Linear Feedback Sensor Applications: The Three M’s

Applications for linear feedback sensors are numerous and varied.  Likewise, linear feedback sensors are available in numerous form factors and with a wide variety of performance characteristics.  Matching your application to the most appropriate sensor can be a daunting proposition.

When choosing the right linear feedback sensor, it is helpful to first define the job the sensor is being tasked to do.  One way to do this is to think in terms of the three M’s: (M)easuring, (M)onitoring, and (M)otion control.  Linear feedback sensor characteristics that are critical for one of these jobs may not matter as much for another job.  We’re going to take a look at each of these jobs and discuss some of the more important linear feedback selection criteria associated with each.

Measuring:  In measuring applications, the linear feedback sensor is asked to perform the job of an “electronic ruler”.  That is, the sensor is a measuring device used to gauge the size (length, width, thickness, etc.) of the part being produced or processed.  Examples of measuring applications include cut-off saws, or any other cut-length applications.  In such applications, it is absolutely critical for the feedback sensor to provide 1) high accuracy (low non-linearity), and 2) fine resolution. Other factors, such as a fast update rates and highly rugged enclosures are typically not as important in most measuring applications.

Monitoring:  Monitoring applications are those that require continuous linear feedback, but which do not require close-loop control.  That is, the feedback sensor is not part of a closed-loop control system that is controlling run-time operations like acceleration and deceleration.  Examples of such an application include shut-height feedback on a metal stamping press.  In this case, the feedback sensor provides continuous position feedback, but it does not play a role in controlling the press as it moves up and down.  Monitoring applications require the feedback sensor to deliver 1) good repeatability, and 2) rugged physical construction.  In monitoring applications, non-linearity is not critically important, as long as the sensor is repeatable.

Motion Control:  Motion control applications are perhaps the most demanding of the three.  Of course, high performance closed-loop motion control requires careful selection of the prime mover components (e.g., hydraulic cylinder, servo or proportional valve, pump, accumulator, etc.).  But it’s also critically important that the feedback sensor be carefully chosen to ensure that it can deliver the necessary performance.  Feedback sensors for a closed-loop control system need to offer 1) high accuracy (low non-linearity), 2) Fine resolution, 3) fine resolution, and 4) deterministic data.

By first defining the job that the sensor is being asked to perform, the subsequent selection process becomes much more straightforward, as the universe of possibilities is narrowed down considerably.

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