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Top1. Introduction
Due to advantages of parallel manipulators over manipulators with serial kinematics chains, they are commonly used in various industrial and research applications. Positioning errors in serial kinematic chains propagate throughout the chain links, while this is not the case in parallel kinematic chains (Tian, Shirinzadeh, Zhang, & Alici, 2009). Furthermore, parallel structures inherently distribute the forces/torques by the actuators providing this class of robots with high bandwidth dynamic characteristics (Alici & Shirinzadeh, 2006). The parallel structure was originally proposed in Gough machine for testing the tires of the airplane (Gough, 1956) and in Stewart machine as a flight simulator (Stewart, 1965). Thereafter, certain parallel architectures with more potential applications in robotics were developed (Hunt & Primrose, 1993). In the past two decades, parallel manipulators have received considerable research attentions and efforts due to the variety of practical applications. General applications of this kinematics configuration include flight simulators (Salcudean, Drexel, Ben-Dov, Taylor, & Lawrence, 1994; Stewart, 1965); shaking tables used to investigate the effects of earthquakes in building structures; support structures for micro/nano-positioning; industrial robots for high speed assembly (Cleary, Brooks, & Hughes, 1993); force and torque sensors (Kerr, 1989); parallel kinematic machines (Zhao, Fang, Li, Xu, & Zhang, 1998); minimally invasive surgery instruments and even entertainment devices (Merlet, 2006). Parallel architectures were widely investigated and utilised in flexure based mechanisms for micro/nano manipulations (Asif & Iqbal, 2011; Liaw, Shirinzadeh, & Smith, 2008b; Tian, Shirinzadeh, & Zhang, 2009) and for frontier applications such as scanning electron microscopy, atomic force microscopy, cell surgery, nano surgery, and micro/nano surface methodology (Li & Xu, 2009; Liaw, Shirinzadeh, & Smith, 2008a; Yi, Chung, Na, Kim, & Suh, 2003). Parallel manipulators have also been developed for large workspace providing macro/micro manipulation capabilities (Alici & Shirinzadeh, 2006; Portman, Sandler, & Zahavi, 2000). Further, parallel micro/nano manipulators may be integrated with parallel macro/micro manipulators through accurate reconfigurable fixturing techniques (Zubir, Shirinzadeh, & Tian, 2009). This will enable large workspace envelope providing coarse to ultra-precision positioning of an end-effector like a surgical tool (Moradi Dalvand & Shirinzadeh, 2011d; Shoham et al., 2003).