Module 3
ACTUATORS
ACTUATION SYSTEMS
Actuation systems are the elements of control
system which are responsible for transforming
the output of a microprocessor or control system
into a controlling action on a machine or device.
It may be mechanical, electrical or
electromechanical type to produce linear or
rotary motion.
CLASSIFICATION OF ACTUATION
SYSTEM
I. Type of medium
1. Hydraulic and pneumatic system
2. Electrical system
3. Mechanical system(cam, lob, etc.)
II Type of motion
1. Linear actuation
2. Rotary actuation
HYDRAULIC AND PNEUMATIC
SYSTEM
These systems are actuated by fluid power. high
power control devices. Hydraulic systems can be
used for higher power control devices than
Pneumatic systems, because hydraulic oil are
incompressible. But it is more expensive.
HYDRAULIC POWER SUPPLY
SYSTEM
HYDRAULIC POWER SUPPLY
SYSTEM
In a hydraulic power supply system pressurised oil is provided by a
pump driven by an electric motor/engine/turbine. This pumped oil from
the sump is passing through a non return valve, pressure relief valve
and an accumulator to the system and return to the sump.
The non return valve prevents the oil being back driven to the
pump. The pressure relief valve release the pressure of the system if it
rises above a safe level and the oil returns to the sump. The
accumulator is to smooth out any short term fluctuations in the output
pressure.
The accumulator is just a container in which the oil is held
under pressure against an external force. A gas pressurised accumulator
is shown in figure. It involves gas within the bladder in the chamber
containing hydraulic fluid. When the oil pressure rises the bladder
contracts thus increases the volume of oil and pressure maintains. If the
oil pressure falls the bladder expands, thus volume of oil reduces and
increases the oil pressure.
PNEUMATIC POWER SUPPLY
PNEUMATIC POWER SUPPLY
In a pneumatic power supply on electric motor
drives the air compressor. The air inlet to the
compressor is filtered and reduces the noise level by
using a silencer. A pressure relief valve provides
protection against the pressure in the system raising
above the safe level. Since the air compressor
increases the temperature of the air , a cooling
system is provided. A water trap arranged to
remove contamination and water from the
compressed air. An air receiver increases the volume
of compressed air in the system and smooth out any
short term pressure fluctuations.
VALVES
The following subjective aspects are needed to be
controlled by the valve.
1. Directional control valves
2. Flow control valves
3. Pressure control valves
DIRECTIONAL CONTROL
VALVES
1.
2.
3.
4.
Spool valve
Poppet valve
Pilot operated valves
Directional valves
SPOOL VALVE
Spool valve is a common type
of directional control valve. A
spool moves horizontally with
in the valve body to control
the flow. In first position the
air supply (or hydraulic oil) is
connected to port 2 from port
1. Port 3 is closed. Thus
compressed air flow from
compressor
to
working
system.
POPPET VALVE
• This valve normally in
closed condition and thus
there is no connection
between pressure supply
(port 1) and the working
system (port 2). When the
push button is depressed
the ball is pushed one of
its seat and flow occurs
from port 1 to port2.
• In poppet valves balls, disc
or cones may be used as
valves.
DIRECTIONAL (CHECK) VALVES
• In directional valves the
flow occurs in only in one
direction by opening the
ball valve by fluid pressure
against spring pressure.
The flow in other direction
is blocked by the spring
forcing the ball against its
seat.
PILOT OPERATED VALVES
• The force required to move the
valve or shuttle in a valve may
be too large for normal or
solenoid operations. To
overcome this problem a pilot
operated system is used where
one valve is used to control a
second valve .
• The pilot valve is of small
capacity and can be operated
manually or by a solenoid. It is
used to allow the main valve to
be operated by the system
pressure. Both pilot valve and
main valve are housed in a single
housing.
PRESSURE COTROL VALVES
• Different types of pressure control valves are ,
1. Pressure relief valve or pressure limit valve.
2. Pressure regulating valve or pressure
reduction valve.
3. Pressure Sequence valve.
PRESSURE RELIEF VALVE OR
PRESSURE LIMITTING VALVE
• When the pressure in the system
exceeds a particular limit, This pushed
up the spool in upward direction
through pilot line. Thus the fluid from
the pump (inlet port) goes out to the
outlet line (either to the reservoir or
to the atmosphere) When the system
pressure reduce to limiting value the
spool lowers by spring pressure and
thus closes the outlet line.
• The pressure limit can be set by
varying the spring tension by using
adjustment screw. This can be used as
safe guard to the system from
excessive pressure.
PRESSURE SEQUENCE VALVE
• A sequence valve is closed
relative to pressure relief valve
and is used where a set of
operation are to be carried out in
a pressure related sequence.
• The sequence value is to direct
flow in a predetermined
sequence. The sequence valve
operates on the principle that
when system pressure exceeds
the spring pressure, the valve
moves up allowing flow to the
secondary port that is connected
to the second operating
hydraulic cylinder.
PRESSURE REGULATING VALVE OR
PRESSURE REDUCTION VALVE.
• These are used to control the
operating pressure in a
circuit and maintain it at
constant value.
VALVE SYMBOLS
• Each position of valves is
shown by squares. Arrow
headed lines are used to
indicate direction of flow
in each direction
• Blocked-off lines shows
the flow is closed.
• The initial position of the
valves has the
connections to the port
as shown. This figure
shows valve has four
ports.
Representation of Valve Ports
•
•
•
•
•
A,B,C,etc ---Working lines.
P
--- Power supply port.
R and S --- Exhaust port (Pneumatic).
T
--- Return port Hydraulic.
Z,Y,X, etc
--- Pilot lines.
• a,b,c, etc
---Valve positions.
VALVE ACTUATION METHODS
Hydraulic Circuit Symbols
Hydraulic Circuit Symbols
CYLINDER
• A hydraulic or pneumatic cylinder is an
example of a linear actuator. The cylinder
consist of a cylindrical tube along with a
piston or ram which can slide.
SINGLE ACTING CYLINDER-EXTENSION
SINGLE ACTING CYLINDER-EXTENSION
In single acting cylinder, control pressure is applied
on one side of the piston, a spring pressure is
provided to oppose the movement of piston.
When a lever is actuated, the valve
switches on the piston and pressure is applied to
move the piston along the cylinder. When the
lever released, the valve returns to its initial
position by spring pressure and the air is vented
from the cylinder.
DOUBLE ACTING CYLINDER
DOUBLE ACTING CYLINDER
• In double acting cylinder control pressure is
applied to each side of the piston a difference
in pressure between the two sides results, the
motor of the piston. Current through one
solenoid causes the piston to move in one
direction, current through the other solenoid
reversing the direction of motion.
AUTOMATIC CYLINDER SEQUENCING
Many control system employs pneumatic or
hydraulic cylinder as the actuators and
requires a sequence of extension and
retractions of the cylinders.
Sequencing of two cylinders (cylinder A
and B) is shown in figure.
AUTOMATIC CYLINDER SEQUENCING
AUTOMATIC CYLINDER SEQUENCING
The sequence of operations are:
1. Initially both cylinders have retracted pistons start
push button on valve is pressed. This applies a
pressure to valve 2 as initially limit switch b- is
activated, valve 3 is switched to apply pressure to
cylinder A for extension.
2. Cylinder A extends, releasing switch a- When
cylinder A is fully extended limit switch a+ operates.
This switches valve 5 and causes pressure to be
applied to valve 6 to switch it and apply pressure to
cylinder B to extend the piston.
AUTOMATIC CYLINDER SEQUENCING
3. Cylinder 3 extends releasing switch b-. When
cylinder B is fully extended limit switch b+
operates. This switches valve 4 and pressure to
be applied to valve 3 and so applies pressure to
cylinder A causing piston to retract.
4. Cylinder A retracts releases limit switch at and
finally operates limit switch a. This switches valve
7 and pressure to be applied to valve 6 and this
pressure applied to cylinder B retracts the piston.
AUTOMATIC CYLINDER SEQUENCING
5. Cylinder B retracts releasing limit switch b+
and operates limit switch bThus a cycle is completed.
The cycle can be restarted by pushing the
start button.
If you want the system to run continuously,
then the last movement in the sequence
would have to trigger the first movement.
PROCESS CONTROL VALVES
DIAPHRAGM ACTUATED PROCESS
CONTROL VALVES
• Process control valves are used to control rate
of fluid flow. The basics of such valves is an
actuator being used to move a plug into the
flow pipe and so alter the cross section of the
pipe through which the fluid can flow.
DIAPHRAGM ACTUATED PROCESS
CONTROL VALVES
• A common form of pneumatic
actuator used with process
control valves is the diaphragm
actuator. It consists of a
diaphragm with input pressure
signal from the controller on one
side and atmospheric pressure
on the other side. The diaphragm
is made of rubber, which is
sandwiched in its centre between
two circular steel disc. The
change in input pressure moves
the central part of diaphragm
and the stem communicates this
movement to the final control
element.
• A single seated valve can be closed more tightly than a
double seated but the force on the plug is much higher
than the double seated. So diaphragm in the actuator has
to exert a higher force on the stem and results in problems
in accurate positioning of the plug.
Plug Shapes
SEMI ROTORY ACTUATORS
A linear cylinder with suitable
mechanical linkage can be used to
produce rotary movement through
angles less than 3600.
A vane type semi rotary
actuator can be used for producing
rotation of angles less than 3600.
here pressurised fluid is supplied to
the casing, on both sides of the vane
causes the vane to rotate and so give
a shaft rotation, which is a measure
of pressure difference between the
fluids from clockwise port and anticlock wise port.
ROTARY actuators
1. Gear motor
2. Lobe motor
3. Vane motor
GEAR MOTOR
• The external gear motor has
two gears meshed together
and rotates in opposite
direction. The high pressure
fluid enters the motor which
is trapped in the gear teeth
spaces between the housing
bore and the outside of the
gears, is transferred from the
inlet side of the motor to the
outlet side. The pressure
energy is absorbed by the
gears and mechanical energy
is given out through output
shaft.
LOBE MOTOR
• Lobe motor are similar to
external gear motor in
operation, in that fluid flows
around the interior of the
casing.
• High pressure fluid travels
around the interior of the
casing in the pockets
between the lobes and the
casing. It does not pass
between the lobes. Pressure
energy is absorbed by the
lobes and given out through
output shaft as mechanical
energy.
VANE MOTOR
• It has an eccentric rotor
contains radial slots splined
on it. Each radial slot
contains a vane, which is free
to slide in or out of the slots
due to centrifugal force. The
vane is designed to mate
with surface of the casing as
the rotor turns.
• High pressure fluid enters at
inlet and then it transfers to
the delivery side through the
space between rotor and
casing and rotates the vane
and rotor.
ELECTRICAL ACTUATION SYSTEM
The electrical actuation system includes the
following 3 types of actuators.
1. Switching devices
2. Solenoid type devices
3. Drive systems
SWITCHING DEVICES
It includes,
1. Mechanical switches
Eg: SPST, SPDT, DPDT, Limit switches, relays.
2. Solid state switches
(eg: diodes, thyristors)
RELAYS
Relays are electrically
operated switches in
which, change in
current in one electrical
circuit switches a
current, on or off, in
another circuit.
SOLID STATE SWITCHES
• There are a number of solid state devices
which can be used electronically to switch
circuits. These include:
• 1 Diode
• 2 Thyristors
DIODE
• A Diode is made by joining two equally doped
P-type and N-type semi-conductor material.
When these two materials are joined together
form a small layer in-between them called the
depletion layer. This is because the P-type
layer has excess hole and the N-type layer has
excess electrons and they both diffuse into
each other forming a high resistance blockage
between both the materials called as
depletion layer.
DIODE
Diode passing current only when forward
biased, i.e. with the anode being positive
with respect to the cathode. If the diode
reverse biased it will not conduct, until a
very high voltage is applied and it will
break down the junction.
• If an alternating voltage is applied
across a diode, it can be regarded as
only switching on when the direction of
the voltage is such as to forward- bias
and it and being off in the reversebiased direction. The result is that the
current through the diode is halfrectified. i.e. the circuit only ‘switches
on’ for positive half cycle and switches
off for negative half cycle.
THYRISTOR (SCR)
• A silicon controlled
rectifier is a four-layer
(p–n–p–n ) solid
state current-controlling
device.
• There are three modes of
operation for an SCR
depending upon the biasing
given to it.
1 Forward blocking mode
2 Forward conduction mode
3 Reverse blocking mode
Forward blocking mode
When a negative voltage is applied to the anode and a positive voltage to the
cathode, , keeping the gate at zero (0) potential, junction J1and J3 are forward-biased,
while J2 is reverse-biased, allowing only a small leakage current from the anode to the
cathode. When the applied voltage reaches the breakover value for J2, it undergoes
avalanche breakdown and starts conducting, but below breakover voltage J2 offers
very high resistance to the current and the SCR is said to be in the off state.
Forward conduction mode
An SCR can be brought from blocking mode to conduction mode in two ways: Either by
increasing the voltage between anode and cathode beyond the breakover voltage, or
by applying a positive pulse at the gate. Once the SCR starts conducting, no more gate
voltage is required to maintain it in the ON state. The minimum current necessary to
maintain the SCR in the ON state on removal of the gate voltage is called the latching
current.
Reverse blocking mode
When a negative voltage is applied to the anode and a
Positive voltage to the cathode, the SCR is in reverse
blocking mode, making J1 and J3 reverse biased and
J2 forward biased. The device behaves as two
reverse-biased diodes connected in
series. A small leakage current flows.
TRIAC
• The triac is a thyristor family of
devices. It is a bidirectional device
that can pass the current in both
forward and reverse biased
conditions and hence it is an AC
control device. The triac is
equivalent to two back to back SCRs
connected with one gate terminal as
shown in figure.
• The triac has three terminals namely
Main Terminal 1(MT1), Main
Terminal 2 (MT2) and Gate (G) as
shown in figure. If MT1 is forward
biased with respect to MT2, then the
current flows from MT1 to MT2.
Similarly, if the MT2 is forward
biased with respect to MT1, then the
current flows from MT2 to MT1.
BIPOLAR TRANSISTOR
• A Bipolar (or Bijunction) transistor (BJT) is
a type of transistor that uses
both electrons and electron holes as charge
carriers. A bipolar transistor allows a small
current injected at one of its terminals to
control a much larger current flowing
between two other terminals, making the
device capable of amplification or
switching.
• BJTs exist as PNP and NPN types, based on
the doping. An NPN transistor comprises
two semiconductor junctions that share a
thin p-doped region, and a PNP transistor
comprises two semiconductor junctions
that share a thin n-doped region.
• N-type means doped with impurities that
provide mobile electrons, while P-type
means doped with impurities that provide
holes that readily accept electrons.
NPN BJT with forward-biased E–B junction
and reverse-biased B–C junction
SOLENOIDS
• A solenoid is a coil of wire. When an electric current
passes through the wire a magnetic field is created around
it.
• Electromagnets have an advantage over permanent
magnets in that they can be switched on and off by the
application or removal of the electric current, which is
what makes them useful as switches and valves and allows
them to be entirely automated.
AC AND DC MOTORS
AC MOTOR
•
•
•
•
•
•
AC motors consume alternating electrical power to
produce mechanical actuation in terms of angular movement.
The principle of operation of all AC motors relies on the
interaction of a revolving magnetic field created in the stator by
AC current, with an opposing magnetic field at the rotor. The
opposing magnetic field is originated by virtue of induction or
by supplying an armature current by a separate DC current
source. Accordingly, AC motors are two types.
- Induction motor
- Synchronous motor.
AC motors are either single phase or multiphase,
depending upon the input signal requirement and internal
construction.
The AC motors are well suited for constant speed
applications but DC motors are suited for variable speed
applications.
SINGLE PHASE SQUIRREL CAGE INDUCTION MOTOR
• A squirrel-cage induction motor has a
rotor, consists of a cylinder of steel
laminations, with aluminium or copper
conductors embedded in its surface. In
operation, the nonrotating stator winding is connected to
an alternating current power source;
the alternating current in the stator
produces a rotating magnetic field.
• The rotor winding has current induced
in it by the stator field, like
a transformer except that the current
in the rotor is varying at the stator
field rotation rate minus the physical
rotation rate. The interaction of the
magnetic fields of currents in the
stator and rotor produce a torque on
the rotor.
SINGLE PHASE SQUIRREL CAGE
INDUCTION MOTOR
THREE PHASE INDUCTION MOTOR
• The three phase induction motor is similar to the single
phase to induction motor but has a stator with three
windings located 120 degree apart, each winding being
connected to one of the three lines of the supply. Because
the three phases reach their maximum currents at different
times, the magnetic field can be considered to rotate round
the stator poles, completing one rotation in one full cycle of
the current.
•
DC MOTOR
• A brush type d.c. motor is essentially a coil of wire which is free to
rotate, and so termed the rotor, in the field of a permanent
magnet or an electromagnet. The magnet being termed as stator
since it is stationary. When a current is passed through the coil, the
resulting forces acting on its sides at right angle to the field causes
forces to act on those sides to give rotation. For the rotation to
continue, when the coil passes through the vertical position the
current direction through the coil has to be reversed and this is
achieved by the use of brushes making contact with a slip-ringcommutator rotating with the coil.
•
In the conventional d.c. motor, coil of wire are mounted in slots
on a cylinder of magnet material called armature. The armature is
mounted on bearing and is free to rotate. It is mounted in the
TYPES OF DC MOTOR
• Dc motor is classified into two types
1 Permanent magnet motors (permanent
magnet stator
2 Electro magnet motors
• (i) Series motors
• (ii) Shunt motors
• (iii) Compound motors
•
Permanent Magnet Motors
• Permanent magnetic motor
• The permanent magnet motor
uses a magnet to supply field flux.
Permanent magnet DC motors have
excellent starting torque capability
with good speed regulation. A
disadvantage of permanent magnet
DC motors is they are limited to the
amount of load they can drive.
These motors can be found on low
horsepower applications.
• Another disadvantage is that
torque is usually limited to 150% of
rated torque to prevent
demagnetization of the permanent
magnets
SERIES MOTOR
• In a series DC motor the field is
connected in series with the
armature. The field is wound
with a few turns of large wire
because it must carry the full
armature current.
• A characteristic of series motors
is the motor develops a large
amount of starting torque.
However, speed varies widely
between no load and full load.
Series motors cannot be used
where a constant speed is
required under varying loads.
SHUNT MOTOR
• In a shunt motor the
field is connected in
parallel (shunt) with the
armature windings. The
shunt-connected motor
offers good speed
regulation. The field
winding can be
separately excited or
connected to the same
source as the armature
COMPOUND MOTOR
• Compound motors have a
field connected in series
with the armature and a
separately excited shunt
field. The series field
provides better starting
torque and the shunt field
provides better speed
regulation.
STEPPER MOTOR
• The stepper motor is a device that produces rotation through
equal angles (steps), for each digital pulse supplied to its input.
It requires sequencers and driver to operate. Sequencer
generates sequence for switching which determines the
direction of rotation and mode of operation. Driver is required
to change the flux direction in the phase winding. A stepper
motor is a motor controlled by a series of electromagnetic coils.
• The centre shaft has a series of magnet mounted on it, and the
coils surrounding the shaft are alternatively given current or
not, creating magnetic fields which repulse or attract the
magnets on the shaft, causing the motor to rotate.
• . A stepper motor has no comutator but it have five or
six wires coming out of the motor, one wire for each
coil and one or two common ground wires. Power
must be applied to one coil after another in the proper
sequence in order to get the motor to turn. To get
maximum torque, two coils are always on at any time.
Each step only turns the shaft a degree or two. This
four step cycle has to be repeated about 50 times for a
full revolution. If all coils are switched off, the motor
will be free to idle. Otherwise it is always locked in its
current position. If the load on the stepper motor is too
great or if the stepping sequences are being cycled too
fast, it will skip a step.