How do I choose the right sensor?

Posted on by Henry Menke

How do I choose the right sensor?

Selecting an industrial sensor can be a daunting task.  With so many different sensing technologies and the endless variety of products in the market, how is it possible to find that one ideal sensor for any given application?

Turns out, it’s not really so much a process of selecting the right sensor…it’s really about eliminating all the wrong choices.  Selecting a sensor is a process of asking a series of questions to eliminate any technology or product that doesn’t fit the application requirements.  For example:

1) Type of Sensing

Am I sensing a process parameter (e.g. temperature, pressure, flow), the presence of an object, the distance to a target, or the position of a mechanism? Let’s say I want to detect the presence of an object. That means I am looking for some kind of proximity sensor (sometimes called “presence sensors” or “object detection sensors”). There are several kinds of sensor technologies that can detect the presence (or absence) of an object. Inductive, photoelectric, capacitive, magnetic, and ultrasonic sensors are all possible candidates at this stage of the selection process.

2) Composition of Target

What is the material composition of the object (metallic, non-metallic, solid, liquid, granular)? Let’s say the object is metallic. Inductive, photoelectric, capacitive, and ultrasonic sensors are all capable of detecting metallic objects, so we need to ask some more questions.

3) Distance to Target

How far away from the object must the sensor be? Well, if I am building a compact piece of automation machinery, I want to keep everything as close together as possible. I expect the sensor to be installed pretty close to the metallic object that I want to detect. In this case, an inductive proximity sensor would be the best choice. Although inductive sensors have rather short sensing distances (typically 1mm up to about 50mm) compared to other sensing technologies, they have some strong advantages: a) they ignore all materials except metal (e.g. water, oil, non-metallic dust) b) they are very robust physically and c) they are relatively inexpensive. Let’s say that I have decided the sensor needs to see the metallic target at a distance of 4mm.

4) Form Factor

What sort of physical form-factor best fits my application? In our example, it’s fairly tight space and there isn’t much room to mount something with a lot of length to it. That eliminates the most common inductive proximity sensor type: the threaded tubular housing. We’re going to be looking at some kind of low-profile, flat sensor, typically called a block style or rectangular type.

5) Control Interface

What kind of controller interface and switching logic is required? These days, most sensors are 3-wire DC types. There are other types out there, such as 2-wire DC and 2-wire AC/DC, but by far the vast majority of control systems will require a 3-wire DC sensor. In our case, we need a “3-wire PNP N.O. sensor,” meaning 3 wires (+24DC, 0VDC, and output), a PNP-type “sourcing” output (current is sourced from the sensor to the controller), and “normally open” switching logic (means the output is “off” when the sensor does not see the target).

6) Special Requirements

Are there any special application requirements? Special application requirements might be things like high temperatures (more than 80 degrees C), nearby welding processes, or high-pressure washdown procedures. In our machine, we don’t expect anything worse than a little machine tool oil getting splashed around. This is completely normal for inductive sensors to work around, so nothing special is required other than an IP67 liquid ingress protection rating (standard on most good-quality sensors).

7) Electrical Connection

How do I want to make the electrical connection? Sensors are typically available with three kinds of electrical connections: a) pre-wired cable with flying leads b) integrated quick-disconnect connector c) a pre-wired cable with a molded-on connector (often called a “pigtail” connector). A fourth connection type – terminal chamber – was once common in the days when proximity sensors were used to replace mechanical limit switches, but has become less common in today’s industrial environment.

Armed with the above information, it’s now possible to visit a sensor manufacturer’s website or catalog and be able to find an appropriate match for nearly any application.  If you’re still not sure, sales people and technical support personnel are always ready to help you find the right sensor for your application.