Single Integrator/High Pass Filter Converts Vibration Data From Acceleration to Velocity Format Configurable for Optimum Signal/Noise Down to Min. 2Hz
The integration process inherent in deriving velocity from acceleration signals produces a 1/f weighting of the frequency spectrum and thus the following properties : progressive attenuation of data vs. frequency thus filtering high frequency signal components which may be spurious ; gain increases asymptotically to infinity @ zero frequency - clearly a minimum frequency constraint must be imposed. The hardware realisation of the process (VM/04) therefore comprises a single integrator proceeded by a high pass filter, confining velocity conversion to above the minimum frequency as defined by the filter. The transient response of the filter generates ripple, limiting the conversion to quasi steady state periodic signals such as emanate from rotating machinery.
Integration of f-½ and (bandwidth limited) white noise, i.e. as inherent to charge amplifiers, is characterised by an output noise voltage proportional to fo-1/2 where fo is the corner frequency as defined by the high pass filter.
Thus a 100 : 1 increase in corner frequency is accompanied by a 20dB noise level reduction in the velocity converter output, 20dB less than would be expected by virtue of the f -1 transfer characteristic of the VM/04.
Two switched high pass filter corner frequencies are provided, together with six switched, 10dB increment velocity scalings. Table1 lists available filter corner frequencies, each with its maximum realizable velocity scaling assuming a CA/04 10pC/g input noise source.
Optimisation for a specific application may call for tradeoffs between the various parameters governing noise and phase/amplitude error, as extending the low frequency response to improve the latter increases noise. Separate phase/gain data for the filter anti integrator sections is provided to help in this subjective area. The dependence of noise level upon filter corner frequency may render the higher sensitivity velocity scalings unuseable at the lower of the two filter corner frequencies.
Stock availability as opposed to custom production, thus reduced cost and leadtime. HP filter frequencies 5, 20Hz, velocity ranges 1, 3.16, 10, 31.6, 100, 316mm/sec.pk. /volt pk. for 10mV pk./g input.