My friends from Pepperl + Fuchs shared this information regarding measurement displacement sensors and I'd like to share it with you:
Measurement displacement sensors are used when you need to know a continuous distance that the sensor is from a selected target. There are two primary output types that can give distance. The discrete digital output is basically a yes or no signal threshold that will change the output of the sensor when the target reaches a certain distance or falls within a certain range.
An analog output will give a continuous analog signal either on 4 mA to 20 mA or 0 V to 10 V (model dependent). This signal is established by teaching in a window on the sensor that will have the near or far limit return 4 mA or 20 mA. This information is then interpreted by a controller or PLC to give you a distance measurement.
The VDM28 Series is a popular distance measurement photoelectric sensor that is relatively low cost for the range and performance it yields. The VDM28 Series has a diffuse sensing range of 8 meters or about 26 feet. The long sensing distance and small light spot diameter is possible through the class 1 or 2 laser (model dependent) that the sensor utilizes. The long range and exceptional accuracy are all due to to pulse ranging technology.
The sensor sends out a short pulse of light, and that light is reflected off the target. The time it takes for the light pulse to reach the sensor's receiver is used to calculate the target's distance. This method is more effective than phase correlation, which is more susceptible to ambient light due to continuously sending out light and evaluating the slight phase change. Some models of the VDM28 Series of lasers also offer dual, independent discrete outputs for applications where switch outputs are needed rather than a continuous distance measurement.
The photoelectric VDM18 Series is offered for more precise applications requiring a very high resolution at a shorter sensing range. The two primary models have a 100 mm range and a 300 mm range, both with exceptional resolution for applications demanding better than 1 mm accuracy. Since this sensor is based on a triangulation method of distance measurement, both VDM18 models have a better resolution than the VDM28, but this unit is also more costly (usually more than double the price of the VDM28).
Ultrasonic sensors are another common sensing method. Pepperl+Fuchs has several models available. Most come with either switching outputs or analog outputs. They can be used for detection based on a window, threshold value, or they can be used for continuous distance measurement using the analog output. Most UC Series ultrasonic sensors offer a 0 to 10 voltage output and a 4 to 20 mA output signal. There are also discrete signal models. The advantages offered by an ultrasonic sensor are in the function of operation. Since ultrasonic sensors operate by transmitting sound waves instead of a light source, they are not sensitive to targets that have different colors and are a good option for detecting transparent objects. This is critical for tank filling applications when the fluid is transparent such as water or where the use of a laser light source is not possible.
Some successful applications include overhead crane operations, level measurement, or direct distance measurement of targets when accuracy is critical. The tank measuring application described earlier involves using an ultrasonic sensor to determine the level in the tank. If the distance is known, you can derive a formula to find the relationship between the distance, and how full the tank is. Continuous measurement will require an analog output (0 V to 10 V or 4 mA to 20 mA). If continuous measurement is not important, a discrete output model can be used. For example, many tank level applications just need a high and a low point to trigger a pump to turn on or off. In these applications, a discrete output model is preferred.