Transcript Electrical actuation systems

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 electronics
 Semi-conductor
 Diode
 Transistor
 Resistor
Electronics
specification and
abbreviation
Expansion
of
abbreviation
British
mains
wiring
name
Description
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.
Two-way
A simple changeover
switch: C (COM,
Common) is connected to
L1 or to L2.
Mechanical switches
SPST
Single pole, single
throw
SPDT
Single pole, double
throw
SPCO
Single pole, centre
off
DPST
Double pole, single
throw
DPDT
Double pole, double
throw
DPCO
Double pole
changeover
or Double pole,
centre off
switches with a stable off
position in the centre
Double pole
Equivalent to
two SPST switches
controlled by a single
mechanism
Equivalent to
two SPDT switches
controlled by a single
mechanism.
Equivalent to DPDT.
Some suppliers
use DPCO for switches
with a stable off position
in the centre
Symbol
Mechanical switches
 Relay - A relay is an electrically operated switch.
Relay
 Electrically operated switches in which changing the
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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
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
 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.
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.
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
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.
 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=Fxb
= 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
Ra
V
a
La = inductance
Vb
 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=KI
= 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 nonpermanent 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.
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
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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
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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 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.
 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.
 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.
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
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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.