**Cheng Shen, Shixun Fan \*, Xianliang Jiang, Ruoyu Tan and Dapeng Fan**

College of Intelligence Science and Technology, National University of Defense Technology, Changsha 410073, China; shensicheng1996@sina.cn (C.S.); jxl123gfkd@163.com (X.J.); tyirorie@hotmail.com (R.T.); fdp@nudt.edu.cn (D.F.)

**\*** Correspondence: shixunfan@nudt.edu.cn; Tel.: +86-158-7428-5588

Received: 12 February 2020; Accepted: 7 March 2020; Published: 11 March 2020

**Abstract:** In the UAV electro-optical pod of the two-axis four-gimbal, the characteristics of a coarse–fine composite structure and the complexity of dynamics modeling affect the entire system's high precision control performance. The core goal of this paper is to solve the high precision control of a two-axis four-gimbal electro-optical pod through dynamic modeling and theoretical study. In response to this problem, we used finite element analysis (FEA) and stress study of the key component to design the structure. The gimbals adopt the aerospace material 7075-t3510 aluminum alloy in order to meet the requirements of an ultralight weight of less than 1 kg. According to the Euler rigid body dynamics model, the transmission path and kinematics coupling compensation matrix between the two-axis four-gimbal structures are obtained. The coarse–fine composite self-correction drive equation in the Cartesian system is derived to solve the pre-selection and check problem of the mechatronic under high-precision control. Finally, the modeling method is substituted into the disturbance observer (DOB) disturbance suppression experiment, which can monitor and compensate for the motion coupling between gimbal structures in real time. Results show that the disturbance suppression impact of the DOB method with dynamics model is increased by up to 90% compared to PID (Proportion Integration Differentiation method) and is 25% better than the traditional DOB method.

**Keywords:** two-axis four-gimbal; electro-optical pod; dynamics modeling; coarse–fine composite
