electromechanical systems - eacass.welbni.org/downloads/27/169_84_electronics-...

ELECTROMECHANICALSYSTEMS

VERSION 1

The Engineering Council10 Maltravers StreetLONDONWC2R 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 permissionis granted to make photocopies of pages for classroom useprovided that the copies are used exclusively within a purchasinginstitution. No other reproduction, storage in a retrieval system ortransmission in any form or by any means may be made withoutprior permission from The Engineering Council.

ACKNOWLEDGEMENTS

Text by John Cave

Layout by Peter Stensel

Line illustrations by James Wilkinson

Series Editor

John CaveMiddlesex University

CONTENTS

Introduction 1

Section 1Motors and gearboxes 3

Section 2DC generator 14

Section 3Shape memory alloy actuator 27

Section 4Linear actuator 45

Study file 1“Bit by bit” controller 49

Study file 2Generator applications 60

Study file 3Solenoids 63

Resources 67

1

ELECTROMECHANICAL SYSTEMS

ELECTROMECHANICAL SYSTEMS - VERSION 1


INTRODUCTION

As the name suggests, electromechanical systems or devicesconvert electrical energy into mechanical movement - andsometimes vice versa. Most of the common electromechanicalcomponents, such as electric motors and solenoids are used incombination with mechanical parts to provide actuation ormovement.

Solenoid Motor

Solenoids are used, for example, as actuators in vendingmachines, cash registers and photocopiers. Electric motors, forexample, are used in linear actuators (providing straight linemovement), in electric window systems, operating tables, androbotic arms.

2



A relatively new branch of engineering design called mechatronicsinvolves integrating the three areas of sensing, electronic controland mechanical actuation . A modern camera is a good example ofa mechatronic product. One of the goals of mechatronics is toreduce the number of mechanical components to an absoluteminimum; nevertheless in any system where movement isneeded, there remains a basic need for some kind of controlledmechanical output.

This book examines four basic electromechanical systems and thecomponents needed to design and construct them. The examplesanticipate the general requirements of pupils and students forpractical prototyping work and focus on the use of basicinexpensive components. Because some of these are unique toTEP, additional product information is given on page 67.

3



SECTION 1

ELECTRIC MOTORS AND GEARBOXES

A combination of electric motor and gearbox providing rotaryactuation is one of the most common electromechanicalproducts. A gearbox is really a method of matching the primarypower input from a motor (high speed, low torque) to therequired output (normally low speed, high torque). (Torque canbe thought of as “turning power”.)

Very often a gearbox is built in as an integral part of a motorunit, and this may also contains sensors to feed back positionalinformation to a control circuit. A good example is a type of DCmotor used in photocopiers. Because its rotor consists only of acoil winding with very low inertia, it can be accelerated orstopped very rapidly.

4



For most practical prototyping work you are likely to be usingcheaper motors with gearboxes that you can design and make togive required output characteristics.

Electric motorsMany types and sizes of electric motor are available - with threemain categories operating in the lower voltage range (6-24volts).These are:

• DC brush motors• brushless motors• stepper motors

DC brushmotor

Steppermotor

Brushless motor

A DC brush motor uses a commutator to cause the magnetic fieldin the armature coils to change so that the coils will rotatebetween permanent magnets.

Commutator

Armature

Brushes

Case

Magnets

End plate

5



A brushless DC motor has a permanent magnet rotor and fixedstator coils - but no commutator. As the rotor turns, one or moresensors close to its edge send a signal to a control circuit thatenergises the stator coils in the correct sequence.

A stepper motor - sometimes called a stepping motor - has apermanent magnet rotor that revolves within fixed stator coils.Unlike a brushless motor, however, there are no sensors. Therotor is driven round by switching the coils on and off in aspecial sequence using a driver circuit.

Stator

Rotor

Rotor

Stator

6



GearboxesA gearbox is an assembly of gears insidea frame or casing. A gearbox has arotary input and a rotary output. Thegears inside mesh together to give arequired output torque and speed. Thegearbox of a cordless electric drill isoften larger than the electric motor in the drill!However, it enables the drill’s very small motor runningat very high speed to be turned into a very powerful drillingaction. A simple gearbox may contain just two gears meshing; amore 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 effectsfor programmes such as 'Spitting Image'. The illustrations show arange of general purpose gearboxes that can be obtained readymade, or as kits.

In one of these examples, thegears are arranged within asmall plastic case. In the othertwo, the gears are mountedaround the motor itself. Allthese gearboxes can beadapted to give differentoutput speeds and torque.

7



There are several methods for making gearboxes; this sectionexamines two of them.

1. TEP gearbox using pre-punched sideplates.This gear box has two plates held together using bolts andspacing pillars - e.g. plastic mouldings, small lengths of tubing ora series of nuts. The driving motor is attached to the outside ofone plate, and a gear train set out between the two plates. Themotor can be either a solar motor or an MM28 type. The solarmotor is more expensive but is quieter in operation, will operatefrom a smaller current (e.g. solar cells) and produces lesselectrical noise.

Technical note:MM28 and similar cheapmotors use graphite brushes onthe commutator; the solarmotor uses very fine preciousmetal brushes which makebetter contact. The differencebetween the two shows upclearly when their rapidlychanging current consumption,due to the commutator action,is shown on an oscilloscope.For the solar motor, thechanging current is seen as arelatively smooth signal; forthe cheaper motor, it is very“noisy”.

Precious metal brushes

Graphite brushes

Gear ratiosThe gearbox side plates arepre-punched to give a choiceof three different gear ratios.The gears supplied are a 16tooth pinion for the motor,two compound gears with 10and 42 teeth and a larger(output) gear with 60 teeth.The illustrations show howthe gears are set out to obtainthe different ratios.

8



Constructing a gearboxStep 1. Secure either an MM28 or a solar motor to the side platewith screw-fastening holes. If a solar motor is selected, packingwashers must be used to prevent the screws touching the motor’sarmature. Push fit the pinion on the motor spindle.

Step 2. Push fit the selected gears on the 2mm diameter axlesand position these approximately between the plates to ascertainwhat spacers need to be added to keep the gears floating(moving) across the gearbox. You can use washers as spacers orsmall lengths of plastic sleeving.

9



Step 3. Assemble the side plates by putting in the gears and thefastening bolts and spacers. Any excess axle material can besnipped off.

Step 4. OPTIONALBecause the output shaft will be subjected to a load, the gear thatdrives it might slip. Also, the shaft is running, effectively, in analuminium bearing. A better arrangement is to ream out thepunched holes that form the output bearing and insert nylonbearing bushes.

10



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 toproduce a rough 'spline' to lock onto the gear. This is done byplacing the shaft on a hard wooden surface (not metal) androlling back and forth with the edge of a file - pressing downhard.

TEP guillotine

An excellent alternative shaft material is 3mm diameter fibre-reinforced pultruded rod. The gear should be drilled to 2.9mmdiameter to make a secure interference fit. The rod itself is cuteither with a hacksaw or the special TEP guillotine.

2. Gearbox using punch-tool methodA two-plate gearbox can be made using a combination of TEP’slarger compound gears. In order to mesh properly the gears needto be positioned at a distance of 26mm between centres. (26mmis also the distance between the motor pinion and the first drivengear centres.)

26 mm

40 teeth

12 teeth

11



If correctly spaced bearing holes can be punched along a pair ofside plates, you can create gearboxes with different ratios just byusing the correct number of gears. Any two gears meshing give aratio 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 ofaluminium plate up to 1mm thick using the special TEP punchtool. After one hole is punched , the aluminium plate is simplymoved along the graduated straight edge by 23mm and anotherhole punched.

12



A simple gearbox with a ratio of 3.3 to 1 can be constructed asfollows:

Step 1. Cut out the gearbox side plates - allowing for thedistances between gear centres and material at both ends forspacing pillars. An example hole layout for the motor side of thegearbox is shown. As well as bearing holes, two holes arepunched for the pillars and two for the motor fastening screws.

Step 2. Locate the motor side plate against a convenient point onthe punch’s graduated scale and punch the first hole. Move theplate along by the required distance and punch the second hole.Do this for all the holes on the diagram. Repeat for the secondplate but leaving out the two holes for the motor screws.

25 7 7 26 26 40

40

150

13



Step 3. Using a hand reamer, open out the punched hole for themotor boss (6.2mm) and secure either an MM28 or a solar motorwith the self-tapping screws. If a solar motor is selected, packingwashers must be used to prevent the screws touching the motor’sarmature. Push fit a pinion onto the motor spindle. Push fitbearing bushes into all the remaining punched holes (see page 9).

Step 4. Push fit the selected gears on 3mm diameter axles andposition roughly between the plates to ascertain what spacersneed to be added to keep the gears floating (moving) acrossbetween the two plates. You can use washers as spacers or smalllengths of plastic sleeving.

Step 5. Assemble the side plates by putting in the gears and thechosen fastening bolts and spacing pillars. The spacers can bemouldings, short lengths of tubing or a series of nuts. Any excessshaft material is cut off prior to assembly.

Note: 3mm diameter fibre-reinforced pultruded rod is an idealshaft material for this gearbox. It can be cut using a smallhacksaw or TEP’s guillotine.

14



SECTION 2

DC GENERATOR

The TEP generator is an electromechanical system consisting of amotorised reduction gearbox working in reverse; i.e. a low speed,high torque input is converted through gearing into a highspeed, low torque drive for the generator. Most miniatureelectric motors work as either motors or generators but some aremore efficient than others. The TEP generator is a solar motor.

The generator principleWhen a conductor such as copper wire is moved within amagnetic field and cuts across the lines of force, an electriccurrent flows. The direction of current flow can be determinedusing Fleming’s right hand rule.

Mot

ion

Flux

Current

Lines of force

Current

If the wire is made into a coil and moved in the field, a largercurrent flows. A current can also be made to flow by moving themagnet rather than the coil.

15



The TEP generator has coils rotating between fixed magnets. Itwas in fact designed as an electric motor but like most miniaturemotors it works as a generator when the spindle is turnedrapidly. The coils are wound around an armature and the currentgenerated in them passes to a pair of terminals by means of acommutator and brushes. The ends of the coils areconnected to commutator segments from whicha direct current (DC) is drawn by means ofbrushes in contact with the segments. Thesimplified diagram of a single coil rotating ina magnetic field shows how the commutatorworks. The segments are mounted on theshaft and rotate with the coil. The current inthe coil flows towards the top segment andaway from the bottom segment. Half a turn laterthe current in the coil has reversed but it is still flowingtowards the top segment and away from the bottom segment. Soalthough the coil is generating an alternating current, thecommutator acts as a mechanical rectifier and supplies a directcurrent from the brushes. The direct current from this generatoralways flows the same way, unless you reverse the direction ofrotation of the shaft.

Ideally, the output of a DC generator should be smooth like thatsupplied by a battery. This appears on a time graph simply as astraight line.

N S

N S

Minimum

Maximum

Cur

rent

Time

0

Flux

Brush

Brush

Commutator

+

-

Cur

rent

Time

0

The TEP generator, however, provides pulsating DC.This is because the current flow rises to a maximumwhen the coils cut directly across the lines of force andthen falls to a minimum when they move along thelines of force.

Commutator

Armature

Brushes

Case

Magnets

End plate

16



Improving the DC OutputIf the TEP generator is connected to a loudspeaker, the pulsationsare heard as a loud sound. The pitch of this sound rises as thegenerator’s speed is increased. If the generator is used to power aradio, these pulsations may seriously interfere with the music orspeech from the radio itself. For this type of application, a wayhas to be found to make the output as smooth as possible.

The unit of capacitance is the Farad which is a very large value. Amicrofarad (µF) is one millionth of a Farad. If you place a 1000µFcapacitor across a 4.5 V battery, it charges up almost instantly. Ifit is then connected - say - to an LED, this will light up, but onlyfor a short period.

The most common method of smoothing a pulsating supply, isto connect a capacitor in parallel with it. A capacitor is a devicethat stores electrical charge and is sometimes thought of as arechargeable battery with very rapid charge and discharge times.

G

+

0

Generator

17



When a capacitor is connected in parallel withthe generator, it charges up during eachpulsation and discharges to 'fill in' the gapsbetween pulsations as the diagram shows. Acapacitor by itself does not give perfect DC butcan turn a pulsating supply into one with onlya small “ripple”.

(In order to produce a perfect DC supply, weneed also to add a device called a voltageregulator which senses the ripples and correctsthem to give almost perfect DC. Voltageregulators are available in a small compactpackage and are now quite low in cost.)

If we need to convert the output from an ACgenerator to DC, it can be done with diodeswhich allow current to flow in one directiononly. This process is called rectification. Thesimplest circuit uses just one diode and giveshalf wave rectification. As the time graph shows,the diode stops current flowing one way andproduces pulsations with a gap between each.This output can be smoothed with a capacitorto fill in the gaps.

Cur

rent

Time

0

A better form of rectification uses four diodesoften supplied as single component. This iscalled full wave rectification.

G

Diode

Generator

0

+

Vol

tage

Time

0

Voltage regulator

G

0

+

Diodebridge

Circuit symbol

Vol

tage

Time

+

-

Cha

rgin

g

Dis

char

ging

Smoothed output

Generatoroutput

18



The TEP Mini-GeneratorThe TEP generator was actually designed as a special motor tooperate from very small sources of current such as solar cells. It islarger in diameter than other miniature motors because it has abigger armature and slightly larger permanent magnets. It alsohas better spindle bearings and brushes that contact thecommutator with very little friction.

To generate useful current, the armature has to be turned rapidly.For example, to generate the output needed to energise astandard LED, the spindle has to be turned at a minimum speedof 300 revolutions per minute (r.p.m.). You can do this for just amoment by spinning it between finger and thumb. To generate acontinuous useful current, the generator spindle has to be rotatedat a speed of at least 1500 r.p.m.

19



A Practical Mini-Generator With GearboxThe TEP generator cannot be driven directly by hand (or fromseveral other power sources) because the speed of rotationprovided is too low. A way has to be found to increase this speedto at least 1500 r.p.m. Connecting a power source to a generatorto get the best performance is called matching. A pulley systemcan be used but unless stepped belts and pulleys are used, thebelts are likely to slip. A step-up gearbox is the most commonmethod employed.

This gearbox is assembled by the following steps:

• choose the gears to be used and position each of these on alength of 2 mm shafting

(If you know the input speed and the required output speed ofa 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 toothpinion, the gear ratio is 6:1. The driven gear will rotate 6 timesfor each revolution of the driver. In other words, it will rotatesix 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.

A gearbox for a the TEP generator is assembled very quickly usingthe two-plate method. The generator is supplied with two pre-drilled side plates and a selection of gears giving three differentratios.

Stepped belt

20



If the outside of the second gear has 50 teeth and meshes witha 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 screwsand the packing washers (the washers are important toprevent the screws touching the generator armature).

• attach spacing bolts on the same plate and add enough nutsto provide spacing between the two plates.

• place the shafts through the bearing holes and fix the secondplate.

To test the gearbox and generator, a length of 2 mm shaft can bebent as a crank. Where it passes through the driving gear, itshould be flattened slightly or upset to give a form of “spline” toprevent the gear slipping round.

21



The TEP generator gearbox side plates are pre-drilled to allow several different combinationsof gears. If you want to drill holes for a differentgear train, use the following method:

Getting gears to mesh properlyThe most important aspect of making a gearbox is making the gears mesh together properly.If they press together too tightly, there is a lotof friction and they may not turn at all. If theyare too far apart, the teeth may jump over oneanother. You need to mark the bearing holes inthe side plates that support the shafts veryaccurately to make sure the teeth mesh withjust enough clearance to turn freely. Calculateit using the simple method below:

1. Place the two gears on a flat surface with ashort length of metal shaft forced into thecentre of each.

2. Push the two gears together between fingerand thumb and then measure the distancebetween the two shafts.

3. Subtract the diameter of one shaft to give thedistance between centres.

4. Add a small allowance to the distance toenable the gears to run freely. As a rule ofthumb (a rough rule) add 1.0 mm to thedistance for large gears and 0.5 mm for smallgears.

Example: The measured distance is 32 mm andthe shaft diameter 3 mm.

32 mm - 3 mm = 29 mm between centres.

Adding allowance of 1 mm for free runninggives 29 mm + 1 mm = 30 mm.

The positions of the bearing holes for the shaftare marked out on just one plate since the twoplates are clamped together for drilling at thesame time.

Mark out the position for the first shaft andmake a small centre punch dot. (The position ofthis first shaft is important because the gearmounted on it has to mesh with one on themotor.)

Open a pair of metalworking compasses to thecorrect distance between centres that you haveworked out for the meshing gear and scribe anarc.

22



You can make a centre punch dot anywhere along this arc for thesecond gear shaft. Its position depends on how you have decidedto set out the gear train. If a bearing hole for a third shaft isneeded, the same procedure is repeated with the compass openedto the correct distance. The centre punch dots can now be madelarger before drilling.

The two side plates are finally clamped together with toolmaker’sclamps and carefully drilled. It is also a good idea at this stage todrill holes for the spacers.

Driving the Mini-DC GeneratorThe generator can be driven directly only if the means ofrotation is very fast (e.g. a small CO2 motor used to power modelaircraft). If the step-up gearbox is used to increase speed - i.e.,matching power source to generator - a variety of power sourcesare available. These include:

• coiled spring or elastic• falling mass• hand turning• air propeller or rotor• water wheel• connection to moving object (e.g. bicycle)

Kgkg

23



The first two of these power sources use storedenergy and may need some form of speedregulation before attachment to the gearboxinput. A spring or elastic band will tend to havehigh torque (‘turning power’) when fullywound up and less when it is run down. Afalling mass will accelerate as it falls, butconnection to the generator and gearboxcauses drag and the mass - providing it is nottoo large - will fall at a more uniform rate.

Although the gearbox can be powered througha 2 mm diameter input shaft, it is preferable touse one of larger diameter if possible. Thisprovides more strength and makes it less likelythat the first gear will slip on the shaft as it isturned. If a 3 mm diameter shaft is used, forexample, the two side plate bearing holes alsoneed to be drilled out to 3 mm. If the gearcentre is drilled out to 2.9 mm, this will providea tight interference fit. Nevertheless, it is anadvantage to roughen the shaft so that it locksmore tightly onto the gear. One way of doingthis is to press the edge of a file against theshaft and slowly roll it back and forth over asmooth surface.

Making an Input CrankThere are a number of methods for makingcranks. These include:

• Bending a long shaft.

• Fitting the end of the shaft and a crankhandle to a third piece of material as shown.The joints must be carefully considered.There are many options. A very easy one isto use metal studding for both the shaft andthe handle.

24



Performance of the TEP Mini-DC GeneratorThe technical term for something connected to and driven by agenerator is called the load. This could be a light bulb forexample, or a radio. The size of the load is measured by the amountof current consumed or drawn and not the physical size. A smalltorch bulb drawing a current of 0.5 A represents a larger loadthan a radio drawing 0.3 A even though the radio itself is muchbigger.

The load connected to a generator has to be correctly matchedto the generator output. A large or heavy load reduces the outputvoltage and will also make the generator very difficult to turn.Remember that the generator is converting mechanical energyinto electrical energy. If you power the generator without a loadconnected, it will turn easily. If you then connect a load such asa bulb, the generator becomes noticeably harder to turn. Thisresistance to turning is the work you have to do to produce electricalenergy.

To examine the performance of the TEP generator, a load has tobe connected. The current flowing through the circuit and thevoltage across it are both measured. For accurate measurements,load resistors are used rather than bulbs etc. A low value resistorrepresents a greater load than a high value one because morecurrent passes. If the load is increased by reducing the value ofthe load resistor, the voltage will drop, because of the increasedvoltage drop in the internal resistance of the generator.

G

A

VLoadresistorGenerator

25



To give a ‘feel’ for the effect of different loads,try connecting the TEP generator to thefollowing:

The TEP generator can be used in the same waybecause it is also an electric motor. It can alsobe used simply as a brake for other devices. Thegenerator offers little resistance to turning withno load. However, as soon as you close a switchto connect - say -a load resistor, it offersconsiderable resistance to turning. When thegenerator is acting as a brake, the work it does iseventually converted into heat in the load (andin the coils of the generator).

Improving the Mini-Generator’s OutputThe pulsating DC output from the TEPgenerator can be improved by connecting acapacitor in parallel across the output terminals.This should be as large as possible but for mostpurposes a 2000 µF capacitor will suffice.Remember, though, to connect the capacitor tothe generator the correct way round in relationto the polarity of the generator.

You can test for positive and negative byoffering an LED to the generator terminals; itwill light up only when the cathode leg isconnected to the negative terminal.

Connecting a load acts as a brake on thegenerator. This effect is used by electric vehiclesto save wear on normal brakes and to conservebatteries. An electric vehicle going up hill isdriven by a motor supplied from batteries.When it is going down hill, the motor isswitched over so that it acts as a generator to re-charge the batteries. In so doing, the motoroffers resistance to turning and has a brakingeffect on the vehicle.

Grain of wheat bulb

Small motor

Small buzzer

LED

Vol

tage

Time

+

-

Cha

rgin

g

Dis

char

ging

Smoothed output

Generatoroutput

G

+

0

Generator

26



For an even smoother DC output from the generator, you shouldselect a suitable voltage regulator from a supply catalogue andmake up the recommended circuit. This usually involves addingjust one or two external components; an example is shown. It isimportant to note that the regulator always needs an inputvoltage higher than the regulated output voltage.

With an AC supply, a transformer can be used to step outputvoltages either up or down. This is not possible with a DCsupply. However, there is now a range of electronic circuits calledDC to DC converters that will either increase or decrease thevoltage from a DC supply. You should consult a supply catalogueunder the heading of “DC to DC converter” to select a suitabledevice.

7805I/P O/P

Com0.22µF

0.47µF4K7

+5V

0V

+7 to25V

0V

Output

Input

DC to DC converter

Voltageregulator

27



SECTION 3

SHAPE MEMORY ALLOY (OR “SMART WIRE”) ACTUATORS

A relatively new type of electromechanical actuator uses shapememory alloy (SMA).

Smart MaterialsMost materials that we use in products have properties whichremain more or less constant in use. 'Smart' materials aredifferent; they respond to external factors such as differences inlight or temperature levels and change in some way. They aredescribed as 'smart' because they seem to be intelligent or have amind of their own.

Smart materials are now being applied in everyday products.Examples include sunglass lenses (and spectacle lenses) whichdarken as light intensity increases and stick-on thermometerswhose colour changes to indicate temperature. Smart materialsare now even used in clothing!

Stick-onthermometer

Reactolightglasses

Shape Memory Alloy (SMA)SMA is a smart material which, as its name suggests, has amemory. The most common SMA is an alloy (mixture of metals)of nickel and titanium - called nitinol. By means of special heattreatment, a piece of SMA can be made to 'remember' a shape.For example, a length of wire can be made to remember that itshould be straight at temperatures above 70°C. If you bend thiswire at normal room temperature into the shape of a paper clip,it stays bent and will continue acting as a paper clip. However, ifyou place it in a glass of water whose temperature is above 70°C,it immediately straightens out! When cool, it remains straightuntil it is bent again.

28



This cycle of bending and then straightening when heated can becontinued millions of times. The temperature at which SMA'remembers' its original form is called the transition temperatureand when this point is reached, it changes shape.

SMA has a relatively high electrical resistance and can beheated to its transition temperature by passing an electricalcurrent through it.

Applications of SMASMA can be used to give a mechanical movement when a settemperature is reached. For example, current applicationsinclude:

• 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 onthe coffee

• air conditioning units - to move louvres or flaps to direct airmovement

• shower units - to control hot water control valves

The advantage of SMA in these and many other applications isthe fact that it provides large forces and movement at a precisetemperature. It is also possible to pre-shape the SMA in differentways - for example as a spring or a flat plate.

29



Smart wire has a relatively high electrical resistance and can beheated to its transition temperature by passing an electricalcurrent through it. Before SMA was available, bi-metallic stripswere really the only simple way of causing mechanicalmovement by change of temperature. A bi-metallic strip consistsof two metal ribbons bonded together. One metal has a high rateof expansion when heated; the other has a low rate of expansion.When the strip is heated, it curls because one side expands morerapidly than the other.

Unlike SMA, bi-metallic strips change shape gradually whenheated - not all at once. Also, in practice, they cannot be made tochange shape when current is passed through them.

Smart WireA common form of SMA is wire available in different diameters.This ranges, for example, from 5 mm diameter down to 50microns (1 micron = 1/1000 millimetre). The SMA wire sampleprovided with this book is Nitol with a diameter of 100 microns.It is heat treated to 'remember' that it has a shorter length whenheated above its transition temperature (70°-80°C) than below it.

If the sample length of wire is held between two points it has alength of approximately 10 cm. When heated to between 70° and80°C, it shortens by about 5% or 1/20 and exerts a useful pullingforce. (The wire becomes shorter and it gets slightly fatter.) Whenthe wire cools down, it relaxes to its longer length of 10 cm.

Bi-metallic strip

Bi-metallic strips arecommonly used to controlthermostats in central heatingsystems and electric kettles.

30



Relaxed

Shortened

10 20

The 5% change in length is constant for any length or diameterof SMA wire. This results in quite small movements for shorterlengths of wire. However, the movement can be increased byincreasing the length of wire. To work out the amount ofmovement for a given piece of wire, you simply multiply itslength 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 largermovement using simple lever systems.

31



SMA wire has to be stretched or biased to return to its longerlength. The force required to do this is much smaller than thepulling force that the wire exerts when it shortens. There are twomain ways of biasing:

• Using a weight• Using a spring

Using a weight

Because SMA has a relatively high electrical resistance, it can beheated to its transition temperature simply by passing currentthrough it. This opens up many possibilities for providingmechanical actuation (movement) without any moving partsother than those the SMA is attached to! Also, for smallerdiameter wires, the currents needed are quite small and can beprovided from smaller batteries.

+V

-V

In a practical design using SMA wire, you need to know whatforce to use to bias it, and what force it will exert when itshortens. If you are heating it with electric current, you also needto know how much current to pass without overheating anddamaging it.

Shape memory alloywire symbol used forthe purposes of thisbook.

Using a spring

32



All these figures (for 100 micron wire) are provided in the tablebelow:

Bias force 0.3 NPulling force 1.5 N

Resistance 150 ohms per metreMax. current 180 milliampsMax. power 5 Watts per metre

Shortening time 0.1 secondRelaxation time 1.0 second

Recommended extension 5%Minimum bend radius 5 mm

Effective transitiontemperature 70°Centigrade

Pulling starts at 68°C.Pulling finishes at 78°C.Relaxation starts at 52°C.Relaxation finishes at 42°C.

Explanation of the TableThe table tells us that at normal room temperature the wireneeds to be stretched with a bias force of 0.3 newtons - which isroughly equivalent to hanging a weight of approximately 30grams on the end. When heated to the transition temperature ofbetween 70° to 80°C, the wire shortens about 5% in length andwill exert a pulling force of 1.5 newtons - roughly equivalent tolifting a weight of 150 grams.

The speed at which the wire shortens when it reaches thetransition temperature is about 0.1 seconds. It takes longer torelax or stretch back to its longer length - about 1 second. Thetable also tells us that when heated, the wire actually startschanging length at 68°C and finishes at 78°C. When it cools,however, the stretching or relaxation does not take place until ithas reached 52°C.

The figures given in the table are the recommended ones for 100micron nitol; if they are exceeded, the useful life of the wire willbe reduced.

33



The supply needed to heat the wire can be determined usingOhm’s Law. This states the relationship between voltage (V),current (I) and resistance (R), as follows:

V = I × RI = V/RR = V/I

The table gives us the resistance of the wire and also states themaximum current. Using Ohm’s Law, we can therefore work outthe voltage needed.

For example, what is the voltage needed to pass the maximumsafe current through the 10 cm length of 100 micron sample wireprovided with this book?

Step 1The 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 2The maximum current is 180 mA or 0.18 A.(1 milliamp = 1/1000 Amp.)

Step 3V = 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 thislength of wire because as current is drawn, its voltage will reduceslightly.

To check that the power rating (the rate of doing work) is notexceeded, we can use the power equation W = I × V.

If we substitute the above figures W = 0.18 × 2.7 = 0.49 Watts fora 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 100micron SMA wire?

How many times per minute could a length of 100 micronSMA wire go through a complete shortening and relaxationcycle?

34



Using SMA WireIt is important to make good electrical and mechanicalconnections to the ends of SMA wire. The wire cannot besoldered and must be joined to other conductors by mechanicalmeans. It is also important to remember that where the wire is incontact with a metal component or surface, some heat will beconducted away and that the whole length of wire may notexhibit the memory effect.

The response times given in the table are for SMA in a normalroom environment. If the wire is enclosed in an insulated sleeve,for example, it will take longer to cool down and relax to itslonger length. If air is blown over it, it will cool more rapidly.

Insulatedsleeve

Wire

Increasing Pulling ForceThe pulling force of SMA wire cannot be increased by supplyingcurrent beyond the recommended limit; this will damage it.However, two or more wires can be run in parallel. Two wires willgive double the pulling force and so on. You must remember,though, that if the wires are connected in parallel, you alsodouble the current needed to heat them up.

35



Power SuppliesCurrent supplied to the SMA wire must be within therecommended limit to avoid any damage. There are several waysof doing this including:

HITA

CH

I

HP7LO

NG

LIFE

HITA

CH

I

HP7LO

NG

LIFEHITACHI

HP7LONG LIFE

• use of an adjustable powersupply unit (PSU).

• use of an appropriatenumber of 1.5 V batteriesconnected in series.

+V

-V

High wattageresistor

SMA

• use of a series resistor toregulate the supply. It maynot be possible to 'fine tune'a number of batteriesaccurately enough or youmay have an unsuitablesupply. In either case,current can be regulated byusing a series resistor in thecircuit. Ohm’s law can beused to work out the valueof this resistor.

[Note: the resistor should bea higher wattage type. Thepower in the circuit can beworked out using W (watts)= I (current) × V (volts). If avariable resistor is used, itshould be a wire-woundhigher wattage type.]

36



• use of a voltage regulator

Vin

Vout

Case also Vout

LM317T

Adj

Control Circuits1. Open loop controlIn open loop control, there is no feedback. The supply is simplyswitched on or off - for example, using a press switch or a timercircuit. Switches that can be used include: reed switches operatedby a magnet, micro switches, membrane panels.

Reed switch and magnet

Membrane panel

Micro switch

37



Supply current can be 'switched' by a thyristor, bipolar transistoror FET (field effect transistor). The example circuits show howsensors can control the supply switching.

+V

0 V

IR530

1mΩ

Touchpads

SMA

+V

0 V

SMA

106

100 KPiezotransducer

Thyristor triggered by shock FET switched on byplacing finger across

touch pads

2 kΩ

+V

0V

BFY51

Probes

SMA

Transistor switched on bywater bridging across

probes

Bipolar transistors and FETs can also be used as the output stageof microelectronic control circuits - e.g. a 555 timer.

555

+V

0V

680 Ω

Set

VR14.7 kΩ

1000 µF

2

7

68 4

3

Setinput

Timeperiodinput

Processtimer

OutputFET

IR 530

SMA

38



2. Closed loop controlClosed loop control involves something feeding back (feedback)from the output to the input of a system. A central heatingsystem turns on and off at a temperature set by a thermostat. Abi-metallic strip in the thermostat heats up and moves to switchoff the heating boiler when an appropriate temperature has beenreached.

Temperaturesetting

Contacts

Bimetallic strip

Thermostat

Because SMA wire changes length when it is heated, themovement can be used as feedback - for example, to switch thesupply on and off. A very simple example involves connecting alength of SMA to a microswitch. When the wire is relaxed theswitch is 'on' and current flows through the wire. The wire thenshortens, depresses the switch contact and turns off the supply.The wire then relaxes and the whole cycle begins again.

Power supply

SMA

Microswitch

39



[Note: Ingenious heat engines have been built from SMAmaterials using a closed loop system. In one example, a wirerelaxes and dips into hot water. This causes it to change shapeand move out of the water to cool down and relax again. Thesame cycle repeats over and over again and turns a smallflywheel.]

Experiments With SMA Wire

• Lifting weights

This experiment simplyinvolves attaching a length ofSMA wire to a weight (e.g. ballbearings in a bag) andobserving the contractionwhen the wire is heated bycurrent. The bias force isautomatically supplied by theweight.

• Amplifying movement withlevers

A simple two dimensionallever system can be assembledon a baseboard usingpolystyrene or card strip forthe lever and a drawing pinpivot. The 'load' on the levercan be supplied by weights ora spring (e.g. elastic band).

Bearing balls

40



The distances from the pivot to (a) the wire attachment and (b)the weights can be expressed as a ratio. In the example shownthe ratio is 5:1. For every millimetre moved by the wire end theweighted end will move through 5 millimetres.

2 cm10 cm

• Amplifying movement using geometry

A weight is attached to the centre of a length of SMA wire so thatit forms two sides of an inverted triangle. Over a range of anglesthe vertical movement of the weight will be greater than thelinear movement of the wire. This effect increases as the angle atx increases (i.e. as the wire becomes closer to horizontal).However, the forces required also increase. Try experimentingwith SMA wire at an angle at x of 140° and plot the movementsof the weight on a piece of paper.

Ball bearings

x

41



Practical Applications of SMA WireLinear actuationSMA wire is most easily used to provide linear or straight linemovement. The example shown uses SMA wire to pull a bolt in asimple lock. In this application very little linear movement isneeded. If its length can be accommodated, SMA wire can oftenbe used in place of a more expensive solenoid. A free-standingactuator can be made by containing the wire in a plastic tube.

Door bolt

Compressionspring keepsSMA wire

stretched andbolt in 'locked'

position

SMA wire used in anelectric door lock

• Angular actuationIn many practical applications of SMA wire, a mechanical systemis used to amplify movement. The barrier prototype modelillustrated uses the lever principle to move and lift up the arm.The same principle can be used to provide the movements of arobot arm.

SMA wire

42



Rotary actuationSMA wire (or a cord extension from it) can be wound around ashaft, drum, pulley, or cam to produce rotary movement. For agiven length of wire, the larger the diameter of the shaft etc., thesmaller the rotation - and vice versa. If the shaft etc., is very smalland expected to rotate through several revolutions, particularattention has to be given to biasing - either with a weight or aspring.

Anthropomorphic actuation'Anthropomorphic' describes something which has humancharacteristics. A lot of robotics research is currently directed atmaking robotic movements - especially hand movements -imitate human ones. This is because of their potential as artificialarms and limbs for disabled people and as precision manipulatorsfor industrial robots. Many of these experimental devices useSMA wire to provide mechanical movement.

It is surprisingly easy to makean actuator that imitates - say- a finger movement. Onemethod is to stretch the SMAwire inside a 'springy' plastictube. This will cause the tubeto curl slightly. When theSMA wire is heated by current,it exerts a stronger pullingforce inside the tube and thiscauses it to curl around evenfurther - closing the 'finger'.When the wire relaxes, the'finger' opens again.

Rotating shaft

Leads

SMA wire inside'finger' segments

43



In many commercial prototype hands the robotic fingers aremade up from hinged segments with small springs to keep themstraight. When SMA wire running through the segmentscontracts, the 'finger' curls just like the tube.

Construction NotesThe most difficult aspect of using small diameter SMA wire isholding it securely and making good electrical contact. These aresome of the methods employed:

• Wire Crimps. These are small fastenings pressed flat aroundwires to be joined; they are available commercially in manydifferent shapes and sizes. The most useful ones for SMA wire areminiature tubes which are closed with special crimping pliers orordinary pliers. The crimps can be placed at the very end of anSMA wire or somewhere along its length.

• Edge and corner crimping. This is a technique for makingwire crimps on the corner or edge of a metal tab - e.g. copper. Anedge or corner is folded over using a pair of pliers. Because themetal at the bend hardens as it deforms it does not close overcompletely and leaves a small opening. The wires can be insertedin this opening which is then finally closed by ‘nipping’ with thepliers. One or more holes punched in the tab can be used tofasten it.

SMA wire fixed at end

Note: The most commonminiature crimps availableare 'bootlace' types - asmall tube but with oneend closed. These willwork for all theapplications shown in thisbook.

44



• Screw, nut and washers. SMA wire and connecting wire canbe fastened to a small screw using two washers and a nut. Thefree end of the screw can also be used to provide a mechanicalanchorage to something else.

• Terminal block. Commercial terminal blocks contain twin-screw brass fastenings in a polythene strip. Individual fasteningscan be removed from the plastic strip as necessary. Note: It is anadvantage to attach a crimp to the SMA wire before securing it inthe terminal block.

Further Reading

Bowyer, M.J. Design and Applications of Ni-Ti ShapeMemory Alloy Springs, Engineering Design,November 1988.

Gilbertson, R.G. Muscle Wires, Mondo-tronics, 1992.

Cave, J.F. (Ed.) TEP Electronics 14-16, The EngineeringCouncil, 1994

SMA

Terminal blockconnector

45



SECTION 4

LINEAR ACTUATORS

A linear actuator is a motorisedunit which often resembles ahydraulic or pneumatic cylinder.It contains a motor, gearbox and ameans of converting the rotary outputfrom the gearbox into a powerful push-pulllinear movement. This movement isnormally obtained by a nut moving along arotating screw thread - the same means used to move the carriageon a manual lathe.

Most larger commercial linear actuators use a ball screw. Thisworks on the same principle as a basic nut and screw but the nutis separated from the screw by ball bearings to minimise friction.

Linear actuators are normally used to provide intermittent ratherthan continuous push-pull movements. They are self-containedunits, and very easy to build into systems such as windowopening mechanisms. However, because the motor is totallyenclosed, they have a limited duty cycle. This means that they canbe energised for only a certain percentage of the time. Forexample, an actuator with a duty cycle of 50% means that itshould only be running for only - say - 2 minutes within a 4minute period. Manufacturers state the precise duty cycleconditions in their literature.

TEP linear actuatorThe TEP linear actuator is an open-frame type that comes almostcompletely assembled. It uses a 5mm diameter screw drivendirectly by a miniature DC motor. The screw engages a brass nutset into a plastic block which also accommodates a push rod. Theend of the screw is supported in a nylon bearing at one end ofthe frame and above this an identical bearing providing supportfor the push rod.

46



If the motor is connected to a 3v - 6v battery supply, the nut willrun rapidly to one end of the frame. Reversing the motor supplywill cause it to run in the opposite direction. If you do thissimple experiment, however, you will find that at the end of itstravel, the nut will lock onto the screw and simply reversing themotor will not be enough to free it. To prevent the nut reachingthe extremity of the thread and to provide proper control, it isnecessary to add two limit switches to the frame. These switch offthe motor when the nut is almost at the end of its travel. Theyalso enable manual or automatic reversing of the nut.

Setting up the limit switchesThe actuator is supplied with two limit switches and small self-tapping screws for fixing. Two leads should be soldered to eachswitch as shown, and the switches fastened to the frame. Thelever of each switch should be bent outwards so that thesupply is switched off well before the end of the nut’s travel.This needs to be done because the motor continues to spin afterthe supply is switched off, and the nut travelling beyond its limitwill jam.

47



As a guide, use only a 3 volt supply either to trial the actuator orrun 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 areconnected to a DPDT (double pole, double throw) switch asshown. When the slide switch, provided with the actuator, is inthe centre position, it is ‘off’. In either of the other two positionsit supplies current to the motor until one of the limit switchesbreaks the circuit. The slide switch can then be thrown to theother ‘on’ position to reverse the nut. Manual switching mightbe used, for example, to cause the actuator to throw a lock bolt.

M3 - 6V

+

—L1

L2

Slideswitch

L1 and L2 are the limit switches. Use connections marked 'con' and 'NC"

The actuator can be controlled electronically by using anappropriate circuit and a DPDT relay (or two SPST relays). Forexample, a “Bit by bit” controller can be programmed to switch apair of SPST relays on and off.

There are many variations on the control theme. For example, asensor 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

48



A very simple example circuit is given. When the water sensor iswet, the relay is energised and the actuator's push rod is ‘parked’in the withdrawn position. If the water level drops, the relayswitches and the rod moves to its extended position and parksthere until the water level rises again.

Note: the screw and the base of the actuator frame should belubricated with light oil. The 3mm diameter push rod is aninterference fit in the plastic nut and can be withdrawnproviding the nut is supported. A longer or specially shapedpush rod, for example, can be substituted.

TEPGenerator/motor

+

-

Shunt brakingIt is possible to stop the travel of the nut very rapidly anywherealong the screw by shunt braking. This involves using one or morerelays to short circuit or shunt across the motor terminalsimmediately the current supply has been interrupted. Whenshunted, the motor (with its spinning armature) is trying to actas both generator and motor. This has the effect of stopping italmost immediately. An example circuit is shown.

6V

+

—

L1

L2

M

Relay coil

BFY 51

1K

Probes

49



STUDY FILE 1 - “BIT BY BIT” CONTROL

All of the electromechanical actuators described can becontrolled with the TEP “Bit by Bit” controller using eithertransistor or relay outputs.

IntroductionThe TEP bit by bit controller is a self-contained electroniccontroller capable of switching on or off up to 8 different outputsover a period of time. It is programmed by setting each of eightsmall DIP switches to either 'on' or 'off' and then committingthese instructions to memory by pressing a push switch. Thememory can hold up to 64 such lines of program. The totalprogram can then be run at different speeds to control a varietyof devices such as lamps, buzzers and electric motors.

Setting UpThe controller is supplied complete to run and program; outputcomponents are added if and as required. The controller requireseither a 6V battery power supply or a supply from a PSU (powersupply unit) which is regulated. THE MAXIMUM SUPPLYVOLTAGE IS 6 VOLTS. IT SHOULD BE NOTED THAT ABATTERY SNAP CAN INADVERTENTLY BE CONNECTED TOA 9V SOURCE.

A 4 × ΑΑ battery box and battery connecting snap is suppliedwith the board. If used, the snap should be soldered to the pointsmarked + and - at the top of the board, if necessary using the twospare holes as mechanical anchorage for the two leads.

Any program will be lost if the battery is disconnected formore than 20 seconds. Because the standby currentconsumption of the board is so small, it is preferable to leavethe battery connected all the time.

LED 1-8

PAUSE INPUT

RESET INPUT

GROUND

RP2

RP

1

MEMORY

1 2 3 4 5 6 7 8

ON DIP

1 2 3 4 5 6 7 8PROGRAM DATA

.1.2

.51

25

1020

RUN SPEED

ONOFF

RE

MO

TE

RU

NS

WIT

CH

1 2

ON

RUN

STOP

PROG ON

PROG OFF

+ –

6v IN

TEPBIT-BY-BITCONTROLLER

(c) 1994 TEP

C2

SPEEDADJUST

+ –C3

IMPORTANTTECHNICAL NOTE:

Under some circumstancesthe bit by bit controller canbe affected by electricalnoise, e.g. from electricmotors. This is discussed onpage 12.

The noise immunity of thecontroller can be improvedby adding an optionalcapacitor at position C3.This should beapproximately 0.1µF.

–

NC2

SPEEDADJUST

+ –C3

50



Basic PrinciplesThe TEP controller uses a single IC (integrated circuit) containinga memory where information can be stored in electronic form. Itis useful to think of this memory as a book having a stack ofpages. Every page represents a line of control programming andhas 8 blank spaces - each one waiting to be filled with a bit ofinformation. Each vertical column of blanks will contain theremembered instructions for a control output. Each controloutput is connected to an LED 'flag'.

1

2

3

4

1 2 3 4 5 6 7 8

ON DIP

LED flags(Outputs)

64

1 2 3 4 5 6 7 8

Page(line of

program)

1

2

3

4

1 2 3 4 5 6 7 8

ON DIP

LED flags(Outputs)

64

1 2 3 4 5 6 7 8

Page(line of

program)on off off on off on off off

The memory locations are filled with individual bits ofinformation - of which there are only two types: logic 1 or logic0. In the controller’s memory these are really instructions whichmean either turn ON an output (logic 1) or turn OFF an output(logic 0).

51



The information is written on each 'page' of memory by settingthe 8 DIP switches to either 'ON' or 'OFF' and then pressing the'MEMORY' press-button switch . Pressing this button 'writes' thePROGRAM DATA switch settings into memory and automaticallyturns over to the next 'page'. This procedure can be repeated upto 64 times - the maximum number of pages or locations in thecontroller’s memory.

The illustration shows a sample 6-line program for the two lefthand outputs. When the controller is instructed to read thisprogram, it turns over the 'pages' at a set speed. An 'ON' bit ofinformation lights up an output LED and an 'OFF' bit turns it off.If, for example, the controller is set to read each page for asecond at a time, the LED on the far left hand side will turn onfor one second off for the next and so on. LED number 2 willturn on for two seconds and then stay off for two seconds. Whenthe program has been run - i.e., all the 'pages' turned over - thecontroller will automatically start again at the first line of theprogram. Unless it is stopped, the program will run over and overagain.

Please note: the remainder of this text will refer to lines ofprogram and not pages.

Outputs 1 2 3 4 5 6 7 8

1

2

3

4

5

6

on on

off on

on off

off off

on on

off on

52



Programming the Bit by Bit ControllerUsing the whole-board diagram as a guide, you should now beable follow these instructions for programming the controller.

1. Make sure the RUN and PROGRAM switches at the top of theboard 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 settingeach 'PROGRAM DATA' switch toeither 'ON' or 'OFF'. This will turn theLED outputs on of off. Because theprogram switches are small, it is moreconvenient 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 intomemory. When you do so, all the LEDs will flash on briefly toconfirm this has happened.

6. Repeat steps 4 and 5 above up to 64 times - once for each lineof 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 beginningafter the final line.

RL 1 RL 2 RL 3 RL 4

COM NO NCRELAY OUTPUT 1




D7 + D8 + D9 + D10 +CUT LINE

CUT LINE

TR1

1

R8 +6v

TR2

2

R9 +6v

TR3

3

R10 +6v

TR4

4

R11 +6v

TR5

5

R12 +6v

TR6

6

R13 +6v

TR7

7

R14 +6v

TR8

8

R15

OPEN COLLECTOR TRANSISTOR OUTPUTS - 800 mA MAX

GND

LOGIC OUTPUTS GND+6v

PROGRAMMING/OUTPUT INDICATORS1 2 3 4 5 6 7 8

LED 1-8

PAUSE INPUT

RESET INPUT

GROUND

RP2

RP

1

MEMORY

1 2 3 4 5 6 7 8

ON DIP


.1.2

.51

25

1020

RUN SPEED

ONOFF

RE

MO

TE

RU

NS

WIT

CH

1 2

ON

RUN

STOP

PROG ON

PROG OFF

+ –

6v INC2


SPEEDADJUST

+ –C3

C4C5

C6C7

C8C9

C10C11

53



Running the Program1. 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 programrun speed. As an example, set the fourth switch from the left to'ON'.

4. Set the program run switch to 'RUN'. The program will nowrun at approximately 1 line per second. (Setting one of theother 'PROGRAM DATA' switches will run the program at adifferent speed - see below.) The LEDs will turn on and offaccording to the stored program in memory.

The speed of execution of the program depends on which'PROGRAM DATA' switch is set to the 'ON' position and also onthe setting of the 'SPEED ADJUST' resistor at the top of the board.The 'PROGRAM DATA' switches provide speed adjustment infixed steps or ratios. The 'SPEED ADJUST' resistor provides overallcontinuous adjustment - faster or slower. To calibrate thecontroller to run at the speeds printed above the 'PROGRAMDATA' switches, create a simple program that turns LED 4 on forone program line, off for the next - and so on (keeping all theother LEDs off). Run this program, and time the result against awatch - altering the 'SPEED ADJUST' so that eventually the LEDturns on and off at one second intervals.

Technical note: capacitorC2, together with the tworesistors at the top right handcorner of the board, controlsthe chip's clock speed. Thisis 390 pF. If it is replacedwith a lower value (no lowerthan 50 pF) the top run speedcan be considerablyincreased. However, it willalso have the effect offlashing the LEDs morerapidly when the memorybutton is pressed and thestandby current consumptionwill increase slightly.

If setting switch 4 provides a run speed of one program line persecond, the switch on the far right will give program steps of 20seconds duration. This adds up to a maximum 64 line programrun time of 20 seconds × 64 lines = 1,280 seconds ORapproximately 21 minutes. This run time can be extendedfurther by adjusting the resistor. Remember, though, this alsoaffects timings provided by the other 'PROGRAM DATA' switches.

Important note: The TEP controller has a volatile memory. Thismeans that a program is lost when the power supply isdisconnected although the larger capacitor at the top centre ofthe board will keep it energised for about 20 seconds. However,the Standby Current Consumption of the controller's chip is solow it can be left connected for most practical purposes.

PAUSE INPUT

RESET INPUT

RP2

RP

1

MEMORY

1 2 3 4 5 6 7 8

ON DIP


.1.2

.51

25

1020

RUN SPEED

ONOFF

RE

MO

TE

RU

NS

WIT

CH

1 2

ON

RUN

STOP

PROG ON

PROG OFF

+ –

6v IN

TEPBIT-BY-BIT

C2

SPEEDADJUST

+ –C3

Speed inseconds

Overallspeedadjustment

54



Using the Controller’s OutputsThe bit by bit controller has 8 LED flags to show the status ofeach output. This enables you to create programs and run thembut not to actually control anything! To switch a load such as amotor on or off a buffer stage has to be added to each output inuse. For convenience, the controller board has additional printedtracks and locations for transistor buffers on all the outputs andtransistor-plus-relay buffers on four of them. (Note: the boardhas only enough room physically for relays on the four left handoutputs.)

Using the Transistor Outputs

The recommended outputtransistor for which the boardhas been designed is theinexpensive BCX38B. This is aDarlington pair device andwill switch on a load of nearly1 amp (800 milliampsmaximum). This is quitesufficient for most filamentbulbs, buzzers and a solarmotor.

CB

EC

BE

BCX38B

OR

CUT LINE

CUT LINE

TR1

1

R8 +6v

TR2

2

R9 +6v

TR3

3

R10 +6v

TR4

4

R11 +6v

TR5

5

R12 +6v

TR6

6

R13 +6v

TR7

7

R14 +6v

TR8

8

R15


GND



LED 1-8

RL 1 RL 2 RL 3 RL 4





D7 + D8 + D9 + D10 +

C4C5

C6C7

C8C9

C10C11

Logicoutputstage

Transistoroutputstage

Relayoutputstage

55



To add a transistor to any required output, fix and solder inposition a 10 K resistor and BCX38B transistor - for example, atthe positions marked R8 and TR1. Make sure the transistor is thecorrect way around by matching the case outline with theoutline on the board. Flying leads to the load are soldered to the+6 V point and open collector output for each transistor. Thediagram shows a lightbulb connected to output 1. A circuitdiagram for this output is also shown.

REMEMBER that when several transistors are used, the total loadcurrent - which can be quite high if all outputs are used - comesfrom the battery powering the controller. This could be depletedvery quickly. Always work out the total load current (or anaverage for outputs switching on and off) and think carefullyabout the type of battery needed.

It is possible, for example, to run the following devices directlyfrom the transistor outputs:

filament bulb buzzer solar motorminiature solenoid stepper motor

Any motors other than the more expensive solar motorshould be run from a relay. This is because they produce ahigh degree of electrical noise which may interfere with theoperation of the chip.

6 Volt lampBCX38B

CUT LINE

CUT LINE

TR1

1

R8 +6v

TR2

2

R9 +6v

TR3

3

R10 +6v

TR4

4

R11 +6v

TR5

5

R12 +6v

TR6

6

R13 +6v

TR7

7

R14 +6v

TR8

8

R15


GND



RL 1 RL 2 RL 3 RL 4





D7 + D8 + D9 + D10 +

C4C5

C6C7

C8C9

C10C11

10KTo chip

+6 V

0 V

56



A motor, solenoid or any other device with a coil is an inductiveload and can produce a high voltage momentarily whenswitched off (back EMF).

To prevent this damaging the transistor, a clamping diode shouldbe added as shown in the diagram. This can be a general purposetype such as IN4001.

The most convenient way of connecting a clamping diode to aninductive load is to use one of the first four left hand outputs andsimply solder in a diode as if you were using a relay. It isIMPORTANT to ensure that the diode is soldered in the correctway round - with the marked end facing towards the right.

10KTo chip

+6 V

0 V

MClamping

diodeMotor(inductiveload)

Miniature solenoidSolar motor

CUT LINE

CUT LINE

TR1

1

R8 +6v

TR2

2

R9 +6v

TR3

3

R10 +6v

TR4

4

R11 +6v

TR5

5

R12 +6v

TR6

6

R13 +6v

TR7

7

R14 +6v

TR8

8

R15


GND



RL 1 RL 2 RL 3 RL 4





D7 + D8 + D9 + D10 +

C4C5

C6C7

C8C9

C10C11

MM28

IMPORTANTTo avoid electricalinterference any smallelectric motor must besuppressed using twocapacitors as shown - 0.22µFceramic, 10µF electrolytic.

It is important to ensurethat the electrolyticcapacitor is correctlyconnected to the powersource. The side markedshould be connected to –ve.

57



Using a transistor output with an external power supplyTo avoid draining the battery powering the board itself, a load canbe connected to a separate power supply, ideally a battery, up to24V. The diagram shows a lightbulb thus connected to output 1.A circuit diagram for this output is also shown.

D7 + D8 + D9 + D10 +

CUT LINE

CUT LINE

TR1

1

R8 +6v

TR2

2

R9 +6v

TR3

3

R10 +6v

TR4

4

R11 +6v

TR5

5

R12 +6v

TR6

6

R13 +6v

TR7

7

R14 +6v

TR8

8

R15


GND



LED 1-8

PAUSE INPUT

RESET INPUT

GROUND

RP2

R

MEMORY

1 2 3 4 5 6 7 8


OFF


– ve

+ ve

10KTo chip

Max 24V

BCX38B

0V

LOAD

The easiest way to add this might be to connect it directly acrossthe load itself; i.e. across the connecting legs of a motor inparallel with the suppression capacitors.

10KTo chip

Max 24V

BCX38B

0V

Remember that if a separate battery is used, the total load currentshould not exceed 800 mA otherwise the transistor will bedamaged.

Please remember that if an inductive load, such as a motor, isconnected, a clamping diode should be added as shown in thediagram below.

58



Using a Relay OutputThe first four left hand outputs are extended at the bottom of thePCB to accommodate relays. To add a relay to any of theseoutputs, first fix and solder in position a 10 K resistor andBCX38B transistor - for example R8 and TR1. Then fix and solderin position a miniature SPDT relay (e.g. Kam Ling KS1P) togetherwith a clamping diode. Also, solder in a 0.22µF suppressioncapacitor across 'com' and 'no' (C5). This is essential. To test therelay, write in a simple on/off program for this output. When theprogram is run, the relay will simply click on and off.

Technical note: a verysignificant problem indesigning industrial controlequipment is the suppressionof 'electrical noise' orelectromagnetic interference.Electromechanical devicesinvariably produceinterference. (Remember thatearly radio transmitters usedan electrical arc to produceradio frequency energy.) Thesuppression capacitor on therelay is very important toprevent any interference to thechip. European Countrieshave a convention - EMC -which sets a standard forprotection fromelectromagnetic interference.

The load is connected to the relay switch outputs at the bottomof the controller board. 'COM' is the pole of the switch, 'NO' isthe normally open contact and 'NC' is the normally closedcontact. The load leads are either soldered to the capacitorlegs on the top of the board or the solder points below it.

10KTo chip

+6 V

0 V

Clampingdiode

no

nc

com

0.22 µF

Curious fact: thedetonation of a nuclearweapon produces a massiveelectromagnetic 'spike'capable of immobilising chipbased equipment. Designersof military communicationsequipment have actuallyconsidered going back tousing valves to avoid thisproblem.

CUT LINE

CUT LINE

TR1

1

R8 +6v

TR2

2

R9 +6v

TR3

3

R10 +6v

TR4

4

R11 +6v

TR5

5

R12 +6v

TR6

6

R13 +6v

TR7

7

R14 +6v

TR8

8R

15


GND



RL 2 RL 3 RL 4





D7 + D8 + D9 + D10 +

C4C5

C6C7

C8C9

C10C11

RL 1

0.22µF capacitor

59



When connecting the load, treat the relay as an ordinary switch. Itsswitches are NOT connected electrically to the controller’s powersupply and you will need to add an external power supply. Youshould avoid exceeding the stated values on the relay.



LED 1-8

PAUSE INPUT

RESET INPUT

GROUND

RP2

RP

MEMORY

1 2 3 4 5 6 7 8


ONOFF


RL 1 RL 2 RL 3 RL 4

D7 + D8 + D9 + D10 +

CUT LINE

CUT LINE

TR1

1

R8 +6v

TR2

2

R9 +6v

TR3

3

R10 +6v

TR4

4

R11 +6v

TR5

5

R12 +6v

TR6

6

R13 +6v

TR7

7

R14 +6v

TR8

8

R15


GND

Solder GNDto track onunderside ofboard

Solder +6 Vto track onunderside ofboard

The output stages of the controller board may be cut off asindicated either if they are not wanted or because the outputstages are to be placed elsewhere in use. For example, the controllermight need to be built into a very tight space. If the output partof the board is separated, the +ve and -ve rails must be connectedbetween the two board halves together with a single wire link foreach output used. Multi-coloured ribbon cable is a useful optionfor making a number of connections between boards.

Technical noteTo avoid electricalinterference any smallelectric motor other than asolar motor must besuppressed using twocapacitors as shown - 0.22µFceramic, 10µF electrolytic.

It is important to ensurethat the electrolyticcapacitor is correctlyconnected to the powersource. The side markedshould be connected to –ve.

CUT LINE

CUT LINE

TR1

1

R8 +6v

TR2

2

R9 +6v

TR3

3

R10 +6v

TR4

4R

11 +6v

TR5

5

R12 +6v

TR6

6

R13 +6v

TR7

7

R14 +6v

TR8

8

R15


GND



RL 2 RL 3 RL 4





D7 + D8 + D9 + D10 +

C4C5

C6C7

C8C9

C10C11

RL 1

+

–

Externalsupply

60



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 otherplaces where an emergency light might beneeded in the event of a power failure.Sometimes, the torch is used rarely and, when itis needed, the batteries are found to have gonepast their shelf life. Inexpensive batteries willprobably last for only two to three years ifunused because of the internal chemicalchanges that take place.

• Battery alternative

In many developing countries, it is possible toobtain small radios but not a reliable supply ofbatteries - which can be both expensive andenvironmentally damaging. The mini-DCgenerator could be used as an alternative bycontinuous turning of a handle or preferably bystoring and slowly releasing energy. Probably theeasiest way of doing this is to “wind up” or raisea mass and then let it fall so that it rotates ashaft. A relatively small mass suitably raised andmatched to the generator with a gearbox cangive several minutes operating time for atransistor radio.

61



• Cell charger

The generator can be used for chargingrechargeable batteries - for example those usedin a cycle lamp. A generator driven by the chainor tyre of the cycle will produce currentwhenever the cycle is used. However,depending upon what batteries are used, thegenerator’s output will almost certainly require(a) a circuit to ensure a smooth charging outputat the correct voltage and (b) a means to ensurethat the batteries do not discharge into thegenerator.

You should take advice from your teacherbefore embarking on a project involvingrechargeable batteries.

• Power transmission

The TEP generator is a reversible device. Thismeans that one generator can be used to driveanother and vice versa. There is some loss ofenergy in such a system, but two generatorsconnected together can be used to replacemechanical linkages and drives in someapplications. The illustration shows a simpletoy. Experimenting with pairs of generator/motor units connected together convincedpioneer electrical engineers that power could betransmitted over distances by means ofelectrical current.

62



Using the TEP Generator as a MotorThe TEP generator can be used with or without its gearbox as anelectric motor. It has the following specification:

NOMINAL NO LOAD AT MAXIMUM EFFICIENCY

Constant Speed Current Speed Current Torque Output Efficiency Stall TorqueVolts rpm A rpm A g-cm W % g-cm

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 increasesin proportion to the amount of work it does. If you try to make amotor do too much work, it slows down and eventually stalls orstops. Because current continues to flow, the armature windingsheat up and may eventually burn out. The small motor in acordless drill is only about two to three times larger than the TEPgenerator/motor and will burn out very quickly if the drill isstalled. A fuse, which melts when a certain current is exceeded,offers some protection.

63



STUDY FILE 3 - SOLENOIDS

A linear solenoid (the most common type) consists of a soft ironplunger within a coil wound on a plastic bobbin. When currentis passed through the coil, the resulting magnetic field pulls theplunger into the coil with a considerable pulling force.

Plunger

Input

These devices are relatively cheap and very simple; however, theusable stroke of a linear solenoid is quite limited and the forceexerted varies according to the position of the plunger within thecoil. When the plunger is at its extreme outside the solenoid, thepulling force is relatively weak; as it moves towards the centre itincreases. This is shown clearly in a graph of force against strokedistance.

0 3 6 9 12 1815 21 2724

Stroke (mm)

30

25

20

15

10

5

For

ce (

N)

64



Various mechanisms are used to increase the stroke length of asolenoid; 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 aplunger and armature plate. The plate is separated from thesolenoid case by three ball bearings each of which runs in a smallinclined plane. When the plunger is pulled into the solenoid coil,it also turns as the ball bearings run down the inclined planes.

Applications of solenoidsSolenoids are used in so many different products, it would take alarge book to list the main applications ! A few examples aregiven below:

Vending machineCoin operated ticket machineCash registerToasterCarPhotocopierDoor lockAutomatic soap dispenserPhoto kioskJuke box

65



Constructing a solenoidIt is very straightforward to construct a solenoid providing thatcare is taken not to break the fine copper wire needed for thecoil. A suitable bobbin can be made from a plastic, such as nylon,turned on the lathe or even from paper - using the TEP “roll-tube” technique. If a paper tube is made, end caps have to befitted to keep the wire in position. The most important feature ofthe bobbin is the wall thickness of the tube; this must be as thinas possible. Mild steel can be used for the plunger and is easilymachined for mechanical connection.

For a typical miniature solenoid, the bobbin can be wound with00 gauge lacquer-insulated copper wire. The overall length usedwill determine the pulling force of the solenoid and the electricalresistance of the coil. The resistance should be as high as possibleif a battery is used to energise the solenoid. The following stepsare a guide to construction:

Step 1Solder a flying lead to the end of the copper winding and passthis through a drilled hole at the end of the bobbin - leavingsufficient inside the bobbin for mechanical anchorage.

66



Step 2Wind the coil neatly backwards and forwards on the bobbin. Ahand drill offers a very convenient method of doing this.

Step 3Solder the end of the winding to a second flying lead and passthis through a drilled hole in the end of the bobbin. Test for coilcontinuity before finally covering the whole winding withadhesive tape.

67



RESOURCES

The main components referred to in this book are listed below.For complete information on the TEP range, a comprehensivecatalogue is available from:

Teaching Resources,Middlesex University,Trent Park,Bramley Road,Oakwood,London N14 4XS

Tel 0181 447 0342

Linear actuatorA powerful motorised miniature actuator capable of a 40mmstroke. The ram rod can be built up in a variety of ways around aplastic block whose movement is controlled at each strokeextremity by limit switches. The actuator comes complete withmotor, unfitted limit switches, instruction sheet - and aminiature slide switch for manual control.Price: £3.50 Code: PAC 1402

Punch toolThis unique self-contained punch tool has been designed andmade in response to the demand for making holes in paper roll-tubes AND for punching aluminium or plastic sheet toaccommodate the nylon bush (stock number CW4 001). Thebrightly plated punch tool comes with an instruction sheetwhich shows how it can be used, for example, to punchaccurately spaced holes in either plastic or aluminium sheet tomake complete gearboxes etc.Price: £17.80 Code: IT5 007

TEP Bit by Bit Programmable ControllerThis is a free standing working control board with 8 LED'flag' outputs. The printed circuit board can be furtherpopulated to add:

1. transistor switched outputs,2. relay switched outputs.

The controller enables sequential control of up to eight outputsturning motors etc. on and off. A programme is entered literallybit by bit using small switches on the board itself - and can berun or modified any number of times. This pack is an idealintroduction to digital control and programming. A TEPhandbook explaining how to use the controller is available

68



These additional components (not supplied) can be soldered tothe board as and when needed. Details of these are contained inthe handbook and can be purchased from Teaching Resourcesand other electronics suppliers. If the basic controller is notextended, the 'spare' part of the board can simply be cut off!

NOTE: TEP’s Bit by Bit controller is complementary to the PLCchip kit. It offers a more basic introduction to digital control anddoes not require a computer for programming.Price: £16.00 Code: PAC BIT

Bit by Bit controller self-assembly kitThis kit provides all the components, including PCB, needed tomake the programmable controller from scratch. Assembly, tothe point of being able to program the device, takesapproximately half an hour.

Comprehensive assembly instructions are provided but a morecomprehensive programming and applications handbook isavailable separately.Price: £12.00 Code: PAC BIT1

Bit by Bit controller - output components packThis pack contains all the parts needed to populate the lower halfof the Bit by Bit controller board with 4 relays. It contains 4transistors (and resistors), 4 relays (and suppression capacitors), 4diodes.Price: £6.00 Code: PAC BIT2

Smart wireShape memory alloy wire - 100 micron diameter.'Smart wire' is a shape memory alloy (SMA) that changes itslength with a useful pulling force when a small current is passedthrough it. A TEP special publication describing SMA and givingapplications for use in design and technology is also available.(Minimum order = 1 metre. This is enough for 10 useablelengths.)£6.00 per metre Code: PAC SW1

Pultruded rod (3mm diam. ××××× 910mm long)This material provides an inexpensive opportunity for pupils touse an advanced composite material. The rod is glass reinforcedpolyester resin with an incredibly high stength to weight ratio. Itcan be used for axles or in structures and is cut either with ajunior hacksaw or TEP’s special guillotine. The guillotineprovides a perfect shearing action without distortion orfracturing of the end of the rod.Price: £0.55 Code: CP9 005

‘Top hat’ bushes CW4 001Compound gears TG1 000

electromechanical systems - eacass.welbni.org/downloads/27/169_84_electronics-...

Documents