Hi Mathew, I found with the knee that best I could do with a 1200 oz-in stepper on a 56VDC power supply and a Gecko driver was 24 ipm with 3:1 driving the right angle shaft, through a bevel gear into the 1" ACME screw with 4 TPI.
I swapped in a 750W Bergerda with 3.2:1 and now I get 150 ipm and that's scary fast for the knee. And I did upgrade the radial ball bearings and bushings for the horizontal shaft to angular contact bearings. The noise now all comes from the angular contact. To figure out what I needed I used the torque arm approach. Hang an arm on the end of the shaft and then add weights X distance from the shaft centerline and see what it takes to move. The 600 oz-in stepper I started with was adequate but torque fell off too much with velocity. The reason I mention all that is although the absolute power might be the same for the stepper and the servo the servos tend to have far less continuous torque but have impressive peak torque values for overcoming say static friction. After that the lower value kinetic friction and moving the load of course, would bring you back into the continuous torque value. So where you are lucky if you get 1000 RPM with a Stepper of a particular power the Servo can do 3x that but with lower torque. At the higher RPM the HP is in effect the same. Ie. Lower RPM, more torque, Higher RPM with less Torque ends up the same power. So I'd keep an eye on the following error from the drive to make sure that you aren't seeing issues due to lack of torque to spin the assembly up to speed. Does your servo drive have a display? Can it show power consumption? John > -----Original Message----- > From: Matthew Herd [mailto:herd.m...@gmail.com] > Sent: June-28-21 2:32 PM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] Advice on Tuning Servo System > > Hi John and Les, > > Thank you for the input. I wasn�t sure how to tune the drives without the > PID. P=0 & FF1=0 will yield a commanded velocity > calculated purely on the commanded velocity and maximum acceleration, then? > If so, that�s easy enough and may actually be all I > need. > > John, I am certain I�ve sized the motors correctly. From all indications > they�re way over-powered. The machine came standard > with NEMA42 steppers and these servos are the same frame size (110mm bolt > circle diameter is a bolt on replacement). They do not > seem to lack for torque whatsoever. I�m fine with turning them down as long > as I get good results and high reliability. I�m hoping > to put the machine back to work as soon as I can get the bugs worked out. I > did some research on the critical speeds and I�m pretty > sure I�m more than safe at anything under 400 inches per minute. I don�t see > any need to push it even that high. I will probably > turn down the Z axis to 150IPM as there are very few use cases where I need > to go even that high. The quill likely won�t tolerate > extreme speeds and the travel is only 5" so I�m able to cover that distance > very quickly regardless of max speed. > > Matt > > > On Jun 28, 2021, at 12:47 PM, John Dammeyer <jo...@autoartisans.com> wrote: > > > > 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 > > > > _______________________________________________ > 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
