Sensorless back_emf speed controller

[Jared Russell]

Originally Posted by Jetweb

To put it simply, its just cool...

It would also be extremely convenient to be able to monitor motor position or speed WITHOUT the encoder.

The jag has a current sensor, but when it comes to trying to monitor position or speed with this you really cant, you can control torque with the addition of an encoder, but we don't want those now do we.

To quote JVN, "Enlighten a man who sometimes has difficulty understanding why others stray outside the box, when the box appears to be an optimized and elegant solution."

I see what you're getting at, that sometimes mounting, wiring, and programming up an encoder is a pain. But measuring back EMF directly is tricky - I am not a fan of removing power from the motor so that I can read its terminal voltage.

It's also going to be less accurate than an encoder, since you need to scale the voltage by some motor constants in order to get the speed. The published motor parameters can vary by 10% or more from model to model, and even more once you consider thermal factors and friction. Lastly, you can't directly get position data from back EMF - you would need to integrate the speed signal, adding to your error.

That said, it might be fun to see for yourself how well this technique works. Here is a website on one way to do it:
http://www.acroname.com/robotics/inf.../back-emf.html