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Experience with Inertia Mismatch of KBM and AKD Combination

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A great benefit of using a direct drive such as TBM or KBM is the stiffness of the mounting to the mechanic.

That results in a better inertia mismatch behavior of the servo system.

With a standard AKM gear box combination I usually set the limit of the inertia ratio (load/motor) to 10.

Is there any information about running applications or solutions with inertia ratio >100, >1000 with Direct Drive and AKD ? 


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Hey Frank!

  1. With AKD, I have no fear of 100:1 inertia mismatch with AKM motors. Even with a belt drive.
    1. That is assuming I use the bode tool to tune the machine. (Old school tuning with the scope can never get there)
    2. I also need to know if the importance is target accuracy or velocity ripple.
    3. Low RPM with low velocity ripple is can be a problem with high inertia mismatch.
  2. We have had a lot of Universities and/or Aerospace companies wanting to use the AKD for satellite tracking or telescope control system. Something like 0.001 RPM. NOPE, doesn’t work.

  3. I have done DDR motors in the 600:1 inertia mismatch.
  4. Another issue is perception. Our DDR literature states that “inertia mismatch is not a problem”. So Customers buy a KBM and mount it with a long drive shaft and lots of load compliance/resonance. And then we have to de-tune the system to a very low bandwidth to keep it stable and the system doesn't’t meet the application requirements for performance.
  5. The statement is based on a very fidget coupling between the motor and the load. It also assumes any mechanical resonance is at least 2~3 times the max system bandwidth required.
  6. Example: you need this system’s settling time to be 20mS. That’s 50Hz bandwidth. So, the lowest resonant frequency will be 100HZ~200Hz but that is if you don’t need much stiffness. If you want to crank up the VL.KI and PL.KP, the lowest resonant frequency will need to be 10 times or 500Hz.
  7. Last one: AKM2G is coming out with a much lower inertia to torque ratio. You will not be able to use its “sales advantage” if you don’t expand your sizing/selection rules to a higher reflected inertia ratio.
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