The 3-axis MEMS accelerometer inside the VSEW_mk2 accurately measures static (DC) and dynamic (AC) acceleration signals with an adjustable sampling rate of up to 4 kHz.

Velocities are calculated in real-time from the acceleration sensor.

The accelerometer’s dynamic range is +-8g.

When measuring accelerations an adjustable-frequency high-pass filter can be added to the measurement path. The high-pass filter is required to measure RMS vibration levels. Otherwise the measured RMS levels would be biased by the DC component. On the other hand, inclination measurements require the measurement of the DC component, so DO NOT use the high-pass filter for inclinations.

When measuring velocities, a high-pass filter is automatically placed in the signal path. The cutoff frequency is adjustable, and has an impact on velocity noise.

The instrument’s power consumption is proportional to the sampling rate, and dependent upon whether the instrument is set to record raw signals or statistics.

Spectrum_flowchart_VSEW

Figure 2        Signal Processing

Figure 2 shows the signal processing flow for one axis of the accelerometer.

The raw acceleration signal is sampled at an adjustable rate. The sampling rate can go up to 4 kHz.

The signal is then optionally high-pass filtered to remove the DC and low-frequency components. This is usually required to capture RMS vibration levels. The cutoff of the high-pass filter is adjustable. Alternately, the acceleration signal is digitally integrated to yield a velocity signal.

•In the Raw-Signal branch the signal (acceleration or velocity) is recorded at the sampling frequency.
•In the Signal Statistics branch Min-Max-Avg processing captures the min, max and average of the acceleration or velocity signal over an adjustable recording (log) interval. For each new interval the statistics are cleared, then the statistics are updated during the course of the interval, then at the end of the interval the statistics are recorded, and a new interval is started.
•In the RMS-Level branch the signal is squared to calculate the instantaneous power. The instantaneous power is low-pass-filtered with an adjustable time constant to produce an average. A short time constant provides an average that is capable of tracking fast transients, while a longer time constant provides a smoother and less noisy average. A typical fast time constant is around 100ms. While a typical long time constant is around 1s. Finally the square-root of the power is taken to present RMS vibration or velocity levels. The RMS vibration levels can be displayed in linear or dB scale.
•In the Level-Statistics branch Min-Max-Avg processing captures the min, max and average vibration levels over an adjustable recording interval. For each new interval the statistics are cleared, then the statistics are updated during the course of the interval, then at the end of the interval the statistics are recorded, and a new interval is started. The RMS vibration levels can be displayed in linear or dB scale.

Irrespective of the setting, the instrument always observes 100% of the acceleration samples.