N/O proximity sensor

[RJS100]

I am ordering a couple proximity sensors. One that counts tool position on an ATC. The other senses clamped/unclamped status for the drawbar used in conjunction with the ATC.

Technically, N/C sensors provide better indications if a wire breaks etc. While I can source one that is N/C (it is also PNP), there are slightly better options available from Automation Direct that are N/O, NPN and have better sensing distance for aluminum which is what I need. Do you think N/O is a deal breaker?

Richard

[cncsnw]

Technically, N/C sensors provide better indications if a wire breaks etc.

A better way to think about it is this:

If triggering the sensor is supposed to cause machine motions to stop (e.g. a limit switch, emergency stop button, cycle-cancel button, etc.) then you should use an N.C. switch or sensor.

If triggering the sensor is supposed to cause machine motions to start or to proceed (e.g. a cycle-start button) then you should use an N.O. switch or sensor.

In both cases, that is the "fail-safe" choice: meaning that if the component or circuit fails -- e.g. due to a broken wire or bad contact -- the resulting condition is more likely to be safe than unsafe.


In an automatic tool change sequence, the tool-clamped and tool-unclamped switches are both "proceed" switches: the PLC program tries to clamp or unclamp the tool, and if it gets positive feedback to show that the action succeeded, then it goes on to the next step in the operation (Z movement, or carousel retraction). Therefore tool-clamped and tool-unclamped switches or sensors should be N.O..

The tool counter sensor can go either way, because the cycle will only proceed if it both opens and closes, in fairly quick succession. For that one, you should select whichever type of sensor will give you the default states, so you don't have to invert the input: the input should be closed if the carousel is aligned and locked in a position, and the input should be open if the carousel is in transit between positions. Whether that requires an N.O. or N.C. sensor depends on where you mount the sensor.

[RJS100]

Thanks. Great explanation! In this case, N/O will work great!

[Ken Rychlik]

For homing proxy's I see no problem with it. If it were going to be set up as a limit proxy, I would lean more towards N/C. For the rotary, if it doesn't stop at the proxy, it doesn't really crash anything either.

[RJS100]

Makes sense.