Intro..Actuator is a device which is used to actuate a process. Actuate is to operate the process.

1.Switching devices – mechanical switches, eg. relay and solid state switches, eg diodes, thyristors and transistors app – switch on or off electrical devices

2.Solenoid – type devices used to actuate valves of hydraulic and pneumatic systems. (flow control)

3.Drive systems – DC motor, AC motor and stepper motor.

Basic electronicsSemi-conductor

Diode Transistor Resistor

Mechanical switches

Electronics specification and

abbreviation

Expansionof

abbreviation

Britishmainswiringname

Description Symbol

SPSTSingle pole, single

throwOne-way

A simple on-off switch: The two terminals are

either connected together or disconnected from each

other. An example is a light switch.

SPDTSingle pole, double

throwTwo-way

A simple changeover switch: C (COM,

Common) is connected to L1 or to L2.

SPCO Single pole, centre off 

 switches with a stable off

position in the centre

DPSTDouble pole, single

throwDouble pole

Equivalent to two SPST switches

controlled by a single mechanism

DPDTDouble pole, double

throw

Equivalent to two SPDT switches

controlled by a single mechanism.

DPCO

Double pole changeover

or Double pole, centre off

 

Equivalent to DPDT. Some suppliers

use DPCO for switches with a stable off position

in the centre

Mechanical switches Relay - A relay is an electrically operated switch.

Relay Electrically operated switches in which changing the

current in one circuit switches a current on or off in another circuit.

NO – normally open , NC – normally closedOutput from controller is small so it is often used with

transistor. Relays are inductancesFree – wheeling or fly back diode.Importance

To operate a device which needs larger current.

solenoidSolenoid is an electromagnet which can be used as an

actuator. Electrically operated actuators.Solenoid valves are used in hydraulic and pneumatic

systems.

Relay

Solid state switchesdiode Transistor Thyristor Triac Bipole transistor MOSFET

Diode

Bipolar TransistorsTransistors are manufactured in different shapes but they have three leads (legs). The BASE - which is the lead responsible for activating the transistor.The COLLECTOR - which is the positive lead.The EMITTER - which is the negative lead.

Transistor as a switch

Bipolar switch

Darlington pair

Transistor needs large base current to switch on.Output from microprocessor has a small input.A second transistor is employed to enable a high current

to be switched on. Such a combination of pair of transistor is called Darlington pair.

MOSFETMetal oxide field effect transistorTwo types

N channelP channel

Three terminals Gate (G)Drain (D)Source (S)

Operation When MOSFET is turned on current flows from source to

drain .Voltage is applied between gate-source to turn on

MOSFET.MOSFET can be turned off by removing gate voltage.Gate has full control over the control of MOSFET.A level shifter buffer required to raise the voltage level at

which the MOSFET starts to activate. Interfacing with µp is simpler then transistor.

Thyristor

Thyristors have three states:Reverse blocking mode — Voltage is applied in the

direction that would be blocked by a diodeForward blocking mode — Voltage is applied in the

direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction

Forward conducting mode — The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current"

Triac

Voltage control

Thyristor dc control

Lamp dimmer

Thyristor dimmers switch on at an adjustable time (phase angle) after the start of each alternating current half-cycle, thereby altering the voltage waveform applied to lamps and so changing its RMS effective value.

R1 is a current limiting resistor and R2 is a potentiometer.By adjusting R2 thyristor can be made to trigger at any

point between 0 deg and 90 deg.

Snubber circuit In order to prevent sudden

change in source voltage, the rate voltage changes with time is dV/dt is controlled by using a snubber circuit.

Drive systemsDC motorAC motorStepper motor

DC motor

Working principleWhen current passes through the coil, the resulting forces

acting on its sides at right angles to the field cause forces to act on those sides to give a rotation.

For the rotation to continue, when the coil passes through the vertical position the current direction through the coil has to be reversed.

Parts Stator (permanent or non permanent magnet)Rotor (electromagnet)Armature Commutator Brush

A brush type dc motor is essentially a coil of wire which is free to rotate - termed as rotor in the field of permanent or non-permanent magnet.

The magnet termed a stator since it is stationery.For the rotation to continue, when coil passes through

vertical position the current direction is reversed which is got by use of brushes making contact with split ring commutator.

Since armature is a rotating magnetic field it will have back emf Vb. The back emf depends on rate of flux induced in coil. Back emf is proportional to angular velocity w

Vb = KwEquivalent circuit diagram for D.C motor

V a Vb

RaLa = inductance

Neglecting the inductance produced due to armature coil, then effective voltage producing current I through resistance R is Va-Vb, hence

I = (Va - Vb)/R = (Va – Kw)/R

T = K I

= k(Va – Kw)/R

Control of brush type DC motorSpeed control can be obtained by controlling the voltage

applied to the armature. Since fixed voltage supply is often used, a variable voltage is obtained by an electronic circuit.

When A.C supply is used a Thyristor can be used to control the average voltage applied to armature.

PWM – pulse width modulation Control of d.c motors by means of control signal from

microprocessors.

Brush type motor with non-permanent magnetSeries woundShunt woundCompound wound Separately excited

Series wound Armature and field

windings are connected in series.

Highest starting torque Greatest no load speedReversing the polarity of

supply will not effect the direction of rotation of rotor.

Shunt woundArmature and field coils

are in parallel.Lowest starting torqueGood speed regulation.Almost constant speed

regardless of load.For reversing direction of

rotation either armature coil or field coil supply has to be reversed.

Compound woundTwo field windings one in

series an another in parallel with armature windings.

High starting torque with good speed regulation.

Separately excited Separate control of

armature and field coils.Speed of these motors can

be controlled by separately varying the armature or field current.

Brush less dc motorIts consists of a sequence of stator coils and a permanent

magnet rotor.Current carrying conductors are fixed and magnet moves.Rotor is ferrite or permanent magnet.The current to the stator coils are electronically switched

by transistor in sequence round the coils.Switching being controlled by position of rotors.Hall effect sensors are used to input signals related to a

particular position of rotor.

A.C motorsSingle phase squirrel cage induction motor

Its consists of a squirrel cage rotor, this being copper or aluminum bars that fit into slots in end rings to form a complete circuit.

Its consists of a stator having set of windings.Alternating current is passed through stator windings an

alternating magnetic field is produced.As a result EMF are induced in conductors in the magnetic

field.Initially when rotor is stationery net torque is zero.Motor is not self starting.

3-phase induction motor3 windings located 120 deg

apart each winding being connected to one of the three lines of the supply.

3 phase reach maximum currents at different times, magnetic field rotates round the stator poles completing one rotation is one full cycle.

Self starting

Synchronous motorsSimilar to that of induction

motor but rotor will be a permanent magnet.

Magnets rotate with the same frequency as that of rotating magnetic field which rotates 360 deg in one cycle of supply.

Used when precise speed is required.

Not self starting.

Speed control of AC motorSpeed control of A.C motor

is done by provision of variable frequency supply.

Torque is constant when ratio of applied stator voltage to frequency ration is constant.

AC is rectified to DC by convertor and inverted back to AC with a selected frequency.

Stepper motorsStepper motor is a device that produce rotation though

equal angles called as steps, for each digital pulse supplied to its input.

Stepper motorsVariable reluctance motor

Rotor is made of soft steel and is cylindrical with four poles, fewer poles than on the stator.

When opposite pair of windings has current switched to them, a magnetic field is produced with line of force pass from stator to nearest poles of rotor.

Rotor will until it is in minimum reluctance position.

Step angle 7.5 deg to 15 deg.

Permanent magnet stepper

Two phase four poles.Coils on opposite pairs of poles

are in series.Current is supplied from dc

source.Rotor is a permanent magnet.Rotor rotates in 45 deg steps.Step angles 1.8, 7.5, 15, 30, 34,

or 90 deg available.

Hybrid stepper motorCombined features of both

variable reluctance and permanent magnet motors.

Permanent magnets are encased in iron caps which are cut to have teeth.

It motor has n phase and m teeth on the rotor, the total number of steps per revolution will be nm

0.9 and 0.8 deg steps available. High accuracy positioning

applications.

Specifications Phase

Number of independent windings on the stator, eg a three phase motor.

Step angleAngle through which the rotor

rotates from one switching change for the stator.

Holding torqueMaximum torque that can

applied to a powered motor without moving it from its rest position and causing spindle rotation.

Pull – in torqueThis is the maximum torque

against which a motor will start for a given pulse rate and reach synchronism without losing a step.

Pull – out torqueMaximum torque against

that can be applied to a motor, running at a given stepping rate, without loosing synchronism.

Pull – in rateMaximum switching rate at

which a loaded motor can start without loosing a step.

Pull – out rateSwitching rate at which a

loaded motor will remain in synchronism as the switching rate is reduced.

Slew rangeRange of switching rates

between pull-in and pull-out within the motor runs in synchronism but cannot start up or reverse.

Bipolar stepper Unipolar stepper

H bridge

Stepper motor control Two phase motors are termed as bipolar motors when they have 4

connecting wires for signals.Solid state switches can be used to switch dc supply between the pair of

stator windings.

Bipolar stepper

Merits and demerits Merits A high accuracy of motion is possible, even under open-loop control. Large savings in sensor (measurement system) and controller costs

are possible when the open-loop mode is used. Because of the incremental nature of command and motion, stepper

motors are easily adaptable to digital control applications. No serious stability problems exist, even under open-loop control. Torque capacity and power requirements can be optimized and the

response can be controlled by electronic switching. Brushless construction has obvious advantages.

Demerits They have low torque capacity (typically less than 2,000

oz-in) compared to DC motors. They have limited speed (limited by torque capacity and

by pulse-missing problems due to faulty switching systems and drive circuits).

They have high vibration levels due to stepwise motion. Large errors and oscillations can result when a pulse is

missed under open-loop control.