either with a weight or a spring. Rotating shaft Anthropomorphic actuation 'Anthropomorphic' describes something which has human characteristics. A lot of robotics research is currently directed at making robotic movements - especially hand movements imitate human ones. This is because of their potential as artificial arms and limbs for disabled people and as precision manipulators for industrial robots. Many of these experimental devices use SMA wire to provide mechanical movement. Leads It is surprisingly easy to make an actuator that imitates - say - a finger movement. One method is to stretch the SMA wire inside a 'springy' plastic tube. This will cause the tube to curl slightly. When the SMA wire is heated by current, it exerts a stronger pulling force inside the tube and this causes it to curl around even further - closing the 'finger'. When the wire relaxes, the 'finger' opens again. ELECTROMECHANICAL SYSTEMS - VERSION 1 SMA wire inside 'finger' segments 42 ELECTROMECHANICAL SYSTEMS In many commercial prototype hands the robotic fingers are made up from hinged segments with small springs to keep them straight. When SMA wire running through the segments contracts, the 'finger' curls just like the tube. SMA wire fixed at end Construction Notes The most difficult aspect of using small diameter SMA wire is holding it securely and making good electrical contact. These are some of the methods employed: • Wire Crimps. These are small fastenings pressed flat around wires to be joined; they are available commercially in many different shapes and sizes. The most useful ones for SMA wire are miniature tubes which are closed with special crimping pliers or ordinary pliers. The crimps can be placed at the very end of an SMA wire or somewhere along its length. Note: The most common miniature crimps available are 'bootlace' types - a small tube but with one end closed. These will work for all the applications shown in this book. • Edge and corner crimping. This is a technique for making wire crimps on the corner or edge of a metal tab - e.g. copper. An edge or corner is folded over using a pair of pliers. Because the metal at the bend hardens as it deforms it does not close over completely and leaves a small opening. The wires can be inserted in this opening which is then finally closed by ‘nipping’ with the pliers. One or more holes punched in the tab can be used to fasten it. ELECTROMECHANICAL SYSTEMS - VERSION 1 43 ELECTROMECHANICAL SYSTEMS • Screw, nut and washers. SMA wire and connecting wire can be fastened to a small screw using two washers and a nut. The free end of the screw can also be used to provide a mechanical anchorage to something else. • Terminal block. Commercial terminal blocks contain twinscrew brass fastenings in a polythene strip. Individual fastenings can be removed from the plastic strip as necessary. Note: It is an advantage to attach a crimp to the SMA wire before securing it in the terminal block. SMA Terminal block connector Further Reading Bowyer, M.J. Design and Applications of Ni-Ti Shape Memory Alloy Springs, Engineering Design, November 1988. Gilbertson, R.G. Muscle Wires, Mondo-tronics, 1992. Cave, J.F. (Ed.) TEP Electronics 14-16, The Engineering Council, 1994 ELECTROMECHANICAL SYSTEMS - VERSION 1 44 ELECTROMECHANICAL SYSTEMS SECTION 4 LINEAR ACTUATORS A linear actuator is a motorised unit which often resembles a hydraulic or pneumatic cylinder. It contains a motor, gearbox and a means of converting the rotary output from the gearbox into a powerful push-pull linear movement. This movement is normally obtained by a nut moving along a rotating screw thread - the same means used to move the carriage on a manual lathe. Most larger commercial linear actuators use a ball screw. This works on the same principle as a basic nut and screw but the nut is separated from the screw by ball bearings to minimise friction. Linear actuators are normally used to provide intermittent rather than continuous push-pull movements. They are self-contained units, and very easy to build into systems such as window opening mechanisms. However, because the motor is totally enclosed, they have a limited duty cycle. This means that they can be energised for only a certain percentage of the time. For example, an actuator with a duty cycle of 50% means that it should only be running for only - say - 2 minutes within a 4 minute period. Manufacturers state the precise duty cycle conditions in their literature. TEP linear actuator The TEP linear actuator is an open-frame type that comes almost completely assembled. It uses a 5mm diameter screw driven directly by a miniature DC motor. The screw engages a brass nut set into a plastic block which also accommodates a push rod. The end of the screw is supported in a nylon bearing at one end of the frame and above this an identical bearing providing support for the push rod. ELECTROMECHANICAL SYSTEMS - VERSION 1 45 ELECTROMECHANICAL SYSTEMS If the motor is connected to a 3v - 6v battery supply, the nut will run rapidly to one end of the frame. Reversing the motor supply will cause it to run in the opposite direction. If you do this simple experiment, however, you will find that at the end of its travel, the nut will lock onto the screw and simply reversing the motor will not be enough to free it. To prevent the nut reaching the extremity of the thread and to provide proper control, it is necessary to add two limit switches to the frame. These switch off the motor when the nut is almost at the end of its travel. They also enable manual or automatic reversing of the nut. Setting up the limit switches The actuator is supplied with two limit switches and small selftapping screws for fixing. Two leads should be soldered to each switch as shown, and the switches fastened to the frame. The lever of each switch should be bent outwards so that the supply is switched off well before the end of the nut’s travel. This needs to be done because the motor continues to spin after the supply is switched off, and the nut travelling beyond its limit will jam. ELECTROMECHANICAL SYSTEMS - VERSION 1 46 ELECTROMECHANICAL SYSTEMS As a guide, use only a 3 volt supply either to trial the actuator or run it with a light load. With a heavier load, you can use a 4.5v 6v supply. For manual operation of the actuator, the limit switches are connected to a DPDT (double pole, double throw) switch as shown. When the slide switch, provided with the actuator, is in the centre position, it is ‘off’. In either of the other two positions it supplies current to the motor until one of the limit switches breaks the circuit. The slide switch can then be thrown to the other ‘on’ position to reverse the nut. Manual switching might be used, for example, to cause the actuator to throw a lock bolt. + M Slide switch 3 - 6V — L1 L2 L1 and L2 are the limit switches. Use connections marked 'con' and 'NC" The actuator can be controlled electronically by using an appropriate circuit and a DPDT relay (or two SPST relays). For example, a “Bit by bit” controller can be programmed to switch a pair of SPST relays on and off. There are many variations on the control theme. For example, a sensor might be used so that the actuator opens: • a vent when a set temperature is reached • a vent above a set light level • a valve when water (or moisture) falls below a fixed level ELECTROMECHANICAL SYSTEMS - VERSION 1 47 ELECTROMECHANICAL SYSTEMS A very simple example circuit is given. When the water sensor is wet, the relay is energised and the actuator's push rod is ‘parked’ in the withdrawn position. If the water level drops, the relay switches and the rod moves to its extended position and parks there until the water level rises again. Relay coil L1 + L2 6V 1K — BFY 51 M Probes Shunt braking It is possible to stop the travel of the nut very rapidly anywhere along the screw by shunt braking. This involves using one or more relays to short circuit or shunt across the motor terminals immediately the current supply has been interrupted. When shunted, the motor (with its spinning armature) is trying to act as both generator and motor. This has the effect of stopping it almost immediately. An example circuit is shown. + TEP Generator/ motor Note: the screw and the base of the actuator frame should be lubricated with light oil. The 3mm diameter push rod is an interference fit in the plastic nut and can be withdrawn providing the nut is supported. A longer or specially shaped push rod, for example, can be substituted. ELECTROMECHANICAL SYSTEMS - VERSION 1 48 ELECTROMECHANICAL SYSTEMS STUDY FILE 1 - “BIT BY BIT” CONTROL All of the electromechanical actuators described can be controlled with the TEP “Bit by Bit” controller using either transistor or relay outputs. SWITCH + PROG ON RUN – C2 6v IN IMPORTANT TECHNICAL NOTE: Under some circumstances the bit by bit controller can be affected by electrical noise, e.g. from electric motors. This is discussed on page 12. ON RUN REMOTE Introduction The TEP bit by bit controller is a self-contained electronic controller capable of switching on or off up to 8 different outputs over a period of time. It is programmed by setting each of eight small DIP switches to either 'on' or 'off' and then committing these instructions to memory by pressing a push switch. The memory can hold up to 64 such lines of program. The total program can then be run at different speeds to control a variety of devices such as lamps, buzzers and electric motors. C3 1 2 PROG OFF ON .2 – The noise immunity of the controller can be improved by adding an optional capacitor at position C3. This should be approximately 0.1µF. STOP RUN SPEED 1 5 20 .5 2 10 DIP 1 2 3 4 5 6 7 8 MEMORY SPEED ADJUST RP1 .1 + ON OFF 1 2 3 4 5 6 7 8 PROGRAM DATA PAUSE INPUT RP2 RESET INPUT TEP BIT-BY-BIT CONTROLLER – C2 N GROUND LED 1-8 (c) 1994 TEP C3 Setting Up The controller is supplied complete to run and program; output components are added if and as required. The controller requires either a 6V battery power supply or a supply from a PSU (power supply unit) which is regulated. THE MAXIMUM SUPPLY VOLTAGE IS 6 VOLTS. IT SHOULD BE NOTED THAT A BATTERY SNAP CAN INADVERTENTLY BE CONNECTED TO A 9V SOURCE. A 4 × ΑΑ battery box and battery connecting snap is supplied with the board. If used, the snap should be soldered to the points marked + and - at the top of the board, if necessary using the two spare holes as mechanical anchorage for the two leads. Any program will be lost if the battery is disconnected for more than 20 seconds. Because the standby current consumption of the board is so small, it is preferable to leave the battery connected all the time. ELECTROMECHANICAL SYSTEMS - VERSION 1 49 + SPEED ADJUST – ELECTROMECHANICAL SYSTEMS Basic Principles The TEP controller uses a single IC (integrated circuit) containing a memory where information can be stored in electronic form. It is useful to think of this memory as a book having a stack of pages. Every page represents a line of control programming and has 8 blank spaces - each one waiting to be filled with a bit of information. Each vertical column of blanks will contain the remembered instructions for a control output. Each control output is connected to an LED 'flag'. ON DIP 1 2 3 4 5 6 7 8 Page (line of program) 1 2 3 4 64 LED flags (Outputs) 1 2 3 4 5 6 7 8 The memory locations are filled with individual bits of information - of which there are only two types: logic 1 or logic 0. In the controller’s memory these are really instructions which mean either turn ON an output (logic 1) or turn OFF an output (logic 0). ON DIP 1 2 3 4 5 6 7 8 Page (line of program) 1 on off off on off on off off 2 3 4 64 LED flags (Outputs) 1 2 3 4 5 6 7 8 ELECTROMECHANICAL SYSTEMS - VERSION 1 50 ELECTROMECHANICAL SYSTEMS The information is written on each 'page' of memory by setting the 8 DIP switches to either 'ON' or 'OFF' and then pressing the 'MEMORY' press-button switch . Pressing this button 'writes' the PROGRAM DATA switch settings into memory and automatically turns over to the next 'page'. This procedure can be repeated up to 64 times - the maximum number of pages or locations in the controller’s memory. Outputs 1 on on 2 off on 3 on off 4 off off 5 on on 6 off on 1 2 3 4 5 6 7 8 The illustration shows a sample 6-line program for the two left hand outputs. When the controller is instructed to read this program, it turns over the 'pages' at a set speed. An 'ON' bit of information lights up an output LED and an 'OFF' bit turns it off. If, for example, the controller is set to read each page for a second at a time, the LED on the far left hand side will turn on for one second off for the next and so on. LED number 2 will turn on for two seconds and then stay off for two seconds. When the program has been run - i.e., all the 'pages' turned over - the controller will automatically start again at the first line of the program. Unless it is stopped, the program will run over and over again. Please note: the remainder of this text will refer to lines of program and not pages. ELECTROMECHANICAL SYSTEMS - VERSION 1 51 ELECTROMECHANICAL SYSTEMS SWITCH + PROG ON RUN – C2 6v IN ON RUN REMOTE Programming the Bit by Bit Controller Using the whole-board diagram as a guide, you should now be able follow these instructions for programming the controller. C3 1 2 PROG OFF SPEED ADJUST – STOP RUN SPEED 1 5 20 .5 2 10 ON DIP RP1 .2 .1 + ON OFF 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 PROGRAM DATA MEMORY TEP PAUSE INPUT RP2 BIT-BY-BIT CONTROLLER RESET INPUT GROUND LED 1-8 1 2 3 4 5 6 PROGRAMMING/OUTPUT INDICATORS +6v 7 8 GND LOGIC OUTPUTS TR1 TR2 1 2 TR3 TR4 TR5 +6v TR6 3 4 5 6 GND OPEN COLLECTOR TRANSISTOR OUTPUTS - 800 mA MAX R15 +6v R14 +6v R13 +6v R12 +6v R11 +6v R10 R9 R8 CUT LINE +6v TR7 TR8 7 8 CUT LINE D7 + RL 1 C4 C5 COM NO NC RELAY OUTPUT 1 D8 + D9 + D10 RL 2 RL 3 RL 4 C6 C7 C8 C9 C10 C11 COM NO NC RELAY OUTPUT 2 COM NO NC RELAY OUTPUT 3 + COM NO NC RELAY OUTPUT 4 1. Make sure the RUN and PROGRAM switches at the top of the board are set at the 'PROG OFF' and 'STOP' positions. 2. Connect the battery or power supply. 3. Set the program switch to 'PROG ON'. 4. Write a line of program by setting each 'PROGRAM DATA' switch to either 'ON' or 'OFF'. This will turn the LED outputs on of off. Because the program switches are small, it is more convenient to operate them with a stylus e.g., the tip of an empty pen. 5. Press the 'MEMORY' switch to write this line of program into memory. When you do so, all the LEDs will flash on briefly to confirm this has happened. 6. Repeat steps 4 and 5 above up to 64 times - once for each line of memory. If you try to go beyond 64 lines of programming, the extreme left hand LED will flash continuously. There is no problem if you write a program less than 64 lines. When the program is run, it will loop back to the beginning after the final line. ELECTROMECHANICAL SYSTEMS - VERSION 1 52 ELECTROMECHANICAL SYSTEMS Running the Program 1. Switch the programming switch to 'PROG OFF'. 2. Set all the 'PROGRAM DATA' switches to the 'OFF' position. 3. The 'PROGRAM DATA' switches will now control the program run speed. As an example, set the fourth switch from the left to 'ON'. 4. Set the program run switch to 'RUN'. The program will now run at approximately 1 line per second. (Setting one of the other 'PROGRAM DATA' switches will run the program at a different speed - see below.) The LEDs will turn on and off according to the stored program in memory. SWITCH + PROG ON RUN – C2 6v IN ON RUN REMOTE The speed of execution of the program depends on which 'PROGRAM DATA' switch is set to the 'ON' position and also on the setting of the 'SPEED ADJUST' resistor at the top of the board. The 'PROGRAM DATA' switches provide speed adjustment in fixed steps or ratios. The 'SPEED ADJUST' resistor provides overall continuous adjustment - faster or slower. To calibrate the controller to run at the speeds printed above the 'PROGRAM DATA' switches, create a simple program that turns LED 4 on for one program line, off for the next - and so on (keeping all the other LEDs off). Run this program, and time the result against a watch - altering the 'SPEED ADJUST' so that eventually the LED turns on and off at one second intervals. C3 PROG OFF .1 ON .2 – STOP Overall speed adjustment RUN SPEED 1 5 20 .5 2 10 DIP 1 2 3 4 5 6 7 8 MEMORY SPEED ADJUST RP1 Speed in seconds + 1 2 ON OFF 1 2 3 4 5 6 7 8 PROGRAM DATA PAUSE INPUT RP2 RESET INPUT TEP BIT-BY-BIT If setting switch 4 provides a run speed of one program line per second, the switch on the far right will give program steps of 20 seconds duration. This adds up to a maximum 64 line program run time of 20 seconds × 64 lines = 1,280 seconds OR approximately 21 minutes. This run time can be extended further by adjusting the resistor. Remember, though, this also affects timings provided by the other 'PROGRAM DATA' switches. Important note: The TEP controller has a volatile memory. This means that a program is lost when the power supply is disconnected although the larger capacitor at the top centre of the board will keep it energised for about 20 seconds. However, the Standby Current Consumption of the controller's chip is so low it can be left connected for most practical purposes. ELECTROMECHANICAL SYSTEMS - VERSION 1 53 Technical note: capacitor C2, together with the two resistors at the top right hand corner of the board, controls the chip's clock speed. This is 390 pF. If it is replaced with a lower value (no lower than 50 pF) the top run speed can be considerably increased. However, it will also have the effect of flashing the LEDs more rapidly when the memory button is pressed and the standby current consumption will increase slightly. ELECTROMECHANICAL SYSTEMS Using the Controller’s Outputs The bit by bit controller has 8 LED flags to show the status of each output. This enables you to create programs and run them but not to actually control anything! To switch a load such as a motor on or off a buffer stage has to be added to each output in use. For convenience, the controller board has additional printed tracks and locations for transistor buffers on all the outputs and transistor-plus-relay buffers on four of them. (Note: the board has only enough room physically for relays on the four left hand outputs.) LED 1-8 1 2 3 4 5 6 PROGRAMMING/OUTPUT INDICATORS +6v 7 Logic output stage 8 GND LOGIC OUTPUTS TR1 TR2 1 TR3 TR4 TR5 +6v TR6 +6v TR7 GND 3 4 5 6 OPEN COLLECTOR TRANSISTOR OUTPUTS - 800 mA MAX 2 R15 +6v R14 +6v R13 +6v R12 +6v R11 +6v R10 R9 R8 CUT LINE TR8 7 Transistor output stage 8 CUT LINE D7 + RL 1 C4 C5 COM NO NC RELAY OUTPUT 1 D8 + D9 + D10 RL 2 RL 3 RL 4 C6 C7 C8 C9 C10 C11 COM NO NC RELAY OUTPUT 2 COM NO NC RELAY OUTPUT 3 + Relay output stage COM NO NC RELAY OUTPUT 4 Using the Transistor Outputs The recommended output transistor for which the board has been designed is the inexpensive BCX38B. This is a Darlington pair device and will switch on a load of nearly 1 amp (800 milliamps maximum). This is quite sufficient for most filament bulbs, buzzers and a solar motor. ELECTROMECHANICAL SYSTEMS - VERSION 1 OR C B E C BC X38B 54 B E ELECTROMECHANICAL SYSTEMS To add a transistor to any required output, fix and solder in position a 10 K resistor and BCX38B transistor - for example, at the positions marked R8 and TR1. Make sure the transistor is the correct way around by matching the case outline with the outline on the board. Flying leads to the load are soldered to the +6 V point and open collector output for each transistor. The diagram shows a lightbulb connected to output 1. A circuit diagram for this output is also shown. 1 2 3 4 5 6 PROGRAMMING/OUTPUT INDICATORS +6v 7 8 GND LOGIC OUTPUTS TR1 TR2 1 2 TR3 TR4 TR5 +6v TR6 3 4 5 6 GND OPEN COLLECTOR TRANSISTOR OUTPUTS - 800 mA MAX R15 +6v R14 +6v R13 +6v R12 +6v R11 +6v R10 R9 R8 CUT LINE +6v TR7 TR8 7 8 CUT LINE D7 + D8 RL 1 C4 C5 COM NO NC RELAY OUTPUT 1 + D9 + D10 RL 2 RL 3 RL 4 C6 C7 C8 C9 C10 C11 COM NO NC RELAY OUTPUT 2 COM NO NC RELAY OUTPUT 3 + COM NO NC RELAY OUTPUT 4 +6 V 6 Volt lamp 10K To chip BCX38B 0V REMEMBER that when several transistors are used, the total load current - which can be quite high if all outputs are used - comes from the battery powering the controller. This could be depleted very quickly. Always work out the total load current (or an average for outputs switching on and off) and think carefully about the type of battery needed. It is possible, for example, to run the following devices directly from the transistor outputs: filament bulb miniature solenoid buzzer stepper motor solar motor Any motors other than the more expensive solar motor should be run from a relay. This is because they produce a high degree of electrical noise which may interfere with the operation of the chip. ELECTROMECHANICAL SYSTEMS - VERSION 1 55 ELECTROMECHANICAL SYSTEMS A motor, solenoid or any other device with a coil is an inductive load and can produce a high voltage momentarily when switched off (back EMF). Solar motor Miniature solenoid To prevent this damaging the transistor, a clamping diode should be added as shown in the diagram. This can be a general purpose type such as IN4001. +6 V Clamping diode Motor (inductive load) M 10K 0V The most convenient way of connecting a clamping diode to an inductive load is to use one of the first four left hand outputs and simply solder in a diode as if you were using a relay. It is IMPORTANT to ensure that the diode is soldered in the correct way round - with the marked end facing towards the right. 2 3 4 5 6 PROGRAMMING/OUTPUT INDICATORS +6v 7 8 GND LOGIC OUTPUTS TR1 TR2 1 2 TR3 TR4 TR5 +6v TR6 3 4 5 6 GND OPEN COLLECTOR TRANSISTOR OUTPUTS - 800 mA MAX R15 +6v R14 +6v R13 +6v R12 R11 +6v R10 R9 R8 CUT LINE +6v +6v TR7 TR8 7 8 CUT LINE D7 + D8 RL 1 C4 C5 COM NO NC RELAY OUTPUT 1 IMPORTANT To avoid electrical interference any small electric motor must be suppressed using two capacitors as shown - 0.22µF ceramic, 10µF electrolytic. It is important to ensure that the electrolytic capacitor is correctly connected to the power source. The side marked should be connected to –ve. To chip 1 MM28 + + D10 RL 2 RL 3 RL 4 C6 C7 C8 C9 C10 C11 COM NO NC RELAY OUTPUT 2 ELECTROMECHANICAL SYSTEMS - VERSION 1 D9 COM NO NC RELAY OUTPUT 3 + COM NO NC RELAY OUTPUT 4 56 ELECTROMECHANICAL SYSTEMS R Using a transistor output with an external power supply To avoid draining the battery powering the board itself, a load can be connected to a separate power supply, ideally a battery, up to 24V. The diagram shows a lightbulb thus connected to output 1. A circuit diagram for this output is also shown. OFF 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 PROGRAM DATA MEMORY TEP PAUSE INPUT RP2 BIT-BY-BIT CONTROLLER RESET INPUT GROUND LED 1-8 – ve 1 2 3 4 5 6 PROGRAMMING/OUTPUT INDICATORS +6v 7 8 GND LOGIC OUTPUTS + ve TR1 TR2 1 TR3 TR4 TR5 +6v TR6 GND 3 4 5 6 OPEN COLLECTOR TRANSISTOR OUTPUTS - 800 mA MAX 2 R15 +6v R14 +6v R13 +6v R12 +6v R11 +6v R10 R9 R8 CUT LINE +6v TR7 TR8 7 8 CUT LINE D7 + D8 + D9 + D10 + Max 24V 10K To chip 0V Remember that if a separate battery is used, the total load current should not exceed 800 mA otherwise the transistor will be damaged. Please remember that if an inductive load, such as a motor, is connected, a clamping diode should be added as shown in the diagram below. LOAD Max 24V 10K To chip BCX38B 0V The easiest way to add this might be to connect it directly across the load itself; i.e. across the connecting legs of a motor in parallel with the suppression capacitors. ELECTROMECHANICAL SYSTEMS - VERSION 1 57 BCX38B ELECTROMECHANICAL SYSTEMS Using a Relay Output The first four left hand outputs are extended at the bottom of the PCB to accommodate relays. To add a relay to any of these outputs, first fix and solder in position a 10 K resistor and BCX38B transistor - for example R8 and TR1. Then fix and solder in position a miniature SPDT relay (e.g. Kam Ling KS1P) together with a clamping diode. Also, solder in a 0.22µF suppression capacitor across 'com' and 'no' (C5). This is essential. To test the relay, write in a simple on/off program for this output. When the program is run, the relay will simply click on and off. 1 2 3 4 5 6 PROGRAMMING/OUTPUT INDICATORS +6v 7 8 GND LOGIC OUTPUTS TR1 TR2 1 TR3 TR4 TR5 +6v TR6 + D8 RL 1 CUT LINE D9 +6v TR7 TR8 7 8 + D10 RL 2 RL 3 RL 4 C6 C7 C8 C9 C10 C11 C4 C5 COM NO NC RELAY OUTPUT 1 + COM NO NC RELAY OUTPUT 2 R15 +6v R14 R13 +6v GND 3 4 5 6 OPEN COLLECTOR TRANSISTOR OUTPUTS - 800 mA MAX 2 D7 +6v R12 +6v R11 +6v R10 R9 R8 CUT LINE COM NO NC RELAY OUTPUT 3 Technical note: a very significant problem in designing industrial control equipment is the suppression of 'electrical noise' or electromagnetic interference. Electromechanical devices invariably produce interference. (Remember that early radio transmitters used an electrical arc to produce radio frequency energy.) The suppression capacitor on the relay is very important to prevent any interference to the chip. European Countries have a convention - EMC which sets a standard for protection from electromagnetic interference. + COM NO NC RELAY OUTPUT 4 0.22µF capacitor The load is connected to the relay switch outputs at the bottom of the controller board. 'COM' is the pole of the switch, 'NO' is the normally open contact and 'NC' is the normally closed contact. The load leads are either soldered to the capacitor legs on the top of the board or the solder points below it. +6 V no Clamping diode com nc 10K To chip 0.22 µF 0V ELECTROMECHANICAL SYSTEMS - VERSION 1 58 Curious fact: the detonation of a nuclear weapon produces a massive electromagnetic 'spike' capable of immobilising chip based equipment. Designers of military communications equipment have actually considered going back to using valves to avoid this problem. ELECTROMECHANICAL SYSTEMS When connecting the load, treat the relay as an ordinary switch. Its switches are NOT connected electrically to the controller’s power supply and you will need to add an external power supply. You should avoid exceeding the stated values on the relay. 1 2 3 4 5 6 PROGRAMMING/OUTPUT INDICATORS +6v 7 8 GND LOGIC OUTPUTS TR1 TR2 1 TR3 2 D7 TR4 TR5 +6v TR6 + D8 C4 C5 COM NO NC RELAY OUTPUT 1 CUT LINE + D9 +6v TR7 8 + D10 RL 3 RL 4 C6 C7 C8 C9 C10 C11 COM NO NC RELAY OUTPUT 3 It is important to ensure that the electrolytic capacitor is correctly connected to the power source. The side marked should be connected to –ve. TR8 7 RL 2 COM NO NC RELAY OUTPUT 2 R15 R14 +6v 3 4 5 6 GND OPEN COLLECTOR TRANSISTOR OUTPUTS - 800 mA MAX RL 1 External supply +6v R13 +6v R12 +6v R11 +6v R10 R9 R8 CUT LINE + COM NO NC RELAY OUTPUT 4 + – The output stages of the controller board may be cut off as indicated either if they are not wanted or because the output stages are to be placed elsewhere in use. For example, the controller might need to be built into a very tight space. If the output part of the board is separated, the +ve and -ve rails must be connected between the two board halves together with a single wire link for each output used. Multi-coloured ribbon cable is a useful option for making a number of connections between boards. RP ON OFF 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 PROGRAM DATA MEMORY TEP PAUSE INPUT RP2 BIT-BY-BIT CONTROLLER RESET INPUT GROUND LED 1-8 1 2 3 4 5 6 PROGRAMMING/OUTPUT INDICATORS +6v 7 8 GND LOGIC OUTPUTS TR1 TR2 1 TR3 TR4 TR5 +6v TR6 GND 3 4 5 6 OPEN COLLECTOR TRANSISTOR OUTPUTS - 800 mA MAX 2 R15 +6v R14 +6v R13 +6v R12 +6v R11 +6v R10 R9 R8 CUT LINE Solder +6 V to track on underside of board +6v TR7 TR8 7 8 CUT LINE D7 RL 1 + D8 + RL 2 ELECTROMECHANICAL SYSTEMS - VERSION 1 D9 RL 3 + D10 + RL 4 59 Technical note To avoid electrical interference any small electric motor other than a solar motor must be suppressed using two capacitors as shown - 0.22µF ceramic, 10µF electrolytic. Solder GND to track on underside of board ELECTROMECHANICAL SYSTEMS STUDY FILE 2 - USES FOR TEP GENERATOR Possible Uses For the Mini-DC Generator • Emergency generator for lighting Many people keep a torch in the home or other places where an emergency light might be needed in the event of a power failure. Sometimes, the torch is used rarely and, when it is needed, the batteries are found to have gone past their shelf life. Inexpensive batteries will probably last for only two to three years if unused because of the internal chemical changes that take place. • Battery alternative In many developing countries, it is possible to obtain small radios but not a reliable supply of batteries - which can be both expensive and environmentally damaging. The mini-DC generator could be used as an alternative by continuous turning of a handle or preferably by storing and slowly releasing energy. Probably the easiest way of doing this is to “wind up” or raise a mass and then let it fall so that it rotates a shaft. A relatively small mass suitably raised and matched to the generator with a gearbox can give several minutes operating time for a transistor radio. ELECTROMECHANICAL SYSTEMS - VERSION 1 60 ELECTROMECHANICAL SYSTEMS • Cell charger The generator can be used for charging rechargeable batteries - for example those used in a cycle lamp. A generator driven by the chain or tyre of the cycle will produce current whenever the cycle is used. However, depending upon what batteries are used, the generator’s output will almost certainly require (a) a circuit to ensure a smooth charging output at the correct voltage and (b) a means to ensure that the batteries do not discharge into the generator. You should take advice from your teacher before embarking on a project involving rechargeable batteries. • Power transmission The TEP generator is a reversible device. This means that one generator can be used to drive another and vice versa. There is some loss of energy in such a system, but two generators connected together can be used to replace mechanical linkages and drives in some applications. The illustration shows a simple toy. Experimenting with pairs of generator/ motor units connected together convinced pioneer electrical engineers that power could be transmitted over distances by means of electrical current. ELECTROMECHANICAL SYSTEMS - VERSION 1 61 ELECTROMECHANICAL SYSTEMS Using the TEP Generator as a Motor The TEP generator can be used with or without its gearbox as an electric motor. It has the following specification: NOMINAL NO LOAD AT MAXIMUM EFFICIENCY Constant Volts Speed rpm Current A Speed rpm Current A Torque g-cm Output W 3.0 1800 0.022 1430 0.085 8.4 0.123 48.3 41 6.0 3700 0.028 3060 0.134 14.5 0.455 56.4 84 The motor can be driven by a battery, power supply unit (PSU), or by a second TEP generator. Its current consumption increases in proportion to the amount of work it does. If you try to make a motor do too much work, it slows down and eventually stalls or stops. Because current continues to flow, the armature windings heat up and may eventually burn out. The small motor in a cordless drill is only about two to three times larger than the TEP generator/motor and will burn out very quickly if the drill is stalled. A fuse, which melts when a certain current is exceeded, offers some protection. ELECTROMECHANICAL SYSTEMS - VERSION 1 62 Efficiency Stall Torque % g-cm ELECTROMECHANICAL SYSTEMS STUDY FILE 3 - SOLENOIDS A linear solenoid (the most common type) consists of a soft iron plunger within a coil wound on a plastic bobbin. When current is passed through the coil, the resulting magnetic field pulls the plunger into the coil with a considerable pulling force. Plunger Input These devices are relatively cheap and very simple; however, the usable stroke of a linear solenoid is quite limited and the force exerted varies according to the position of the plunger within the coil. When the plunger is at its extreme outside the solenoid, the pulling force is relatively weak; as it moves towards the centre it increases. This is shown clearly in a graph of force against stroke distance. 30 Force (N) 25 20 15 10 5 0 3 6 9 12 15 18 21 Stroke (mm) ELECTROMECHANICAL SYSTEMS - VERSION 1 63 24 27 ELECTROMECHANICAL SYSTEMS Various mechanisms are used to increase the stroke length of a solenoid; the simplest of these is a lever. In a rotary solenoid, a spindle turns through a specific angle - e.g. 45’ - when the solenoid is energised. This type of solenoid has a plunger and armature plate. The plate is separated from the solenoid case by three ball bearings each of which runs in a small inclined plane. When the plunger is pulled into the solenoid coil, it also turns as the ball bearings run down the inclined planes. Applications of solenoids Solenoids are used in so many different products, it would take a large book to list the main applications ! A few examples are given below: Vending machine Coin operated ticket machine Cash register Toaster Car Photocopier Door lock Automatic soap dispenser Photo kiosk Juke box ELECTROMECHANICAL SYSTEMS - VERSION 1 64 ELECTROMECHANICAL SYSTEMS Constructing a solenoid It is very straightforward to construct a solenoid providing that care is taken not to break the fine copper wire needed for the coil. A suitable bobbin can be made from a plastic, such as nylon, turned on the lathe or even from paper - using the TEP “rolltube” technique. If a paper tube is made, end caps have to be fitted to keep the wire in position. The most important feature of the bobbin is the wall thickness of the tube; this must be as thin as possible. Mild steel can be used for the plunger and is easily machined for mechanical connection. For a typical miniature solenoid, the bobbin can be wound with 00 gauge lacquer-insulated copper wire. The overall length used will determine the pulling force of the solenoid and the electrical resistance of the coil. The resistance should be as high as possible if a battery is used to energise the solenoid. The following steps are a guide to construction: Step 1 Solder a flying lead to the end of the copper winding and pass this through a drilled hole at the end of the bobbin - leaving sufficient inside the bobbin for mechanical anchorage. ELECTROMECHANICAL SYSTEMS - VERSION 1 65 ELECTROMECHANICAL SYSTEMS Step 2 Wind the coil neatly backwards and forwards on the bobbin. A hand drill offers a very convenient method of doing this. Step 3 Solder the end of the winding to a second flying lead and pass this through a drilled hole in the end of the bobbin. Test for coil continuity before finally covering the whole winding with adhesive tape. ELECTROMECHANICAL SYSTEMS - VERSION 1 66 ELECTROMECHANICAL SYSTEMS RESOURCES The main components referred to in this book are listed below. For complete information on the TEP range, a comprehensive catalogue is available from: Teaching Resources, Middlesex University, Trent Park, Bramley Road, Oakwood, London N14 4XS Tel 0181 447 0342 Linear actuator A powerful motorised miniature actuator capable of a 40mm stroke. The ram rod can be built up in a variety of ways around a plastic block whose movement is controlled at each stroke extremity by limit switches. The actuator comes complete with motor, unfitted limit switches, instruction sheet - and a miniature slide switch for manual control. Price: £3.50 Code: PAC 1402 Punch tool This unique self-contained punch tool has been designed and made in response to the demand for making holes in paper rolltubes AND for punching aluminium or plastic sheet to accommodate the nylon bush (stock number CW4 001). The brightly plated punch tool comes with an instruction sheet which shows how it can be used, for example, to punch accurately spaced holes in either plastic or aluminium sheet to make complete gearboxes etc. Price: £17.80 Code: IT5 007 TEP Bit by Bit Programmable Controller This is a free standing working control board with 8 LED 'flag' outputs. The printed circuit board can be further populated to add: 1. transistor switched outputs, 2. relay switched outputs. The controller enables sequential control of up to eight outputs turning motors etc. on and off. A programme is entered literally bit by bit using small switches on the board itself - and can be run or modified any number of times. This pack is an ideal introduction to digital control and programming. A TEP handbook explaining how to use the controller is available ELECTROMECHANICAL SYSTEMS - VERSION 1 67 ELECTROMECHANICAL SYSTEMS These additional components (not supplied) can be soldered to the board as and when needed. Details of these are contained in the handbook and can be purchased from Teaching Resources and other electronics suppliers. If the basic controller is not extended, the 'spare' part of the board can simply be cut off! NOTE: TEP’s Bit by Bit controller is complementary to the PLC chip kit. It offers a more basic introduction to digital control and does not require a computer for programming. Price: £16.00 Code: PAC BIT Bit by Bit controller self-assembly kit This kit provides all the components, including PCB, needed to make the programmable controller from scratch. Assembly, to the point of being able to program the device, takes approximately half an hour. Comprehensive assembly instructions are provided but a more comprehensive programming and applications handbook is available separately. Price: £12.00 Code: PAC BIT1 Bit by Bit controller - output components pack This pack contains all the parts needed to populate the lower half of the Bit by Bit controller board with 4 relays. It contains 4 transistors (and resistors), 4 relays (and suppression capacitors), 4 diodes. Price: £6.00 Code: PAC BIT2 Smart wire Shape memory alloy wire - 100 micron diameter. 'Smart wire' is a shape memory alloy (SMA) that changes its length with a useful pulling force when a small current is passed through it. A TEP special publication describing SMA and giving applications for use in design and technology is also available. (Minimum order = 1 metre. This is enough for 10 useable lengths.) £6.00 per metre Code: PAC SW1 Pultruded rod (3mm diam. × 910mm long) This material provides an inexpensive opportunity for pupils to use an advanced composite material. The rod is glass reinforced polyester resin with an incredibly high stength to weight ratio. It can be used for axles or in structures and is cut either with a junior hacksaw or TEP’s special guillotine. The guillotine provides a perfect shearing action without distortion or fracturing of the end of the rod. Price: £0.55 Code: CP9 005 ‘Top hat’ bushes CW4 001 Compound gears TG1 000 ELECTROMECHANICAL SYSTEMS - VERSION 1 68