Encoder versus resolver

  • Thread starter Progettazione elettrica
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Thread Starter

Progettazione elettrica

I have a motion control application where the motion profile is like this: constant speed during 100 ms, stop in 15 ms and acceleration to constant speed in 15 ms: this cycle is continuous.
I need a minimum position error. Unlike the encoder, the position feedback from resolver has a time lag when it encounters great acceleration (tracking process of Resolver Digital Controller). Please give me some information useful to estimate this lag comparing to solution with encoder feedback.

Ivan Messori
An encoder will update the feedback register ever time an incremental pulse of distance comes along. You can't ask for anything smoother than that. A resolver working in the phase shift mode of digitizing will update the feedback register once every carrier cycle. If the carrier frequency is high enough, there will be no problem. For instance, with a 5 KHz carrier,
the feedback register will be upgraded every 200 microseconds. During the 15 ms acceleration, it will be upgraded 75 times. The servo position error will be more affected by the gain in the system. Also, this error will change during deceleration from what it was at the maximum velocity to zero at turnaround. There will also be an abnormality in the feedback register
that looks like a sawtooth signal superimposed on the feedback. The peak-to-peak value of this abnormality will be the same as the distance
that the axis moves in 200 microseconds. One key is to get the carrier frequency as high as possible. To help you determine actual errors, much more info will be needed. What is the gain of your position loop? Are you using a velocity loop, too? Torque loop? How fast are you going during the 100 ms? Are you using feedforward or PID? If you are experiencing unacceptable errors, my gut feeling is that it is due to the position servo loop rather than the lag of the resolver carrier frequency.
Tom Bullock
Bull's Eye Marketing, Inc.
Industrial Controls Consulting Division
[email protected]

Setbacken, Bob


I think the best way to handle the lag is to treat it as a type two servo. Most RDC's give you a transfer function that is more or less representative of the loop dynamics for the tracking RDC converters. You should then do a
Bode analysis to see how you can compensate it.

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David Lawton Mars

From the info you give I think I'd be more concerned with the mechanics than the electronics.

I'm no resolver expert, but motor positions are normally updated once every servo cycle (typ. 1ms). If you're concerned about the encoder emulated signal update time from the resolver I think the best bet would be to speak to the drive manufacturer. Sorry I can't be of more help

You have to give more information to get this parameter. You have to know what the RDC (Resolver to Digital Converter) is, and some of the filter parameters used in the circuit. You can get information on some common RDCs from Analog Devices and DDC. They make some monolithic chips for this purpose (e. g. 2S80 or RDC19220).


Willy Smith

Daniel Boudreault

Hello Ivan,

Your Servo Processor should take care of that automatically. What I am trying to say is that during the servo tuning, you can specify feed-forward, and other parameters that will minimize position error during such moves. The servo processors that I have used (Warner TDC series) allows all kinds of acceleration types. If yours is capable of an S-curve style, then I think that would give you the best position tracking. But of course, I don't know which servo processor that you are using...

If that is not possible, then maybe you can do it in programming.

For example:
Do your own S-curve acceleration profiling in the program...

You know when you are accelerating positively, your position is most likely to be lagging, so you can start your acceleration at a higher rate than necessary, then halfway to your constant speed point, your can reduce the acceleration (so that you won't over shoot your target point, by allowing the servo to be able to respond to the
delayed position signal.) And vice-versa for the decel.

Does your servo processor allow you to plot out the position error as you do your moves?

Dan Boudreault.

Tanweer Ahmed

Hello Ivan,
For the purpose you can also use SIMOVERT AC/DC drive from Siemens with a technology board T100/T300 for the purpose. These technology boards takes care most of the functions you described.


Tanweer Ahmed
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