Control System Components
Topic: Actuators and Valve Positioner
Prepared by :Prof. Rajesh Zadfiya
Instrumentation & Control Engg.
Institute of Technology
Nirma University
Actuators
• A valve actuator is a
device that produces
force to open or close
the valve utilizing a
power source.
• This source of power
can be manual (hand,
gear, chain-wheel, lever,
etc.) or can be electric,
hydraulic or pneumatic.
Contd..
• Basic actuators turn valves
to either fully opened or
fully closed positions.
• But modern actuators have
much
more
advanced
capabilities. They not only
act as devices for opening
and closing valves, but also
provide
intermediate
position with high degree of
accuracy.
Type of Actuators
• Two types of actuators are common: pneumatic and electric
actuators.
• Pneumatic:
• Pneumatic actuators utilize an air signal from an external control device to create a
control action. These are commonly available in two main forms:
• Diaphragm actuators and
• Piston actuators
• Electric:
• Electric actuators are motor driven devices that utilize an electrical input signal to
generate a motor shaft rotation.
• This rotation is, in turn, translated by the unit’s linkage into a linear motion, which drives
the valve stem and plug assembly for flow modulation.
• In case of electric signal failure, these actuators can be specified to fail in the stem-out,
stem-in, or last position.
• Commonly used motors for electric actuators include steppers and servos.
Contd..
Diaphragm actuators –
 Diaphragm actuators have compressed air applied to a flexible
membrane called the diaphragm.
 These types of actuators are single acting, in that air is only supplied
to one side of the diaphragm, and they can be either direct acting
(spring-to-retract) or reverse acting (spring-to-extend).
Contd..
Flapper nozzle amplifier
• A pneumatic control system operates with air.
• The signal is transmitted in form of variable air pressure (often in the
range 3-15 psi, i.e. 0.2 to 1.0 bar) that initiates the control action.
• One of the basic building blocks of a pneumatic control system is the
flapper nozzle amplifier.
• It converts very small displacement signal (in order of microns) to
variation of air pressure.
• The basic construction of a flapper nozzle amplifier is shown below.
Contd..
Characteristics of a flapper nozzle amplifier
Limitations of Flapper Nozzle Amp.
• The major limitation of a flapper nozzle amplifier is its limited air
handling capacity. The variation of air pressure obtained cannot be
used for any useful application, unless the air handling capacity is
increased.
• Another problem of a flapper nozzle amplifier is its sensitivity
variation.
Air Relay
The principle of operation of an
air relay can be explained using
the schematic diagram shown
here.
It can be seen from Fig. that the
air relay is directly connected to
the supply line (no orifice in
between). The output pressure of
the flapper nozzle amplifier (p2)
is connected to the lower
chamber of the air relay with a
diaphragm on its top.
Contd..
The variation of the pressure p2 causes the movement (y) of the
diaphragm. There is a double-seated valve fixed on the top of the
diaphragm. When the nozzle pressure p2 increases due to decrees in
xi, the diaphragm moves up, blocking the air vent line and forming a
nozzle between the output pressure line and the supply air pressure
line. So more air goes to the output line and the air pressure
increases. When p2 decreases, the diaphragm moves downward,
thus blocking the air supply line and connecting the output port to
the vent. The air pressure will decrease.
Limitations of Air Relay
Problem with of an air relay is its sensitivity variation.
Flapper Nozzle Amplifier with Feedback
Flapper nozzle amplifiers are never used in open loop; it is always
used in closed loop. The scheme shown below is a torque balance
arrangement.
Contd..
Anticlockwise moment:
and
Clockwise moment:
Where AB1 and AB2 are the areas of the two bellows, a and b are
the corresponding lengths of the link segments.
Thereby at balance:
solenoid
• Solenoid is an electromagnet which can be used as an actuator.
• Electrically operated actuators.
• Solenoid valves are used in hydraulic and pneumatic systems.
It moves a rod by electromagnetic
energy
Contd..
Applications
Contd..
Contd..
Applications (Combined)
Contd..
Advantages of Pneumatic Actuators
• Weight
• Cylinders much lighter than motors
• Simple
• Much easier to mount than motors
• Much simpler and more durable than other for linear motion
• Fast on/off type tasks
• Big forces with elasticity
• No leak problems
Disadvantages of Pneumatic Actuators
• All the components are quite expensive
• A properly designed system is more complex than an equivalent
electromechanical system (electric motors, power screws and other
linear actuators).
• All these components take up quite a bit of valuable space (For
example within a robot).
• No weight advantage if only one cylinder used (still need compressor,
reservoir, pressure sensors, regulator)
Operators
Manual
General manual
Lever
Push button
Pedal
Pull button
Treadle
Push/pull button
Rotary knob
Operators
Mechanical
Plunger
Pressure
Spring normally
as a return
Pilot pressure
Roller
Differential pressure
Uni-direction
or one way trip
Detent in 3 positions
Operators
Electrical
Solenoid
direct
Solenoid pilot
Solenoid pilot
with manual
override and
external pilot
supply
Solenoid pilot
with manual override
and integral pilot
supply
When no integral
or external pilot
supply is shown it
is assumed to be
integral
Pneumatic Rotary Actuators
Pneumatic Valve Positioner
• Pneumatic valve positioner is another important component used in
process control.
• The control valve should be moved up or down, depending on the air
pressure signal (3-15 psi).
• The valve postioner can be of two types, (a) direct acting type and (b)
feedback type.
• The direct acting type valve positioner is shown below.
Direct acting type valve positioner
Contd..
• Here the control pressure creates a downward pressure on the
diaphragm against the spring, and the stem connected to the
diaphragm moves up or down depending on the control pressure p.
At equilibrium the displacement of the stem can be expressed as:
pA=Kx ---------------(1)
where A is the area of the diaphragm and K is the spring
constant.
• But the major shortcoming of this type of positioner is the nonlinear
characteristics.
• Though ideally, the stem displacement is proportional to the control
pressure (from (1)), the effective area of the diaphragm changes as it
deflates.
Contd..
• The spring characteristics is also not totally linear. Moreover, in (1) we
have neglected the upward thrust force exerted by the fluid.
• The change in thrust force also causes the change in performance of
the positioner.
• Besides the force exerted on the control valve is also not sufficient for
handling valves for controlling large flow.
• As a result, the use of direct acting type valve positioner is limited to
low pressure and small diameter pipelines.
Feedback type valve positioner
Contd..
• The feedback type valve positioner has a pilot cylinder with which the
diaphragm is attached.
• The piston of this pilot cylinder opens or closes the air supply and vent
ports to the main cylinder whose piston is connected to the stem of the
control valve (not shown).
• There is a mechanical link connected to the stem that adjusts the fixed end
of the spring connected to the diaphragm. This link provides the feedback
to the postioner.
• As the control pressure increases, the diaphragm moves down, so is the
piston of the pilot cylinder. This causes the lower chamber of the main
cylinder to be connected to the 20 psi line and the upper chamber to the
vent line.
• Compressed air enters the bottom of the main cylinder and the piston
moves up.
Contd..
• As the piston moves up, the feedback link compresses the spring
further and this causes the diaphragm to move back to its original
position.
• The air supply and the vent ports are now closed and the piston of
the main cylinder remains at its previous position. The relationship
between the control pressure and movement of the stem in this case
is more or less linear.
• Moreover due to presence of power cylinder, the scheme is more
suitable to position large control valves.
Cylinder types:
Hydraulic actuators: cylinders
Double acting piston:
Single acting:
work can be done only in one direction
Plunger
Work is done in both
directions
Piston rod on both sides
Piston
Tandem
Telescopic
Fast moving
Telescopic
Fast moving
Hydraulic cylinders
Properties:

The cylinders have to be good quality steel with close tolerances.

There have to be good sealing both at the piston rod and at the cylinder.

With time dirt may come in and damage the surfaces. This has to be possibly reduced.

In this case, the leakage will increase all the time.
Hydraulic cylinders
Calculation of cylinders
FCmax  FLmax  FF
Fcmax  FLmax  FF  F1
maximum load
friction forces
inertial forces
slow motion, can
be often neglected
Outward:
Backward:
FC 0  A1 p1  A2 p2
FCB  A2 p2  A1 p1
Q
v0 
A1
Q
vB 
A2
A2
A1
p2
p1
Q
vB
v0
Hydraulic cylinders
Calculation of cylinders
1.
2.
3.
Stick-slip
Transition
Normal behaviour
Ff
1
2
3
Hydraulic cylinders should be possibly
operated in the 3rd region for smooth
operation.
v
If the cylinder is new, the leakage losses are
negligibly small so that:
ηc = ηmech
c 
v  FL
FL

p1Q1  p2Q2 p1 A1  p2 A2
c
 0,85  0,92
ηc
outwards
inwards
max
at higher pressures
Δp
Hydraulic cylinders
Checking for buckling
Maximum permissible force:
Fmax L
1
 
n  lk
2

  E  I1

n: safety factor: 1-3,5
lk: buckling length
I1: moment of inertia of the piston rod
E: elasticity modulus of the rod material
d 4
64
Hydraulic cylinders
Cushioning of cylinders:
A hard impact of the piston at the end surfaces has to be inhibited
– kinetic energy has to be absorbed.
This is done by increasing the hydraulic resistance at the end of
the stroke.
Rotary hydraulic actuators
Swivel vane rotary actuator:
Limited angle in both directions
Maximum angle always smaller than 360°
Parallel
piston rotary
actuator
Párhuzamdugatty
ús lengőhajtás
The same torque in both directions
Limited angle rotary
actuator
Piston rotary actuator:
With rack and gear coupling
Here maximum angle may be larger than 360°
Limited angle rotary
actuator