Electrical actuation systems

Intro..Actuator is a device which is used to actuate a process. Electrical actuation system consists of1.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 electronics

Semi-conductorDiode 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 closedOutput from controller is small so it is often

used with transistor. Relays are inductances

Relay

Relay

solenoid

• Solenoid is an electromagnet which can be used as an actuator.

• Electrically operated actuators.• Solenoid valves are used in hydraulic and

pneumatic systems.

Solenoid

Solid state switches

• diode • Thyristor • Triac • Bipole transistor • MOSFET

Diode

Thyristor• 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).

Thyristor

• 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 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.

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.

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.

Transistor as a switch

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.

• 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"

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 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.

• 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

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)/RT = 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.

SPEED CONTROL DC MOTOR

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 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 motors

• Stepper motor is a device that produce rotation though equal angles called as steps, for each digital pulse supplied to its input.

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 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.