On Sunday, October 09, 2011 10:00:38 AM Bruce Klawiter did opine: > Jon Elson <elson@...> writes: > > Yeah, actually, I'd like to see one of these spikes zoomed in on the > > time scale so it is > > See image 9 here: https://sites.google.com/site/bmklawt/home/pid-tuning > > > Bruce says he has run it back > > and forth for > > a half hour, and it was within one encoder count of proper position > > (.0005") It seems VERY > > unlikely it could do that without errors accumulating. > > I am not following this, why can't it do this without errors > accumulating.
If his position feedback is entirely from the encoders, these are generally digital and less subject to noise pickup. So each time it is disturbed, the position feedback corrects it. It could run for days that way with no accumulated error. > > I think Bruce has an oscilloscope, > > I do not have one, wouldn't have a clue how to use it if I did. The interpretation is similar to what you see with the halscope. Time is the horizontal axis, voltage (or whatever you are measuring) is the vertical axis. > I unhooked the Y and Z axis amps and only had the X axis plugged into > the DAC board and when running the X axis it still had random jerks. This looks as if something is disturbing the D/A output to the motor driver. The position feedback is then correcting it. > Here are two videos of the jitter I get in the Z and Y axis. > The > first one showing the Z axis jitter when the X axis is moving back and > forth. The knob is on the end of the servo motor. > http://www.youtube.com/watch?v=7wZQwee8bec > > The second one is of the Y axis while the X axis is homing, as soon as > the X axis stop so does the Y jitter > http://www.youtube.com/watch?v=O66oefYwhp0 Now, this is a small clue. And I would read that as meaning that somewhere, it appears the ground connection from the D/A might be sharing a path with some motor current. A ground loop maybe that defeats the intention of the star topology? With the power removed, you should be able to lift each circuits ground conductor from the 'star' connection point, and measure between the star point and the lifted connection without finding there is still a low resistance path between them. Low being defined as below 20 ohms generally but this isn't a hard and fast rule. Basically, each circuits common point, and each servo _is_ likely 2 circuits, one for power & one for control in which case each should be connected to this star point with no connection between them except at the 'star' point, and a path to earth ground then is nothing more than another wire leaving this 'star' common point. It is there for your safety, or should be, but the rest of the system shouldn't care. If you are using shielded cables, don't use the shield for ground currents, tie it to the 'star' and cut it loose and back for insulation at the other end. Any grounds in that cable also get connected to the star point. The idea being that if there is only one common point, then any noise on this common point is impressed equally on all connections common to this 'star' point, thus becoming 'longitudinal' noise that because all the circuitry works on differences, is not seen and has no effect. I hope this helps. Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Your heart is pure, and your mind clear, and your soul devout. ------------------------------------------------------------------------------ All of the data generated in your IT infrastructure is seriously valuable. Why? It contains a definitive record of application performance, security threats, fraudulent activity, and more. Splunk takes this data and makes sense of it. IT sense. And common sense. http://p.sf.net/sfu/splunk-d2dcopy2 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
