To enable widespread use of vibration sensors for condition monitoring, two factors are of great importance: low cost and small size. Where previously piezoelectric sensors were frequently used, MEMS-based accelerometers are increasingly being used today. They feature higher resolutions, excellent drift and sensitivity characteristics, and a better signal-to-noise ratio, and they enable detection of extremely low frequency vibrations nearly down to the dc range. They also are extremely power saving, which is why they are also ideal for battery-operated wireless monitoring systems.
Another advantage over piezosensors is the possibility of integrating entire systems in a single housing (system in package). These so-called SiP solutions are growing to form smart systems through incorporation of additional important functions: analog-to-digital converters, microcontrollers with embedded firmware for application-specific preprocessing, communications protocols, and universal interfaces, while also including diverse protective functions.
Integrated protective functions are important because excessively high forces acting on the sensor element can often result in sensor damage or even destruction. The integrated detection of a possible overrange delivers a warning or deactivates the sensor element in a gyroscope by switching off its internal clock and thus protecting the sensor element.
As demands in the CBM field increase, so do the demands on the sensors. For useful CBM, the requirements regarding the sensor measuring range (full-scale range, or FSR for short) are already in part greater than ±50 g. Because the acceleration is proportional to the square of the frequency, these high acceleration forces are reached relatively quickly. This is proven by Equation 1:
a = –4 × π² × ƒ² × d
Variable a stands for acceleration, f for frequency, and d for the amplitude of vibration. Thus, for example, for a 1 kHz vibration, an amplitude of 1 µm already yields an acceleration of 39.5 g.
Regarding noise performance, this should be very low over as wide a frequency range as possible, from nearly dc to the middle two-digit kHz range, so that beyond other artifacts, bearing noise can also already be detected at very low speeds. Yet it is precisely here that the manufacturers of vibration sensors are currently facing a huge challenge, especially for multiaxis sensors.
Only a few manufacturers offer adequate low noise sensors with bandwidths of greater than 2 kHz for more than one axis. Analog Devices, Inc. (ADI) has developed the ADXL356/ADXL357 three-axis sensor family, especially for CBM applications. It offers very good noise performance and outstanding temperature stability. Despite their limited bandwidth of 1.5 kHz (resonant frequency = 5.5 kHz), these accelerometers still deliver important readings in condition monitoring of lower speed equipment such as wind turbines.
The single-axis sensors in the ADXL100x family are suitable for higher bandwidths. They offer bandwidths of up to 24 kHz (resonant frequency = 45 kHz) and g ranges of up to ± 100 g at an extremely low noise level. Due to the high bandwidth, the majority of faults occurring in rotating machines (damaged plain bearings, imbalance, friction, loosening, gear tooth defects, bearing wear, and cavitation) can be detected with this sensor family.
