Chapter 1
Introduction to Instrumentation
1. Instrumentation
An instrument is a device that transforms a physical variable of interest (the measurand)
into a form that is suitable for recording (the measurement). In order for the measurement
to have broad and consistent meaning, it is common to employ a standard system of units
by which the measurement from one instrument can be compared with the measurement
of another. An example of a basic instrument is a ruler. In this case the measurand is the
length of some object and the measurement is the number of units (meters, inches, etc.)
that represent the length.
The work or responsibility of an instrumentation engineer ranges from designing,
developing, installing, managing equipment that are used to monitor and control
machinery. Equipment in almost everywhere, at home and industry is only because of the
science of instrumentation. And this discipline of instrumentation engineering has been
studied since early part of 1970s as a branched out of the streams of electrical and
electronic engineering. Whereas, instrumentation engineering is the branch of engineering
that specialises on the principle and operation of measuring instruments that are used in
fields of design, configuration of automated systems in electrical, pneumatic domains, etc.
The required tasks are very domain dependent; instrumentation engineers typically work
for industries with automated process with the goal of improving the productivity, reliability,
safety, optimisation and stability. Instrumentation engineers are commonly responsible for
integrating the sensors with the recorders, transmitters, displays or control systems. They
may design or specify installation, wiring and signal conditioning. They may be responsible
for calibration, testing and maintenance of the system.
2. Introduction to Process Control
2.1. Introduction
The technology of controlling a series of events to transform a material into a desired end
product is called process control. For instance, the making of fire could be considered a
primitive form of process control. Industrial process control was originally performed
manually by operators. Their sensors were their sense of sight, feel, and sound, making
the process totally operator-dependent. To maintain a process within broadly set limits,
the operator would adjust a simple control device. Instrumentation and control slowly
evolved over the years, as industry found a need for better, more accurate, and more
consistent measurements for tighter process control. The first real push to develop new
instruments and control systems came with the Industrial Revolution, and World Wars I
and II added further to the impetus of process control. Feedback control first appeared in
1774 with the development of the fly-ball governor for steam engine control, and the
concept of proportional, derivative, and integral control during World War I. World War II
saw the start of the revolution in the electronics industry, which has just about
revolutionized everything else. Industrial process control is now highly refined with
computerized controls, automation, and accurate semiconductor sensors [1].
2.2. Process Control
Process control can take two forms: (1) sequential control, which is an event-based
process in which one event follows another until a process sequence is complete; or (2)
continuous control, which requires continuous monitoring and adjustment of the process
variables. However, continuous process control comes in many forms, such as domestic
water heaters and heating, ventilation, and air conditioning (HVAC), where the variable
temperature is not required to be measured with great precision, and complex industrial
process control applications, such as in the petroleum or chemical industry, where many
variables have to be measured simultaneously with great precision. These variables can
vary from temperature, flow, level, and pressure, to time and distance, all of which can be
interdependent variables in a single process requiring complex microprocessor systems
for total control. Due to the rapid advances in technology, instruments in use today may
be obsolete tomorrow. New and more efficient measurement techniques are constantly
being introduced. These changes are being driven by the need for higher accuracy, 1
quality, precision, and performance. Techniques that were thought to be impossible a few
years ago have been developed to measure parameters.
2.2.1 Sequential Process Control
Control systems can be sequential in nature, or can use continuous measurement; both
systems normally use a form of feedback for control. Sequential control is an event-based
process, in which the completion of one event follows the completion of another, until a
process is complete, as by the sensing devices. Figure 1.1 shows an example of a process
using a sequencer for mixing liquids in a set ratio [2]. The sequence of events is as follows:
1. Open valve A to fill tank A.
2. When tank A is full, a feedback signal from the level sensor tells the sequencer to turn
valve A Off.
3. Open valve B to fill tank B.
4. When tank B is full, a feedback signal from the level sensor tells the sequencer to turn
valve B Off.
5. When valves A and B are closed, valves C and D are opened to let measured quantities
of liquids A and B into mixing tank C.
6. When tanks A and B are empty, valves C and D are turned Off.
7. After C and D are closed, start mixing motor, run for set period.
8. Turn Off mixing motor.
9. Open valve F to use mixture.
10. The sequence can then be repeated after tank C is empty and Valve F is turned Off.
2.2.2 Continuous Process Control
Continuous process control falls into two categories: (1) elementary On/Off action, and (2)
continuous control action. On/Off action is used in applications where the system has high
inertia, which prevents the system from rapid cycling. This type of control only has only
two states, On and Off; hence, its name. This type of control has been in use for many
decades, 2 Introduction to Process Control Liquid A Liquid B Liquid level A sensor Liquid
level B sensor Sequencer Mixture out Mixer Tank A Tank B Valve A Valve B Valve C
Valve D Valve F Tank C Figure 1.1
Valve A Valve B
Liquid
Liquid level B
level A A B
sensor
Mixture
Sequencer
Valve F
Figure 1.1 Sequencer used for liquid mixing.
Sequencer used for liquid mixing. long before the introduction of the computer. HVAC is
a prime example of this type of application. Such applications do not require accurate
instrumentation. In HVAC, the temperature (measured variable) is continuously
monitored, typically using a bimetallic strip in older systems and semiconductor elements
in newer systems, as the sensor turns the power (manipulated variable) On and Off at
preset temperature levels to the heating/cooling section. Continuous process action is
used to continuously control a physical output parameter of a material. The parameter is
measured with the instrumentation or sensor, and compared to a set value. Any deviation
between the two causes an error signal to be generated, which is used to adjust an input
parameter to the process to correct for the output change. An example of an
unsophisticated automated control process is shown in Figure 1.2.
A float in a swimming pool is used to continuously monitor the level of the water, and to
bring the water level up to a set reference point when the water level is low. The float
senses the level, and feedback to the control valve is via the float arm and pivot. The valve
then controls the flow of water (manipulated variable) into the swimming pool, as the float
moves up and down. A more complex continuous process control system is shown in
Figure 1.3, where a mixture of two liquids is required. The flow rate of liquid A is measured
with a differential pressure (DP) sensor, and the amplitude of the signal from the DP
measuring the flow rate of the liquid is used by the controller as a reference signal (set
point) to control the flow rate of liquid B. The controller uses a DP to measure the flow rate
of liquid B, and compares its amplitude to the signal from the DP monitoring the flow of
liquid A. The difference between the two signals (error signal) is used to control the valve,
so that the flow rate of liquid B (manipulated variable) is directly proportional to that of
liquid A, and then the two liquids are combined [3]. 1.2 Process Control 3 Fluid in Valve
Pivot Float (Level Sensor) Measured variable (Level) Feedback Manipulated variable
(Flow) Figure 1.2 Automated control system. DP DP Liquid A Controller Liquid B Mixture
out Figure 1.3 Continuous control for liquid mixing.
3. Instrumentation Diagrams
3.1. Definition of the Elements in a Control Loop
In any process, there are a number of inputs (i.e., from chemicals to solid goods). These
are manipulated in the process, and a new chemical or component emerges at the output.
To get a more comprehensive look at a typical process control system, it will be broken
down into its various elements. Figure 1.4 is a block diagram of the elements in a
continuous control process with a feedback loop. Process is a sequence of events
designed to control the flow of materials through a number of steps in a plant to produce
a final utilitarian product or material. The process can be a simple process with few steps,
or a complex sequence of events with a large number of interrelated variables. The
examples shown are single steps that may occur in a process. Measurement is the
determination of the physical amplitude of a parameter of a material; the measurement
value must be consistent and repeatable. Sensors are typically used for the measurement
of physical parameters. A sensor is a device that can convert the physical parameter
repeatedly and reliably into a form that can be used or understood. Examples include
converting temperature, pressure, force, or flow into an electrical signal, measurable
motion, or a gauge reading. In Figure 1.3, the sensor for measuring flow rates is a DP cell.
Error Detection is the determination of the difference between the amplitude of the
measured variable and a desired set reference point. Any difference between the two is
an error signal, which is amplified and conditioned to drive a control element. The
controller sometimes performs the detection, while the reference point is normally stored
in the memory of the controller. Controller is a microprocessor-based system that can
determine the next step to be taken in a sequential process, or evaluate the error signal
in continuous process control to determine what action is to be taken. The controller can
normally condition the signal, such as correcting the signal for temperature effects or
nonlinearity in the sensor. The controller also has the parameters of the process input
control element, and conditions the error sign to drive the final element. The controller can
monitor several input signals that are sometimes interrelated, and can drive several control
elements simultaneously. The controllers are normally referred to as programmable logic
controllers (PLC). These devices use ladder networks for programming the control
functions.
1.4 Instrumentation and Sensors 4
Set point
Error
Control signal Comparator
signal Controller
Variable
amplitude
Feedback signal
Manipulated Controlled
variable variable
Control Measuring
Process
element Output element
Input
Figure 1.4. Block diagram of the elements that make up the feedback path in a process control
loop.
Control Element is the device that controls the incoming material to the process (e.g., the
valve in Figure 1.3). The element is typically a flow control element, and can have an
On/Off characteristic or can provide liner control with drive. The control element is used to
adjust the input to the process, bringing the output variable to the value of the set point.
The control and measuring elements in the diagram in Figure 1.4 are oversimplified, and
are broken down in Figure 1.5. The measuring element consists of a sensor to measure
the physical property of a variable, a transducer to convert the sensor signal into an
electrical signal, and a transmitter to amplify the electrical signal, so that it can be
transmitted without loss. The control element has an actuator, which changes the electrical
signal from the controller into a signal to operate the valve, and a control valve. In the
feedback loop, the controller has memory and a summing circuit to compare the set point
to the sensed signal, so that it can generate an error signal. The controller then uses the
error signal to generate a correction signal to control the valve via the actuator and the
input variable. The function and operation of the blocks in different types of applications
will be discussed in a later chapter. The definitions of the terms used are given at the end
of the chapter.
Summary
This chapter introduced the concept of process control, and the differences between
sequential, continuous control and the use of feedback loops in process control. The
building blocks in a process control system, the elements in the building blocks, and the
terminology used, were defined.
Definitions
Absolute Accuracy of an instrument is the deviation from true expressed as a number.
Accuracy of an instrument or device is the difference between the indicated value and
the actual value.
Actuators are devices that control an input variable in response to a signal from a
controller.
Automation is a system where most of the production process, movement, and inspection
of materials are performed automatically by specialized testing equipment, without
operator intervention.
Controlled or Measured Variable is the monitored output variable from a process, where
the value of the monitored output parameter is normally held within tight given limits.
Controllers are devices that monitor signals from transducers and keep the process
within specified limits by activating and controlling the necessary actuators, according to
a predefined program.
Converters are devices that change the format of a signal without changing the energy
form (e.g., from a voltage to a current signal).
Correction Signal is the signal that controls power to the actuator to set the level of the
input variable.
Drift is the change in the reading of an instrument of a fixed variable with time.
Error Signal is the difference between the set point and the amplitude of the measured
variable.
Feedback Loop is the signal path from the output back to the input, which is used to
correct for any variation between the output level and the set level.
Hysteresis is the difference in readings obtained when an instrument approaches a signal
from opposite directions.
Instrument is the name of any various device types for indicating or measuring physical
quantities or conditions, performance, position, direction, and so forth.
Linearity is a measure of the proportionality between the actual value of a variable being
measured and the output of the instrument over its operating range.
Manipulated Variable is the input variable or parameter to a process that is varied by a
control signal from the processor to an actuator.
Offset is the reading of the instrument with zero input.
Precision is the limit within which a signal can be read, and may be somewhat subjective.
Range of an instrument is the lowest and highest readings that it can measure.
Reading Accuracy is the deviation from true at the point the reading is being taken, and
is expressed as a percentage.
Repeatability is a measure of the closeness of agreement between a number of readings
taken consecutively of a variable.
Reproducibility is the ability of an instrument to repeatedly read the same signal over
time, and give the same output under the same conditions.
Resolution is the smallest change in a variable to which the instrument will respond.
Sensitivity is a measure of the change in the output of an instrument for a change in the
measured variable.
Sensors are devices that can detect physical variables.
Set Point is the desired value of the output parameter or variable being monitored by a
sensor; any deviation from this value will generate an error signal.
Span of an instrument is its range from the minimum to maximum scale value.
Transducers are devices that can change one form of energy into another.
Transmitters are devices that amplify and format signals, so that they are suitable for
transmission over long distances with zero or minimal loss of information.
References
[1] Battikha, N. E., The Condensed Handbook of Measurement and Control, 2nd ed., ISA, 2004, pp. 1–8.
[2] Humphries J. T., and L. P. Sheets, Industrial Electronics, 4th ed., Delmar, 1993, pp. 548–550.
[3] Sutko, A., and J. D. Faulk, Industrial Instrumentation, 1st ed., Delmar Publishers, 1996, pp. 3–14.
[4] Johnson, C. D., Process Control Instrumentation Technology, 7th ed., Prentice Hall, 2003, pp. 6–43.
[5] Johnson, R. N., “Signal Conditioning for Digital Systems,” Proceedings Sensors Expo, October 1993, pp.
53–62.
