Chris Radek wrote: > > I see you're using bare H bridges which means you don't have a > velocity loop. I think this problem with FF1 is because of that, and > it is a fairly fundamental problem with a torque-mode setup. > > My PWM servo system is very similar, essentially a voltage-mode amplifier commanded by PWM output. So, PWM duty cycle commands a voltage to the motor. Back-EMF is pretty dominant, so it works like a velocity servo with low gain, and therefore low stiffness. But, I find that FF1 works VERY well to correct for the finite gain. I only have experience with this on my minimill, and I have a lot of reduction between motor and table - 4:1 belt reduction and 16 TPI screws. I can imagine a case where the motor is less well matched where motor resistance would tend to bog it down at higher speeds. I seem to recall from your pic that the motors were directly driving the leadscrews, and they did not look to be very large motors.
Once you have FF1 correcting the cruise error, then a TINY amount of FF2 fixes the spikes during deceleration. I have 128,000 encoder counts/inch on my minimill, and can usually get the error down to about .00005", or about 6 encoder counts, during acceleration, and even less during cruise, up to above 60 IPM. Jon ------------------------------------------------------------------------------ This SF.net email is sponsored by Sprint What will you do first with EVO, the first 4G phone? Visit sprint.com/first -- http://p.sf.net/sfu/sprint-com-first _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
