An accelerometer is a sensor used to measure acceleration. Acceleration is defined as the rate of change of velocity i.e. how quickly a body is speeding up or slowing down while travelling in a given direction.
Physically, an accelerometer is a microelectromechanical (MEMS) device which is machined using microfabrication technology. Accelerometers are fabricated in a multilayer wafer process, measuring acceleration forces by detecting the displacement of a mass relative to fixed electrodes.
Capacitance is commonly used as a sensing approach in accelerometers. This works on the principle that acceleration is related to the change in the capacitance of a moving mass (Figure 1). This accelerometer sensing technique is known for its high accuracy, stability, low power dissipation and simpler construction. It is not prone to noise and variation with temperature. The bandwidth of a capacitive accelerometer is only a few hundred Hertz because of its physical geometry (spring and air trapped inside the IC which acts as a damper).
Figure 1. A moving mass creating variable capacitance
The capacitance sensing structure can be either single-sided or a differential pair (a movable mass on either side of the fixed electrode).
Since the change in capacitance for a single mass/electrode pair is tiny, multiple movable and fixed electrodes are connected together in a parallel configuration. This creates a much greater change in capacitance, which is detected more easily and increases accuracy of measurement.
The signal from the capacitance sensing structure then undergoes charge amplification, signal conditioning, demodulation, and lowpass filtering before being converted into a digital domain using a sigma-delta ADC. The serial digital bit stream from the ADC is then sent to a host controller (using an appropriate protocol like SPI or I2C) for further processing.
A shock sensor is a type of accelerometer that can detect high acceleration and deceleration.