Can Scales be used to close the loop?

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20 Oct 2011 22:21 #14081 by DroopG96
I am having a hard time finding out what is needed to have real time position control of a machine like the ones I operate at work. Is it possible to have emc2 maintain a machines position based on real time data from a scale?

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20 Oct 2011 23:47 #14082 by BigJohnT
Servo motors with encoder or resolver ( which needs converting to encoder signals) feedback is the norm for closed loop everything else is way out is left field and becomes a time sink to try and implement and use.

John

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21 Oct 2011 00:45 #14083 by andypugh
DroopG96 wrote:

Is it possible to have emc2 maintain a machines position based on real time data from a scale?

Yes, but you need a very fine scale, or some other feedback to get a tight enough loop.
Work on the basis of at least 4 scale-counts of dither.

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22 Oct 2011 18:44 #14106 by doug6949
Years ago it was common to see linear scales on CNC machines. Motor mounted encoders became popular because they are cheaper and have fewer instability issues from backlash. High resolution rotary encoders with modern AC drives also eliminate the need for tachometers.

Linear scales eventually became a high dollar option to improve accuracy. They worked together with rotary encoders.

Nowadays most manufacturers prefer to use temperature controlled ball screws with mapping algorithms instead of linear scales. It is simpler and more reliable.

Like Andy said, you will need high resolution scales if you intend to use them without also having rotary encoders. You will also need tachometers (if using DC drives), and very little backlash in your screws if you hope to avoid instability issues.

Linear scales can be a real pain.

Doug

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21 Nov 2011 22:54 #15000 by DroopG96
Thanks for the help! I keep reading about a board called a MESA for a true closed loop system. The Cnc Fusion kits ballscrews are only good for .003" per foot but I am told the software can tighten that up.

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21 Nov 2011 23:05 #15002 by BigJohnT
You can map the screw with a comp file as shown here:

www.linuxcnc.org/docview/html/config_ini...b:%5BAXIS%5D-section

You do need a very low backlash system to be able to get tight servo control.

John

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21 Nov 2011 23:48 #15008 by andypugh
DroopG96 wrote:

Thanks for the help! I keep reading about a board called a MESA for a true closed loop system.

There are other options as well as Mesa, The Pico cards are well supported too. I see that Jon just posted a nice summary in the Pico section of the forum.

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22 Nov 2011 05:12 #15012 by jmelson
DroopG96 wrote:

I am having a hard time finding out what is needed to have real time position control of a machine like the ones I operate at work. Is it possible to have emc2 maintain a machines position based on real time data from a scale?

There are two areas of concern. First, if there is even a little backlash in the ballscrew, then the motor will be
constantly banging back and forth to try to control position of the table. You'd really want the backlash to be well
less than .001" or it will drive you crazy.

The second is the resolution. My Bridgeport conversion with shaft encoders has 25,000 counts/inch on
X and Y, and 50,000 on Z. This is good, and can be done with relatively inexpensive encoders. This
machine has tachometers for velocity feedback to the velocity servo amplifiers, so it has quite smooth
motion.

My minimill has 128,000 counts/inch (due to belt reduction and fine-pitch leadscrews) and no tachometer.
it gets smooth motion due to the very high encoder resolution.

Typical linear scales for DRO use often have 10 um resolution, or about .0004" or 2540 counts/inch.
While this will work for CNC use, it is fairly coarse. An old rule of thumb was you wanted the encoder to
have 10X the displayed resolution. If the readout is .0001", then 10X that is 100,000 counts/inch.
I think you will find 2 um direct-reading linear scales start getting quite expensive. There are also
interpolating scales that have a lower physical resolution and use electronic interpolation to
increase resolution. If you select one of these for CNC motion control, you need to make sure there
is not excessive lag in the interpolation unit. A lag of even a couple milliseconds will make it totally
unsuitable for CNC.

Jon

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22 Nov 2011 05:18 #15013 by jmelson
DroopG96 wrote:

I am having a hard time finding out what is needed to have real time position control of a machine like the ones I operate at work. Is it possible to have emc2 maintain a machines position based on real time data from a scale?

Hmmm, perhaps we have been so focused on linear scales, we are only answering a part of your question.

So, there are several vendors of hardware (like Mesa and Pico Systems) that can interface position sensors
to EMC2. You generally need hardware to do this, as the encoder resolution * feedrate will likely overwhelm
software reading of the encoder signal. Of course, reading the position from an encoder is only part of the
job, you also have to interface movement commands to some sort of motor drive. Depending on your motors
and drives, that guides the choice of the interface board.

Jon

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03 Dec 2011 05:43 #15317 by garymcrobertpdx
Hi
I also have considered using linear scales for a feedback source but I have yet to try it.
one source of scales is DRO-PROS www.dropros.com/ TTL quadrature outputs
and up to 1 micron res for around $200 - $300 a pop depending on length.
You will have to write some HAL code to make this work.

I did some thing roughly like this when I rigged my mill spindle with a DC servo and rerouted
the spindle encoder in the HAL code for feedback to position the spindle rotation to allow an
automatic tool change.

Lead screw lash is a problem but if you use ball screws this will make a big difference
I am using ball screws from these guys roton.com/ and they work especially well
for the price.

I made my own ball nut assembly by rigging two ball nuts back to back in such a way that
I can pre load them which reduced the lash to the point that my 1/10th reading dial indicator
is unable to detect it. Im sure this will loosen up after many hours of use but I will readjust
the ball nut again.

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