Pneumatic sensors are used to measure the displacement as well as to sense the proximity of an object close to it. The displacement and proximity are transformed into change in air pressure.
The Figure shows a schematic of construction and working of such a sensor. It comprises of three ports. Low pressure air is allowed to escape through port A.
In the absence of any obstacle / object, this low pressure air escapes and in doing so, reduces the pressure in the port B.
However when an object obstructs the low pressure air (Port A), there is rise in pressure in output port B. This rise in pressure is calibrated to measure the displacement or to trigger a switch.
These sensors are used in robotics, pneumatics and for tooling in CNC machine tools.