position sensor OR displacement sensor OR level sensor

The measurement of position, displacement or level is very essential for many vivid applications such as process feedback control, transportation traffic control, robotics, security systems and more. Here term position refers to determination of object's co-ordinates (either linear or angular) with respect to a selected reference. The term displacement refers to moving from one position to the another position for a specific distance or angle. A critical distance is measured by proximity sensor.

A proximity sensor is a threshold version of a position detector. A position sensor is a linear device whose output signal represents a distance to the object from a certain reference point. A proximity sensor is a simpler device which generates the output signal when a certain distance to the object becomes essential for an indication. A displacement sensor often is part of a more complex sensor where the detection of movement is one of several steps in a signal conversion. An example is a pressure sensor where pressure is translated into a displacement of a diaphragm. Later the diaphragm displacement is converted into an electrical signal which represents pressure. Hence the position sensors are essential for the design of many other sensors.

Position and displacement sensors are static devices whose speed response usually is not critical for the performance. Following specifications need to be consider while selecting or designing the displacement or position sensors.

•  How large is the displacement and of what type (linear, circular)?
•  Amount of resolution and accuracy needed
•  Material used to construct measured object (such as metal, plastic, fluid, ferromagnetic etc.)
•  Space available to mount the detector
•  Amount of play available in the moving assembly and required detection range
•  environmental conditions available such as humidity, temperature, sources of interference, vibration, corrosive materials etc.
•  How much power is available for the sensor?
•  Total mechanical wear expected over the entire lifetime of the machine
•  What is the production quantity of the sensing assembly (limited number, medium volume, mass production)?
•  What is the target cost of the detecting assembly?

Potentiometer as position sensor

position or displacement sensor

Figure-1 depicts potentiometer as displacement sensor. A position sensor can be built with a linear or rotary potentiometer or a pot for short.

R = δ (L/A)
Where R is the resistance of a conductor, A is the cross sectional area and L is the length of the conductor.

As mentioned in the equation above, resistance linearly proportional to wire length. Hence displacement measurement can be performed by making object to control the length of the wire as carried out in potentiometer.

As shown in the figure-1 stimulus is coupled to the pot wire, whose movement will cause the resistance to change accordingly. Most of the electronic circuits will use resistance measurement as a function of voltage drop.

The voltage drop across the wiper of linear potentiometer V = E (d/D),
Where, D is full scale displacement, E is voltage across the pot , d is the displacement. Here the output signal is proportional to the excitation voltage applied across the sensor.

Fluid level sensor

fluid level sensor

Figure-2 depicts gravitational fluid level sensor using a float. As the liquid level changes either on upward direction or downward direction than float position changes. This results into variation in the wiper arm across the resistance. This results into measurement of level position.


Also refer IoT compliant sensors which includes ambient light sensor, optical sensor, gesture sensor, proximity sensor, touch sensor, fingerprint sensor etc.

Refer Sensors and transducers article which covers types, basic functions and provide links to sensors and transducers. It include temperature sensor, humidity sensor, strain sensor, proximity or occupancy sensor, force or load sensor, voltage and current sensor, pressure sensor, speed sensor, resistance sensor, power sensor, level sensor, event or state sensor etc.

RF and Wireless Terminologies