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False limit trip
« on: February 03, 2011, 09:29:28 AM »
I am battling a long standing problem with getting false limit trips. I am running a G540 and changed all my limit switch cable to mil spec shielded cable yet I still get false trips.
I have tried setting de bounce form 10 to 6000 and it still does it. It usually occurs at high RPM or under high load. Any thoughts??????
"Gravity, its not just a good idea its the law"

Offline Hood

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Re: False limit trip
« Reply #1 on: February 03, 2011, 09:50:44 AM »
Switches vibrating open?
Hood

Offline Tweakie.CNC

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Re: False limit trip
« Reply #2 on: February 03, 2011, 10:37:26 AM »
Are you using a VFD ?

Tweakie.
KEEP SAFE !
Re: False limit trip
« Reply #3 on: February 03, 2011, 11:58:24 AM »
is it possible to change the limit/reset/estop to normally open, just to see if it's noise related.
Re: False limit trip
« Reply #4 on: February 03, 2011, 06:13:03 PM »
Switches vibrating open?
Hood

Not sure I will have to test each one.
"Gravity, its not just a good idea its the law"
Re: False limit trip
« Reply #5 on: February 03, 2011, 06:13:47 PM »
is it possible to change the limit/reset/estop to normally open, just to see if it's noise related.
Are you using a VFD ?

Tweakie.

No I am not.
"Gravity, its not just a good idea its the law"
Re: False limit trip
« Reply #6 on: February 04, 2011, 08:55:57 PM »
I ran into a similar problem when I first started testing my machine.  I was getting limit trigger but couldn't actually seeing it happening on the input screen.  I am not for sure what your set up is but mine were being caused by one of the stepper motors when it was just in the locked postion.(Machine setting idle not moving)  I thought I had a voltage drop causing it but everything checked good.  After pulling my hair out for awhile I pulled the fuses on my stepper motors and then started powering  them up one at a time.  Sure enough it ended up being noise across my limit switch circuit.  I isolated it by running the limit circuit through a relay and then brought the signal wire from the bob across the contacts.  Basically the relay stays pulled in until you break a limit switch.  I don't consider myself an electronics guru but thought the info may be useful.
Re: False limit trip
« Reply #7 on: February 05, 2011, 08:46:40 AM »
I ran into a similar problem when I first started testing my machine.  I was getting limit trigger but couldn't actually seeing it happening on the input screen.  I am not for sure what your set up is but mine were being caused by one of the stepper motors when it was just in the locked postion.(Machine setting idle not moving)  I thought I had a voltage drop causing it but everything checked good.  After pulling my hair out for awhile I pulled the fuses on my stepper motors and then started powering  them up one at a time.  Sure enough it ended up being noise across my limit switch circuit.  I isolated it by running the limit circuit through a relay and then brought the signal wire from the bob across the contacts.  Basically the relay stays pulled in until you break a limit switch.  I don't consider myself an electronics guru but thought the info may be useful.

I think that is my problem as well. As I am using a Gecko G540 I thought it was designed to prevent this from happening. I will have to do a little more research.
Thanks
"Gravity, its not just a good idea its the law"

Offline Sage

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Re: False limit trip
« Reply #8 on: February 05, 2011, 05:18:59 PM »
 It is best to arrange your limit switches so that they supply HARD signals  to the Breakout board under running (or normal) conditions.
 In other words, the signal being supplied to the breakout board (BOB) should be either a solid ground or 24v (or whatever logic level is required) supplied by the switches when the switches are in their normal runnning state.
 It IS NOT good to have open wiring with a pullup resistor (or pull down) on a BOB input supplying the normal operating conditions. You can have pullup or pulldown resistors only if the normal signal condition is a hard ground or logic high supplied by your switch circuitry.
 If you have to have an open circuit arrangement then use that to control a relay which then can supply the hard signal to the BOB. The relay being a low resistance device cannot be triggered by noise or induced voltages on your wiring.
If you accomplish this then any type of wire can be used because a solid ground or solid supply voltage will not be able to be induced into by noise sources.
 For those with an electronics background this is called keeping everytihng as low impedance as possible.

In my case all of my limit switches are normally closed and connected in series to the 24v supply. This drives a relay which is then normally energized. The contacts of the relay are connected to the BOB and supply it with a ground for normal operation. If any switches get triggered (open) the relay drops out and only then does the BOB see a high level at it's input because it has a pullup. The pullup is not left to float around supplying the logic input.

 By this arrangement the pullup resistor IS NOT used to supply the normal operating conditions. (becasue it is suceptable to triggering by noise).

Regardless of what type of switches you are using there will always be a way to arange it into a low impedance circuit. It may take a relay to do so.

Sage



 
« Last Edit: February 05, 2011, 05:24:20 PM by Sage »
Re: False limit trip
« Reply #9 on: February 05, 2011, 10:55:45 PM »
Thats the same set up that took care of my problems.  My home switches are set up the same way execept not in a series which basically took care of any potential noise from getting to my bob.  I ran several programs today with no erratic behavior whatsoever and I am using a vfd as well.