Electrical actuation systems

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Electrical actuation systems
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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.

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Basic electronics

• Semi-conductor
• Diode
• Transistor
• Resistor

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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
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Mechanical switches

• Relay - A relay is an electrically operated switch
  .

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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.

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solenoid

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

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Relay
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Solid state switches

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

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Diode
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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.
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Transistor as a switch
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• Bipolar switch

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Darlington pair
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• 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.

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MOSFET

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

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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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Thyristor
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• 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"

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Triac
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Voltage control
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Thyristor dc control
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Lamp dimmer
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• 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.

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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.

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Drive systems

• DC motor
• AC motor
• Stepper motor

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DC motor
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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.

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Parts

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

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• 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.

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• 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

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• 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

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• 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

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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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Brush type motor with non-permanent magnet

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

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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.

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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.

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Compound wound

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

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Separately excited

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

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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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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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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

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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.

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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.

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Stepper motors

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

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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.

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• 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.

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• 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.

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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.

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• 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.

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• 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.

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• Bipolar stepper

• Unipolar stepper

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H bridge
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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.

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Bipolar stepper
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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.

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• 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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