Workspace Analysis of a New Parallel Manipulator

This paper studies the workspace of a six-DC)F parallel manipulator of three-PPSR (prismatic-prismaticspheric-revolute) type. It is well recognized that the most significant drawback of parallel manipulators is their limited workspace. To develop new parallel mechanisms with a larger workspace is of interest to additional applications. The mechanism of the three-PPSR manipulator and its variations are briefly analyzed first. The workspace is then determined and the effects of joint limit and limb interference constraints on the workspace shape and size are studied. The constituent regions of the workspace corresponding to different classes of manipulator poses are discussed. It is shown that the workspace of this parallel manipulator is larger than that of a comparable Stewart platform especially in the vertical direction. INTRODUCTION In the recent decade, many researchers have shown interest in parallel manipulators. Compared with the more commouly used serial manipulators, the parallel ones have attractive advantages in accuracy, rigidity, capacity and load-to-weight ratio. A parallel manipulator consists of a moving platform, a base platform and several branches connecting both platforms through appropriate kinematic Transactions of NAMRI/SME 305 joints with appropriate actuators. The most well-known parallel manipulator is the Stewart platform (Stewart, 1965), which has been widely studied. In a Stewart platform, six bars connecting moving and base platforms are extensible to control the position and orientation of the moving platform. Many different six-DOFparallel manipulators have been proposed. More recently, a novel parallel manipulator is introduced and studied (Tsai and Talnnasebi, 1993; Talnnasebi and Tsai, 1994a, 1994b). This mechanism consists of an upper and a lower platforms and three inextensible limbs. The lower end of each limb connects through a ball-and-socket joint to an actuator. The actuator is of a linear stepper type but capable of moving in both x and y directions simultaneously on the base platform. The upper end of each limb is connected to the moving platform by a revolute joint. The manipulator is therefore a three-PPSR mechanism, where P denotes the prismatic pair, S the spherical pair, and R the revolute pair. The output motion of the planar linear stepping motors is similar to that of two cross prismatic pairs on the base platform. The desired motion of the upper platform is obtained by moving the actuators, to which the lower ends of the three limbs are attached, on the base platform. Besides the