For measuring linear or angular displacement, potentiometers offer engineers a reliable sensor choice. Potentiometers are available in a variety of sizes and configurations for many different measurement applications. What is a potentiometer sensor, and how does it work?
What is a Potentiometer?
A potentiometer is a resistor with three terminals, two that are fixed and one that slides, forming an adjustable voltage divider. The first patent for a rheostat (variable resistor) was granted to Thomas Edison in 1872 (US patent 131,334).
When one of the fixed terminals of the potentiometer is used along with the sliding terminal, the resistance provided by the resistor is variable. When the position of the sliding terminal is connected to an outside object, the motion of that object changes the position of the sliding terminal, also called a wiper, and therefore the resistor value scales with position of the outside object.
The resistive element of a potentiometer is often made from graphite to provide a low-friction electrical contact. Resistive elements can also be fabricated from cermet or a carbon/plastic matrix.
Types of Potentiometers
Potentiometers can be used to measure both rotary displacement and linear displacement
- Rotary Displacement - Rotary potentiometers couple the input directly to the central shaft of the potentiometer, so one rotation of the input corresponds to one rotation of the wiper within the potentiometer. For multiple input rotations, the input can be geared to the central shaft, so that multiple input rotations equal one wiper rotation, or a helical resistive element can be used.
- Linear Displacement - A string potentiometer uses the same internal component as a rotary potentiometer, but a string is wrapped around the central shaft and spring loaded, so that extension and retraction of the string corresponds to a rotation of the central shaft.
While most potentiometers have a linear relationship between input displacement and resistance, the resistive element can be designed such that the relationship is logarithmic.
Applications for Potentiometers as Sensors
Engineers like to use potentiometers for a variety of measurement applications because they are very reliable and repeatable sensors. String potentiometers are available in a wide range of displacement values, allowing engineers to measure displacement of a few inches or several feet. Rotary potentiometers can measure angular displacements with excellent accuracy, and are relatively inexpensive.
In addition to their function as displacement sensors, potentiometers are used extensively as adjustment mechanisms for electronic systems. Potentiometers are often incorporated into other sensors to act as a trim mechanism to offset baseline measurements for calibration.
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