Kinematics, Workspace and Singularity Analysis of a Parallel Robot with Five Operation Modes *

International audienceMost multi-mode parallel robots can change operation modes by passing through constraint singularities. This paper deals with a comprehensive kinematic study of a 3-DOF multi-mode 3-PRPiR parallel robot developed at Heriot-watt University. This robot is able to reach several operation modes without crossing any constraint singu-larity by using lockable Pi and R joints. Here a Pi joint may act as a 1-DOF planar parallelogram if its lockable P (prismatic) joint is locked or a 2-DOF RR serial chain if its lockable P joint is released. The operation modes of the robot include a 3T operation mode and four 2T1R operation modes with two different directions of the rotation axis of the moving platform. The inverse kinematics and forward kinematics of the robot in each operation mode are dealt with in detail. The joint space and workspace analysis of the robot allow us to know the regions of the workspace that the robot can reach in each operation mode. It is shown that the robot is able to change assembly mode in one operation mode by passing through another operation mode. 1 Introduction Reconfigurable Parallel Manipulators (PMs), such as multi-mode PMs [1–12], have received much attention from a number of researchers in the past decade [13]. The main characteristics of multi-mode PMs include [7–9]: a) fewer actuators are needed for the moving platform to realize several specified motion patterns; and b) less time is needed in reconfiguring the PM since there is no need to disassemble the PM in the process of reconfiguration. A number of interesting multi-mode PMs have been proposed, such as multi-mode PMs with a 3-DOF (degrees-of-freedom) translational mode and a 3-DOF planar mode or a 4-DOF 3T1R mode [7, 8], PMs with two 3T1R (also Schönflies motion) operation mode which has three translational DOF and one rotational DOF operation modes [4], PMs with two 2T1R operation modes [5, 6], and 2-DOF 3-4R PMs with both spherical translation mode and sphere-on-sphere rolling mode [9]. References [14, 15] show 4-DOF 3T1R PMs that have an extra 2-DOF or 3-DOF motion mode in addition to the required 4-DOF 3T1R motion mode. It is noted the metamorphic PMs based on reconfigurable U (universal) or S (spherical) joints [16,17] and the multi-mode PMs with lockable joints [18,19] are in fact kinematically redundant PMs in nature and do not belong to multi-mode PMs that this paper focuses on. In addition, several methods have been proposed [14, 15, 19–25] to the reconfiguration analysis of multi-mode PMs for identifying all the operation modes and the transition configurations of a PM