Can Scales be used to close the loop?

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?

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

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.

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

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

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.