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Author Topic: Homing operation does not pulls back enough to get away from switch?  (Read 1730 times)

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Hello i am fighting a pretty annoying problem on my benchmill i installed a 3 axis kit on , just installed switches for limit and homing

i installed limit switches but they are  industrial NO/NC switches , spring loaded contacts wich have quite a lot of travel in them

 - the machine sees every signals correctly
-the cables are shielded and grounded
- if i disable the limit switches input  it homes and backups fine
- if i disable the home switches pins the limits stops the machine like they should
- if i have all the inputs enabled the machine will home one axis back off and sometimes just not pulling back from the switch enough to bring back the NO switch in open position and then it locks the machine .probably the travel needed times out the homing routine and gets out of the homing operation still on the limit switch locking the machine
-i did install capacitors to clear a potential noise problem but it isnt the case apparently



The axises sometimes clicks and unclicks the switches well but sometimes it doesnt so it is not constant. i can just push a bit on the carriage or the tab the switch sits on and i make it unclick from her activated position so looks like the travel of the homing back off is about a few thousands too short . i tried giving a bit more of acceleration to the axises but it is not helping.


so now i am trying to figure out if there is a way to edit the amount of distance the homing operation has to recognise the home switch click back out ??
tried modifying a few values in the homing setup screen and it didnt helped :-(
« Last Edit: March 16, 2018, 01:40:33 PM by Letomoto »
Re: Homing operation does not pulls back enough to get away from switch?
« Reply #1 on: March 16, 2018, 07:19:18 PM »
Hi,
my recommendation, and will see if you read any of my other posts, repeatedly make the case foe separate home switches.

Leave all your existing switches in place but as limit switches only. Get another three switches an fit them as Home switches alone. It avoids all the
trouble that you are talking about.

Note that its not necessary to have a home switch at the very end of travel. Its common to have them just in from the end, a few mm or more, but they
don't have to be.

Craig
My wife left with my best friend...
     and I miss him!
Re: Homing operation does not pulls back enough to get away from switch?
« Reply #2 on: March 16, 2018, 08:41:14 PM »
well i got some micro switches at the store today , most people seemed to have success with their combined switches so i was trying to understand why the hell i wasnt able to , so yeah simply add more switches will definately work , i can also just take my wires from the switches i already installed no need to rewire
Re: Homing operation does not pulls back enough to get away from switch?
« Reply #3 on: March 16, 2018, 08:52:38 PM »
Hi,
in Machs early days everyone used a parallel port and need to be very frugal with inputs, theres oly five in a parallel port. Combining limit and home switches
was required. It caused problems with compatibility then and it still does now.

With extra BoBs and external motion controllers as cheap as they are theres no need to do it any more.

Craig
My wife left with my best friend...
     and I miss him!
Re: Homing operation does not pulls back enough to get away from switch?
« Reply #4 on: March 17, 2018, 02:22:51 AM »
Hi,
a lot of switches, including snap action microswitches have hysteresis. It means that the switch makes at position x=0 but when it unmakes it has to travel
a bit further say x=-0.1. For home switches this is a good thing.

When Mach homes an axis it drives to the home switch in the direction and speed that you specify on the setup page until the switch makes. When the switch makes Mach
will stop and back up until the switch unmakes, ie the hysteresis distance. You've already noted that if that distance is too small it is possible for flexure in the machine to cause the
switch to re-trigger. Thus for home switch a small but repeatable hysteresis is desirable.

Note that switches with vanishing small hysteresis are available but are expensive and actually unsuitable for this purpose.

I not also in your post that when you set the Home Off distance on the setup page you expected the axis to drive backwards that distance. Mach doesn't do that.
What Mach does in fact is drive until the switch makes, then reverse until the switch unmakes and then set the machine coordinate of the axis to zero if no
Home Off distance is specified or the Home Off distance if it is specified. Note Mach DOES NOT drive to that remote location, it just sets the machine coordinates
to some remote location.

This has caused a lot of confusion over the years. Note also just to complicate matters a bit there is at least one external motion control board (PoKeys) that
can drive to that remote location if you program it to do so. It is not standard Mach behavior. Note also that with a macro you can cause Mach to go to a remote
location and achieve the same result, it just requires a short program.

Craig
My wife left with my best friend...
     and I miss him!

Offline Davek0974

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Re: Homing operation does not pulls back enough to get away from switch?
« Reply #5 on: March 17, 2018, 03:51:40 AM »
If this is a stepper system, simply use one of your existing switches on each axis (at the home end) as a home switch, forget limits they are not needed on a stepper drive.

disconnect the other switches.

configure Mach to HOME input only, not limit or combined, get the machine homing correctly and then turn on soft limits, job done.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives.
Plasma table, Mach3 V062, Step motors, C&CNC THC.

Offline Filou

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Re: Homing operation does not pulls back enough to get away from switch?
« Reply #6 on: April 04, 2018, 12:17:46 PM »
" ... forget limits they are not needed on a stepper drive".
the simplification you are proposing presents the advantage of simplicity. How do you motivate this choice ?

Offline Davek0974

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Re: Homing operation does not pulls back enough to get away from switch?
« Reply #7 on: April 04, 2018, 03:10:02 PM »
" ... forget limits they are not needed on a stepper drive".
the simplification you are proposing presents the advantage of simplicity. How do you motivate this choice ?

Unless you are running geared reduction or high reduction belt drives, i.e. an unusual setup, the steppers do not have the power to do too much damage if they run away or you jog into the end stop.

I would always go for a home switch on each axis, correctly set soft-limits on each axis and thats it, simple and easy. When set correctly soft-limits are the perfect answer - less wiring, less hardware, less hassle.

On a servo drive then no, hard limits AND soft-limits all the way - theres some awesome destructive power in a servo of reasonable size and you really do not want that smashing the ends off your mill table etc.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives.
Plasma table, Mach3 V062, Step motors, C&CNC THC.

Offline Filou

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Re: Homing operation does not pulls back enough to get away from switch?
« Reply #8 on: April 04, 2018, 04:25:51 PM »
Davek0974, all right, your argument leverages on the "limited" power of stepper motors AND on the presence of sufficiently strong end stops.

Here the story of a (thus far unique) unexplained event that convinces me to combine HW AND soft limits on a open loop rig without encoders. On a long job, about a square meter of working area, one axis gets a sudden shift of about 200 mm. I double checked (after the "crash" into the support) that this shift WAS NOT in the G-code in the form of a global or fixture offset. The axis passed beyond it SW limit (don't ask me why/how) and was stopped by the hardware limit switch placed 3 cm before the physical limit of this axis.
Without this switch, I agree that the damage (to the machine) would have been limited, but the job would not have stopped and the work piece would have been wasted.

Has anyone else experienced "sudden unexplained" shifts ?     

Offline Davek0974

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Re: Homing operation does not pulls back enough to get away from switch?
« Reply #9 on: April 04, 2018, 04:42:25 PM »
Interesting, no, i have never seen a sudden shift like that, especially going past soft limits.

Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives.
Plasma table, Mach3 V062, Step motors, C&CNC THC.