Replacing limit/homing switches.

[domini...@gmail.com]

Hi everyone, long time no posting...

So one thing that's been bothering me is the repeatability of the homing of the machine, and it occurred to me that perhaps the microswitches could be replaced with better ones.

I'm wondering if a switch with a lower actuation force will allow the machine to detect the limits sooner, and thus stop the motor sooner resulting in a more repeatable and accurate homing position.

So:

1) is my theory correct?

2) has anyone replaced there switches with different ones, if so which ones and did you get more repeatable homing?

The ones in the machine are Omron SS-5GL2's which have an actuation force of 0.49N.

Well, I assume they do, because the full part number might not be written on the side of the switch...

I'm considering replacing them both with SS-01GL2-E which are 10V rated and have an operating force of 0.08N.  They are from the same series, so should fit fine, and are also hinge roller lever operated with the same solder-terminals and switch connections.

Thoughts?

Dominic

[Nathan Seidle]

An interesting question!

My mechanical brain says the switch force shouldn't matter that much. The motors don't care, they are far more powerful than 10x any switch force rating. I suspect the important bit is the mechanical bits on the end of the switch (the leaf metal) so that when the switch does activate, it activates repeatedly and reliably in the same way, so that you have a known position when it does.

But I'm all ears to anyone else's experience/opinion.

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[John]

I believe most systems home with the following (close switch first, using a higher speed so as not to take forever to, slightly over travel to make sure the mechanical switch, which are user snap or over-center, have fully traveled), then reverse the motor and move much more slowly in reverse. This minimizes several effects, switch bounce (mechanical and electrical) momentum, system reaction speed and finally backlash. I suspect that the mechanical switch (if the homing sequence is like this) is not introducing much variability. If you are attempting to measure this, you should get a test indicator that reads to 0.0001", no a dial indicator that typically reads to 0.001", which is 25 microns. You don't need to get a $150+ name brands (Mitutoyo, Starrett, Tesa), test indicator you will want a good way to hold it like an adjustable mag base. In all honesty, I suspect there are many more and larger sources of positioning error other than the limit switch.

[domini...@gmail.com]

Just as a quick update, today I finally replaced both the homing micro-switches with new SS-01GL2-E variants in the machine, it still works afterwards and I'll be monitoring if the repeatability of the machine origin/homing is improved or not over the coming weeks and will let you guys know if it made any difference.

I will say that that changing the micro-switches is a PITA as you have to remove the head and turn it up-side down, which involves removing the belts, which in turns means you have to recalibrate the entire machine after you re-assemble everything.  Still, I learnt more about the machine in the process so I guess that's useful too.

Dominic