Direct load position sensing with secondary feedback encoders

Motion control system designers have found a way to eliminate or reduce common sources of position error, such as mechanical backlash, non-linearity, and hysteresis.  The method is called direct load position sensing and it employs linear encoders as a source of secondary position feedback.  Secondary feedback encoders supplement the indirect position measurement taken by a rotary shaft encoder by measuring the position of the moving load directly.

This method can save money by delivering the specified motion system performance at lower initial cost, and helps maintain system performance over time by getting around the problem of mechanical wear and tear degrading the accuracy of position measurements taken at the motor.

If you’d like to know more, there’s a White Paper available called “Motion Control Primer: Direct load position sensing with secondary feedback encoders”.


Linear Position Sensor Terminology

Hysteresis, resolution, repeatability, non-linearity, null-point, temperature coefficient, accuracy.  These are but a handful of the many terms associated with linear position sensors.  To the uninitiated, it can be rather daunting.  And, unfortunately, there is a lot of room for ambiguity and confusion.

For example, let’s take a look at the term “accuracy”, as in “how accurate is this this linear position sensor?”  It seems like a fairly straightforward
question, right?  But in reality, it’s not that simple.   Whenever I get asked that question, my response is “what do you mean by accuracy?”  To which, I usually get a response something like “what do you mean what do I mean by accuracy?”  The fact is that the term “accuracy” means different things to different people.   The person asking the question may want to know the absolute straight-line, absolute positional accuracy (non-linearity) of the sensor.  Or, they may be referring to how accurately the sensor can repeat the same indicated value at the same position over subsequent moves (repeatability).  Or, perhaps what they’re really interested in is the smallest amount of position change that the sensor can detect (resolution).  So, as you can see, it’s not a simple question after all.

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What is the hysteresis of your magnetic field sensor?


I received a call the other day from a customer who wanted to use a magnetic field sensor on a cylinder, and evidently was requiring very precise results. He asked, “what is the hysteresis of your sensors? I notice that it is listed in your catalog as a percentage and I need to know the exact value in millimeters.” My response was, “well it depends”,  upon which he was not overly pleased. I then continued to explain my answer which leads me to the contents of this posting.

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