Final Control Element
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LECTURE OUTLINE
 1.The Final Control Element
 2. Electric Motor
 3. Relay
 4. Pneumatic actuator
 5. Hydraulics actuator
 6. Power electronics
 7. SCR Actuator Switching Principles
 8. Control valve
*Major part of this presentation are from available
web resource from corresponding manufacturers
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The Final Control Element
 The Final Control Element
Carries out control actions through control
mechanism produced by actuators.
Actuator

Actuator
A peripheral which converts control signals
from the controller to the moving part
(mechanically) of the final control
element.
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The Final Control Element..
. Electrical
motors, pneumatic actuators,
hydraulic pistons, relays, comb drive,
piezoelectric actuators, and electro
active polymers are some examples of
such actuators.
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The Final Control Element…
 Motors are mostly used when circular motions
are needed, but can also be used for linear
applications by transforming circular to linear
motion with a bolt and screw transducer. On
the other hand, some actuators are intrinsically
linear, such as piezoelectric actuators.
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2. Electric Motor
 An electric motor converts electrical energy
into kinetic energy. The reverse task, that of
converting kinetic energy into electrical
energy, is accomplished by a generator or
dynamo. In many cases the two devices differ
only in their application and minor
construction details, and some applications
use a single device to fill both roles. For
example, traction motors used on
locomotives often perform both tasks if the
locomotive is equipped with dynamic brakes.
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Motor operation
Most electric motors work by electromagnetism, but motors
based on other electromechanical phenomena, such as
electrostatic forces and the piezoelectric effect, also
exist. The fundamental principle upon which
electromagnetic motors are based is that there is a
mechanical force on any current-carrying wire contained
within a magnetic field. The force is described by the
Lorentz force law and is perpendicular to both the wire
and the magnetic field. Most magnetic motors are rotary,
but linear motors also exist. In a rotary motor, the rotating
part (usually on the inside) is called the rotor, and the
stationary part is called the stator.
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Motor operation..
The rotor rotates because the wires and magnetic
field are arranged so that a torque is developed
about the rotor's axis. The motor contains
electromagnets that are wound on a frame.
Though this frame is often called the armature,
that term is often erroneously applied. Correctly,
the armature is that part of the motor across
which the input voltage is supplied. Depending
upon the design of the machine, either the rotor
or the stator can serve as the armature.
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ชนิดของมอเตอร์
สามารถแบ่ งตามหลักการทํางานได้ 2 ชนิดคือ
Direct Current Motor
Alternating Current Motor
มอเตอร์ ไฟฟ้ ากระแสตรงแบ่ งออกเป็ น 3 ชนิดได้ แก่
1. Series Motor
2. Shunt Motor
3. Compound Motor
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มอเตอร์ ไฟฟ้ากระแสสลับ(Alternating Current Motor)
 มอเตอร์ ไฟฟ้ ากระแสสลับแบ่งออกเป็ น 2 ชนิ ดได้แก่
1.มอเตอร์ไฟฟ้ ากระแสสลับชนิด 1 เฟส (A.C. Single Phase)
- Split-Phase motor
- Capacitor motor
- Repulsion-type motor
- Universal motor
- Shaded-pole motor
2.มอเตอร์ไฟฟ้ ากระแสสลับชนิด 3 เฟส (A.C. Three phase Motor)
 แบ่งออกตามโครงสร้างและหลักการทํางานของมอเตอร์ ได้ 2 แบบ คือ
1. 3 Phase Induction Motor
2. 3 Phase Synchronous Motor
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DC Motor……
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Wound field DC motor
The permanent magnets on the outside (stator) of a DC
motor may be replaced by electromagnets. By varying the
field current it is possible to alter the speed/torque ratio of
the motor. Typically the field winding will be placed in
series (series wound) with the armature winding to get a
high torque low speed motor, in parallel (shunt wound)
with the armature to get a high speed low torque motor, or
to have a winding partly in parallel, and partly in series
(compound wound) for a balance that gives steady
speed over a range of loads. Further reductions in field
current are possible to gain even higher speed but
correspondingly lower torque, called "weak field"
operation.
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Squirrel Cage rotors
Most common AC motors use the squirrel cage rotor, which
will be found in virtually all domestic and light industrial
alternating current motors. The squirrel cage takes its
name from its shape - a ring at either end of the rotor, with
bars connecting the rings running the length of the rotor. It
is typically cast aluminum or copper poured between the
iron laminates of the rotor, and usually only the end rings
will be visible. The vast majority of the rotor currents will
flow through the bars rather than the higher-resistance and
usually varnished laminates. Very low voltages at very high
currents are typical in the bars and end rings; high
efficiency motors will often use cast copper in order to
reduce the resistance in the rotor.
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Squirrel-cage rotor
Three-phase wound rotor
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Three-phase AC induction motors
Three phase AC
induction motors
rated 1 Hp (746
W) and 25 W with
small motors from
CD player, toy
and CD/DVD
drive reader head
traverse
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The speed of the AC motor
 The speed of the AC motor is determined
primarily by the frequency of the AC supply and
the number of poles in the stator winding,
according to the relation:

Ns = 120F / p
 where



Ns = Synchronous speed, in revolutions per
minute
F = AC power frequency
p = Number of poles per phase winding
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Stepper motors
Closely related in design to three-phase AC synchronous
motors are stepper motors, where an internal rotor
containing permanent magnets or a large iron core with
salient poles is controlled by a set of external magnets
that are switched electronically. A stepper motor may also
be thought of as a cross between a DC electric motor
and a solenoid. As each coil is energized in turn, the rotor
aligns itself with the magnetic field produced by the
energized field winding. Unlike a synchronous motor, in
its application, the motor may not rotate continuously;
instead, it "steps" from one position to the next as field
windings are energized and deenergized in sequence.
Depending on the sequence, the rotor may turn forwards
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or backwards
Stepper motors….
The top
electromagnet (1)
is charged,
attracting the
topmost four teeth
of a sprocket.
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Stepper motors…..
The top
electromagnet (1)
is turned off, and
the right
electromagnet (2)
is charged, pulling
the nearest four
teeth to the right.
This results in a
rotation of 3.6°.
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Stepper motors……
The bottom
electromagnet (3)
is charged;
another 3.6°
rotation occurs.
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Stepper motors…….
The left electromagnet
(4) is enabled,
rotating again by
3.6°. When the top
electromagnet (1) is
again charged, the
teeth in the sprocket
will have rotated by
one tooth position;
since there are 25
teeth, it will take 100
steps to make a full
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rotation.
Nano-motor
Nanomotor
constructed at UC
Berkeley. The
motor is about
500nm across:
300 times smaller
than the diameter
of a human hair
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3. Relay
A relay is an electrical switch that opens and closes
under control of another electrical circuit. In the
original form, the switch is operated by an
electromagnet to open or close one or many sets
of contacts. It was invented by Joseph Henry in
1835. Because a relay is able to control an output
circuit of higher power than the input circuit, it can
be considered, in a broad sense, to be a form of
electrical amplifier.
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Automotive style miniature relay,
Small relay as used in electronics
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Solid-state relay
By analogy with the
functions of the original
electromagnetic device, a
solid-state relay is made
with a thyristor or other
solid-state switching
device. To achieve
electrical isolation, a light
emitting diode (LED) is
used with a photo
transistor. (No moving
part)
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Relay contacts
 Relay contacts can be either Normally Open (NO),
Normally Closed (NC), or change-over contacts.



Normally-open contacts connect the circuit when the relay is
activated; the circuit is disconnected when the relay is
inactive. It is also called Form A contact or "make" contact.
Form A contact is ideal for applications that require to switch a
high-current power source from a remote device.
Normally-closed contacts disconnect the circuit when the relay
is activated; the circuit is connected when the relay is inactive.
It is also called Form B contact or "break" contact. Form B
contact is ideal for applications that require the circuit to
remain closed until the relay is activated.
Change-over contacts control two circuits: one normally-open
contact and one normally-closed contact with a common 36
terminal. It is also called Form C contact.
Relay contacts..
Circuit symbols of
relays. "C"
denotes the
common
terminal in
SPDT and
DPDT types.
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Relay contacts….
 Since relays are switches, the terminology
applied to switches is also applied to relays.
According to this classification, relays can be of
the following types:
 SPST - Single Pole Single Throw. These have
two terminals which can be switched on/off. In
total, four terminals when the coil is also
included.
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Relay contacts….
SPDT - Single Pole Double Throw. These have one row of
three terminals. One terminal (common) switches between
the other two poles. It is the same as a single change-over
switch. In total, five terminals when the coil is also
included.
DPST - Double Pole Single Throw. These have two pairs of
terminals. Equivalent to two SPST switches or relays
actuated by a single coil. In total, six terminals when the
coil is also included. This configuration may also be
referred to as DPNO.
DPDT - Double Pole Double Throw. These have two rows of
change-over terminals. Equivalent to two SPDT switches
or relays actuated by a single coil. In total, eight terminals
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when the coil is also included.
Relay contacts….
A DPDT AC coil
relay with "ice
cube" packaging
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4. Pneumatic actuator
 A pneumatic actuator converts energy (in the form of
compressed air, typically) into motion. The motion can be
rotary or linear, depending on the type of actuator. Some
types of pneumatic actuators include:









Tie Rod Cylinders
Compact Air Cylinders
Rotary Actuators
Grippers
Escapement mechanisms
Rod less Actuators with Magnetic linkage
Rod less Actuators with Mechanical linkage
Specialty actuators that combine rotary and linear motion-41
frequently used for clamping operations
Vacuum Generators
Pneumatic cylinder
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5. Hydraulics actuator
In the hydraulic technology we transmit and control
forces and velocities by transmitting and
controlling pressure and flow.
The principles of hydraulic technology are not new.
In the 18 Th. century in London a hydraulic press
was built and the Eifeltower was adjusted by
water hydraulic jacks. About 200 years BC the
Greek already used machines that were driven
by water hydraulics.
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Examples of uncertainty
statements
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Variable frequency drive
A Variable Frequency Drive (sometimes
abbreviated VFD) is a system for controlling the
rotational speed of an alternating current (AC)
electric motor by controlling the frequency of the
electrical power supplied to the motor. A variable
frequency drive is a specific type of adjustable
speed drive. Variable frequency drives are also
known as adjustable frequency drives (AFD),
variable speed drives (VSD), AC drives or inverter
drives.
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Variable frequency drive..
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Variable frequency drive…
Operating principle
 Variable frequency drives operate under the
principle that the synchronous speed of an AC
motor is determined by the frequency of the AC
supply and the number of poles in the stator
winding, according to the relation:
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Variable frequency drive….
where
RPM = Revolutions per minute
f = AC power frequency (Hertz)
p = Number of poles (an even number)
Synchronous motors operate at the synchronous
speed determined by the above equation. The
speed of an induction motor is slightly less than the
synchronous speed.
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Variable frequency drive…..
Example
 A 4-pole motor that is connected directly to 60 Hz utility
(mains) power would have a synchronous speed of 1800
RPM:

 If the motor is an induction motor, the operating speed at
full load will be about 1750 RPM.
 If the motor is connected to a speed controller that provides
power at 40 Hz, the synchronous speed would be 1200
RPM:

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VFD system description
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VFD Controller..
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Single Phase, Solid State Relays
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Three Phase Power Controller
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8. Control valve
What Is A Control Valve?
 The most common final control element
in the process control industries is the control
valve. The control valve manipulates a
flowing fluid, such as gas, steam, water, or
chemical compounds, to compensate for the
load disturbance and keep the regulated
process variable as close as possible to the
desired set point.
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What Is A Control Valve?..
Many people who talk about control valves or
valves are really referring to a control valve
assembly. The control valve assembly typically
consists of the valve body, the internal trim parts,
an actuator to provide the motive power to
Operate the valve, and a variety of additional
valve accessories, which can include positioners,
transducers, supply pressure regulators, manual
operators, snubbers, or limit switches.
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Control Valve Terminology
 Accessory: A device that is mounted on the
actuator to complement the actuator’s function
and make it a complete operating unit. Examples
include positioners, supply pressure regulators,
solenoids, and limit switches.
 Actuator*: A pneumatic, hydraulic, or electrically
powered device that supplies force and motion to
open or close a valve.
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Control Valve Terminology..
 Capacity* (Valve): The rate of flow through a
valve under stated conditions.
 Control Range: The range of valve travel over
which a control valve can maintain the installed
valve gain between the normalized values of 0.5
and 2.0.
 Equal Percentage Characteristic*: An inherent
flow characteristic that, for equal increments of
rated travel, will ideally give equal percentage
changes of the flow coefficient (Cv).
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Inherent Valve Characteristics
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Control Valve Terminology…
 Installed Characteristic*: The relationship
between the flow rate and the closure member
(disk) travel as it is moved from the closed
position to rated travel as the pressure drop
across the valve is influenced by the varying
process conditions.
 I/P: Shorthand for current-to-pressure (I-to-P).
Typically applied to input transducer modules.
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Control Valve Terminology….
 Positioner*: A position controller
(servomechanism) that is mechanically connected
to a moving part of a final control element or its
actuator and that automatically adjusts its output
to the actuator to maintain a desired position in
proportion to the input signal.
 Quick Opening Characteristic*: An inherent flow
characteristic in which a maximum flow coefficient
is achieved with minimal closure member travel.
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Control Valve Terminology…..
 Sizing (Valve): A systematic procedure
designed to ensure the correct valve capacity for
a set of specified process conditions.
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Sliding-Stem Control Valve
Terminology
Control Valve
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Sliding-Stem
Control Valve
Terminology..
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SlidingStem
Control
Valve
Terminol
ogy…
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Sliding-Stem
Control Valve
Terminology
….
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Characterized Cages for GlobeStyle Valve Bodies
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Rotary-Shaft Control Valve
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Rotary-Shaft Control Valve.
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Rotary-Shaft Control Valve…
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Control Valve Functions
and Characteristics Terminology
 Fail-Closed: A condition wherein the valve closure
member moves to a closed position when the
actuating energy source fails.
 Fail-Open: A condition wherein the valve closure
member moves to an open position when the
actuating energy source fails.
 Fail-Safe: A characteristic of a valve and its
actuator, which upon loss of actuating energy
supply, will cause a valve closure member to be
fully closed, fully open, or remain in the last
position, whichever position is defined as necessary
to protect the process.
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Rotary Actuators
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Linear Actuators
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Valve Sizing (Finding Cv)
Sizing Valves for Liquids
Draft estimate only
Q = Cv√(Δp/SG)
Where
Q : U.S. gallons per minute flow
Cv : Valve capacity
Δp : Pressure drop across the valve psi
SG : Specific gravity of liquid unit less
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Valve Sizing (Finding Cv)..
Example, find the proper Cv for a valve that must
allow 150 gal/min of ethyl alcohol with specific
gravity of 0.8 at maximum pressure of 50 psi.
Q = Cv√(Δp/SG)
Cv = Q √(SG/Δp)
= 150√(0.8/50)
= 18.97
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