Hi Andrew,
I don’t have anything specifically for hal configs yet, but it is high on my to do list. Your inquiry has helped me to get off my butt and write up what I’ve been doing and experiencing. I have a knee mill with one micron scales, servo motors & drives configured in velocity mode and ball screws.
The ball screws are from Rockford ballscrew they have an adjustable design that is supposed to be able to adjust the backlash out. It kind of works. The problem is instead of backlash they have “wind-up” I guess technically its not backlash like “open space” but the wind-up causes problems very similar. Because adjusting the ball nuts takes a lot of time I put it off and that’s when things got really interesting. The wind-up became more and more, up to nearly +/- ten thousandths. I could measure it because I integrated the original hand wheel dials in my retro fit. The dials are an excellent reference for tuning, you can literally see the oscillations and overshoot. As you probably know because of the windup all of the direction changes in circular interpolations had oscillations which could be seen the halscope but also in the surface finishes but not really measurable. Going point to point was and still is spot on within a tenth. With so much easy windup in the ball nut the cutter would occasionally grab and take a really heavy bite (climb cutting) and pull the wok piece/table into the cutter. It was easy to hear and almost visually perceivable when it happened. When it happens it is so very fast, so fast that I believe it’s beyond the linear scale encoders ability to read and it misses counts and results in a position losses.
I’m pretty sure this is happening because when cutting a pocket that steps down in increments the pocket migrates. Since the grab usually happens in the same place on each level it migrates the same direction, it’s very repeatable. After a few pocket increments I would command the tool to go to program Zero and it go to where the control displays zero but it is NOT at work piece zero. When I back-off the cut parameters so there is no tool grabbing (really light cuts)there is no loss of position. My gut has been telling me this is most likely NOT a problem for large machines with very heavy tables and very little backlash and I’m not sure how much a dual loop feed back will help with the grab and missing counts situation with my machine. If the ballscrews were truly zero backlash there would be no problem, potentially no need for dual loop feedback. I’m going to try some things, hopefully I’ll get some time to do it this winter.
-Doug