ME 1402 – MECHATRONICS (UNIT – II)ACTUATORS

ACTUATION SYSTEM:

The actuation systems are the elements of the control system

and they are responsible for transforming the output of a

microprocessor into a controlling action on a machine or device.

Actuators produce physical changes such as linear and angular

displacement.

There are four types of actuators.

1. Mechanical actuators.

2. Electrical actuators.

3. Hydraulic actuators.

4. Pneumatic actuators.

Example:

In a CNC milling machine, there may be an electrical signal

output from the CNC controller to move the milling table in the x

direction for a certain length. There you need an actuation system.

PNEUMATIC AND HYDRAULIC SYSTEMS:

Power from one point to another point can also be transmitted

using air as medium called pneumatic transmission or liquid as

medium called hydraulic transmission. In case of hydraulic system,

liquid, which may be water or hydraulic oil is pressurized to 20 to 250

atm pressures and transmitted through pipe line. The pressurized

liquid is made to actuate rotary or linear actuator through control

valves to get required function. Hydraulic system of power

transmission is preferred over mechanical or electrical system on the

following grounds.

1

1. Compact size.

2. Less moving parts.

3. Less wear and tear & self lubricating.

4. Controlled motion.

5. Adaptability for automatic control.

However, the initial cost of hydraulic transmission will be high.

Improperly filled hydraulic system will give maintenance problem and

cost of spares will be high. Some of the applications of hydraulic

system are hydraulic presses, fork lifts, hydraulic jacks and hydraulic

shaper etc.

In hydraulic actuation system, the hydraulic signals are used to

control device but are more expensive than pneumatic system. Oil leak

is another problem in hydraulic system.

Basic components of hydraulic system are

1. Reservoir to hold oil,

2. Hydraulic pump normally positive displacement type,

3. Electric motor to drive the pump,

4. Actuator, which may be rotary or linear,

5. Control valves for controlling flow, direction and pressure, and

6. Pipe lines and fittings to transmit oil power.

In pneumatic control system, the moister should be separated, to

avoid presence of free moisture during expansion. Besides, this

moisture will pose problems in line especially in pilot operator solenoid

valves. Pneumatic system is fast comparable to hydraulic system. But

positioning and speed control is difficult because of compressibility of

air.

In the pneumatic actuation system pneumatic signals are used to

control the system. The pneumatic signals can be used to actuate 2

large valves and other high power control device and so it can be used

to move heavy loads. Pneumatic system consists of a compressor,

control valves and actuators. Since air is used as medium, reservoir is

not required.

Power supplies:

Hydraulic power supply:

Hydraulic systems are design to move large loads by controlling a

high pressure fluid in distribution lines and piston with mechanical or

electromechanical valves.

The basic components of a hydraulic system are,

In a hydraulic system, pressurized oil is provided by a hydraulic

pump driven by an electric motor.

The hydraulic pump pumps the oil from a sump through a non-

return valve and an accumulator to the system.

A pressure relief valve is circulated to release the pressure when

it rises above the safe level.

The non return valve is to prevent the oil returning back to the

pump.

The accumulator is a reservoir in which the oil is held under

pressure.

The accumulator is used to store the oil and provides a smooth

drive during any short term fluctuation in the output oil pressure.

3

Fig. Hydraulic Power Supply

Pneumatic System:

The basic components of a pneumatic system are,

In a pneumatic power supply an electric motor drives an air

compressor.

Before the air enters the compressor, it passes through a filter

and a silencer.

In the filter all the dust particles present in the inlet air is

removed.

Fig. Pneumatic power supply

4

In the silencer the noise level is reduced.

A pressure relief valve is provided to protect the system in case

of pressure rises above the safe level.

Since the air compressor increases the temperature of the air, a

cooler is provided to reduce the temperature of air.

In the filter and water trap, the water from the air and other

unwanted particles in air are removed.

An air receiver increases the volume of air in the system and

smoothens out any short term pressure fluctuation.

DIRECTION CONTROL VALVES:

The direction control valves are used in the pneumatic and

hydraulic system to direct the flow of liquid through a system. They are

used for varying the rate of flow of liquid. They are either completely

open or closed.

There are two types of direction control valves. They are.

1. Spool valve.

2. Poppet valve.

Spool Valve:

A spool moves horizontally within the valve body to control the

flow.

In fig a, the air supply is connected to port 1. The port 3 is closed.

5

The device is connected to port 2, and device is pressurized.

In fig. b when the spool is moved to the left, the air supply is cut

off.

Port 2 and port 3 are connected.

So the air in the system connected to port 2 is allowed to go out

to the atmosphere through port 3.

In fig. a air is allowed to flow into the system.

In fig. the air is allowed to flow out of the system.

Poppet Valve:

This valve is normally in the closed

condition.

The port 1 is connected to pressure

supply.

The Port 2 is connected to the system.

Initially there is no connection between

port 1 and port 2.

Here balls, discs, or cones are used as a

valve to be seated in the valve seat to

control the flow.

Here a ball is used as shown in fig.

When the push button is depressed, the ball is pushed out of its

seat.

This allows the flow from port 1 connected to port 2.

When the button is released, the spring forces the ball back to its

seat and so closes off the flow.

6

Valve Symbols

The symbol used for control valves consists of a square for each

of its switching positions.

A two position valve will have two squares; a three position valve

will have three squares.

The arrow headed lines are used to indicate the direction of flow

in each of the position.

The blocked-off lines indicate the flow is closed.

In the fig the valve has four ports.

The ports are labeled by a number or a letter according to their

function.

The ports are labeled 1 (or P) for pressure supply.

The ports are labeled 3 (or T) for hydraulic return port, 3 or 5 (or

R or S) for pneumatic exhaust port and 2 or 5 (or B or A) for

output ports.

7

Example

The following are some of the illustrations of how these various

symbols can be combined to describe how a valve operates. The Fig.

is a 2/2 valve, because it has 2 ports and 2 positions. The first number

(numerator) indicates the number of ports. The second number

(denominator) indicates the number of .positions. The valve symbol in

Fig. is 2/2, solenoid operated, push button valve.

Fig. 2/2 Valve

The valve symbol in Fig. , is 3/2 because it has 3 ports and 2

positions.

Fig. 3/2 Valve

The valve symbol in Fig. is 4/2 valve because it has 4 port and 2

positions.

Fig. 4/2 Valve

8

The following is an example for the application of valves in a

pneumatic lift system.

The push button 2/2 valves are used. When the up valve is pressed

the load is lifted. When the bottom valve is pressed the load is

lowered. An open arrow is used to indicate a vent to the

atmosphere.

Pilot Operated Valve:

The force required to move the ball or shuttle in a valve can

often be too large for manual or solenoid operation. To overcome this

problem a pilot operated system is used. Where one valve is used to

control second valve. Figure illustrates this. The pilot valve is small

capacity and can be operated manually or by a solenoid. It is used to

allow the main valve to be operated by the system pressure. The pilot

pressure line is indicated by dashes. The pilot and main valve can be

operated by two separate valves but they are often combined in a

single housing.

9

Direction Valves

Figure shows a simple direction control valve and its symbol.

Free flow can occur in one direction through the valve, that which

results in the ball being pressed against the spring. Flow in the other

direction is blocked by the spring forcing the ball against its seat.

PRESSURE CONTROL VALVE:

There are three types of pressure control valve.

1. Pressure regulating valves.

2. Pressure limiting valves.

3. Pressure sequence valves.

1. Pressure regulating valves:

This is used to control the operating pressure in a circuit and

maintain it at a constant value. The compressed air produced by the

compressor may fluctuate. Changes in the pressure may affect. The

switching characteristics of the cylinder the running times of the

cylinders. The timing characteristics of flow control valve. Thus the

constant pressure level is required for the trouble free operation of a

pneumatic control. A pressure regulator is fitted downstream of the

compressed air filter. It provides a constant set pressure at the outlet

of the regulator. The pressure regulator is also called as pressure

reducing valve or pressure regulating valve. There are two types of

pressure regulators. They are

10

1. Diaphragm type pressure regulator (with or without vent holes).

2. Piston spool type pressure regulator.

1. Diaphragm type pressure regulator:

Two types of pressure regulators with diaphragm are available.

(a)With vent holes and

(b) Without vent holes.

(a)Diaphragm type pressure regulator (with vent holes):

A diaphragm type pressure regulator is shown in figure (a) &

(b).In this type pressure is regulated by a diaphragm. The output

pressure acts on one side of the diaphragm. On the other side of the

diaphragm, a spring (set spring) force acts. The spring force can be

adjusted by an adjusting screw provided at the bottom of the

regulator.

When the pressure output increases:

The diaphragm moves against the spring force. Due to this, the

outlet area of cross-section at the valve seat reduces or closes

entirely. Thus the quantity of air flowing is regulated.

When the air drawn off on the outlet side:

The operation pressure drops. The spring force opens the valve.

Thus, the continual opening and closing of the valve seat

regulates the preset output pressure. a damper spring is provided

above the valve disc to avoid fluttering. A pressure gauge is fitted to

the outlet of the regulator for monitoring and setting of the circuit

pressure.

11

If the pressure on the outlet side increases considerably:

The diaphragm is pushed down against the spring force. The

center piece of the diaphragm opens. The compressed air flows to the

atmosphere through the vent holes in the housing.

(b) Diaphragm type pressure regulator (without vent holes):

A diaphragm pressure gauge without vent holes is shown in the fig.

With these valves, it is not possible to exhaust the compressed air.

The spring is pre-stressed by means of adjusting screw. Thus the

diaphragm is also pre-stressed. The plunger is raised with the

diaphragm to a greater or lesser extent from the seat. Therefore, the

flow from the primary to the secondary side increases or decreases

depending on the setting of the spring. If no air is drawn off on the

outlet side, the diaphragm moves down against the compression

spring. The damper spring moves the plunger downward to its seat.

Thus the flow of air is closed off at the sealing seat. The compressed

12

air can continue to flow only when the air is drawn off on the outlet

side.

Piston - spool type pressure regulator:

A piston type pressure regulator is shown in fig.

The valve is of piston type. It is kept on its seat by a spring force.

The spring force can be adjusted screw provided at the bottom. In the

normal piston, the valve is open and the compressed. Air freely flows

from inlet A and outlet B. The valve spool is kept in equilibrium by the

spring force on one side and air pressure on the other side through

secondary circuit. When the pressure in the secondary side rises, the

pressure on the spool face increases. The spool moves and partly

closes the outlet side. This reduces the volume of air going to the

secondary side and hence pressure is reduced. If the pressure in the

secondary side increases considerably, the outlet port is completely

closed. The flow is completely closed.

13

2. Pressure limiting valves:

These are used as safety devices to limit the pressure in a circuit

to below some safe value. The valve opens and vents to the

atmosphere or back to the sump, if the pressure rises above the set

safe value. A simple pressure relief valve is shown in the fig. it consists

of conical poppet valve, spring, adjusting screw. The force exerted by

the spring on the poppet can be varied by the pressure adjusting

screw.

Under normal conditions:

The spring presses the conical poppet valve in its seat. The oil flow

path is closed.

When the system pressure exceeds the set value:

The increased pressure presses the poppet against the spring

force. Oil flow through the exhaust port T to the reservoir. Thus the

excessive pressure is released. When the pressure drops below the

set value, the poppet again closes.

14

3. Pressure Sequence Valves:

The sequence valve helps two or more cylinders to work in a

particular sequence. It makes sure that the operation of one cylinder is

completed before the start of the operation of another cylinder. For

example, consider two hydraulic cylinders which operate in sequence.

The sequences of operations to be performed are

(i) Lifting the weight up to the floor level by the first cylinder

(ii) Pushing the weight into the floor by the Second Cylinder

The sequence valve is connected in the hydraulic circuit as shown

in the fig.

DCV is shifted in one extreme position

The fluid from the pump enters into the inlet of the sequence valve

and comes out and enters into the first cylinder through a check valve

and causes the piston to rise up.

Now the load is lifted up to the floor level. During this operation fluid

on its top is going back to the reservoir. After lifting the load, the piston

comes to rest. Thus the first operation is over. As soon as the piston

has come to rest, the oil does not find any passage for its flow.

15

Thus the pressure in the sequence valve increases. The increase in

pressure lifts the valve piston and the oil is now entering to the second

cylinder. The piston of the second cylinder pushes the load into the

floor.

During this operation the fluid on the left side is discharged to the

reservoir. Thus the secondary operation is completed.

DCV is shifted in another extreme position

Now the outlet port in sequence valve is closed as the piston of the

sequence valve move down. The fluid now entering into the second

cylinder causes the piston to move from the left to right while the fluid

on the other side is connected to the reservoir through the check valve

After the second cylinder piston has come to rest, the pump supply

enters into the top of the first cylinder. The piston in the first cylinder

lower down while fluid at its bottom is flowing to the reservoir through

the check valve and sequence valve. The pressure setting of the

sequence valve is adjusted by adjustment screw.

CYLINDERS

The hydraulic or pneumatic cylinder is an example of a linear

actuator. The principles for both hydraulic and pneumatic versions are

the same. Only difference is big size cylinder are used in hydraulic due

to high pressure.

Construction

A cylinder consists of a cylindrical type along which a piston/ ram can

slide. There are two types of cylinder. They are.

1. Single acting cylinder.

2. Double acting cylinder.

16

1. Single acting cylinder:

The simple level of control for the single acting cylinder involves

direct control signals. Direct control is used when. The flow rate

required to operate the cylinder is relatively small. The size of the

control valve is small with low actuating forces. The circuit for the direct

control of single acting cylinder is shown in the fig.

A 3/2 directional control valve is used. The cylinder is of small

capacity and the air consumption is low. Hence the operation can be

directly controlled by a push button 3/2 direction control valve with

spring return. When the push button is pressed, the air passes through

the valve from pressure port (P) the cylinder port (A). The piston rod

extends against the force of the cylinder return spring. Thus the work

piece is clamped. When the push button is released, the valve spring

returns the 3/2 D.C.V. to its initial position. The cylinder retracts by the

return spring force. The air form the cylinder returns through the

exhaust port (R).Cylinder is the only working element or actuator.

17

2. Double acting cylinder:

They are used when the control pressures are applied to each side

of the piston.

A difference in pressure between the two sides, result in motion

of the piston. The circuit includes a double acting cylinder with 5/2

D.C.V.

When the spool of the DCV is at extreme left:

Air flows from pressure port (P) to the working port (B).Then air is

allowed to enter the right end of the cylinder. The piston moves from

right to left. At the same time air in the left end of the cylinder flows into

the valve through port 'A' and exhausted through exhaust port 'R'. The

other exhaust port 'S' is blocked.

When the spool of DCV is at extreme right:

Air flows from inlet pressure port (P) to the working port (A). Air is

allowed to enter the left end of the cylinder. The piston moves from left

to right .At the same time the air in the right end of the cylinder flows

into the valve through the exhaust port 'S'. The other exhaust port 'R' is

blocked. This is the principle of working of the double acting cylinder.

18

CYLINDER SEQUENCING:

Many control system employ pneumatic (or) hydraulic cylinders as

actually elements and require a sequence of extensions and retraction

of the cylinder to occur.

For example,

There are two cylinders A & B. When the start button is pressed,

the piston of cylinder A extends. When it is fully extended, the piston of

cylinder B extends. The sequence of operation of these two cylinders

is explained.

Each cylinder is given a reference letter A & B. To indicate the

extension of the piston in cylinder. A, a+ sign is used. To indicate the

retraction of the piston in cylinder A a - sign is used. Similarly for

cylinder B, b+ (for extension) and b - (for retraction) are used.

19

Sequence of operation:

Initially both the cylinder has retracted pistons. Start push button on

valve 1 is pressed. This applies pressure to valve 2 as the limit switch

b – is activated. So the supply is given to valve 3, and from valve 3 the

air enters the left side of the piston in cylinder A. As the pressure built

up on the left side, the piston starts extending towards right side till the

limit switch a+ operates. Once limit switch a+ is operated, it gives a

supply to valve 5 and then causes pressure to valve 6. Then air enters

in the left side the piston in the cylinder B through valve 6.So the piston

moves towards right side, and then the piston actuates the limit switch

b+. Once limit switch b+ is actuated, it gives a supply to valve 4 and

then causes pressure to valve 3. So the air enters the cylinder A,

through the left end of the piston. It moves in the piston from left to

right through retraction and finally the switch a - is actuated. Once limit

switch a - is actuated, it gives a supply to valve 7, then gives a

pressure to valve 6. Now air enters the cylinder B, through the left end

of the piston and the piston moves from left to right end then it will

retract. This cycle can be started again by pushing the start button. If

we want to run the system continuously, then the last movement in the

sequence should be used to trigger the first movement.

PROCESS CONTROL VALVES:

Process control valves are used to control the rate of fluid flow. For

example it can be used to control the rate of flow of a liquid into a tank.

Principle:

The basis of such valve is an actuator used to move a plug into the

flow pipe and alter the cross section of the pipe. Therefore the liquid

flow through. The cross section can be increased or decreased. A 20

common type of pneumatic actuator used with the process control

valves is the diaphragm actuator. The diaphragm is made of rubber

which is sand winched in its centre between two circular steel disc, as

shown in the fig.

The effect of changes in the input pressure results in the movement

of the central part of the diaphragm. This movement is communicated

to the final control element by a shaft which is attached to the

diaphragm.

The force F acting on the shaft is the force that is acting on the

diaphragm and it is equal to the gauge pressure P multiplied by the

diagram area.

F= PA 1

Restoring force is provided by the spring. Assume shaft moves

through a distance x and the compression of the spring is proportional

to the force F,

(ie) F = kx. 2

Where k is a constant.

Therefore comparing equation (1) and (2)

kx = PA

Thus the displacement of the shaft is proportional to the gauge

pressure.21

The above fig. shows the cross-section of a valve for the control of

rate of flow of a fluid. The pressure change in the actuator causes the

diaphragm to move and results in the movement of the stem. When

the stem moves, it results in the movement of the inner valve plug

within the valve body. The plug restricts the fluid flow and the position

of the plug determines the flow rate.

Valve bodies

There are many types of valve bodies. They are

i) Single seated valve.

ii) Double seated value.

22

through the valve and so just one plug is needed to control the

flow. In double seated valve, the fluid after entering the valve splits into

two streams and this needs two plugs.

Valve plugs:

The shape of the valve plug determines the relationship between

the stem movement and the effect on the flow rate. There are three

commonly used types:

a) Quick opening type - A large change in flow rate occurs for a small

movement of the valve stem. Such a plug is used where on/off control

of flow rate is required.

Linear - Contoured type - The change in flow rate is proportional to

the change in displacement of the valve stem

Equal Percentage type - Equal percentage changes in flow rate

occur for equal changes in the valve stem position.

23

ROTARY ACTUATORS:

There are many methods to achieve the rotary motion. Three

types of Rotary activators are discussed below.

A linear cylinder to produce rotation.

As shown in the fig. 3.25 a linear cylinder with the help of suitable

mechanical linkages can be used to produce rotary movement through

angles less than 360°.

Semi - Rotary Actuator

It has two ports clockwise port and anticlockwise Port, and also

has a vane attached with rotor of the actuator. The pressure difference

24

between the two ports causes the vane to rotate. When the vane

rotates, the shaft attached to the vane also rotates. Therefore the shaft

rotation is a measure of the pressure difference between the two ports.

Depending on the pressures, the vane can be rotated clockwise (or)

anticlockwise.

Vane motor:

The vane motor is a pneumatic motor through which a rotation

angle greater than 360° can be achieved. It has an eccentric rotor with

slots in which vanes are forced outwards against the wall of the

cylinder by the rotation. The vane divides the chamber

In to separate compartments which increase in size from the inlet

port round to the outlet port. The air entering each compartment exerts

a force and the vane and causes the rotor rotates. The motor can be

made to rotate in both clockwise and anticlockwise directions.

25

MECHANICAL ACTUATION SYSTEMS

Mechanical systems

Mechanisms are devices which can be considered to be

motion converters in that they transform motion from one form to

other form. They might, for example, transform linear motion in

to rotational motion, or motion in one direction into a motion

direction at right angles, or perhaps a linear to into rotary motion,

as in the internal combustion the reciprocating motion of the

pistons is cons of the crank and hence the drive shaft. Mechanical

elements can include the use of gears, rack-and-pinion, chains,

belt drives; etc rack-and-pinion can be used to convert rotation

motion. Parallel shaft gears might be used to re Bevel gears might

be used for the transmission through 90°. A toothed belt or chain

drive might form rotary motion about one axis to motion at and

linkages can be used to obtain motions which are prescribed to

vary in a particular manner. This chapter is the basic characteristics

of a range of such mechanisms.

Many of the actions which previously were obtained by use

of mechanisms are, however, often nowadays the use of

microprocessor systems. For example rotating shafts were

previously used for machines in order to give a timed

sequence. Let us opening a valve to water into the drum,

switching a heater on/off, etc. Modern washing microprocessor-

based system with the programmed to switch on outputs in the

required Mechanisms still, however, have a role in me For example,

the mechatronic system in us camera for adjusting the aperture for

correct exposures involves a mechanism for adjusting the size of

26

the diaphragm. While electronics might now be used often for

many functions that previously were fulfilled 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.

3Transfer 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

mechanism.

The term kinematics is used for the study of motion without

regard to forces. When we consider just the motions without any

consideration of the forces or energy involved then we are carrying

out a kinematics analysis of the mechanism. This chapter is an

introduction to such a consideration.

Types of motion

A rigid body can have a very complex motion which might seem

difficult to describe. However, the motion of any rigid body can be

considered to be a combination of translational and rotational

motions. By considering the three dimensions of space, a

translation motion can be considered to be a movement which can

be resolved into components along one or more of the three axes

(Fig. (a)). A rotation can be considered as a rotation which has

components rotating about one or more of the axes (Fig. (b)).

27

A complex motion may be a combination of translational and

rotational motions. For example, think of the motion required for

you to pick up a pencil from a table. This might involve your hand

moving at a particular angle towards the table, rotation of the hand,

and then all the movement associated with opening your fingers

and moving them to the required positions to grasp the pencil. This

is a sequence of quite complex motions. However, we can break

down all, these motions into combinations of translational and

rotational motions. Such an analysis is particularly relevant if we

are not moving a human hand to pick up the pencil but instructing

a robot to carry out the task. Then it really is necessary to break

down the motion into combinations of translational and rotational

motions so that we can design mechanisms to carry out each of

these components of the motion.

For example, among the sequence of control signals sent to a

mechanism might be such groupings of signals as those to instruct

joint I to rotate by 20° and link 2 to be extended by 4 mm for

translational motion.28

Freedom and constraints

An important aspect in the design of mechanical elements is

the orientation and arrangement of the elements and parts. Body

that is free in space can move in three, independent, mutually

perpendicular directions and rotate in three ways about those

directions (Fig). It is said to have six degrees of freedom. The

number of degrees of freedom are the number of components of

motion that are required in order to generate the motion. If a joint is

constrained to move along a line then its translational degrees of

freedom are reduced to one.

Figure (a) shows a joint with just this one translational

degree of freedom. If a joint is constrained to move on a

plane then it has two translational degrees of freedom.

Fig. (b) Shows a joint which has one translational

degree of freedom and one rotational degree of freedom.

The problem in design is often to reduce the number of degrees

of freedom and this then requires an appropriate number and

orientation of constraints. Without any constraints a body would

have six degrees of freedom. A constraint is needed for each

degree of freedom that is to be prevented from occurring.

Provided we have no redundant constraints then the number of

degrees of freedom would be 6 minus the number of constraints. 29

However, redundant constraints often occur and so for constraints

on a single rigid body we have the basic 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 no redundant 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 of constraints should be used, i.e. there should

be no redundancies. This is often referred to as kinematics design.

For example, to have a shaft this only rotates about one axis with

no translational motions. We have to reduce the number of

degrees of freedom to 1. Thus the minimum number of

constraints to do this is 5. Any more constraints than this will

give redundancies. The mounting that might be used to mount the

shaft has a ball bearing at one end and a roller bearing at the

other (Fig.).

30

The pair of bearings together prevents translational

motion at right angles to the shaft. The y-axis and rotations

about the z-axis and the Y-axis. The ball bearing prevents

translational motion along the x-axis and along the z-axis.

Thus there is a total of five constraints. This leaves just one

degree of freedom, the required rotation about the x-axis. If there

had been a roller bearing at each end of the shaft then both the

bearings could have prevented translational motion along the x-

axis and the z-axis and thus there would have been redundancy.

Such redundancy might cause damage. If ball bearings are used at

both ends of the shaft, then in order to prevent redundancy one of

the bearings would have its outer race not fixed in its housing so

that it could slide to some extent in an axial direction.

Loading

Mechanisms are structures and as such transmit and

support loads. Analysis is thus necessary to determine the

loads to be carried by individual elements. Then consideration

can be given to the dimensions of the element so that it might, for

example, have sufficient strength and perhaps stiffness under such

loading.

Kinematics chains

When we consider the movements of a mechanism

without any reference to the forces involved, we can treat the

mechanism as being composed of a series of individual links.

Each part of a mechanism which has motion relative to some

other part is termed a link. A link need not necessarily be a rigid

body but it must be a resistant body which is capable of

31

transmitting the required force with negligible deformation. For

this reason it is usually taken as being represented by a rigid body

which has two or more points of attachment to other links, these

being termed nodes. Each link is capable of moving relative to its

neighboring links. Figure shows examples of links with two, three

and four nodes.

A joint is a connection between two or more links at their

nodes and which allows some motion between the connected

links. Levers, cranks, connecting rods and pistons, sliders,

pulleys, belts and shafts are all examples of links.

A sequence of joints and links is known as a kinematics chain.

For a kinematics chain to transmit motion, one link must be fixed.

Movement of one link will then produce predictable relative

movements of the others. It is possible to obtain from one

kinematics chain a number of different mechanisms by having a

different link as the fixed one.

32

As an illustration of a kinematics chain, consider a

motor car engine where the reciprocating motion of a piston is

transformed into rotational motion of a crankshaft on bearings

mounted in a fixed frame (Fig. (a)). We can represent this as

being four connected links (Fig. (b)). Link I is the crankshaft, link 2

the connecting rod, link 3 the fixed frame and link 4 the slider, i.e.

piston, which moves

Relative to the fixed frame The designs of many mechanisms

are based on two basic forms of kinematics chains, the four-bar

chain and the slider-crank chain. The following illustrates some

of the forms such chains can take.

The four-bar chain

The four-bar chain consists of four links connected to give four

joints about which turning can occur. Figure shows a number of

forms of the four-bar chain produced by altering the relative

lengths of the links. If the sum of the length of the shortest link

plus the length of the longest link is less than or equal to the sum of

the lengths of the other two links then at least one link will be

33

capable of making a full revolution with respect to the fixed link. If

this condition is not met then no link is capable of a complete

revolution. This is known as the Grashof condition. in Fig. (a), link 3

is

fixed and the relative lengths of the links are such that links l

and 4 can oscillate but not rotate. The result is a double-

lever mechanism. By shortening link 4 relative to link 1, then link 4

can rotate (Fig. (b)) with link 1 oscillating and the result is termed a

lever-crank mechanism. With links 1 and 4 the same length and

both able to rotate (Fig. (c)), then the result is a double-crank

mechanism. By altering which link is fixed, other forms of mechanism

can be produced.

34

The above Figure illustrates how such a mechanism can be

used to advance the film in a cine camera. As link 1 rotates so the

end of link 2 locks into a sprocket of the film pulls it forward

before releasing and moving up and back to lock into the next

sprocket.

Some linkages may have toggle positions. These are

positions where the linkage will not react to any input from one of its

links. Figure illustrates such a toggle,

being the linkage used to control the movement of the

tailgate of a truck so that when link 2 reaches the horizontal

position no further load on link 2 will cause any further

movement. There is another toggle position for the linkage and

that is when links 3 and 4 are both vertical and the tailgate is

vertical.

The slider-crank mechanism

This form of mechanism consists of a crank, a connecting rod

and a slider and is the type of mechanism described in Fig. which

35

showed the simple engine mechanism. With that configuration,

link 3 is fixed, i.e. there is no relative movement between the

centre of rotation of the crank and the housing in which the

piston slides. Link 1 is the crank that rotates, link 2 the

connecting rod and link 4 the slider which moves relative to the

fixed link. When the piston moves backwards and forwards, i.e.

link 4 moves backwards and forwards, then the crank, link 1, is

forced to rotate. Hence the mechanism transforms an input of

backwards and forwards motion into rotational motion.

The above Figure. Shows another form of this type of

mechanism, a quick-return mechanism. It consists of a rotating

crank, link AB, which rotates round a fixed centre, an oscillating

lever CD, which is caused to oscillate about C by the sliding of

the block at B along CD as AB rotates, and a link DE which

causes E to move backwards and forwards. E might be the ram

of a machine and have a cutting tool attached to it. The ram will be

at the extremes of its movement when the positions of the crank are

AB, and AB2. Thus as the crank moves anti-clockwise from B, to

B2 the ram makes a complete stroke, the cutting stroke. When

the crank continues its movement from B2 anti-clockwise to B, then 36

the ram again makes a complete stroke in the opposite

direction, the return stroke. With the crank rotating at constant

speed, then, because the angle of crank rotation required for the

cutting stroke is greater than the angle for the return stroke, the

cutting stroke takes more time than the return stroke. Hence the

term, quick-return for the mechanism.

Cam

A cam is a body which rotates or oscillates and in doing

so imparts a reciprocating or oscillatory motion to a second body,

called the follower, with which it is in contact (Fig.).

As the cam rotates so the follower is made to rise,

dwell and fall, the lengths of times spent at each of these positions

depending on the shape of the cam. The rise section of the cam

is the part that drives the follower upwards, its profile determining

how quickly the cam follower will be lifted. The fall section of the

cam is the part that lowers the follower, its profile determining how

quickly the cam follower will fall. The dwell section of the cam is the

part that allows the follower 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

37

required to produce a particular motion of the follower will

depend on the shape of the cam and the type of follower

used.

The above Figure. Shows the types of follower

displacement diagrams that can be produced with different

shaped cams and either point or knife followers. The radial

distance from the axis of rotation of the carp to the point of

contact of the carp with the follower gives the displacement of the

follower with reference to the axis of rotation of the cam. The

figures show how these radial distances and hence follower

displacements. Vary with the angle of rotation of the cams.

The eccentric cam Fig. (a) is a circular cam with an offset

centre of rotation. It produces an oscillation of the follower which is

38

simple harmonic motion and is often used with pumps. The

heart-shaped cam Fig.(b) gives a follower displacement which

increases at a constant rate with time before decreasing at a

constant rate with time, hence a uniform speed for the follower.

The pear-shaped cam Fig.(c)) gives a follower motion which is

stationary for about half a revolution of the cam and rises and

falls symmetrically in each of the remaining quarter revolutions.

Such a pear-shaped cam is used for engine valve control. The dwell

holds the valve open while the petrol/air mixture passes into

the cylinder. The longer the dwell, i.e. the greater the length of

the cam surface with a constant radius, the more time is allowed

for the cylinder completely charged with flammable vapors.

The above Figure shows a number of examples of different

types of cam followers. Roller followers are essentially ball or

roller bearings. They have the advantage of lower friction than a

sliding contact but can be more expensive. Flat-faced followers are

often used because they are cheaper and can be made

smaller than roller followers. Such followers are widely used with

39

engine valve cams. While cams can be run dry, they are often

used with lubrication and may be immersed in an oil bath.

Gears

Rotary motion can be transferred from one shaft to another

by a pair of rolling cylinders (Fig.) however; there is a possibility of

slip. The transfer of the motion between the two cylinders

depends on the frictional forces between the two surfaces in

contact. Slip can be prevented by the addition of meshing teeth to

the two cylinders and the result is then a pair of meshed gear

wheels. Gears can be used for the transmission of rotary motion

between parallel shafts Fig. (a) and

for shafts which have axes inclined to one another Fig.

(b). the term bevel gear is used when the lines of the shafts

intersect, as illustrated in Fig. (b). When two gears are in mesh,

the larger gear wheel is often called the spur or crown wheel

and the smaller one the pinion.

40

Gears for use with parallel shafts may have axial teeth with the

teeth cut along axial lines parallel to the axis of the shaft Fig. (a). Such

gears are then termed spur gears. Alternatively they may have helical

teeth with the teeth being cut on a helix Fig. (b) and are then termed

helical gears. Helical gears have the advantage that there is a gradual

engagement of any individual tooth and consequently there is a

smoother drive and generally prolonged life of the gears. however, the

inclination of the teeth to the axis of the shaft results in an axial force

component on the shaft bearing. This can be overcome by using

double helical teeth.(fig.)

Gear trains

The term gear train is used to describe a series of

intermeshed gear wheels. The term simple gear train is used

for a system where each shaft carries only one gear wheel, as in

Fig. For such a gear train, the overall gear ratio is the ratio of the

angular velocities at the input and output shafts and is thus

Consider a simple gear train consisting of wheels A, B and C,

as in Fig. with A having 9 teeth and C having 27 teeth. Then, as

the angular velocity of a wheel is inversely proportional to the

number of teeth on the wheel, the gear ratio is 27/9 = 3. The

effect of wheel B is purely to change the direction of rotation of the

output wheel compared with what it would have been with just

41

the two wheels A and C intermeshed. The intermediate wheel,

B, is termed the idler wheel.

We can rewrite this equation for the overall gear ratio G as

But is the gear ratio for the first pair of gears and the

gear ratio for the second pair of gears. Thus the overall gear ratio

for a simple gear train is the product of the gear ratios for each

successive pair of gears.

The term compound gear train is used to describe a gear train

when two wheels are mounted on a common shaft. Figure (a) and

(b) shows two examples of such a compound gear train. The gear

train in Fig. (b) Enables the input and output shafts to be in line.

42

When two gear wheels arc mounted on the same shaft they

have the same angular velocity. Thus, for both of the compound

gear trains in Fig. The overall gear ratio G is thus

For the arrangement shown in Fig. (b), for the input and

output shafts to be in line we must also have for the radii of the

gears:

Consider a compound gear train of the form shown

in Fig. (a). with A, the first driver, haying 1 5 teeth. B 10

teeth. C 18 teeth and D. the final driven wheel. 36 teeth.

Since the angular velocity of a wheel is inversely

proportional to the number of teeth on the wheel. The

overall gear ratio is

43

Thus, if the input to wheel A is an angular velocity of

160 rev/min, then the output angular velocity of wheel D is 160/4

=40 rev/min.

A simple gear train of spur, helical or bevel gears is usually

limited to an overall gear ratio of about 10. This is because of the

need to keep the gear train down to a manageable size if the

number of teeth

on the pinion is to be kept above a minimum number which is

usually about 10 to 20. Higher gear ratios can, however, be

obtained with compound gear trains. This is because the gear

ratio is the product of the individual gear ratios of parallel

gear sets.

Ratchet and Pawl

Ratchets can be used to lock a mechanism when it is

holding a load. Figure shows a ratchet and pawl. The

mechanism consists of a wheel, called a ratchet, with saw-shaped

teeth which engage with an arm called a pawl. The arm is

pivoted and can move back and forth to engage the wheel. The

shape of the teeth is such that rotation can occur in only one

direction. Rotation of the ratchet wheel in a clockwise direction

44

is prevented by the pawl and can only take place when the pawl is

lifted. The pawl is normally spring loaded to ensure that it

automatically engages with the ratchet teeth. Thus a winch used

to wind up a cable on a drum may have a ratchet and pawl to

prevent the cable unwinding from the drum when the handle is

released.

Belt drives

Belt drives are essentially just a pair of rolling cylinders,

as described in Fig. with the motion of one cylinder being

transferred to the other by a belt. Belt drives use the friction that

develops between the pulleys attached to the shafts and the belt

around the arc of contact in order to transmit a torque. Since

the transfer relies on frictional forces then slip can occur.

The transmitted torque is due to the differences in tension

that occur in the belt during operation. This difference results in a

tight side and a slack side for the belt. If the tension on the tight

side is Ti, and that on the slack side T. then

with pulley A in Fig. as the driver:

Torque on A = (T1 – T2)rA

Where rA is the radius of pulley A. For the driven pulley B we

have:

45

Torque on B = (T1 – T2)rB

Where re is the radius of pulley B. Since the power transmitted

is the product of the torque and the angular velocity, and since the

angular velocity is v/rA for pulley A and v/re for pulley B, where v is

the belt speed, then for either pulley we have:

Power = (T1 - T2)v

As a method of transmitting power between two shafts, belt

drives have the advantage that the length of the belt can easily be

adjusted to suit a wide range of shaft-to-shaft distances and the

system is automatically protected against overload because

slipping occurs if the loading exceeds the maximum tension that

can be sustained by frictional forces. If the distance between

shafts is large, a belt drive is more suitable than gears, but over

small distances gears are to be preferred. Different size pulleys

can be used to give a gearing effect. However, the gear ratio is

limited to about 3 because of the need to maintain an adequate arc

of contact between the belt and the pulleys.

The belt drive shown in Fig. gives the driven wheel

rotating in the same direction as the driver wheel. Figure a & b

46

shows two types of reversing drives. With both forms of drive,

both sides of the belt come into contact with the wheels and so

V-belts or timing belts cannot be used.

Types of belts

The four main types of belts (Fig.) are:

1. Flat

The belt has a rectangular cross-section. Such a drive has an

efficiency of about 98% and produces little noise. They can

transmit power over long distances between pulley centres

crowned pulleys are used to keep the belts from running off the

pullets.

2 Round

The belt has a circular cross-section and is used with grooved

pulleys.

3 V-belts

V-belts are used with grooved pulleys and are less efficient than

flat belts but a number of them can be used on a single wheel and

so give a multiple drive.

4. Timing

Timing belts require toothed wheels, having teeth which fit into he

grooves on the wheels. The timing belt, unlike the other belts,

does not stretch or slip and consequently transmits power

47

at a constant angular velocity ratio. The teeth make it possible

for the belt to be run at slow or fast speeds.

Chains

Slip can be prevented by the use of chains which lock into

teeth on the rotating cylinders to give the equivalent of a

pair of intermeshing gear wheels.A chain drive has the

same relationship for gear ratio as a simple gear train. The

drive mechanism used with a bicycle is an example of a chain

drive. Chains enable a number of shafts to be driven by a single

wheel and so give a multiple drive. They are not as quiet as timing

belts but can be used for larger torques.

Bearings

Whenever there is relative motion of one surface in

contact with another, either by rotating or sliding, the resulting

frictional forces generate heat which wastes energy and results in

wear. The function of a bearing is to guide with minimum

friction and maximum accuracy the movement of one part relative

to another.

Of particular importance is the need to give suitable

support to rotating shafts, i.e. support radial loads. The term

thrust bearing is used for bearings that are designed to

withstand forces along the axis of a shaft when the relative

motion is primarily rotation. The following sections outline the

characteristics of commonly used forms of bearings.

Plain journal bearings

48

Journal bearings are used to support rotating shafts which

are loaded in a radial direction. The term journal is used for a

shaft.

The bearing basically consists of an insert of some

suitable material which is fitted between the shaft and the

support (Fig.). Rotation of the shaft results in its surface sliding

over that of the bearing surface. The insert may be a white

metal, aluminum alloy, copper alloy, bronze or a polymer such as

nylon or PTFE. The insert provides lower friction and less wear

than if the shaft just rotated in a hole in the support. The bearing

may be a dry rubbing bearing or lubricated. Plastics such as

nylon and PTFE are generally used without lubrication, the

coefficient of friction with such materials being exceptionally low.

A widely used bearing material is sintered bronze, This is

Bronze with a porous structure which allows it to be impregnated

with oil and so the bearing has a `built in' lubricant. The lubricant

may be:

1. Hydrodynamic

The hydrodynamic journal bearing consists of the shaft

rotating continuously in oil in such a way that it rides on oil and

49

is not supported. by metal (Fig.). The load is carried by the

pressure generated in the oil as a result of the shaft rotating.

2. Hydrostatic

A problem with hydrodynamic lubrication is that the shaft

only rides on oil when it is rotating and when at rest there is metal-

to-metal contact. To avoid excessive wear at start-up and when

there is only a low load, oil is pumped into the load-bearing

area at a high-enough pressure to lift the shaft Off the metal when

at rest.

3. Solid-film

This is a coating of a solid material such as graphite or

molybdenum disulphide.

4. Boundary layer

This is a thin layer of lubricant which adheres to the surface of the

bearing.

Ball and roller bearings

With this type of bearing, the main load is transferred from

the

50

rotating shaft to its support by rolling contact rather than sliding

contact. A rolling element bearing consists of four main elements:

an inner race, an outer race, the rolling element of either balls or

rollers, and a cage to keep the rolling elements a part (Fig.). The

inner and outer races contain hardened tracks in which the

rolling elements roll.

There are a number of forms of ball bearings:

1. Deep-groove

This is good at withstanding radial loads but is

only moderately good for axial loads. It is a versatile bearing

which can be used with a wide range of load and speed

2. Filling-slot

This is able to withstand higher radial loads than the deep-

groove equivalent but cannot be used when there are axial loads.

3. Angular contact

This is good for both radial and axial loads and is better for axial

loads than the deep-groove equivalent.

4.Double-row

Double-row ball bearings are made in a number of types and are

able to withstand higher radial loads than their single-row

equivalents. The figure shows a double-row deep-groove ball

bearing, there being double-row versions of each of the above single-

row types.

5.Self-aligning

Single-row bearings can withstand a small amount of shaft

misalignment but where there can be severe misalignment a self-

51

aligning bearing is used. This is able to withstand only moderate

radial loads and is fairly poor for axial loads.

6. Thrust grooved race

These are designed to withstand axial loads but are not

suitable for radial loads.

There are also a number of forms of roller bearing, the

following being common examples:

1. Straight roller

This is better for radial loads than the equivalent ball bearing but is

not generally suitable for axial loads. They will carry a greater load

than ball bearings of the same size because of their greater

contact area. However, they are not tolerant of misalignment.

2. Taper rollers

This is good for radial loads and good in one direction for axial

loads.

3. Needle rollers

This has a roller with a high length/diameter ratio and tends to be

used in situations where there is insufficient space for the

equivalent ball or roller bearing.

Selection of bearings

In general, dry sliding bearings tend to be only used for

small diameter shafts with low load and low speed situations, ball

and roller bearings, i.e. bearings involving rolling, with a much wider

range of diameter shafts and higher load and higher speed, and

hydrodynamic bearings for the high loads with large diameter

shafts. Figure shows a chart indicating the selection of

52

bearings based on their load-shaft speed characteristics for a

number of different diameter shafts. Thus suppose we want a

bearing for a 25 mm diameter shaft rotating at 10 rev/s and

carrying a radial load of 10 000 N. This is beyond the limit for a dry

sliding bearing and is a point on the graph below the line for

rolling bearings for such a diameter and speed, hence rolling

bearings can be used.

Failure of ball and roller bearings generally occurs as a result of

fatigue. With fatigue failures there is always a scatter of values at

which failure of an individual item occurs. The life of a bearing is thus

defined as the number of millions of shaft revolutions that 90% of

the bearings are expected to exceed before failing. This life L10

depends on the applied load F. For ball bearings the

relationship is:

where C is a constant for a particular form of bearing. For roller

bearings:

Manufacturers often tabulate data for bearings in terms of the

number of hours of life at a particular speed given in units of

rev/min. The life in hours = 106/(3600 x n/60) x Lo in millions of revs

= (16 667/n) x Lo in millions of revs; n is the number of

revolutions per minute. For example, a particular ball bearing

53

may be rated as 3000 h at 500 rev/min for a radial loading of

10 kN. This gives L1, as 90 million revs and hence C as 44.8 kN.

Thus with a load of, say, 20 kN at 400 rev/min then the life we

can expect is 11.2 million revolutions or 468 h. If this is not long

enough we need to select a ball bearing with a higher rating.

54

ELECTRICAL ACTUATION SYSTEMS

Any discussion of electrical systems used as actuators for

control. The discussion has to include:

1. Switching devices such as mechanical switches. e.g. relays, or

solid-state switches, e.g. diodes, thyristors, and transistors, where the

control signal switches on or off some electrical device, perhaps a

heater or a motor.

2. Solenoid type devices where a current through a solenoid is used to

actuate a soft iron core. as, for example, the solenoid operated

hydraulic/pneumatic valve where a control current through a solenoid

is used to actuate a hydraulic/pneumatic flow.

3. Drive systems, such as d.c. and a.c. motors, where a current

through a motor is used to produce rotation. This chapter is an

overview of such devices and their characteristics.

Mechanical switches

Mechanical switches are elements which are often used as sensors

to give inputs to systems. e.g. keyboards. In this chapter we are

concerned with their use as actuators to perhaps switch on electric

motors or kiting elements, or switch on the current to actuate solenoid

valves controlling hydraulic or pneumatic cylinders. The electrical relay

is an example of a mechanical switch used in control systems as an

actuator.

Relays

The electrical relay offers a simple on/off switching action in

response to a control signal. Figure illustrates the principle. When a

current flows through the coil of wire a magnetic field is produced. This

pulls a movable arm, the armature that forces the contacts to open or

55

dose; usually there are two sets of contacts with one being opened

and the other closed by the action. This action might then be used to

supply a current to a motor or perhaps an electric heater in a

temperature control system.

As an illustration of the ways relays can be used in control systems,

Fig. shows how two relays might be used to control the operation of

pneumatic valves which in turn control the movement of pistons in

three cylinders A, B and C. The sequence of operation is:

1. When the start switch is closed, current is applied to the A and B

solenoids and results in both A and B extending, i.e. A+ and B+.

2.The limit switches a+ and b+ are then closed, the a+ closure results

in a current flowing through relay coil 1 which then closes its contacts

and so supplies current to the C solenoid and results in it extending,

i.e. C+.

3. Its extension causes limit switch c+ to close and so current to switch

the A and B control valves and hence retraction of cylinders A and B,

i.e. A- and B-.

4. Closing limit switch a- passes a current through relay coil 2; its

contacts close and allows a current to valve C and cylinder C to

retract, i.e. C-. ,

56

The sequence thus given by this system is A+ and B+ concurrently,

then C+, followed by A- and concurrently and finally C-.Time-delay

relays are control relays that have a delayed switching action. The time

delay is usually adjustable and can be initiated when a current flows

through the relay coil or when it ceases to flow through the coil.

SOLID STATE SWITCHES:

Here are number of solid-state devices which can be used to

electronically switch circuits. These include:

1. Diode.

2 .Thyistors and triacs.

3 .Bipolar transistors.

4. Power MOSFETs

DIODES

The diode has the characteristic shown in Fig. and so allows a

significant current in one direction only. A diode can thus be regarded

as a 'directional element', only passing a current with forward biased.

i.e. with the anode being positive with respect to the cathode. If the

diode is sufficiently reversed biased, i.e. a very high voltage, it will

break down. If an alternating voltage- is applied across diode, it can be

regarded as only switching on when the direction of the voltage is such

as to forward biased it and being off in the reverse biased direction.

The result is that the current through the diode is half –rectified to

become just the current due to the positive halves of the input voltage

(Fig.).

57

Thyristors and triacs

The thyristor, or silicon-controlled rectifier (SCR), can be regarded

as a diode which has a gate controlling the conditions under which the

diode can be switched on. Fig shows the thyristor characteristic. With

the gate current zero. The thyristor passes negligible current when

reverse biased (unless sufficiently reverse biased, hundreds of volts,

when it breaks down).

When forward biased the current is also negligible until the forward

breakdown voltage is exceeded. When this occurs the voltage across

the diode falls, to a low level, about 1 to 2 V, and the current is then

58

only limited by the external resistance in a circuit. Thus, for example, if

the forward breakdown is at 300 V then. When this voltage is reached

the thyristor switches on and the voltage across it drops to I or 2 V. If

the thyristor is in series with a resistance of. say. 20 ohm (Fig.)

then before breakdown we have a very high resistance in series

with the 20 and so virtually all the 300 V is across the thyristor and

there is negligible current. When forward breakdown occurs, the

voltage across the thyristor drops to, say, 2 V and so there is now 300

– 2 = 298 V across the 20 Ω resistor, hence the current rises to 298/20

= 14.9 A. When once switched on the thyristor remains on until the

forward current is reduced to below a level of a few milliamps. The

voltage at which forward breakdown occurs is determined by the

current entering the gate, the higher the current the lower the

breakdown voltage. The power-handling capability of a thyristor is high

and thus it is widely used for switching high power applications.

59

Triac

The triac is similar to the thyristor and is equivalent to a pair of

thyristors connected in reverse parallel on the same chip. The triac can

be turned on in either the forward or reverse direction. Figure shows

the characteristic.

Bipolar transistors

Bipolar transistors come in two forms, the npn and the pnp. Figure

shows the symbol for each. For the npn transistor, the main current

flows in at the collector and out at the emitter, a controlling signal being

applied to the base. The pnp transistor has the main current flowing in

at the emitter and out at the collector, a controlling signal being applied

to the base.

npn Transistor pnp Transistor

For an npn transistor connected as shown in Fig.(a) These so

termed common-emitter circuit, the relationship between the collector

current IC and the potential difference between the collector and emitter

VCE is described by the series of graphs shown in Fig.(b)

60

When the base current Is is zero the transistor is cut off. In this state

both the base emitter and the base collector junctions are reverse

biased. When the base current is increased, the collector current

increases and VCE decreases as a result of more of the voltage being

dropped across Rc. When VCE reaches a value VC the base-collector

junction becomes forward biased and the collector current can

increase no further, even if the base current is further increased. This

is termed saturation. By switching the base current between 0 and a

value that drives the transistor into saturation, bipolar transistors can

be used as switches. When there is no input voltage V, then virtually

the entire VEC voltage appears at the output. When the input voltage is

made sufficiently high the transistor switches so that very little of the

VCC voltage appears at the output (Fig. (c)).

61

MOSFETs

MOSFETs (metal-oxide field-effect transistors) come in two

types the n-channel and the p-channel. Figure shows the symbols. The

main difference between the use of a MOSFET for switching and a

bipolar transistor is that no current flows into the gate to exercise the

control. The gate voltage is the controlling signal. Thus drive circuitry

can be simplified in that there is no need to be concerned about the

size of the current.

n- Channel p- channel

SOLENOID

It is used to provide electrically operated actuator. Example:-

solenoid valves are used in hydraulic and pneumatic valves. A current

passes through a coil. Due to this current, a soft iron core is pulled into

62

the coil. In doing so it can open or close the ports to allow the flow of a

fluid.

D.C Motor

Electric motors are frequently used as the final control element in

positional or speed-control systems. Motors can be classified into two

main categories: d.c. motors and a.c. motors. Most motors used in

modern control systems being d.c. motors

Construction of d.c motor

* YOKE:

1. It is the outermost covering of the machine. 2. It provides

mechanical support for the poles. 3. It is a stationary part. 4. It carries

magnetic flux produced by the poles. 5. It is made of cast iron.

* FIELD SYSTEM:

(a) Pole core

(b) Pole shoe

(c) Field coil.

(a) Pole core:

1. They are fabricated by laminations of steel.

2. They are laminated so as to avoid eddy current loss.

63

(b) Pole shoe:

1. They act as a mechanical support to the field coil.

2. They reduce the reluctance of the magnetic flux.

3. They spread out the flux in the air gap uniformly.

(c) Field coil:

1. These coils are wound on the pole core. 2. When current is passed

through this coil, they electromagnetic.

* Inter poles:

1. They are fixed between the main poles.

2. They are in-line with the neutral axis.

3. They are smaller in size than main poles.

4. They are used for spark less commutation.

* Armature:

1. It is the rotating part of the machine.

2. It is cylindrical in shape.

3. It is fabricated by means of steel laminations.

4. It is laminated to avoid the eddy current loss.

5. The periphery of the armature is cut into slots and teeth's.

6. The conductors are placed in the slots.

7. Due to loss, heat is developed in the armature.

8. Therefore to dissipate heat a fan is provided at one end of the

armature.

* Commutator:

1. It is made up of copper segments insulated from each other by mica

sheets.

2. The armature conductors are soldered to Commutator.

3. It is used to convert bidirectional current to unidirectional current.

64

* Brushes:

1. These are made of carbon. It is rectangular in shape.

2. The brush holders are kept passed against the Commutator.

3. It collects current from the line to the Commutator.

Principle of Working:

The armature is made up of magnetic material with coils of core

wound on it. The armature is mounted on bearings and is free to

rotate. They are field coils wound and permanent magnet W

electromagnets fixed to the carrying [w] starter. The ends of the

armature coil are connected to Commutator. When the current is

applied to the field coil it cuts the magnetic flux near to armature, and

armature start rotating. The direction of rotation of the D.C motor can

be changed by reversing either the armature current (or) the field

current.

Types of D.C. motor

1. Series wound motor (fig. a)

With the series wound motor the armature and fields coils are in

series. Such a motor exerts the highest starting toque and has the

greatest no-load speed. With light loads there is a danger that a series

wound motor might run at too high a speed. Reversing the polarity of

the supply to the coils has no effect on the direction of rotation of the

motor. It will continue rotating in the same direction since both the field

and armature currents have been reversed.

65

2. Shunt wound motor (fig.b)

With the shunt wound motor the armature and field coils are in

parallel. It provides the lowest starling toque, a much lower no-load

speed and has good speed regulation. Because of this almost constant

speed regardless of load, shunt wound motors are very widely .used.

To reversed the direction of rotation. either the armature or field

supplied must be reversed. For this reason, the separately excited

windings are preferable for such a situation.

3. Compound motor (fig. c)

The compound motor has two field windings. One in series with the

armature and one in parallel. Compound wound motors aim to get the

best features of the series and shunt wound motors, namely a high

starting torque and good sped regulation.

4. Separately excited motor (fig. d)

The separately excited motor has separate control of the armature

and field currents and can be considered to be a special case of the

shunt bound motor.

66

Control of D.C. Motor:

1. The speed of the permanent magnet motor depends upon the

current through the armature coil.

2. In a field coil D.C motor, the speed can be changed by varying the

armature current or the field current.

3. Generally it is the armature current that is varied.

4. To obtain a variable voltage at the armature an electric circuit is

used.

5. Usually the D.C motors are controlled by the signals coming from

microprocessors.

6. In such cases the technique knows as pulse width modulation

(PWM) is used, to obtain a variable voltage.

7. This PWM can be obtained by means of a basic transistor circuit.

8. This technique can be used to drive the motor in one direction only.

9. By involving four transistors which is know as H - circuit, the

direction change in rotation of motor can be obtained

In a closed loop control system, the feed back signals are

used to modify the motor speed. There are three methods for doing it.

Method - I

.

67

1. Here the feed back signal is provided by a tachometer.

2. The analogue signal from the tachometer is converted into digital

signal by using ADC.

3. This digital signal is given as input to the microprocessor is

converted into analogue by using DAC.

4. This signal is used to vary the voltage applied to the armature of the

D.C. motor.

Method – II

1. The feed back signal is coded using a encoder.

2. In the code converter, the digital output is obtained.

3. This digital signal is given as an input to the microprocessor.

Method – III

68

1. The system is completely digital.

2. PWM is used to control the average voltage applied to the armature.

A.C MOTOR

Types of A.C motor:

1. Squirrel cage induction rotor.

2. Slip ring (or) wound rotor.

Construction of A.C Motor

STATOR:

1. It is the stationary part of the machine.

2. It is made of high grade silicon steel laminations.

3. it is laminated so as to avoid eddy current loss.

4. The stator windings are placed in the slots on the inner surface of

stator core.

5. The windings are wound for a particular number of poles.

6. The three phase stator windings are fed from three phase supply.

7. The stator windings are sometimes known as primary windings.

69

ROTOR:

Squirrel cage rotor:

1. The construction is very simple.2. It is the rotating part of the

machine.

3. It is cylindrical in shape with slots on its outer surface.

4. The rotor conductors are heavy bars of copper or aluminum.

5. All the ends of the bars are short circuited by means offend rings on

both sides.

6. The slots are slightly angled to prevent hum noise and locking of

stator and rotor.

Slip ring or wound rotor:

1. The rotor is wound for the same numbers of poles as that of the

stator.

2. The rotor is made up of silicon steel laminations.

3. The open ends of the rotor windings are brought out and they are

connected to three slip rings.

70

4. The three slip rings are mounted on the shaft.

5. The slip rings are insulated from each other.

6. The slip rings are made of phosphor bronze.

Principle of Operation:1. A three phase supply is given to the stator winding.

2. A rotating magnetic field is produced in the stator which rotates in

synchronous speed.

3. Synchronous speed depends upon supply frequency and number of

poles.

4. In rotor short circuited copper bars are provided.

5. The rotating magnetic field cuts the short circuited copper

conductors, thereby inducing an emf in the rotor conductors.

6. Hence a magnetic field is setup in the rotor.

7. Due to the interaction between the stator and rotor magnetic flux,

the rotor rotates.

8. The direction of rotation of rotor is same as that of rotating magnetic

field but with the speed lesser than the synchronous speed.

71

Control of A.C Motor:

1. The speed control of A.C motor is more complex than the D.C

motors.

2. The speed of the A.C motor is determined by the frequency of

supply.

N= 120 F/P Where, N = speed in rpm.

F = frequency

P = no. of poles.

3. Therefore the control of A.C motor is based on the variable

frequency supply.

4. The change in the frequency can be achieved by two methods.

A. using a converter and an inverter.

B. using cyclo converter.

Using a converter and an inverter:

1. The three phase A.C is rectified to D.C by a converter.

2. Then it is inverted back to A.C. again but at a frequency that can be

selected.

Cyclo converter:1. It is used to operate slow speed motors.

2. This converts A.C at one frequency directly to A.C at another

frequency without the intermediate D.C conversion.

72

STEPPER MOTORS:

1. It is a device that produces rotation through equal angle called steps

for each digital input.

2. for example :-Suppose say one phase input produces 6O of rotation,

then60 phase of input produces 360° of rotation.

Construction and working principle:

1. Stepper motor is a special type synchronous motor.

2. It converts electrical pulses applied to it into discrete rotor

movements called steps.

3. A 30° per step motor will require 12 pulses to move through one

revolution.

4. From diagram, A, B and C are the three stator coils placed at 120°

apart around the circumference of the stator.

5. A four pole rotor made of soft iron is placed in between stator coils

so as to rotate.

6. When coil A is excited, the rotor teeth 1 & 3 are aligned along 'A'

axis as shown in figure (i).

7. When excitation of coil A is removed and if coil B is excited, the

rotor teeth 2 is attracted by coil B and thus rotor teeth2 and 4 are

aligned 'B' axis as shown in figure (ii).

73

8. Now when the coil C is excited after removing the excitation of coil

B, the rotor teeth 3 gets aligned along 'C' axis as shown in figure

(iii).

9. Hence a clockwise motion of the motor is produced when pulses

are given in the order of A, B, c, A, B, C.... etc.

10. For each pulse, the rotor moves 30° per step.

11. If no coil is excited, then the rotor stands at any position.

12. When pulses are given for the coils in the order A, C, B, A, C, B....

etc., the rotor rotates in anticlockwise direction.

Application:

1. Used in X - Y plotters.

2. Used in machine tools.

3. Used in robots.

4. Used in computer peripherals like floppy disc drives line printers etc.

5. Used in watches.

Types of stepper motor:

1. Variable reluctance of stepper motor.

The rotor is made of soft steel and is cylindrical in the four poles.

The rotor will rotate until rotor and stator poles line up. This is termed

as position of minimum reluctance. This form of stepper gives step

angle of 7.50(or) 150

74

2. Permanent magnet stepper motor:

1. The rotor is a permanent magnet.

2. The motor has a stator with four poles.

3. Each pole is wound with a field winding.

4. The coils in position pair of poles are connected in series.

5. When the current is supplied to the stator windings, the rotor which

is a permanent magnet will move to line up with the stator poles.

6. From the figure shown, the rotor will move to the 450osition.

7. When the polarity is reversed for the current supply to stator coil,

thus the rotor will move on the reversed side to450positions.

8. Thus by switching through the coils the rotor rotates in 450teps.

9. The usual step angles are 1.80 7.50 150 30°, or 90°.

75

3. Hybrid stepper

1. It combines feature of both the variable reluctance and permanent

magnet motor.

2. It has a permanent magnet encased in iron caps.

3. The iron caps are cut to have teeth.

4. The typical steps angles are 0.90to1.80

5. Used in high accuracy positioning application.

76

ADDITIONAL QUESTIONS IN TEXTBOOK (Mechtronics By W.Bolton)

HYDRAULIC & PNEUMATIC ACTUATION SYSTEMS.

77

MECHANICAL ACTUATION SYSTEMS

78

ELECTRICAL ACTUATION SYSTEMS

79

ME1402MECHATRONICS

(Common to Mechanical and Production- VI Semester)L T P M3 0 0 100

OBJECTIVE

To understand the interdisciplinary applications of Electronics, Electrical, Mechanical and Computer Systems for the Control of Mechanical and Electronic Systems.

1. MECHATRONICS, SENSORS AND TRANSDUCERS 9

Introduction to Mechatronics Systems – Measurement Systems – Control Systems – Microprocessor based Controllers.Sensors and Transducers – Performance Terminology – Sensors for Displacement, Position and Proximity; Velocity, Motion, Force, Fluid Pressure, Liquid Flow, Liquid Level, Temperature, Light Sensors – Selection of Sensors

2. ACTUATION SYSTEMS 9

Pneumatic and Hydraulic Systems – Directional Control Valves – Rotary Actuators.Mechanical Actuation Systems – Cams – Gear Trains – Ratchet and pawl – Belt and Chain Drives – Bearings.Electrical Actuation Systems – Mechanical Switches – Solid State Switches – Solenoids – D.C Motors – A.C Motors – Stepper Motors.

3. SYSTEM MODELS AND CONTROLLERS 9

Building blocks of Mechanical, Electrical, Fluid and Thermal Systems, Rotational – Transnational Systems, Electromechanical Systems – Hydraulic – Mechanical Systems.Continuous and discrete process Controllers – Control Mode – Two – Step mode – Proportional Mode – Derivative Mode – Integral Mode – PID Controllers – Digital Controllers – Velocity Control – Adaptive Control – Digital Logic Control – Micro Processors Control.

4. PROGRAMMING LOGIC CONTROLLERS 9

Programmable Logic Controllers – Basic Structure – Input / Output Processing – Programming – Mnemonics – Timers, Internal relays and counters – Shift Registers – Master and Jump Controls – Data Handling – Analogs Input / Output – Selection of a PLC Problem.

80

5. DESIGN OF MECHATRONICS SYSTEM 9

Stages in designing Mechatronics Systems – Traditional and Mechatronic Design - Possible Design SolutionsCase Studies of Mechatronics Systems, Pick and place robot – automatic Car Park Systems – Engine Management Systems.

TOTAL : 45TEXT BOOKS1. W. Bolton, “Mechatronics”, Pearson Education, Second Edition, 1999.

REFERENCES1. Michael B. Histand and David G. Alciatore, “ Introduction to Mechatronics

and Measurement Systems”, McGraw-Hill International Editions, 2000.2. Bradley D. A., Dawson D., Buru N.C. and. Loader A.J, “Mechatronics”,

Chapman and Hall, 1993.3. Dan Necsulesu, “Mechatronics”, Pearson Education Asia, 2002 (Indian

Reprint).4. Lawrence J. Kamm, “Understanding Electro – Mechanical Engineering”, An

Introduction to Mechatronics, Prentice – Hall of India Pvt., Ltd., 2000.

81