Types of Temperature Sensors

Abstract

There are three main types of temperature sensors: thermometers, resistance temperature detectors and thermocouples. These sensors measure a physical property that changes as a function of temperature, and temperature sensors are classified into contact and non-contact sensors. Contact sensors detect the degree of hotness or coldness of an object when placed in direct contact with the object. It can be used to sense the degree of hotness or coldness in liquids, solids or gases in a wide range of temperatures. Contact temperature sensors include thermometers, thermocouples and thermistors. A thermometer detects the body temperature of human beings, and a thermocouple is a thermoelectrical thermometer that works on the principle of the Seebeck effect; they are cheap; hence, their model and basic materials are easy to get, and non-contact sensors are not placed in contact with the object that it measures; however, they measure the temperature by utilizing the radiation of the heat source. IR sensors detect the energy of an object remotely and emit a sign to an electronic circuit that senses the object’s temperature by a specific calibration diagram. Other types of temperature sensors are available and produced based on the working principle, size, temperature range and their function and application.

1. Introduction

A temperature sensor is an electronic device that measures the temperature of its environment and converts the input data into electronic data to record, monitor or communicate temperature changes. A temperature sensor is an electronic device that monitors the temperature of its surroundings and turns the input data into electronic data. Temperature sensors come in a wide variety of forms [1].

Temperature sensors are electrical/electronic physical sensing device which transforms an input signal from a specific environment into an equivalent output signal [2].

According to the amount of general literature on the topic, thermocouples are the most often employed type of temperature measuring in industry. Its widespread acceptance, reasonable accuracy over a wide measurement range, and relatively inexpensive sensors all contribute to its appeal. Narrower measuring ranges can handle accuracy closer to 0.1 degrees Celsius, whereas accuracy over wide ranges is comfortably between 0.5 and 2 degrees Celsius [3].

As long as the Seebeck coefficients of material A and material B for the two materials are known, these thermoelectric devices use the Seebeck effect in dissimilar metal wires linked at the thermoelectric junction representing T1 to determine a temperature gradient down the wire [4]. The temperature can be gauged at the terminus connections T0 by measuring the net electromotive force between T0 and T1 within the wires, which is voltage of the order of microvolts. Cold junctions are frequently utilised in the form of a fixed physical temperature or electronically mimicked via cold junction compensation because a temperature gradient must be constructed to produce a net voltage output signal (CJC).

Due to the non-linear temperature-resistance connection of thermistors, which are composed of semiconductor materials, calibration is even more crucial [5]. Although routine calibration is required to prevent the impacts of sensor drift, the use of semiconductor materials allows them to deliver a far better level of sensitivity [6] than other sensor types.

According to Schweiger’s 2007 argument, if the right sensors are chosen and calibrated properly, a quick multichannel precision thermometer might compete with Precision Thermometers using thermistors [7]. Deviations of less than 30 mK were seen in tests conducted in the temperature range of −50 to 10 C. Improvements have been made in spatial resolution of surface temperature measurement compared to standard soldered type K thermocouple using an electrochemically etched microtip [8]. Thin film thermocouples can also be deposited onto a surface and have been used to measure heat generated in the friction between sliding surfaces [9]. Non-linearity of sensors can be an issue, although one study showed it to be possible to correct for this using a neural network approach in type K thermocouples [10].

Industrial thermocouple measurements can be further enhanced by improving high-temperature alloys and more intelligent electronics [11].

Figure 1 shows an illustration of temperature-sensing using human hands as a sensor and its digital equivalent, while Figure 2 shows a temperature sensor formed by joining two different materials. There are many different types, sizes and shapes of temperature sensors. In general, temperature sensors can be categorised into two groups: contact sensors and non-contact sensors [15].

Figure 1.

Temperature-Sensing illustration [12, 13, 14].

Figure 2.

Temperature sensor [15].

2. Contact sensors

When positioned close to an object to be detected for heat or cold, contact sensors are used to measure the object’s temperature. These sensors can determine the concentration of liquids, solids or gases throughout a wide temperature range.

Thermocouples and thermistors are good examples of contact temperature sensors.

Thermocouples are inexpensive, and it is easy to find the basic materials needed to manufacture thermocouples [15, 16].

Contact Sensors are devices that measure temperature by placing it in direct contact with the object being measured or the desired measurement environment. They can be used to detect temperature changes in gases, liquids or solids in a range of temperature measurements. Thermocouples and thermistors are two contact sensor types. Its model and fundamental components are straightforward, and thermocouples are frequently inexpensive.

Additionally, thermocouples have the broadest temperature range of any temperature sensor, ranging from well below -200°C to well over 2000°C [16].

Thermocouples are thermoelectric sensors that are essentially made of two welded or crimped junctions of dissimilar metals, such as copper and constantan. The reference (cold) junction and the measuring (hot) junction are the two junctions that are maintained at the same temperature. As illustrated below, a voltage is created across the junction when the two junctions are at different temperatures. This voltage is used to measure the temperature sensor [16].

2.1 Construction of a thermocouple

Figure 3 shows how a thermocouple is constructed by joining two metals of iron and constantan.

Figure 3.

Construction of a thermocouple [16].

2.2 Working principle of a thermocouple

The thermocouple’s working principle is quite straightforward and fundamental. When two different metals, such as copper and constantan, are fused together, a “thermoelectric” effect results, producing a constant potential difference between the two materials of only a few millivolts (mV). The “Seebeck effect” refers to the voltage differential between the two junctions because an electromagnetic field (emf) is created when a temperature gradient develops between the conducting wires. The output voltage of a thermocouple is then dependent on temperature variations [17].

If both junctions in Figure 3 are at the same temperature (zero potential difference across the junctions), and there is no voltage output because V1 = V2. But when the junctions are linked together in a circuit and operate at different temperatures, a voltage output, V1 - V2, corresponding to the temperature differential between the two junctions, will be noticed. This is because the characteristics of the two different metals employed influence how much of a voltage difference will increase with temperature until the junction reaches its maximum voltage level [17].

Extreme temperatures between −200°C and over +2000°C can be recorded using thermocouples, which can be constructed from various materials. Internationally recognised standards have been created with thermocouple colour codes to help users select the best thermocouple sensor for a given application due to the wide variety of materials and temperature ranges available. Below is a list of the standard thermocouple colours used in Britain [17].

Figure 4. shows the thermocouple colour codes that were used in the manufacturing of different types of thermocouples. Thermistor contacts are the second kind of contact temperature sensor. The resistance of thermistors is dependent on temperature change, as opposed to other types of resistors whose value is determined by the colour code [18].

Figure 4.

Thermocouple colour codes [17].

Thermistors are available in two types which are:

1. Positive temperature coefficient (PTC)

2. Negative temperature coefficient (NTC)

A PTC thermistor’s resistance rises with temperature, but an NTC thermistor’s resistance falls with temperature. Therefore, an NTC thermistor is the most common type of thermistor.

Temperature sensors include thermocouples. They can be found in common appliances, including ovens, refrigerators and fire alarms. Thermometers and numerous other vehicle appliances also include them [18].

Figures 5 and 6 show PTC (left) and NTC (right) thermistor electrical symbols and a typical NTC thermistor.

Figure 5.

PTC (left) and NTC (right) thermistor electrical symbols [19].

Figure 6.

A typical thermistor [19].

2.3 Advantages of a thermistor

• Less expensive

• Can measure changes in a small temperature range

• They are more sensitive than other temperature sensors

• They provide a fast response

• They are easy to use

They are small and can fit into any smallest space [19].

A bi-metallic strip is created when two distinct metals, such as nickel, copper, tungsten, or aluminium, are bonded together to create the thermostat, an electro-mechanical contact type temperature sensor. When the strip is heated, the differing linear expansion rates of the two dissimilar metals cause a mechanical bending action.

The bi-metallic strip is frequently used to control hot water heating elements in boilers, furnaces, hot water storage tanks and vehicle radiator cooling systems. In addition, it can be used as an electrical switch on its own or as a mechanical method of operating an electrical switch in thermostatic controls [16].

Figure 7 shows two metals with distinct thermal properties bonded back-to-back to form the thermostat. The connections are closed when it is cold, allowing current to flow through the thermostat. However, the bonded bi-metallic strip bends up (or down) and opens the contacts when it gets hot because one metal expands more than the other, blocking the current flow [16].

Figure 7.

Bi-metallic strip.

2.4 Thermostat

A thermostat is a temperature-sensing tool that gauges engine coolant temperature. In order for internal combustion engines to operate at an efficient temperature, the component is intended to know when to open and close.

If the coolant is not hot enough, the thermostats stay closed. However, when the coolant reaches a certain temperature, a valve opens, letting hot coolant flow into the radiator. The thermostat therefore functions similarly to a gate by allowing or preventing the passage of coolant from the engine to the radiator.

Modern automobile engines operate within a specific temperature range; typically, they operate between 194 degrees Fahrenheit, or 90 degrees Celsius, and 221 degrees Fahrenheit. The thermostat determines when to open and close based on the coolant temperature [20].

Figure 8 shows the on/off the thermostat; there are two main types of bi-metallic strips with respect to their movement when subjected to temperature changes. They are:

1. snap-action

2. creeper types

Figure 8.

On/off thermostat [16].

Both the faster “creep-action” types gradually adjust their position as the temperature changes, and the snap-action types generate an instantaneous “ON/OFF” or “OFF/ON” type action on the electrical connections.

Snap-action type thermostats are frequently used in our houses to regulate the temperature set point of ovens, irons, immersion hot water tanks, as well as the domestic heating system. They can also be found mounted on walls [16].

In most creeper varieties, a bi-metallic coil or spiral slowly unwinds or coils up in response to temperature changes. Since the creeper-type bi-metallic strips are longer and thinner than the conventional snap ON/OFF varieties, they are typically more sensitive to temperature changes, making them perfect for use in temperature gauges, dials, and other similar devices [16].

Standard snap-action-type thermostats have a significant hysteresis range between the time the electrical contacts open and the time they close again, which is a drawback despite their low-cost and wide operating range when used as temperature sensors. It might be set to 20°C, for instance, but not open until 22°C or close again until 18°C [16].

Therefore, the temperature swing range might be rather wide. Bi-metallic thermostats that are sold for residential usage contain temperature adjustment screws that enable more exact pre-setting of the appropriate temperature set point and hysteresis level [16].

Contact sensors are employed in industries to control various automation temperature processes; hence, it is advantageous to use sensors in the industry, offices and home to regulate the environment’s temperature.

2.5 What is a temperature controller?

Temperature controls make sure a process gets the desired temperature and keeps it there. These are typically employed for closed-loop control, in which the temperature controller compares the actual temperature with the set point established by the programmer using data from a temperature probe (thermocouple, resistance thermometer or temperature transmitter). It then modifies its output signal to the appropriate control element as necessary (electrical heater, cooling circuit, steam control valve, etc.). A variable output, where the output signal to the process is between 0 and 100%, and a straightforward ON/OFF control, working like a thermostat, are possible. The latter is also called a 2-point, binary, or bang-bang control [21].

2.6 How does a temperature controller work

The heating circuit is turned on for ON/OFF control when the temperature is below the set point and off when it is above. Additionally, a cooling circuit may be activated above and deactivated below the specified point. A proportional–integral–derivative (PID) controller frequently performs variable control (three-term controller). In order to attain and keep the set point with the least amount of overshoot and to retain it as steadily as possible, this controller applies a revised algorithm on the error (the difference between the set point and the measured value) [21].

2.7 What is a PID controller

Depending on the needs of the process, three-term or PID controllers (proportional–integral–derivative) can be employed for proportional alone (P), PI or PID control. In proportion to the departure from the set point, proportional control modifies the output. A defined proportionate band is below and/or above the set point. The output for cooling (above) or heating (below) is 100% outside of this band. It decreases linearly within the band, reaching 0% at the set point. The integral term can then further alter the output based on the rate-of-change of the mistake because this can result in a sluggish approach to the set point (achieving the set point quicker). Due to the possibility of overshooting the fixed point, the derivative term predicts future errors and modifies the output [21].

2.8 Advantages of temperature sensors

Temperature sensors are possible when an object needs to be heated, cooled, or both, and it must maintain the desired temperature (setpoint) despite changes in its surroundings.

Open-loop and closed-loop controls are the two fundamental methods of temperature control.

Open-loop systems apply continuous heating and cooling without considering the actual temperature output. It is comparable to a car’s interior heating system. You might have to set the heat all the way up on a chilly day to get the car up to 75 degrees. However, during warmer weather, the same setting would leave the inside of the car much warmer than the desired 75° [22].

Temperature sensors can control a given situation using the open and closed loops, as shown in Figures 9 and 10, respectively.

Figure 9.

Open-loop temperature control diagram [23].

Figure 10.

Close loop temperature controller block diagram [13, 23, 24].

Regardless of sophistication, all temperature sensors and controllers operate in essentially the same way. A controller keeps a variable or parameter constant at a predetermined value. The actual input signal and the desired setpoint value are the two variables that the controller needs. The input signal is also known as the process value. The controller determines how frequently the input is sampled [25].

The input or process value is then compared to the setpoint value. If the process value deviates from the setpoint, the controller changes the output signal based on the difference between the process value and the setpoint and whether the process value is getting closer to the setpoint or moving further away from it. The actual value is then changed in response to the output signal in order to bring it into compliance with the setpoint. Typically, the control algorithm updates the output power value before applying it to the output [25].

The control action is based on the type of controller being used. The controller decides whether the output should be turned on, off or left in its current state, for example, if it is an ON/OFF control [25].

One of the easiest control kinds to use is the ON/OFF control. By establishing a hysteresis band, it operates. To regulate the temperature inside a room, for instance, a temperature controller might be used. An error signal would display a − 1° difference if the setpoint temperature was 68° and the actual temperature was 67°. The temperature would then be raised back to the setpoint of 68° by the controller sending a signal to increase the applied heat. The heater turns off when the room reaches 68 degrees. The controller does nothing, and the heater stays off for a temperature between 68° and 67°. The heater will, however, start up once the temperature hits 67° [25].

Unlike ON/OFF control, PID control determines the precise output value required to maintain the desired temperature. Power output ranges from 0–100%. When an analogue output type is used, the output drive is proportional to the output power value. If the output is a binary output type, such as a relay, Solid State Relay driver or triac, it must be time-proportional in order to provide an analogue representation [25].

A system that uses cycle time to proportion output values is called time-proportional. A system requiring 50% power will have its output on for 4 seconds and off for 4 seconds if the cycle time is set to 8 seconds. The time values would not change as long as the power value remained constant. The power is gradually averaged to the requested 50% amount, which is evenly split between on and off. The output would be on for two seconds and off for six seconds over an eight-second cycle if the output power needed to be 25% [25] as shown in Figure 11.

Figure 11.

Output time proportioning [25].

A shorter cycle time is desired, barring any other factors, because the controller can react to changes in the process and the output’s condition more quickly. Due to the way relays operate, which may shorten their longevity, a cycle duration of less than 8 seconds is not recommended. For solid state switching components like an SSR driver or triac, quicker switching times are preferred. Longer switching times allow for higher process value variation regardless of the output type. A longer cycle time is typically desirable when employing a relay output, but only if the process allows it [25], as shown in Figure 12.

Figure 12.

Overview of contact temperature sensor controller [1].

Table 1 shows the comparison between NTC thermistor and thermocouple.

NTC Thermistor	Thermocouple
Effect of lead resistance on accuracy	Very low	None
Linearity	Non-linear-output requires linearisation	Non-linear requires conversion
Stability	Epoxy coated: 0.2 °C/year Hermetically sealed: 0.02°C/year	>1 °C/year
Response time	0.12–10 s (depending on size and packaging)	0.2–10 s (depending on size and packaging)
Temperature range	−50 to 250°C (dependent on type)	−200 to 1250°C, dependent on type
Temperature range	−50 to 250°C (dependent on type)	−200 to 1250°C, dependent on type
Response time	0.12–10 s (depending on size and packaging)	0.2–10 s (depending on size and packaging)
Stability	Epoxy coated: 0.2°C/year Hermetically Sealed: 0.02°C/year	>1°C/year
Linearity	Non-linear-output requires linearisation	Non-linear requires conversion
Effect of lead resistance on accuracy	Very low	None

Table 1.

A brief comparison of thermistor and thermocouple [19].

2.9 Non-contact sensors

Non-contact sensors are not in contact with the object that it measures; however, they measure the temperature by utilising the radiation of the heat source. An example of a non-contact sensor is the infrared (IR) sensor. IRs detect the energy of an object remotely and emit a sign to an electronic circuit that senses the object’s temperature by a specific calibration.

Non-contact temperature sensors generally rely on technologies that are based on electrical, magnetic, optical, sonic or other principles rather than depending on physical contact or mechanical movement to obtain the measurements. The sensor often emits a form of energy such as radiation that can be used to detect a condition without physical contact.

2.10 Working principle of non-contact sensors

Non-contact sensors detect changes in physical environmental conditions without physical contact with the measured object. There are several types of non-contact sensors, including optical, capacitive, magnetic, ultrasonic and many other types of sensors.

The specific working principle of non-contact sensors can vary based on the type of sensing pattern; however, they all depend on detecting changes in the environment, converting the required information into electrical signals which can be processed or analysed.

Manufacturing process of non-contact sensors.

Non-contact sensors are manufactured using a variety of different technologies depending on the specific application and the type of sensor being produced.

Optical sensors: Optical sensors use light to detect changes in position or distance. They can be made using a variety of.

2.11 Applications of non-contact sensors

The field is progressing thanks to innovation. Active-matrix flexible temperature sensors and self-powered flexible temperature sensors are two examples of flexible temperature sensors that have recently been studied and optimised. Flexible temperature sensors also include flexible thermocouples, flexible thermistors, and flexible thermochromic types [26].

Patients’ temperatures have been monitored using printable, flexible sensors with excellent sensitivity. There is a trend toward creating wearable sensors that can measure temperature, avoiding conventional problems with heavy equipment and measuring inaccuracies caused by a variety of factors such as the wearer’s movement.

Other prominent research in the field of non-contact infrared temperature sensors is recent work on creating a low-cost, more accurate Arduino-based infrared thermometer for body temperature detection. Arduino is an open-source electronics platform that converts input to output. This research aims to circumvent the problems inherent with non-contact infrared sensors currently on the market [26] (Table 2).

Type	Advantages	Disadvantages	Max working distance
MMW-Radar	1) Long working distance	1) Unapplicable for static objects	5 m–200 m
	2) Available for radial velocity	2) Generating false alarms easily
	3) Applicable for all-weather
Camera	1) Excellent discernibility	1) Heavy calculation burden	250 m (depending on the lens)
	2) Available lateral velocity	2) Light interference
	3) Available for colour distribution	3) Weather susceptible
		4) Unavailable for radial velocity
LiDAR	1) Wide field of view (FOV)	1) Insufferable for bad weather	200 m
	2) High-range resolution	2) High price
	3) High-angle resolution
Ultrasonic	1) Inexpensive	1) Low resolution	2 m
		2) Inapplicable for high speed
DSRC	1) Applicable for high speed(up to 150 km/h)	1) Low data rate	300-1000 m
	2) Relatively mature technology	2) Small coverage
	3) Low latency (0.2 ms)
LTE-V2X	1) Long working distance	1) High latency in long distance (> 1 s)	Up to 2 km
	2) Relatively high data transmission rate(Up to 300 Mbps)	2) Inapplicable for time-critical events
5G-V2X	1) Ultra-high data transmission rate	1) Immature application	100 m - 300 m
	2) Low latency(< 80 ms)
	3) High bandwidth
	4) Applicable for high speed (up to 500 km/h)

Table 2.

Comparison of different types of non-contact sensors.

Advantages of non-contact temperature sensors.

1. They are used in measuring hard-to-reach or very hot objects.

2. They have very short measurement and response time.

3. They are used in the non-destructive measurement

4. They have longevity of measuring point.

5. They have the option of measuring even at high voltages, electromagnetic fields or aggressive materials.

Examples of non-contact temperature sensors

1. Thermal imagers

2. Furnace monitoring cameras

3. Infrared thermometers

4. Hall effect sensors technology

5. Ultrasonic sensors technology

6. Photonic sensors technology

7. Capacitive sensors technology

8. Inductive sensors technology

9. Laser displacement sensors technology

10. Radiation thermometers

11. Optical pyrometers

Applications of temperature sensors.

Some temperature sensor applications include;

• Motorsport and other vehicles – within motorsports, there are many temperature sensor applications. These include; ensuring motors do not overheat, surface plate temperature, exhaust gas temperature, oil temperature, etc.

• Industrial equipment – most machinery used in manufacturing will contain a temperature sensor for safety reasons. Temperature sensors used within this environment must be highly robust and resistant to dirt and moisture.

• Medical Applications – temperature sensors are used for patient monitoring and within machines and devices for a range of medical procedures. In this industry, temperature sensors will require various safety standards and approvals.

• Food and beverage industry – temperature sensors are used within this environment as part of food safety standards, ensuring food is kept at the correct temperature. They are also used on various manufacturing equipment used within this sector.

• Home appliances and white goods – many appliances within the home will contain a temperature sensor, oven, toaster, kettles, washing machines, coffee machines, dishwashers, electric radiators, boilers, etc.

• Computers and devices – temperature sensors are used within computers and other devices to ensure they do not overheat and become dangerous.

More temperature sensor applications and areas:

• Calibration and Instrumentation

• Transit – refrigerated vans and lorries

• HVAC – Heating ventilation and air conditioning

• Power and utilities

• Renewable energy

• Heat Exchangers

• Drilling

• Laboratory and testing applications

3. Recent development in the temperature sensors

There is a recent development in temperature sensors, thanks to innovation. Active-matrix flexible temperature sensors and self-powered flexible temperature sensors are two examples of flexible temperature sensors that have recently been studied and optimised. Flexible temperature sensors also include flexible thermocouples, flexible thermistors, and flexible thermochromic types [26].

Printable, high-sensitivity flexible sensors have been explored to provide temperature monitoring of patients. There is a trend toward developing wearable sensors that can monitor temperature, circumventing traditional issues with bulky equipment and errors in measurement due to numerous factors such as the wearer’s movement [26].

Gems sensors are made to detect or measure a media (air, gas, oil, water, steam, etc.). It may occasionally be essential to modify its attributes (level, volume, flow, pressure and temperature). Sometimes all that is required is to observe or record the media properties.

In order to represent the measurement or detection of the media, sensors send an output signal. After receiving the output, west controllers’ devices can display, record, and/or control the process to modify the media’s attributes to suit the application [27].

Figure 13 shows gems continuous measurement sensors deliver a linear output to reflect the whole sensor range. DC voltage, current and frequency outputs are the three most typical linear outputs for sensors [27].

Figure 13.

Gems measurement continuous sensor [27].

On the majority of versions, west controllers include a universal input. This supports most linear output kinds from gems sensors. However, it would help if you made sure the needed output is supported because west controllers do not support all output types offered by gems sensors [27].

Figure 14 shows the Gems 3100 Series Pressure Transducer, which may deliver 4–20 mA (milliamp) current or DC voltage outputs of 0–5, 1–5 and 0–10 VDC. The scale of these numbers corresponds to the pressure range that the transducer was designed. For example, the transducer would supply 4 mA at 0 PSIG (pounds per square inch, gauge) and 20 mA at 750 PSIG (pounds per square inch, gauge) if the 4–20 mA output for 0–750 PSIG was used, respectively. The universal input for 4–20 mA and the complete range of the 0–750 scale are both programmed into the West 6100 Plus Series Controller.

Figure 14.

The gems 3100 series pressure transducer.

3.1 Common non-contact sensor technologies and real-world applications

There have been some common non-contact sensor technology developments in recent years, through a series of innovations, research and development.

3.1.1 Capacitive sensor technology

These non-contact sensor varieties track changes in capacitance to gather important details about the movement or location of a specific target. A capacitor can store energy in an electric field between two plates known as electrodes. This technology targets the other capacitor plate; the capacitance sensor is the first. The amplitude of the AC voltage, when a fixed frequency AC current is delivered, serves as a gauge of the separation between the sensor and target [28].

Position sensing and dynamic and thickness measuring are typical uses for capacitive sensor technology. In addition, on workstations, conveyors and robots, capacitive sensors can be utilised to detect parts and count and monitor liquid levels.

Everyday devices, such as digital audio players, smartphones and tablets, leverage capacitive sensing touchscreen as input devices. These sensors can also replace mechanical buttons [28].

3.1.2 Laser displacement sensor technology

The high accuracy of distance, position and displacement measurements of targets at long ranges are well suited for laser displacement sensors, also known as laser triangulation sensors.

These sensors are utilised for displacement measurement in a wide range of applications and sectors, from automated process control and research and development testing to Original Equipment Manufacturer integration, inventory management and more.

They are designed to measure and check the levels of liquid and bulk materials as well as the position, size, surface profile, vibrations, and sensing of technical items [28].

3.1.3 Inductive sensors technology

Inductive sensors employ magnetic fields produced in the coil to assess a target’s motion or location.

When targets are conductive, one kind of inductive sensor technology uses Eddy currents.

This kind of sensor creates an alternating magnetic field by applying an alternating current to a coil.

The field causes currents—Eddy currents—in the target when it gets close to the sensor.

A secondary magnetic field is created by these currents and opposes the sensor’s magnetic field.

The interaction can be gauged and utilised to calculate how far away the sensor is from the target.

Due to its resistance to grease, filth, dampness, magnetic interference fields and harsh industrial settings, Eddy current sensors are appropriate for use in places with limited access.

The measurement of internal combustion engine cylinder vibrations or sheet metal thickness in roller gaps is two instances of this technique in action [28].