Wiring home and limit switches

[irving2008]

The reason is that the inputs to the board have to be switched between 0v and +5v. generally the inputs are pulled up to +5v by a resistor on the board and the switches then short the input to ground. An alternative approach is to use both poles of the switch to switch the input line between a 0v wire and a +5v wire. This is how the home switches in the first diagram are wired, as individual inputs. But the same approach is used for the limit switches in the lower part of the diagram and this is just wrong... in fact the way that is wired when one limit switch is activated it puts a short from 0v to +5v, which will probably have the effect of stopping everything but for the wrong reasons.... I would ignore that and go for the single chain of switches. However I would seperate the PANIC switch from the limit switches. A panic switch should always control a relay which supplies power to the motors. That relay MAY have a contact that does the panic input to the breakout board and thence to MACH3 as per these diagrams... one should NEVER rely on MACH3 to stop the motors in an emergency, the panic switch should always act directly...

[Colin Barron]

The reason is that the inputs to the board have to be switched between 0v and +5v. generally the inputs are pulled up to +5v by a resistor on the board and the switches then short the input to ground. An alternative approach is to use both poles of the switch to switch the input line between a 0v wire and a +5v wire. This is how the home switches in the first diagram are wired, as individual inputs. But the same approach is used for the limit switches in the lower part of the diagram and this is just wrong... in fact the way that is wired when one limit switch is activated it puts a short from 0v to +5v, which will probably have the effect of stopping everything but for the wrong reasons.... I would ignore that and go for the single chain of switches. However I would seperate the PANIC switch from the limit switches. A panic switch should always control a relay which supplies power to the motors. That relay MAY have a contact that does the panic input to the breakout board and thence to MACH3 as per these diagrams... one should NEVER rely on MACH3 to stop the motors in an emergency, the panic switch should always act directly...

It is interesting what hitting the Emergency stop will do, if too heavy a cut is going to damage the spindle then killing power would probably be ok, but if someone has a hand trapped then that is not the best option.

I worked on japanese grinding machines where all end of travel prox switches and the emergency stops were wired into the omron c500 machine controller. If the emergency stop button was pressed, the spindle would turn off, the table would retract in case someone was trapped, then power would go off turning off hydraulics and all power.

[Danielroyal25]

Hi guys, I'm working on the control panel for my cnc 4 axis router (2nd x axis to be slaved). I will be running a 24v safety system and within this will be 2x e-stops, limits and home switches.
When triggered the 24v system will cut power to the 70v supply to the steppers but I'm confused how the triggering item I.e. e-stop will communicate to the bob to show Mach 3 what has caused the stop if wired in series as surely any item within the parallel wiring loop could be seen as the cause? Or is this the case, Mach just sees a fault and its down to the operator to find the cause?
Are relays typically used from the 24v system to send the control signal to the 5v break out board inputs?
I am working on a diagram for my system which could do with being looked over by someone that knows what they are talking about, should have this posted mid next week if you could comment. I have seen loads of diagrams of set ups but want to draw one for my system to get it clear in my head as this side of it is not my strong point. Cheers.

[JAZZCNC]

Hi Dan,

The way it's done is to series wire the E-stop and limits thru the 24V coil (coil not contact) of a relay. Then take the +5V signal from the BOB and run thru the contact of same relay and back to -5V on BOB.
This then informs Mach an Estop has happened so stop the G-code from running.
Inside mach you just set inputs up to watch same pin for both e-stop and limits. This does mean fault LED's in Mach will show both E-stop and limits tripped at same time. If you want to know which individual switch tripped then only way round this is to use an input for each switch. Has you can see this will need many inputs which you don't have and In practice this doesn't matter much has you'll know what's caused it.

The other contacts of the relay can then be used to either control other relays or directly turn something off IE: 70V supply.

Home switches shouldn't be run thru relays has the delay is too long and makes them inaccurate. Just use the 5V signal from bob for relays has your only using it when homing so very rarely do they get affected by noise etc so ok to do so.
They can also be wired in series and just use 1 input has Mach only homes one Axis at a time and actually doesn't care where the switch is pressed because it just records the coordinates for the Axis it's referencing at the time when it see's the input trigger then does the same again for next Axis when sees input trigger again.
In theory and practice when wired in series you can set the home position for X axis by pressing Z or Y axis switch because it just records the position it's at for Axis it's referencing when input fires.

Note If your using 2 motors slaved to an axis then you must have each motor on separate inputs. One can be wired in series with other axis but the other must be on it's own separate input. If not then Mach can't individually home each motor for squaring gantry.

[Danielroyal25]

Cheers Dean! That's a big help.

I don't know if this should be posted in another section really, but its all related.

When switching off the 70v supply should it be switch before or after the transformer or does this not matter?

Do many people have an input to the 24v system from the pc incase the computer crashes / turns off for some reason acting as an e-stop or would the code just stop running the anyway not harming the machine?

So that the drain resistor isn't constantly connected during the 70v supply is it connected via a relay so that when power is removed the resistor is engaged to drain the motor supply system?

[JAZZCNC]

When switching off the 70v supply should it be switch before or after the transformer or does this not matter?

It's arguable but you could break circuit after transformer but me personally for safety sake then I want power killed. Same goes with VFD.? you could just send a stop signal to vfd but again for safety sake just kill power to vfd.
It's like with the drives you could just kill the enable signal and keep the drives powered but that's not safe enough for me and I want the machine off line full stop.
This is why you have a master relay.? One relay controls all other relays and if the other relays which are controlled by limits, E-stop,Door,coolant switches etc are not safe or ready then master relay won't allow a reset.

Do many people have an input to the 24v system from the pc incase the computer crashes / turns off for some reason acting as an e-stop or would the code just stop running the anyway not harming the machine?

Well that's the job of a charge pump feature if BOB has it, usually from pin1. Charge pump monitors pin 1 and if it doesn't see a signal shuts down it's outputs etc. Not full safety but enough to E-stop the system if PC crashes which then if designed correctly should shut down the master relay which kills power to every thing else and not allow restart.

So that the drain resistor isn't constantly connected during the 70v supply is it connected via a relay so that when power is removed the resistor is engaged to drain the motor supply system?

Yes exactly just use a NC contact so when working it's disengaged from circuit and put back in to drain caps when off.

[Danielroyal25]

If I use separate home switches for x,y,z & x2 linked directly to the bob and then have my individual limit switches + and - for x,y & z linked via the 24v circuit, where are the home switches to be positioned?
Originally I was going to use the home switches to act as limits on one end and have limit switches on the other but if I go for the setup above I'm confused as to where the homes should be placed.