8

Capacitive Sensors

Xiujun Li and Gerard C.M. Meijer

8.1 Introduction

Capacitive sensors are similar to thermal sensors in that the transduction of the physical input signal to the output signal is performed in two steps: firstly, by transducing a physical quantity into a change of electric capacitance; then, by measuring and converting the capacitive signal into an electric output signal. For this reason, many thermal sensors can be replaced by capacitive ones and vice versa. The majority of capacitive sensors can be found in applications for the detection of mechanical quantities of moving objects such as position, speed, and acceleration, as well as force and pressure [1, 2]. Another important application area is the measurement of liquid levels and dielectric properties of materials. Capacitive sensors have attractive features such as low energy consumption and simple structure. Moreover, in a clean environment, the accuracy and resolution of capacitive sensors can be very high [3].

Capacitive sensing elements can be made in micromachined silicon technology as well as in conventional nonsilicon technology. In the past, measuring low capacitor values was only possible with laboratory instruments. Nowadays, such measurements can be performed using low-cost smart sensors systems, which accurately and rapidly perform the measurement of capacitive signals while using a microcontroller to control the measurement procedure and to perform data processing.

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