Stiffness Enhancement and Motion Control of a 6-DOF Wire-driven Parallel Manipulator with Redundant Actuations for Wind Tunnels

As is well known, a wire-driven parallel manipulator is a manipulator whose end-effector is driven by a number of cables instead of rigid links. It shows several promising advantages over its rigid-link counterpart, such as simple light-weight mechanical structure, low moment inertia, large reachable workspace and high-speed motion. In the 1980s, the National Institute of Standards and Technology (NIST) in America invented a wire-driven parallel manipulator named RoboCrane for shipyards (Albus et al, 1993). So far, wire-driven parallel manipulators have been applied in load lifting, industrial machining, virtual reality and astronomic observation (Dekker et al, 2006; Ning et al, 2006; Ma & Diao, 2005). Because of the advantages and unique features of wires, wire-driven parallel manipulators have attracted a great attention in robotics literature. The first general classification was given by Ming and Higuchi (Ming and Higuchi, 1994). Based on the number of wires (m) and the number of degrees of freedom (n), wire-driven parallel manipulators were classified into three categories, i.e. the incompletely restrained positioning mechanisms (mn+1). Yamamoto et al. presented basic dynamics equations and a feedback control method based on exact linearization for the incompletely restrained positioning mechanisms (Yamamoto et al, 2004). Hithoshi et al. studied a robust PD control using adaptive compensation for translational wire-driven parallel manipulators of a completely restrained type (Hithoshi et al, 2007). Zi Bin et al. developed a fuzzy plus proportional-integral control method for the cable-cabin mechanism of 500m aperture spherical radio telescope (Zi et al, 2008). Yu Kun considered active stiffness control schemes as optimization problem with different criteria for redundantly restrained positioning mechanisms (Yu, 2008). In essence, a wire-driven parallel manipulator can be considered as a complex, time-varying, strong-coupled, multiple input and multiple output, and nonlinear system. Since the wires can only pull and not push on the platform, dynamics and control are key issues for high-precision motion of wire-driven parallel manipulators. Wind tunnel tests of aircraft models are widely utilized to investigate the potential flight dynamics and aerodynamic characteristics of aircrafts at their early developing stage. Wiredriven parallel manipulators have been introduced to wind tunnels as flexible suspension systems of aircraft models in recent years (Liu et al, 2004). The posture of the scale model corresponding to the stream line of airflows can be adjusted by controlling the length of