There are three general classes of position sensors that – taken together – form a position sensing hierarchy. This hierarchy applies to any underlying sensing technology, for example inductive, capacitive, ultrasonic, or photoelectric. Going from the most basic to the most advanced sensor operation, the hierarchy includes:
Discrete – These are sensors with a fixed switching point. The detection point is set at the time of manufacture and is not user-adjustable.
Adjustable Setpoint – These sensors allow the user to adjust the detection sensitivity so that the switchpoint occurs at a user-defined setting. The classic means of setpoint adjustment is a potentiometer on the sensor that is turned with a small screwdriver. Sometimes, the sensor has a higher level of intelligence, allowing the setpoint to be locked-in through a pushbutton procedure. These pushbutton sensors are often called “teach” or “teach-in” sensors because the sensor “learns” the switchpoint when the button is pushed. Adjustable setpoint sensors sometimes include an analog output but not always. Some teach-in sensors have multiple outputs that can be adjusted to different setpoints, and some can be set up for more complex operation such as off-on-off “cam” or “detection window” profiles.
Continuous Output – These are sensors that provide a continuous output signal that changes in response to changes in position. The vast majority of sensors with a continuous output provide an analog signal, typically 0 – 10 VDC or 4 – 20 mA DC, but more and more are delivering their data digitally. They are usually “absolute”, meaning that they output the correct measured value when powered up and do not require session calibration to a known reference.
For basic applications, discrete sensors fit the bill because of their economical price, simple interface, and easy set up. Adjustable setpoint sensors are often employed where it is necessary to “dial in” a sensor’s detection characteristics, but changing programming in the controller is not feasible or desired. Continuous output sensors allow for flexible operation because the setpoints are defined in the controller program, not in the sensor itself. Continuous output sensors also allow tolerance bands to be defined in the controller, rather than fixed go/no-go detection points.
For more information on any of these sensor types, click here.