Brad Ratto, Jeff Kissel
Continuing in the design process of the HXDS damping loops, we have created the modeling infrastructure to recreate the open loop gain function we measured, summarized in LHO:62585, and predict noise performance ala typical level 2 design, as specified by T1100232.
We want to confirm that our model can adequately predict reality by comparing it to our measurements of the open loop gain (OLG) and loop suppression for ZM6 under the influence of the currently installed damping loops LHO:61255. We also cross checked the model with plant measurements taken for ZM(1-6) to verify that the dynamical model of the double suspension itself is representative of the physical system in addition to understanding the calibration of the senors and actuators, as summarized by Jeff in LHO:62779.
Takeaway: We confirm that the current damping loop is not ideal, but use it to build and verify the modeling infrastructure before moving forward with a future desgin. Below are a few observations we made during said process.
Looking at the loop stability for all three DOFs, the model confirms that the Yaw and Longitudinal damping loops are stable whereas the Pitch insabilities that we suspected from the measurement, LHO:62585, are indeed present. See page 10, the impluse response, and page 11, loop design metrics (1/(1+G), G, P, K), found in the PDF report: dampingfilters_HXDS_ZM6_2022-01-10.pdf. Furthermore, a comment made by Georgia, (LHO:61294), reafirms that there does exisist a instability at 6Hz. Further the instability in Pitch is present in the current design regardlesss of whether the loop gain is changed by an order of magnitude because the freqeuency response phase is so close to -180 (Deg) in that 6Hz frequency region. Now we can do better, stay tuned.
Primary toplevel script to reproduce the existing design:
/ligo/svncommon/SusSVN/sus/trunk/HXDS/Common/FilterDesign/Scripts/
design_damping_HXDS_20220110_H1SUSZM6.m
Relies on the generic HXDS design infrastructure:
/ligo/svncommon/SusSVN/sus/trunk/HXDS/Common/FilterDesign/Scripts/
plothxdsdampingcontroldesign.m
plothxdsactuatornoise.m
And a common-to-all SUS type tool:
/ligo/svncommon/SusSVN/sus/trunk/Common/MatlabTools/
createdampingfilterstruct.m
