Optimal Design And System Development Of A Flexible Parallel High-precision Pointing Machanism

With the rapidly development of precision engineering, multi-DOF(degree-of-freedom) ultra-precise pointing mechanisms are required urgently, whichare widely employed in laser communication, antenna pointing, astronomicaltelescope and infrared tracking and sighting system. In order to ensure the accuracyand reliability of engineering tasks and convenience of system design, the designtargets of rotation repeatability, angular range and angular resolution are explicitlyset in advance as1rad,100rad, and0.1rad, respectively.First of all, parallel mechanisms are widely employed in ultra-preciseengineering, due to their advantages of high accuracy, high stiffness, high loadcapacity and high dynamic performance. In addition, flexure hinges are applied aspassive joints instead of conventional rigid connectors, since they are free ofbacklash, wear and friction. Flexure hinges realize high precision moving utilizingmaterial’s compliance and manipulator’s profile. Moreover, the application ofhigh-precision actuators ensures the instruments high resolution, accuracy andstability. Based on the techniques, a unique high-precision pointing mechanism ispresented for inter-satellite optical communication, which emploies flexure hingesas passive joints and parallel mechanisms as basic type, and positions its movingplatform along all six degrees of freedom by means of six independent piezoelectricactuators.Secondly, Castigliano’s theorem and Labotto quadrature formula are utilized toestablish mechanical model and derive the compliance matrices of the free end andgeometrical center. Three evaluation indexes are defined, i.e. rotation capacity,relative flexibility and relative rotation error, which are used to assess the quality offlexure hinges. Compared with various longitudinal sections (V-shaped, circular,elliptic, parabolic, and hyperbolic etc.) under the same key parameters,corner-filleted flexure hinge is considered as the best candidate due to itsdistinguished mechanical characteristics and processability. An exclusivelongitudinal section of the corner-filleted flexure hinge is proposed and the curve ofthe cross-section is expressed by basic design parameters (r0, d and l) and machiningerror parameter (η). The influences of parameters to indexes are analyzed and theoptimal solution is obtained. The proposed method is verified by finite elementmethod and experimental testing within a5%margin of error.Thirdly, the parameters of parallel mechanism are optimized by multi-objectiveoptimization based on inverse kinematics model. The optimization model is built byemploying non-domination sorting based genetic algorithms (NSGA-II) and using moving platform’s angular displacements and resolutions in the directions of x, y, zas optimization objectives. The global optimized solution is selected eventuallyfrom the optimized population by adjusting the proportion of weight with preference.The maximum range and resolution of the optimized solution meet the designrequirements theoretically. Moreover, the workspace, rigidity, intensity anddynamical modal are analyzed and the feasibility of the proposed mechanism isverified.Finally, a high-precision pointing prototype is established based onprogrammable multi-channel control algorithm. The performance testing has beenimplemented and the experiments reveal the pointing system can provide highrotation repeatability, high resolution and large pointing ranges, which validates theproposed system satisfies the design requirements. The concept and approachoutlined can be extended to a variety of applications.