Hi Mathew, I'll lend my two cents to this too. I'm running DC servos on two axis and an AC Servo on the knee. The DC Servo drives are HP_UHU kits with the CPU an add on card from Henrik Olsson in Sweden. The AC knee is a Bergerda 750W drive already set up from the factory for that size motor. All three motors are running step/dir and tuned internally. In the case of X and Y given the motor specifications of 3000 RPM with 3:1 or 4:1 the MAX_VELOCITY is based on the pulleys, motor and leadscrew pitch.
The torque on servos compared to stepper motors is pretty low for the same size but as Les stated, it's pretty well constant over the entire RPM range. Are you certain that you have actually sized the motors correctly for the load? And I agree with Les. Set step/dir, P=0, FF1=0 and first tune the drives without LCNC. John > -----Original Message----- > From: Les Newell [mailto:les.new...@fastmail.co.uk] > Sent: June-28-21 8:03 AM > To: emc-users@lists.sourceforge.net > Subject: Re: [Emc-users] Advice on Tuning Servo System > > Hi Matthew, > > That FF1 is probably causing the increasing following error. Again I'd > strongly recommend using just stepgen rather than a combination of > stepgen and PID. At the moment LCNC and your drives are fighting each other. > > The oscillation could be the drive tuning or the loops fighting each other. > > > I�d be fine with a max rapid speed of 250 IPM but I�d hope to be able to > > have enough torque to cut at the full 250 IPM > > From your numbers I'd say you'll have plenty of torque at 250IPM. The > motor torque should be pretty flat from 0 to 250IPM. > > As far as speeds are concerned the absolute limit is the critical speed > of the screws. You can calculate it using this calculator > <https://www.lintechmotion.com/sizing/critical-speed/critical-speed-inputs.php>. > Exceeding the critical speed is a really bad idea. The next limiting > factor is the nuts. If you spin the screws too fast the balls skid in > the nuts and you get accelerated wear. Without the manufacturer's data > sheets this is much harder to estimate. About all you can do is look up > data sheets from various manufacturers and make estimates based on their > numbers for similar sized screws. By the way the Z axis screw is a bit > of an oddity. It has a very large diameter and fairly fine pitch. It's > lubricated from the central lube system. The combination of large > diameter and thick way oil means you get a lot of viscous friction > losses in the nut. If I remember correctly I limited mine to somewhere > around 2 - 3m/min 80-120IPM. > > Les > > > On 28/06/2021 15:27, Matthew Herd wrote: > > Hi Les, > > > > I�m jogging at 60IPM, 120IPM, and 250IPM for testing purposes. Depending > > on the PID parameters I was seeing increasing ferror > with jog time at all three speeds. The delay in stopping also seemed to > correlate with the ferror. Tuning eventually brought it down > to constant ferror, but results were still subpar. > > > > The motors seem to be capable of far more than they�re currently doing > > because when the ferror oscillated during a stop from > 250 IPM the machine would shake. During normal jogging moves at 60 or 120 > IPM they apparently struggled to keep up (per the > ferror) > > > > The max rated RPM for these motors is 2500 RPM, which equals 500 IPM. Max > > continuous is supposed to be 2000 RPM (400 IPM). > I�d be fine with a max rapid speed of 250 IPM but I�d hope to be able to have > enough torque to cut at the full 250 IPM. I�m hoping > to achieve good high speed machining performance so I�d also like to get the > accelerations up pretty high if possible. Cutting speed > isn�t too big a concern, but the NEMA 42 steppers I used to use were quite > slow to accelerate. Your max acceleration values (30 > in/s/s) seem like a good target for me. > > > > Unfortunately, although the drives support PWM and analog, my USC boards > > don�t support them. In hindsight I would have gone > with PWM or analog control of the drives, but I already had the USC boards. > > > > Matt > > > >> On Jun 28, 2021, at 9:51 AM, Les Newell <les.new...@fastmail.co.uk> wrote: > >> > >> Hi Matthew, > >> > >> I'd recommend setting this up as a stepper system and ignore the encoder > >> feedback for now. The feedback loop inside the drive > is likely to fight with the PID loop inside LinuxCNC. The encoders can be > used for position feedback but that's not essential and is just > making it harder to figure out what is going on. Later once you have the > machine running you can come back to this. > >> > >> You can use Halscope and monitor the axis commanded position and encoder > >> position to assist setting the PID values within the > drive. > >> > >> Alternatively if the drives can accept PWM torque commands I'd do that and > >> have LinuxCNC doing all of the PID work. > >> > >>> Extended jogs tend to result in the machine moving awhile after I release > >>> the button and ferror is all over the place > >> That's a symptom of the machine not being able to keep up with the > >> command. Either your speed or acceleration are too high. > Looking at your numbers I'd guess you are maxing out the speed. > >> > >>> What is a reasonable acceleration value for a Bridgeport BOSS5 (Series 1 > >>> type factory CNC machine) with 1kW 5Nm AC servos? > Are my accelerations too low? Raising them by orders of magnitude seemed to > have no effect. I freely admit I�ve been too lazy to > try to calculate it. > >> It depends on a huge number of variables. I generally work on trial and > >> error. 1kW is a bit overkill for a S1 BP so assuming your > gearing is correct the limiting factor is probably your ball screws. I'd feel > a bit uncomfortable going much over 250IPM. If you spin > the screws too fast you'll damage them. How fast will your motors be running > at that speed? Could you simply be running out of revs > on your motors? > >> > >> To give you a rough idea my Hurco, which is considerably bigger than a S1 > >> CNC, uses 750W servos and does 10m/min (~400IPM) > easily. From my ini file: > >> MAX_VELOCITY = 166.66666 > >> MAX_ACCELERATION = 750.0 > >> Note these are metric so divide by 25.4 to get inches. > >> > >> Looking at your ini file: > >>> MAX_VELOCITY = 8.33 > >>> MAX_ACCELERATION = 10.0 > >> Your velocity looks way out. That's 500IPM. While 1kW servos should be > >> able to run the machine this fast with the right gearing I > can't see it lasting very long. > >> > >> Les > >> > >> > >> On 28/06/2021 14:08, Matthew Herd wrote: > >>> Hi Les, > >>> > >>> The drives accept step/direction from the USC board to command position, > >>> not velocity. The encoders are pass-through to the > USC board and then to LinuxCNC. > >>> > >>> Thanks! > >>> Matt > >>> > >> > >> _______________________________________________ > >> Emc-users mailing list > >> Emc-users@lists.sourceforge.net > >> https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
