Capacitive Displacement Sensor for a Self-Sensing Shock-Absorber Piston-Cylinder Mechanism

Measurement of piston displacement is a common problem for any pneumatic or hydraulic device, like shock-absorber. Direct measurements are not always feasible because of mechanical constraints; most recent techniques rely on magnetic phenomena, introducing considerable complexity. In an attempt to achieve an economical and feasible solution, an intrinsic capacitive sensor is developed. Such sensors measure the capacitance between piston and cylinder, which is directly proportional to displacement. It is developed an oscillator stage to measure the unknown capacitance. The oscillator’s output is acquired by a microcontroller, conditioned and transformed into the estimated displacement. This paper focuses on the design methodology of the measurement stage, highlighting tradeoffs and optimizations. The sensor was developed for an automotive application in a commercial shock absorber: however, it can be extended to other devices where proper electrical isolation between cylinder and piston is provided. Mathematical models and experimental results are reported compared to a commercial position sensor.