Re: [CAD_CAM_EDM_DRO] Limit switches revisited

On November 14, 2005 07:23 pm, yet_another_kent wrote:

> I could imagine a mixed system using mechanical switches for detecting
> hard limits, while using some other kind of sensor, say an optical
> slot sensor triggered by a moving vane, to detect home position. Same
> questions about performance data....

I use a typical mechanical switch for Limit with a ramp-type actuator on the
moving member such that the ramp "wipes" the button. Therefore, overtravel
does not trash the switch.

I wire them up so that they are normally closed, and open upon overtravel. In
this manner, a disconnected cable, cut cable or corroded contacts will all
cause the machine to not move. (Nothing worse to find that the machine has
gone past its limit and was not stopped because the limit switches had
oxidized beyond normal operation or because the cable was pulled out .)

I also use a fairly small value pull-up resistor so that some measureable
amount of current is always running through the switch. (Contacts are
supposed to last longer that way)

But I use slotted optical switches for Home. While I've heard all sorts of
good things about the repeatability of mechanical switches, the opticals are
just about as easy to use (for a guy with a wee bit of electronics skill) and
just _have_ to be more repeatable, long term.

Last ones I bought were

Omron EE-SX4009-P1

cheap, easy to mount.

While perhaps not as reliable as a good-ole mechanical switch, should they
fail, they are more likely to fail entirely, causing a limit switch to
activate. This is, to my mind, preferable to a slow drift in Home position
that a mechanical switch _might_ be prone to.

My two cents.


Alan
--
Alan Rothenbush
Academic Computing Services
Simon Fraser University
Burnaby, B.C., Canada


Before me things create were none, save things
Eternal, and eternal I endure.
All hope abandon ye who enter here.