Position and Displacement: Potentiometric Sensor

The measurement of position and displacement of physical objects is essential for many applications: process feedback control, performance evaluation, transportation traffic control, robotics, and security Systems—just to name the few. By position, we mean the determination of the object’s coordinates (linear or angular) with respect to a selected reference. Displacement means moving from one position to another for a specific distance or angle. In other words, a displacement is measured when an object is referenced to its own prior position rather than to another reference.

Position and displacement sensors are static devices whose speed response usually is not critical for the performance. In this article, we do not cover any sensors whose response is a function of time, which, by definition, are dynamic sensors. They are covered in other article in this site.

Potentiometric Sensors

A position or displacement transducer may be built with a linear or rotary potentiometer or a pot for short. The resistance of  potentiometer linearly relates to the wire length. Thus, by making an object to control the length of the wire, as it is done in a pot, a displacement measurement can be performed. Because a resistance

measurement requires passage of an electric current through the pot wire, the potentiometric transducer is of an active type; that is, it requires an excitation signal, (e.g., dc current).

A stimulus (displacement) is coupled to the pot wiper, whose movement causes the resistance change (Fig. 1A). In most practical circuits, the resistance measurement is replaced by a measurement of voltage drop. The voltage across the wiper of a linear pot is proportional to the displacement d:

                    V = E (d/D)               (1)

where D is the full-scale displacement and E is the voltage across the pot (excitation signal). This assumes that there is no loading effect from the interface circuit. If there is an appreciable load, the linear relationship between the wiper position and the output voltage will not hold. In addition, the output signal is proportional to the excitation voltage applied across the sensor. This voltage, if not maintained constant, may be asource of error.

It should be noted that a potentiometric sensor is a ratiometric device; hence the resistance of the pot is not a part of the equation. This means that its stability (e.g., over a temperature range) virtually has no effect on accuracy. For the low-power applications, high-impedance pots are desirable; however, the loading effect must be always considered. Thus, a good voltage follower is required. The wiper of the pot is usually electrically isolated from the sensing shaft.  

Fig.2A shows one problem associated with a wire-wound potentiometer. The wiper may, while moving across the winding, make contact with either one or two wires, thus resulting in uneven voltage steps (Fig. 2B) or a variable resolution. Therefore, when the coil potentiometer with N turns is used, only the average resolution n should be considered:

                    n = 100 / N%                  (2)

The force which is required to move the wiper comes from the measured object, and the resulting energy is dissipated in the form of  heat.Wire-wound potentiometers are fabricated with thin wires having a diameter on the order of 0.01 mm. A good coil potentiometer can provide an average resolution of about 0.1% of FS (full scale), whereas the high-quality resistive film potentiometers may yield an infinitesimal resolution which is limited only by the uniformity of the resistive material and noise floor of the interface circuit.

The continuous-resolution pots are fabricated with conductive plastic, carbon film, metal film, or a ceramic–metal mix which is known as cermet. The wiper of the precision potentiometers are made from precious metal alloys. Displacements sensed by the angular potentiometers range from approximately 10° to over 3000° for the multiturn pots (with gear mechanisms).

Advantage and disadvantage of potentiometer:

Advantage:
      1. Easy to use 

      2. Low cost 

      3. Proven technology

      4. High-amplitude output signal

Disadvantage:

     1. Noticeable mechanical load (friction)

     2. Need for a physical coupling with the object

     3. Low speed

     4. Friction and excitation voltage cause heating of the potentiometer

     5. Low environmental stability.

Type and Material:
     •  Type of displacement:
             •  Liniear 
             •  Rotary 
             •  String potentiometer
     •  Material:
             •  Wirewound (high power dissipation) 
             •  Non-wirewound
 
Application:
      •  Industri              •  Automotive       •  Medical 
      •  Aeronautic       •  Space               •  Military
Fig. 3 shows example of the potentiometers  :

Reference Books About Sensor