James Louis wrote: > Hello everyone. I had this on the forum for a while: This question goes out > to any "servo loop" experts out there in EMC2 world. > I understand that EMC2 uses PID to form a control loop when using > servomotors, but . . . > what if you use a drive that already has its own loop? For example, my > machine uses Yaskawa SGM-0xA31x series servos, Yaskawa SGDA-0xAS series > analog drives, and a Pico Systems PPMC (ENC, DAC, DIO) interface. > > These drives receive the encoder signals and redirect them to the PPMC ENC > card, but have their own PID terms which are set during Autotuning. So is it > loop or loops? > > What effect do the PID terms in the EMC2 *.ini file have? I am using just > the P term right now and it works. The motors move OK, but I would like to > know HOW. Like what is an optimal value for the P term? Is HALscope the > right tool for this? If I use values in the other terms will this form a > second nested loop? Is that better? What about other INI entries like > DEFAULT_ACCELERATION, etc? > Yes, lots of people use a variety of servo amps with EMC2's PID. In fact, this is the classic scheme. The amps I use on my Bridgeport have P and D, but no I. But, it is more complicated, as they have an inner torque loop (with its own P and D) and an outer velocity loop, with ITS own P and D! If the amp has gain, then you need less gain in EMC's PID to get tight control. I suspect your Yaskawa is much the same.
Assuming the Yaskawa amp performs perfect control, then all you need in EMC is P, so that the velocity command to the amp exactly matches the voltage it needs to provide that velocity. The I and D terms accommodate less than perfect control of velocity and help keep the error smaller, You can't put servo loops in a nutshell, it is a very big topic, with many ramifications. There are resonances all over the place, finite gain and dynamic stability concerns. For instance, on a machine with a leadscrew, the resonant frequency of the torsional moment of the screw varies as the nut runs down the screw from one end to the other, so you can't just damp out one resonant frequency! Jon ------------------------------------------------------------------------------ This SF.net email is sponsored by Make an app they can't live without Enter the BlackBerry Developer Challenge http://p.sf.net/sfu/RIM-dev2dev _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
