Choosing the Proper Sensor.


Okay, I know I said monday that I would make some comments on proximity switches and photoeyes “tomorrow” and that that tomorrow was yesterday. Between seasonal depression due to cold wet weather and some other items I needed to take care of it didn’t get done. I’m still working on it.

What brought this up was the customer I was working with had some bad experiences with the proximity switches they were using prior to this equipment upgrade and decided to use only photoeyes on the upgrade. That’s sort of like saying that you’ve had bad luck using open-end wrenches in the past and you are only going to use boxed-end wrenches in the future. Both will tighten and loosen a nut but there are applications where either one will be inappropriate.

Proximity switches are great for locating an object in a small specific area where photo eyes are great for finding out if a boundry has been crossed. Each type of sensor can be broken down into a dozen or more specific sensors for a very specific job.

Proximity switches can work off of inductance or capacitance. Inductive proximity switches are only going to be able to sense metallic objects while capacitance type sensors can detect almost anything.

Photoeyes can be broken down into multiple groups. Through photoeyes use an emmiter and a reciever mounted opposite of each other and provide a clearly defined line that it will sense objects crossing. Reflective photoeyes use their emmited light reflected back to them from a reflector to indicate a "clear" condition and are useful when a clearly defined line isn’t necessary and the objects being sensed may have holes that a through photoeye could "look" through. Diffuse photoeyes use reflective light to indicate a "blocked" condition. Light reflected back from the object being sensed is how these work.

The thing is that these are specialty devices that need to be chosen based on the application. If you are having problems with photoeyes or proximity switches in your application it is likely that the proper devices was never specified. The makers of these devices, like Banner Engineering or IFM Efector, are very good at helping you choose the device suitible for your sensing needs. Listen to their advice.

Oh, and when a controls engineer that has been specifiying sensors for the last eighteen years tells you you need to use a prox instead of a photo eye for an application, listen to him.