Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: professorburnout on March 09, 2012, 09:16:22 AM
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Hello,
I have an encoder attached to a stepper my plan is to use it to determine if the stepper loses steps. But I am having a problem. The encoder and stepper DROs match when moving in one direction but when the machine returns back to where it started the DROs do not match. For example Starting from 0 and moving to 1” the DRO will match after the move but when the machine is returned to 0 the DROs will no longer match. This is also true if the move is in the negative direction 0 to -1" and back to 0.
The calibration per unit is about 12000 for the encoder. Could the encoder be to high of a resolution (but then why would this happen in only the return)?
Any thoughts on the problem.
Thanks,
Paul
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Backlash
(;-) TP
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+1
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The encoder is connected directly to the back of the motor with a helical beam coupler.
This setup up is on the Z axis where the spindle motor and assembly (greater than 300 pounds) on the ball screw. When checking with a indicator the Z axis is returning to the same location.
The problem is interesting in that it is not accurate on the second move and is not dependent when moving in the positive or negative direction.
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Is that 12000 pulses/1"? also what is the resolution of the motor in revolutions/inch?
If it were backlash I would expect one direction error only?
Nosmo.
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The encoder resolution is 12000/inch the motor is about 5200/inch. The reason I am thinking it is not related to backlash is because the the encoder is directly coupled to the motor and I am comparing the Z-axis dro value to the encoder dro value. Also, When I test moving in the positive and negative direction for the first movement the error is in the second direction.
Will be doing more testing this weekend.
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How much error are we talking ?
(;-) TP
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What I was getting at is how many motor revolutions to the inch?
Ok I see it would be around 2.3?
N.
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I think what he is alluding to IS the encoder is attached to the stepper motor shaft. AND the encoder should read what the DRO says IF the two are properly calibrated and IF the steps per rev indicate close enough to be repeatable.
(;-) TP
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I have found through testing that the DRO is more reliable than most indicating devises. The DRO is relative to distance. Actualy if you want to be anal about it one should count the pulses sent by the computer to the drive, from the drive to the motor, and if you have a BOB the in and out of that device should also be counted. In our testing they were all compared via a calibrated multi channel counter accurate to 20 parts per million. So all the measuring done was apples to apples.
A stepper is a eltro-mechanical device and as you know they are all not alike in how they react to a pulse when you get down to the single pulse senerio. So you could for instance find that it it will take more pulses to move it a small distance and it will behave dfferently going the other way. Just test jogging at 0.0001" increments. At 5200 the resolution is
0.00019".
If one adds distance to pulses then the variables associated with the distance must also be callibrated to a known standard. Note that one must be able to decipher what is
contributing to any distance inaccuracy measured and that could be due to a number of things. ie; Backlash can be due to a number of different causes and it can be difficult to narrow it down.
In the end, my point is, one must know exactly what they are measuring and if comparing know exactly what they are comparing and to what degree of accuracy they can compare.
FWIW,
RICH
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THis could also be a lost steps senario IF the only thing you are looking at is the DROS. Just because Mach3 puts out the pulses does not MEAN that the motor actually follow it perfectly all the time. In this case IF the motor lost steps the Axis Dro would say one thing BUT the encoder would say another.
Just a thought, (;-) TP
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Hi Rich,
I built a system that works great using Mach 3 and steppers.
I want to put encoders on this system and have no idea how to do this.
What can you tell me?
Please let me know,
Thanks,
Brian
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Brian,
What do you want to do accomplish by adding the encoders?
RICH
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Well, the machine is pretty darn accurate right now. Cutting circles though I think can be just a bit better, but
let's just say you crash and the drives go into a fault or protection shutdown. I'd like to be able to just ref zero and start up again.
It only happens once in a while but it does happen; or let's say I'm free machining a block and I keep resetting the 3 axis's; after I'm done I would like to just hit ref zero and then be able to punch in my left hand corner vise coordinates and just start machining instead of having more time wasted by edge finding the corner again.
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What you want is home switches, not encoders....
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I take it you are trying to replicate the ABS encoders found on moderm machine tools on a MACH3 budget ??
Install encoders route them to Mach3 DROs,
Calibrate the encoder to machine travel
When you Refhome the machine to Machine 0,0 then zero the encoder DROs as well.
IF you munch the machine and want to get back to zero do a compare of where you are to the Encoder DROs then calculate how far to move to get back to encoder 0,0 then move and setmachine Zero to 0,0.
THat is the simple version,
Home switches would be simpler(;-)
Personelly I would study and practice more with Gcode and the CNC so you did NOT munch the machine as often.
(;-) TP
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ALSO I believe the Rogers Machine encoder board WITH their plugin is what you are looking for. If my memory serves me their setup has a auto correction function built in.
http://romaxxcnc.com/encoderinterface.html
(;-) TP
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Well as you put it "munching" the machine only happens once in a great while and frankly it's all been due to a bug in Mastercam 9 that we are trying to figure out.
As far as home switches are concerned, I have them and they work great, but what I want is something that will give feedback to the machine while it's cutting to make sure it's in the right place.
The machine I built out of a 1965 wells index is holding a tolerance of +/-.0001-2 with no problem. I just think it skips a beat once and a while.
I take it you are trying to replicate the ABS encoders found on moderm machine tools on a MACH3 budget ??
Install encoders route them to Mach3 DROs,
Calibrate the encoder to machine travel
When you Refhome the machine to Machine 0,0 then zero the encoder DROs as well.
IF you munch the machine and want to get back to zero do a compare of where you are to the Encoder DROs then calculate how far to move to get back to encoder 0,0 then move and setmachine Zero to 0,0.
THat is the simple version,
Home switches would be simpler(;-)
Personelly I would study and practice more with Gcode and the CNC so you did NOT munch the machine as often.
(;-) TP
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Interesting, thanks.
ALSO I believe the Rogers Machine encoder board WITH their plugin is what you are looking for. If my memory serves me their setup has a auto correction function built in.
http://romaxxcnc.com/encoderinterface.html
(;-) TP
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I have them - but, if you ref zero and then indicate the corner with an dial indicator - then ref zero and indicate it again over and over or after parts are cut it moves just a hair......0001-.0002'
I cut production parts all day long with this and it works great, I just want something that is 100% reliable, not 99.9% like I have now.
I cut parts that are .03 x .025 x .06 with tabs on them that are .008 x .015 x .015
If you're off by .001 you'll see it.
What you want is home switches, not encoders....
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Just want to put this discussion into perspective.
There is part tolerance and then there is a some reference point tolerance.
You currently say the reference point can be found at a repeatable tolerance of 0.0001 to 0.0002" and you desire a
"last word " or confirmation of that. So if using the encoder,12000 pulses, and the resolution would be
0.000083". So if you had a dro which displayed .000100 difference based on a move and another that displayed .000083 there would be a difference of 0.000017" just based on resolution. I won't even get into the +- of the resolution and practicality for talking purposes.
So from an absolute value which one is correct? Answer is neither.
One needs a standard to relate to. What is the standard you want to use?
The only traceable standard I have for reference in the micro range ( $1250 for a calibration and i think around $3500 new and calibrated) glass standard. You need to view using a 1000x microscope. It's accurate to .000003" @ some defined calibration conditions.
So one may touch off say a 100 times, and then using some displayed value compared to the traceable standard, figure out just how how accurately / what tolerance / repeatablity they have in touching off. Unless you just want to use the encoder as your "standard" which really dosen't have a absolute defined basis and is rather meaningless when compared yet to another non defined value.
Nice stuff to play with on a rainy day and nothing to do......... ;)
RICH