This paper focuses on high precision leveling control of an underwater heavy load platform, which is viewed as an underwater parallel robot on the basis of its work pattern. The kinematic of platform with deformation is analyzed and the dynamics model of joint space is established. An adaptive backstepping controller according to Lyapunov's function is proposed for leveling control of platform based on joint space. Furthermore, the “lowest point fixed angle error” leveling scheme called “chase” is chosen for leveling control of platform. The digital simulation and practical experiment of single joint space actuator are carried out, and the results show high precision servo control of joint space. On the basis of this, the platform leveling control simulation relies on the hardware-in-loop system. The results indicate that the proposed controller can effectively restrain the influence from system parameter uncertainties and external disturbance to realize high precision leveling control of the underwater platform. 1. Introduction An underwater parallel heavy-load platform based on tension-leg platform is proposed here, and it will be used to carry some special underwater tools for underwater experiments at designated depth. From the viewpoint of topology, the platform can be seen as an underwater flexible parallel robot with advantages of both parallel mechanism and flexible-driven mode. From the viewpoint of control, the platform is a multi-input and multioutput (MIMO) nonlinear system with complex coupling dynamic. Hence, it has certain difficulty to realize the high precision leveling control of platform. At present, the studies on underwater parallel robot control are relatively lacking, so it can be just referred to rigid platform leveling control and flexible parallel robot control technology. The leveling control technology is widely used in engineering machinery, aerospace, military engineering, and other aspects, and its precision will directly affect the accuracy of overall system. Zhai and Ni proposed two leveling methods, respectively, based on position error and angle error aiming at the leveling issue of six supports hydraulic platform on stationary base [1]. Sheng and Qiu designed six-support hydraulic Stewart platform bearing heavy load, proposed computer parameterization leveling algorithm, and constructed electrohydraulic servo automatic leveling system [2, 3]. Ye focused on leveling control of the six-support platform bearing heavy load and then put forward a leveling scheme called “fixed point” scheme [4]. Zhang chose a “highest point
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