Last week, we performed the following activities on the SDB1 bench:
· On Tuesday Feb 2nd, we installed an additional electronic circuit in the SDB1 rack that allows the monitoring of the current sent to the primary LVDT coils and host a resistor and a capacitance which modify the transfer function between the DAC channels and the coils. We measured the transfer function between the excitation channels and the readout of the secondary LVDT coils by sending white noise. This allowed us to check that the coil response is maximum and almost flat in the frequency band between 9000 and 10000 Hz, and accordingly to select the frequencies of the excitation lines within this window.
· Then for each of the 8 LVDT, we adjusted the demodulation phases used to extract the error signal from the secondary coils. And we checked the resulting calibration factors for the readout of the LVDT secondary coils. This calibration factor, now implemented in the SDB1_Pr_LC configuration file, is equal to 2e-4 m/(V/V) for the FLH, FLV, FRH, FRV, BLH, BLV and BRV coils, and to 2.4e-4 m/(V/V) for the BRH coil. The LVDT error signals are now calibrated into equivalent displacements given in meters,with ~10% errors.
· On Tuesday afternoon and Wednesday Feb 2 and 3, we suspended the SDB1 bench and adjusted the bench weight and its balancing with the help of Paolo. When the bench was freely suspended we noticed a rotation around the vertical axis (so called yaw angle) by ~15 mrad. Correcting this rotation yaw angle probably requires an intervention by the experts at the level of the suspension. This has been discussed with Franco Frasconi and will be followed up.
· Based on the results of previous measurements made during the week of Jan 11 (these results will be posted in the logbook soon), we estimated that the feet of the bench were not at their nominal position, which according to VIR-0267B-15, should correspond to the center of SDB1 located at Y = -0.5 mm. Instead we estimated that the bench was located at Y = -9 +/-2 mm. A cross-check of this result was performed by measuring the distances between the bench and the north and south flanges of the tower with a laser meter. This extra measurement favours the lower side of the error bar. Thus we decided to move the feet of the bench by order of 7 mm towards north, which was done on Wednesday afternoon (see Fig 1).
· On Friday morning Fabio and Francesco entered in the detection tower to fix a broken LVDT wire (this was the FLV coil that we broke incidentally on tuesday). The coil is now operative again.
· At the end of the week we left the bench suspended and free. We are now analyzing remotely the data provided by the LVDT and doing some tests.
The attached figure (Fig 2) shows a preliminary spectrum of each degrees of freedom when the bench is suspended and free (please note that the calibration of the angular degrees of freedom is not yet accurate). For now the channels refer to the VSR coordinates. This will be changed to refer to the local referential of the bench at a later time.
