My simple drives are the same way. Requiring a few percent of pwm. I thought it was you that mentioned you could enter negative deadband into emc (in wichita)... I have used this in testing but have not actually tried it on a machine. I defiantly makes the dead spot at rest smaller - if not non existant. If it doesn't work well - I was planning on using awallins hal component that jepler posted.
sam Jon Elson wrote: > I've been tinkering with some brushless drives and have found something > that I'd like to improve. My brushless amps have a minimum pulse width > they will respond to. Depending on which transistors I am using, I have > different deadtime selections in the CPLD, so it goes from about 400 ns > to 1 us. I am using the stock EMC2 PID routine with my PWM controller > and the hal_ppmc.c driver. With my new version of the universal PWM > controller, that gives me pulse width resolution of 25 ns and minimum > pulse widths well below what the drive can respond to. > > The problem is there is a dead band where the P term produces a pulse > width that is too narrow for the servo amp to produce any output. > If you deflect the motor until the pulse width is able to produce output > from the servo amp, then there is a discontinuity in the transfer function, > and the motor jumps when deflected across that "edge". I have tinkered > with negative values of the dead band parameter to PID, and it definitely > seems to help narrow the width of the motor's dead band, but I can't get > it down to zero. If I try, it causes it to be very unstable, as a tiny > movement > of the motor jumps from + to - output of the servo. (So, not a surprise.) > Maybe a very fine tweaking of this value to allow a tiny dead band to > exist would help. > > Anyway, I'm not asking for a "solution", just a discussion of what tricks > might be useful in reducing the deadband. Right now, it varies from > a couple degrees to maybe 5 degrees or more of motor shaft movement. > You can easily feel the zone where there is no motor current, and the > sharp transition when the minimum pulse width causes the transistors > to turn on. > > I know that if I could turn up the P gain, it would help narrow this zone. > But, I have P turned up as high as I can while maintaining reasonable > stability with these voltage-mode amps. I've been trying to figure out > if there's an inexpensive way to turn these simple PWM-in equals > PWM-out amps into some kind of torque-mode amps, but so far I > haven't come up with anything that sounds promising. (It might look > something like smooth the incoming PWM into an analog voltage, > compare to motor current and generate PWM from that. But, that > would be adding a bunch of op-amps and comparators to the > board.) > > Jon > > ------------------------------------------------------------------------------ > Join us December 9, 2009 for the Red Hat Virtual Experience, > a free event focused on virtualization and cloud computing. > Attend in-depth sessions from your desk. Your couch. Anywhere. > http://p.sf.net/sfu/redhat-sfdev2dev > _______________________________________________ > Emc-developers mailing list > Emc-developers@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-developers > > ------------------------------------------------------------------------------ Join us December 9, 2009 for the Red Hat Virtual Experience, a free event focused on virtualization and cloud computing. Attend in-depth sessions from your desk. Your couch. Anywhere. http://p.sf.net/sfu/redhat-sfdev2dev _______________________________________________ Emc-developers mailing list Emc-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-developers
