Back emf tends to be only useful with brushes motors where you have additional 'inputs'. As you only drive two at a time you can measure back emf from the free input.
When driving a normal dc motor from a h bridge the problem you probably cam across was flyback when turning the h bridge between energized states.
It is possible I guess to use back emf with a h bridge by monitoring the motor terminals when the bridge is high z, giving you a peak at the motors output when rotating free. But for assuming a dc motor doing 6k rpm, and a 10k pwm of 95% you will get 5us of sample time which is not much of a 10ms period on the motor rotation, and nyquist might have something to say too.
It is possible extract the frequency if the signal is clean enough, but processing might become an issue.
What methods have you tried? It is fairly early for me so I could easily have missed something.
Sent from my HTC
----- Reply message -----
From: "Beng Kiat Ng" <firstname.lastname@example.org>
Date: Tue, Dec 14, 2010 02:24
Subject: Back emf control of DC motor
Has anybody done any work on bemf control on dc motor?
I'm looking for working circuits or ideas to extract the bemf from the
H-bridge. Tried to design one but was not successful. Not as easy as it
appears to be. Any help will be appreciated.
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