Absolute Positioning

[Cartierusm]

OK so I've battled repeat positioning and setup on my cnc for a while and I've a pretty good system down but I'm ready to take it to the next level, of which I have no idea. I'm currectly using a partial 80/20 and solid aluminum cnc that is prettty solid but I'm going to build a new one out of steel. I guess the issues I have to deal with are welding or epoxy and pins, what kind of steel (I hope just mild as it's the cheapest) and how to stree relieve the metal at home and then flatten the plane for a good surface to work off of if I weld up the frame (I tig and mig pretty good).

The main reason for this post is how to get absolute positioning like a huge Haas VMC for repeat setup after I turn off the machine and then start it up, currently my cnc during a run of course is very accurate. First question do they use home switches that are super accurate, I have a feeling not, they probably go by either encoders or glass scales. SO how do I get absolute posistioning and repeatable setup?

If I add glass scales or modify rotatry encoders to my STEPPER system how do I use that to accurately resetup after I turn off the machine then back on?

If I use a servo system do I need the glass scales to achieve what I just said above and be able to repeat my setup accurately?

Thanks.

[ger21]

If you join the Yahoo group CNC_Building_Blocks, there's a manual of a board that homes using a combination of switches and encoders. Here's a link.

http://f1.grp.yahoofs.com/v1/8EbpSM5...ccuStepv21.pdf

They no longer have a website offering them for sale, but I believe he is still selling them if you contact him directly. I don't recall the price. PM me and I can give you his email address if your interested.

[Mariss Freimanis]

A little bit of background:

1) Servos use rotary incremental encoders only because they cannot run without them. It's only there because the servomotor cannot be controlled (be stable) without it.

2) Step motors are open-loop positioners that can run without any encoders at all.

3) A servo system with a linear encoder is almost always a dual-loop servo. A servo still needs a rotary encoder on the motor to close the motor loop while the linear encoder closes the exterior position loop, thus dual-loop. The linear encoder removes leadscrew error and backlash error while the interior rotary encoder stabilizes the servomotor.

4) A step motor with only a linear encoder is the same as (3) except the step motor doesn't need the rotary encoder. It positions by itself just fine. It is still treated by the CNC program as if it were a dual-loop servo.

5) Dual-loop servos give the best accuracy. The linear encoder cancels all machine accuracy errors (leadscrew tolerance) and goes a long way to eliminate backlash. Backlash errors can never be fully eliminated or compensated for because they theoretically require the impossible from motors: Infinite speed between backlash travel limits.

6) No hobby CNC programs and only some professional CNC programs support dual-loop servos. They are complicated, expensive and cannot completely solve for backlash. No dual-loop CNC program used on a bad machine can beat a well-rigged machine using single-loop feedback.

About absolute versus incremental encoders: Incremental encoders say "you have moved 1 count CW or 1 count CCW'. They depend on the CNC program to add or subtract that count from the machine axis position register. This works very well 99.999% of the time. Simple book-keeping on the program's part.

Absolute encoders report back the exact location of the machine, maybe even 24 bits. No book-keeping is ever required; here's the axis position, just save it and use it.

The problem is absolute encoders cost an arm and a leg and then some. Maybe 100 to 1,000 times what an incremental encoder does. They cost so much because they may have 24 output bits (and wires) instead of the 2-bits an incremental encoder needs.

If you work with the Space Shuttle and you must have 99.999999% guaranteed reliability, use an absolute encoder; lives depend on it. If you can live with 99.999% reliability, use an incremental encoder which will be 100 to 1,000 times cheaper.

Be careful of some 'absolute' encoders, particularly if they cost less than $1,000. Some of them are incremental encoders with the 'book-keeping' up/down counters included inside the encoder. That's cheating from my perspective.

Mariss

[Cartierusm]

Mariss, thanks for the info, very helpful, but what I was after and I'm sure I wasn't too clear --

Let's say I build a jig and the point I need to reference the point is 4.5" x 3". Now when I turn off the machine it's lost that position, when I turn the machine on again I want to be able to just type in 4.5" x 3" and it will go to that position and I can zero out my DRO and run the code. I currently use an electronic edge finder to find the edge of my jig and then zero out and type in a known coordinate and zero out again and I've got my position, but this give a little play doing this way as it's all by touch and feel. I don't really care about glass scales and encoders I was just wondering if they have a zero position that is perfect, so I can turn the machine on tell it to goto 'Home' and then move the gantry to the exact position I need for the first time, then if I turn off the machine I can home it and then go back to that exact position.

EXAMPLE: I have a pair of digital dial calipers, just baisc ones nothing fancy. When it's closed and says '0' i can move the calipers and get a reading. If I turn the calipers off and move the slide and turn the calipers back on it will give me the reading of where it's at NOT where is was when I turned the calipers off. I hope I'm explaning this right. Thanks.

[Cartierusm]

Ger, the link didn't work but I subscribed to the site and found the manual, thanks. As far as I understand it talks about counting the pulses it received after hitting a home switch, which is all fine and good but if my switches are not very accurate, what would I do about that?

[ger21]

I don't know exactly how it works, but from what I've read, it's the most accurate way to home using Mach.

[Mariss Freimanis]

Cartierusm,

When you turn on a machine for the first time in the morning, I would expect it to seek out the home position first off using hard-wired "home" switches. Once homed, I would expect that machine to find any X,Y,Z Cartesian coordinate previously programmed into it.

Mariss

[Geof]

This has been discussed before so if you did a search and a bit of wading through numerous hits you may find something. I have posted a description of how commercial machines find home, but that will be buried in the archives so here it is again.

To home (or zero) an axis the controller runs the axis slightly past the zero position until it opens the limit switch. Then the controller reverses the axis travel and as soon as the switch closes starts counting encoder pulses and looking at the 'Z channel' on the encoder; the Z channel gives one pulse every revolution. When the control senses the Z pulse it stops and that axis is zeroed (or homed). The controller counts the pulses as a safeguard against the switch sticking or something going wrong. The count has to fall within limits that are defined by a Parameter and if the count is outside these limits it will alarm.

This homing procedure is accurate to within one, or a few, encoder count and does not rely on the precision of the switch.

Quite a while back I suggested it would be possible to mimic this procedure on a home built machine by mounting a disk with a very thin slit on the feedscrew and having an optical sensor looking through the slit; this optical sensor would give the equivalent of the Z channel pulse. Since I did this post I have read a few posts which suggest that Mach has something available that does this, or has the software to work with this kind of setup.

[Cartierusm]

I really do appreciate the help and I understand the concepts but it still seems a little confusing. The way I think of it is if the encoder on a servo makes sure it's in the right position why isn't there a marker on the encoder or glass scale that says hey this is the mid point and once it passes this you can reset to the center, I believe some glass scales are this way. As I mentioned above with the dial calipers, they operate thay way and are very cheap to buy.

I guess I could always buy a manual DRO and hook that up and keep it on all the time or once I find a home of 0,0 once on the CNC zero out the manual DRO then make an offset for the jigs I use. At the end of the day I could bring the CNC back to 0,0 turn off the machine, then turn off the manual DRO. Then at the beginning of the day turn on the manual DRO first, then the CNC so if the CNC kicks upon startup the DRO will register it. Seems like a lot of trouble for something that should be so simple if you already have a device that can generate position like a glass scale or an optical encoder.

[Geof]

Originally Posted by Cartierusm .... The way I think of it is if the encoder on a servo makes sure it's in the right position why isn't there a marker on the encoder or glass scale that says hey this is the mid point and once it passes this you can reset to the center, I believe some glass scales are this way. As I mentioned above with the dial calipers, they operate thay way and are very cheap to buy.....

Did you type this before or after reading my post?

[Cartierusm]

After, did I miss something? I understand what you're saying I was just kind of commenting on the fact that there should be a simpler way. Now, you're way might be simple because you understand it, I understand the concept but not the execusion, but I do appreciate the help. I'm going to poke around here a little more and see what I can come up with. Thanks.

[Geof]

I was puzzled because you write:

why isn't there a marker on the encoder or glass scale that says hey this is the mid point and once it passes this you can reset to the center,

The Z channel is this marker.

But it is necessary to have the limit switch in the picture because this marker (Z pulse) gives a signal every revolution of the encoder; the limit switch sequence ties the home position down to a specific Z pulse.