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Unread 05-05-2009, 20:54
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Mentor :Contol System Engineer
AKA: Chris
FRC #0599 (Robodox)
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Re: Sensorless back_emf speed controller

Quote:
Originally Posted by Al Skierkiewicz View Post
Chris,
Don't forget that the coast/brake mode only affects the turn on of the two lower pairs of FETs when the PWM goes to a value that would represent "off" i.e. no direction. During that time, the two FET groups (six FETs in all) would present a short to the motor. Effectively, this is about 6-8 mohm at the output of the JAG. The Jag also produces a charge pulse to charge up the cap in the voltage doubler of the FET driver chip periodically.
Al, the short is what I would expect in the brake mode. It seems to me that the two FET groups would be open in the coast mode and the current would discharge through the protection diodes. When the current is zero , then the voltage would jump to the back_emf. This is what I saw with the vex controller and also agrees with other waveforms that have been posted for H bridges online. So I'm still vexed ...seems the coast mode might not be operational.