On 01/19/2016 09:45 AM, TJoseph Powderly wrote: > > clear me up on in this... > about very slow motion with steppers and servos > > assuming the minimum position change is .001mm > ( commanded step or feedback unit > so: a single step moves .001" > and any feedback has .001 resolution ) > > when the system gets a command to move another .001mm, > what happens when the velocity requested is .001mm per second? > > in a stepper system, does it go clunk and wait 999millisecss? > (or wait 999 then go clunk, or at 500mSec etc.)? Yes. > in a servo system does it crawl towards the destination over the whole > second? > > When I first converted my Bridgeport over to using velocity servo amps with tachometer feedback, I wanted to measure this. I hooked a scope up to the current sensing for the motor, and moved at slower and slower velocities until the motor current got "lumpy", indicating some stick-slip friction. This showed up at about .01 IPM! I did some calculation to figure the tach was producing 7 uV at that speed. Just to see what would happen, I shorted the tach out with a screwdriver. The servo amp immediately faulted. That proved that is was still closing the velocity loop, even at 7 uV. The encoder was producing 3.33 quadrature counts/second. I verified that the stick-slip was NOT harmonically related to that frequency. So, the velocity servo system WAS maintaining the steady motion velocity until the slide just got too sticky to keep it steady any slower.
So, with a properly tuned velocity servo system, it WILL maintain smooth motion down to insanely low speeds. Without tach feedback, such as on my PWM systems, the only way to know velocity is from the encoder counts. If you have a high resolution encoder, it can give a steady stream of counts at low speed. With a low res encoder, there are long periods between counts, where the position and velocity are unknown to the PID, and so it has to ASSUME that the velocity has not changed. Jon ------------------------------------------------------------------------------ Site24x7 APM Insight: Get Deep Visibility into Application Performance APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month Monitor end-to-end web transactions and take corrective actions now Troubleshoot faster and improve end-user experience. Signup Now! http://pubads.g.doubleclick.net/gampad/clk?id=267308311&iu=/4140 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
