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dachopper
03-06-2017, 01:04 PM
Hello guys,

I've been trying to diagnose a skipped steps problem, that I seem to be having part way through operations, after around 20 - 30 minutes of machining.
The X axis will randomly jam for 3-4 seconds, performing an identical cut that it just completed 70-80 times successfully, and then resume moving partway through the cut, now displaced be a few cm's.

I thought maybe it was caused by a lack of torque, and now I am wondering if it is the case, due to motor overheating causing a loss in torque?

Aside from reducing the peak current from the driver, what other things reduce heating?

Reduce acceleration? or speed?


Nick

Neale
03-06-2017, 06:35 PM
Not sure that speed and acceleration make much difference to heating with stepper motors. Might be worth seeing if your drivers have a "half current" option for when the motor is stationary, though, as this can help reduce heating. I don't think that heat effects in the motor would cause it to suddenly stop and start either. Might be driver overheating and faulting, though? Does your machine monitor fault output from the drivers? What drivers/motors/supply voltage are you using? Any obvious signs of binding/mechanical stickiness (motor trying to move but just making noises)?

Last time I saw this effect was on my 3D printer and in that case it was excessive motor current causing the drivers to overheat and go into shutdown until they cooled and started up again - having lost sync.

dachopper
04-06-2017, 04:22 PM
Hi,
Am882 drivers , 68 Volts, KL23H2100-35-4B motors.

I had peak current set to 4.5 then 4.9 Amps, but now I've lowered it to 3.5 Amps for the next cut. I emailed the company to try and find out if 3.5 amps was RMS or Peak, and they said try both and see if the stepper gets hot. At 3.5 Amps it was cold, so I increased to 4.9 assuming the rating was 3.5 RMS..... but the motors got bloody hot to touch ( as in almost could not hold hand on there for more than 3-4 seconds ) !, so I have backed them back to 3.5 for now.

If the motor stalls at low speed ( when reversing direction for example where it is fighting the machine inertia + cut resistance, ) then at low speed no fault is displayed by the AM882, and the motor will pickup position once it decelerates and commands a reverse, or after a couple seconds of being stalled, it will just start moving again and continue to the new incorrect position.

The Motor coupling is definitely not slipping, and there is no binding, so for some reason at times the motor torque required, is too much for the torque output sometimes, and the only factor I could think of is that over time the stepper gets hot, and that in turn reduces torque. So therefore decreasing speed will decrease cut resistance, or decreasing acceleration will decrease the torque required to overcome inertia in a given time period.

Neale
04-06-2017, 06:18 PM
General view seems to be that motor case temp of about 100C is about the limit, and that's based on manufacturer recommendation that wire insulation temp should not exceed 130C and there's about 30C difference between wire and case temps. Manufacturers do not seem worried about magnetism. My own steppers run at about 60-65C (based on the "just to hot to keep hand on" test) and that seems to be pretty normal.

I have two motors driving X and the only stalling has been when I've had too-rapid "rapids" configured - max X speed is at about the corner speed for my steppers so they are starting to lose torque and tend to stall. The driver concerned tripped with a stall fault. Low-speed stall is clearly something else. What kind of moving mass are you talking about, and what acceleration settings? Odd that it only happens after running happily for a while, though.

dachopper
04-06-2017, 06:38 PM
I only have 1 motor driving the X axis , I would estimate the weight to be around the 30kg mark. My max speed with low acceleration was around 18,000, so I set 8000, which is about 70% torque figure.
acceleration settings around 1500. It takes about 1 to 1.5 secs to stop from rapid speed max acceleration is about 3000-4000 at low speeds, and it will stall on reversing ( or on accel / decel ). I've not seen evidence it stalls mid move at constant speed ( I'm doing 3D profiling so there are other speed csonstraints beside my X max speed

dachopper
04-06-2017, 07:21 PM
My specs below...

If I reduce the microstepping amount, say from 3200 steps/rev to 1600 or 800 steps/rev, that will increase resolution, but will it provide any greater torque using the AM882?

100 RPM 190 N cm 1000mm/min
300 RPM 174 N cm 91% 3000mm/min
500 RPM 150 N cm 80% 5000mm/min
700 RPM 120 N cm 63% 7000mm/min
750 RPM 105 N cm 55% 7500mm/min
800 RPM 85 N cm 45% 8000mm/min

RAPIDS / ACCEL SETTINGS
X 8000, Accel 1200, single motor ~30 kgs....... 10mm pitch
Y 7500, Accel 1000, dual motor ~ 60-70kgs..... 10mm pitch
Z 3500, Accel 1200, single motor ~ 15-20 kgs.. 5mm pitch

Bottom Chart is my motor
21842

Neale
04-06-2017, 11:21 PM
I'm not an expert in this area, and I only have limited experience based on my own machine, so please take that into account in whatever I say here! However, I have spent some time kicking around figures for my machine, so these numbers aren't just plucked out of the air.

First - microstepping. You currently have 10/3200 mm = 3um resolution. I'm using 5mm pitch with 1600 usteps, giving the same resolution. This is way better resolution than is needed (for woodworking, at least) and probably better than the backlash in your ballscrews, unless you are using rather better quality screws than most of us! I use x8 microstepping as a balance between torque and smoothness; it seems to be generally accepted that this is a good target figure as higher microstep ratios (such as you are using) cause torque to fall off; lower ratios give a rougher drive. I once played with a machine with no microstepping configured and it sounded like marbles in a cement mixer. I reconfigured to x8 and it was much, much better. I suspect that if you go to 1600 ustep/rev you might get better torque and so less chance of stalling.

My gantry weighs, at a guess (never weighed it, but I know what it felt like lifting it on and off while building the machine, and then it has the Z axis assembly and spindle motor added) around 60kg. That is driven by two 3Nm NEMA 23 steppers so roughly speaking each motor is driving about the same mass as your motor. You appear to be using nominal 1.9Nm motors, so they are a bit down in torque compared with mine. At the same time, you are running at about twice the speed (my rapid max is 4500mm/min as I get stalling problems above this) so have more drag from guide rails (although I realise that your problems do not happen at higher speeds so this is probably not too relevant) but also using around twice the acceleration figure. My machine runs with acceleration set to 600 (whatever units that is in). According to the little graph in the Mach3 config page, that gives 0-full speed in about 0.15s. You are trying to accelerate at twice this rate, with higher pitch ballscrews which means twice the torque required, with lower torque motors. Someone might well argue that my machine is conservatively configured and they might be right, but I do a fair bit of fine 3D machining and I worry as much as anything about things like pulley-ballscrew fixings handling this amount of torque with a lot of axis reversals and coming loose.

Your other axis (we have different conventions re X and Y!) has similar loads, but lower speed and acceleration. Is there a reason for this? Based on testing, gut feel, or inspired guesswork? (I used the last of these with a bit of testing to confirm, BTW...)

Given that this is an intermittent problem, that seems to come after a period of running, and tends to happen at low rather than high speeds (which is actually the higher average current situation for the steppers) I would still be chasing something like thermal shutdown in the driver, unlikely though this seems. Other possibilities are noise (but why only after a significant period of running and only one axis?) and something odd on the Mach3 PC causing the pulse stream to glitch (but again, why one axis and only after running for a time?)

Have you been able to try swapping drivers from one axis to the other and see if the problem follows the driver or not?

I suspect that you have already gone through much of this thinking, and I put it forward less as a probable diagnosis and more in the hope that as you say to yourself, "this guy's talking nonsense - but maybe if I check..." you might get a blinding flash of inspiration! I hope so - good luck with it.

m_c
05-06-2017, 12:02 AM
Changing the micro-stepping setting won't have any practical effect on torque at the motor when moving.

The Leadshine website isn't working for me just now, but don't AM drives have stall detection?
Do you have the error outputs connected, so you know if a drive has faulted?

dachopper
05-06-2017, 04:43 AM
Yeah they have stall detection, but it only operates when / if my motors are above 3000 mm/min. And due to my cut rate being 5000, then I can summize, that the error must be occuring at lower speed prior to stall detect kicking in, IE on decel / acceleration.


I came up with my numbers by trial and error, finding the max unloaded speed with low acceleration, then the max acceleration to low speed with lots of reversals. , and setting a % of this, like 50 as the max machine settings.

dachopper
05-06-2017, 07:45 PM
Did some more testing to check the acceleration settings - Checks performed at 8000 mm/min
Y axis failed at 1100 mm/s/s Now set at 550
X axis failed at 1600 mm/s/s Now set at 800. ( this is the axis that jammed, but was also doing most of the work at the time )




Servo case temp - could touch case for 5/6 seconds, and my turkey cooking probe was at 52 degrees C

Neale
05-06-2017, 08:08 PM
That case temp sounds fine to me. Out of the four steppers on my router, three spent the best part of five years driving my previous machine, usually at or around that sort of case temp. They are all working fine still!