# Design and Implementation of Attitude Stabilization System for Marine Satellite Tracking Antenna
## 5 Experiment Setup and Results  
The controller designed in Section 4 was tested on an experimental system, as shown in Figure 12. A Stewart platform was used as a ship simulator for disturbance generation. The rotation movement of the Stewart platform is the sinusoidal function in Equation (20). The elevation part of the MSTA was installed on a test bench, where the rotation torque, motor current, rotation velocity and angle can be measured. The robust velocity controller together with the PI position controller were converted into discrete form and implemented in C code, which is put in the Interrupt Service Routine (ISR) of the timer interrupt in ARM chip. The timer interruption period was set to be 1 ms. Several important variables, such as current rotation angle, rotation velocity, reference angle, and torque, were sent to the computer through UART port and displayed in Simulink in real-time mode. During the experiment, the reference pitch was 50 deg and the running time was about 85 s. The running results are shown in Figure 13.  
In Figure 13, it can be seen that the designed robust controller can stabilize the antenna dish with high precision, which makes the two curves shown in the top part of Figure 13 almost overlapped. The angle error between the reference pitch and measured pitch is shown in the bottom part of Figure 13 and the pitch error is in the range of \([-0.15\ 0.15]\) deg, whose RMS value is definitely less than the 0.2 deg performance requirement of Ka-band MSTA. The rotation velocity of antenna dish is shown in Figure 14 to give more information about the running states of the system. From the rotation velocity curve, it can be seen that, most of the time, the rotation velocity of antenna dish is in the range of \([-2\ 2]\) deg/s while the maximum velocity disturbance from Stewart platform can reach about 26.2 deg/s, which again shows the high performance of designed control system. Figure 11: Bode plot of the frequency response of \(K_{\mathrm{p}}G_{\mathrm{res}}\). Figure 12: Experimental system for testing robust controller. Figure 13: Running results of attitude stabilization system of MSTA. Note that the controller designed above is actually only one of the two controllers for this system. In the current stage, when the MSTA is started, a normal Proportional-Integral-Derivative (PID) controller is firstly applied to adjust the antenna dish to the command attitude, and then the controller in this paper is switched on, which means the angle error is actually very small when the controller in this paper starts working. In addition, note that the data shown in Figures 13 and 14 are sampled when the system has already reached the stable state, which is why we cannot see the transient stage at the beginning of these figures. In this experiment, only sinusoid input was applied, the frequency (1 rad/s) and amplitude (25 deg) of which correspond to the worst case specified in the product performance requirements. In this worst case, the maximum ship rotation velocity was 26.2 deg/s and the control system could keep the RMS value of tracking angle error less than 0.2 deg. According to loop gain in the bode diagram in Figure 11, the designed robust controller could reject any combination of irregular input if the rotation velocity of the input was less than 26.2 deg/s, which was validated through the simulation experiment stated below in detail. In this simulation, the disturbance input of pitch was set as,  
\[Input=A_{1}\sin\left(\frac{2\pi}{T_{1}}\cdot t\right)+A_{2}\sin\left(\frac{2 \pi}{T_{2}}\cdot t\right)+A_{3}\sin\left(\frac{2\pi}{T_{3}}\cdot t\right)\]  
where \(A_{1}=5\), \(A_{2}=5\), \(A_{3}=5\) (unit: deg/s) and \(T_{1}=10\), \(T_{2}=8\), \(T_{3}=6\) (unit: s). These values were chosen with the requirement that the rotation velocity of input should not be larger than 26.2 deg/s. The curve of the input is shown in Figure 15.  
Figure 14: Rotation velocity of antenna dish during experiment. Figure 15: Disturbance input of pitch.