Preliminary measurements
On June 10 and 11 we installed the setup and performed preliminary measurements in order to check the instrumentation and optimize the measurering setup.
We installed two shakers on the exterior of the NI chamber: the large shaker is placed om the NI tower base, NW corner, and the small shaker clamped to the North big flange (thank you Antonio for helping in this installation). The shakers (in turn, manually) are connected to the amplifier which is driven from a DAC ch in the TCS room.
After a careful cleaning of all the tools, we moved inside the NI.
As a test we performed measurement of mechanical modes of the the frame shown in Figure 1. We used the monoaxial accelerometer PCB352C68 on the vacuum chamber (attached with double tape, which Antonio aknowledged, asking for a careful cleaning with Acetone) and the triaxial accelerometer PCB356B18 ("golden cube") attached to the frame in different positions, aslo with double tape. For the data acquisition we used the CoCo80X.
We tested (the full set of measurements is detailed in the attached .txt file):
- injecting different levels of colored noise 10-1200 Hz to the shaker (0.02,0.03,0.04 V). For reference, inside ENVnoise.cfg:
ACL_NOISE_CH noise_white "V" 5 SAMP_FREQ 1.0
ACL_FILTER_CH noise_colored "V" 5 SAMP_FREQ noise_white 0.03 "flt3"
and inside ENVnoise_Filters.cfg:
ACL_FILTER_SET "flt3" 1 1 600 20 # --> sets gain 1 @ f0 Hz (must be the same f0)
ACL_FILTER_BUTTERWORTH "flt3" "bandpass" 4 3 600 590 - use of the small or the big shaker
- two configurations for the monoaxial acc: vertical or horizontal (see pictures)
- two positions of the cube acc. on the frame: (1) inner ring, (2) outer ring close to holding point
For each configuration we measured the TF of the monoaxial acc versus each axis (X,Y,Z) of the gold cube acc, and compared.
Figures 1-3 are pictures of the frame with accelerometers in different positions.
Figure 4 compares spectra during quiet and during shaking. The shakers were able to excite above the quiet noise in the whole expected range.
Figures 5-9 refer to the setup with monoaxial acc. horizontally placed on the vessel and the cube acc on the inner ring of the frame. They show: spectra, coherence, TF between monoaxial acc and each of the three direction of the cube acc.
Figures 10-end is the same set, but when the cube acc. was positioned on the outer ring of the frame, near to the frame's holding point.
Measurements were satisfactory, the setup works.
A number of peaks were excited especially in the range 50Hz to a few hundred Hz. Good coherence is measured in this range.
TFs look satisfactory: we observe phase rotation in correspondence of the main excited modes. The red points in the TF plots correspond to coherence > 0.4.
Moving the cube accelerometer from the inner to the outer ring of the frame, some peaks are no longer excited (for example the 130Hz peak is no more observed). As expected, this position is more rigid and low frequencies modes are not easily detectable from here.
Additional observations:
Tests indicate a slightly better preference for using the small shaker, with level 0.03V. The placing of the accelerometer on the vacuum chamber, horizontal or vertical, does not make significant difference in the measured TF.
