On Wednesday 25 March 2015 22:48:36 Leonardo Marsaglia wrote: > 2015-03-25 21:12 GMT-03:00 Gene Heskett <ghesk...@wdtv.com>: > > If I can butt in here Leonardo, when I was setting up the spindle > > speed servo in my toy lathe after switching from a non-linear and > > failure prone driver to drive the 1hp treadmill motor my 7x12 now > > sports, to a slightly modified version of the Pico Systems servo > > driver, and keeping in mind I am using one of Peters 5i25 > > interfaces, which means I no longer needed a base thread in the > > setup. But the speed servo was hunting badly enough to keep the > > back gears in the headstock rattling pretty good when the servo > > thread was running at nominally 1 kilohertz. > > > > I had to reduce the P in the pid to the point it was essentially > > worthless at sub 300 rev speeds. So for S&G, thinking the control > > was too slow, I did a rockhopper diagram and re-arranged the hal > > file so that I was doing a straight fall thru of the control path so > > it was not wasting a period or more because it was out of order when > > the modules were in the wrong order. That helped but the amount of > > P seemed to be limited yet, so the only other way to get a faster > > control loop was to raise the servo thread speed. Nominally 2 > > kilohertz made an obvious difference, and at 4 kilohertz, it was > > lots quieter. So that is where it has been running at for several > > months now. P in the speed PID is now high enough that I can peel > > off a pretty good sized string of blued steel at 150 revs, or even a > > dimly glowing string at 500 revs, which gives a "more better" > > finish. > > Hello Gene! > > I always like to here from your experiences. > > From what I've been doing I got really good results but I don't recall > trying to raise the speed of the servo thread to make any difference > so I'll be trying tomorrow. > > The idea that was spinning in my head is if it will make any > difference (if possible) first to close a speed loop and then feed > with that the position loop all using the same encoders for the two > feedbacks. > > Here's an example picture: > > http://s23.postimg.org/6xyj2q0zf/reductor.jpg > > That's the rotary axis, the picture is before I mounted the encoder > but you can see how it works. I know It would be the best to have > another encoder on the main shaft to really know the exact position > but, since this is not thar critical and the worm and gear have almost > none backlash we decided this setup would be ok. > > What I'm doing here is to sense directly on the shaft of the motor > (this would give me a really accurate reading of the speed). But It > would be great if this same encoder could feed also the position loop > in LinuxCNC.
That does look as if the tut for using two PID's in a previous message might be helpful. I' make the comment that the tension on the belt is pretty important to reduce tha backlsash. I have it tight enough on the mills home made z drive to potentially damage the motors bearings. I'd estimate the current tension is well north of 75 lbs. The screw is solidly anchored into the top of the heads sled casting, and I am turning the doubled nut assembly, squeezed between two angular ball bearings with the timing belt. This gets me down to around 1.5 thou of backlash on a 10 tpi nook acme screw when the double nuts are dragging. If there is any deflection of the belt in the middle of the pulley to pulley span, it will translate directly into some pretty horrible backlash readings. If I just push the motor until its snug, the measured backlash will easily top 10 thou. Another thought on that setup might be to add a smooth drum above the driven pulley, to wind up a tape measure spring on, put considerable tension on the far end of the tape, and attach a linear scale to the tape. I'd think it would not be too difficult to get .1 degree accuracy out of that once it was scaled properly with the usual circumference math. Same idea as was used on old floppy drives back in the 80's to step the heads. Then you wouldn't need to care about worm to bull lash as that would just be something the servo would self correct for. But you'd still want it as low as possible to prevent to table from being driven by the cutting force IF there was cutting force. >From your description of what its being used for that doesn't sound as if there would be much if any of that sort of force involved as its basically non-contact. Sometimes its amazing where you find amplified slop. Thanks for the flowers. :) Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
