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Keyence Sensors in Parallel

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Has anyone used 2 Keyence PZ series or similar retroreflective sensors in parallel to detect on the closest distance of irregular shaped objects?

It should work if my sensor-fu is correct, but in actual use, has anyone had experience with it, and are there any 'gotchas' to watch out for?

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according to simplified circuit shown in datasheet, PZ series has open collector output. if this is correct and all sensors use same polarity output (all PNP or all NPN), there should no issues paralleling the outputs to drive common external load (PLC input of whatever). if the output stage is different (has built in load or stage is push-pull type etc.) then this would not the a good idea because output current would need to be limited. this could still be used by adding diode to each output.  note that adding diode increases output voltage drop but for usual operation at 24V this should be no problem.

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Can't say about the Keyence PZ in particular, but using two retroflective sensors in parallel has been done many times that I know of.  Especially when the target object presents a curved face to the sensor.

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yes... as long as sensor outputs are compatible for such use. also sensing function (reflective, optical, capacitive, inductive etc.) is independent from output type. 

but sensor market is changing and the number of choices is becoming greater. and today it is not uncommon to see sensors with different type of output. in the past vast majority of sensors was sold as open collector/drain of either "NPN" or "PNP" style. but times have changed, sensors with PP (Push-Pull), IO link etc are not exactly rare. so knowing the nature of the beast is important, and this is why one should read the datasheet and understand what output stage and configurations are possible.

when sensors have open collector/drain output and all of them are using same polarity ("NPN" or "PNP") then they can be wired in parallel without issue. but ... other possibilities exist...

nice characteristic of PP outputs is that one does not have to commit to one polarity. same sensor will be useful in installations that use either of the polarities. but connecting two or more such outputs together would create short circuit. this is why some sensors with flexible outputs choose to con bring this output on one terminal. using two separate terminals for this (black/white wire or contact 4/2) still allows paralleling without issues. others use configuration that depending on selection enables only one of the two output transistors. some have decided to just make it 2-wire sensor. this is nice but has leakage current and voltage drop is bigger than with 3-wire sensors. if such sensors are cascaded (wired in series) voltage drop is quite significant. 

but there are also 3-wire sensors where output is on single terminal. and if such sensor output stage is PP type, then paralleling is still possible, but external diodes are required. diodes can always be used in paralleling DC sensors but they too increase voltage drop slightly (using Schottky it is even lower).

hope this helps a bit. note instead of PNP/NPN i used term in quotation marks "PNP" or "NPN" because actual transistor output may be different but still performing same function.

sorry for using external site for image, there was a problem uploading attachment here and forum is undergoing some rework

https://pasteboard.co/JAyreyh.png

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