Introduction to Industrial Instrumentation
Instrumentation is the science of automated measurement and control. Applications of this science abound in modern research, industry, and everyday living. From automobile engine control systems to home thermostats to aircraft autopilots to the manufacture of pharmaceutical drugs, automation surrounds us. This chapter explains some of the fundamental principles of industrial instrumentation. The first step, naturally, is measurement. If we can’t measure something, it is really pointless to try to control it. This “something” usually takes one of the following forms in industry:
• Fluid pressure
• Fluid flow rate
• The temperature of an object
• Fluid volume stored in a vessel
• Chemical concentration
• Machine position, motion, or acceleration
• Physical dimension(s) of an object
• Count (inventory) of objects
• Electrical voltage, current, or resistance
Once we measure the quantity we are interested in, we usually transmit a signal representing this quantity to an indicating or computing device where either human 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 usually takes one of the following forms:
• Control valve (for throttling the flow rate of a fluid)
• Electric motor
• Electric heater
Both the measurement device and the final control device connect to some physical system which we call the process. To show this as a general block diagram:
The common home thermostat is an example of a measurement and control system, with the home’s internal air temperature being the “process” under control. In this example, the thermostat usually serves two functions: sensing and control, while the home’s heater adds heat to the home to increase temperature, and/or the home’s air conditioner extracts heat from the home to decrease temperature. The job of this control system is to maintain air temperature at some comfortable level, with the heater or air conditioner taking action to correct temperature if it strays too far from the desired value (called the setpoint).
Industrial measurement and control systems have their own unique terms and standards, which is the primary focus of this lesson. Here are some common instrumentation terms and their definitions:
Process: The physical system we are attempting to control or measure. Examples: water filtration system, molten metal casting system, steam boiler, oil refinery unit, power generation unit.
Process Variable, or PV: The specific quantity we are measuring in a process. Examples: pressure, level, temperature, flow, electrical conductivity, pH, position, speed, vibration.
Setpoint, or SP: The value at which we desire the process variable to be maintained at. In other words, the “target” value of the process variable.
Primary Sensing Element, or PSE: A device that directly senses the process variable and translates that sensed quantity into an analog representation (electrical voltage, current, resistance; mechanical force, motion, etc.). Examples: thermocouple, thermistor, bourdon tube, microphone, potentiometer, electrochemical cell, accelerometer.
Transducer: A device that converts one standardized instrumentation signal into another standardized instrumentation signal, and/or performs some sort of processing on that signal. Often referred to as a converter and sometimes as a “relay.” Examples: 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). Note: in general science parlance, a “transducer” is any device that converts one form of energy into another, such as a microphone or a thermocouple. In industrial instrumentation, however, we generally use “primary sensing element” to describe this concept and reserve the word “transducer” to specifically refer to a conversion device for standardized instrumentation signals.
Transmitter: A device that translates 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, Fieldbus digital signal packet, etc., which may then be conveyed to an indicating device, a controlling device, or both.
Lower- and Upper-range values, abbreviated LRV and URV, respectively: 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 300 degrees Celsius and ending at 500 degrees Celsius, 300 degrees would be the LRV and 500 degrees would be the URV.
Zero and Span: alternative descriptions to LRV and URV for the 0% and 100% points of an instrument’s calibrated range. “Zero” refers to the beginning-point of an 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 300 degrees Celsius and ending at 500 degrees Celsius, its zero would be 300 degrees and its span would be 200 degrees.
Controller: A device that receives a process variable (PV) signal from a primary sensing element (PSE) or transmitter, compares that signal to the desired value for that process variable (called the setpoint), and calculates 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, or FCE: A device that receives the signal from a controller to directly influence the process. Examples: variable-speed electric motor, control valve, electric heater. Manipulated Variable, or MV: Another term to describe the output signal generated by a controller. This is the signal commanding (“manipulating”) the final control element to influence the process.
Automatic mode: When the controller generates an output signal based on the relationship of process variable (PV) to the setpoint (SP).
Manual mode: When the controller’s decision-making ability is bypassed to let a human operator directly determine the output signal sent to the final control element.
The following links are some practical examples of measurement and control systems and other types of basic instruments so you can get a better idea of these fundamental concepts.
Instrument technicians maintain the safe and efficient operation of industrial measurement and control systems. As this chapter shows, this requires a broad command of technical skill. Instrumentation is more than just physics or chemistry or mathematics or electronics or mechanics or control theory alone. An instrument technician must understand all these subject areas to some degree, and more importantly how these knowledge areas relate to each other.
The all-inclusiveness of this profession makes it very challenging and interesting. Adding to the challenge is the continual introduction of new technologies. The advent of new technologies, however, does not necessarily relegate legacy technologies to the scrap heap. It is quite common to find state of- the-art instruments in the very same facility as decades-old instruments; digital fieldbus networks running alongside 3 to 15 PSI pneumatic signal tubes; microprocessor-based sensors mounted right next to old mercury tilt-switches. Thus, the competent instrument technician must be comfortable working with both old and new technologies, understanding the relative merits and weaknesses of each.
This is why the most important skill for an instrument technician is the ability to teach oneself. It is impossible to fully prepare for a career like this with any amount of preparatory schooling. The profession is so broad and the responsibility so great, and the landscape so continuously subject to change, that life-long learning for the technician is a matter of professional survival.
Perhaps the single greatest factor determining a person’s ability to independently learn is their skill at reading. Being able to “digest” the written word is the key to learning what is difficult or impractical to directly experience. In an age where information is readily accessible, the skilled reader has the advantage of leveraging generations of experts in any subject. Best of all, reading is a skill anyone can master, and everyone should.
My advice to all those desiring to become self-directed learners is to build a library of reading material on subjects that interest you (hopefully, instrumentation is one of those subjects!), and then immerse yourself in those writings. Feel free to “mark up” your books, or take notes in a separate location, so as to actively engage in your reading. Try as much as possible to approach reading as though you were having a conversation with the author: pose questions, challenge concepts and ideas, and do not stop doing so until you can clearly see what the author is trying to say. I also advise writing about what you have learned, because re-phrasing key ideas in your own words helps you consolidate the learning, and “makes it your own” in a way few other activities do. You don’t necessarily have to write your own book, but the act of expressing what you have learned to the best of your ability is a powerful tool not only for building confidence in what you know, but also for raising your own awareness of what you do not (yet) know.
Lipt´ak, B´ela G., Instrument Engineers’ Handbook – Process Software and Digital Networks, Third Edition, CRC Press, New York, NY, 2002.
written by Administrator, January 11, 2014
written by mattflintop, February 25, 2016
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