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 ------------------------------------------------------------------------------ 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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
