Thread: Wiring home and limit switches
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.
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.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.
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?Dan
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.
The Following User Says Thank You to JAZZCNC For This Useful Post:
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.Dan
The home switches are normally positioned just before the limit switches and so they home to a corner or end you would like to designate Zero position in Machine coordinates. That said they can be positioned anywhere you like really.!!
Personally I set my machines up so they home to corner then all my moves are always positive in machine coordinates. Mach is then setup to always home negative, this way it can't ever home in the wrong direction.
Now to answer the question of how do you put a home switch before limit switch.? It's easy really.!! . . . . You position The switch so gets triggered by a ride over Ramp not a hard stop. The ramp allows the switch to ride over while cutting and still have access to Limit switches at the ends. (Thou better other way round so static switch with mobile ramp triggering.!)
Home switch inputs are only watched or checked while homing so when it rides over the ramp it triggers, rest of time it's ignored.
This does mean the Home switches need an input of there own, they can still be wired in series but separate input to limits.
This is the reason why you can use Limit switches has home switches if sharing the same input or switch because the opposite is true while homing.!. . Limit's are ignored.
So if set to do this Mach treats the input differently while homing and instead of seeing the switch trigger and setting E-stop condition it now sets the current position has Zero Machine coordinate and resets Axis DRO's to zero.
Really micro switch type switches are not the best for Home switches unless very high quality due to them not being very repeatable. MS are fine for limits has they just work has ON/OFF triggers.
Inductive proximity switches or hall affect switches are better being much more accurate/repeatable and with contact less operation so they work without ramps etc.
Hope this helps clear it bit more.!
The Following User Says Thank You to JAZZCNC For This Useful Post:
awesome, cheers Dean. That's what i was unsure of during normal operation the home switch being hit / operated, but that clears it up.
With X2 i just meant the home switch on the slaved X axis to get the gantry square (4 switches in total).
I have some Omron micro switches for the limits and Honeywell switches for the homes. Cheers!Dan
I got a bit annoyed with my machine stripping the delrin on the Z axis, so I decided to put limit switches on it. At the same time I put a couple of extra home switches to give home and limits on X++, X--, Y++, Y-- and Z++. While I was about it I put in status LED's (C'mon it's my toy )
The Limits had to go on a second BoB that I had that worked on 5v, so I basically OR'd each module to operate a reed relay, to give a Ground signal to indicate a limit switch had been triggered.
My main control board worked on 12v for the inductive prox switches and again each axis was OR dioded on the extra inputs for the X and Y axis's.
The only stupidity was the Override to kill the limit signal was a push to break switch. This means I have to hold it pressed while I reset Mach3 and keep it pressed while I jog off the limit. I am changing this to a SPDT switch which puts power onto a buzzer when the override is on. I know it is overkill but hey - I have retired and I need all the excuses I can find to stop other people finding me jobs.
OK - If anybody wants more detail I am happy to provide it, but right now I have to go and clean up in the Kitchen
I'm just about to order my cy cable for the limit switches. If using a multi core cable I.e 4 core, is only one single core used to physically wire the switches together and the other cables are left unconnected?
Is the minimum 2 core cable that you can get?Dan
By vputz in forum LinuxCNC/EMCReplies: 4Last Post: 15-10-2014, 08:51 AM
By Wal in forum Resources & HelpReplies: 6Last Post: 17-11-2013, 04:52 PM
By manofgresley in forum General ElectronicsReplies: 30Last Post: 02-01-2013, 02:08 PM
By HankMcSpank in forum General DiscussionReplies: 2Last Post: 13-11-2011, 12:42 PM
By NICKMODELMAKER in forum Gantry/Router Machines & BuildingReplies: 1Last Post: 29-05-2010, 12:38 PM