forum.cncdrive.com

i was running low on inputs so wired all limits and homes to 1 pin
when homing- if you set a slow back off speed (in metric 50 ) the homing completes but it activates limit fault
set this low as i wanted maximum accuracy/repeatability.
i had to back off at least 125 mm/minute speed to get it to work without reporting a limit switch was hit
(running old stable release 47)

Is the home and limit switch the same? Probably, it has a floating signal when you move off the switch. What kind of switch do you have?

(I use optical sensors and 25mm/min back off works fine, but I had problems in the past, because while homing the next axis the resonation triggered a limit. In the new test versions there's an option to move the axes after homing, but in the latest stable version it in not implemeted yet. Try this macro for homing, maybe, it solves your problem until the test versions come final.)

All normally closed micro switches wired in series

That should have stable states... Does it click normally? Maybe, the "click mechanism" went wrong in it? Is all of them unstable or only one?

All 3 are the same
They home but trigger limit if backing off too slowly.
At 150mm/ minute backoff they work fine
At 50mm/ minute they trigger limit

Sorry, I have no more idea... I never used micro switches, I don't trust them. Perhaps you could test some other type of switches, but I know, it's not easy to change.

It is possible that the mechanical switch is sliding off the contacts and the change in contact resistance is being picked up as debounce and it's tripping the limit switch status and not the "home" switch status?

Have you tried to add a little debounce to see what happens?

I know that this is self defeating in that the debounce will slow down the reaction time, but it may be that the mechanical switch just is the way it is?

50mm/min = 0.833 mm/sec, there is probably a switch pivot point (the click in the mechanical switch) that is creating the issue.

I'm not sure it would help, but you can try things like using a comparator (LM339) to square up the wave again.
http://www.ti.com/lit/an/snoa654a/snoa654a.pdf

Mechanical switches are never clean in "break", "make" is normally more rapidly achieved as the switch clicks its contracts in position.

Is there any merit in changing to linear encoders {the machine always knows where it is then..... no need to home?} ..... I noticed reinshaw do some 0.1uM ( 0.1 micro meter) accuracy encoders. Others are available with less accuracy and lower cost
https://www.ebay.co.uk/sch/i.html?_from ... r&_sacat=0

I will be adding home to encoder via the drives.
Was just pointing out what I found .
Most probably wouldn’t lower the backoff that slow.
( I do on the big router but that has separate switches limits/ homes)

There is no back off parameter for homing in the 1.2047, but I think the down speed instead?
And since there is no back off distance in that version yet I think what happens at you is that with slow speed the axis stops closer to the home switch, probably too close.
I mean the switch has probably instable switching point or in other words it's on/off state is still instable at that mechanical point and so it switches off and the controller stops the homing process but then it still goes back to the on state at least for a short time and so it triggers the limit.
The limit switch function is only disabled until the homing finishes and when it's done then it is enabled back on.

So, I think probably this will be the problem, but if you post your profile file and let me know how do you exactly doing the homing, I mean which axis and what way (button, macro call etc.) then we can test it here.

FYI I have an identical problem but with NPN NC closed prox sensors individually wired to each limit...

The backoff in the test version is clearly the way to address this - Is there a release schedule that might give an indication when this might become a release version?

(I acknowledge there are macros to achieve the same result with the current stable)

Cheers - N