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  1. #1
    Join Date
    Oct 2010

    Dual loop settings

    Does anyone here have a dual (rotary & linear encoders on same axis) loop setup with a Kflop that is working well?

    I have tuned my single loops (rotary motor encoders) and they work quite well, but nothing seems to be stable when adding in a outer loop with the linear encoders.

    I am using the ch1->Dest += ch0->Output in a C program to add the loops together, but it is very hard to get anything to move stable.

    I have set the outer loop to be encoder input and no output in the config settings.

    Am I supposed to match the acceleration / velocity settings between the axis channels?
    I think the outer loop should be all P and I and the inner loop should be all D for stability, but how would this work?
    If you make an axis with only D values then it will not work, and having P values in both loops makes it unstable.

    What kind of limits should the ch0 output and integrator have?

    If I understand this theory it should be:
    A motion command is send to Ch0, this output is conditioned with the PI values.
    The output is then sent to the Ch1 destination.
    Ch1 combines this with is encoder and adds D to stabilize the axis.

    Any help would be appreciated.


  2. #2
    Junior Member
    Join Date
    May 2006
    Hi Dave,

    There are numerous ways to do the dual loop. But here is my suggestion.

    First tune the inner loop (motor shaft feedback) for maximum performance. This may involve PID+Filters etc as required. You can set the Velocity, Acceleration, and Jerk to good values for testing and for any single loop mode but these wont actually be used in dual loop mode as the outer loop will be controlling the trajectory.

    Next enable the dual loop. Note that loops connected together with:

    ch1->Dest += ch0->Output;

    means that the target position of the inner loop moves at a velocity proportional to the output of the outer loop. So the Outer loop can be tuned in the same manner as if it was connected to say a +/-10V Analog Velocity Commanded Amplifier. Only P Gain should be stable. A servo with simple P only Gain commanding velocity usually works well. As the error decreases the velocity decreases which should result in a nice exponential approach to the target. If the system is unstable decrease the P gain until it is stable.

    As stated previously the Outer Loop's trajectory will determine the motion, so set the Velocity, Acceleration, an Jerk here for your system.


  3. #3
    Join Date
    Sep 2011
    WHich linear encoders are compatible? Renishaw? Typical glass scale?


  4. #4
    Junior Member
    Join Date
    May 2006
    Hi SwampDonkey,

    As long as the encoders output digital quadrature and don't exceed 1 million quadrature counts/sec at max speed they should work. See also here


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