Episode #21: Linear Encoder Error and Accuracy

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The Motion Control Show

You may be at the point of looking at adding feedback to your mechanics in order to make them more precise or to capture some of the errors I discussed last episode in Episode 20.  But, you may also be wondering about some of the problems that can cause.  There are two things that I mentioned in the last episode: one was that it can make your electronics more sophisticated and therefore more expensive; the other one is that the feedback devices may not be as accurate as you think.  So, I am going to talk about linear encoders today just to give you an idea of some of the things to be aware of.  First, I have a poll question for you to answer: Do linear encoders solve more problems than they cause?  Or do they cause more problems than they solve?  Or are you still wondering  what a linear encoder is?  Check out the poll. I am kind of curious.  I am Corey foster of Valin Corporation.  Let us learn things.

Again, last episode I discussed different types of linear encoders.  There are optical, magnetic, and inductive.  They each have their pros and cons.  They also specify things differently.  The question here is:  Does a linear encoder solve drivetrain accuracy?  You would think “yes” and it may be.  Linear encoders take the need for high accuracy drivetrain elements, such as ground ball screws, away.   However, the linear encoder performance must be accounted for: linearity compared to accuracy.  Some encoders specify a linearity; other ones specify accuracy.  The point of measurement, and where you are measuring, can really also matter as well because then the angular errors come into play. 

Linearity is usually specified for steel or magnetic strip encoders and there is no accuracy of the scale, only the linearity or the variation from the straight-line.  The advantage of these types of encoders, as I mentioned last episode, is the cost, the ease of installation, and the robustness for different environments.  If we look at this linearity example, over half of a meter, 500 millimeters, the position error goes up.  Even though the accuracy is not specified, in this case it is 20 microns.  But, the linearity is specified +/- 3 microns.  This example can be easily corrected with some slope correction in the electronics. 

Then you have glass scale types that have a true accuracy and the scale specification is actually called accuracy.  The advantage of these types of encoders is the precision and the thermal stability.  They might be a little more expensive, but they are more precise.  If you look at this, there is no linear increase of the accuracy error over the half a meter, but we do have an accuracy specified and it's only +/-3 microns for this one. 

Then there are sub-divisional errors (SDE).  This is the signal error that occurs within one signal period of the encoder.  The period is based on the lines or the magnetic pole pitch of the encoder.  The most common signal periods are 20 microns for the optical encoders and 2 to 5 millimeters for magnetic.  The typical SDE is +/-1% of the signal period which is 0.2 microns, or 200 nanometers, for the optical and 20 microns for the magnetic.  This can be improved with finer signal periods or by correctional electronics in the encoder.  If we look at this example, if we have a 3 second move here at 40 microns per second, you can see the period of the error.  This scale here is in millimeters so this is 0.1 microns here.  That is 0.1 microns and this is for a standard 20 micron pitch encoder.  If we apply this same thing with some correctional electronics, we can cut this 0.1 just about in half for the same encoder. 

How does the SDE affect your application?  If you are scanning at slow speeds, it causes some velocity variation, or also known as velocity ripple.  It can also cause some position triggering errors.  Depending upon the scale of magnitude of the SDE, it can cause some audible noise in the application. 

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form