Types of Noncontact Sensors

Image credit: rozdemir/Shutterstock.com

Sensors are critical devices that allow for monitoring, detecting, and reacting to conditions in an environment or within a process such as an automated production line or machine. By providing feedback on critical parameters to a controller such as a PLC, sensors play a key role in maintaining highly efficient systems that operate autonomously and are central hardware elements in the Internet of Things (IoT). While many types of sensors exist that are intended to monitor different conditions, such as temperature, humidity, or pressure, all sensors can be characterized as being of one of two primary types – contact sensors and non-contact sensors. This article will focus on the latter of these and will review some of the different technologies, types, and applications of non-contact sensors. To learn more about other types of sensors, see our related guide Sensors – A Complete Guide (Types, Applications, and Suppliers).

Contact Sensors vs. Non-Contact Sensors

Measuring, monitoring, or detecting a specific condition or state can be accomplished by means of either direct physical contact or indirect sensing. When direct physical contact is used with a sensor, the sensor is characterized as being a contact sensor. A simple thermometer that is immersed in a swimming pool to measure the water’s temperature or a float that sits inside of a tank and used to drive a gauge that shows the level of fluid in the tank are both examples of contact sensors. The sensing device in each case needs to be in physical contact with the object or substance being measured in order for that sensor to function.

In contrast, non-contact sensors refer to ones that have the ability to function without the need to physically touch the object being monitored. For example, rather than using an oral thermometer to measure a patient’s temperature (which requires the thermometer to be physically placed in the patient’s mouth so that the thermometer bulb can make contact), a nurse or physician may employ an infrared thermometer. This type of thermometer is an example of non-contact sensor technology that uses infrared radiation to establish a temperature reading – thus avoiding the need for direct physical contact to occur.

Non-contact sensors, therefore, 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 readings. The sensors often emit a form of energy such as radiation that can be used to detect a condition without needing physical contact. The object being sensed or detected is usually referred to as the target.

Non-Contact Sensor Technologies

Below are a few of the common sensor technologies used in the design of non-contact sensors. The technology employed in a specific sensor will very much depend on the parameter or condition being monitored (e.g. temperature, pressure, vibration, position, etc.)

Air Sensing or Gauging Technology

With air gauging, a flow of pressurized air is used to measure the dimensions of parts or distances such as diameters or tapers. The technology relies on the principle that as the distance between the air gauge and the object being measured decreases, the flow rate drops while the air pressure increases. Similarly, increasing the separation distance increases the flow rate and decreases pressure. These changes can be measured and used to establish measured values of distance for parts.

Hall Effect Sensor Technology

The Hall effect allows non-contact sensors to be created that can detect the presence of a magnetic field and generate an electrical output signal whose value is proportional to the strength of the field. Hall effect sensors use a thin, rectangular piece of p-type semiconductor material to which a constant DC voltage is applied. When the sensor material is brought in proximity to an external magnetic field, the lines of magnetic flux exert a net force on the charge carriers in the sensor, resulting in a deposition of like charge carriers (electrons or holes) to accumulate on opposite sides of the semiconductor material. This charge accumulation results in an electrical potential difference to develop between these sides of the material, which can be measured, and which is known as the Hall voltage. The Hall voltage can then be used as a proxy for how close the sensor is to the target object, for example.

Ultrasonic Sensor Technology

An ultrasonic sensor makes use of high-frequency sound waves as a detection or sensing mechanism. For example, to measure distance, the ultrasonic sensor emits a sound wave towards the target, some portion of which reflects off of the target and is detected by the sensor. By measuring the time that it takes for the sound to make a round trip from the sensor to the target and back again, the distance to the target can be easily determined. These types of non-contact sensors can work over relatively long distances, but how well they perform will be a function of the material composition and shape of the target. A frequent application is to measure the level of a liquid in tanks.

Photonic Sensor Technology

Photonic or optical sensors use light energy directed through fibers as a means of determining the displacement from or the distance to the target object by measuring the intensity of the light that is reflected off the target. Photonic-based sensors have the advantage of being unaffected by the presence of EMI emissions or high voltage, but the characteristics of the target’s surface finish, which determine its reflectivity, can impact the sensor operation. Additionally, environmental conditions may play a role in determining if this sensor type is suitable for use in a given application.

Capacitive Sensor Technology

Capacitive sensors rely on detecting a change in capacitance to provide information about the movement or position of a target. A capacitor is a device that has the ability to store energy in an electric field between two plates that are known as electrodes. With capacitive sensors, the sensor functions as one plate of the capacitor with the target functioning as the other plate. If a fixed frequency AC current is applied to the sensor, the amplitude of the AC voltage will provide a proportional measure of the distance between the sensor and target.

In liquid level sensing, a capacitive sensor will establish the level of liquid by having the liquid behave as a dielectric material between the plates of the capacitor in the sensor. Measuring the capacitance therefore will establish the value of the dielectric which can be translated into a value for the liquid level in the container.

In other types of capacitive sensor designs, such as for those that measure position, movement of the target relative to the sensor causes a change in either the amount of dielectric (which changes the dielectric constant of the capacitor) or the overlapping area of the capacitor plates (which changes the value of capacitance).

Inductive Sensor Technology

Inductive sensors make use of magnetic fields generated in coils to detect motion or the position of a target. One common type of inductive sensor is a Linear Variable Differential Transformer or LVDT. A set of coils, one primary and two secondaries, are contained in the LVDT. When the primary coil is energized with an AC voltage, induced EMFs are triggered in each of the secondary coils. By measuring the voltage difference between the two secondary coils, the sensor can establish the movement of the target and determine its position. More information about LVDTs can be found in our related guide All About Position Sensors.

Another type of inductive sensor technology makes use of Eddy currents and is used when targets are conductive. An Eddy current sensor uses an alternating current applied to a coil to generate an alternating magnetic field. When the conductive target approaches the sensor, the magnetic field induces currents in the conductive target, called Eddy currents. These currents cause the generation of a secondary magnetic field which opposes the primary field of the sensor. The field interaction can be measured and used as an indicator of the distance that the sensor is from the target.

Laser Displacement Sensor Technology

Laser displacement sensors, also called laser triangulation sensors, are a non-contact sensor technology that uses the triangulation of a reflected light beam to establish movement or position of a target across a measurement range. A semiconductor laser generates a light beam that is sent through a transmitting lens towards the target. Light reflected off the target passes through a receiving lens in the sensor and is focused onto a detector element such as a CCD (charge-coupled device) array. As the target changes its position relative to the sensor, the angle of the reflected light will change which results in that light focusing on a different part of the detector element. The change in light position can be analyzed and used to establish the position of the target.

Non-contact sensor uses for thermal measurement

In many industrial applications requiring thermal measurement, resistance temperature detectors (RTDs) and thermocouples are common types of contact sensors that can be utilized. But there are a number of non-contact sensor types that can be used to perform measurement of temperatures. These include radiation thermometers, thermal imagers, optical pyrometers, and fiber optic temperature sensors.

Radiation Thermometers

Radiation thermometers gauge temperature based on the radiation released from an object. An object’s ability to release radiation is called its emissivity—the more emissive an object is the more radiation a sensor has to work with. Sensors in this category include spot measuring devices that can produce 1-D and 2-D temperature readings, as well as thermal imaging thermometers which can display temperature data as a 2-D image. These types of visual representations are useful in terms of industrial processes because they can help identify potential problems or inconsistencies. In turn, the resulting feedback can provide increased quality and productivity. Radiation thermometers are seen in a vast number of fields, including playing key role in the medical industry where they monitor human temperature. Additionally, they can be used to help control and monitor building temperature and to maintain certain types of power generation.

Thermal Imagers

Although thermal imagers are a type of radiation thermometer, they possess several unique characteristics that set them apart. Instead of measuring temperature based on the radiation at a given point on an object, a thermal imager can measure a two-dimensional space, in essence providing an accurate picture of both the source of radiation and the space around it. Thermal imagers can be used to locate areas in a cord that are overheating, as well as by firefighters to locate people amid smoke and fire. The devices can also be used to locate heat leaks in buildings with weak insulation.

Optical Pyrometers

The name pyrometer comes from the Greek, literally meaning “to measure fire.” Optical pyrometers receive such a meaningful name because they can gauge temperatures that are too bright to see with the naked eye. A pyrometer has two parts: an optical system and a detector. To conduct a proper temperature reading, the optical system works to focus the chosen region of thermal radiation onto the detector, which in turn translates data into a readable temperature. This particular type of measuring device is especially helpful for measuring the temperature of moving objects, or objects that cannot be touched—it is a useful tool in smelting, where the temperature of the metal is an essential part of the operation.

Fiber Optic Temperature Sensors

Many fiber optic temperature sensors are merely variations of radiation thermometers. Relatively simple in design, the fiber optic features an active sensing device attached to a system that processes the radiation data and produces a temperature reading. They are extremely useful in automotive applications because they can set a temperature limit signal for engines.

Summary

This article provided information on non-contact sensors, including a review of the technologies underlying their use and examples of non-contact sensors used for thermal measurement. For information on other topics, consult our additional guides or visit the Thomas Supplier Discovery Platform where you can locate potential sources of supply for over 70,000 different product and service categories, including over 130 suppliers of non-contact sensors.