From: Jon E. <el...@pi...> - 2009-10-31 23:40:08
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Chris Radek wrote: > On Fri, Oct 30, 2009 at 10:11:49PM -0500, Jon Elson wrote: > >> One thing that has been common in the Fanuc systems is they use servo >> amps that need velocity >> feedback from the motors, but they don't have tachs on them. The >> controller synthesizes an analog >> velocity signal from the encoder and sends that to the amps. >> > > That sounds odd and makes me wonder (again) about something on my > machine. > > I have tachs, but they were not wired to the amps. They went to "some > part" of the main computer of the Yasnac control [which was missing]. > Then "something else" came from the control board and went to the > velocity feedback input on the amps. > Yup, that was the way the old Fanuc controls worked. They used either the L290 or a similar quadrature to voltage converter chip. The L290 was a frequency to voltage converter specifically designed for this purpose. It used 2 identical sections feeding into a difference amp so encoder dither doesn't count as velocity. Unfortunately, it is no longer available. > I do not know what was there - it's possible they were just wired > together. I wired them straight through. > > Any guesses whether something was there? I had some trouble getting > stable velocity loops, and it makes me wonder. > The problem is the real Fanuc machines had no tachs, so it makes the retrofit harder. If you DO have tachs, then no problem. One possible setup had a circuit that compared absolute velocity from the encoder to absolute velocity from the tachs. If the encoder velocity was 20% greater than the tach showed, it caused an E-stop. This was used in the days of encoders with incandescent light bulbs, where an encoder failure was a yearly ocurrence. The Allen-Bradley 7320 had this. Jon |