The Motion Control Show, Episode 20: Do I need feedback?

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The Motion Control Show

I have a question for you:  Do you embrace linear encoders?  Or do you avoid them whenever possible?  Or are you wondering what a linear encoder even is?  After all this sizing and selecting of mechanics, and even designing your own mechanics, you might be considering putting a feedback device on there and whether you should or not.  I am Corey foster of Valin Corporation.  Let us see what we can learn.  If you like what you are learning, follow my #MotionControlShow and check out this website.

You might want to start with “what is feedback?”  Feedback is when you have a command going to your motor or mechanics and you put a device on there to give you information back to your electronics.  That is a feedback device.  It is feeding information back.  If you take that off this, it is called “open loop” where you are giving a command and then you are just expecting your motor to do what you have told it to do.  It is a lot like throwing a bowling ball down an alley.  You throw the ball.  You put it into motion and then you get a strike, maybe a gutter ball, or somewhere between.  If you had a feedback device, you could nudge it over on the way down the lane.  This is much more like a car.  When you are driving a car, you are the feedback device because you are watching the road and you are making constant adjustments.  Your cruise control is a feedback device that is adjusting the cruising speed.  The bowling ball is open loop.  Your driving a car, that is closed loop.  That is what the feedback device providing information back is.  If we look at this, restructure this a little bit, the command comes through and it goes through some electronics.  In this case, “A” is for amplifier and that amplifier powers the motor.  We put a feedback device onto the back of the motor and that provides the information back to a summing junction.  The command comes in and then it gets subtracted.  The information coming back gets subtracted from it; that is why there is a plus and a minus there.  The error between the two is what goes on to the amplifier.  If we had a positive there where the information is coming back, then they would just add together and that would be a feedback loop.  Just picture a screeching microphone where the sound gets out of control.  That is why we subtract that feedback.  That is really the basics of feedback.  There is a lot more.  There are a lot of different kinds, but that is just the basics for the moment that helps us to ask the question “why would you want feedback.”

Let us say you put a motor on a ball screw and you have bearings here carrying the load.  You can put a linear scale along the side and this read head runs along the scale.  This read head would be attached to this carriage here and that would be providing feedback.  Maybe you are worried about whether or not your load is actually getting to the location you are telling it to.  What happens if the coupler slips or the coupler breaks?  Or maybe you are concerned about whether your ball screw actuator is actually precise enough and you want to get a little more precision out of it.  Maybe you want to compensate for the mechanical backlash.  If you look at this chart here, distance is here in the Y, time is on the X.  If you are expecting this black line here, you are expecting to get to this distance.  But, because of the mechanical backlash, it lags a little bit and so it never quite gets there.  If you had that feedback, you could go a little bit further and you could pick up this slop.  You could tell if your coupler was broken.  You could take a rolled ball screw and make it a little bit more precise instead of needing a ground ball screw.  You would have to take a look at that design and whether the cost is worth it for you.  You could take a ground ball screw make it more precise. 

There are different types of linear feedback and each one of them has their pros and cons.  There is optical, there is magnetic, there is inductive.  Some are a little bit more expensive because they are a little more precise.  Some are a little bit less expensive and not as precise.  Some are more robust and better for machine cutting tool applications for example.  Or for vibration or high temperature. 

But, before you assume too much about linear encoders and think they are going to solve all of your problems, two things you might want to consider: one is it makes your electronics more complicated.  Leave that for the electronics group.  That is right.  Let them deal with it.  But, it does make your electronics more complicated, more sophisticated, and therefore more expensive.  So, think about that.  The other thing to think about is that linear encoders, as do rotary encoders, do have errors in them.  Just because you have a 1-micron linear encoder doesn't necessarily mean you could get one micron out of it.  It is going to be close, but we are going to talk about that in the next episode.  I hope that gives you some idea of what you are looking at for feedback and whether or not you want it.

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