1. How do linear potentiometers work?
Linear potentiometers are variable resistors used as voltage dividers to measure linear position e.g. for applications in industrial machinery. The potentiometer consists of a body that resembles either a cylinder or an engineering profile and a piston-like rod which is free to move axially. The potentiometer body contains a long resistive strip (typically a metal oxide thick film resistive device). The rod, which is guided by the potentiometer body to prevent lateral movement, has an electrically conducting slider (sometimes called a wiper) attached to its end. The slider always maintains contact with the resistive strip.
The potentiometer includes three electrical terminals; two at the fixed end terminals A and B at either end of the resistive strip and the third is the slider, attached to the rod and always in contact with the resistive strip. A fixed DC supply voltage is applied to the resistive strip such that the voltage across the two end terminals is constant and equal to the supply voltage. The slider position is determined by measuring the voltage VBS between end terminal B and the slider. If for example VBS is measured to be 12% of the fixed supply voltage and the stroke of the sensor is 100 cm, then the rods displacement must be 12 cm. Likewise, if VBS is measured to be 47% of the fixed supply voltage, then the rods displacement must be 47 cm. Note that the voltage difference across the end terminals is equal to the supply voltage.
2. General characteristics of linear potentiometer position sensors
Potentiometers are low cost and easy to use sensors that do not require any complicated electronics for their operation. They provide reasonably accurate measurement of displacements up to 100 cm at velocities as high as 10 m/s. However, sliding contact between the slider and resistive strip results in a shorter lifetime than other rod type position and sensors. Furthermore, whereas most analog sensors have a hypothetically infinite sensing resolution, the friction between the slider and resistive strip limits the potentiometers spatial resolution to about 10 microns. A displacement of below 10 microns will not result in relative movement between the slider and the resistive strip. The potentiometers output is relatively insensitive to changes in ambient temperature as long as the entire resistive strip is of uniform temperature.
3. Input and output signals
The potentiometer requires a fixed 5-48 Vdc supply voltage. The unconditioned output signal of a potentiometer is a DC voltage proportional to the linear displacement of the rod i.e. a V/cm output signal. Many potentiometers include built-in electronics to convert the output signal to a standard 0-10 V or 4-20 mA output signal. In applications where high accuracy is not required, potentiometers can be used without any signal conditioning because a fixed 10 Vdc supply voltage will yield a 0-10 V analog output signal. The output signal it not sensitive to changes in temperature because the potentiometer automatically self-compensates in a similar manner to a half-bride Wheatstone bridge circuit.
4. Applications of linear potentiometer position sensors
Due to their low cost and ease of use, potentiometers are the most widely used rod type position sensor in industry. They can be used in most applications where there is no requirement for extremely high accuracy, a high number of cycles before failure or a high linear velocity. A typical example of their use is to provide feedback of the position of a hydraulic of pneumatic piston to the control system in industrial machinery.
5. Typical specification
|Measurement range||0-100 cm|
|Repeatability||0.1-0.5 % F.S.|
|Linearity||0.1-0.5 % F.S.|
|Lifetime (cycles to failure)||100,000 to 10,000,000 cycles|
|Ambient temperature||-30 to 100°C typical, as high 200 °C available|
|Full scale resistance||1-10 kΩ|
|Supply voltage:||5-48 Vdc|
|Output signal||0-10 V, 4-20 mA, V/mm|
|Vibration resistance||10 g|
|Shock resistance||50 g|
|Ingress protection||IP40 typical, IP67 possible|
|Passive / active:||Active|
|Contact / non-contact:||Sliding contact|
6. Purchasing tips
- Rod end joint: Most potentiometers include a rod end ball joint (fish-eye joint) at the end of the rod to absorb miss-alignment between the rod and the target object. Potentiometers may also be purchased with an additional ball joint on the cylinder body, enabling the entire potentiometer to pivot.
- Spring-loaded: Many potentiometers have a spring-loaded rod that automatically returns to its initial position once the actuating force is removed. The spring is useful for reducing backlash or for applications where the actuating force acts in only one direction. Furthermore, a spring-loaded rod will maintain contact with the target, without the need for mechanical coupling.
7. Advantages of linear potentiometers
Linear potentiometer position sensors:
- Are of low cost and simple design in comparison to other rod type position sensors. Furthermore, they do not require any complicated signal conditioning (it is possible to use them without signal conditioning).
- Achieve a reasonably high measurement accuracy, close to that of higher cost rod type position sensors such as LVDT and strain gauge based sensors.
- Are available with full scale measurement ranges from 1 to 100 cm, a similar measurement range to more expensive rod type position sensors.
8. Disadvantages of linear potentiometers
Linear potentiometer position sensors:
- Have a limited lifetime (number of cycles to failure) due to the mechanical friction between the slider and the resistive strip. Although some manufactures report lifetimes as high as 10 million cycles, others report lifetimes as low as 100,000 cycles.
- Are limited to linear velocities of about 10 m/s due to the mechanical friction between the slider and the resistive strip.
- Require a force of several Newtons to overcome the mechanical friction between the wiper and the resistive strip, even if the rod is not spring loaded. This may be undesirable for some applications and also results in a minimum resolution of about 10 microns.
- Are generally less immune to shock and vibration and have lower ingress protection ratings than other rod type position sensors. The most common ingress protection rating for potentiometers is IP40, though there are models for purchase with IP67 protection.
9. Application tips
- Temperature sensitivity: Potentiometer readings are not significantly affected by temperature as long as the temperature of the resistive strip is uniform. Do not use a potentiometer in applications where a spatial temperature gradient is likely to form across the resistive strip (e.g. if heat is conducted through the rod) as this will result in a large measurement error.
- Resistance: Potentiometers are available with a variety of resistance ratings (typical 1,000 – 10,000 ohms), the magnitude of which will affect the current consumption. Potentiometers operate at low current, no higher than about 10 mA.
- Sensing accuracy: To maintain a high sensing accuracy, aim to design your system in a manner that VBS never approaches 0 V as it will result in a very poor signal to noise ratio. One option is to not use the full measurement range of the sensor.
- Non-axial forces: Ensure that only axial forces are applied to the potentiometer rod. Non-axial forces that cause bending of the rod, including those that arise from misalignment, can damage the potentiometer and reduce measurement accuracy.