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:

  • What kind of object am I detecting?
  • How far away, what kind of material, can I contact the part?
  • How much space do I have for mounting? Is the sensor easily accessible?
  • What kind of voltage am I using for I/O?
  • What kind of environment is the sensor to be used in?
  • What kind of wiring method am I using? (conduit entry, cable, quick disconnect, etc.)
  • Are there special requirements such as color detection, background suppression or precise location?
  • Is it fast and easy to obtain a replacement in case of failure?
  • 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!


    Electrical Engineer and business owner from the Nashville, Tennessee area. I also play music, Chess and Go.

    1 Comment on “Choosing the Right Sensor

    1. A couple comments from my experience:

      When testing your sensors, try to find and test the widest variety of potential targets as possible. It’s easy to test a machine with the small number of sample parts, used over and over again. Then, when the machine hits production, it has problems because the real world parts are different (e.g. much more out of spec) than the test parts.

      The same sensor can act very differently with different targets; for example, Photo-Electric and fiber sensors will be affected by the target’s reflectivity. If you have to reliability detect a wide range of targets, it can be tough to find one setting that works for all. This leads to my second comment….

      I hate Photo-Electric amplifiers that don’t have a manual setting mode. For example, the Keyence PS01 setpoint can only be set using one of 4 calibration routines. When trying to detect a widely varying range of targets, it can be vary hard to get it set to work right in all cases. After wasting many hours setting up the PS01, I was ready to dump it for all future machines — until I checked my alternate vendors, and discovered none of them had an acceptable replacement with manual setpoint settings.