Quote:
|
Originally Posted by ChrisH
I'd have to look at the robot to be sure, but as I recall it was a 256 step optical encoder with quadrature. The arm was rotating around 4 rpm and the jack shaft was about 5 times that due to a 5:1 reduction between it and the arm. The arm position was what we were trying to control.
So say we had 20 rpm and 256 steps/rev, that works out to about 17Hz. Well within the capability of the sound card scope anyway  |
Sorry Chris, I tend to disappear from CD for days at a time, so I apologize for the late response.
http://www.virtins.com/ shows a little diode circuit to clip the signal at about 1V, which Al alluded to as roughly the limit of the sound card. Additionally, an amplifier set up as a unity gain buffer (doesn't actually amplify) provides some isolation, and impedance matching. I don't have any schematics for you handy, but I'm sure they can be found on the net, or someone here can provide them. You could probably get away with just the diode circuit in the link above, if you're using it on the 5V signals from your sensors. I can't make any promises though =). I haven't used the sound card oscilloscope myself yet.
Lastly, we used 256 ppr quadrature encoders on our drivetrain last year (Grayhill 63R series). We initially had a lot of problems when we bumped into things, or our drive chains backlashed. We worked out our max RPMS, and pulse rates, and they were all well below our maxes...
What we didn't account for were "jolts." Momentary hits that moved the encoder just far enough to record a count or two (at 256 ppr, this isn't much movement), and fast enough that the RC would read the quadrature signal incorrectly. The end result was a count going in one direction was being misread in the other direction - this is what happens when you read a quadrature signal too slowly. This caused havoc with our PID loops, which functioned great until you smacked the robot, and things would go haywire until you settled everything down again.
We actually worked around this with a quadrature stretching circuit, the schematics of which you can find
here, courtesy of Kevin Watson.
-SlimBoJones...