UNIT 2Electrical actuation systems

Intro..• Actuator is a device which is used to actuate a process. • Actuate is to operate the process.• Switching devices – mechanical switches, eg. relay and

solid state switches, eg diodes, thyristors and transistors app – switch on or off electrical devices

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

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

Basic electronics• Semi-conductor

• Diode • Transistor

• Resistor

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 closed• Output from controller is small so it is often used with

transistor. • Relays are inductances• Free – wheeling or fly back diode.• Importance

• To operate a device which needs larger current.

Solenoid• Solenoid 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 switches• diode • Transistor • Thyristor • Triac • Bipole transistor • MOSFET

Diode

Transistor as a switch

Thyristor

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

that would be blocked by a diode• Forward 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

Bipolar Transistors

Transistors 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.

MOSFET

• Metal oxide field effect transistor• Two types

• N channel• P 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.

Drive systems• DC motor• AC motor• Stepper motor

DC motor

Working principle

• When 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.

• For an armature conductor of length l and carrying a current I, the force resulting from a magnetic flux of density B at right angles to the conductor is given by

• F = BIL • Torque produced along the axis of the conductor due to

force F is • T = F x b

• = nBIL x b• = KI

• 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 = Kw• Equivalent circuit diagram for D.C motor

• 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 motor• Speed 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 magnet

• Series wound• Shunt wound• Compound wound • Separately excited

Series wound

• Armature and field windings are connected in series.

• Highest starting torque • Greatest no load speed

• Reversing the polarity of supply will not effect the direction of rotation of rotor.

Shunt wound• Armature and field coils

are in parallel.• Lowest starting torque• Good 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 wound• Two 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 motor• Its 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 motors• Single 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 motor

3 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 motors

Similar 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 motor

• Speed 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 motors• Stepper motor is a device that produce rotation though equal

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

How it works (VR)

1. The upper electromagnet is activated and the teeth of the central cog line up accordingly.

2. The upper electromagnet is deactivated and the right one turned on. The closest cog teeth then jump to line up with this. This causes a step (e.g. 1.8° turn).

3. The right electromagnet is deactivated and the lower one is turned on. The cog teeth then jump to line up with the bottom electromagnet. This causes another step.

4. The bottom electromagnet is deactivated and the left-most one turned on. The cog teeth then jump to line up with this. This causes another step. On a motor which has a step angle of 1.8°, 200 steps are required for a full rotation.

Stepper motors

Variable 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 motor

Combined 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.

Servo Motor• DC motors with built in gearing and feedback control loop

circuitry, require no driver• Extremely popular with robot• Most servo motors can rotate about 90 to 180 degrees or a full

360 degrees or more• unable to continually rotate – e.g. driving wheel• precision positioning makes them ideal for robot arms and legs,

rack and pinion steering• To use a servo, simply connect the black wire to ground, the red

to a 4.8-6V source, and the yellow/white wire to a signal generator (e.g. from microcontroller)

• Vary the square wave pulse width from 1-2ms and your servo is now position/velocity controlled.

THANK YOU…