Re: Home Switches

William Scalione wrote:

> From: William Scalione <scalione@...>
>
> Anyone know what the best type of switch to use for a home switch?
> I am using mechanical "Cherry" microswitches but they don't always
> actuate at the exact same position, which makes them kind of useless.
>
> Limit switches don't seem to matter, a couple thousaths either way, as
> long as they work but my hone switches need to be right on
>
> Proximity sensors? Photoelectric sensors? Other? or just better
> quality mechanical switches?

The way the home switch works on encoder-equipped machines with
servo control, is that the rotary encoders have an 'Index' channel,
usually called the 'Z' channel (while the quadrature channels are A and B)
which generates a short pulse once per revolution. Linear encoders
have similar functions. The home switch and the encoder are aligned
so that the home switch closes just before the encoder passes the Index
location. The Home switch uniquely identifies one pulse of the index
signal as the true home, other index pulses at other locations are
ignored. This way, as long as the home switch can click in and out
in, say, .1" movements, there will be perfect selection of the correct
index pulse with a .2" lead screw.

If you are using a stepper system, you could put an optical sensor and
a disc with one very narrow slot on the leadscrew, and use that to refine
the home switch signal. That should get you to within a few thousandths.

Jon