dave wrote: > On Sun, 2011-06-05 at 10:39 -0500, Chris Radek wrote: > >> On Sun, Jun 05, 2011 at 07:58:52AM -0700, dave wrote: >> >>> Plan 1. go to velocity mode and add a tach. The tach is low output. >>> only about 2.5 V/1000 rpm. For some reason I can't get the system >>> stable. When enabled it oscillates for a few seconds and faults. >>> That is with the analog input at zero (shorted). >>> I've moved the tach gain all over the place with no luck so far. >>> >> Are you sure it's not hooked up backwards? >> > > Yep! No control at all .... just takes off in one direction if I reverse > the connections. > OK, that's a sign that the tach is doing the most basic function properly. I built my own velocity servo amps over a decade ago, and STILL end up tweaking the performance every once in a while, trying to eliminate the jitters that happen every now and then. So, I sure don't have a magic recipe for tuning a velocity servo.
But, if it oscillates with no input, that clearly means either there is too much gain on the tach input or it needs more compensation. Try turning down the tach gain or velocity gain (they label these differently on different makes). Oh, you said you did that, already! Possibly, you need a resistor attenuator to drop the tach feedback some more. Perhaps try a 2:1 divider, with 1 K Ohm resistors in series with the tach output and then one across the tach inputs of the servo amp. Well, what frequency of oscillation was it? Really fast buzzing or slow rocking back and forth? That might give some hints whether it is compensation or not. Is there a velocity compensation pot, or do you have to solder caps onto a header? (some amps make you do this). And, finally, are you sure the tach is giving a clean output? It could have brush noise causing intermittent signals. Also, you need to be sure there is no slack between the tach and the motor. If there is a sloppy coupling or belt, it could allow the motor to rock some amount before the tach starts to move, that would guarantee oscillation. Jon ------------------------------------------------------------------------------ Simplify data backup and recovery for your virtual environment with vRanger. Installation's a snap, and flexible recovery options mean your data is safe, secure and there when you need it. Discover what all the cheering's about. Get your free trial download today. http://p.sf.net/sfu/quest-dev2dev2 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
