On May 20, 2012, at 1:56 PM, Jon Elson wrote: > Jeshua Lacock wrote: >> >> Well lets just say the only thing that I am sure about is the drive is >> under-damped. >> >> ;) >> >> Tuning is proving to be insanely difficult for me, I guess mostly because I >> don't really have much of a clue what I am doing. It is also kind of scary >> because if I turn a POT on the Gecko just a tad in the wrong way the drive >> violently rattles the whole table. >> >> When it was just the Gecko, that was something I had a grip on. But now it >> is a combination of the Gecko and the PID settings. >> >> > So, first you need to tune the Gecko, by itself, so that it works OK. > Set DIP switches > 1-4 to OFF, to go to open-loop mode. This allows you to generate step > pulses without > LinuxCNC trying to respond to the movement.
Thanks Jon. Actually, I really have only been using open-loop mode. I am a little confused however because changing the PID settings still seems to affect the drive. > Get the drives set stably, > and you may > just want to turn down the "gain" setting until it is stable. Gain is > not so critical, as you > will be increasing it when you go to closed-loop mode. Once the Gecko > is working > OK in the open-loop mode, set the switches to ON, and observe performance. I do have the drives working stably, just the X gantry is underdamped unless I turn the acceleration way down. But as soon as I try closed loop mode, I get a ton of noise and the drive is super jittery. It starts nearly stable but seems to run away until it faults. When I say run away it becomes increasingly jittery. > Start with low settings of P, and minimal I and D. Increase P in small > steps until > the following error gets better, and then add a bit of I and D to get > the best > performance. That procedure is described in the document I sent the > link to. The sample .ini file uses two different values for 2 different axises. When you say low P settings, what would you start with? And what I and D is good to start with? Also, what would you consider small steps to increase for P, I and D? >> It may be that I am just trying to accelerate the drive faster than the >> system can adequately damp it. This gantry is probably close to 200 pounds, >> and I just don't know how fast I can expect to be able to accelerate it. >> > Yes, possibly. You might put an amp meter in the motor wires and see how > much it draws, but you can actually detect this with Halscope. By slowly > increasing the acceleration until following error spikes, you will know the > acceleration the system is capable of. When the error spikes, wouldn't that also indicate that it needs more gain and or damping? >> Does gearing a drive help with this? I seems in my mind it should. Gearing >> it down further would give me increased torque and resolution so seems like >> a win win. >> > Yes. Gearing down increases torque at the leadscrew. The only downside is > that the motor rotational inertia is increased by the gearing down. To > reach > the same linear speed, the motor needs to accelerate X times faster, where > X is the reduction ratio. But, when you simply run out of torque, then > the reduction helps. But, there's no way to know for sure whether this > is really your problem. Note that I am using pulleys and timing belts for all my axises except for the Z. I see. Currently my main gantry moves about 2 inches for one motor rotation (which is geared 4:1). I just ordered Kelling's 5:1 planetary gear which would make it geared 20:1 with the current pinion (12 tooth) or 10:1 with a 24 tooth pinion... Thanks again, Jeshua Lacock Founder/Engineer 3DTOPO Incorporated <http://3DTOPO.com> Phone: 208.462.4171 ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
