On Wednesday 25 March 2015 19:02:16 Karlsson & Wang wrote: > To get higher accuracy you need more accurate measurement, some extra > resolution above the accuracy is also useful. > > For better dynamic perfomance it may make sense to close the inner > loop if the inner loop is faster than the outer loop. If the electic > response is much faster than the mechanical response then there is > rather little to gain from faster control loop. If motor inductance is > around 1mH, applied voltage is 48 volt and current is 10A then > U=L*di/dt <=> dt = L*di/U = 1mH*10A/48V = 0.2ms = 200µs is the time to > change motor torque from zero to maximum which put an upper limit on > dynamic response, there is also a rotational speed dependent back emf > reducing available voltage. > > If you do not like the math. Change servo loop peridicity adjust > control loop and see if it make any real difference. > > > Nicklas Karlsson
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. My $0.02. Works well here, might not be worth the electron wiggle to send it to someone else. The phrase YMMV applies here. > > > > > On Wed, 25 Mar 2015 14:45:42 -0300 > > Leonardo Marsaglia <[email protected]> wrote: > > Hello to all! > > > > I just managed to turn the second motor of my machine (another AC > > motor with a Powerflex 40 drive) and works just fine for positioning > > the part. Tomorrow I'll be uploading some videos. > > > > Although I need to do some fine tuning on both the rotary and the > > linear axis I was thinking about a concept to improve the accuracy > > of the servo loops. > > > > As I have seen on many machines including the Mazak that we have > > here, the servo systems close the velocity loop within the servo > > drive, and then the CNC applies a voltage to control the velocity of > > the motor based on the PID of the position loop. > > > > So as I see it, it's like there is one pid nested inside the other, > > or sort of. Now the VFDs that I'm using can close the velocity loop > > but I would have to use a special module for that. > > > > My question is, based on your experience. Could this improve the > > accuracy of the joints? I guess this could be done within LinuxCNC > > without closing the loop on the VFDs. > > > > Thanks as always for your help!! > > > > -- > > *Leonardo Marsaglia*. > > -------------------------------------------------------------------- > >---------- 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 > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/emc-users > > ---------------------------------------------------------------------- >-------- 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 > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users 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 [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
