One of the great things about ultrasonic sensors is that they can provide
analog feedback over a long range. They are commonly applied in big
tanks to measure how much volume is inside the tank. An equally common
application is unwind applications, where the anlog signal can be used
to determine how much material is left on a roll. As the roll unwinds,
it becomes smaller and smaller in diameter; therefore, the ultrasonic
sound waves provide a varying signal back to the processor. A tension
control system might "follow" the signal from the sensor,
for example. Any number of devices are available to read linear signals,
from Red Lion or Dynapar meters, drives, or analog cards in plc's.
