Slow Speed Motor Control

[rsatterlee]

Posted on October 27, 2015 at 21:37

I am having troubles controlling the speed of my motor below ~100 RPM (~10Hz on the halls). I am having similar issues with both Hall Effect and sensor-less modes. I could understand this being an issue with the sensor-less mode due to the current signal getting hard to interpret at these levels. But there doesn't seem to be much I can tweak other than the kp and ki to get this working better with the Hall Effects, and I get minimal response in this range playing with the Rev Up config in sensorless mode. I continually get Speed Feedback errors, which often can be attributed to gain configuration it seems when the PIDs get out of control, but other times it's not terribly obvious what is wrong...

Ideally I need to be able to creep the motor along rather slowly (~20-40 RPM) for this application, so overshoot on startup isn't terribly acceptable...The motor seems to run just fine above 100 RPM to it's max rated speed otherwise.Perhaps there is something I am missing or should be looking at closer?Are there other operational limits for this firmware that may be relevant here as well??Thank you for your time.

[Gigi]

Posted on October 29, 2015 at 12:22

Ciao Raddix

I think that with the implemented FW you can't achieve your target. I explain better. The control loop is done considering as measurement unit the tenth of mecchanical herz (0.1Hz) this mean that in digit the uint corresponds to 6 rpm. So in the range of 20-40 RPM the speed controller is working with target and measurement speeds value that have very very small for a good regulation. A change of measurement unit is mandatory to achieve a good control but this modification has big impact on the code. Ciao Gigi

[rsatterlee]

Posted on October 29, 2015 at 14:51

Thank you for getting back to me so soon.

I was somewhat afraid that would be an issue. I did notice however that I was able to drive the motor in this low speed area using the rev up period if I manually set the rev up segments to be rather long with a larger torque request (you can tell it's fighting the magnetics and notching a bit at this level). This likely isn't terribly efficient and obviously there is not much for feedback options here, but would this possibly be a way to make things work in this range if we had access to the full source or by manipulating our calls to the API?My concern here is handling the torque command in this area with the given software and the eventual (hopefully smooth) transition into a feedback control mode.If this isn't terribly feasible we may need to re-evaluate how we will approach this application...Thank you again for your feedback.

[Gigi]

Posted on October 29, 2015 at 15:38

Maybe I missunderstand your proposal,

you are thinking to run the motor in open loop at fixed speed giving a lot of current to spin it away? If your final application doesn't take care of the global efficiency, ok this can be an option. Othwerwise the other option shall be to modify the code changing the speed unit to, for instance, 0.01 mechanical Hz, or less, but I agree that this is not an easy task. Ciao Gigi

[rsatterlee]

Posted on October 29, 2015 at 16:15

Of course we would rather not burn a ton of current in this area if we don't have to, but I am starting to wonder if we have too many other options here. The nature of the application could require rather high torque at pretty low speeds.

Worse case if we can't make it work in the SDK, I suppose we could have a bootstrap or low speed mode where we would manually control the torque with our own PID. I would still be concerned about the hand off or transistion into a feedback mode. Would something like this be feasible??Speaking of source for the SDK, I have tried 3 different ST offices near me and have not been able to get through to a real person to discuss getting access to it. Could you possibly give me a contact in the Midwest, US or elsewhere?Thank you for your help and insight.

[Gigi]

Posted on November 02, 2015 at 11:13

Ciao

I can't help you so much. Try to contact SCHAUMBURG or SANTA CLARA. Ciao Gigi