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• Actuators: An actuator is the part of a final control device that causes a
physical change in the final control device when signalled to do so. The most
common example of an actuator is a valve actuator, which opens or closes a
valve in response to control signals from a controller. Actuators are often
powered pneumatically, hydraulically, or electrically.
• Diaphragms, bellows, springs, gears, hydraulic pilot valves, pistons, or electric
motors are often parts of an actuator system.
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SENSORS/PRIMARY ELEMENTS
• A sensor is a device that detects or measures a physical quantity and
produces an electrical signal proportional to the sensed input.
• The greatest ingenuity in the process control field is apparently in sensing
devices. Because sensing devices are the first element in the control loop to
measure the process variable, they are also called primary elements.
• Sensors have contributed significantly to recent advances in manufacturing
technology. Using a sensor makes a process or system more automated and
removes the need for human operators to monitor and control the situation.
• The three main categories of sensors are;
• Limit switches
• Proximity sensors
• Photoelectric sensors.
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LIMIT SWITCHES
• A limit switch is an electromechanical device. A part of the limit switch, called
an actuator, is placed in the path of an oncoming object, such as a box on a
conveyor. When the object contacts the actuator, the contacts in the limit
switch are opened (or closed, depending on the limit switch’s design) to stop
(or start) the flow of current in the electrical circuit.
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PROXIMITY SENSOR
• This type of sensor uses an electronic field to detect when an object is near. There
is no physical contact between the object and the sensor.
• Inductive proximity sensors detect only metal objects. Capacitive proximity
sensors can sense both metallic and non-metallic objects.
• In a manufacturing process where the alignment of a part is critical. A proximity
sensor can be used to make sure the part is aligned within a certain tolerance. If
the part is not properly aligned, the proximity sensor will be triggered. This type
of sensor is generally used to sense at distances less than one inch.
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PHOTOELCETRIC SENSOR
• Photoelectric sensor uses light to detect the presence or absence of an object.
• A Thru-Beam photoelectric sensor uses two devices (a light source and a
detector) facing each other. Detection occurs when an object blocks or breaks the
beam of light passing between them.
• A Retro-Reflective (Reflex) sensor emits a light beam that is reflected back to the
sensor from a retro reflector. When an object blocks the beam between the
sensor and the retro reflector, detection occurs.
• A Diffuse Reflective sensor emits a light beam that must be reflected back to it by
the target object itself for detection to occur .
• Most electric garage door openers include a photoelectric sensor for safety
reasons. If the photoelectric sensor’s beam is broken (by a child for example) as
the door is going down, the sensor signals the door opener to reverse the
direction of the door.
• While environmental factors can affect photoelectric sensors, these devices have
along sensing range. The objects they detect can be of any material.
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TRANSMITTER
• A Transmitter is a device that converts the signal produced by a sensor into a
standardized instrumentation signal such as 3‐15 PSI air pressure, 4‐20 mA DC
electric current, Fieldbus digital signal etc., which may then be conveyed to
an indicating device, a controlling device, or both.
• The indicating or controlling device is often located in a centralized control
room.
• The transmitter often combines a sensor and the transmitter in a single piece.
• The sensor measures the process variable and generate a proportional signal.
The transmitter then amplifies and conditions the sensor signal for
transmission to the receiving or controlling device.
• Transmitters used in Process Instrumentation can be broadly divided into two
broad groups:
(a) Electronic Transmitters
(b) Pneumatic Transmitters
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• We can further group transmitters according to the types of signals they
produce namely:
(a) Pneumatic Transmitters
(b) Analog Transmitters
(c) Digital Transmitters
Pneumatic Transmitters
• Pneumatic transmitters output a pneumatic signal corresponding to the
process variable. The pneumatic signal range that is commonly used in
industrial plants today is the 3 – 15psig. 3psig corresponds to the lower range
value (LRV) and 15psig corresponds to the upper range value (URV). It is still
commonly used today especially in remote locations where electric power is
not readily available.
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Analog Transmitters
• Analog transmitters are mostly electronic in nature. They output an electrical
signal (current or voltage) whose magnitude represents a physical
measurement or a control quantity.
• The transmitter is classified as being analog by virtue of the fact that it uses
an analog signal standard to communicate information.
• The most common standard for transmitting an analog signal is the 4‐20 mA
current signal. With this signal, a transmitter sends a small current,
proportional to the physical measurement, through a set of wires.
• In this signal standard, 4mA represents the lowest possible measurement or
the LRV (Lower Range Value) while the 20mA represents the highest possible
measurement or URV (Upper Range Value).
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Digital Transmitters
• Digital transmitters produce digital signals that are combined in a variety of
ways to enhance communication with the devices.
• Enhances diagnostic capabilities of the device and makes control of the
devices and processes relatively easy and smooth.
• Digital signals are discrete levels or values that are combined in specific ways
to represent process variables and also carry other important information,
such as diagnostic information.
• Digital transmitters combine the digital signals in a variety of ways leading to
various communication protocols such as Fieldbus, HART etc.
• Most digital transmitters may be referred to as smart instruments. They have
inbuilt microprocessors that helps in signal conditioning and processing and
gives the devices some diagnostic capability.
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Electronic Transmitters
• Electronic transmitters are grouped according to the number of wires
necessary to provide transmitter power. Accordingly, there are 2‐wire, 3‐wire
and 4‐wire transmitters.
Two wire transmitters:
• These are the simplest and most economical transmitter devices. They are
often called loop powered instruments. In a two wire system, the only source
of power to the transmitter is from the signal loop.
• The 4 mA zero‐end current is sufficient to drive the internal circuitry of the
transmitter and the current from 4 to 20 mA represents the range of the
measured process variable. The power supply and the instruments are
usually mounted in the control room.
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Three wire transmitters
• These transmitters require more power than the signal loop (4‐20mA. etc)
can supply their internal circuitry.
• A DC common wire is run from the instrument to the transmitter. This
permits the transmitter to draw whatever power it needs from the power
supply and produce the desired signal current at the transmitter output.
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Four wire transmitters
• These transmitters have their own internal power supply hence they are
often referred to as self‐powered instruments.
• They require no connection to the DC power supply. A 120 Vac sources is
connected only to the receiving instrument.
• These are often used where an instrument is added to the load of the DC
supplies. The disadvantage is the need for AC power at the instrument site.
