[Miyakawa, Kamiizumi, Tomura, Shimode, Okutomi]

• We replaced the LVDT combiner for the ETMX BF LVDTs to a modified one.
  • The modified chassis has rotated inductors by 90 deg on one of the two boards.
  • This modified board is assigned to the horizontal LVDTs, while the verticals have the initial board-mounting configuration.
• We measured the transfer function in the longitudinal DoF of the BF.
  • The result was similar to the previous measurement, nothing improved. (see the figure below)
  • It means that the inter-channel couplings in the LVDT combiner was not the main reason of the spurious coupling at higher frequencies.
• I suggest some scenarios to explain this spurious coupling.
  • The main coupling path could be the LVDT-actuator unit itself where the receiver coil of sensing and the actuator coil are shared.
  • The mechanical transfer function has poor overall gain due to the position the actuation force applies on.
  • Geometrical configuration of the sensing and actuation coils can cause the coupling.
    • Because the IP and GAS coils also have the coupling with similar characteristics even without sharing the sensing and actuation coils.
• A better way to understand this spurious coupling is to measure the pure actuator-to-sensor coupling and investigate the setup to reduce it.
  • A prototype BF LVDT unit (could be in NAOJ Mitaka) sounds useful to test it.