Structure design, dynamic analysis and test of FPSM of APT system in free space laser communication

As one of the best solutions for future space communication, free space optical communication possesses such advantages as high data rate, large capacity and high secrecy as compared with microwave communication. However, the extremely narrow beam width calls for severe acquisition, pointing and tracking requirements. As a key component of laser beam Acquisition, Pointing, and Tracking (APT) system for free space optical communication, the compact Fine Pointing Steering Mirror (FPSM) determines the performances of the communication system. This paper presents a new type structural design of FPSM - a piezoelectrically-driven micro-positioning deflective mechanism where the tilt movements are implemented by the elastic deformation of flexible rings. It provides fast and precise control in tilt movements around X-axis and Y-axis. The mechanism is machined from one solid alloy steel block cut by wire-EDM (electric discharge machining). The flexure rings promise zero friction, zero clearance and excellent guiding accuracy. The mirror, 36mm in diameter, is driven by four piezoelectric actuators (two pairs) spaced at 90° intervals. Each actuator pair works as a unit in push/pull mode. The differential design exhibits excellent angular stability over a wide temperature range. The stiffness model of the flexible ring was setup through the analysis of structural mechanics. The FSPM has been simplified to springs-particle system. A dynamic model was then presented based on Lagrange Equation, and modal analysis and experiments were then performed. Dynamic calculation demonstrates that the first order, the second order and the natural frequency of the FSPM is 1.278 KHz, 1.653 KHz and 1.653 KHz. Modal analysis shows that the first, second and third order natural frequency is 1.308 KHz, 1.525 KHz and 1.530 KHz. The experimental test displays that the first, second and third order natural frequency of the FPSM is 1.28 KHz, 1.56 KHz and 1.58 KHz. The error between three of methods is less than 10%. It demonstrates that the novel FSPM has excellent performance and can meet the dynamic requirements for establishing optical communication link.