Input Sensors – Your eyes, fingers and…more?

No need to "eyeball" it any more. Input sensors take the place of our human senses… and do a great job of it, to boot.<br><br>There
 are several types of input sensors. One of the first I encountered was 
the photo electric sensor or photo eye. These eyes controlled remote 
relays with a set of integrated mechanical contacts. Located inside the 
sensor body, the contacts changed state with a tiny magnetic relay. Schneider Electric (Square D)

Steve Maurer, IME

No need to "eyeball" it any more. Input sensors take the place of our human senses… and do a great job of it, to boot.

There are several types of input sensors. One of the first I encountered was the photo electric sensor or photo eye. These eyes controlled remote relays with a set of integrated mechanical contacts. Located inside the sensor body, the contacts changed state with a tiny magnetic relay.

Some could be fitted with a small logic board, located in the sensor head. This allowed the output to be timed, waiting for a set interval to pass before changing state.

One setup was nicknamed the traffic cop. Monitoring boxes on converging conveyor belts, it kept "traffic accidents" to a minimum. On longer belts, photo eyes were used in tandem on a single conveyor. Latching circuits using remote relays kept boxes out of the "intersection" until incoming boxes cleared out.

More complex systems integrate PLCs (programmable logic computers) and DCSs (distributed control systems) to control both machinery and processes.

A vertical packaging machine I help develop contained no less than 33 sensors. Some photo electric sensors were thru-beam, others were retro reflective. A few were diffused photo electric, "seeing" the light reflected off the target instead of a reflector.

Other sensors on the equipment were inductive metal sensing proximity switches. These prox sensors monitored component location and were used in conjunction with timers controlled by PLCs.

Intelligent automation required new sensor technologies
In the past, the only signal needed was a "yes or no" indication. Called discrete sensors, they used an on/off logic. Was the device in place, yes or no? Many were contact type sensors, such as limit switches.

But, as automation evolved, positioning was not enough. Now it was necessary to know "how much." Ultrasonic sensors were developed to handle this process question. Often used to measure the liquid or solid levels in tanks, these are often positioned above the media to measure fill capacity. Think sonar for containers.

Because they are not affected by color or reflected light, they are valuable for bottling or packaging applications as well.

Transducers monitor pressure, both positive and vacuum.

Another technology is also employed for level sensing. Capacitive sensors don't require container penetration. This eliminates the possibility of leaks in the sensor mounting.

These sensors aren't just used in process applications. In fact, if you've ever hung a picture at home, you've probably used one. Electronic stud finders use capacitance sensors. Some advanced sensors even warn of live circuits behind a wall.

After all, you don't want to get buzzed when driving a nail, now do you.

There are many considerations when choosing a sensor. Application is one. Environment is quite another. And that's a topic for the next article.


Photo courtesy of Schneider Electric (Square D)

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