Hiedenhain glass scales

[davesnd]

Just wondering if anyone out there has used glass scale feedback succesfully with camsoft and hiedenhain scales in particular. I have a machine with LS 404 scales and need to know if they are ttl differential output - anyone have a pinout on this?

[Al_The_Man]

I do not have info on the LS404, the LS405 & LS406 however, show 11µa sine wave output that many of Heidenhain scales are. Which without the conversion box, it cannot be input to the Galil card.
If they are the TTL 5v output it should be no problem.
You should be able to get confirmation from Heidenhain Canada, they are usually good at getting back to you.
Al.

[unterhaus]

I would assume any machine with sine wave encoders has the box somewhere. That part number sounds familiar. You can't really tell by the number though, there are very similar numbers that output totally different signals.

[Al_The_Man]

Originally Posted by unterhaus I would assume any machine with sine wave encoders has the box somewhere.

In some cases the scale goes directly into the Heidenhain DRO/CNC so there is no external box.
Al.

[CarbideBob]

Ignore this post if you wish, seems that most here think I'm nuts anyway.
My first question would be ---- Why?
Feedback from linear scales seems logical. In reality it is full of problems and the odds are you will end up with a system that performs worse than when you started.

If you are serious look at the Galil website for info on dual closed loop control.
Be aware that it is much more difficult than they make it look.

If you are well versed in digital control loops, Z-transforms and such be my guest and give it a try.
Otherwise my advice would be to not do this. It took me years to make this work well.

I'm sure others will have a different opinion.
Bob

[Karl_T]

Originally Posted by CarbideBob If you are serious look at the Galil website for info on dual closed loop control. Be aware that it is much more difficult than they make it look.

Could you expand on your experience? I'd like to add a scale and dual closed loop. I've read up on the two methods explained in the Galil literature.

Karl

[davesnd]

Just to be clear, this machine is not dual closed loop. The machine had a Heienhain TNC 155 control factory installed with glass scales as the only method of closing the position feedback loop. The glass scales plugged directly into the back of the controller - no external conversion boxes. I felt the simplest way (and likely cheapest way) to retrofit this machine should be to use this glass scale feedback as it should be quite accurate, free from any backlash due to ballscrew or timing belt wear. I pulled some more covers off the machine and discovered that the x axis is a LS 403 scale - the LS 404 is on the y axis. Mitutoyo makes a converter box to convert their sine wave scale signals into A/B quadrature - maybe heidenhain does too - I need to do more research.

[Al_The_Man]

You can get 1,2 & 3 axis conversion boxes from Heidenhain, the last one I bought some time ago was around $500.00 for a single.
They come up on ebay periodically.
If you are handy with electronics, you could make your own from an op amp and Schmidt Trigger.
Al.

[davesnd]

Heidenhain Canada has model EXE 100 converter selling for $769.00 each.
Since I need one per axis, $2300.00 seems like a lot of money to me - maybe it's just my scottish heritage but I am inclined to try something else.....

[Al_The_Man]

The Heidenhain manuals show an example of subsequent electronics which shows a few components and a RC4157 IC. it could be squared up and conditioned with a RS422 driver maybe.
Al.

[Karl_T]

Originally Posted by Al_The_Man The Heidenhain manuals show an example of subsequent electronics which shows a few components and a RC4157 IC. it could be squared up and conditioned with a RS422 driver maybe. Al.

I see digikey has this IC for 16 cents each. If you can get a copy of this manual print you should be able to save about $2275 out of the $2300 expense.

Karl

[CarbideBob]

Originally Posted by Karl_T Could you expand on your experience? I'd like to add a scale and dual closed loop. I've read up on the two methods explained in the Galil literature. Karl

Unfortunately I can't disclose some of the details in our dual loop system as we hold them as "trade secrets" and our competitors are still trying to figure out what we have done.

Most importantly do everything you can to remove any backlash in the system, otherwise you will have to "soften" the loop tuning up so badly that you'll wonder why you tried this method.

Another thing that helps is to locate the reader head as close as possible to the ballnut. This helps remove instabilities resulting from abbe error (twisting of the table).

Be warned that this will appear to be tuned ok with an unloaded axis but have problems when cutting. Be sure to apply a load by pushing on the table when setting up your tuning.
Also make sure you try a wide range of speeds when tuning. You may also encounter problems with high decell rates if you have very free slides as the screw transfers from tension to compression. We use a gain break which changes the tuning parameters at higher speeds and standstill.

Very close inertia matching of the motor and reflected load helps quite a bit. It's a lot of math but it's best to calculate the values for the PID loop before starting the tuning. If you don't have a copy I'd recommend Dr. Jacob Tal's "Step-by-Step Design of Motion Control Systems" (he's the founder of Galil).


On the original posters question on the sine wave scales it it very easy to reshape and change the sine wave to square waves with an op amp but you may not like the resolution that you get.
Hiedenhain and Mitutoyoboth both incorporate interpolation units which chop up the sine wave into little pieces to give the final resolution. This is why their boxes cost so much.
Bob