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Electrical actuation systems

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Title: Electrical actuation systems


1
Electrical actuation systems
2
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.

3
Basic electronics
  • Semi-conductor
  • Diode
  • Transistor
  • Resistor

4
Mechanical switches
Electronics specification and abbreviation Expansionofabbreviation Britishmainswiringname Description Symbol
SPST Single pole, single throw One-way A simple on-off switch The two terminals are either connected together or disconnected from each other. An example is a light switch.
SPDT Single pole, double throw Two-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
DPST Double pole, single throw Double pole Equivalent to two SPST switches controlled by a single mechanism
DPDT Double pole, double throw Equivalent to two SPDT switches controlled by a single mechanism.
DPCO Double pole changeoveror Double pole, centre off   Equivalent to DPDT. Some suppliers use DPCO for switches with a stable off position in the centre
5
Mechanical switches
  • Relay - A relay is an electrically operated switch
    .

6
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.

7
solenoid
  • Solenoid is an electromagnet which can be used as
    an actuator.
  • Electrically operated actuators.
  • Solenoid valves are used in hydraulic and
    pneumatic systems.

8
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10
Relay
11
Solid state switches
  • diode
  • Transistor
  • Thyristor
  • Triac
  • Bipole transistor
  • MOSFET

12
Diode
13
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.
14
Transistor as a switch
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16
  • Bipolar switch

17
Darlington pair
18
  • 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.

19
MOSFET
  • Metal oxide field effect transistor
  • Two types
  • N channel
  • P channel
  • Three terminals
  • Gate (G)
  • Drain (D)
  • Source (S)

20
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.

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22
Thyristor
23
  • 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"

24
Triac
25
Voltage control
26
Thyristor dc control
27
Lamp dimmer
28
  • 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.

29
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.

30
Drive systems
  • DC motor
  • AC motor
  • Stepper motor

31
DC motor
32
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.

33
Parts
  • Stator (permanent or non permanent magnet)
  • Rotor (electromagnet)
  • Armature
  • Commutator
  • Brush

34
  • 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.

35
  • 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

36
  • 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

37
  • 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

38
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.

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41
Brush type motor with non-permanent magnet
  • Series wound
  • Shunt wound
  • Compound wound
  • Separately excited

42
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.

43
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.

44
Compound wound
  • Two field windings one in series an another in
    parallel with armature windings.
  • High starting torque with good speed regulation.

45
Separately excited
  • Separate control of armature and field coils.
  • Speed of these motors can be controlled by
    separately varying the armature or field current.

46
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.

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49
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.

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51
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

52
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.

53
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.

54
Stepper motors
  • Stepper motor is a device that produce rotation
    though equal angles called as steps, for each
    digital pulse supplied to its input.

55
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.

56
  • 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.

57
  • 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.

58
Specifications
  • Phase
  • Number of independent windings on the stator, eg
    a three phase motor.
  • Step angle
  • Angle through which the rotor rotates from one
    switching change for the stator.
  • Holding torque
  • Maximum torque that can applied to a powered
    motor without moving it from its rest position
    and causing spindle rotation.

59
  • Pull in torque
  • This is the maximum torque against which a motor
    will start for a given pulse rate and reach
    synchronism without losing a step.
  • Pull out torque
  • Maximum torque against that can be applied to a
    motor, running at a given stepping rate, without
    loosing synchronism.

60
  • Pull in rate
  • Maximum switching rate at which a loaded motor
    can start without loosing a step.
  • Pull out rate
  • Switching rate at which a loaded motor will
    remain in synchronism as the switching rate is
    reduced.
  • Slew range
  • Range of switching rates between pull-in and
    pull-out within the motor runs in synchronism but
    cannot start up or reverse.

61
  • Bipolar stepper
  • Unipolar stepper

62
H bridge
63
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.

64
Bipolar stepper
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66
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.

67
  • 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.

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