Linear Sensor Basics: Absolute vs. Incremental

When it comes time to choose a linear position sensor, there’s a dizzying array of options and terminology to wade through.  In this series of articles, we’re going “back to basics” to try to shed some light on the sometimes confusing world of linear sensing options, technologies, and terminology.

First up, we’re going to take a look the two basic linear sensor measurement types: absolute measurement and incremental measurement.

Linear position sensors that employ incremental measuring technology provide an output signal based on relative movement from a known, or reference, position.  Often, incremental position sensors provide a series of pulses, usually on both an A channel and a B channel.  The number of pulses is relative to the amount of linear position movement from the starting, or home, point.  The phase relationship of the A channel and B channel provide an indication of direction of movement.  The potential disadvantage of an incremental system is that the actual position value is lost if system power is lost, which requires that the system be re-homed.  On the other hand, incremental systems are well-suited for high-speed applications and position measurement over very long distances.  And, depending on the application, homing the system may not be a problem.   For an example of an incremental measuring system that uses a magnetically encoded tape and a non-contact read head, click here.

Absolute linear position sensors, such as magnetostrictive displacement transducers provide a unique output at a given position.  For example, a linear position sensor with analog voltage (0-10 Vdc) output produces a constant, repeatable output at any position within its total travel.  Continuing the example, assume a linear transducer with 100” of total travel, over which it produces a 0 to 10 Vdc output.  At 50” (half its total travel), the sensor would indicate 5Vdc on the output.  If power is lost and re-applied, and the system hasn’t moved, the output would still be 5 Vdc.  Perhaps more importantly, if the system was moved during the power-down condition, the sensor would still indicate actual position when power was restored, so re-homing is not required.

Both incremental and absolute position sensors have a place in industrial automation.  The individual application requirements will dictate the proper choice.

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