In order to maintain accurate target tracking, it is essential to counteract the effects of hull disturbances caused by waves. These disturbances lead to random swaying—such as rolling, pitching, and yawing—which can cause a narrow beam antenna to drift, reducing its tracking performance or even causing loss of the target. To ensure stable tracking in inertial space, an anti-disturbance stabilization system must be implemented, isolating the antenna from the hull’s movements.
Traditional systems rely on multi-mode compensation, requiring at least six rate gyros to detect three-dimensional hull disturbances and active antenna rotation. Based on the three axes of the antenna (azimuth, pitch, and cross-cut), both feedforward open-loop and feedback closed-loop compensations are used to isolate these disturbances. However, this approach results in complex designs with high gyro usage and limited redundancy.
The influence of hull three-dimensional disturbance on the visual axis of a three-axis antenna is significant. The triaxial system includes the azimuth axis (A), pitch axis (E), and transverse axis (C). When the pitch angle E is 0°, the transverse axis aligns with the azimuth axis; when E reaches 90°, the transverse axis becomes perpendicular to the azimuth axis. This dynamic structure makes the system sensitive to the ship's angular velocity vector ωz = (ωp, ωy, ωh), where ωy represents roll speed, ωp represents pitch speed, and ωh represents heading speed.
As the ship moves, the disturbance translates into velocity components along the heel, azimuth, and pitch axes. The relationship between these parameters and the antenna’s movement is critical for maintaining stability. The total rotational speed of each antenna axis is calculated considering both the ship’s motion and the antenna’s active drive. Equations describing these relationships help the servo control system implement either open-loop or closed-loop compensation to reduce the impact of disturbances.
Anti-disturbance design involves detecting interference and using either closed-loop or open-loop methods to minimize its effect. Feedforward compensation is particularly effective as it adds measured disturbance information to the input, allowing the antenna to rotate in the opposite direction of the ship’s movement. Unlike feedback control, which may affect the system’s response time and stability, feedforward maintains system bandwidth and ensures better stability.
The compensation principle involves rotating the antenna in the opposite direction to counteract the disturbance. By measuring the movement of the three axes relative to inertial space, the system can apply feedforward adjustments that improve tracking accuracy. The control implementation uses a combination of position and speed feedback, with the rate gyro feeding forward disturbance data to the speed loop. This setup enhances the system’s ability to track targets accurately, even under challenging conditions.
In practical applications, the installation of rate gyros is carefully planned to measure the effects of hull movement on each axis. For example, the tilt gyro is mounted on the azimuth turret, aligned with the pitch axis, while other gyros are placed to capture different components of the disturbance. Through mathematical calculations based on the pitch angle, the system can indirectly determine the disturbance on the heel and azimuth axes.
Testing and analysis of a shipboard three-axis antenna control system showed impressive results. During sea trials, the system demonstrated a 46.4 dB isolation of ship shake and a tracking accuracy of 0.031°, confirming the effectiveness of the open-loop compensation scheme. The use of feedforward compensation combined with feedback control not only simplifies the system but also improves reliability and longevity.
In conclusion, feedforward compensation offers a reliable method to enhance tracking performance without affecting the system’s stability or bandwidth. Its integration with feedback control ensures efficient operation, making it a valuable solution for stabilizing antennas in dynamic environments.
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