On Tue, 7 Nov 2017, Gene Heskett wrote:
Date: Tue, 7 Nov 2017 12:28:31 -0500
From: Gene Heskett <[email protected]>
Reply-To: "Enhanced Machine Controller (EMC)"
<[email protected]>
To: [email protected]
Subject: Re: [Emc-users] PID refresher for Gene?
On Tuesday 07 November 2017 10:08:33 Peter C. Wallace wrote:
On Tue, 7 Nov 2017, Gene Heskett wrote:
I fail to see why a 2 or 3 percent error in duty cycle and or
quadrature results in so much noise. There simply is no way in hell
that spindle can be moving at 200 rpms on one servo-thread sample,
and 600 rpms on the next sample one millisecond later while its
filtered display on a pyvcp tach dial says 400 rpms. Thats
physically impossible due to the mass of spinning material involved.
One could expand the size of the fifo, but eventually the lag in
seeing a real change in the velocity due to a tap starting to
actually cut will become a factor in the Nyquist response causing
even more instability since thats a direct Pgain input in any
feedback calculations. That of course is self defeating.
It may be that you have a EMI issue, false counts will play havoc with
the velocity estimation so impulse noise generated edges should be
eliminated by quadrature input filtering
Not too likely Peter, there is a 74ls04 used to clean up, square up, the
waveforms from the photointerrupters. about 3" of cable away from them.
Because of the limited space under the cover, and the longer lines to
the electronics box I had to make it in two pieces. The square waves are
nice and clean at the bob inputs on a 100 mhz dual trace scope, and at
the encoder a/b inputs on the halscope. Limited there of coarse by the
bandwidth of the halscope which shows a clean ramp. The little monster,
with only 4 feet of cable between the same photointerrupter and the bob
inputs. And it has the same problem, in fact it was on TLM that I first
made this fifo based averager. And I incorporated it in the hal file in
the pi version on the Sheldon, which has ATS667's watching the 60 tooth
bull gear. It however has a vfd, which doesn't need the feedback to
maintain within a couple percent of the requested speed.
Can you plot the encoder velocity signal via halscope at a known constant
speed and post a screen shot?
I asked about EMI because I dont know of any other customers with this kind of
issue with velocity estimation.
Peter Wallace
Mesa Electronics
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