Does anyone have experience to share
about using a photoelectric sensor as a homing button?
My current microswitches are not repeatable enough to pause and continue any finishing-passes. Mostly because the axis sometimes doesn’t stop fast enough and runs them over for a few millimeter.
Not a problem with an optical interruptor well before the end of the axis. You can run through it at full speed and nothing happens at all.
I have some sitting in a draw that I was going to use but have never gone any further. Main concern would be dirt getting in and setting off the limit.
Would you still have problems if your machine homed until it hit the switch, backed off a bit and re-homed at a much slower speed?
3D printers use them a lot. I’ve got packets of them. Easy.
The worst part about using them is that they nearly always come with a JST-style connector, rather than a solderable or screwable link. Not a problem for me as I have a few hundred in every size, but if you don’t and don’t have a crimp tool, you need to factor in those costs, too.
I’m not sure why you think they will be any more likely to stop your machine than a microswitch - if it’s running on, it’s either a faulty switch, faulty wiring or controller settings. They’re pretty infallible. What is the orientation of your switch? You should be acting directly down onto the switch, not running across the face of the metal switch actuator.
@Kyle_Kerr @Kyle_Kerr It does that already and it already broke the switch once. Setting an extremely slow speed is not much of an option because the axis is rather long.
When the switch triggers, the motor can not always stop the axis within a fraction of a millimeter when running directly into the switch. It needs space to properly decelerate the moving mass.
My plan is to keep the mechanical switches as limit switches to cause an error condition and shut down the current program but have an optical switch 3-5cm before and use that for very accurate homing. This is sufficiently away from the chips, so I don’t have to worry about them interrupting the beam. Mach3 runs until it interrupts the beam, then slowly backs off and uses the location where the beam is no longer interrupted.
I’m asking for experience of people who already use these switches. Things. I may not have considered.
I really like the idea of optical switches. My homing speed is really slow, in order to prevent that overshooting condition you mentioned. Your “flag” that breaks the light beam can be as long as it needs to so that the homing move doesn’t go completely past the optical sensor, switching back off.
I’ve bent the snot out of my limit switch roller arms with accidental manual moves. I think this would help with my human error situation.
I’ve need to program the software limits too which might also help…
Hitachi SCARA arm robots used to use OMRON photomicro slot sensors for home. http://www.ia.omron.com/products/family/436/
You use a little metal or plastic flag to slide through the slot. Shielded inductive proximity sensors in “pass by” configuration are also very repeatable and often used as home switches. Another option is to use traditional mechanical micro switch with a bump trapezoidal style dog to trip the switch in pass by configuration. Look here -
http://www.ia.omron.com/product/cautions/30/safety_precautions.html
I may be in the minority here, but my experience is that I have poorer repeatability with optical limit switches than with mechanical ones. My machine is small so I don’t mind slow home speeds. You might consider putting the limit switch on a pivot with a spring holding it against a fixed stop, and homing in until it closes, then homing out until it (hits the fixed stop and) opens and using the open as your homing signal.
My friend Ed did some repeatability tests with a microswitch: https://softsolder.com/2010/04/17/ugliest-tool-length-probe-switch-repeatability/ and that was part of what convinced me to stick with them.
@John_Bump Frankly when I do WOC=0.05mm finishing cuts to get the scalop down, having an error of 0.01mm in my homing to resume the cut on the next day does not sound very good. That’s a step in the part-surface you can clearly feel and see.
With a spring I’m afraid it would not expand to exactly the same postion it had before. So when upgrading anyway, a contactless method that will read the same value in a week as it does now sounds reasonable. It’s also trivial to build. Just 2 custom, 3D printed parts, 2 resistors (for LED and signal-pulldown) and it’s done.
Why re-home at all? Just keep the controller and motors powered?
We measured about 40 microns of repeatability in the standard lever micro switches at Ultimaker. Which is quite “ok”, more expensive switches can be better, but a switch that has < 1 micron is expensive.
I don’t know for the IR light gate switches (common in 2D printers) might be an option for you, I don’t know exactly how they are called. But as they are so common in 2D printers, I kinda expect a decent repeatability. And you can pass trough them without touching them, solving your momentum issue.
@Daid_Braam I’m not comfortable with keeping this powered while I am 800km away. I had powered but not moving stepper drivers catch fire once whil I thought the machine was off for a lecture just one hall away. It was quite a memorable thing at the congress.
The IR light gates are what I am currently installing and what I asked about in the posting you are commenting on. I never asked for alternatives. I asked for people who have already used them to come forward with experiences in how well they worked out and pitfalls I may have missed.
I should have constrained my comments to optical. My experiments were a while ago, and I used the slotted photodiode units from optical mice. This was with a custom motion control board I built for one of the slots in my Amiga 2000. I didn’t like the repeatability. I don’t know exactly how I’d do this, but if I could make the same structure a photo-interruptor mouse has, with a thin slit that lets light through, and blocked off the rest of the time, I’d feel like I could get better resolution.
On servo driven machines I have been involved with, the prox is never used for the actual home position - just the rough home position. Typically home until you see the prox, then reverse until you get the zero pulse from the encoder. That is accurate - because rotatory movement is finer than linear. Alternatively, when the prox goes on, stop and creep really slowly off it again. These methods get better results from poor sensors.