Mach ignores limit switches

[ssutton]

Why does Mach ignore the limit switches when you do a Ref All Home? My machine has separate homing sensors and limits. My home sensors are hall effect magnetic type and my limits are mechanical switches. When my machine is homing is does not evaluate the limits, I can manually trip them during the home and the machine just keep on going. My fear is that if I have a hall effect sensor failure my machine will go into the limit switch and continue to plow into the mechanical end of travel and just sit there and keep grinding away.

-Scott

[ger21]

Because some machines have the home switch behind the limit switches. And some machines use one switch for both home and limits. If mach3 didn't ignore the limits while homing, it would be much less flexible, as neither of these two options would be possible. And a LOT of people use a shared home/limit switch.

[robe_uk]

I guess the next question is can mach3 be configured to see or ignore the limit switches when homing or not?

Rob

[rowbare]

AFAIK the homing routine is a VB Script. You could write your own homing script that does whatever you want it to do. For instance some servo drives have encoders with an index pulse. For best repeatability one could write a custom homing routine where after triggering the home switch the axis would return to the index pulse.

bob

[ger21]

AFAIK the homing routine is a VB Script.

Yes and no. It is a VB script, but it just calls a built in Mach3 function.

Someone used to make a board that would allow homing to an index, but it was transparent to Mach3, and still used the standard Mach3 home routine.

And I don't believe that ignoring limits is configurable.

[ger21]

My fear is that if I have a hall effect sensor failure my machine will go into the limit switch and continue to plow into the mechanical end of travel and just sit there and keep grinding away.

If you use normally open home switches, switch failure will not allow homing, so no damage can occur.

But if you're machine uses steppers, it's unlikely to cause any damage by running into the end stops, as the motors should just stall.

[ssutton]

Early on after getting the power up, I did crash and if was not pretty. I have a rack and pinion drive and the steppers would overcome the tension springs and begin to jump out of the rack and then slam back into them and repeat this until I killed power. This would damage the rack/pinion in very short order.

I have a work-around to the ignoring limit issue. (I realize that it is not an issue for most users). All of my limits are wired together and all of them are open, if any limit is tripped the G540 gets a ground signal on one of its input pins. That ground input now also feeds ground to one side of a 12 volt relay coil. The other side of the coil connects to 12 volts in my control cabinet. The relay energizes when a limit is tripped. My estop is wiring thru the normally closed contacts on the relay so that when any limit is tripped my estop now opens and stops all machine movement. I did install a flyback diode on the relay coil so that the G540 would not see any counter electromechanical force on the input. This has solved my problem.