PROCESS CONTROL (IT62)
Chapter 4
CONTROLLERS, TRANSMITTERS, CONVERTERS AND
RELAYS
by
Dr. Mallikarjun S. Holi
Professor & Head
Department of Biomedical Engineering
Bapuji Institute of Engineering & Technology
Davangere-577004
1
Chapter 4 : Controllers, Transmitters, Converters and Relays
4.1 Introduction
In this chapter we study the fundamental concepts, design and working of signal
converters, signal generators, computing relays, transmitters and annunciators,
which are the part of process control. One should have knowledge about different
types such devices which will help designing appropriate control system based on
the application.
4.2 Objectives
At the end of this chapter you will be able to understand:
• Principle of signal conversions and working of different signal converters
• Fundamental concepts of signal generators and their design and working
• Principle and working of computing relays
• Basic concepts and working of different type of transmitters used in process
control.
• Different types of annunciators and their working.
4.3 Converters
The signal conversion refers to the modifications that must be made to the control
signal to properly interface with the next stage in the control loop. Thus, if a valve
control element is to be operated by an electric motor actuator, then a 4-20 mA dc
control signal must be modified to operate the motor. If a dc motor is used,
modification might be current to voltage conversion and amplification. The
devices that perform such signal conversions are often called transducers because
they convert control signals from one form to another, such as pneumatic to
electronic (i.e. pressure to current), electronic to pneumatic (i.e. current to
pressure), current to voltage, etc. The principal objective of signal conversion is to
convert the low-energy control signal to a high-energy signal to drive the actuator.
Pneumatic-to-Electronic Converters
The pneumatic-to-electronic transducer is used wherever pneumatic signals must be
converted to electronic signals for anyone of the following reasons:
• Transmission over large distances
• Input to an electronic logger or computer
• Input to telemetering equipment
• Instrument air not available at the receiver controller
In principle, any of the electronic pressure transmitters could be used, but in practice,
special devices are used to improve accuracy. The air signals are at low pressure levels
(3-15 PSIG), and many of the pressure detectors are not sensitive or not linear enough at
these pressures. A P/I transducer should be at least ½ percent accurate and preferably ¼
percent to preserve the integrity of the initial signal. Since the total error is the square
root of the mean squares or the individual component errors, the greater the precision of
the P/I transducer, the better the signal.
2
Because of this need for accuracy, most P/I transducers use a bellows input and a motion
balance sensor. A typical high-quality P/E converter is shown in Figure 4.1.
When 3-15 PSIG input pressure is applied to pressure capsule or bellow, it leads to
movement of the LVDT core between the primary and secondary coils. The primary coil
has input excitation from a square wave oscillator. The movement of the core leads to
generation of differential potential across two secondary which is demodulated to obtain
dc voltage output, which will be directly proportional to applied input pressure.
Fig. 4.1 Pneumatic to electronic converter
Millivolt-to-Current Converters
Millivolt-to-current converters are widely used in the measurement of temperature, using
thermocouples or other millivolt generating sensing elements. They are also utilized in
converting the output signals of analyzers into higher level transmission signals. A
typical millivolt-to-current converter is illustrated in Fig.4.2.
When these devices are used to convert thermocouple outputs, they are also referred to as
temperature transmitters.
Voltage-to-Current Converters
Voltage-to-Current Converters are used for conversion of higher voltages into
transmission signals, preferably in the form of current. These circuits as shown in
Fig. 4.3 usually consist of voltage dividers & rectifiers to reduce the voltages to a
level compatible with the receivers.
3
Fig. 4.2 Millivolt to current converter
Fig. 4.3 Solid-state voltage to current converter
4
Current-to-Current Converters
Current-to-Current transducers are available to convert AC signals to DC (or DC
to DC) and to amplify or reduce their levels as necessary. Levels of alternating
current can be changed, when necessary, by current transformers. A common
signal levels in power industry is 0-5 amps. Direct currents are re-ranged by
putting a series of resistors in the circuit and reading the voltage drop across it. A
converter of AC to DC milliamperes is shown in Fig. 4.4. The figure shows three
separate devices, a current transformer, an AC to DC milliampere converter, and a
current-to-air converter. The function of the transformer is to scale down the
current to the range normally used for direct AC metering. The AC/DC converter
makes this signal compatible with the usual DC milliampere transmission system.
DC to DC converters are sometimes used for isolation of electrical circuits, such as
with intrinsically safe systems.
Fig. 4.4 AC to DC milliampere converter with integral I/P transducer
Current-to-Air Converters
Electro-pneumatic transmitters are also called converters and transducers. They are
extremely important, since they form the link between electrical measurements and
pneumatic control systems. They also convert electronic controller outputs into air
pressures for operation of pneumatic valves.
Figure 4.5 illustrates one of these converters and also lists the various electric devices
with which it is commonly combined. The input is usually a DC current in the range of
1-5, 4-20, or 10-50 milliamperes. An permanent magnet creates a field that passes
through the steel body of the transmitter and across a small air gap to the pole piece. A
multi-turn, flexure-mounted voice coil is suspended in the air gap. The input current
flows through the coil creating an electromagnetic force that tends to repel the coil and
thus converts the current signal into a mechanical force.
Since the total force obtainable in a typical voice coil motor with such small current
5
inputs is only in the order of some ounces, a different approach, namely, the use of a
reaction nozzle, is employed to convert the force into a pneumatic output pressure. In this
circuit, supply air flows through a restriction and out the detector nozzle. The reaction of
the air jet as it impinges against the nozzle seat supplies the counterbalancing force to the
voice coil motor. The nozzle back pressure is the transmitted output pressure.
In order to make the transmitter insensitive to vibration, the voice coil is integrally
mounted to a float, submerged in silicone oil. The float is sized so that its buoyant force
equals the weight of the assembly, leaving a zero net force.
Zero is adjusted by changing a leaf-spring force. Span is adjusted by turning the rangeadjusting screw to change the gap between the screw and the magnet, thus shunting some
of the magnetic field away from the pole piece.
Fig. 4.5 Electronic to pneumatic transducer – transmitter with typical input sources
Resistance-to-Current Converters
Resistance measurements are common in temperature measurements and in resistance or
strain gauge sensors. The circuits used are similar to those of the millivolt-to-current
converters, except that the front end is a resistance bridge instead of a voltage bridge as
shown in Fig.4.6. In the strain gauge bridge, the strain gauge elements may take the place
of two of the resistors in the resistance bridge shown in Fig.4.6.
6
Fig. 4.6 Resistance to current converter
4.4 Function Generators
Time Function Generators (Programmers)
The simplest and the least expensive analog time function generator is the cam-type
programmer. These are assembled in large-case circular chart recorder housings as shown
in Fig. 4.7, and consist of a motor-driven cam that moves the set-point index, to which a
motion transmitter is connected. The output is usually a 3-15 PSIG pneumatic set-point
signal. Electric outputs are also available. The time base is a function of motor speed, and
a wide selection of speeds is available. The cams can be made of plastic or metal. It is
also common to incorporate an integral controller, direct-sensing element, and circular
chart recorder in the same housing.
Cam programmers are usually applied to batch processes that are repeated time after
time. These units are not as accurate as the profile tracer and line follower types of more
recent manufacture. The cam rise is also limited for mechanical reasons to about 50degree cam rotation for full-scale movement of the index. Curvilinear coordinates make
the cams more difficult to lay out as compared with programming a device with
rectilinear coordinates.
7
Fig. 4.7 Cam type programmer
Adjustable Ramp-and-Hold Programmers
For batch processes in which the controlled variable must be made to rise at a controlled
rate, then hold at some preset value and, possibly, fall at a controlled rate, programmers
such as that in Fig. 4.8 are often preferable to cam types, particularly if the program must
be changed periodically. These, too, are usually packaged as large-case circular chart
recorders. In this type programmer, the set-point index is driven by a constant speed
motor. The rate of rise is set by adjustment of an interrupter timer, which makes contact
for a set percentage of the basic timer cycle time. The movement of the index is,
therefore, actually in steps, hut the steps are so small that the operation is, for all practical
purposes is continuous. The set point rises until it coincides with the hold point index, at
which point hold timer is energized while the interrupter timer is de-energized.
Controlled cooling rate requires driving the set-point index in reverse.
8
Fig. 4.8 Adjustable ramp-and-hold programmer
Profile Tracer Programmers
Profile tracer programmers come in 150 x 150 mm miniature pneumatic recorder type
cases as shown in Fig. 4.9. The program is stored on a laminated, endless belt plastic
master. It combines an analog set-point program with up to 25 synchronized digital tracks
for operation of logic circuits, auxiliary equipment, solenoid valves, lights, etc. There is
no limit to the slope the programmer can follow-even slopes of 90 degrees are
accommodated.
Since the master program can be quickly changed, these programmers are often used
where the program does require periodic change and where accurate reproduction of the
program is essential, as in textile dyeing processes. The complete program is stored on
the master, thus eliminating the need for having an operator make various program
settings for each change and, therefore, eliminating the chance for human error in setting
the program. These programmers are accurate to within ¼ percent of full scale, which
makes them applicable when accuracy alone is the critical requirement of the operation.
The endless belt master is made up by plotting the desired analog program on the
rectilinear chart and cutting the top portion away with scissors. The second layer serves
as a backing and is also used to program the synchronized digital tracks. At any point of
the program where a switch action is desired, a hole is punched in with a conductor's
punch. The back of the analog program has a pressure-sensitive adhesive that joins the
two sections. A splice finishes the make-up of the master,
9
In operation motor drives the master program. A cable-mounted tracer
nozzle senses the step on the analog program profile. The back-pressure of the nozzle
actuates a servo, which, through the cable drive, keeps the tracer following the profile.
Operating from the same servo drive is an accurate force balance type motion detector.
Sensing the back side of the digital master are a series of vertically aligned nozzles.
Normally, their hack pressure is high since the master baffles the nozzles. However, if a
punched hole presents itself, the hack pressure of that particular nozzle drops to zero,
actuating the connected pressure switch.
Fig. 4.9 Pneumatic profile tracer programmer with synchronized on-off sequence control
switches
Electric Line and Edge Programmers
Electric line and edge follower programmers will perform with less than ±¼ percent
error. In the electrostatic line follower type, Fig. 4.10, the desired program curve is
etched into a conductive surface chart, dividing it into two electrically isolated surfaces.
The surfaces are energized by oppositely phased AC voltages establishing a gradient
across the gap. A non-contacting probe senses the electrostatic field developed by the
surfaces and energizes a servo amplifier to keep the probe tracking the line, which is at
zero potential. Attached to the servo drive is the wiper of a potentiometer whose output is
proportional to line position. The photoelectric line follower type functions to keep the
line centered between two slightly overlapping pickup heads. The detector must he
manually set over the line at start-up, and slope rate is limited by the speed of the
follower mechanism.
The photoelectric edge follower consists of a chart which is divided into a transparent
and an opaque section at the program line. A photocell detector senses the edge and a
servo system tracks it. Up to eight digital tracks are available with the electric
programmers.
10
Fig. 4.10 Electric line follower programmer
11
4.5 Computing Relays
Relay: An electrical switch that allows a low power to control a higher one. A small
current energizes the relay, which closes a gate, allowing a large current to flow through.
In general, it is device that receives information in the form of one or more instrument
signals, modifies the information or its form, or both, and if required, sends out one or
more resultant signals.
Electromechanical Relay: An electrically controlled mechanical device that opens and
closes electrical contacts when a voltage (or current) is applied to a coil. A relay provides
isolation of control signals from switched signals.
Solid State Relay: A Solid State relay is a switching device that completes or interrupts a
circuit electrically and has no moving parts. A Mechanical relay is an electromechanical
device that closes contacts to complete a circuit or opens contacts to interrupt a circuit.
Computing Relays: The relays which are able produce suitable output based on the
fundamental formulation/equation on which they are designed in analog / digital /
pneumatic form.
Pneumatic Multiplying and Dividing
In the force bridge multiplier-divider shown in Figure 4.11 input pressures act on bellows
in chambers A, B, and D. The output is a feedback pressure in chamber C. The bridge
consists of two weigh-beams that pivot on a common movable fulcrum, with each beam
operating a separate feedback loop. Any unbalance in moments on the left-hand beam
causes a movement of the fulcrum position until a moment-balance is restored. An
unbalance in moments on the right-hand beam results in a change in output pressure until
balance is restored. Equations which characterize the operation of the force bridge are
A x a = B x b and
Dxa=Cxb
(4.1)
The equation can be reduced to
AxC=BxD
(4.2)
C = (B x D) / A
(4.3)
Multiplication results when the two input variables are connected to chambers B and D,
Division results when the dividend is connected to either chambers B or D, with the
divisor connected to A. Simultaneous multiplication and division results when B, D, and
A chambers are used.
Electronic Multiplying and Dividing
In Fig. 4.12, inputs e1 and e2 are multiplied in the diode bridge. Conduction of the diodes
in the bridge is dependent upon the relative magnitude of the inputs with respect to the
constant slope of the sawtooth input. The output of the diode bridge is trapezoid, which
has an area equivalent to
Area = e1e2 tan θ
(4.4)
The angle θ is established by the constant slope of the sawtooth, and thus
Area = Ke1e2
12
(4.5)
The output voltage, e0, is amplified and filtered to a DC signal, and its voltage level will.
therefore be proportional to the area and, consequently, to the product of e1 and e2.
Adding another diode bridge to the multiplier circuit produces a multiplier/divider as
shown in Fig.4.13. The input to the amplifier is the output difference from the two bridge
networks.
e0 = A (K e1e2 - Ke3e0)
where A = gain of the amplifier. Rearranging the terms we will get
e1e2 = [(e0)/AK + e3e0]
The term e0/AK is very small if the amplifier gain is high, and thus
e0 = (e1e2)/e3
Fig. 4.11 Pneumatic force bridge multiplying and dividing relay
13
(4.6)
(4.7)
(4.8)
Fig. 4.12 Electronic multiplier
Fig. 4.13 Electronic multiplier-divider
Pneumatic Adding, Subtracting, and Inverting
In the force balance arithmetic computing relay as shown in Fig. 4.14, a signal pressure in
chamber A acts downward on a diaphragm with unit effective area. A signal in chamber
B also acts downward on an annular diaphragm configuration, likewise having an
effective area of unity. Signal pressures in chambers C and D similarly act upward on
unit effective diaphragm areas. Any unbalance in forces moves the diaphragm assembly
with its integral nozzle seat. The change in nozzle seat clearance changes the nozzle back
pressure and hence, changes the output pressure, which is fed hack into chamber D until
force balance is restored. The basic equation which describes the operation of the relay is
T=A+B–C±K
(4.9)
K is the spring constant, which is adjustable to give the required pressure.
14
Fig. 4.14 Pneumatic adding, subtracting, inverting, and biasing relay
Electronic Adding, Subtracting, and Inverting
In Fig.4.15, the two input potentials e1 and e2 are compared in the multiple comparator,
which produces a proportional output to the amplifier. The current paths of the two inputs
can be the same or opposite, resulting in either an adding or subtracting circuit,
respectively.
Inverting is accomplished by biasing the comparator to produce maximum output with no
input. Applying a reverse input (i.e., a reverse current input with respect to bias current)
causes the output to decrease with increasing input. The feedback signal is such that the
amplifier acts as a unity gain network.
Electronic Scaling and Proportioning
Simple electronic scaling or proportioning involves combining a voltage divider circuit
with an amplifier. The voltage divider circuit is connected to either the input or output
side, depending upon whether the gain is to he greater or less than unity.
15
In Fig. 4.16, the amplifier comes to balance when ∆ei equals zero. Since the voltage
divider is on the output, only a portion of the amplifier output is fed back to
counterbalance the input voltage. Therefore, the output will rise above eI, resulting in
gains greater than one. The operation can be expressed as
(4.10)
e0 = eI (R1 + R2) / R1
Fig. 4.17 shows the electronic scalar with gain less than one. The operation can be
expressed as
e0 = eI R2/(R1 + R2)
(4.11)
Fig.4.15 Electronic adder, subtracter, and inverter
Fig.4.16 Electronic scalar with gain greater than one
Fig.4.17 Electronic scalar with gain less than one
16
Electronic Differentiating
The input amplifier in Fig.4.18 is capacitor coupled so that only the rate of change of the
input signal is seen by the amplifier. Two diodes in the feedback of the amplifier allow
its output to go positive or negative (depending on the direction of the rate of change) by
an amount equal to the forward drop across the diodes (only a few tenths of a volt). The
output amplifier inverts and amplifies this signal by its open loop gain.
A small positive feedback is applied to the last amplifier to prevent output from
"chattering" at the diodes' switching point.
Fig.4.18 Electronic differentiator
Electronic Integrating
The first amplifier in Figure 4.19, a Simple inverting type, performs the integration
function as the charge accumulates across the capacitor of the RC network. The second
amplifier is an inverting, general purpose type, which output directly to the input.
Fig.4.19 Electronic integrator
Electronic Square Root Extracting
The square root converter, Fig. 4.20, combines a DC amplifier with a negative feedback
diode network. As current into the amplifier increases, the amplifier gain decreases with
decreased feedback resistance in the diode network. The gain varies according to,
typically, seven straight line segments that approximate a square root function. This is
accomplished by having seven diode-resistance paths in the feedback network
automatically parallel each other with increasing input. The output stabilizes when the
diode network modified feedback counterbalances the input.
17
Fig. 4.20. Electronic square root extractor
Electronic High- and Low-Voltage Selector and Limiter
The higher of the two positive inputs in Fig.4.21 causes a higher negative potential at the
cathode of one of the diodes (CRl or CR2). The forward bias of this diode passes the
higher input and reverse biases the other diode to isolate the lower input. Thus if signal e1
drops below signal e2, CR2 is forward-biased to pass signal e2 and CRI is reverse-biased
to isolate signal e1. All the amplifiers are unity-gain inverter types.
Substituting a fixed input for one of the variables produces a low-limit relay.
To obtain a low-voltage selector as shown in Fig.4.22, the diodes are inverted and a
negative supply (e) is used. Thus, the least positive input forward-biases one of the diodes
(by the least negative potential applied to the anodes of the diodes). This automatically
reverse biases the other diode and isolates the higher input from the output. Substituting a
fixed input for one of the variables produces a high-limit delay.
Fig. 4.21. Electronic high-voltage selector
18
Fig. 4.22. Electronic low-voltage selector
4.6 Transmitters
4.6.1 Electronic and Intelligent Transmitters
A transducer is a device that receives information in one form and generates an output in
response to it. A transmitter is transducer that responds to a measurement variable and
converts that input into a standardized transmission signal.
An example of a simple transmitter or transducer is a thermocouple for measuring
temperature. In the thermocouple, the temperature difference between the hot
junction and the reference junction creates a DC voltage directly proportional to the
temperature difference.
Force-Balance Transmitters
Fig.4.23 illustrates a force-balance differential pressure transmitter, in which the
measurement that produces a force tends to move the top of the force bar. This tiny
motion, acting through levers, moves the ferrite disc closer to the transformer, changing
its output. This changes the amplitude output of the oscillator, which is rectified and then
amplified to generate a DC milliampere transmitter signal. This output signal is fed back
through the voice coil on the armature of the force motor, which is in series with the
output terminals. When this feedback moment is equal to the moment created by the
measurement force F2, the force bar is again in its original position and the amplifier
signal stabilizes.
The advantage of force-balance units over motion-balance devices is that by reducing
motion, one minimizes the effect of pivot friction. Further, by always returning to the
same position, hysteresis is minimized and greater accuracy can be obtained. In general, a
force-balance cannot be used to produce digital signals without the use of a
19
supplementary device external to the transmitter, such as an analog-to-digital
converter (ADC).
Fig.4.23 Motion-balance electronic transmitter
Motion-Balance Transmitters
In a motion-balance transmitter (Fig.4.24), the process measurement produces
motion against a calibration spring, resulting in a change of position corresponding
to a change in the process variable. This position is detected by a transducer. The
output of the transducer is amplified and an electric feedback signal is used to
stabilize the amplifier. Depending upon the type of transducer and the signal level it
generates, the amplifier may not be required but may be a part of the receiver.
20
Fig.4.24 Motion-balance electronic transmitter
Differential Transformer
One of the most frequently used transducer principles is known as the differential
transformer as shown in Fig.4.25.
The linear variable differential transformer (LVDT), illustrated in of Fig. 4.25(a),
consists of a transformer coil with a single primary winding and two symmetrically
spaced secondary windings. The core or armature is a cylinder of magnetic material, such
as ferrite, which can be moved within the "air gap" of the windings.
When AC excitation is applied to the primary winding and the armature slug is centered,
or is in the "null" position, the induced AC voltage in the secondary windings is equal
and is in the same or opposite direction, depending on the method of winding. If the two
secondary windings are connected in series with the voltages opposed (Fig. 4.25(b)), they
will cancel out and give a zero or null reading when the slug is in the null position. As the
slug is moved closer to coil A and further from coil B, as shown in Fig. 4.25(c), the
output voltage increases in the direction of the coil A output, and this increase is
proportional to the displacement of the slug.
Another typical hookup is shown in Fig. 4.25(d), with a rectifier in the output of each
secondary coil hooked up to a DC zero center meter. The meter will read zero at the null
slug position and plus or minus for slug positions displaced from the center.
The DC output of such a differential transformer can be amplified and used directly as a
transmitted signal as shown in Fig. 4.24 or it can be used as a position detector of other
devices as shown in Fig. 4.23.
The excitation in a differential transformer must he supplied by an AC circuit. The AC
source can be the receiving instrument using an AC transmitted signal or it can be a part
of the amplifier in the transmission part of the transmitter, powered by a DC supply taken
from the receiver.
21
Fig.4.25 Linear variable differential transformer
Photoelectric Transducer
Fig.4.26 shows a typical schematic of a photoelectric transducer where the position of
the photocoder is proportional to the motion of a primary sensing element. Light from
the source shines through perforations in the shutter to energize photoelectric cells. The
output of these cells is scanned and the pulses are amplified to produce a digital signal,
or they are rectified to produce DC analog signal.
Fig.4.26 Photoelectric encoder transducer
22
Other popular transducers which are used in process control are capacitance type,
potentiometric type and piezoelectric type transducer.
Intelligent Transmitter
An intelligent transmitter incorporates a microcontroller or a microprocessor in which not
only the measurement signal is transferred from the transmitter to a receiver, such as an
indicator or controller, but also the microprocessor implements the smart functions and
provides a communication facility. This enables data specific to the transmitter itself, such as
its type, serial number, etc, to be stored at the transmitter and accessed via the measurement
loop in which it is installed, as shown in Fig.4.27. Other functions, such as setting or resetting
the zero and span, details of the location and application, and running diagnostic routines to
give warning of malfunctioning, can also be implemented. The term "intelligent" has come to
be used to identify such transmitters.
A further evolution which is now under way is to multiplex the transmitter outputs onto a
network or "fieldbus" instead of connecting the transmitters via individual circuits to the
control room.
Fig.4.27 General schematic of an intelligent transmitter
4.6.2 Fiber Optic Transmitters
Electricity and pneumatics are commonly used to measure, control, and transmit
information among process, controller, and operator. Fiber optics provides an alternate
method of process measurement and information transmission. Although fiber optic (FO)
transmission hardware and systems are readily available to the user, FO instruments are
not yet available in the same broad spectrum as are electrical or pneumatic instruments.
This is because the efforts of the FO industry have been primarily directed toward
23
meeting the needs of their prime customers, namely AT&T (commercial FO communication) and the military (special purpose FO instrumentation). Today the industrial and
commercial world lags behind the military in FO instrumentation development.
Instrumentation and Fiber Optics
Fiber optics technology provides the instrument engineer with analog and digital sensors
that measure process variables, operator actuation devices, interfaces to instrument
(pneumatic, electrical, electronic, mechanical) sensors (hybrid devices), and
communication capability between system analog or digital controllers.
One of fiber optic's most compelling features for the instrumentation engineer is its
intrinsically safe characteristics for use in hazardous environments. Moreover, the advent
of digital electronics, the microprocessor, and distributed control systems has resulted in
all awareness of problems associated with electronics in an industrial environment. These
problems include voltage isolation between equipment, ground isolation, EMI, RFI,
noise, lightning susceptibility, and EMP susceptibility. Since FO cable is a nonconductor
and does not radiate energy, it can provide a built-in solution to these problems. Fiber
optics are also being used today in the analog realm to transmit 0-10V and 0-20mA
information, thus minimizing use of shielded cables.
The microprocessor is now implanted in all levels of the instrument born system building
block (i. e., process, controller, and operator interface). Information interchange between
these blocks is implemented by various methods of communication, including 20mA
two-wire loops, serial, parallel, baseband, and broadband communication. The ability of
fiber optics to meet high information throughput requirements, while maintaining its
advantages in its operating and environmental capabilities is making FO an attractive
alternate in communication applications.
Conventional instrumentation can be converted or expanded utilizing fiber optics because
of the advances in optoelectronics. The marriage of conventional sensors and transmitters
with fiber optics via optoelectronics is common and is sometimes referred to as a hybrid
fiber optic link.
Fiber Optic Principles
Optical fiber bandwidth (dispersion) is one of the basic parameters considered in all
applications. When analyzing dispersion in an optical fiber, the index of refraction (n) is
considered as follows:
n=C/V
(4.12)
C = speed of light in a vacuum (3 x 108 m/sec), and
V = speed of light within the optical fiber, selected.
The index profile of an optical fiber defines how the index of refraction varies as a
function of radial distance from the fiber’s center, Step index profile refers to a fiber
having an index of refraction with an abrupt change (step) at the core radius. The optical
fiber divides into two sections, with the inner light-carrying optical conductor called the
core and the outer jacket called the cladding, as shown in Fig.4.28.
Optical energy is inserted into the core and travels along the core through internal
reaction at the core through internal reflection at the core cladding interface. Reviewing
24
the figure, the comparative index of refraction for core (n1) and cladding (n2) is shown in
Fig.4.29. Note the index of refraction for the cladding (n2) is less than that of the core (nl)
Fig.4.28 Fiber optic cable cross section and light transmission through fiber.
Fig.4.29 Index of refraction comparison.
The core to cladding index ratio is a prime factor in defining optical transmission
dispersion. To, illustrate this, a discussion of multimode propagation characteristics is
required. In a multimode fiber, Fig.4.28 might be modified as illustrated in Fig. 4.30.
Two factors must be recognized in this figure. First, not all, of the light travels down the
core. Some of the light is lost in the cladding. The amount of light loss (dispersion) is a
function of the angle that the light within the core hits the cladding, Snell’s law provides
the minimum angle that supports internal reflection, as follows:
Sin θmin = n2 / n1
(4.13)
θmin depends on the cable selected and the indexes of refraction of the cladding and the
core. Rays striking the core-cladding interface at angles less than θmin will he lost in the
cladding (Fig.4.31).
25
Fig. 4.30 Light transmission in a multimode fiber
Fig. 4.31 Minimum angle for internal reflection depends upon the indexes of refraction of
cladding and core
Referring to Fig.4.30 note that many rays are traveling down the core, varying from the
axial ray to various rays striking the cladding at various acceptance angles. Because each
ray travels a different distance at the same speed, their arrival times will vary. Fig.4.30
illustrates a cable that has numerous propagation modes and an abrupt change in index
profile; this fiber is called a multimode step index type fiber. Characteristically,
multimode step index fibers can cause bit smearing or intersymbol interference in a
digital data system and delay distortion in an analog-modulated system.
To decrease mode volume, small glass fibers with n1/n2 as small as practical are
constructed. If "V" is less than 2.405 m/sec, only a single-mode (axial ray) can propagate.
Called a single-mode fiber, this design exhibits no modal dispersion at all.
Single-mode construction offers the best bandwidth features but provides design
challenges in injecting light into the small-diameter core as well as fiber-to-fiber splicing.
Fig.4.32 illustrates single-mode transmission.
Fig. 4.32 Single-mode (monomode) fiber
The fiber core can be constructed with an index of refraction that decreases parabolically
from the center of the fiber. Light propagation now occurs through refraction, a continual
bending of the ray toward the fiber’s optical axis. This manufacturing technique provides
a large-diameter fiber than single mode and a wider bandwidth fiber than multimode step
26
index, thus reducing the coupling problem while enhancing the distance handling
capability. This product is called graded index fiber.
Fiber Optic Components:
Fig.4.33 illustrates components in a simple fiber optic system. These are identified as
source, connector, cable and detector.
(d)
Fig.4.33 Components of a fiber optic system (a) Source (LED) emission profile.
(b)Acceptance cone for step-index fiber. (c) Source/fiber interface. (d) fiber optic link
Source: Light emitting diode (LED) or Injection laser diode (ILD)
Connector: Fiber optic
Cable: Single mode, multimode step index, multimode graded index
Detector: PIN diode or Avalanche photodiode (APD)
27
4.7 Annunciators and Alarms
Principle of Operation
The basic annunciator system consists of multiple individual alarm points, each
connected to a trouble contact (alarm switch), a logic module, and a visual indicator
(Fig.4.34). The individual alarm points are operated from a common power supply and
share a number of annunciator system components, including an audible signal generator
(horn), a flasher, and acknowledge and test push buttons. In normal operation the
annunciator system and individual alarm points are quiescent.
The trouble contact is an alarm switch that monitors a particular process variable and is
actuated when the variable exceeds preset limits. In electrical annunciator systems it is
normally a switch contact that closes (makes) or opens (breaks) the electrical circuit to
the logic module and thereby initiates the alarm condition. In the alert stage the
annunciator turns on the visual indicator for the particular alarm point and the audible
signal and the flasher for the system. The visual indicator is usually a backlighted
nameplate engraved with an inscription to identify the variable and the abnormal
condition, but it can also be a bull’s-eye light with a nameplate. The audible signal can be
a horn, a buzzer, or a bell.
The flasher is common to all individual alarm points and interrupts the circuit to the
visual indicator as that point goes into the alert condition. This causes the light to
continue to flash intermittently until either the abnormal condition returns to normal or it
is acknowledged by the operator. The horn acknowledgment pushbutton is provided with
a momentary contact; when it is operated, it changes the logic module circuit to silence
the audible signal, stop the flasher, and turn the visual indicator on "steady." When the
abnormal condition is corrected, the trouble contact returns to normal and the visual
indicator is automatically turned off. The lamp test pushbutton with its momentary
contact tests for burned-out lamps in the visual indicators. When activated, the
pushbutton closes a common circuit (bus) to each visual indicator in the annunciator
system, turning on those lamps that are not already on as a result of an abnormal
operating condition.
Operating Sequences
The operation of an individual alarm point in the normal, alert, acknowledged, and
return-to-normal stages is the annunciator sequence. A wide variety of sequences can be
developed from commercially available logic components; many special sequences have
been designed to suit the requirements of particular process applications. The most
commonly used annunciator sequences is given below in the Table
28
Fig. 4.34 Elements of basic electronic type annunciator system
Pneumatic annunciators
Pneumatic annunciators consist of air-operated equivalents of the trouble contact, logic
module, and visual indicator stages of an electrical annunciator system. A single-point
system furnishing high tank level monitoring is shown in Fig.4.35. Power supply to the
system is instrument air at 80 to 100 PSIG, which is reduced to the required operating
pressure by pressure regulator (1). The operating pressure is indicated on pressure gauge
(2). A 3 to 15 PSIG analog input signal from a direct-acting level transmitter (LT-9)
enters high-pressure limit relay (4), which is normally closed and set to open when the
high level limit is exceeded. When this happens (alert condition), an input at supply
pressure from (4) turns on a pneumatic visual indicator (3), and a normally open highpressure limit relay (6) allows supply air flow to air horn (7), turning it on.
Simultaneously, the air output from (4) enters normally closed high-pressure limit relay
(5) and momentary contact pushbutton (8), which is a normally open acknowledgment
pushbutton for the system. In the alert condition, the pneumatic indicator and horn are
both on.
One acknowledges the alert condition by pushing button (2), closing it, and thereby
opening high pressure limit relay (5). Supply air pressure from (5) closes high-pressure
limit relay (6), which cuts off the operating air to the horn, thereby turning it off.
Simultaneously, operating air pressure from (5) is fed back to the inlet of (5). The
feedback pressure locks up (5) so that it will not close when the acknowledgment
pushbutton (8) is released.· In the acknowledged condition, the pneumatic indicator is on
and the horn is off. The system returns to normal when the 3 to 15 PSIG analog input
falls below the set point. This closes high-pressure limit relay (4) which turns off the
pneumatic indicator (3). It also closes high pressure limit relay (5) by venting the lock-in
circuit through relay (4).
Pneumatic annunciators are used when one or two alarm points are needed but electrical
29
power is not readily available and in hazardous electrical areas where an electrical
annunciator might not be practical. Pneumatic annunciator require a substantial amount
of installation space and are expensive to manufacture.
Fig.4.35 Pneumatic annunciator circuit
Summary:
In this chapter the fundamental concepts about the converters, function generators,
computing relays and annunciators are discussed along with their design and working
principle.
1. The signal conversion refers to the modifications that must be made to the
control signal to properly interface with the next stage in the control loop.
The devices that perform such signal conversions are often called
transducers because they convert control signals from one form to another.
The principal objective of signal conversion is to convert the low-energy
control signal to a high-energy signal to drive the actuator.
2. Function generators can provide an output as per the requirement of a process.
The function generators can be designed and programmed in both analog and
digital form.
3. Relay is device that receives information in the form of one or more instrument
signals, modifies the information or its form, or both, and if required, sends out
one or more resultant signals The relays which are able produce suitable output
based on the fundamental formulation/equation on which they are designed in
analog / digital / pneumatic form.
4. A transmitter is transducer that responds to a measurement variable and converts
that input into a standardized transmission signal.
5. Annunciators and alarms provide an indication about the process condition either
30
in the form of visual or audible signal.
Suggested Readings and Websites:
1. Instrument Engineers Handbook: Volume 2-Process Control, by Bela J. Liptak,
Chilton Book Company.
2. Computer based industrial control by Krishna Kant, PHI, 2002
3. Computer Aided Process Control by S.K.Singh, PHI
4. www. controlmagazine.com
5. www. icsmagazine.com
6. www.honeywell.com
7. www.controlguru.com/
8. www.processautomationcontrol.com
Glossary:
Absolute Alarm: An alarm caused by the detection of a variable which has exceeded its
high or low limit condition.
Adder: A device whose output is a representation of the sum of the inputs.
Alarm: An audible or visible signal that indicates an abnormal or out-of-limits condition
in the plant or control system.
Computing relay: A device that performs one or more calculations or logical functions or
both, and sends out one or more resultant signals.
Error squared: The technique of using the square of the error on which to make the
control calculation so as to produce a non-linear correction.
Relay: An electrical switch that allows a low power to control a higher one. A small
current energizes the relay, which closes a gate, allowing a large current to flow through.
Signal Conversion: The signal conversion refers to the modifications that must be
made to the control signal to properly interface with the next stage in the control
loop.
Switch: A device that connects, disconnects or transfers one or more circuits and is not
designated as a controller, a relay, or a control valve.
Keywords:
Signal conversion, converters, function generators, relays, computing relays, transmitters,
annunciators, alarms.
31