## 5 Experimental Verification  
Tables 1 and 2 respectively descript parameters of the linear MEMS accelerometers and the CCD. Two linear MEMS accelerometers work as a group to get the angle acceleration of one direction in a differential configuration [12]. Figure 7 shows the experimental devices. The FTOS is a two-axis system. Due to the symmetry of the two axes, we focuses on one axis. To verify the above analysis, we use two FTOS platforms driven by voice coil motors. The upper stabilization platform to stabilize the LOS is mounted on the below disturbance platform which is utilized to simulate the outer disturbance. The outer disturbance is a sine signal produced by the dynamic signal analyzer. The light source emits light as a reference of LOS and the CCD receives the reflected light to provides the last LOS error. The MEMS accelerometers or FOG (XW-FG70-20, Beijing StarNeto Technology Co Ltd, Starneto, Beijing, China) are mounted on the stabilization platform to detect its angular acceleration or velocity, while the eddy is installed on the below platform to measure the given disturbance angle. All of the inertial sensors and the eddy have a sampling rate of 5000 Hz and the CCD updates in 100 Hz with 20 ms (2 frames) time delay. To get the DSA of the system, the stabilization platform should work on closed-loop mode and the disturbance platform works on open-loop mode. Figure 8 gives the acceleration open-loop bode response with the accelerometers, in which the fitting curve highly matches the actual. The acceleration open-loop transfer function is presented as follows:  
\[\widetilde{G}_{a}=\frac{0.0022s^{2}}{0.0007s^{2}+0.0185s+1}\cdot\frac{1}{0.00 04s+1} \tag{19}\] \begin{table}
\begin{tabular}{c c} \hline \hline
**Brand** & **Silicon Designs Inc.*** \\ \hline Model & Model 1221 \\ Sensitivity & 400 mV/g \\ Input Range & \(\pm 10\) g \\ RMS & 10 \(\mathrm{\SIUnitSymbolMicro g}/\sqrt{Hz}\) \\ \hline \hline \end{tabular}  
* Kirkland, WA, USA.  
\end{table} 
Table 1: MEMS accelerometers parameters. \begin{table}
\begin{tabular}{c c} \hline \hline
**Brand** & **Pulnix*** \\ \hline Model & TMC-6740 CL \\ Pixels & 640 \(\times\) 480 \\ Pixel Size & 7.4 \(\mathrm{\SIUnitSymbolMicro m}\) \\ \hline \hline \end{tabular}  
* National Instruments Corporation, Sunnyvale, CA, USA.  
\end{table} 
Table 2: CCD parameters. As in the previous analysis, we design the acceleration controller to be as Equation (20):  
\[C_{a}=150\cdot\frac{0.0007s^{2}+0.0185s+1}{s(1+0.00077s)} \tag{20}\]  
In keeping with Equation (9), the actually used DOB controller is presented as Equation (21). \[C_{f}=\frac{0.00077s+1}{0.33s} \tag{21}\]  
Figure 8: The acceleration open-loop bode response. Figure 7: Experimental devices. The velocity and position controller are described as follows:  
\[C_{v}=0.05\cdot\frac{0.33s+1}{s(1+0.001s)} \tag{22}\]  
\[C_{p}=0.012\cdot\frac{1}{1+0.0078s} \tag{23}\]  
Figure 9 shows the time-domain curves of the composite velocity and FOG at different frequencies. Below 5 Hz, the phase lag brought by CCD's time delay is too small to effect the DSA of FTOS. AS the frequency goes up, the negative effect will become bigger. However, according to Section 2.3, since the DOB method has greatly reformed the platform, the impact can be ignored. The total disturbance attenuation performance of the four methods are presented in Figure 10. The green line represents the pure ADLC method, whose DSA in low and medium frequency is not satisfied. Compared with the pure ADLC method, the only introduction of DOB could enhance the DSA in low and medium frequency, while the improvement in low frequency is a little, which is coincident with the previous analysis. After the composite velocity loop added to the system, the DSA below 2 Hz has increased a lot, which is very lacking in accelerometers. Although the time delay brought by the correction of the CCD would partly decrease the DSA in medium frequency, the last DSA in medium frequency still can be acceptable.