Capacitive accelerometer

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

In these accelerometers, the mass (made of conductive material) constitutes one armature, while the other is realized on the fixed structure of the device, in the immediate vicinity of the mass. The mass is suspended on a relatively rigid elastic element (typically a diaphragm). A special circuit detects the capacitance of the resulting capacitor and generates an electrical signal proportional to the position of the mass. This type of accelerometer is manufactured using micro-electro-mechanical systems (MEMS) technology for typical applications such as airbags and mobile technological devices. Manufacturing technology with high-volume processes and therefore lower production costs.

The capacitive accelerometers are low cost with a signal to noise ratio and less than optimal dynamic response. A common 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, which is always present in the output signal. The noise present is high frequency and generally 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 bandwidth of the capacitive accelerometer 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 sensing 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, especially high quality devices, offer excellent linearity and high stability of the output signal.

Capacitive accelerometers are better suited for monitoring applications. They are ideal for measuring low-frequency motion where the level g is also low, such as vibration measurements in civil engineering.

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