[Encyclios]

Capacitive accelerometer

The capacitive accelerometer uses, as the operating principle for detecting mass displacement, the variation of the electric capacity of a capacitor to vary the distance between its armatures.

In these accelerometers, the mass (made with conductive material) constitutes an armature, while the other is realized on the fixed structure of the device, in the immediate proximity of the mass. The mass is suspended on a relatively rigid elastic element (typically a membrane). A particular circuit detects the capacitance of the capacitor thus created and generates an electrical signal proportional to the position of the mass. This type of accelerometer is made for typical applications such as air-bags and mobile technological devices, with Micro-Electro-Mechanical Systems (MEMS) technology. Manufacturing technology with high volume processes and therefore lower production costs.

The capacitive accelerometers are of low cost with a signal-to-noise ratio and non-optimal dynamic response. An intrinsic feature of all capacitive elements is the internal clock circuit. The frequency of this circuit is high (about 500 kHz) and is an integral part of the current detection circuit, always present in the output signal. The noise present is at high frequency and in general outside the acceleration measurement range. Thanks to its built-in amplifier/IC, the 3 wires (or 4 wires for differential output) are the connection to a stable voltage source.

The capacitive accelerometer bandwidth is limited to a few hundred Hertz due to the gas damping that reacts to the element due to the damping effect. The structure of the capacitive sensor element favors the low acceleration measurement range. The maximum range is generally limited to less than 100 g. Apart from these limitations, modern capacitive accelerometers, in particular, high-quality devices, offer excellent linearity and high stability of the output signal.

Capacitive accelerometers are more suitable for monitoring applications. They are ideal for measuring low-frequency movement where level g is also low, such as vibration measurements in civil engineering.