BLDC rotary position control

[zappedagain]

I'm very familiar with speed control for a BLDC. Is it possible, without a mechanical brake, to stop a BLDC at a given position? Maybe decelerate very slowly and then hold the motor phases at a given state to lock the motor at a fixed angle?

i suspect I may need a linear system to do this, unless the PWM frequency is high enough that it doesn't cause me to jitter.

If this works, how accurately can I control the stopping position? Degrees? Arc-seconds?

Or is a BLDC just the wrong technology to attempt this?

My application has a mirror mounted on the motor shaft. I'd like to be able to stop the motor at a fixed position for calibration on occasion. Presently we do this by blocking the beam from the mirror so it can keep spinning without affecting the optical signal. For simplicity, I'd rather just stop the motor.

Z

 

[itsmoked]

Same as any three phase motor with a VFD. You should be able to stop it nicely and in the same spot. It sounds like you'll definitely want position info.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[blacksea]

Simplest way is using position loop with any position sensor (encoder, etc)

 

[cswilson]

Brushless permanent magnet motors are the most common technology today for precision positioning. These motors do have high-resolution position sensors on them, and you typically want higher resolution on the sensor than your required positioning accuracy. With this, closing a position loop as blacksea recommends is a straightforward exercise in servo control.

If you don't want to invest in a high-resolution position sensor, you can, as you suggest, force current into phases in a known pattern. When you do this, you are basically operating the motor as a stepper motor. If you only want to be able to stop the motor at one position in the revolution, and there are low external loads on the motor, this could well be a good solution that permits very high repeatability.

I wouldn't worry about PWM "jitter". The mechanical system just cannot respond to PWM frequencies of multiple kHz.

Curt Wilson
Delta Tau Data Systems

 

[zappedagain]

Thanks, this looks like a worthwhile experiment. I'll make sure the driver and software can support position control. My requirements are changing drastically, so two of the requirements for position control have dropped, but one is still hanging on!

Z