Choosing the Right Input Sensor

With so many types of input sensors available, you need to narrow down 
your selection to what will give you the feedback required. As with 
human "input sensors," each type has a dedicated function. You need to 
match the sensor with the object being sensed and what that feedback 
does.<br><br>Another consideration is the action taken when the sensor 
notices a change of state. In our human example, a sudden bright light 
sends a signal to the brain, causing us to shield our eyes. Schneider Electric (Square D)

Steve Maurer, IME

With so many types of input sensors available, you need to narrow down your selection to what will give you the feedback required. As with human "input sensors," each type has a dedicated function. You need to match the sensor with the object being sensed and what that feedback does.

Another consideration is the action taken when the sensor notices a change of state. In our human example, a sudden bright light sends a signal to the brain, causing us to shield our eyes.

Knowing what you're targeting is the first step. It may be as simple as a mechanical sensor with a trigger. When the trigger moves by physical contact, a contact is either opened or closed. Obviously, an inductive sensor requires a metal target. On non-metallic objects, a photo sensor or ultrasonic sensor can sense the object.

The sensing element itself might not need power. However, the contact will be a part of circuit to convey the message. For example, a closed contact may complete the circuit, turning on a warning light, sounding a buzzer, or activating a similar function.

With inputs sensors, you not only consider what to sense and how, but how the information is communicated and acted upon. Perhaps that input needs to trigger a more complex process.

You may need a photo electric sensor, an inductive prox or ultrasonic sensor (using sound wave feedback) to start a process. These often send the output to a "brain" made up of PLCs or similar devices.

Input devices may be discrete, meaning a Yes/No or On/Off logic. Or, they may be analog sensors. Analog sensors are used when you need to know "How Much?" of a state is present. The output of these inputs consists of a range, not a simple yes or no.

Often their output is measured in milliamps (transmitter types) or in voltage measurements such as 0-10 VDC (for transducers). While more granular information is possible about the actual state of the measured process is possible, there is one caveat. These sensors must be calibrated with the process controller.

Once your input sensor type is determined, you're still not done. Consideration must be given to the environment the sensor will be place into. For that, you need to check the IP rating.

The IP codes consists of at least two numbers – IP##. The protection against solid objects is denoted by the first number. The larger the number, the smaller the openings in the sensor.

Protection against water and liquids is shown by the second number. The more hostile the environment, the higher the rating number should be. The "1" rating corresponds to dripping water… the "8" means protection from prolonged immersions under certain conditions.


Photo courtesy of Schneider Electric (Square D)

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