Accelerometers

Accelerometers are employed to measure accelerations. Several measurement principles can be used for this. In addition to e.g. piezoelectric accelerometers or strain gauges, microsystems (MEMS) have established themselves in recent years.

The ideal accelerometer features a large linear frequency range (allows even static measurements), very low noise and is inexpensive. Unfortunately, such an accelerometer does not exist. Each measurement principle above possesses advantageous as well as disadvantageous properties.

As for example, the microelectromechanical systems (MEMS). These miniature accelerometers include a spring-mass system of silicon. When accelerations occur, the only few micrometers large spring shifts itself (see fig. 1). Similiar to a plate capacitor, this causes a change in capacity. An integrated electronic evaluates the capacity changes and allows conclusions to the occuring accelerations. MEMS accelerometer can be produced inexpensively in large numbers and are very reliable. Furthermore, they allow static measurements. However, the linear frequency range of these accelerometers is often very limited and they feature a relatively high noise. Therefore, they are not suitable for precision measurements with high bandwith.

MEMS accelerometer apply mainly as tilt and orientation sensor in electrical devices as mobile phones, tablet computers, gamepads or as a trigger for airbag or ESP deployments in the automotive field. Currently research focuses on highly sensitive MEMS accelerometers that are required to detect accelerations in the ng range (10^-9 g). The main task is to reduce the basic noise of the accelerometers drastically.

In piezoelectric accelerometers, a seismic mass affects to the piezoelectrical crystal. This shifts the charge centers in the crystal and an electrical signal is triggered (see fig. 2). This signal is proportional to the applied acceleration.

The output of piezoelectric accelerometers is charge or voltage. For evaluation of the transducer signal, the former has to be connected to a charge amplifier. IEPE compatible accelerometers are equipped with an integrated electronic. They are powered by constant current source and emit an alternating voltage signal. The factor for conversion of voltage (charge) to acceleration is called sensitivity. The unit of this value is mV/m/s² (pC/m/s²) and depends on the the material properties and the structure of the sensor.

All accelerometers have a similar frequency response. Thus, the transducer features a wide frequency independent range. Above this range, the accelerometer has a specific resonance frequency at which the sensitivity rises sharply. This resonance is caused by the interactions of the inner components (seismic mass, piezoceramics). The sensitivity decreases with low frequencies and the cut-off frequency depends on the connected electronic.

In contrary to MEMS accelerometers, piezoelectric accelerometers feature a low basic noise and a wide temperature range. So, sensors with charge output can be used even in the presence of ambient temperatures above 250 °C. Therefore, piezoelectric accelerometers are suitable for various applications: From recording low-frequent building vibrations to detecting high-frequent shock impulses in the bearing diagnostics