On Wed, Mar 3, 2010 at 10:17 AM, Jon Elson <[email protected]> wrote:
> Chris Morley wrote: > >>> > >> The servo drives are from Advanced Motion Controls. They are Direct PWM > >> Brushless DC Servo Drives Model# BD30A8. The datasheet can be read at > >> http://www.a-m-c.com/download/datasheet/bd30a8.pdf . I talked to their > tech > >> support and there is no current loop or velocity loop in the drive. They > >> told me the PWM signal directly controls the amount of time the MOSFETs > are > >> turned on. If it was a brushed DC motor, I understand that the PWM duty > >> cycle would have a linear relationship to the average voltage and hence > >> average speed of the motor, but since it's a brushless DC motor, I don't > >> understand if that same relationship holds or not. > >> > > > > > > AMC has instructions on their web site (little hard to find the right > one) > > > > on how to tune the servo drive itself. > > > > Are you sure the drive is set right for velocity mode? > > > If you look up the pdf he links to above, it is clear this is NOT > inherently a servo amplifier, it is > just a PWM amplifier, with no internal loop other than the current > limit. There is no tach input, > no encoder input, other than PWM from the controller. Therefore, other > than setting the current limit, > there is no tuning that can be done. I make a very similar servo amp, > and maybe I shouldn't call it a servo amp, as it has no inherent loop, > either. > > The OP's last question, yes, there is still an approximate linear > relation between PWM duty cycle and speed. > The motor's resistance throws it off a little, speed will drop slightly > under load. Tuning of such drive systems is a bit different than on > velocity servo systems, and the inherent stability is less. You don't > want to turn up gain until you have "wiggles", you want to tune first > for stability, then use FF1 and FF2 to reduce following error. See my > page on servo tuning for these types of servos at : > http://wiki.linuxcnc.org/cgi-bin/emcinfo.pl?PWM_Servo_Amplifiers > > > Jon > > Thanks for the link to your servo tuning page, I will play with the PID settings to see if I can tune for stability first and then add in the FF1 and FF2 as you suggest. Lawrence ------------------------------------------------------------------------------ Download Intel® Parallel Studio Eval Try the new software tools for yourself. Speed compiling, find bugs proactively, and fine-tune applications for parallel performance. See why Intel Parallel Studio got high marks during beta. http://p.sf.net/sfu/intel-sw-dev _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
