Linear Output

Linear Output

So far, all the different types of sensors we've discussed have had one thing in common; they all feature switching outputs. They're either "high", or they're "low". They're "on", or they're "off". The target is either present or it is absent. We have explored all the different modes of sensing, from inductive proximity sensors to photoelectrics, and we have laid down a preferred order of things. Use an inductive prox if you can. Look next to capacitive prox. If you need to use photoelectrics, look first to opposed mode pairs, etc. But there are times when knowing the presence or absence of a target is not enough. What if you need to know how close, or how far away, the target is. What if you need to know the thickness of the target, such as detecting the variance in thickness of a piece of material? What if you need to know not only whether or not there is material in a tank, but in fact you need to measure exactly how much material is in a tank? Or on a roll that is winding down? For these applications a switching output is not sufficient. You must use a sensor with a linear output. A linear output is commonly measured in milliamps (4-20 Ma) or in Volts (0-10Volts, or 0-5 Volts). The signal varies according to the distance from the target, and the signal is processed by an analog card in a programmable controller, or by a drive, or by a meter, such as a Red Lion or a Veeder-Root panel meter. Sometimes you might just want to take this signal and get a readout. Or maybe a readout needs to be accompanied by a response to the signal. A drive needs to speed up or slow down. A pump needs to cut on or cut off when the material in a tank reaches a certain level. A line needs to shut down when a roll of material is empty. Tension might need to vary as the material on the roll increases or decreases. The list goes on. The point is, you need a sensor which will furnish a linear output.

Photoelectrics and Prox Switches

There are photoelectric sensors and even prox switches with linear outputs. Certainly they cannot provide the resolution of a laser, but often your application does not require resolution that precise.
Ultrasonic Sensors
Ultrasonic sensors are a very commonly used type of sensor with analog output. Ultrasonic sensors are great for applications that call for a linear signal over a long range, such as measuring the unwind process of a roll, or measuring product in a large tank.

Lasers

Lasers generally have short sensing ranges, but they can provide extremely accurate analog feedback. Lasers are used when you need , for example, multiple outputs over a very tiny range. Measurement is a good laser application. With lasers you can sense minute changes in thickness of a material.