In this article, we are going to familiarize you with the sorts of sensors that we frequently use in industrial equipment. We intend this to be an introductory article for a new engineer just getting started, or for anyone looking for a general understanding of some general controls concepts.

As a controls engineer, we often tend to see things from the electrical end of things, so in this exploration, we will look at two broad categories of sensors: Digital sensors that return on/off signals, and Analog sensors that return a range of values.

Digital sensors

Our first stop is Digital sensors. These are by far the most commonly used sensors in the industrial world. So, what is a “digital” or “binary” sensor? In theoretical terms, we are referring to something that returns one or more bits of information per sensor.

Initially, they were simple: A contact that touched another contact when something got where it was supposed to stop. In those old machines, this often meant 110 Volts exposed for the operator to touch or passing through the machine frame – Unsafe under any condition, and probably illegal in today’s safety conscious world.

Later these became a switch that flipped when something got to a position. These are referred to as “limit switches,” and are still in use. We use these sorts of sensors for anything that we divide into two states – On and Off, True and False, Is and Isn’t.

For example: In Position, Full, Empty, Power On and Running. Let’s look at a few examples of these kinds of sensors. First, as mentioned before, mechanical switches of various kinds are still around. Limit switches are still used in ugly, dirty environments thanks to their “armor-plated” construction. One big reason they have become less popular over the years is that they are huge compared to many of the other sensors available.

Proximity sensors

Proximity sensors, very often called “proxes,” are used for detecting close metal objects using magnetic fields. In many environments, these have replaced limit switches in position sensing applications. Optical sensors have a much longer range than proximity sensors, but they are susceptible to dirt and other environmental and mechanical issues because they use light for sensing.

We often use them where we are not picky about exactly where the target is, but we need to know it is “there,” like boxes on a conveyor where we don’t care whereon the conveyor it is, just that it is passing by. Capacitive proximity sensors are like a proximity sensor, but for detecting non-conductive materials. They are very sensitive to contamination and historically have not been very dependable. Ultrasonic proximity detectors detect solid objects using high-frequency sound but are very susceptible to environmental conditions and dirt. We don’t use them often, but they can solve sensing problems nothing else can.

An auxiliary contact is a part of a relay. These tell us when whatever is controlling the relay has turned it on or off. A pushbutton senses the operator’s action. For now, that is all we will say about digital sensors. Most controls are still designed around on/off signals, so these are the “bread and butter” of a controls engineer’s life. At one time, cars had no fuel gauge, and you had to have a reserve tank – a gas can – so that when you ran out, you could get to a fuel station. Now cars all have fuel gauges, and we will next look at the sensors that make that possible along with many other measurements that automation requires.

Analog sensor

An analog sensor is one that converts a variable physical quantity into a signal that the control system can understand – a voltage or current. By physical quantity, we mean Temperature, Pressure, Humidity, Distance and Speed among others. There is a general category of sensor for each of these. Some sensors combine quantities, like temperature and humidity or distance and speed into a single instrument generating two signals. There are a few general categories of signals generated by these devices.

For temperature sensing, the devices themselves produce either millivolt-range signals in the case of thermocouples, or variable resistances in the case of Resistance Temperature Detectors (RTDs.) Because of their higher accuracy and repeatability, RTDs are generally a better sensing element when we can use them. PLCs have cards that are specifically designed to handle both of these kinds of devices.

The rest of the signal categories are converted locally into a more generally understood form of signal, either voltage or current before being connected to the control system. If the temperature signals have to travel very far to the control system, we usually convert them like this also.

The most commonly used standard today is the 4-20mA signal because of noise immunity and other characteristics, and every type of analog sensor I have mentioned can generally be purchased in that type of output. We have just taken a whirlwind tour of the primary sensors used in almost every industrial control system. With these sensors, we sense everything from which buttons the operator pushes to the height of liquid in a tank to the pressure and temperature of steam in a boiler. These sensors and a few others, with their signals processed by hardware and software, control the industrial processes of the world.

Most likely you’ve heard about “RTDs”, “Thermocouples”, “Thermistors”, “Semiconductor” type elements and so on, which will be addressed here. Before I go into details of this subject, let’s see what a “Temperature Sensor” (Temperature Transducer) is and what does a “Temperature Transmitter” mean.

Generally, a sensor or transducer is a physical device which is capable of transforming one type of process variable to my favorite signal type. To elaborate on this generalized sentence, let me give you an example. Temperature, pressure, flow, etc, are some process variables and actually, they are physical characteristics of our real world. With modern technology and because of tremendous advances in Electrical Engineering in the past century, we like to transform every measurable process value into an electrical signal and a temperature sensor is a device which will transform the temperature into an electrical signal, no matter how tiny the amount of this signal might be!

So far I took a big “First Step” which was the transformation of “Temperature” into “Electrical Signal”. Based on different sensor technologies, this signal may have different ranges and for industrial applications, I need to have my signals limited to some universally accepted electrical “signal-ranges”. Today some of these globally accepted electrical signal-ranges are 4-20 mA , 1-5 V , 0-10 V , etc.

A “Temperature Transmitter” is a device which transforms the tiny output of a “Temperature Transducer” to one of these standard signal ranges. Now let’s get back to different “Temperature Transducer” technologies. RTD or “Resistance Temperature Detector” is a device the resistance of which varies with the temperature. Since it is a passive device, an external electrical current should be applied to it and then the voltage drop across it can be measured. This voltage is a good indication of the temperature. When referring to such a device as “passive”, it means that the device needs external current (or voltage) source. To state the obvious, a big amount of external current can cause power dissipation in the resistor of RTD and lead to excess heat, so to avoid this type of error, the current should be kept at a minimum level.

There is 2 wire, 3 wire and 4 wire wiring configuration for RTD. More accurate reading calls for 3-wire or 4-wire configurations. In reality, the distance between the temperature sensing point and measuring system calls for wiring and since the real wiring has its own resistance, some measurement error sneaks in hereby! 3-wire and 4-wire solutions are developed to remove this error. One of the most common RTDs is “PT100” which consists of a thin film of Platinum on a plastic film and shows a resistance of 100Ω at 32°F. Its resistance varies with temperature and it can typically measure temperatures from -330 to 1560°F. The relationship between resistance and temperature of PT100 is relatively linear. PT100 is just an example of platinum RTDs and in the industry you may find different RTD types suitable for various applications, e.g.: Copper, Nickel, Nickel-Iron, etc.

Thermistors are temperature-dependent resistors and are widely used in industrial purposes, such as over-current protection, self-regulating heating elements, inrush current limiters and so on. Thermistors can be NTC or PTC. In NTC (Negative Temperature Coefficient) thermistors, resistance decreases as temperature rises. NTC’s are commonly used as “inrush” current limiters. And with PTC (Positive Temperature Coefficient) thermistors, resistance increases as temperature increases. PTC thermistors are commonly used as “overcurrent protection” and in resettable fuses.

A thermocouple or simply “TC” is comprised of a couple of specific dissimilar wires joined together, forming the “sensing point” or “junction”. Based on physical characteristics called “Thermoelectric Effect”, when this junction is placed at different temperatures, different millivolt signals are generated which can be interpreted as an indication of the temperature. In comparison with RTDs, Thermocouples are self-powered and require no external excitation current source. Thermocouples are commonly used for furnaces, Gas Turbine combustion chamber, high-temperature exhaust ducts, etc. The main restriction of Thermocouples is the “accuracy” which doesn’t make it the best solution for precise applications. Also, Thermocouples need a reference measurement point called “Cold Junction”. The thermocouple junction is often exposed to extreme environments, while the cold junction is often mounted near the instrument location. Based on “range” of temperature measurement, “sensitivity” and some other factors in each application, different types of Thermocouples are available, for example E, J, K, M, N, T and so on. For instance, Type “J” is made up  of “Iron-Constantan” combination with a range of −40°F to +1380°F and sensitivity of about 27.8 µV/°F while Type “K” (Chromel-Alumel) is one of the most common general-purpose thermocouples with a sensitivity of approximately 22.8 µV/°F. Type K is inexpensive and a wide variety of probes are available in its −330°F to +2460°F operating range. Since the functionality of thermocouple sis based on Thermoelectric Effect in different types of conductors, when the location of a thermocouple is far from the “measuring instrument” (e.g. electronic transmitter), the proper type of conductors should be used for extension purpose. Otherwise, the tiny signal generated by thermocouple will be added with some error at the point where thermocouple wires are connected to the extension wire! “Semiconductor Temperature Sensor” is based on the fact that the junction voltage across ap-n combination of semiconductors, like a diode junction or “base-emitter” junction of regular transistors, is a function of temperature. This technology is vastly used in electronic devices and IC technologies. Linear characteristic, small size, and low cost are advantages of this technology, but it should be noted that the limited range of around -40°F to 248°F makes it suitable for specific applications. To wrap up this video, the comparison between different types of temperature sensor technologies is a multi-facet task. For example, if “accuracy” is considered as the key performance indicator, usually RTD’s are better than Thermocouples; approximately 10 times more accurate. From the “sensitivity” point of view, while both RTDs and Thermocouples respond quickly to temperature changes, at similar costs, thermocouples are often faster. If I have to measure electronic PCB and/or IC temperature, silicon-based types are the best choices.