Process Control Instruments and their Classification

Process Control Instruments and their Classification

In the physical sciences, process engineering, and product quality assurance, measurement is the activity of obtaining and comparing physical quantities. Established standards are used as units, and the process of measurement gives a number relating the item under study and the referenced unit of measurement. Measurement generally involves using an instrument as a physical means of determining the quantity or variable. The instrument serves as an extension of manual capabilities and enables a person to determine the value of an unknown quantity which unaided person cannot measure.

An instrument can be defined as a device for determining the value or magnitude of a quantity or variable. Measuring instruments, and formal test methods which define the instrument’s use, are the means by which the variables and the relations between variables are found.

Instrumentation is the basis for the control of a process. It provides the various indications to the operator for controlling the process. In some cases, operator records these indications for evaluating the current condition of the process and to take actions if the conditions are not as expected. Because of the continuous interactive nature of the most of the processes, manual control is not feasible and is unreliable. With instrumentation, automatic control of such processes can be achieved.

Instrumentation is used in almost every industrial process and generating system, where consistent and reliable operations are required. Instrumentation provides the means of monitoring, recording and controlling a process to maintain it at a desired state.

The first step in any process control, naturally, is measurement. Anything which cannot be measured cannot be controlled. The  common parameters which need measurement are (i) fluid pressure, (ii) fluid flow rate, (iii) the temperature of an object, (iv) fluid volume stored in a vessel, (v) chemical concentration, (vi) machine position, motion, or acceleration, (vii) physical dimension(s) of an object, (viii) count (inventory) of objects, and (ix) electrical voltage, current, or resistance etc.

After the parameter quantity has been measured, a signal representing this quantity is then transmitted to an indicating or computing device where either manual or automated action then takes place. If the controlling action is automated, the computer sends a signal to a final controlling device which then influences the quantity being measured. This final control device normally takes one of the forms namely (i) control valve (for throttling the flow rate of a fluid), (ii) electric motor, and (iii) electric heater. Both the measurement device and the final control device are connected to the process. Fig 1 shows a block diagram of a simple instrument control system.

Fig 1 Block diagram of a simple instrument control system

Industrial measurement and control systems have their own unique terms and standards. Some of the common instrumentation terms and their definitions are given below.

Process – It is the physical system which is to be controlled or measured.

Process variable (PV) – It is the parameter whose quantity is to be measured in a process. Examples of parameters are pressure, level, temperature, flow, electrical conductivity, pH, position, and speed etc.

Set point (SP) – This is the value at which the process variable is to be maintained at. In other words, this is the ‘target’ value for the process variable.

Primary sensing element (PSE) – It is a device directly sensing the process variable and translating the sensed quantity into an analog representation (electrical voltage, current, resistance, mechanical force, or motion etc. Examples are thermocouple, thermistor, bourdon tube, microphone, potentiometer, Electro-chemical cell, and accelerometer etc.

Transducer – It is a device which converts one standardized instrumentation signal into another standardized instrumentation signal, and/or performing some sort of processing on that signal. Transducer is often referred to as a converter and sometimes as a ‘relay’. Examples are I/P converter (converts 4 – 20 mA electric signal into 3 – 15 PSI pneumatic signal), P/I converter (converts 3 – 15 PSI pneumatic signal into 4-20 mA electric signal), square-root extractor (calculates the square root of the input signal).

Programmable logic controllers (PLC) – These are the devices which are used in process-control applications, and are microprocessor-based systems. Small systems have the ability to monitor several variables and control several actuators, with the capability of being expanded to monitor 60 to 70 variables and control a corresponding number of actuators, as may be required in an industrial process. PLCs, which have the ability to use analog or digital input information and output analog or digital control signals, can communicate globally with other controllers, are easily programmed on line or off line, and supply an unprecedented amount of data and information to the operator. Ladder networks are normally used to program the controllers.

Error signal – An error signal is the difference between the set point and the amplitude of the measured variable.

Correction signal – A correction signal is the signal used to control power to the actuator to set the level of the input variable.

Transmitter – It is a device translating the signal produced by a primary sensing element (PSE) into a standardized instrumentation signal such as 3 – 15 PSI air pressure, 4-20 mA DC electric current, and field bus digital signal packet, etc. The standardized signal is then conveyed to an indicating device, a controlling device, or both.

Lower range values (LRV) and upper range values (URV) – The values of process measurement deemed to be 0 % and 100 % of a transmitter’s calibrated range. For example, if a temperature transmitter is calibrated to measure a range of temperature starting at 200 deg C and ending at 600 deg C, its LRV is 200 deg C and its URV is 600 deg C.

Zero and span – It is alternative descriptions to LRV and URV for the 0 % and 100 % points of the instrument’s calibrated range. ‘Zero’ refers to the beginning point of the instrument’s range (equivalent to LRV), while ‘span’ refers to the width of its range (URV − LRV). For example, if a temperature transmitter is calibrated to measure a range of temperature starting at 200 deg C and ending at 600 deg C, its zero is 200 deg C and its span is 400 deg C.

Controller – It is a device receiving a process variable (PV) signal from a primary sensing element (PSE) or transmitter, comparing that signal to the desired value (called the set point) for that process variable, and calculating an appropriate output signal value to be sent to a final control element (FCE) such as an electric motor or control valve.

Final control element (FCE) – It is a device receiving the signal output by a controller which influences the process directly. Examples are variable-speed electric motor, control valve, and electric heater etc.

Feedback loop – It is the signal path from the output back to the input to correct for any variation between the output levels from the set level. In other words, the output of a process is being continually monitored, the error between the set point and the output parameter is determined, and a correction signal is then sent back to one of the process inputs to correct for changes in the measured output parameter.

Controlled or measured variable – It is the monitored output variable from a process. The value of the monitored output parameter is normally held within tight given limits.

Manipulated variable (MV) – It is the quantity in a process which is adjusted or otherwise manipulated in order to influence the PV. It is also used to describe the output signal generated by a controller; i.e. the signal commanding (manipulating) the final control element to influence the process.

Automatic mode – Automatic mode is when the controller generates an output signal based on the relationship of PV to the SP.

Manual mode – Manual mode is when the controller’s decision making ability is by-passed to let a human operator directly determine the output signal sent to the final control element.

Classification of instruments

The instruments can be classified as (i) mechanical, electrical, and electronic instruments, (ii) absolute or primary and secondary instruments, (iii) manual and automatic instruments, (iv) self operated and power operated instruments, (v) self contained and remote indicating instruments, (vi) contact type and non-contact type instruments, and (vii) analog and digital instruments.

Mechanical, electric, and electronic instruments

Mechanical instruments – The instruments which came into existence in early days were of mechanical nature. The principles on which these instruments worked are even in use today. The earliest scientific instruments used the same three essential elements as the modern instruments use. These elements are (i) detector, (ii) intermediate transfer device, and (iii) indicator, recorder or a storage device. These instruments are very reliable for static and stable conditions. There are a large number of possibilities for mechanical instruments.  For example, the instruments can be calipers, micrometers, scales, measuring tapes, and lasers, etc. for measuring distances, pressure gauge for measuring pressure, strain gauges for measuring how much a part is stretched or compressed when a load is applied, tachometer for measuring the rotational speed, multi-meter for measuring electrical voltages and currents, and many others.

The disadvantage associated with the mechanical instruments is that they are unable to respond quickly to measurements of dynamic and transient nature. These instruments have several moving parts which are rigid, heavy and bulky and thus they have a large mass. The mass presents inertia problems and hence these instruments cannot follow the rapid changes which are involved in dynamic measurements. Another disadvantage of mechanical instruments is that most of them are a potential source of noise and cause sound pollution.

Mechanical instruments are simple in design and application. They are normally durable and relatively cheaper. No external power source is needed for their operation. They are normally quite reliable and accurate for measurements under stable conditions.

Electrical instruments – Electrical methods of indicating and transmitting the output are faster when compared with the respective mechanical methods. However, an electrical system normally depends upon a mechanical pointer movement as an indicating device. Thus owing to the inertial of mechanical movements, these instruments have a limited time and frequency response. As an example, the majority of industrial recorders have response time ranging from of 0.5 seconds to 24 seconds though some electrical recorders can give full scale response in 0.2 seconds. Some of the galvanometers can follow 50 Hz variations, but as per present day requirements of fast measurements, these are also considered to be slow.

Electrical instruments are light and compact. Amplification produced is higher than what is produced by the mechanical methods. These instruments provide greater flexibility and are lighter in construction. Also, they consume less power and thus cause lesser load on the system.

Electronic instruments – Majority of the modern instruments used for scientific and industrial measurement applications require very rapid responses. The mechanical and electrical instruments and systems cannot meet these requirements. There is a requirement of decreasing the response time and also the detection of dynamic changes in certain parameters. The monitoring time needed can be of the order of milli-seconds and many a times, in micro- seconds. This has led to the development of electronic instruments and their associated circuitry. These instruments involved vacuum tubes or semi-conductor devices. The present day practice is to use semi-conductor devices owing to their having many advantages over the vacuum tube counterparts.

Since in the electronic instruments, the only movement involved is that of electrons and the inertia of electrons being very small, the response time of these instruments is extremely small. For example, a cathode ray oscilloscope (CRO) is capable of following dynamic and transient changes of the order of a few nano seconds.

Electronic instruments are steadily becoming more reliable because of improvements in design and manufacturing processes of semi-conductor devices. Another advantage of using electronic instruments is that very weak signals can be detected by using pre-amplifiers and amplifiers. The foremost importance of the electronic instruments is the power amplification provided by the electronic amplifiers. Additional power can be fed into the system to provide an increased power output beyond that of the input. This has been only possible through the use of electronic amplifiers, which have no important mechanical counterpart. This is particularly important where the data presentation devices use stylus type recorders, galvanometers, CROs and magnetic tape recorders.

It is a fact that hydraulic and pneumatic systems can be used for power amplification of signals. However, their use is limited to slow acting control applications like servo-systems, chemical processes and power systems. Electronic instruments find extensive use in detection of electro-magnetically produced signals such as radio, video, and micro-wave. Electrical and electronic instruments are particularly useful in the intermediate signal modifying stage. Electronic instruments are light, compact and have a high degree of reliability. Their power consumption is very low.

Electronic instruments make it possible to build analog and digital computers without which the modern developments in science and technological are virtually impossible. Computers require a very fast time response and it is only possible with use of electronic instruments. The mathematical processing of signal, such as, summation, differentiating, and integrating, is possible with electronic measurements. With these instruments non-contact or remote measurements are also possible.

Absolute or primary and secondary instruments

Electrical measurements of different parameters like current, voltage, power, energy, etc. are very necessary for the industry. These are being used since a long time for all measurements. The various electrical instruments used are broadly divided into two categories namely (i) absolute or primary instruments, and (ii) secondary instruments

Absolute or primary instruments – These instruments are those instruments which give the value of electrical quantity to be measured in terms of the constants of the instruments and their deflection only e.g. tangent galvanometer. These instruments are rarely used except in standard laboratories, especially for calibration of secondary instruments. Working with absolute instruments for routine work is time consuming since every time a measurement is made, it takes a lot of time to compute the magnitude of the quantity under measurement. The use of the absolute instruments is simply confined within laboratories as standardizing instruments.

Secondary instruments – It is the secondary instruments which are most generally used in everyday work.  Secondary instruments are those in which the values of electrical quantity to be measured can be determined from the deflection of the instruments only when they have been pre-calibrated by comparison with an absolute instrument. Without calibration, the deflection of such instruments is meaningless. It is the secondary instruments which are most generally used in everyday work. Typical examples of secondary instruments are voltmeter, glass thermometer, and pressure gauge. Secondary type of measuring instruments has been classified in three categories namely (i) indicating instruments, (ii) recording instruments, and (iii) integrating instruments.

Indicating instruments are those which indicate the instantaneous value of the variables being measured, at the time at which it is being measured. Their indications are given by pointers moving over calibrated dials or scales. Examples are ammeter, voltmeter, and watt-meter. This movement of pointer or the deflection is not constant and depends on the quantity it measures. As the needle deflects and indicates the amount of current, voltage, or any quantity. The instruments are also called deflection type of instruments.

Recording instruments are those which give a continuous record of variations of the measured variable over a selected period of time. The moving system of the instrument carries an inked pen which rests tightly on a graph chart. These instruments go on recording on a graph sheet fixed on the instrument all the variations of the quantity in the time it is connected in the circuit. These recordings are normally for a day and the recorded sheets are kept as a record of variation of the quantity with time.

Integrating instruments are the instruments which add up the quantity as the time passes or in other words give a total account of quantity over a period of given time for which it is connected in a circuit. As an example, an electric meter measure and register, by a set of dials and pointers, either the total quantity of electricity (in ampere-hours) or the total amount of electrical energy (in watt-hours or kilowatt-hours) supplied to a circuit over a period of time and are known as  ampere-hour meters, watt-hour meters, and energy meters etc. More examples are domestic water meter or piped gas meter.

In deflecting type instruments, deflection is normally with in 90 degrees, though circular scale instruments are also available which give a deflection of around 250 degrees. All the deflecting instruments have marked on scale to indicate its working principle by symbols. Deflecting type instruments are again classified depending upon (i) working principle, such as, moving coil, moving iron, dynamometer, electrostatic type, or induction type etc. (ii) the quantity it measures, such as, voltmeter, ammeter, ohm meter, power factor meter, and energy meter etc. and (iii) the shape of the instruments, such as, portable, panel board type with flush mounting, or surface mounting etc.

Manual and automatic instruments

Manual instrument needs the services of an operator, where as in automatic instruments there is no need for an operator. As an example, measurement of rotational speed by a hand operated tachometer an operator is required to make the contact of the instrument with the rotating shaft. For measurement of temperature by a resistance thermometer by wheat-stone bridge in its circuit an operator is required to indicate the temperature being measured. On the other hand in measurement of temperature by mercury-in-glass thermometer, no operator is required.

Self operated and power operated instruments

A self operated instrument does not need any external power source for its operation. In such instruments the output energy is supplied by the input signal such as a dial indicator, or mercury in the glass type thermometer.

In power operated instruments some auxiliary power source is needed for its operation. This external power source can be electricity, or compressed air etc. In such cases the input signal supplies only the insignificant portion of the output power. Example is an electro-mechanical measurement system.

Self contained and remote indicating instruments

A self contained instrument has all the physical elements in one assembly. Examples are an analog ammeter, or mercury in the glass thermometer etc. In case of a remote indicating instrument, the primary sensory element and the secondary indicating element are located at two different locations linked by transmitting element. These locations can have a long distance between them. In modern instrumentation technology such type of arrangement is quite necessary and is in use.

Contact type and non-contact type instruments

In contact type instruments, the sensing element of the instrument contacts the control medium for the measurements. Example is mercury in the glass thermometer. On the other hand in the non-contact type instruments, the sensor does not contact the control medium. The non-contact type measurement includes optical, radioactive, or radiation measurements. Examples are radiation or optical pyrometer, and non-touch tachometer etc.

Analog and digital instruments

Analog and digital instruments are normally secondary instruments. The terms analog and digital represent the working signal on which the instruments work. Hence, the analog instruments work on analog signal while the digital instruments work on digital signal.

Analog instrument with analog signal – Analog signal (Fig 2) is one which varies in a continuous fashion and takes on infinity of values in any given range. The devices which produce these signals are called analog devices. Analog is normally thought of in an electrical context. However, mechanical, pneumatic, hydraulic, and other systems can also convey analog signals. An analog signal uses some property of the medium to convey the signal’s information. As an example, an aneroid barometer uses rotary position as the signal to convey pressure information. Electrically, the property most frequently used is the voltage followed closely by frequency, current, and charge. The devices which produce such signals are called analog devices.

Any information can be conveyed by an analog signal. Frequently, such a signal is a measured response to changes in physical phenomena, such as temperature, pressure, sound or position and is achieved using a transducer. An analog signal is one where at each point in time, the value of the signal is significant. For example, in sound recording, fluctuations in air pressure (that is to say, sound) strike the diaphragm of a microphone which induces corresponding fluctuations in the current produced by a coil in an electromagnetic microphone, or the voltage produced by a condenser microphone.

Digital signal – In contrast the analog signal which varies in a continuous fashion and takes on infinity of values in any given range, the digital signal (Fig 2) varies in discrete steps and thus takes up only finite different values in a given range. The devices which produce such signals are called digital devices. In an analog system, the function varies continuously. On the other hand, the digital system values are discrete and vary in equal steps. The Fig 2 illustrates how both an analog voltage and a digital voltage vary with time.

Fig 2 Analog and digital signals

Comparison of analog signal and digital signal

The analog signal is one where at each point in time the value of the signal is significant, where as the digital signal is one where at each point in time, the value of the signal is to be above or below some discrete threshold. The display of the quantity to be measured in analog instruments is in terms of deflection of a pointer, where as digital instruments indicate the value to be measured in terms of decimal number. The main advantage of the analog signal is its fine definition which has the potential for an infinite amount of signal resolution. Compared to digital signals, analog signals are of higher density.

One of the advantage with analog signals is that their processing can be achieved more simply than with the digital equivalent. An analog signal can be processed directly by analog components, though some processes are not available except in digital form.  The analog instruments are less costly and simple in design as compared to their digital counter parts.

The main disadvantage of analog signaling is that any system can have noise, that is, random unwanted variation. As the signal is copied and re-copied, or transmitted over long distances, these apparently random variations become dominant. Electrically, these losses can be diminished by shielding, good connections, and several cable types such as coaxial or twisted pair. The effects of noise create signal loss and distortion. This is impossible to recover, since amplifying the signal to recover attenuated parts of the signal amplifies the noise (distortion/interference) as well. Even if the resolution of an analog signal is higher than a comparable digital signal, the difference can be over shadowed by the noise in the signal.

The digital devices have high speed and they also eliminate the manual error. With increasing use of digital computers for data handling and automatic process control, the importance of digital instruments is increasing. It has become necessary to have both analog to digital converter at input to the computers and digital to analog converters at the output of the computers.

In order to convert an analog quantity into a digital number, the vertical displacements in Fig 3 are divided into equal parts. The vertical quantities are divided into 10 equal parts with each part having a length of 1 unit. While dealing with digital numbers, a quantity between 0 to 0.5 are considered as 0, while a quantity between 0.5 to 1.5 is considered as 1 and similarly a quantity between 1.5 to 2.5 is considered as 2. It is apparent that if digital system is adopted, there are errors involved. But if the vertical quantities are further divided with each of the steps into 2 equal parts, then there are 20 steps instead of 10. And if these 20 steps are further divided into 2 parts each, then there are 40 steps. By doing this, much better accuracy is achieved in converting analog quantities into digital numbers. It is possible to go on subdividing each step further and further, till the desired accuracy is achieved.

However, it is to be remembered that a digital number is still a sum of equal units. And in a digital system, magnitudes lying within one of these steps lose their identity and are all defined by the same number. For example, if there are ten steps, all the numbers lying between 2.5 to 3.5, that is, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, are being read as 3.

Thus it can be concluded that the difference between analog and digital information is that the analog output is a continuous function while the digital output is a discrete number of units. The last digit of any digital number is rounded to 0.5 of the last digit. It is also be understood that the magnitude of the digital quantity is measured only at the instant the reading is taken. One reading persists till another reading is taken, unlike the analog quantity which is a continuous function.

Fig 3 Analog to digital conversion of signals

Analog measuring device

An analog device is one in which the output or display is a continuous function of time and bears a constant relation to its input. The analog instruments find extensive use in present day applications although digital instruments are increasing in number and applications. The areas of application which are common to both analog and digital instruments are fairly limited at present. Analog instruments are expected to remain in extensive use for some time and are not likely to be completely replaced by digital instruments for certain applications.

Classification of analog and digital instruments

Broadly, the analog and digital instruments can be classified according to the quantity the instrument measure. As an example, an instrument meant for measurement of current is classified as an Ammeter while an instrument which measures voltage is classified as a voltmeter. Thus there are watt-meters, power factor meters, and frequency meters etc. Electrical instruments can also be categorized as per the kind of current which can be measured by them, such as, DC (direct current), AC (alternating current), and DC/AC.  Also, there are three categories of instruments on the same pattern and the instruments can also be classified as indicating, recording, and integrating type.

Indicating instruments are those instruments which indicate the magnitude of a quantity being measured. These instruments generally make use of a dial and a pointer for this purpose. Examples are ordinary voltmeters, ammeters, and watt-meters. The analog indicating instruments can be further divided into two groups consisting of (i) electro-mechanical instruments, and (ii) electronic instruments. Electronic instruments are constructed by addition of electronic circuits to electro-magnetic indicators in order to increase the sensitivity and input impedance.

Recording instruments give a continuous record of the quantity being measured over a specified period. The variations of the quantity being measured are recorded by a pen, attached to the moving system of the instrument. The moving system is operated by the quantity being measured on a sheet of paper carried by a rotating drum. Example is a recording voltmeter in a sub-station which keeps record of the variations of supply voltage during the day.

Integrating instruments totalise measurements over a specified period of time. The summation, which these instruments give, is the product of time and an electrical quantity. Examples are ampere hour and watt hour (energy) meters. The integration (summation value) is generally given by a register consisting of a set of pointers and dials.

On the basis of method used for comparing the unknown quantity (measured) with the unit of measurement, the analog instruments can also be grouped into two categories of instruments namely (i) direct measuring instruments and (ii) comparison instruments.

The direct measuring instruments convert the energy of the measurement directly into energy which actuates the instruments and the value of the unknown quantity is measured or displayed or recorded directly. Examples of this category of instruments are ammeters, voltmeters, watt-meters and energy meters.

The comparison instruments measure the unknown quantity by comparison with a standard. Examples of this category of instruments are DC bridges and AC bridges.

Direct measuring instruments are the most commonly used in engineering practice because they are the most simple and inexpensive. Also their use makes the measurement possible in the shortest time.

Digital measuring device

A digital measuring device is that in which the value of the measured physical quantity is automatically represented by a number on a digital display or by a code, that is, a set of discrete signals. Digital measuring devices can be divided into digital measuring instruments and digital measuring transducers. Digital measuring instruments are self-contained devices which automatically present the value of the measured quantity on a digital display. Digital measuring transducers lack a digital display; and the measurement results are converted into a digital code for subsequent transmission and processing in measuring systems. The most common types of digital measuring devices are those used to measure electrical quantities, such as current, voltage, and frequency. These devices can be used to measure non-electrical quantities such as pressure, temperature, speed, and force, if the non-electrical quantity is first converted into an electrical quantity. Examples are digital multi-meters, digital thermometers, and digital flow meters etc.

The operation of digital measuring devices is based on the digitization, that is, quantization with respect to level and coding of the value of the measured physical quantity. The coded signal is fed either to a digital display or to a data transmission and processing system. In a digital display the coded measurement result is converted into a number expressed by numerals, usually in the decimal number system. The most widely used digital displays give two to nine digits. Digital measuring instruments can use electric, cathode-ray, gas-discharge, or liquid-crystal displays.

Analog to digital (A/D) conversion

The majority of present day instruments are of analog type. The importance of digital instruments is increasing, mainly because of the increasing use of digital computers in both data reduction and automatic control systems. Since digital computer works only with digital signals which means that any information supplied to it must be in digital form. The computer’s output is also in digital form. Thus working with a digital computer at either the input or the output, only digital signals are to be used. However, most of the present day measurement and control apparatus produces signals which are of analog nature, it is thus necessary to have both analog to digital (A/D) converters at the input to the computer and digital to analog (D/A) converters at the output of the computer.

A/D converter is a device which converts a continuous quantity to a discrete time digital representation. The reverse operation is performed by a D/A converter. Typically, A/D converter is an electronic device which converts an input analog voltage or current to a digital number proportional to the magnitude of the voltage or current. However, some non-electronic or only partially electronic devices, such as rotary encoders, can also be considered A/D converters. The digital output can use different coding schemes. Typically, the digital output is to be a binary number which is proportional to the input, but there are other possibilities.

An analog to digital converter inputs an analog electrical signal such as voltage or current and outputs a binary number. A digital to analog converter on the other hand, inputs a binary number and outputs an analog voltage or current signal. The block diagrams are given in Fig 4.

Fig 4 Block diagrams of analog to digital converter and digital to analog converter

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