Hello Guest it is May 08, 2021, 07:19:58 AM

Author Topic: Mach4 positioning error  (Read 2376 times)

0 Members and 1 Guest are viewing this topic.

Re: Mach4 positioning error
« Reply #10 on: December 12, 2019, 06:55:49 PM »
what Russ is proposing is that open collector outputs from your BoB will result in 'missed' pulses by the drive, given that the drive is optimized
for differential inputs.

This would be detected by the same test as I proposed earlier.

If you command a move of 10mm and x number of steps were missed by virtue of open collector signaling, then surely a command of 20mm would
miss twice as many steps.

What I'm trying to establish is the error proportional to the distance moved or the number of moves.

Pactual=a x Pcommanded + d

In an ideal world a would equal one and d would be zero. What we are trying to do is find the actual and existing inaccuracies described by the two
parameters a and d.

If Russ's proposal is correct then a will be close to but not exactly equal to one. If my contention is correct, ie zero window error, or I suppose some
type of backlash compensation error, then d will be non zero.

My wife left with my best friend...
     and I miss him!
Re: Mach4 positioning error
« Reply #11 on: December 13, 2019, 04:20:49 AM »
Hi again,

Yes, i will perform the following test today;

gonna send 1 turn pulse from Mach4 to one of the servo drives and check the pulse reaches the driver, if it displays 10000(4x2500) it means there is something wrong with the lead of the balscrew(its not actually 25mm, its something like 24,89..), but if the pulse number is below 10000 then it means there is a loss in pulse signal.

Personally i dont think Mach4 or ESS causing this, since the error is persistent in every axis, its always short and its not adding up with the distance i am leaning more towards the differential thingy side. We will see..


Re: Mach4 positioning error
« Reply #12 on: December 13, 2019, 07:22:05 AM »
Hi Overloaded,
The differential line driver u connected was something like this? and how did u connect it to pulse and direction signals? how is the wiring? i never used one, if all other fixing attempts fail i am definitely gonna try it, thanks.


These weren't available when I needed them so I made up my own. Otherwise, I would have used these. Here is more info from the CNC4PC website for that item.

Offline RICH

  • *
  •  7,419 7,419
    • View Profile
Re: Mach4 positioning error
« Reply #13 on: December 13, 2019, 08:52:51 AM »
Interesting thread and I'll post some info later for a reality check.

Re: Mach4 positioning error
« Reply #14 on: December 13, 2019, 11:17:04 AM »

none of us was right.

I sent the necessary pulse signal for 1 full turn of the ballscrew to every axis which has servo installed, checked the settings and made measurements with precision 1micron marcator and tesa indicators;

drivers received exactly 10000pulses(4x2500) for 1 full turn, received and actual encoder pulses are same(10000), so nothing wrong with the pulse signals or signal generators,

measured the 25mm(X axis), 10mm(Y axis) and 5mm(Zth axis) ballscrew pitches, they are exactly 25.001mm, 10mm and 5.001mm, i assume 1micron is the measuring error of the dial indicator, so nothing wrong with the ballscrews,

before measured backlash is still there and its not the problem,

i made many measurements again with +1mm, +2mm, +3mm, +4mm, +5mm, +10mm and +25mm command set, it is still repeating that odd positioning error in most of the time, but in couple times it made the exact positioning within 1-2microns for the whole 1 set in every axis.

without a hope i tried to adjust the count per unit value with the wizard and of course it didnt fix anything.

i still dont know why i am reading this positioning error in the dial indicators, when every axis commanded to go 500mm and return back to 0 all of them returns back the exact point within couple of microns.

before this error presented itself i was making a test cut which has couple of circles, tilted square and rectangles on it, weird thing is this; for the rectangle(35mmx25mm) machine made it within 20microns, it was 35.02 in the X and 25.01 in the Y direction. But according to the measurements i am reading it should have done that cut with 50-60 micron error. The rectangle and squares was ok but the circles were not true circles, they were reflecting the errors i am measuring and +60micron wrong.

I am gonna make the same test cut again, after tuning the servos and aligning the ballscrews within 10micron to guideways.

I wonder what u guys read on the dial indicator when u move one of the axis of your machines +1mm for example?



Re: Mach4 positioning error
« Reply #15 on: December 13, 2019, 01:23:53 PM »
hmmmm..... another possible culprit could be the mounting and condition of the screws, endblock bearings and shaft couplings.
Can you run your indicator test at 2 vastly different speeds to see if there is a variation ? This might show if there is a "mechanical/inertial" aspect to the problem.
Re: Mach4 positioning error
« Reply #16 on: December 13, 2019, 01:52:03 PM »
i checked all mechanical aspects for a week everything is fixed as it should be, screws are new, couplings are german made R+W, every screw tightened with torque wrench to the specified torque values. End bearings are korean and they are also new.

I already tried different speeds, speed difference had no effect on readings, 6000mm/min and 50mm/min gives same results. Inertia was the first thing came to my mind because X axis is relatively heavy(400-500kg)and there were jitters on some side cuts in the test cut so i tuned the X axis servo and they are all gone now axis moves very smooth in all speeds also i am checking both encoder displays(commanded pulse and actual encoder feedback) they are identical in every command. While driving the heaviest axis motor only use 1-2Amp, there is no noise, no heat, no sign of overload.

Also, while dial indicator touching the axis i push the axis back and forth and can see the position value on servo driver display, it is very rigid and when i push hard it fixes the error immediately within 1-2microns. While doing that i am not even reading any backlash.

By the way thank u guys for helping out, if noone else getting this kind of odd readings clearly i am missing something thats for sure.


Re: Mach4 positioning error
« Reply #17 on: December 13, 2019, 07:36:33 PM »
Since you are dealing in microns, take a closer look at the apparent angle of the indicator axis in relation to the slide rails. To bet accurate indications, these must be precisely parallel in both perpendicular planes. As illustrated, a 3 deg misalignment will result in an untrue reading.
If ALL else is perfect ......... this might be lying to you.
Could be a photo anomaly, but it doesn't look to be in line from here.

If angled, it would likely read LONGER than commanded tho .... not shorter ? ? ? ?
« Last Edit: December 13, 2019, 07:42:33 PM by Overloaded »
Re: Mach4 positioning error
« Reply #18 on: December 13, 2019, 07:40:48 PM »
God I love this stuff:) ;D
Re: Mach4 positioning error
« Reply #19 on: December 13, 2019, 07:45:00 PM »
God I love this stuff:) ;D
Are you serious ? ?  or just pokin fun   :D
Any ideas  :)