On Thu, Mar 28, 2013 at 10:51:12AM -0500, Stuart Stevenson wrote: > I am trying to dumb down the drive so the LinuxCNC tuning is directly > effecting the motor motion instead of manipulating the tuned servo system.
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. 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. For those interested in the results (these were apparently taken at a time when the real tach was working right): http://timeguy.com/cradek-files/emc/real-vs-generated-tach.jpg http://timeguy.com/cradek-files/emc/real-vs-generated-tach-upclose.jpg But this is all background information for what I propose. The first thing the amp does, as has been pointed out, is take the difference of the command and feedback signals. I suspect you could do exactly the same in HAL and use just one DAC to feed an amp like this. One of the inputs would be just zeroed (shorted). The signal to the amp would now be velocity error, not velocity command. The amps on my machine also have at least one safety feature that uses the tach input: it has a max speed setting and just compares the tach input to a setpoint and faults if it exceeds. I think this is a basic runaway detection. I think this still works in my setup now but I didn't test it. I say this only to remind people to watch for similar gotchas if you try to "trick" the amp. Shorting the tach input would disable this safety feature, but perhaps shorting the command input and hooking my error signal to the tach wouldn't!? This might help you scratch your itch of simplifying the parts that are outside linuxcnc/hal in the hardware realm, but it doesn't eliminate dependence on tuning in the amp. I have had such good success with velocity mode amps (sometimes after tuning them with a soldering iron) that I don't share your goal of making them dumb, but if you succeed at that (likely the only way is also by using a soldering iron), I would love to hear your results. Chris ------------------------------------------------------------------------------ Own the Future-Intel® Level Up Game Demo Contest 2013 Rise to greatness in Intel's independent game demo contest. Compete for recognition, cash, and the chance to get your game on Steam. $5K grand prize plus 10 genre and skill prizes. Submit your demo by 6/6/13. http://p.sf.net/sfu/intel_levelupd2d _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
