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08-01-2017 #1
I'm just commissioning my new router and have been testing limit/home switches. I am using NPN NC proximity switches, running off 24V. I started by doing basic limit switch testing - jog gently towards the limit switch and check that it worked. I have combined home/limit switches at one end of each axis (XYZ), limit at the other (XY), and just at one end for Z. Pretty standard configuration; for me it was easier to mount two switches than have one travelling switch and I had bought a box of 10 off eBay anyway. I am using three separate inputs on a CSMIO-IP/M so each axis has its own input with cascaded pairs of switches on X and Y.
Limit switch functions worked fine - good reliable stop. Then I moved to checking homing function with Mach3 and that's where I started seeing problems. Using "Ref All Home", Z homes to the top of travel, overshoots slightly, and then moves off the switch to find home. Usually. However, I get frequent "limit switch" trips and when I look closely, I see that the LED indicating on/off on the Z prox switch is flickering slightly. I had noticed something like this when I first fitted the switches and was just testing them by winding the relevant axis by hand, using the LED as an indicator as the IP/M was not connected at the time. Some of them seemed to have a pretty sharp cutoff, and some seemed to have a slight area of uncertainty. I put this down to them not being fully wired in and that the input circuitry of the motion controller would sharpen up the switching point.
What I think is happening is that Z moves to home, overshoots as expected, and then backs slowly off. However, the IP/M detects "home" the first time the switch trips, but because the switch then changes state a few more times (and goes on doing this) this is detected as a limit switch event. Even if the Z homing works OK (maybe 50% of the time) as the other axes home, the vibration (???) shakes the Z axis slightly, and the limit function trips again. On X, I have made the target block as square-edged as possible for the most accurate triggering; on Y I use the edge of the ballnut but that is also fairly square; on Z I am using the end of the Hiwin rail which has something of a bevel on it. Don't know if this is a contributory factor.
It might be noise related as I can have the machine sitting with the Z switch LED flickering and then hit e-stop (which turns off power to the motors, amongst other things). The flickering stops, but restarts when I reset Mach3. I shall try taking the switch cable out of the drag chain it shares with the Z motor cable (CY screened) to see if that helps, but I ran out of time this evening.
It might be a cheap and nasty set of switches, of course - one of them was completely faulty from new. Has anyone seen anything like this before? I know that JazzCNC has demonstrated excellent repeatability of these switches but maybe that was with a better quality switch!
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