Stephen Wille Padnos wrote: > > EMC2 has a component called "at_pid", which is an autotuning PID. It > uses something like Z-N to get the coefficients. In testing, it seemed > to create usable sets of coefficients, but the results were pretty > heavily dependent on initial conditions. You could tune an axis twice, > and you'd get two very different sets of coefficients. Both seemed like > they would work OK, but the fact that the results were so dependent on > initial conditions led most everyone, including the author of the > component, to question whether it was a good idea to use it.
I was writing the same thing, but Stephen beat me to it. > I don't know if auto-tuners can tune feedforward - I've never heard of > one that does (though I haven't looked hard). Feedforward lets the PID > feed control commands straight through (mroe or less), and then adds on > the error correction terms. In many cases, feed-forward doesn't need to be "tuned". The idea behind feed-forward is to provide an open-loop command that gets you very close to the desired result, and then let the PID correct for any remaining errors. By doing incremental design and measurements, and setting the scales right, you can arrange it so that the most important FF term will be 1.000. For example, assume your servo drives have a velocity loop, which means you will be wrapping EMC's position loop around the drive's inner loop. Note - in this case you MUST get the velocity loop tuned first! If the drive has a velocity loop, the dominate feedforward term will be FF1, the velocity term. Once the drive is tuned, there will be a very specific relationship between the voltage from EMC and the speed of the motor. For example, +3V to the servo drive might result in 6 inches per second movement of the axis. In that case, the DAC scale should be set so that a PID output of 6 generates +3 volts at the DAC, which will make the axis move at 6 inches/sec. Then you can set the FF1 term in the PID loop to 1.0 - a commanded position that is changing at 6 inches/sec will result in a PID output of 6 * FF1 = 6, the DAC will output +3V, and the axis will move at 6 inches/sec. Of course it won't be perfect - analog drift, shaft load, etc, might make the resulting speed be 6.02 or 5.97 inches/sec - that is why we have feedback. If the servo drives accept a current (or torque) command instead of velocity, then the dominate feedforward term will be FF2 (acceleration). A specific input voltage to the drive would result in a specific output current and thus motor torque, and that torque would accelerate the axis at a rate which depends on its mass. You would set the DAC scaling so that a PID output of 1.0 would result in enough torque to accelerate the axis at 1.0 inches per second per second. Then set FF2 to 1.0, so that when the position command accelerates, the PID output is automatically whatever is needed to achieve that acceleration. Again, there will be errors between the open loop accel from the drive, and the exact accel you want - probably worse than with a velocity loop in the drive, since there are more things that can change the open loop response. Put a heavy workpiece on the table, and the same amount of torque will give you less acceleration. So again, feedback and the PID terms takes care of the errors. > There's just no substitute for having a brain in the process somewhere - > computers can't do it all (yet). Exactly. IMHO this is a good thing - if computers could do all the thinking for us, we would let our brains rot. Then who would tell the computers what to do? Regards, John Kasunich ------------------------------------------------------------------------------ Check out the new SourceForge.net Marketplace. It is the best place to buy or sell services for just about anything Open Source. http://p.sf.net/sfu/Xq1LFB _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
