> On Thu, Mar 28, 2013 at 11:19 AM, Chris Radek <ch...@timeguy.com> wrote: > > >> >> My VMC uses velocity mode amps with tachs, and I had one tach that >> would sometimes give a "bumpy" output that would cause a banging >> once per screw revolution. >> >> Maxon and Escap motors are made with exactly the same design as their high-quality tachs. The only difference is the motors have slightly more robust brushes, the tachs have silver wires instead of brushes. They make excellent tachs, and I have used them a few times to repair gear where the silver wires have worn through. >> It is a high accel machine with high res encoders. >> >> I had extra DACs so I fixed it by unhooking the real tach and >> instead sending the hostmot2 high quality encoder velocity feedback >> out a DAC and hooking that to the tach input on the drive. I had to >> adjust the gains on the drive to compensate for the much lower >> "tach" input (now +-10). I don't remember whether I replaced >> components, or just turned knobs. >> I have thought this was possible for some time, especially after discovering Fanuc had been doing this back in the mid 80's. Moving the entire velocity loop back into the PC has a lot going for it. I made my own velocity servo amps in 1998, and there are WAY too many trimpots on it, 11, I think! The interaction between the current loop, velocity loop and positioning loop leads to a whole bunch of ways to make it unstable. I have FINALLY managed to make the whole system work better, but it took me a number of years where the system would occasionally break into a howl or vibrate after a fast move. And, I can only use Halscope to infer what might be going on in the two inner loops.
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