Challenges going faster than certain rpm

[Antun_Skuric]

Hey @daniel,

Yes, your MCU is fast and you can have loop times even much higher than 2.5kHz if you use a different sensor. I2C based AS5600 is really not meant for high speeds or fast loop times. But even an ideal sensor you will still not be able to come to 500rad/s.

@daniel

In open loop velocity mode I can get it to around 200 rad/s, by manually stepping up both the target velocity and current limit. I haven’t tried to go above that as the sweet spot for the current limit becomes smaller and smaller, so it just becomes tedious.

The open-loop control is really really fast. cca 10x-100x faster then the FOC and it makes a lot of difference.

@daniel

With this setup (and basically any other I’ve tried with the same motor), it seems that there is an ‘invisible wall’ to get the motor to spin at more than ~100 rad/s in closed loop velocity control or about 150 rad/s in closed loop torque control mode. Raising values above that just makes the motor more noisy, draw a lot more current and make rad/s more unstable or even go down.

Velocity control algorithm takes longer time to execute and that is probably the reason why you see such a high difference. Try using the motion downsampling to let more time to the FOC algorithm and less for your motion control as your task for the moment is very simple. With some motion downsampling you should be able to reach at least this 150rad/s.

@daniel

I don’t think the limitation is the speed of the controller; my main loop runs at a fixed 2.5 kHz (both move and foc)

You are using a timer?

@daniel

I don’t think the limitation is the speed of the controller; my main loop runs at a fixed 2.5 kHz (both move and foc) I’ve read at other posts here that foc should work well with at least 4 loop iterations per pole pair (or was that per pole, so 8 per pair?), meaning that purely from that aspect I think I should be able to reach ~560 rad/s (2500hz / 7pp /4 its/pp * 2 * pi), so that’s quite a way to go from 100 or 150.

There is part of the equation that influences a lot and that is power supply voltage. As when going faster your back-emf rises motor’s torque fill diminish and it will not be able to reach higher speeds. So one simple way you can increase your speed range would be to use a higher power-supply voltage.

@daniel

So, 1: is there anything obvious that I should be doing differently?
And 2: could this possibly be an effect of the delay in the read-out of sensor values? I haven’t calculated or measured the actual delay, but as this is generally the limiting factor in loop iteration speed, I think it’s save to assume that by far most of the iteration time is spend in waiting for I2C communication back and forth, meaning that the sensor value was actually sensed around half of the iteration time earlier, let’s say 200uS in my case, by the time by the voltages are adjusted based on that reading. I would imagine that at low rpm this delay is no problem as it is only as small fraction of the time between two passing poles, but this fraction of course grows as rpm goes up and therefore time between passing poles reduces. Is there any compensation for this delay in the library?

We do not do the compensation of the delay in the library in this way. We do have the adaptive time sampling which will help in motion control applications but not in the field vector setting. There is something that you can do if you are motivated though, we do use the motor.zero_electric_angle which basically determines the calibrated offset in between sensor and the motor 0 angles. For higher velocities (due to this delay you are interested in) we will have a certain offset in between where the motor is and where we this it is which will depend on velocity.
So you could add your estimation of the offset to the motor.zero_electric_angle:

You could do something like this in your loop

float normal_zero_angle = motor.zero_electric_angle;
float alpha = 0.1; // something small - this is up to you to find depending on your motor and sensor
void loop (){
   motor.zero_electric_angle=normal_zero_angle + alpha*motor.shaft_velocty;
   ...
   motor.loopFOC();
   motor.move();
}