Can HomeAll act as a limit switch prior to homing the machine?

[clough42]

Is it possible to make the home switches stop the machine movement when jogging prior to homing?

I have a slaved dual-drive gantry machine set up with three homing switches (-X, -Y master, +Z) configured on a HomeAll input, a slave axis homing switch (-A) configured as SlavedHomeInput, and limit switches on +X and +Y, configured as FirstAxisPlusLimitOK and SecondAxisPlusLimitOK.

Prior to homing the machine, if I jog it, the limit switches stop the movement, but the machine will happily jog right past the home switches and crash into the hard stop.

Do I need separate limit switches to protect the machine from crashing prior to homing?

I suppose it would be possible to wire the switches in such a way that the home switches trigger both HomeAll and LimitAll, but that feels like I'm trying to do something the hard way.

[ShawnM]

After you power up the machine and press the reset button on the VCP the first thing you do is HOME the machine. Why are you jogging it around? And if you are jogging it around you should be aware that until it's HOMED the machine doesn't know where the HOME switches are. Simply press reset and then cycle start to HOME the machine and then you don't have to worry about crashing it.

[clough42]

After you power up the machine and press the reset button on the VCP the first thing you do is HOME the machine. Why are you jogging it around?

I understand that if I do everything properly in the right sequence, I won't crash the machine. I'm trying to understand if I can set it up to protect it or at least reduce the consequences if I make a mistake.

[ShawnM]

I understand that if I do everything properly in the right sequence, I won't crash the machine. I'm trying to understand if I can set it up to protect it or at least reduce the consequences if I make a mistake.

I get that but if you follow the proper procedure and home the machine FIRST as the software expects then you reduce the consequences of making a mistake to ZERO.

[clough42]

I get that but if you follow the proper procedure and home the machine FIRST as the software expects then you reduce the consequences of making a mistake to ZERO.

You're using a different definition of "mistake" than I am if you rely on the operator to always follow any procedure--even a simple one.

Back to the original question: If I want an absolute limit that works even if the machine isn't homed, do I have to install a second switch, or is there a way to make the home switch perform that function?

[Yucholian]

I understand that if I do everything properly in the right sequence, I won't crash the machine. I'm trying to understand if I can set it up to protect it or at least reduce the consequences if I make a mistake.

I get that but if you follow the proper procedure and home the machine FIRST as the software expects then you reduce the consequences of making a mistake to ZERO.

You NEVER, EVER design any equipment with the assumption the user will follow expected behavior 100% of the time. We are not perfect, mistakes happen despite the operator knowing what the “right thing” is. It’s called an expected use and even the FDA requires medical device companies to not only design to specifications, but what a human may do.

[RogDC]

Is it possible to make the home switches stop the machine movement when jogging prior to homing?

You might be able to run the limit switches thru relays and modify the PLC so once the homing sequence starts the relays will place the switches in the correct sequence for the auto-squaring.

If you 'Park' the machine with the default park macro, it will be very close to the homing position when you start up the machine and will only take a minute to home.

I have dust hoses to the rear of the machine to my Z Axis so I need to park my machine when I am done to the rear of the machine to prevent my hoses from stretching out when it is not active. If I don't do this my hoses will stretch and interfere with my ATC tool rack at the rear of the machine if I don't have the Dust Collection on. My travel is Y-73, X 61.5, so I need to park near X-30.75, Y 72.5 when I shut down.

My plan is to modify the 'Park' macro for shutting down the machine which will go to the correct X-Y location, then a startup 'Pre-Home' macro with button that I can move closer to the home position on start up prior to homing, then home. When I park the machine it will go to X-30.75, Y 72.5, then Pre-Home position would move to to X1 Y1, then I can press Home. It will take a little longer to home than using the default Park command, however I figure 1" should be enough to allow any bumps that may happen when the machine is not powered up.

[cncsnw]

You could also write PLC logic that triggers a fault on a falling edge of HomeAll, only when home is not set and no program cycle is running.

E.g.

Code: Select all

IF !HomeAll THEN (HomeAllTrippedPD)
IF HomeAllTrippedPD && !SV_PC_HOME_SET && !SV_JOB_IN_PROGRESS
  THEN FaultMsg_W = your-fault-message-identifier,
       SET OtherFault_M

[clough42]

Is there an issue if I wire the machine so the one switch triggers both HomeAll and LimitAll at the same time? Will that interfere with the homing cycle?

[cnckeith]

Is there an issue if I wire the machine so the one switch triggers both HomeAll and LimitAll at the same time? Will that interfere with the homing cycle?

yes that will interfere with homing cycle as the home cycle will see a limit tripped and think something is wrong.