Basically, a distance sensor – also known as a displacement, proximity or position sensor – measures the distance between a reference point on the sensor and a target object. Then, the sensor transmits the changing distance value via an interface to a control unit (e. g. a programmable logic controller or PLC). Generally, the faster the measured distance changes, the more often the sensor has to measure in order to correctly map the distance change. As a result, the measurement rate of a distance sensor is an important quality feature, as is its measurement accuracy and range.

Also, there is a difference between distance and proximity sensors (or proximity switches): In contrast to distance sensors, proximity sensors cannot determine an exact distance value. However, proximity switches can detect an object at close range, e. g. as soon as a defined threshold is reached.

Contactless distance sensors measure the distance to a target object without touching it. In brief, the sensor emits a signal in the form of sound waves or electromagnetic waves (respectively light). When the signal is thrown back by the target, the sensor subsequently receives the signal back and measures its runtime. As a result, the distance can be calculated using the exact length of time and known parameters such as the speed of sound or light. Both ultrasonic and laser distance sensors work according to this principle. However, FMCW radar sensors determine the distance by measuring the frequency shift between the emitted and received radar signal. Moreover, very precise laser sensors calculate the distance via triangulation.

By comparison, tactile displacement or position sensors measure the distance to a target object by touching it or are connected to the object via a cable pull. These sensors include, for example, draw wire sensors or inductive displacement sensors.

Whether for dimensional measurements, level control or collision avoidance: Distance, displacement or position sensors are undoubtedly used in many applications and industries. In manufacturing, for instance, displacement sensors are used for the precise measurement of products in quality control. Furthermore, contactless distance sensors control the level of liquids or solids in silos and tanks – for instance in the food, chemical or pharmaceutical industries. Moreover, displacement or position sensors also measure bulk material heights or throughput on conveyor belts. They are also used to position material handling equipment, robots, cranes or mobile robots. Similarly, distance measurement sensors can also detect obstacles to avoid collisions in the automotive, transport and logistics sectors.

How accurately contactless distance measurement sensors measure depends not least on the measurement technology used. For instance, OndoSense radar sensors feature a measurement accuracy in the micrometer range. This accuracy is also achieved by triangulation-based laser distance sensors. However, even fine dust particles, raindrops, rough materials or surfaces in certain colors can prevent the measurement of laser sensors. Radar-based distance measurement sensors, on the other hand, measure reliably even with steam, dust, extreme temperatures or vibrations. Consequently, they are particularly well suited for harsh environments or outdoor use.

With ultrasonic distance sensors, reliable measurements with sub-millimeter precision are only possible to a very limited extent, because the sound wave velocity changes depending on temperature, pressure or humidity. Also, in contrast to radar, dirt or dust on the lens of the ultrasonic sensor has a major impact on the measurement precision.

In short, the range of a distance sensor is the smallest and largest possible distance range in which the sensor can detect an object. However, this measurement range varies greatly depending on the type of sensor. Generally, ultrasonic distance sensors have a range that starts below 100 mm and is usually limited to about 15 meters. Nevertheless, dust, noise, high humidity and sound-absorbing targets can further reduce this range. By comparison, FMCW radar sensors can measure up to 50 meters and more. For very long distances with small targets, the use of radar reflectors may be necessary for a strong radar signal. The minimum measuring range of distance radars is 0.3 to 0.5 meters, however OndoSense radar sensors can already measure from 0.1 meters.

With laser triangulation sensors, precise distance measurements are possible from below 100 mm - but only up to a few meters. In contrast, time-of-flight laser sensors offer a much longer range – sometimes up to one kilometer – nevertheless with significant limitations in terms of measurement accuracy and object recognition (special reflective tapes are usually needed).

In general, there are two types of laser sensors for distance measurement: Firstly, triangulation laser sensors, which are very precise, and secondly time-of-flight laser sensors, which have a longer range. In brief, time-of-flight lasers determine the distance between the sensor and the target by measuring the time of travel of the signal. For this purpose, the sensor emits a laser beam and measures the time required for the signal to return to the sensor after being reflected by the target object. Then, the laser distance sensor calculates the distance using the signal's runtime and the speed of light.

By comparison, the triangulation laser measures the distance using an angle calculation: In short, the sensor emits a laser beam, which is thrown back by the target object. Depending on the distance, the beam hits the receiver of the laser sensor at a certain angle. Subsequently, the sensor determines the exact distance to the object based on the angle and the distance between the transmitter and receiver.

Basically, ultrasonic distance sensors measure distances by emitting sound pulses at a very high frequency and evaluating their time of flight. Then, the sound waves are reflected on the target object and received by the sensor. Since the speed of sound (about 340 m/s in air) is known, the ultrasonic sensor can calculate the distance based on the time between the emission and reception of the sound waves. Due to their limited measurement range and low measurement rate, ultrasonic sensors are not suitable for measuring fast-moving targets.

Almost all radar sensors for distance measurement are based on the frequency-modulated continuous wave radar. In short, FMCW radar sensors emit a radar frequency that increases or decreases steadily over a defined frequency band range (e. g. from 122 to 123 GHz). This radar signal is reflected by the target object and subsequently received by the radar sensor. Then, the exact distance can be calculated from the frequency shift between the transmitting and receiving signals. More information can be found on our FMCW radar page.

Radar sensors for distance measurement have some advantages compared to laser or ultrasonic sensors when it comes to reliable, precise and fast distance measurements. First of all, radar sensors operate at a significantly higher wavelength than laser sensors, so that e.g. dust particles or steam do not interfere with the radar signal. Therefore, radar distance sensors measure reliably even in difficult conditions with dirt, rain, snow or smoke. Moreover, OndoSense radar sensors offer a real-time capable measurement rate as well as a measurement accuracy in the micrometer range, as it is the case for precise laser sensors.

In addition, the propagation speed of radar waves is constant and does not depend on temperature, pressure or humidity, as with ultrasonic waves. Therefore, precise distance measurements are possible even in adverse environments and outdoors. Also, dirt on the lens does not affect the distance radar. Furthermore, there are no moving parts or sensitive optics in radar sensors compared to lidar sensors. As a result, radar technology is very low-maintenance and robust.

In the following graph you will find a comparison of different sensor technologies for distance measurement (laser, ultrasound and radar):