Re: [CAD_CAM_EDM_DRO] Home /limit switches

Robert Allen & Marsha Camp wrote:

> Jon Elson wrote:
>
> > Robert Allen & Marsha Camp wrote:
> >
> > > Anyone have a recommendation for high accuracy home
> > > switches, will regular micro switches work? I have CNCpro
> > > which apparently goes to the switch then backs up a set
> > > distance so the home switch must be repeatable to at least
> > > .001" I would think maybe closer.
> >
> > You may want to add a second sensor, probably an optical
> > 'slot' sensor that detects a disc with one slot on the motor.
> > An and gate can combine the coarse switch and the fine
> > sensor to get a more repeatable position.
> >
> > Jon
> >
>
> I think the encoder has an index mark (ch. A, ch. B and I) but I don't think
> CNCpro can use it.

Well, of course, EMC can use it!

> I wonder if there is a way to use it. I don't quite see how I can get
> movement back to the index mark without building some kind of circuit that
> stops the input to the gekkos when the home switch is reached and the table
> backs up a set distance.

It depends a bit on exactly how the program expects the hardware to
function.

> I would have to set the back up distance to go further than the index mark
> and stop the gekko input, how, a transistor shorting the input from the
> software high or low as needed. This might be accurate if the homing speed
> was slow enough. I see this getting a lot more complicated than I thought it
> would be.

This sounds dangerous. (Not physically, just that it could cause lost
steps, sometime.) I think some sort of trick that would make the home switch
look like what the program is looking for is in order. If it moves until it
detects the home switch, then backs up until the home switch releases,
and calls that exact position home, then here's how to do it.

Have a flip-flop of the RS type which sets when the home switch transitions
to indicate at home, and then clears when it sees the index pulse. An RS
flip flop can be made with a pair of NAND gates, with each output wired
to one input of the other gate. The home switch would need to cause a
momentary pulse when it closes. This can be done with an RC differentiator
circuit.

Jon