Embed Size (px)
DESCRIPTION
Brief Description on Actuators
Citation preview
ACTUATORS
Actuators
Hardware devices that convert a controller command signal into a
change in a physical parameter
• The change is usually mechanical (e.g., position or velocity)
• An actuator is also a transducer because it changes one type of
physical quantity into some alternative form
• An actuator is usually activated by a low-level command signal, so
an amplifier may be required to provide sufficient power to drive
the actuator
Actuators
Signal Processing
& Amplification
Mechanism
Electric Hydraulic
Pneumatic
Final Actuation Element
Actuator Sensor
Logical
Signal
Sensors
Microprocessor
or
Microcontroller Actuator
Plant
(Robot, AGV, NCM, Consumer products, Conveyor systems, Assembly
system, Cranes, Defense equipments, Air craft engines, etc…)
Sensing signal
Control code
Command signal
Mechanical
Actuation Parameter, variables
Figure 1) A simple sensor actuator connection
1. Electro-mechanical
2. Fluid-power
3. Active material based
Classification of Actuators
Electro-mechanical actuators:
Switches
• Relays, Diode, Transistors, Thyristor
Solenoid type
• Hydraulic & pneumatic valve
Drive systems
• DC motor
• AC motor
• Stepper motor
Fluid power actuators:
• Hydraulic
• Pneumatic
Active material based actuators:
• Piezoelectric
• Magnetostrictive
• Shape-memory alloys
Selection parameters
Volume
Hydraulic Pneumatic DC motor
AC
Stepper motor
a) Torque Vs Volume of actuators b) Weight Vs power of actuators
Power
Hydraulic
Electrical
c) Cost Vs power of actuators
Power
Hydraulic
Electrical
d) Torque Vs speed of actuators
Speed / pulse
Stepper motor DC motor
Solenoid Actuator
Solenoid is an electromagnetic device, that works by the
magnetization of a coil, which operates a soft iron core.
A solenoid consists of a coil and a movable iron core called
armature.
When the coil is energized with current, the core moves to
increase the flux linkage by closing the air gap between the cores.
The movable core is usually spring loaded to allow the core to
retract when the current is switched off.
The force generated is approximately proportional to the square of
the current and inversely proportional to the square of the width of
the air gap.
Movable
armature core
Spring
Coil
a) Plunger type Stationary iron core
Movable
armature core
Spring Coil
b) Non plunger type Stationary iron core
Force available from a solenoid, N – No. of turns on the coil
I – Current through the coil
A – C/s of air gap
0 – permeability of air
X – width / length of air gap
Movable
armature core
Spring
Coil
2 2
02
1 N IF = A
2 x
How it works (continued)
Example 1 –Valve
A spring maintains the valve in its closed or
open position.
When a current is passed through the coils
around the core, it will produce a
magnetic force, that pushes the valve to
the Open or closed position.
When the current is stopped The force is
removed and The valve moves to its
original Position.
Major Specifications – Selection Factors
• Voltage
• Duty cycle - Specifies the length of time the solenoid coil is to be
electrically energized and de-energized.
• Current and Power
• Temperature
• Stroke – Distance the plunger must travel
• Force – Push or pull energy the actuator must exert
• Mounting and Environment – Coil heat is bad for the Solenoid
Limitations
• Temperature of device may increase very fast
• Limited to current input possible
• Limited to force of actuator
• Large force = Lots of money
• Must be mounted very firm
• Must control with PWM or AC
Advantages
• Very strong
• Very fast
• Very customizable to specifications
• Several Manufacturers
• Great for high power short bursts
Relay An electromechanical relay is used to make or break mechanical
contact between electrical leads.
An electrical relay (also called as contactor) is an electrically
operated ON/OFF type of switching device, based on a control signal
input.
It consists of a magnetic coil, a moving armature and a set of
electrical contacts.
When the current flows through the coil, a magnetic is generated,
which in turn attracts the armature.
This causes the internal contacts to change position(open to closed,
or closed to open).
Electromechanical Relays: What’s Inside
This diagram shows the basic parts of an electromechanical relay: a spring, moveable armature, electromagnet, moveable contact, and stationary contact. The spring keeps the two contacts separated until the electromagnet is energized, pulling the two contacts together.
Moveable Armature Moveable Contact
Electromagnet Spring Stationary Contact
Wiring Up an Electromechanical Relay
Spring
To Control Circuit
Moveable Armature Moveable Contact
Load
Power Supply
Electromagnet
This diagram shows how to wire an electromechanical relay. When the control circuit turns the electromagnet on, the moveable armature is drawn towards the electromagnet and connects the moveable contact and the stationary contact. This completes the circuit and delivers power to the load.
Stationary Contact
Advantages of Relays
The complete electrical isolation improves safety by ensuring that high voltages and
currents cannot appear where they should not be.
Come in all shapes and sizes for different applications and they have various
switch contact configurations.
Easy to tell when a relay is operating - you can hear a click as the relay switches on and
off and you can sometimes see the contacts moving.
• Their parts can wear out as the switch contacts become dirty - high voltages and currents
cause sparks between the contacts.
• They cannot be switched on and off at high speeds because they have a slow response
and the switch contacts will rapidly wear out due to the sparking.
• Their coils need a fairly high current to energise,
•The back-emf created when the relay coil switches off can damage the components that
are driving the coil.
Disadvantages of Relays
START / STOP
V
Relay 2 Relay 1
Figure 1) Relay controlled system
a- a+ b- b+ c+
Relay contact 1
Relay contact 2
A+ A- B+ B- C+ C-
DC motor
Converts electrical energy in to mechanical energy.
Conductor placed in a magnetic field, current is passed
Lorentz force acts on the conductor.
Fleming’s left hand rule for direction of the force
Conductor moves in the direction of force
DC Motors
N S
DC motor basics
B F
I
Field pole
Armature
Armature
conductors
DC motor
DC Motor:
Fleming’s Left hand rule
Armature
Figure 1) Permanent magnet
DC motor
S
N
N
S
Stator
Rotor
F F
Figure 2) Armature
Permanent magnet DC motor:
• Magnetic flux remains constant at all levels of the armature current
• speed - torque characteristics is linear.
Torque – speed characteristics of a DC motor
For an armature conductor of length ‘L’, carrying current ‘i’, placed
in a magnetic field of flux density ‘B’
Force (F) acting on the conductor is,
F B i L
For ‘N’ such conductors,
B N F i L
For ‘N’ such conductors,
B N F i LTorque (T) about coil axis,
B iN ) T ( LK b
tT i K ………… (1)
K Proportionality constant
Kt Torque constant
Since an armature coil is rotating in a magnetic field,
Electromagnetic induction will occur, back emf is produced.
Vb V
Figure 3) Equivalent circuit of dc motor
R L
bbV K Kb Torque constant
Figure 3) Equivalent circuit of dc motor
Vb V
R L
bbV K
Neglecting inductance of an armature coil, current through the resistor is,
b V -i
V
R
V - bK
R
………… (2)
tT i K ………… (1)
Sub. eqn.(2) in eqn.(1) we get,
Rotational speed ()
To
rqu
e (
T)
Figure 4) Torque – speed characteristics of a dc motor
tK(V - T )b
RK
Tacho-generator
Va Set speed
Vf
Figure: Closed loop dc motor Speed Measurement system
DC servomotor
Servo principle :
Na set speed
Va Reference signal
Vf Feedback signal
V
V Applied voltage
Tacho-generator
Va Set speed
Vf
DC servomotor Servo principle :
V
At time t = 0,
a f-V V V aV V
At time t = t1,
When motor picks up speed (N2),
If Vf > Va , ‘V’ is negative [Motor reduces speed]
If Vf < Va , ‘V’ is positive [Motor increases speed]
(N2 > Na)
(N2 < Na)
Stepper Motor
A stepper motor is an electromechanical device which converts
electrical pulses into discrete mechanical movements.
The shaft or spindle of a stepper motor rotates in discrete step
increments when electrical command pulses are applied to it in the
proper sequence.
Step motors are different from all other types of electrical drives in the sense that they operate on discrete control pulses received and rotate in discrete steps.
AC and DC drives are analog in nature and rotate continuously depending on magnitude and polarity of the control signal received.
The sequence of the applied pulses is directly related to the
direction of motor shafts rotation.
The speed of the motor shafts rotation is directly related to
the frequency of the input pulses and
The length of rotation is directly related to the number of
input pulses applied.
The discrete nature of operation of a step motor makes it suitable
for directly interfacing with a computer & direct computer control.
These motors are widely employed in industrial control, specifically
for CNC machines, where open loop control in discrete steps are
acceptable.
Specifications
• Phase: Number of independent windings on the stator.
• Step Angle: Angle through which the rotor rotates for one
switching change for the stator coils
• Holding torque: Maximum torque that can be applied to a
powered motor without moving it from its rest position and
causing spindle rotation.
• Pull in Torque: Maximum torque against which motor will start,
for a given pulse rate, and reach synchronism without losing a
step.
• Pull-out Torque: Maximum torque that can be applied to a motor,
running at a given stepping rate, without losing synchronism
• Pull-in Rate: Maximum switching rate at which a loaded motor
can start without losing a step.
• Pull-out Rate: Switching rate at which loaded motor will remain in
synchronism as the switching rate is reduced
• Slew Range: Range of switching rate between pull-in and pull-out
within which the motor runs in synchronism but cannot start up
or reverse
Step motors are normally of two types:
(a) permanent magnet and
(b) variable reluctance type.
Also there is Hybrid type stepper motor
In a step motor the excitation voltage to the coils is DC and the
number of phases indicates the number of windings.
In both the two cases the excitation windings are in the stator.
In a permanent magnet type step motor the rotor is a permanent
magnet with a number of poles.
In a variable reluctance type motor the rotor is a cylindrical
structure with a number of projected teeth.
Rotor is cylindrical with 4 poles
Poles on stator > rotor
Variable reluctance Stepper motor
N
S
Current flows through opposite pair of windings
Magnetic field is produced
Magnetic lines of force move from stator to nearest poles on rotor
Position of minimum reluctance
Step angles 7.50/ 150
Variable reluctance type step motors
• Variable reluctance type step motors do not require reversing of current through the coils, but at the same time do not have any holding torque.
• Step angle as low as 1.8o can be achieved with this type of motors.
• Rotor is a cylindrical soft iron core with projected teeth.
• When a particular stator coil is excited, the rotor aligns itself such that one pair of teeth is along the energised stator coil, at the minimum reluctance path.
• When phase-1 is energised,
the rotor will align itself as
shown in the figure.
• In the next step, if phase-1 is
switched off and phase-2 is
switched on, the rotor will
rotate in CCW direction by
an angle of 15o.
Three-phase single-stack VR step motor with twelve stator poles (teeth) and eight rotor teeth.
Field pole
Rotor
Figure 1) Permanent magnet stepper motor
(Permanent magnet)
Permanent magnet Stepper motor
S
N
– Rotor is a permanent magnet
– Stator consists of coils
– Different stator winding combinations are excited by current
– Magnetic field is produced
– causes rotor to move in different directions
– Polarities of stator needs to be changed
– Step angles 1.80,7.50,150,300,340, 900
Permanent magnet Stepper motor
S
N
Permanent Magnet Step Motor
• Figure shows permanent magnet step
motor;
two-phase two-pole
• Winding A is split into two halves A1 and A2. They are excited by
constant d.c. voltage V and the direction of current through A1
and A2 can be set by switching of four switches Q1, Q2, Q3 and Q4.
• Similar is the case for the halves B1 and B2 where four
switches Q5-Q8 are used to control the direction of current
Let Winding A be energised and the induced magnetic poles are as shown in Fig (we will denote the switching condition as S1=1). The other winding B is not energised. As a result the moving permanent magnet will align itself along the axis of the stator poles as shown in Fig.
In the next step, both the windings A and B are excited simultaneously, and the polarities of the stator poles are as shown in Fig. 3(b). We shall denote S2=1, for this switching arrangement for winding B. The rotor magnet will now rotate by an angle of 45o and align itself with the resultant magnetic field produced
In the next step, if we now make S1=0 (thereby de-energising winding A), the rotor will rotate further clockwise by 45o and align itself along winding B, as shown in Fig.
In this way if we keep on changing the switching sequence, the rotor will keep on rotating by 45o in each step in the clockwise direction.
A pair of switch (say Q7-Q8) remains closed during consecutive 3 steps of rotation and there is an overlap at every alternate step where both the two windings are energised. This arrangement for controlling the step motor movement is known as half stepping. The direction of rotation can be reversed by changing the order of the switching sequence.
More on Stepper Motors
• Animation shows how coils are energized for full steps
• The advantage of a permanent magnet step motor is that it has a
holding torque. This means that due to the presence of permanent
magnet the rotor will lock itself along the stator pole even when the
excitation coils are de-energised.
• But the major disadvantage is that the direction of current for each
winding needs to be reversed. This requires more number of
transistor switches that may make the driving circuit unwieldy.
• Another way of reducing the number of switches is to use unipolar
winding. In unipolar winding, there are two windings per pole, out of
which only one is excited at a time. The windings in a pole are
wound in opposite direction, thus either N-pole or S-pole,
depending on which one is excited.
More on Stepper Motors
• Full step sequence showing how binary numbers can control the motor
Half step sequence of binary control numbers
Hybrid stepper motor
Permanent magnet
N S
Teeth on end caps
– Combines the features of both the variable reluctance & PM motors
– Permanent magnet placed in iron end caps containing the teeth
– Energising pair of stator coils rotates rotor to min. reluctance position
– Step angles are 0.90 & 1.80
– Computer hard discs
Fig : Hybrid stepper rotor
AC motor
Classification of a AC motor:
– Single phase
– Poly phase
1. Induction
2. Synchronous
Single induction motor
Pole Pole
Stator Rotor
Figure 1) Single – phase induction motor
End view of squirrel cage
Three induction motor
Rotor
Figure 1) Three – phase induction motor
Stator
Three Synchronous motor
Rotor
Figure 1) Three – phase synchronous motor
Stator
N S
Fluid Power Actuators
Fluid Power Actuators
– Any actuator which actuates a system on receiving the input
power in the form of air, oil
Classification:
Pneumatic Actuators
Hydraulic Actuators
Power supply:
Sump
Control valve Cylinder
Accumulator
Filter
Motor Pressure relief valve
Oil
Figure 1) Hydraulic power supply
Pump
P
E
A
B
Power supply:
Cooler Control valve
Cylinder
Air receiver
Filter
Motor Pressure relief valve
Air inlet
Figure 1) Pneumatic power supply
Filter &
Water trap
To atmosphere
Compressor
P
E
A
B
Hydraulic Pneumatic
Pressurised liquid (oil) Compressed air
Pressure (7 Mpa – 21 Mpa) Pressure (500 Kpa – 1 Mpa)
Pump Air compressor
Large loads / force Less force
Heavy construction equipment,
large m/c Open systems, process new air
Self lubricating Cleaner than oil
Precise control at low speeds Compressible
High pressure hazzards Low operating pressure
Large infrastructure needed Costs less
Hydraulic & pneumatic systems
Selection criteria
1. Force
2. Speed
3. Size
4. Type of motion
5. Service life
6. Sensitivity
7. Safety & reliability
8. Controllability
9. Handling & storage
10.Energy costs
Signal flow
Energy supply source
Input elements & signals
Processing elements
Actuating devices
Final control element
System symbols
System symbols
Water separator Filter Lubricator
Pressure regulator
2(A)
3(R) 1(P)
Air service unit
Air service unit
Simplified symbol
System symbols
System symbols
Control Valves
1) 2) 3)
4) 5)
6)
2(A)
1(P)
7)
System symbols
P Pressure port
R , S Exhaust port
A , B Outlet ports
2(B)
1(P)
4(A)
3(R) 5(S)
5/2 Normally closed valve
P Pressure port
R , S Exhaust port
A , B Outlet ports
2(B)
1(P)
4(A)
3(R) 5(S)
5/3 Normally closed valve
Types of switch actuation for DCV’s
Manual Push button Lever operated
Foot paddle operated
Detent lever operated Spring return
Roller operated Solenoid operated Double Solenoid operated
Flow control valves
Flow
Check valve
Spring loaded Check valve
Non – Return valves
Two way pressure valve (AND)
Shuttle valve (OR)
Flow control valves Non – Return valves
Two way pressure valve (AND)
Shuttle valve (OR)
Quick exhaust valve
Flow control valves
Adjustable
One way adjustable
Pressure control valves
Adjustable pressure regulating valve
Adjustable pressure regulating valve (Relieving type)
Sequence valve
Actuators
Single acting cylinder with spring return
Double acting cylinder
1(P)
2(A)
3(R)
1 2
2(B)
1(P)
4(A)
3(R)
1 2
Valves
– Device for closing or opening the passage through a pipe in
order to stop, allow or control the flow of a fluid
– Act as a control element to control flow of fluid in the chamber
of cylinder
– Classified based on how they work
– Normally closed & normally open valves
– 2-way, 3-way & 4-way valves
2 way Valves
– 2 ports
– 1 inlet & 1 outlet
3 way Valves
– 3 ports
– 1 inlet , 1 outlet & exhaust
4 way Valves
– 4 ports
– 1 inlet , 2 outlet & exhaust
5 way Valves
– 5 ports
– 1 inlet , 2 outlet & 2 exhaust
Valve operating conditions
1. Inlet open to the outlet with exhaust blocked
2. Inlet blocked with outlet connected to exhaust
1(P)
2(A)
3(R)
1 2
Direction Control Valves (DCVs)
Classification of Direction control valves
1. Spool type
2. Poppet valve
3. Directional valve / Check valve / Non return valve (NRV)
4. Pilot operated valve
Spool valve (solenoid operated)
Solenoid
(not energized)
Port 3 Port 2
Port 1
Vent to atmosphere
Figure 1) Working of a spool valve
Solenoid Port 3 Port 2 Port 1
Air supply
Spool valve (solenoid operated)
– Common type of a direction control valve
– Spool moves horizontally with in the valve body to control the flow
– Air supply is connected to port 1 , port 3 is closed
– When spool moves to the left,
– Air supply is cut off & port 2 is connected to port 3.
– Port 3 opens to atmosphere
– Pressurised air in the system goes out
Poppet valve
Figure 1) Poppet valve
1
2
2(A)
1(P)
– Normally closed condition
– No connection between port 1 & 2
– Balls, discs or cones are used with valve seats to control the flow
Poppet valve
Figure 1) Poppet valve
2(A)
1(P)
– When PB is depressed, ball will be pushed out of its seat
– Port 1) connected to port 2), Flow occurs,
– When button is released, no flow occurs
Directional valve (check valve)
Figure 1) Poppet valve
No flow Symbol
Directional valve (check valve)
– Free flow occurs in one direction through valves
– Ball is pressed against the spring
– Flow in other direction is blocked by spring forcing the ball
against the seat
Pilot operated valve (Impulse valve)
Figure 1) Pilot operated System
2(B)
1(P)
4(A)
3(R)
1(P)
2(A)
3(R)
– One valve is used to control another valve
– Pilot valve is operated manually or by solenoid
– Double pilot valves are called MEMORY VALVES (Bistable valve)
Pressure Control Valves (PCVs)
Classification of Pressure control valves
1. Pressure regulating valve
2. Pressure limiting / relief valve
3. Pressure sequence valve
Pressure regulating valve P
A
Fig 2) Symbol
Diaphragm
Poppet valve
Body
Main spring
Fig 1) Pressure regulator
(Normally open)
Adjustable screw
– To control the operating pressure in a circuit and maintain it
at a constant value
Function :-
– To regulate the incoming pressure to the system
– Air flows at a desired pressure in to the cylinder
– 2 openings, primary & secondary
Pressure regulating valve
Application:
- Pneumatic circuits
Working :-
– Poppet valve opening = desired level (Adjustable screw)
– Screw moves the diaphragm & air flows to outlet
– 2 openings, primary & secondary
– Vent hole openings
– Spring compression Pressure
Pressure regulating valve
Pressure limit valve/ Relief valve
Fig 1) Pressure relief valve
Fig 2) Symbol
System pressure (P)
Ball
Spring
Adjustable support
– Safety devices
– Limits the pressure below the safe value
– Normally closed
Pressure limiting valve
Working:
- Inlet pressure > Spring pressure
- Built-in spring closes the valve
Pressure sequence valve
A
B
P
T
Fig 2) Symbol
1 (P) 2 (A)
P A
Cylinder 1
Cylinder 2
1
2
Fig 1) Sequential system
– To sense the pressure of an external line & give a signal
when it reaches some preset value
Pressure sequence valve
Working principle:
- Inlet pressure > Spring pressure
- Built-in spring closes the valve
Lift system
Pressure supply
UP
DOWN
Vent
Load
Vent
1(P)
2(A)
3(R)
1
2
Start
4 5 7
3 6
Cylinder A Cylinder A
Limit switches Limit switches
b- b+ a+ a-
a- a+ b- b+
Figure 1) Two-actuator sequential operation
2(B)
1(P)
4(A)
3(R) 5(S)
2(B)
1(P)
4(A)
3(R) 5(S)
Cylinder Sequencing
THYRISTOR
or
SILICON CONTROLLED RECTIFIERS
THYRISTOR
P
N
P
N VG
VD
C
J1
J2
J3
Fig 1 a) SCR
A
GATE
C
b) Symbol
THYRISTOR
P
N
P
N VG
VD
C
J1
J2
J3
SCR current
Reverse breakdown voltage
V Gate
voltage
Forward conduction
Forward Breakdown
voltage
A
h
Figure 2) V-I characteristics of a SCR
APPLICATION
THYRISTOR V
t
0 2
1 2 3 t Gate pulses
Average DC voltage
Figure 2) SCR firing
TRIAC
TRIAC
GATE
b) Symbol
T2
T1
T1 T2
G1
G2
Fig 1 a) Triac equivalent circuit
V
T2 (+ve)
VBR h
Figure 2) V-I characteristics of a Triac
TRIAC
VBR
T1 (+ve)
h
