Editorial Feature

What is a Position Sensor?

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Sep 19 2014

A position sensor is any device used for measuring the distance traveled by a body starting from its reference position. It measures linear or angular position in reference to a fixed point or arbitrary reference. The sensor can also be used to detect the presence or absence of an object.

If the position or proximity information is combined with time measurements, then speed, velocity, and acceleration can be calculated for motion control.

Types of Position Sensors

Position sensors use different sensing principles to sense the displacement of a body. Depending on the different sensing principles used for position sensors, they can be classified as follows:

Potentiometric Position Sensor

Potentiometric position sensor is operated based on the principle of resistive effect. A resistive track acts as a sensing element, and a wiper is attached to the body or part of the body whose displacement is to be measured. The wiper is also in contact with the track.

The movement of wiper changes the resistance between one end of the track and wiper. Therefore, the resistance becomes a function of the wiper position, and the change in resistance per unit change in wiper position is linear.

The conductive track can be made linear or angular depending upon the requirements. The tracks are made from carbon, resistance wire or piezoresistive material. The main advantage of this sensor is its ease of use.

Capacitive Position Sensor

Capacitive position sensors are operated under the following two configurations:

By changing dielectric constant – In this configuration, a body whose displacement is to be measured is connected to the dielectric material between the plates. The effective dielectric constant between the plates during the movement of the body is the resultant of the dielectric constant due to air and dielectric materials.
By changing overlapping area – In this configuration, a body is connected to one of the plates, and the other plate remains fixed. The overlapping area between the plates changes with the movement of the body, and the changing overlapping area results in change in capacitance between the plates.

Magnetostrictive Position Sensor

A magnetostrictive position sensor primarily consists of components such as a position magnet, a magnetostrictive position sensor that measures the distance between the head end of a sensing rod and the position magnet, an electronics module and a waveguide.

A current pulse is transmitted down the waveguide from the electronics module. The magnetic field generated by the current pulse interacts with the magnetic field from the position magnet thereby generating sonic wave or torsional strain wave in the waveguide.

The strain wave travels towards the head end where the pickup device senses its arrival. The time difference between the generation of the interrogation pulse and the arrival of strain wave indicates the location of the position magnet.

Eddy Current-Based Position Sensor

When an alternating current is applied to the coil, a primary magnetic field is induced which leads to the formation of eddy currents in the electrical conducting material. Eddy currents, in turn, induce secondary field that has a coil impedance effect.

The presence or absence of the conducting material alters the secondary field and the coil impedance. Changes in the coil impedance can be used to measure the distance of the electrical conducting body.

Hall Effect-Based Magnetic Position Sensors

In Hall-effect position sensors, a moving part is linked to a magnet housed with a sensor shaft thereby forming a Hall element. With the movement of the body or its part the magnet also moves which leads to the formation of magnetic field and hence Hall voltage. Thus Hall voltage becomes a function of the position of the moving part.

Optical Position Sensor

Optical sensors are operated based on one of the two mechanisms. In the first type, light is transmitted from one end and received at the other to monitor the change in one of the characteristics such as wavelength, intensity, phase or polarization. In the second type, the transmitted light is reflected from the object and light returned towards the source is monitored.