hall effect home switches

My concept of a “home” switch is one that is closed when at, or close to, the home position; and on either side of the home position and at a small distance away from it, the switch is open. The homing sequence on the AXIS simulator seems to reinforce this concept in that it seems to try to find the limits on either side of the home switch and then center over it.

While trying to conceive of an appropriate cam mechanism to effect this, I realized that the easiest and most reliable mechanism for achieving this would be a hall-effect switch and a magnet. This is what I have implemented on my LMS3960 mini-mill X and Y axes.

Now that I’m past the stage of configuring the hardware inputs, I’ve found that the implementation of the logic doesn’t appear to support this configuration. In particular, after reading Chris Morley’s write-up about homing and limit switches ( forum.linuxcnc.org/32-documents/12146-an...oming-hope-its-clear ), it appears that LinuxCNC, or at least the “firmware” for the Mesa 5i25 board that I’m using, doesn’t think the same way. In particular, it appears that the homing switch is expected to be like a limit switch, except not at the limit, but rather far enough away from center/home so that the axis can be moved in a single direction to find it rather than using a gradually increasing amplitude back and forth motion to find the home switch more or less in the center of the range of motion.

Can somebody please confirm that this is true? Or, if it isn’t, either provide documentation that will enable me to use the home switches that I’ve implemented, or direct me to it?

Unfortunately, the combination of scattershot organization of the information that is available for LinuxCNC and the lack of clarity in the user interfaces of the software make it almost impossible to catch these issues prior to getting there, and make dependence on folks like you extremely important.

Thanks for your help!

Have you looked at:

linuxcnc.org/docs/html/config/ini-homing.html ?

AFAIK LinuxCNC always homes in one direction, this direction is selected by ini constants

Your choice of limit switch shouldn't matter (switch closed = tripped, or switch open = tripped) as the signal can be inverted within LinuxCNC to gain the required logic polarity to indicate that the axis is at limit or home positions.

There are, however good reasons to use limit switches which are closed when not tripped, the primary one being that LinuxCNC will fail safe if the supply to them is accidentally disconnected.

I think you should forget about your concept of homing and take the time to read the homing configuration documentation PCW has linked to. Note that if you are using V 2.8 (master), then homing is a bit different so make sure you read the correct documentation. I found this to be an excellent article and is easy to find from the documentation menu.

There is nothing mentioned about back and forth motion and as PCW says, it is one way. Also the homing switch is not designed to be a pulse and must stay active until the axis decelerates to a stop as part of the homing process. In my case, I allow about 100mm for this to happen.

Alas, another logical idea about how things ought to work in the absence of good documentation has bit the dust. Meanwhile, I think that the Chris Morley documentation is the best that has been referred to. It's the clearest that I've found. Too bad it's so far to find.

So, now I'll have to figure out how to make an intrinsically less reliable method of providing a home switch. Or, perhaps just make it a limit switch. It seems like it doesn't matter which.

The thing about LinuxCNC is that, when it works, it works really well. But getting it to work is maddeningly difficult. Oh well.

Thanks to all for your help.

Dr. John wrote: The thing about LinuxCNC is that, when it works, it works really well. But getting it to work is maddeningly difficult. Oh well.

It took me about 10 minutes from when the ubuntu ( emc2 at that time) started to having a working cnc when i first did it a looooooong time ago, albeit i had one of the motors moving in the wrong direction so ended up with the "EMC2" written upside down, and i did not even know that i could reverse the rotation in the config or ini file or hal file, so i powered down and reversed one side of the wiring to the stepper motor. This was without reading anything ever about Linuxcnc ( emc2 ). I refuse to refer to something that easy as "difficult". By the end of that evening i had read a lot about hal and how it functions and got enlightened by the scope of possibilities it has.
Oh and it does work, very well. Extremely well. It works much better than a lot of industrial software solutions i had experience with.
Back on topic.
You can use your implementation of limit switches as is, it will work properly. What i do not understand is where you got the idea that the limit switches should be passed back and forth and stop at the switch ? That does not work, and it does not exist in any machine i have used or worked on. Stopping at the switch will inhibit all motion, so it has to find it, move back and stop as soon as it passes the switch. Linuxcnc has also the option of moving fast to find the switch, backing off and moving again slowly to find it again and back off. That provides for more precise homing and it is very useful for large machines.