Service Communautaire d'Information sur la Recherche et le Développement - CORDIS

Report on the laboratory tests of the optical seismometers (including calibration on a shaker table)

The combined tri-axial sensor was tested in the Dynamic Tests Laboratory of the Institute of Solid Mechanics in Bucharest, which belongs to the Romanian Academy of Sciences. A computer-controlled shaker table VEB RFT MESEELEKTRONIK 'OTTO SCHON' type 11075 was used. The reference accelerometer used was Bruel & Kjaer type 4368.

Hundreds of tests were performed. From these tests a frequency response curve resulted for the HiRes sensors, practically identical with the one obtained at UKC with a different shaker table and reference accelerometer. The resonance frequency resulted of about 580 HZ, leading to a usable bandwidth of about 200 Hz. This was a good confirmation concerning the calibration method. With the acquisition system used (16 bits resolution), a sensitivity of about 0.91 microg in the bandwidth of 200Hz was calculated. This is quiet close to the required sensitivity specified at the beginning of the project. The linearity of the HiRes response referred to the reference sensor resulted of about 3%.

Similar measurements were performed in order to determine the frequency response curve of the LoRes optical sensor. In a sinusoidal movement regime (on the shaker table) the resonance frequency resulted even higher than for HiRes, in the range 700-800Hz, leading to a usable bandwidth of at least 300 Hz. The determined phase shift sensitivity of 0.5rad/g is 4 times smaller than the theoretical value of 2rad/g. This leads to a sensitivity of about 5.4mg, which is much better than the value calculated from the spectral density of the sensitivity specified for LoRes at the beginning of the project, for a bandwidth of 200Hz. The linearity of the HiRes response referred to the reference sensor resulted of about 5%.

Even for sinusoidal regimes of shaking, it became clear that only signal versus time analysis is not sufficient. First of all, the shaking table movement is slightly different from a pure sinusoid in different manners at different acceleration values, so one should not expect to see such a sinusoid on any sensor.

Moreover, the mechanical characteristics of the optical and reference sensors are different (resonance frequency, damping factor, etc.). Therefore, spectral analysis turned out to be very useful and it was used for laboratory tests and field tests as well, revealing a better sensitivity and signal-to-noise-ratio for the optical sensors, compared to the classical reference sensors used.

Reported by

University "Politehnica" of Bucharest (UPB)
Spl. Independentei 313
060042 Bucharest