Using an encoder on z-axis as a failsafe

[jderou]

I have a stepper system (coverted Boss mill) with an intermittent problem on the z-axis. Occasionally, with no explanation, the axis will 'slip' and not move on command. When this happens it has always started at the beginning of a rapid z move, so when it loses position, it loses big. I haven't noticed any problem of missing 1 or 2 steps, its always either on or way off. Biggest annoyance about this is I am afraid to walk away from the machine, even though through the last 20 or 30 hours of use it has only done it once (crashed the part though). Rapid feedrate is set well under the max it is capable of, I haven't found any explanation for it.
Here is what I am wanting to accomplish. Mount an encoder on the z-axis motor. Develop a system (PIC?) that would monitor incoming steps and compare to encoder pulses. If the difference between the two becomes more than a set amount (could probably be several thousandths worth), e-stop the machine.
Does this sound like a PIC would be the ticket, or should I go another route, or forget it completely. Unfortunately a servo system is out of the question for now.

Joe

[valmet58]

what about acceleration of your stepper.if it is too fast it can cause that..i would check if it can be lowered..

[jderou]

I could lower it, not sure it would help. I used to have this problem regularly when applying tool offsets and first moving Z axis. Lowering acceleration and velocity would not fix the problem. I ended up having to put a G04 (dwell) between applying tool offset and the first Z move. Its weird, but it fixed that problem. That was at least consistent. This is once in a rare while, and if I run the program again it won't happen.
The dwell thing seems like a processing problem, don't know if this could be related or not. I really just want something else 'watching my back'.

[snoopy27]

Originally Posted by jderou I have a stepper system (coverted Boss mill) with an intermittent problem on the z-axis. Occasionally, with no explanation, the axis will 'slip' and not move on command. When this happens it has always started at the beginning of a rapid z move, so when it loses position, it loses big. I haven't noticed any problem of missing 1 or 2 steps, its always either on or way off. Biggest annoyance about this is I am afraid to walk away from the machine, even though through the last 20 or 30 hours of use it has only done it once (crashed the part though). Rapid feedrate is set well under the max it is capable of, I haven't found any explanation for it. Here is what I am wanting to accomplish. Mount an encoder on the z-axis motor. Develop a system (PIC?) that would monitor incoming steps and compare to encoder pulses. If the difference between the two becomes more than a set amount (could probably be several thousandths worth), e-stop the machine. Does this sound like a PIC would be the ticket, or should I go another route, or forget it completely. Unfortunately a servo system is out of the question for now. Joe

Just curious, what kind of counter-balance do you have on Z, and have you been able to notice if this problem occurs in the same location or part of your cutting file?

[jderou]

No counterbalance. Not sure if it happens in the same spot of the Z travel, but I haven't run the same program enough times to know if it happens in the same spot more than once. I know it doesn't do it every time in that spot of the program.

[LeeWay]

Not sure what type machine you have, but could it be trash in the ball nut or linear slides or sticking gibbs?

On my mill, I had a kind of open way cover. Stuff would accumulate under the Y axis. One of the linear slides mounting holes filled up with swarf and would actually make the axis stutter. That was a bit difficult to locate. I now have proper way covers, so that won't happen again.

The thing with trash in a ball nut is sometimes it won't hurt anything, then it gets jammed between the balls just right and it would be enough to cause missed steps.
Something to check anyway.

[123CNC]

Have you been following the CNC Brain thread? Your scenario sounds like a great test for the CNC Brain which basically provides the necessary processing power/speed to make real time corrections. I may be wrong, but I'm pretty sure I read stepper and servo compatibility. In fact you can individually configure each axis. So say in your scenario, just add one or two sensor(s) and feedback signal(s) on the Z and leave x and y alone (for the time being.) A linear scale on the quill and rotary encoder on the stepper for double closed or linear scale alone for single closed.

Obviously its still in beta, so I'm expressing my interpretation. But are you curious enough to look www.cncbrain.com?

I also whole heartedly agree with recent posters, check your accel/decel rates, max speed, and anything that may be occassionally or consistently hampering the axis movement (even old way oil gumming/varnishing your slide).

[jderou]

Wow, I've been away for awhile. I completely missed the CNC Brain thing. That's exactly what I need. A little pricey for me, but looks like a good investment. Thanks for the link!
About the dirty screw/slide. It's a bridgeport quill, so the screw is completely enclosed. Quill is lubed pretty well, hard to protect it any better. That's not to say a brief sticking of something isn't causing this, it is very possible, and that's why I need a closed loop!

[Oldmanandhistoy]

Hi,
The CNC brain sounds like the answer to many problems but I would look to solve the problem first.
Sounds to me to be a mechanical problem and I would start with sticktion and check out if the motor has enough power without a counter balance?
Also do you have a tight spot in your ways?
Does this happen when you jog the axis at high speed?

[jderou]

As it is only moving the quill, and I don't know of any other Boss BP having this problem, I don't think it needs a counterbalance.
I have the max velocity and acceleration set at about half of what I can jog it at without problem.
I may tear into it this weekend and see if I can turn the screw manually throughout the travel without any binding.
Still, a closed loop would make me feel a whole lot better about walking away from the machine.

[LeeWay]

Steppers and open loops are only as good as the code too. I often run the same codes many times. It's only after it has run a time or two that I leave it unattended. Very seldom do I leave the shop though and then only for a couple minutes.
If I was running a larger machine that would be damaged if it crashed hard or even a lathe, I would feel better with a closed loop system. I too am keeping an eye on the Brains.
Cool stuff.