5 Sensors, measuremen technology and test options
Using hybrid accelerometers
Specifically for the wind energy sector, PRUFTECHNIK has developed a hybrid
accelerometer that can measure from 0 Hz (type VIB 6.216). The characteristic lines of the new Triax accelerometer and a picture of the sensor are shown in Figure 8. The sensor has no damping along the X and Y axes and its sensitivity is 500 mV/g. Users of accelerometers should note that only special accelerometers have such ideal characteristic lines. In the past, the most commonly used piezoelectric accelerometers could only measure from 2 Hz. This dropped to 0.5 Hz and, today, devices that can measure from 0.1 Hz are already available at affordable prices. Users should first have sensor manufacturers provide them with the characteristic lines of the sensors in order to see the magnitude of the damping. For instance, at first glance, deviations of 3 dB or 9 dB might not seem like much numerically, but when viewed in relation to pod vibrations of 100 mm/s, they would lead to erroneous measurements of 70.8 mm/s (3 dB) or even 35.5 mm/s (9 dB). This results in the rotor blade and pod vibration readings being too low on the measuring device by orders of magnitude. Roller bearing condition analyses are, however, different. Here, piezoelectric accelerometers are unparalleled; they make it possible to measure acceleration shocks of up to 100 g and can even handle the high-frequency resonance frequencies of rolling elements.
With the new hybrid Triax acceleration sensors, PRUFTECHNIK has combined the proven TANDEM-PIEZO technology with MEMS technology. This equipment uses a single sensor to perform not only machine diagnoses along the x, y and z axes, but also additional roller bearing diagnostics (along the z axis) and the previously described directionally-dependent acceleration rate diagnoses (along the x and y axes).
The measurement chain determines the quality of the results
Sensors alone are not enough to precisely measure low frequencies. DIN ISO 2159 and other standards specify that the entire measurement chain should operate with an inaccuracy of no more than 2 dB in specific frequency ranges. A measurement chain consists of the vibration sensor, the measurement system and the analysis software. It starts with the AD conversion and continues through to measurement processing and integration before finally providing vibration velocities. Hardware and software manufacturers require suitable testing equipment in order to adapt the entire measurement chain, right up to the software, to specific applications and to control it throughout.
Necessary quality controls
The quality of low-frequency vibration measurements can be examined internally and externally, and can even be confirmed by the PTB (Physikalisch-Technische Bundesanstalt, Germany‘s national metrology institute). In addition to „conventional“ measurement and test facilities, PRUFTECHNIK has of its own 3-axis motion simulator (Fig. 9), which makes it possible to rotate, position, wobble, tip, pivot and pitch items at freely-selectable rotational speeds, frequencies and signal waveforms. We will now report on verification measurements taken with VIBGUARD XP in the motion simulator, where positioning procedures, rotational movements and pitch movements can be simulated – much like in a wind turbine. In order to obtain an idea of which signal deviations can be expected in the low-frequency range, other accelerometers were also tested at the same time.