Proximity switch "contact bounce"

[Neale]

I bought a box of ten a couple of years ago (this machine has taken longer to finish than I expected!) I have fitted 6, so I have gone through the rest of the box testing each of the other four. I bolted each one in turn to the machine so I could trigger it by turning the ballscrew by hand and connected it to a bench power supply. So, completely independent of the control box, motion controller, etc.

One does not work at all. One turned out to be an "EX" version which only had two wires instead of a three-wire AX. The other two both have this sharp switch-on/switch-off point with a microscopic movement of the ballscrew needed to go from on to off with a flickering stage between. As I understand it, this is not what a proximity switch should do - there should be a difference between on and off trigger points to avoid exactly this situation. I think I have a bad batch of switches.

[Boyan Silyavski]

You must have been unlucky with that batch/ seller. Yes, there is not a flickering stage normally. Its either on or off. Hysteresis means once you are ON and the back to OFF, like you noted there is a ~0.4mm distance / at least on my switches.

[Neale]

The more I look, the more certain I am that that is the problem. Fortunately I only need to replace three of them - the switches are fine as limits where the lack of hysteresis doesn't matter. But how do you find a seller that has good ones in stock! Maybe not looking for the cheapest would be a good start...

[Clive S]

The more I look, the more certain I am that that is the problem. Fortunately I only need to replace three of them - the switches are fine as limits where the lack of hysteresis doesn't matter. But how do you find a seller that has good ones in stock! Maybe not looking for the cheapest would be a good start...

Neale if you pm me your address I will send you one

[Neale]

Clive - that's a very kind offer but I have just ordered 4 more which should be here by the end of the week. For the moment, I can get by with homing each axis in turn (using the buttons on the Diagnostics page). I home one axis, then jog it a tiny bit away from the home position. Doing each axis in turn like this is a bit of a chore but it does mean that I can continue with testing. Next job is to sort out squaring the gantry and then test my ideas for dual-motor homing using an IP/M which does not support this.

[Neale]

I ordered some new switches, and I also put the controller box in the machine frame where it was supposed to go. That meant that I could connect the star point earth terminal in the control cabinet to the frame. This completely cured all the homing problems I was seeing. Reliable homing, and not a single limit switch trip since. Was this a shielding/noise/ground loop problem? No idea, but I seem to have fixed it. I was testing the proximity switches by connecting them to a bench power supply and just watching the built-in LEDs; in retrospect I'm not sure that this is a valid test and I should probably have connected the switch output to some kind of load (like motion controller input). When the new switches arrived, I tested those and still did not see any hysteresis but again this was "off load" and might not be a valid test. I do see the axis overshoot very slightly and then return slowly to the home position (which is what you would expect) on two of the three axes; it might be happening on the third but the movement is too small to see easily.

I did have a problem with one limit switch apparently triggering as the cable chain containing motor cables moved near its cable but I rerouted the cable and that seems OK now.

I haven't measured resetting accuracy when homing yet. Be interesting to see what these cheap switches can achieve.

[paulus.v]

I had the bouncing problem when the distance between the sensor and the triggering metal was too high and the triggering metal was aluminium. I have the sensor passing by the metal not towards it.
In regard to repeatability, I have one sensor mounted too close to the stepper and the triggering position differs with the position of the stepper rotor, but only by a few hundredths of a millimeter.