[Tanaka, Dan, Ushiba, Komori]
Abstract:
We attempted to implement a new filter to achieve high-bandwidth control of DHARD yaw, but have not succeeded yet.
As the next step, we will pursue a more moderate-bandwidth control scheme.
Details:
We began implementing a new filter to control DHARD yaw with a higher unity gain frequency (UGF).
This approach is expected to significantly reduce fluctuations in the arm transmission, which currently seem to be limited by TM yaw motion.
Based on the measured transfer function from DHARD yaw to the TM oplev signal (klog:36289), we designed a new filter consisting of ten zero–pole phase compensation stages.
The figure shows the estimated open-loop transfer function, with the UGF set to 4 Hz.
When we enabled DHARD yaw control using this new filter, an oscillation around 20 Hz occurred immediately, leading to lock loss.
To suppress this oscillation, we had to reduce the gain by a factor of approximately 3e3.
However, with such a low gain, the control was insufficient to drive the error signal to zero, even for a long period.
In addition, another oscillation appeared at approximately 80 mHz at this reduced gain, which also resulted in lock loss.
One possible cause of the high-frequency oscillation is coupling from other degrees of freedom.
The ten zero–pole phase compensation stages significantly amplify signals at high frequencies, which likely enhances such couplings.
Therefore, our next plan is to implement a moderate-bandwidth control scheme.
The target UGF will be between the second and third yaw resonances, approximately 2.5 Hz.
