Choosing the Right Sensor
With all of the different types of sensors available and so many manufacturers and vendors competing for your business sometimes it can be difficult or confusing to choose the correct sensor for an application. In many cases such as detecting the end position of a cylinder it is easy to simply choose the hall effect sensor or proximity switch suggested by the manufacturer. There are times this may not be such a good idea however; I remember using a lot of Phd cylinders on packaging equipment in the mid 90s. At this time they had some serious problems with PNP prox switches burning out in the field. Of course the vendor or manufacturer wasn’t going to tell you that there was a problem until it became widespread so we ended up learning by trial and error.
In the case of cylinders there is also the question of whether to sense the cylinder position or the tooling. This is often dictated by the mechanical configuration of the actuator, size of target, accessibility and ease of maintenance and the environment around the cylinder. There are times when even a fiber optic photoeye may detect a small flag or slot rather than using a prox or hall sensor.
Photoeyes can also present a wide variety of options. Besides the standard configurations of thru or opposed beam, retroreflective and diffuse there are further choices such as color of LED light, polarized retroreflective with a corner cube reflector, lasers, convergent beam or fixed-focus, clear object detection and a host of other special purpose sensors.
Limit switches are used for more rugged applications and also come from a wide variety of manufacturers and with a large range of sizes and configurations. Whisker switches, plungers and rollers are some of the types of limit switches used for product and actuator detection. There are also precision switches available with very short strokes and precise actuation points.
Capacitive proxes are often used for level and non-metallic object detection. As with inductive proximity switches they come on a wide range of sizes and form factors, voltage types and materials.
Some things to think about when choosing a sensor:
Sensors are discussed further in the Primer under sensors. Catalogs from some of the major manufacturers can also be quite useful as well as some of the distributor sales people. Of course they can be somewhat biased toward their own products but I suppose thats to be expected!