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Author Topic: Small Values in DRO's  (Read 2520 times)

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Offline SMA

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Small Values in DRO's
« on: April 22, 2012, 01:49:44 PM »
Hello All,

I find with MACH that it is not unusual to have small values let in the DRO's.  What has prompted me to post this is that it even occurs when entering a value into the MDI's. 

I enter G00 X100.00 and at end of move DRO's read X=100.001.  I understand this "extra" is very very small but just wondering why.  I have no machine attached in this case just "dry" testing MACH settings on my laptop.

So small values...just a relative of CnC life or something else?

S

Offline Hood

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Re: Small Values in DRO's
« Reply #1 on: April 22, 2012, 02:00:59 PM »
All depends on your steps per unit and where you are starting from. Mach will move as close as it can to the commanded point but if your steps per unit are very course then it may not be where you commanded exactly. Mach can only put out a min of a pulse so if your commanded position is not exactly X amount of full pulses away from where you are it is impossible to get exactly to that point, the finer the resolution (greater steps per unit) the closer Mach will get.
Hood

Offline BR549

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Re: Small Values in DRO's
« Reply #2 on: April 22, 2012, 02:22:30 PM »
But in his case MACH went OVER the commanded point and that is a nono.

(;-) TP

Offline Hood

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Re: Small Values in DRO's
« Reply #3 on: April 22, 2012, 02:25:14 PM »
As far as I am aware Mach will get closest to the commanded point.
Hood

Offline SMA

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Re: Small Values in DRO's
« Reply #4 on: April 22, 2012, 05:32:07 PM »
Ok well at least I had that thought for once and so I completely understand the answer.  I am wondering then what is consider high resolution.  As I understand it 200 is max but what is considered high...90 and more?

Offline Hood

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Re: Small Values in DRO's
« Reply #5 on: April 22, 2012, 05:47:48 PM »
It depends what your units are, ie mm or inch. If inch 200 is a poor resolution as it means you can only resolve to 0.005  inch assuming everything is accurate. If mm then 200 means you can resolve to 0.005mm which is 0.00019 inch which is not too bad.
 My machines are set up in metric and steps per unit are 1600 so the resolution theoretically is 0.000625mm or 0.0000246 inch.
If you need to have an accuracy of 0.001" then you would really need a resolution of 0.0001 to be sure.

The tradeoff on steppers is between resolution and velocity, have a high resolution and your velocity will suffer and vice versa. Devices like the SmoothStepper can help to a certain extent by allowing you to  have a high pulse per second  but steppers are still relatively slow motors (typically in the 1000rpm range) so you are still in that trade off place.
Servos differ in that the resolution is determined by the encoder count and with devices such as the SmoothStepper you are no longer limited by the kernel speed of the parallel port so you can have the best of both worlds.

As to what Terry was saying, to me it doesnt make any difference if Mach goes over or under to be closest. If you are over on one part it may be scrap, on another under may make it scrap so it all boils down to the previous point, if you need to resolve to a certain figure your steps per unit have to allow Mach to do that.

Hood

Offline SMA

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Re: Small Values in DRO's
« Reply #6 on: April 22, 2012, 05:57:20 PM »
Thanks.  I am only in need of .25mm so I am of course fine but I would like to be as accurate as the equipment allows.  Why not be?

I just posted another post with my motor settings but a different question.  Dilligently working through the manuals and tyring to get some help with questions that arise in the mind.

again thanks
S
Re: Small Values in DRO's
« Reply #7 on: April 24, 2012, 12:38:02 PM »
Mach3 (.060) is doing the same for me.  I can start a program from X0.000 Y0.000, run to completion with no problems, then with final program command to return to 'G00 X0.0000 Y0.0000', and both DROs stop at 0.0001.  Hitting my GoTo 0 button, and both X and Y motors will "click" and DROs return to 0.0000.

I have not reverted to previous versions, but I don't remember seeing this before on my machine.  Not a problem for me as I don't need the accuracy, but something does appear to have changed.  I need to use latest Development version for M10/M11 commands for laser diode control.

John Champlain
www.picengrave.com