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Author Topic: Calibrating each axis?  (Read 3538 times)

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Calibrating each axis?
« on: May 06, 2008, 11:48:39 PM »
I have all three axis so that when you move 1 inch in Mach 3, a dial indicator shows 1 inch of travel within .0005". BUT what I don't understand is if I tell it to move .1" it ends up about .007" short of that .1" and then I tell in to move another .1" it picks up a little of that distance lost and is about .006" short of .2". I keep going by .1" increments and by the time it gets to 1" of travel I am back to dead on 1" of movement on the dial indicator. What do I need to do to make the movements under 1 inch correct also.

Offline Chip

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Re: Calibrating each axis?
« Reply #1 on: May 07, 2008, 01:46:16 AM »
Hi, Archerks

Is this when moving in one direction or both,

If It's in one direction, It's a backlash issue, Lost motion in your drive train screw/nut.

If your step per is less than 1000 steps " .0001 per step", Then Mach gets as close as it can in steps and holds the partial step/move value to be applied on to the next move and so on till it gets to your 1 inch.

What are the Step's Per values in Motor Tuning for your axises.

More info needed, Chip
Re: Calibrating each axis?
« Reply #2 on: May 07, 2008, 02:17:25 AM »
The steps started out at 10,160 but have been fine tuned for a few more than that for an inch of travel. I am using precision
ball screws and zero backlash ball nuts. This testing was mainly done on the X axis although Y has similar results.
The measurements are the same going in either the + or - direction.

Offline jimpinder

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Re: Calibrating each axis?
« Reply #3 on: May 07, 2008, 04:56:45 AM »
In your 10,000 "steps" are you including micro stepping by the driver - maybe 8 or 10 or even 16 microsteps. Stepper motors do not stop accurately on microsteps.

However to echo Chip - this is backlash.

Your problem is in your fine tuning. You have tuned this problem into the system.

Back lash is the non-movement of the axis - and from what you say appears to be about 7 thou on your system. That 7 thou is a constant on any change of direction - and should be taken up before the axis moves.

There should be no need to "fine tune" an axis drive - it is a mathematical formula - x number of steps per rev of motor - y number of revs per 1 inch of movement - answer x times y. (Mine is 60,000 - if you are using a metric screw for 1 inch of movement the result is not as neat - but is still calculable)

What you have done is fine tune out the backlash and shared it along 1 inch of movement, so you are actually moving maybe 0.1007 for every 0.1 of an inch. Move 10 times for the inch and this gets rid of your backlash. However - for the first tenth, there is still a full 7 thou of backlash -maybe nearer 8- and you have only compenstated for 0.7 of a thou. On the second tenth you have compensated a bit more, but you are still short - and so on one up to 1 inch where you are accurate.

What you must do is calculate your steps per inch - and put this in your motor tuning.
Check then to see if there is any backlash. Move the axis to the right, Stop, Setup your dial test indicators and Zero them. Move to the right a further inch using a G0 X1 command on the MDI line and then move back using a G0X0 command. Check your DTI and the result is the backlash - that distance the axis doesn't move as the gears etc, settle in to push the other way. (You dont have to actually move as far as an inch - backlash is not dependent on distance - half and inch or quarter would do - as long as the movement is equal.)

7 thou actually seems pretty good - I would hate to tell you what mine is - and you may be able to adjust it out mechanically - but despite what everybody says, there will still be some. If everything was so accurate - nothing would move - it might, however, be so small as not to be noticeable or measurable.

Enter this amount in the backlash table and turn the backlash on. You should then get accuracy all along the range, because the backlash is taken up first by Mach3.






« Last Edit: May 07, 2008, 04:59:52 AM by jimpinder »
Not me driving the engine - I'm better looking.
Re: Calibrating each axis?
« Reply #4 on: May 07, 2008, 09:26:38 AM »
Thanks for the come backs....
I believe I am calculating the steps correctly....
200 steps on motor
 10 driver micro steps
=2000 total steps
/ 5  5mm ball screw
=400 steps to go 1 mm
*25.4 convert mm to inch
=10160 steps to move 1 inch

What I wasn't doing, is tuning the axis right. I was tunning the backlash into the 1" test movement. I was moving right, then moving left, then moving right, so I was putting the backlash into every move.
I played with it last night and with backlash enabled, using .006 as the setting I was able to do the 1 inch movements and .1 inch moves accurately. Haven't tried to cut a circle yet to see what happens. It seems to have a lag when Mach3 is adjusting the backlash. I read about changing the backlash speed per cent and shuttle accel, so I will try those settings tonight.

Thanks,
Dave

Offline jimpinder

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Re: Calibrating each axis?
« Reply #5 on: May 07, 2008, 09:51:15 AM »
Yes - you have calculated your steps per inch correctly - that does NOT now need fine tuning. What you need to do is TURN OFF backlash compensation. Move to the right.Stop. Set up your DTI on zero. Move 1 inch to the right on an MDI command (to ensure that Mach 3 puts out the correct number of pulses). Move BACK one inch on an MDI command.

In theory your DTI should read 0 - but I don't think it will. Whatever your DTI reads is the backlash you have in your system.

Enter this in the backlash and switch it back on.

Do not expect Mach3 to apply backlash so you don't notice it. It starts, stops, starts again, stops and then finally gets going. You are quite right, you can adjust the speed at which it does it, but I wouldn't put it up too fast - maybe 50% if your top speed is not too fast. It does not matter in the scheme of things - what you are after is accuracy - not a ballet. Backlash is accurate - mine is about 56 thou - (I,m working on it - maybe these super dooper ball screws are called for) - and in the end my system gets there. I did some ball shapes on a rod the other day (in brass) and you couldn't see the join.
« Last Edit: May 07, 2008, 09:57:21 AM by jimpinder »
Not me driving the engine - I'm better looking.