Linear position sensors that provide continuous, typically analog, feedback are used extensively in a variety of applications in many different industries and markets. Linear position sensors employ various technologies, but at the most basic level the technologies can be classified as being either non-contact or contact based.
For the purpose of this article, when we talk about contact based technology, the example we’re using is resistive linear potentiometers. And for non-contact technology, we’re talking about magnetostrictive sensors.
In industrial linear position sensing applications, both ultimately do the same job; provide variable analog signals that represent the linear position of a machine or a process. The difference is how the signal is derived.
Resistive linear potentiometers employ a resistive element upon which a spring-loaded contact rides:
The output of the sensor represents the position of the slider along the resistive element and typically ranges from 0-10Vdc or -10 to +10Vdc. Out of the box, performance and accuracy is pretty good. But after repeated cycles, wear can start to place that affects the connection between the resistive element and the contact. The end result is signal anomalies and worsening performance over time, as can be seen in the image below.
Other external factors, such as dirt and/or moisture only serve to accelerate this declining performance.
Non-contact technology, such as is incorporated into magnetostrictive linear position sensors, isn’t vulnerable to mechanical wear and subsequent performance degradation.
Unlike, resistive sensors, magnetostrictive sensors operate on the principle of magnetism. Interacting magnetic fields define the output value, which changes as a moving magnet travels along a sensing element, called a waveguide. There is no mechanical contact, so there is no mechanical wear. The result is greatly enhanced life expectancy and consistently excellent performance
Cost Considerations
Generally, resistive linear position sensor cost a bit less than magnetostrictive sensors. However, that doesn’t tell the whole story. True cost of ownership has to be considered. For a more complete discussion about cost of ownership, take a few minutes to review the Sensortech article The True Cost of Low Cost.
