Low speed position control, ok under duty control poor rpm control

General topics and discussions about the VESC and its development.
AusRobotics
Posts: 2
Joined: 14 Sep 2016, 06:13
Location: Newcastle

Low speed position control, ok under duty control poor rpm control

Postby AusRobotics » 14 Sep 2016, 07:17

We are trying to use lunyee skateboard/balance scooter Hub motors, in an application, not too different to a robotic vacuum cleaner. We believe the direct drive brushless hub motors should be upto the task, as we have ridden a balance scooter with these motors and they have very smooth and high torque low speed operation. So we thought they would be ideal for a low speed two wheel robot platform.
We are using the vesc controller because we would like the silent FOC commutation it offers.

So We have two inertion VESC, hardware version 4.12. and running firmware version 2.18.
and are testing with 2 x 3inch and 2 x 4inch hubmotors.

We can't get the 3inch motors to detect the hall sensors reliably.

We have had much more luck with the 4inch motors.
Hall sensor detection works in both BLDC and FOC mode.
but we have very jerky speed oscilations when telling the motor to turn at low rpm, say under 500rpm.
But when we use the left keyboard arrow to command the vesc (10% pwm duty cycle) the motor turns very smoothly at a similar rpm.

Can't work out how to set the pole number, thought this would be required, so we could then say turn at 1rpm and it would turn at 1rpm, without this setting, how does it know how many poles make up a complete revolution? This is just a minor thing that would be great as we get closer to controlled speed or positioning.

basically we can't get anything like smooth low speed performance from anything but keyboard control, which I believe to be PWM duty control. We have been trying to tune the PID velocity loop but not having any luck and always get massive over shoot before we even get close to desired speed.

All the graphs in BLDC tool are VERY noisy and when looking at our hall sensors the signals are square and 0 to 5v but have large swing narrow spikes of a similar magnitude.

The big question is, they turn smoothly under duty cycle command, is there a chance that duty cycle doesn't use the hall sensors or some other difference? or the fact that duty cycle works means that hall sensor and all other settings are correct and that the issue is just in the PID values?

Thanks guys, any help is appreciated.

pf26
Posts: 278
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Low speed position control, ok under duty control poor rpm control

Postby pf26 » 14 Sep 2016, 10:27

I think there are as many pole pairs as magnets on the motor. I.e poles = magnets count / 2
Did you try with both Vescs, to make sure the issue is not hardware related ?
If the running is smooth in current controlled and PID mode, than the issue is related to speed PID parameters. Otherwise, it might be due to noisy hall sensors. Did you measure the hall input signals after or before the input hardware RC filters ?

rew
Posts: 904
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Low speed position control, ok under duty control poor rpm control

Postby rew » 14 Sep 2016, 10:40

The VESC counts RPM as "electrical RPM", sometimes shown as ERPM. My motor has 7 pole pairs, so I specify 140 (e)RPM when the motor physically makes 20RPM....

To debug your problem I would set duty even lower, and see what happens. Especially measure the hall sensors. Those spikes are not good.... Keep the wires away from the motor wires.... See if that helps....

AusRobotics
Posts: 2
Joined: 14 Sep 2016, 06:13
Location: Newcastle

Re: Low speed position control, ok under duty control poor rpm control

Postby AusRobotics » 27 Sep 2016, 07:16

I have been playing around with the PID values and have it driving smooth for now (may need fine tuning later).
New question. I am running two motors and want to be able to drive them both through USB has anyone been able to achieve this.

jyl087
Posts: 1
Joined: 23 Sep 2017, 00:21

Re: Low speed position control, ok under duty control poor rpm control

Postby jyl087 » 23 Sep 2017, 00:24

How did you get your VESC to operate your motors at low RPMs smoothly? In your last post you mentioned that you got it to work by "playing with the PID values". Can you say more?


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