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pca
MemberIf you don’t have end stops, I suspect that may be your problem right there. Without some method to hard-zero it at some point, if it DOES drift for whatever reason, there’s no way to correct for that. It would only take a small amount too little available torque for it to lose a step here and there and end up all over the place. If it rezeros ever time it homes, that’s going to be minimized or eliminated.
Another thought strikes me as well; if you have a variable tightness on your belts, caused, for example, by one of the pulleys not quite being round, or the drive hole not being square to the center of the thing, and your torque is a little too low combined with the cutting forces, you can end up with missed steps from that. Each time it goes around you could lose a step or two, and it would add up to a considerable offset which would tend to cause similar results.
Whatever it turns out to be, I wouldn’t be surprised to learn it was purely mechanical. Some of those sorts of problem can be very subtle.
Good luck with it in any case!
MemberAs an update to my post, I would suggest you create a gcode file by hand which is just a series of back and forth moves on one axis and accurately measure how far it goes in both directions over a number of repetitions at different speeds and distances. Check it’s doing the same travel both ways, and very carefully observe your pulleys, stepper motor shafts, and any mechanical connections for unwanted movement. Do each axis in turn, then try diagonal movements too.
Hope this helps 🙂
MemberHow are you zeroing the machine? It might be that when it returns to the home position, it mechanically hits an end stop or obstruction before it hits a limit switch, even only by a tiny amount, which causes the stepper motors to lose a few steps in a predictable manner each time. Essentially, as an example, you could go 1000 steps right, 1000 steps left again, but after 998 steps you end up hitting something that causes you stop moving and lose those last two steps.That might only be a rough spot on the carriage mechanism if the torque is too low.
The end result would be each time you repeated the process, you’d move another two steps further out, giving something not dissimilar to what you’re seeing.
I’ve had problems on a laser cutter that turned out to be something a little like that caused by some detritus on the rail which acted like a one way bearing, so the carriage would go over it in one direction without noticing and momentarily stall in a fairly consistent manner in the other.
Just a thought.
MemberIt would need careful setup, the switches would need to be adjustable and the whole thing would require alignment in the first place with a set square. But I don’t see it being particularly difficult.
One thing to remember is that mechanically it would be constrained to be MOSTLY square right from the start or it wouldn’t run smoothly. Once initial alignment is achieved, this would allow it to be automatically reset every time to overcome some external force knocking things out of place. In some ways, I think it’s more important that everything is repeatable rather than perfectly square.
For instance, with a laser cutter, once you’ve aligned the mirrors you can overcome small differences in beam path from perfect 90 degree turns, but it absolutely HAS to stay like that. Especially for a big one.
In the case of my machine it was all carefully set up to be as close as possible to a perfect rectangle as I could make it, but because the y gantry is nearly one and a half meters long, it’s certainly possible to manage to get one end slightly advanced compared to the other one if something momentarily jams it. Or if it simply skips a step, although that’s never happened in practice. So the ability to reset everything to a known good position is important and doing this at both ends is better than doing it at one and hoping the other is all right.
I’ll get around to trying the switch modification sometime soon, but I’m busy with another project right now so it might take a little while.
MemberIt doesn’t really need to be complicated, I think. The mechanical constraints of the gantry mechanism would prevent either side being more than a small amount out, but it’s certainly possible to have that happen especially on a very large system. Homing to only one switch if there IS a misalignment won’t correct that misalignment since both motors stop at that point.
The obvious way to sort it out is to be able to define two switches as end-stops, then continue the initial homing process until both close/open. Since the misalignment is small, the fact that one motor is now slipping for a few steps shouldn’t an an issue.
Once the initial homing is done like this, the final homing process of moving back out a little then slowly coming to rest at the final position could use either switch, since by then in theory everything is square again and both switches should operate simultaneously.
In fact, now that I’m thinking about it a similar response might effectively be produced simply by wiring both switches in series, so they both have to close to register as active. I’ll have to try that. It’s not quite the same but it might work.
MemberI don’t think it is yet. I have a large (1.5m x .6m) laser cutter using a similar design with 2 Y motors and I have found that once it is correctly set up, it stays square and true with no issues. Manually pulling both ends of the gantry to the end stops, then sending Gcode to move x and y out 10mm, before homing the axes, will leave it in a state that is completely reproducible.
One only needs to do this once, as after that a normal homing procedure works every time.
While having dual homing would be very nice and I’d like to see it added, so far my tests have shown me that it’s not essential for usable work.
MemberIt’s possible it’s a hardware issue. Are you certain that you have your stepper motor controller set to the same microstep value in each case?
If one was, for example, at x8 and the other was at x16, they would move different distances for the same number of steps.
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