Optical switch, on when I expected off

[will gilmore]

I'm having an electronics problem and I'm hoping someone can help. I ordered three of these optical switches : http://search.digikey.com/scripts/Dk...ds=365-1234-nd to use as spindle index pulse and two home switches. Data sheet here: http://www.optekinc.com/datasheets/OPB930-940-L&W.PDF

I got OPB941 buffered open collector (top right page 2 of data sheet). To test I wired from 5VDC to LED (w/ resistor) to "output". I thought with the slot unblocked the output transistor would be on and LED would be lit. Turns out the opposite is true. Slot unblocked = LED off, Slot blocked = LED on.

For the spindle pulse this is fine but for the home switches I want the "normally closed" logic so if I have a broken wire I don't crash. I can just order the inverted open collector but I'm curious how the circuit actually works. Obviously I'm misreading the diagram.

[H.O]

Hi Will,
Try this: Wire the resistor from Vcc to Out. Wire the LED from Out to GND. How's that?

/Henrik.

[mactec54]

Hi will gilmore

You can get a proximity switch from DMM Tec for $10 this will be much better for a home switch than what you want use

[will gilmore]

H.O -- Yes that reverses the logic but the circuit still relies on the transistor going on when the slot is blocked. If a wire is broken the output is still held high.

mactec54 -- How about a link? EDIT: found it, http://www.dmm-tech.com/Pricing.html , still normally open

[mactec54]

Hi will gilmore

(still normally open)

Thats the way they should be, for home & limit switches, all machining centers Etc are this way

[will gilmore]

I have no experience with this stuff but I thought it was common practice at least with limits to have them NC. What happens if the wire to the limit switch is cut? If switch is NO You wouldn't know until you crash. With NC when the wire is cut it triggers that limit and you investigate.

[Al_The_Man]

Doesn't Mach look at the same switch for home and overtravel?
As opposed to Commercial systems that use two separate switches, where the home is N.O. and the O.T is N.C.?
I don't use Mach, but I believe this is the method they can use i.e. one N.C.
Al.

[mactec54]

Hi Al The Man

That is right for most older systems, new systems/machines don't have hard limits anymore, (some still do) but they don't have soft limits

They have/ use soft limits which works a lot better, less wiring Etc

Will
You only need your home switches in Mach as they have soft limits to set in Mach that you can set for your safety fence, If a wire breaks it does not matter, as you have it in the control software, so the machine can't go any were out side the fence if set right, But you always have to home the machine for soft limits to work, unless you have absolute encoders

[Al_The_Man]

Fanuc and Mitsubishi still use discrete limits as well as soft O.T.
It is my understanding that in Mach you can have 6 N.C. switches in series 3 will operate as home & limit the other three limit.
This allows one input for all 9 functions.
The down side to soft limits only, if a servo runs away a soft limit will not stop it.
Al.

[mactec54]

Hi Al The Man

Only some as I said offer both

If a servo is in a properly engineered system it can not run away as you say

( Years ago Yes, To Day No) It has to have a signial from the control to the drive before power can get to the motor, to make it move, & if it was out of control the drive would fault out & stop it as well, Also if a soft limit is in place it to will will stop a run away servo if the drive did not as well

What will make a servo run away, a Bad encoder, so what happens when you get a bad encoder, modern drives will fault out & stop the run away servo, no hard limits needed

Its like most things its extra security to have both soft & hard, I once thought to that you needed to have both soft & hard limits, but over the years of machine building you get to no that both are not needed, with modern drives & control software