December 18, 2019 by Emmanuel Ikimi
From automobile assembly to chemical plants and refineries—namely industries where automated process control is required—precise measurements are crucial. This article will introduce the basic concept of electrical instrumentation and some essential equipment for electrical engineers.
Electrical and instrumentation engineering (EIE) is a subfield of electrical engineering that deals with the measurement of process variables within industrial facilities and the management of equipment for automated control. Instrumentation engineers measure process variables (such as temperature, pressure, and volume) within an industrial plant or facility by using various measuring components or equipment to control them.
Some of the most essential instrumentation terminologies to electrical engineers include:
Sensitivity: the sensitivity of an instrument is the degree to which an output changes in response to an applied input
Accuracy: the accuracy of an instrument is the degree to which a measured value conforms to a specified standard (aka the ‘true value’). For example, if a sensor has an accuracy of +/−1%, this means that it is up to 99% accurate
Reliability: the reliability of an instrument is the extent to which it accurately measures a specified process variable before it begins to deviate from the true value
Span: the span of a given measurement is the difference between its upper and lower range of values
Transducer: a transducer is a device that converts an input signal (e.g., mechanical pressure, heat, or light) into another form (e.g., an electric signal or change in resistance)
Transmitter: a transmitter is a device that converts one form of energy into another.
Calibration: calibration is the process of configuring an instrument to measure process variables as accurately as possible.
A close-up of a laboratory function generator with a frequency counter. Image Credit: Bigstock.
Measurement of process variables is achieved using electronic devices and equipment, as covered in the following sections.
Temperature sensors detect and measure ambient or process temperatures using an electrical signal. Common examples are thermocouples and resistance temperature detectors (RTDs). Thermocouples measure temperature via two dissimilar metals that generate a voltage when the temperature across the junction changes.
RTDs, on the other hand, utilise a variable resistor (aka varistor), which varies its resistance values in response to temperature changes across its terminals. RTDs are often used to protect sensitive electronics by preventing them from operating beyond their maximum rated temperatures.
Electronic pressure sensors have become favoured over mechanical sensors in industrial applications due to their high accuracy and reliability. Pressure sensors measure values by converting the mechanical pressure applied onto a diaphragm into an electrical output signal.
The output signal is then sent to a data monitoring, acquisition system, or computerised display and recorded in the specified unit of measurement. Electronic pressure sensors can detect pressures ranging from a few millibars up to thousands of millibars—with a negligible margin of error.
Temperature loggers, also known as temperature data loggers or temperature monitors autonomously record the values of ambient or process temperatures over a given period. Instrumentation engineers can utilise such historical data to evaluate process efficiencies for a wide range of temperatures or to troubleshoot failures that occur during the relevant processes.
Temperature loggers are typically battery-powered devices with internal thermocouples or thermistors (resistors whose resistance values vary with changes in temperature) that generate an electrical signal. Sample data is recorded on a small LCD screen on the device or transmitted to RFID (or other wireless systems) for storage.
A tabletop digital multimeter (the 1057a from Datron Electronics). Image Credit: Flickr.
Digital multimeters (DMMs) are handheld instrumentation equipment used to measure circuit variables (such as voltage, current, and resistance). Electrical engineers also utilise them for troubleshooting electric circuit failures.
DMMs consist of internal analogue to digital converters (aka ADCs), which utilise a successive approximation register (SAR) with resolution levels consisting of bits (e.g., 8 or 16 bits). The SAR works by sampling successive values of voltage that come in from the DMM test probes, before displaying the value on its LCD screen.
Two electronic engineers testing a prototype on an electronic test bench. Image credit: Bigstock.
Function generators are electronic equipment used to generate various kinds of repetitive signals. They are particularly useful in automobile manufacturing plants for testing the durability of vehicle components. For example, the chassis of a vehicle may be subjected to a vibration test to ascertain its reliability by simulating typical driving conditions on a bumpy road.
There are two main types of function generators:
Standard signal generators: these are used to produce signals that have a square, triangular, or sinusoidal waveform (with additional functions for triggering and gating)
Arbitrary waveform generators: these are special function generators capable of producing complex and/or high-speed waveforms
Frequency counters are used to measure the frequency of signals in radio-frequency applications, such as industrial remote-controlled and smart sensor applications. The most complex and accurate variant is the heterodyne frequency counter (or heterodyne wavemeter) used for measuring periodic values (with a negligible margin of error) for frequency-sensitive applications.
Frequency counters work by counting the number of times an electrical signal passes through a voltage point (or ‘trigger point’) in a given time. These devices may be either handheld or mounted on a dedicated instrumentation panel or electronics test bench.
All in all, instrumentation is crucial to electrical engineering (a closely-related field is control engineering, which deals with the design and utilisation of hardware and/or software to monitor and control the behaviour of industrial dynamic process systems). This is, ultimately, because circuit and process variables (which again, include voltage, resistance, current, temperature, and so on) cannot be quantified using one’s physical senses.
