Design and experiment of hybrid stabilized tracking platform and its control system

Abstract: In the process of navigation, because of the influence of the waves, ships will generate the movement of rolling, pitching, heading, and so on. This movement will disturb the using of the TV (television) tracking equipment and change the direction and bring some danger to equipment and personnel. In order to overcome the effects of ship swaying, and broaden the application scope of TV tracking equipment, a novel five-degree-of-freedom stabilized tracking platform using two-layer structure is introduced, which is of great significance and illustrates a good prospect of application. The upper layer is the two-degree-of-freedom tracking platform, which uses the series structure of U-U (the U-shaped outer frame revolving around the outer ring axis and the U-shaped inner frame revolving around the inner ring axis). It can ensure that the TV tracking equipment is aimed at the dynamic target in real time. The down layer is the three-degree-of-freedom stabilized platform, which uses the parallel structure of 3UPS/PU because of its characteristics of good stiffness, large carrying capacity, high precision, compact conformation, and so on. But control algorithm complexity and poor real-time performance cause the low accuracy of the stable platform, which seriously affects the further development of the stable platform of parallel mechanism. So the RTX(real-time extension) operating system is combined in order to improve the real-time performance of the control system. The stabilized platform can effectively isolate the disturbance of ships and provide a stable erection platform for TV tracking equipment. UPS is the driving branch and PU is the constraint branch. The upper platform's radius is 500 mm. The down platform's radius is 700 mm. The height of stabilized platform is 900 mm. The angle of adjacent spherical joints is 120° on the upper platform. The angle of adjacent spherical hinges is 120° on the down platform. Mathematical models of tracking platform and stabilized platform are established respectively and the algorithm of tracking platform guidance and the kinematics of stabilized platform are derived. A set of IMU(inertial measurement unit) is installed on the outer frame of the tracking platform, which can dynamically acquire the position of the tracking platform. GPS (global position system) signal source is installed on the target and the target information is transmitted to the tracking platform computer through the wireless network. By guiding the solution method, the rolling and pitching angle of the tracking platform are calculated. And then computer controls the outer frame and inner frame axis to achieve dynamic tracking. Now the control strategy of the stable platform of parallel mechanism is based on kinematics or dynamics. By contrast, the control strategy based on kinematics is easier to implement and has a better real-time performance. So the control strategy of the stable platform of parallel mechanism uses the one based on kinematics. The stabilized platform has the motions of 2 directions i.e. rolling and pitching. The sensitive component installed on stabilized platform is aware of the ship's rolling, pitching attitude and other parameters. These motion parameters are transmitted to the motion control computer. The motion control computer calculates through the real-time kinematics, and the motion quantity of the electric cylinder is obtained and the control instruction is generated. The control instruction is output after signal conditioning by the servo control unit, which drives the movement of the servo system to realize the desired movement attitude. At the same time, the servo control unit can collect the displacement of the electric cylinder in real time, realize the monitoring and protection of the self-stabilized platform, and display the various information. Experimental results show that tracking accuracy of tracking platform is better than that of 0.02°, stable frequency of stabilized platform is better than that of 1 Hz, and dynamic stability precision is better than that of 0.3°. It confirms the self-stability and tracking performance of the designed platform. This study combines the tracking function with the stabilizing function, which can provide reference for the development of other ship-borne and vehicle-mounted equipment.