In order to extend the capabilities for seismic calibration in a frequency range down to 10 mHz, the existing multi-component calibration facility in PTB was modified with several improvements over the past months.
The current multi-component calibration facility for set up for primary seismometric calibration is presented in the figure below. The device under test (DUT) is located on the table of the multi-component shaker. As reference three infrared Laser-vibrometers are utilized. Compared to the previously used HeNe-Lasers, the infrared vibrometer has a higher output power, which guarantees the vibration measurement tasks even on rough surfaces and/or with long measurement distance.
Two kinds of output signal are available from each Laser-vibrometer:
- a velocity-proportional voltage for secondary calibration
- a frequency-modulated signal traceable to the wavelength suitable for primary calibration.
During the calibration, the data acquisition and signal generation are realized using a PXIe system.
After demodulation of the frequency-modulated signal, the measured displacement of the excited vibration is directly linked ( traceable) to the wavelength of the Laser of the vibrometer. However, unlike the HeNe-Laser whose wavelength is already well known with documented stability [CCL-advice], the wavelength of infrared vibrometer is, in fact, tunable and therefore needs calibration and monitoring of stability. For the calibration facility this can be accomplished simultaneously to the vibration measurements using a four-channel wavemeter and fibre-optic auxiliary connection to the three Laser-vibrometers.
In the figure on the right the measured wavelengths together with the room temperature are presented. Despite the apparent temperature dependency, the relative variation in the wavelength of all three vibrometers is within 3ˑ10-6 Which is more than sufficient for the intended purpose of primary vibration calibration. This measurement result indicates a stability of 2·10-6 .
To excite the shaker to the desired vibration, a closed-loop multi-input-multi-output control system was used. However, this system was not intended nor designed for very low frequenciesy below 1 Hz. Therefore, instead of the closed-loop control system, a simple signal generator is applied to generate the drive voltage below 1.0 Hz. With this open loop control using the signal generator, a well evaluable sinusoidal vibration at 10 mHz can still be excited as is shown in the figure on the left.
Very recently, the first tests for the primary seismometer calibration were carried out. The DUTs were two single-axial seismometers of type Geotech GS-13, one was configured for measuring horizontal seismic vibration while the other one was fit for vertical direction. The sensitivity given by the manufacture is 2180 ± 545 V/(m/s). According to the bandwidth limitation and the high sensitivity of the seismometer, the shaker was set to excite the sinusoidal vibration with 1 mm/s velocity amplitude in the frequency range of 30 Hz down to 0.1 Hz. In the figure on the right, the calibrated transfer functions of the two seismometers are shown. The relative standard deviation for the magnitude of sensitivity in this first test was about 1 %.
To summarize, the sufficient output power of the infrared Laser vibrometer makes the closed-loop control more robust and the wavelength of the installed vibrometers is stable within 2·10-6.
For frequencies below 1 Hz the signal generator was used for generating the drive voltage in open loop configuration and the multi-component shaker can successfully be excited with frequenciesy down to 10 mHz. First tests of the primary seismometer calibrations for magnitude were carried out with a relative standard deviation of about 1 %.