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Motor Polarity


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I'm using a PPMAC with a handful of servo motors connected to several Acc24E3 cards and I'm trying to flip the drive direction (reverse the polarity of DACA+ and DACA-) of some of them. Is there a card or channel parameter to do this?


I understand that I can drive the opposite direction by issuing a negative command, but this only works for open loop commands. I have also been able to flip the direction of output when using a user servo algorithm by simply flipping the sign of the output before returning the command value. But in this case I'd like to be able to change polarity of a closed loop delta tau command, e.g. "#1j=10".


Is there any way to do this without physically re-wiring?


I'm using IDE and firmware

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I'm a little confused as to whether you are trying to change the full direction sense or just the output sense. It can make a big difference.


Usually to change the direction sense you change the processing of the position feedback somehow, and the earlier in the processing chain the better. If you have incremental feedback you can change the decode direction sense with Gate3.Chan[j].EncCtrl (usually from 7 to 3 or 3 to 7). Alternately, you could change the sign of EncTable[n].ScaleFactor, regardless of the type of feedback.


Remember that the feedback direction sense must match the output direction sense, so that a positive open-loop command causes the position to increment in the positive direction, and a negative open-loop command causes the position to increment in the negative direction. Otherwise, you will get a dangerous runaway condition when you close the loop.


We do not have a good way of changing just the output polarity of a PPMAC motor that is not commutated by PPMAC. In theory it could be done by setting all of the servo gains negative, but I worry about the long-term maintainability of that solution.

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I too would like a method to reverse the direction of a motor in software. My application is somewhat unusual in that the machine can be installed by the operator in 2 different orientations and it is necessary to change the polarity depending upon the chosen orientation (and it's not possible to change the wiring each time!).


Currently in UMAC I do this by changing the decode control from 3 to 7 (or vice versa) and changing the proportional gain to a negative value. This has worked fine in the past for me. However, as I understand it in PPMAC, the proportional gain no longer scales all of the other gains, so simply changing this one term is no longer possible. It seems to me very undesirable to flip all of the gains (there are so many that it would be easy to forget something). Surely there should be some simple way to do this?

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