Contents I. Introduction
Parallel manipulators have some significant advantages over conventional serial manipulators, such as more rigidity and accuracy, higher force and torque capacity, and simpler inverse kinematics, the latter of which is an advantage in real-time control. Over the past decades, parallel mechanisms have received more and more attention from researchers and industries. They can be found in several practical applications, such as aircraft simulators, adjustable articulated trusses, mining machines, pointing devices and micro-positioning devices[1]. Some parallel manipulators with three to five degrees of freedom have been used in many practical applications[2]. Among them, some parallel manipulators with n SPS-type active legs and an unactuated passive constrained leg have attracted more attentions. However, because the theories and technologies for these devices are not sufficiently developed, most of the parallel manipulators existing today are more expensive and less accurate than conventional ones[3].
