English for mechatronics engineering

Anh văn chuyên ngành cơ điện tử ĐHBKDN. TABLE OF CONTENTS TABLE OF CONTENTS 1 PART 1: DIGITAL LOGIC 3 CHAPTER 1: INTRODUCTION 3 LOGIC GATES 3 CHAPTER 2: APPLICATIONS OF LOGIC GATES 8 INTEGRATED CIRCUIT 8 APPLICATIONS 9 CHAPTER 3: BASIC ELECTRONIC COMPONENTS 11 RESISTANCE 11 CAPACITOR 12 DIODE 14 LIGHT EMITTING DIODE 16 BIPOLAR JUNCTION TRANSISTOR (BJT) 19 CHAPTER 4: MICROCONTROLLER 21 INTRODUCTION 21 IMPORTANT FEATURES 22 POWER SUPPLY CIRCUIT 23 HOW TO START WORKING? 24 PART 2: MECHANICAL ACTUATION SYSTEMS 27 CHAPTER 1: INTRODUCTION 27 MECHANISMS 27 TYPES OF MOTION 29 DEGREE OF FREEDOM 29 CHAPTER 2: CAMS 30 ECCENTRIC CAM 31 DROP CAM 33 FLAT CAM 34 CHAPTER 3: GEARS 36 SPUR GEAR 37 HELICAL GEAR 38 DOUBLE HELICAL GEAR 39 BEVEL GEAR 40 WORM GEAR 41 RACK AND PINION 43 GEAR TRAIN 44 CHAPTER 4: BELT AND CHAIN DRIVES 46 PROS AND CONS 47 FLAT BELTS 48 ROUND BELTS 49 VEE BELTS 49 TIMING BELTS 50 CHAIN DRIVE 51 CHAINS VERSUS BELTS 53 CHAPTER 5: BEARINGS 54 DEEPGROOVE 55 FILLING SLOT 56 ANGULAR CONTACT 57 DOUBLEROW 58 SELFALIGNING 59 STRAIGHTROLLER BEARING 61 TAPER ROLLER 62 NEEDLE ROLLER 64

TABLE OF CONTENTS

PART 1: DIGITAL LOGIC CHAPTER 1: INTRODUCTION Many control systems are concerned with setting events in motion or stopping themwhen certain conditions are met For example, with domestic washing machine, the heater isonly switched on when there is water in the drum and it is to the prescribed level Suchcontrol involves digital signals where there are only two possible signal levels Digitalcircuitry is the basis of digital computers and microprocessor controlled systems There aretwo input signals which are either 1 or 0 signals and an output signal which is 1 or 0 signal.The controller is here programmed to only give a 1 output if both the input signals are 1.Such an operation is said to be controlled by a logic gate Logic gate is the basic buildingblocks for digital electronic circuits The term combinational logic is used for the combining

of two or more basic logic gates to form a required function

LOGIC GATES Logic gates are the basic components in digital electronics They are used to createdigital circuits and even complex integrated circuits For example, complex integratedcircuits may bring already a complete circuit ready to be used – microprocessors andmicrocontrollers are the best example – but inside them they were projected using severallogic gates In this tutorial we will teach you everything you need to know about logic gates,with several examples

As you may already know, digital electronics accept only two numbers, “0” and “1.”Zero means a 0 V voltage, while “1” means 5 V or 3.3 V on newer integrated circuits Youcan think “0” and “1” as a light bulb turned off or on or as a switch turned off or on

input B are high; for all other conditions it gives a low output This is an AND logic gate.

We can visualize the AND gate as an electric circuit involving two switches in series Only when switch A and B are closed, there is a current

The relationship between the inputs and the outputs of an AND gate can be expressed inthe form of an equation, called Boolean equation The Boolean equation for the AND gate iswritten as

b OR gate: An OR gate with inputs A and B gives an output of a 1 when A or B is 1.

We can visualize such a gate as an electric circuit involving two switches in parallel Whenswitch A or B is closed, then there is a current OR gates can also have more than inputs

We can write the Boolean equation for an OR gate as: A

we have a digital input which varies with time, the output variation with time is the inverse The Boolean equation describing the NOT gate is

A bar over a symbol is used to indicate that the inverse, or complement, is being taken;thus the bar over the A indicates that the output Y is the inverse value of A

d NAND gate: The NAND gate can be considered as a combination of an AND gatefollowed by a NOT gate Thus when input A is 1 and input B is 1, there is an output of 0, allother inputs giving an output of 1

The NAND gate is just the AND gate truth table with the outputs inverted An alternativeway of considering the gate is as an AND gate with a NOT gate applied to invert both theinputs before they reach the AND gate The figure below shows the symbols used for theNAND gate, being the AND symbol followed by the circle to indicate inversion

The Boolean equation describing the NAND gate is:

e NOR gate: The NOR gate can be considered as a combination of an OR gatefollowed by a NOT gate Thus when input A or input B is 1 there is an output of 0 It is justthe OR gate with the outputs inverted An alternative way of considering the gate is as an

OR gate with a NOT gate applied to invert both the inputs before they reach the OR gate.The figure below shows the symbols used for the NOR gate; it is the OR symbol followed

by the circle to indicate inversion

The Boolean equation for NOR gate is:

Y = A ⊕ B is the Boolean equation for the XOR gate

The following is the truth table for the XOR gate

1 1

g XNOR gate: XNOR stands for exclusive NOR and is an XOR gate with its outputinverted So, its output is at “1” when the inputs have the same value and “0” when they aredifferent XNOR operation is represented by the symbol (·) The Boolean equation forXNOR gate is:

A (·) B = Y

CHAPTER 2: APPLICATIONS OF LOGIC GATES

INTEGRATED CIRCUIT Logic gates are available as integrated circuits The different manufacturers havestandardized their numbering schemes so that the basic part numbers are the same regardless

of the manufacturer For example, Fig 1(a) shows the gate systems available in integratedcircuit 7408; it has four two-input AND gates and is supplied in a 14-pin package Powersupply connections are made to pins 7 and 14, these supplying the operating voltage for allfour AND gates In order to indicate at which end of the package pin 1 starts, a notch is cutbetween pins 1 and 14 Integrated circuit 7411 has three AND gates which each havingthree inputs; integrated circuit 7421 has two AND gates with each having four inputs.Figure 1(b) shows the gate systems available in integrated circuit 7402 This has four two-input NOR gates in a 14-pin package, power connections being to pins 7 and 14 Integratedcircuit 7427 has three gates with each having three inputs

A3 B3

A2 B2

A1 B1

A0 B0

or both 1 the output is 0, and if they are not equal the output is a 1 To obtain a 1 outputwhen the bits are the same we need to add a NOT gate, this combination of XOR and NOTbeing termed an XNOR gate To compare each of the pairs of bits in two words we need anXNOR gate for each pair If the pairs are made up of the same bits then the output fromeach XNOR gate is a 1 We can then use an AND gate to give a 1 output when all theXNOR outputs are ones Figure 2 shows the system

Figure 3: T he traffic lights

The Fig 3 shows a simple system by which a controller can send a coded digital signal

to a set of traffic lights so that the code determines which light, RED, AMBER OR GREEN,will be turned on To illuminate the RED light we might use the transmitted signal A = B =

0, for the AMBER light A = 0, B = 1 and for the GREEN light A = 1, B = 0 We can switch

on the lights using these codes by using three AND gates and two NOT gates

CHAPTER 3: BASIC ELECTRONIC COMPONENTS

RESISTANCE The electrical resistance of an object is a measure of its opposition to the passage of asteady electric current An object of uniform cross section will have a resistance proportional

to its length and inversely proportional to its cross-sectional area, and proportional to theresistivity of the material

Discovered by Georg Ohm in the late 1820s, electrical resistance shares some conceptualparallels with the mechanical notion of friction The SI unit of electrical resistance is theohm, symbol Ω Resistance's reciprocal quantity is electrical conductance measured in

Siemens, symbol S

The resistance of a resistive object determines the amount of current through the objectfor a given potential difference across the object, in accordance with Ohm’s laws:

R

where

R is the resistance of the object, measured in ohms, equivalent to J·s/C2

V is the potential difference across the object, measured in volts

I is the current through the object, measured in amperes

We all know that voltmeter and ammeter are used for measuring the voltage and thecurrent respectively For the resistance, the meters that use to measure it is the ohmmeter.But what if we don't have an ohmmeter to use?

Color coding system for resistors consists of three colors to indicate the resistance value

in ohms of a certain resistor, sometimes the fourth color indicate the tolerance value of theresistor By reading the color coded in correct order and substituting the correct value ofeach corresponding color coded as shown in the table below, you can immediately tell allyou need to know about the resistor Each color band represents a number and the order ofthe color band will represent a number value The first 2 color bands indicate a number The

3rd color band indicates the multiplier or in other words the number of zeros The fourth band

certain precision resistors have 5 bands or have the values written on them, refining thetolerance value even more

5th Band (Tolerance)

When there is a potential difference (voltage) across the conductors, a static electric fielddevelops across the dielectric, causing positive charge to collect on one plate and negativecharge on the other plate Energy is stored in the electrostatic field An ideal capacitor is

characterized by a single constant value, capacitance, measured in farads This is the ratio ofthe electric charge on each conductor to the potential difference between them

The capacitance is greatest when there is a narrow separation between large areas ofconductor; hence capacitor conductors are often called "plates," referring to an early means

of construction In practice, the dielectric between the plates passes a small amount ofleakage current and also has an electric field strength limit, resulting in a breakdown voltage,while the conductors and leads introduce an undesired inductance and resistance

Capacitors are widely used in electronic circuits for blocking direct current whileallowing alternating current to pass, in filter networks, for smoothing the output of powersupplies, in the resonant circuits that tune radios to particular frequencies and for many otherpurposes

A capacitor consists of two conductors separated by a non-conductive region Thenonconductive region is called the dielectric In simpler terms, the dielectric is just anelectrical insulator Examples of dielectric mediums are glass, air, paper, vacuum, and even asemiconductor depletion region chemically identical to the conductors A capacitor is

assumed to be s elf-contained and isolated, with no net electric charge and no influence fromany external electric field The conductors thus hold equal and opposite charges on theirfacing surfaces, and the dielectric develops an electric field In SI units, a capacitance of onefarad means that one coulomb of charge on each conductor causes a voltage of one voltacross the device

The capacitor is a reasonably general model for electric fields within electric circuits Anideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio ofcharge ±Q on each conductor to the voltage V between them:

diode, the most common type today, is a crystalline piece of semiconductor materialconnected to two electrical terminals A vacuum tube diode (now rarely used except in somehigh-power technologies) is a vacuum tube with two electrodes: a plate and a cathode

direction (called the diode's forward direction), while blocking current in the oppositedirection (the reverse direction) Thus, the diode can be thought of as an electronic version of

a check valve This unidirectional behavior is called rectification, and is used to convert

alternating current to direct current, and to extract modulation from radio signals in radio

receivers—these diodes are forms of rectifiers

A Zener diode is a special kind of diode which allows current to flow in the forwarddirection in the same manner as an ideal diode, but will also permit it to flow in the reversedirection when the voltage is above a certain value known as the breakdown voltage, "Zenerknee voltage" or "Zener voltage." The device was named after Clarence Zener, whodiscovered this electrical property A Zener diode exhibits almost the same properties, exceptthe device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage By contrast with the conventional device, a reverse-biased Zener diodewill exhibit a controlled breakdown and allow the current to keep the voltage across theZener diode close to the Zener breakdown voltage For example, a diode with a Zenerbreakdown voltage of 3.2 V will exhibit a voltage drop of very nearly 3.2 V across a widerange of reverse currents The Zener diode is therefore ideal for applications such as thegeneration of a reference voltage (e.g for an amplifier stage), or as a voltage stabilizer forlow-current applications

A diode bridge is an arrangement of four (or more) diodes in a bridge circuitconfiguration that provides the same polarity of output for either polarity of input When

used in its most common application, for conversion of an alternating current (AC) input intodirect current a (DC) output, it is known as a bridge rectifier A bridge rectifier provides full-

to a rectifier with a 3-wire input from a transformer with a centertapped secondary winding The essential feature of a diode bridge is that the polarity of the output is the sameregardless of the polarity at the input The diode bridge circuit is also known as the Graetzcircuit after its inventor, physicist Leo Graetz

HW: Describing the basic operation of Diode Bridge?

LIGHT EMITTING DIODE

A light-emitting diode (LED) is a semiconductor light source LEDs are used as indicatorlamps in many devices and are increasingly used for other lighting Introduced as a practicalelectronic component in 1962, early LEDs emitted low-intensity red light, but modernversions are available across the visible, ultraviolet, and infrared wavelengths, with very highbrightness

When a light-emitting diode is forward-biased (switched on), electrons are able torecombine with electron holes within the device, releasing energy in the form of photons

This effect is called electroluminescence and the color of the light (corresponding to the

energy of the photon) is determined by the energy gap of the semiconductor LEDs are oftensmall in area (less than 1 mm2), and integrated optical components may be used to shape its

radiation pattern LEDs present many advantages over incandescent light sources includinglower energy consumption, longer lifetime, improved robustness, smaller size, and faster

more precise current and heat management than compact fluorescent lamp sources ofcomparable output

Light-emitting diodes are used in applications as diverse as replacements for aviationlighting, automotive lighting (in particular brake lamps, turn signals, and indicators) as well

as in traffic signals LEDs have allowed new text, video displays, and sensors to bedeveloped, while their high switching rates are also useful in advanced communicationstechnology Infrared LEDs are also used in the remote control units of many commercialproducts including televisions, DVD players, and other domestic appliances

A seven-segment display (SSD), or seven-segment indicator, is a form of electronic

display device for displaying decim al numerals that is an alternative to the more complex

dot-matrix displays Seven-segment displays are widely used in digital clocks, electronicmeters, and other electronic devices for displaying numerical information

A seven segment display, as its name indicates, is composed of seven elements.Individually on or off, they can be combined to produce simplified representations of theArabic numerals Often the seven segments are arranged in an oblique (slanted) arrangement,

which aids readability In most applications, the seven segments are of nearly uniform shapeand size (usually elongated hexagons, though trapezoids and rectangles can also be used),though in the case of adding machines, the vertical segments are longer and more oddlyshaped at the ends in an effort to further enhance readability

In a simple LED package, typically all of the cathodes (negative terminals) or all of theanodes (positive terminals) of the segment LEDs are connected and brought out to acommon pin; this is referred to as a "common cathode" or "common anode" device Hence a

7-segment plus decimal point package will only require nine pins (though commercialproducts typically contain more pins, and/or spaces where pins would go, in order to matchindustry standard pinouts)

Integrated displays also exist, with single or multiple digits Some of these integrateddisplays incorporate their own internal decoder, though most do not – each individual LED isbrought out to a connecting pin as described Multiple-digit LED displays as used in pocketcalculators and similar devices used multiplexed displays to reduce the number of IC pinsrequired to control the display For example, all the anodes of the A segments of each digitposition would be connected together and to a driver pin, while the cathodes of all segmentsfor each digit would be connected To operate any particular segment of any digit, thecontrolling integrated circuit would turn on the cathode driver for the selected digit, and theanode drivers for the desired segments; then after a short blanking interval the next digitwould be selected and new segments lit, in a sequential fashion Often in pocket calculators

odd results on the multiplexed display

By scanning across rows, quickly flashing the LEDs on and off, it is possible to createcharacters or pictures to display information to the user By varying the pulse rate per LED,the display can approximate levels of brightness Multi-colored LEDs or RGBcolored LEDspermit use as a full-color image display The refresh rate is typically fast enough to preventthe human eye from detecting the flicker

A dot matrix display is a display device used to display information on machines, clocks,railway departure indicators and many other devices requiring a simple display device oflimited resolution The display consists of a matrix of lights or mechanical indicatorsarranged in a rectangular configuration (other shapes are also possible, although notcommon) such that by switching on or off selected lights, text or graphics can be displayed

A dot matrix controller converts instructions from a processor into signals which turns on oroff lights in the matrix so that the required display is produced

BIPOLAR JUNCTION TRANSISTOR (BJT)

A bipolar junction transistor (BJT) is a three-terminal electronic device constructed ofdoped semiconductor material and may be used in amplifying or switching applications

NPN TYPE NPN is one of the two types of bipolar transistors, consisting of a layer of P-dopedsemiconductor (the "base") between two N-doped layers A small current entering the base isamplified to produce a large collector and emitter current That is, an NPN transistor is "on"when its base is pulled high relative to the emitter

Most of the NPN current is carried by electrons, moving from emitter to collector asminority carriers in the P-type base region Most bipolar transistors used today are NPN,because electron mobility is higher than hole mobility in semiconductors, allowing greater

NPN transistor is not pointing in, based on the arrows in the symbol and the letters in thename That is, the NPN transistor is the BJT transistor that is "not pointing in"

PNP TYPE The other type of BJT is the PNP, consisting of a layer of N-doped semiconductorbetween two layers of P-doped material A small current leaving the base is amplified in thecollector output That is, a PNP transistor is "on" when its base is pulled low relative to theemitter

The arrows in the NPN and PNP transistor symbols are on the emitter legs and point inthe direction of the conventional current flow when the device is in forward active mode

A mnemonic device for the remembering the symbol for a PNP transistor is pointing in(proudly), based on the arrows in the symbol and the letters in the name That is, the PNPtransistor is the BJT transistor that is "pointing in"

P-N-P Transistor

CHAPTER 4: MICROCONTROLLER

INTRODUCTION When we have to learn about a new computer we have to familiarize about the machinecapability we are using, and we can do it by studying the internal hardware design (devicesarchitecture), and also to know about the size, number and the size of the registers

A microcontroller is a single chip that contains the processor (the CPU), non-volatilememory for the program (ROM or flash), volatile memory for input and output (RAM), aclock and an I/O control unit Also called a "computer on a chip," billions of microcontrollerunits (MCUs) are embedded each year in a myriad of products from toys to appliances toautomobiles For example, a single vehicle can use 70 or more microcontrollers

The Intel MCS-51 (commonly referred to as 8051) is a Harvard architecture, single chipmicrocontroller (µC) series which was developed by Intel in 1980 for use in embeddedsystems Intel's original versions were popular in the 1980s and early 1990s While Intel no

longer manufactures the MCS-51, binary compatible derivatives remain popular today Inaddition to these physical devices, several companies also offer MCS-51 derivatives as IPcores for use in FPGAs or ASICs designs

Intel's original MCS-51 family was developed using NMOS technology, but laterversions, identified by a letter C in their name (e.g., 80C51) used CMOS technology andconsumed less power than their NMOS predecessors This made them more suitable forbattery-powered devices

IMPORTANT FEATURES The 8051 architecture provides many functions (CPU, RAM, ROM, I/O, interrupt logic,

timer, etc.) in a single package

• 8-bit ALU, Accumulator and 8-bit Registers; hence it is an 8-bit microcontroller

• 8-bit data bus – It can access 8 bits of data in one operation

• On-chip RAM – 128 bytes (data memory)

• On-chip ROM – 4 kByte (program memory)

• Four byte bi-directional input/output port

• Two 16-bit Counter/timers

• Two-level interrupt priority

• Power saving mode (on some derivatives)

functioning In this project we are using external A.C supply (220 v) as input, this highvoltage is converted into 12 Volts A.C by step down transformer, then we use voltageregulators and filters with bridge rectifier to convert the A.C into D.C voltage For voltageregulation we are using LM 7805 and 7812 to produce ripple free 5 and 12 volts D.Cconstant supply

MCS-51 based microcontrollers typically include one or two UARTs, two or three timers,

128 or 256 bytes of internal data RAM (16 bytes of which are bit-addressable), up to 128bytes of I/O, 512 bytes to 64 kB of internal program memory, and sometimes a quantity ofextended data RAM (ERAM) located in the external data space The original 8051 core ran

at 12 clock cycles per machine cycle, with most instructions executing in one or twomachine cycles With a 12 MHz clock frequency, the 8051 could thus execute 1 million one-cycle instructions per second or 500,000 two-cycle instructions per second Enhanced 8051cores are now commonly used which run at six, four, two, or even one clock per machinecycle, and have clock frequencies of up to 100 MHz, and are thus capable of an even greaternumber of instructions per second

Features of the modern 8051 include built-in reset timers with brown-out detection,onchip oscillators, self-programmable Flash ROM program memory, built-in external RAM,extra internal program storage, SPI, and USB host interfaces, CAN or LIN bus, PWM

generators, analog comparators, A/D and D/A converters, RTCs, extra counters and timers,more interrupt sources, and extra power saving modes

POWER SUPPLY CIRCUIT There are two things worth attention concerning the microcontroller power supply circuit: Brown out is a potentially dangerous state which occurs at the moment themicrocontroller is being turned off or when power supply voltage drops to the lowest leveldue to electric noise As the microcontroller consists of several circuits which have different

operating voltage levels, this can because it’s out of control performance In order to prevent

it, the microcontroller usually has a circuit for brown out reset built-in This circuitimmediately resets the whole electronics when the voltage level drops below the lower limit Reset pin is usually referred to as Master Clear Reset (MCLR) and serves for externalreset of the microcontroller by applying logic zero (0) or one (1) depending on the type ofthe microcontroller In case the brown out is not built in the microcontroller, a simpleexternal circuit for brown out reset can be connected to this pin

HOW TO START WORKING?

A microcontroller is a good-natured “genie in the bottle” and no extra knowledge isrequired to use it

In order to create a device controlled by the microcontroller, it is necessary to provide thesimplest PC, program for compiling and simple device to transfer the code from PC to thechip itself Even though the whole process is quite logical, there are often some queries, notbecause it is complicated, but for numerous variations Let’s take a look

In order to write a program for the microcontroller, a specialized program in theWindows environment may be used It may, but it does not have to When using such asoftware, there are numerous tools which facilitate the operation (simulator tool comes first),which is an obvious advantage But there is also another ways to write a program Basically,text is the only thing that matters Any program for text processing can be used for thispurpose The point is to write all instructions in such an order they should be executed by themicrocontroller, observe the rules of assembly language and write instructions exactly asthey are defined In other words, you just have to follow the program idea That’s all!

To enable the compiler to operate successfully, it is necessary that a document containingthis program has the extension, .asm in its name, for example: Program asm When aspecialized program (mplab) is used, this extension will be automatically added If any otherprogram for text processing (Notepad) is used then the document should be saved andrenamed For example: Program.txt -> Program.asm This procedure is not necessarilyperformed The document may be saved in original format while its text may be copied tothe programmer for further use

Compiling a program

The microcontroller “cannot understand” the assembly language That is why it isnecessary to compile the program into machine language It is more than simple when aspecialized program (mplab) is used because a compiler is a part of the software Just oneclick on the appropriate icon solves the problem and a new document with hex extensionappears It is actually the same program, only compiled into machine language which themicrocontroller perfectly understands Such documentation is commonly named “hex code”and seemingly represents a meaningless sequence of numbers in hexadecimal numbersystem

In the event that other software for program writing in assembly language is used, specialsoftware for compiling the program must be installed and used as follows - set up thecompiler, open the document with asm extension and compile The result is the same- a newdocument with extension hex The only problem now is that it is stored in your PC

Programming a microcontroller

In order to transfer a “hex code” to the microcontroller, it is necessary to provide a cablefor serial communication and a special device, called programmer, with software There areseveral ways to do it

A large number of programs and electronic circuits having this purpose can be found onthe Internet Do as follows: open hex code document, set a few parameters and click the iconfor compiling After a while, a sequence of zeros and ones will be programmed into themicrocontroller through the serial connection cable and programmer hardware What's left is

to place the programmed chip into the target device In the event that it is necessary to makesome changes in the program, the previous procedure may be repeated an unlimited number

of times

Development systems

A device which in the testing program phase can simulate any environment is called adevelopment system Apart from the programmer, the power supply unit and themicrocontroller’s socket, the development system contains elements for input pin activationand output pin monitoring The simplest version has every pin connected to one push buttonand one LED as well A high quality version has LED displays, LCD displays, temperaturesensors and all other elements which can be supplied with the target device Theseperipherals can be connected to the MCU via miniature jumpers In this way, the wholeprogram may be tested in practice during its development stage, because the microcontrollerdoesn't know or care whether its input is activated by a push button or a sensor built in a realdevice

PART 2: MECHANICAL ACTUATION SYSTEMS

CHAPTER 1: INTRODUCTION

MECHANISMS Mechanisms are devices which can be considered to be motion converters in that theytransform motion from one form to some other required form They might, for example,transform linear motion into rotational motion, or motion in one direction into a motion in adirection at right angles, or perhaps a linear reciprocating motion into rotary motion, as in theinternal combustion engine where the reciprocating motion of the pistons is converted intorotation of the crank and hence the drive shaft

Mechanical elements can include the use of linkages, cams, gears, rack-and-pinion,chains drives, belt drives, etc For example, the rack-and-pinion can be used to convertrotational motion to linear motion Parallel shaft gears might be used to reduce a shaft speed.Bevel gears might be used for the transmission of rotary motion through 900 A toothed belt

or chain drive might be used to transform rotary motion about one axis to motion aboutanother Cams and linkages can be used to obtain motions which are prescribed to vary in aparticular manner

Many of actions which previously were obtained by the use of mechanisms are, however,often nowadays being obtained, as a result of a mechatronics approach, by the use of

domestic washing machines in order to give a timed sequence of actions such as opening avalve to let water into the drum, switching the water off, switching a heater on, etc Modernwashing machines use a microprocessor-based system with the microprocessor programmed

to switch on outputs in the required sequence

While electronics might now be used often for many functions that previously werefulfilled by mechanisms, mechanisms might still be used to provide such functions as:

1 Force amplification, e.g that given by levers

2 Change of speed, e.g that given by gears

3 Transfer of rotation about one axis to rotation about another, e.g a timing belt

4 Particular types of motion, e.g that given by a quick-return mechanisms

TYPES OF MOTION The motion of any rigid body can be considered to be a combination of translational androtational motions By considering the three dimensions of space, a translation motion can beconsidered to be a movement which can be resolved into components along one or more ofthe three axes A rotational motion can be considered as a rotation which has componentsrotating about one or more of the axes A complex motion may be a combination oftranslational and rotational motions For example, think of the motion which is required foryou to pick up a pencil from a table This might involve your hand moving at a particularangle towards the table, rotation of the hand, and then all the movement associated withopening your fingers and moving them to complex motions

DEGREE OF FREEDOM

An important aspect in the design of mechanical elements is the orientation andarrangement of the elements and parts A body that is free in space can move in three,independent, mutually perpendicular directions and rotate in three ways about thosedirections It is said to have six degrees of freedom (DOF) The number of degrees offreedom is the number of components of motion that are required in order to generate themotion

The problem is design is often to reduce the number of degrees of freedom and this thenrequires an appropriate number and orientation of constraints Without any constraints abody would have six degrees of freedom A constraint is needed for each degree of freedomthat is to be prevented from occurring Provided we have no redundant constraints then the

redundant constraints often occur and so for constraints on a single rigid body we have thebasic rule

6 – number of constraints = number of degrees of freedom – number of redundancies Thus if a body is required to be fixed, i.e have zero degrees of freedom, then if noredundant constraints are introduced the number of constraints required is 6

A concept that is used in design is that of the principle of least constraint This states that

in fixing a body or guiding it to a particular type of motion, the minimum number ofconstraints should be used, i.e there should be no redundancies This is often referred to askinematic design

profile determining how quickly the cam follower will fall The dwell section of the cam isthe part that allows the followers to remain at the same level for a significant period of time.The dwell section of the cam is where it is circular with a radius that does not change The cam shape required to produce a particular motion of the follower will depend on theshape of the cam and the type of follower used The radial distance from the axis of rotation

of the cam to the point of contact of the cam with the follower gives the displacement of thefollower with reference to the axis of rotation of the cam

ECCENTRIC CAM The eccentric cam is a circular cam with an offset centre of rotation It produces anoscillation of the follower which is simple harmonic motion and is often used with pumps.The diagrams (1 to 7) which are seen below show the cam rotating in an anticlockwisedirection As it rotates it pushes the flat follower upwards and then allows it to drop

A mechanical toy based on a series of eccentric cams is seen below As the handle is

turned, the shaft and the cams fixed to it rotate Placed above the cams are a number ofsegments representing a “snake” As the cams rotate some of the flat followers are pushedupwards whilst others drop down This gives the impression that the snake is moving

DROP CAM Eccentric cams generally allow for a slow rise and fall of the follower However, a snaildrop cam is used where the drop or fall of the follower must be sudden

The example snail/drop cam shown opposite rotates in an anticlockwise direction Rotating

in a clockwise direction would probably lead to the entire mechanism jamming Thishighlights one possible disadvantage of using this type of cam profile Also, to ensure therotation is smooth, the vertical centre line of the snail/drop cam is positioned slightly to theleft of the slide (see diagram)