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Stall torque in sensorless mode

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cromoly
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Stall torque in sensorless mode

In my application, I need the motor to apply a small stall torque in some cases. The stall torque is needed to overcome some friction in the system. RPM of the motor at that working point is obviously zero or very close to zero for some limited time.

Currently I can't get that stall torque from my VESC 6. If I set the current to some small amount (using COMM_SET_CURRENT) and hold the rotor still, the controller tries to rotate the rotor few times back and forth and then stops. The motor works perfectly fine otherwise, when no stall torque needed, just regular motor operation-with the same command (COMM_SET_CURRENT).

I wonder if sensorless mode is appropriate for such mode of operation. Do I need to use sensors to make it work? Any experience, ideas?

Thanks!

spork
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I'm definitely not the expert, but I've been poking around with this recently.  I built an off-road board with two brushless outrunners and found that starting in sensorless mode was a very iffy proposition.  Without sensors, the controller simply doesn't know the orientation of the motor until it's moving.  So to get it moving it tries some different things.  Sometimes that give you a reasonable start, but a lot of times it results in the motors simply stuttering.  If you give the board the slightest push (about 1 mph) it's then able to read the back emf and you're off to the races.

So I just added internal hall sensors on my motors and I'm a whole lot happier.  The other thing you can do is add the magnetic encoder to the motor shaft.  This gives much higher resolution and apparently a much smoother start (not sure if it gives better 0 RPM torque or not; haven't tried it yet).

 

But if you want torque at 0 RPM I don't think you have much option but to use one or the other type of sensors.

galp
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Stall torque works perfectly with encoder or hall on HW4.12.

cromoly
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Thanks! Will try to use a hall...

spork
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>> Stall torque works perfectly with encoder or hall on HW4.12.

Not sure what you mean when you say "works perfectly".  Mine certainly works *much* better with internal hall sensors (VESC 4.12) than without, but it definitely doesn't give me as much torque at 0 RPM as it does once it's turning.