A novel kinematic structure for a parallel manipulator with 6-DOF is proposed. It consists of a platform which is connected to a fixed base by means of 3-PPSP (P:prismatic joint, S:spherical joint) subchains. Each subchain is connected to a passive prismatic joint at the one end and a passive spherical joint at the other. The spherical joint is then attached to perpendicularly arranged prismatic actuators which are fixed to the base. Due to the efficient architecture the unique solution of the platform position and orientation can be computed from the active input displacement, since the closed form solution is possible for the forward kinematics which is similar to the case of a serial robot. Especially, the general closed-form solutions of forward kinematics derived in this study can be applied directly to any other mechanism which has three prismatic joints constrained to move on the top plate, as long as axes of those three prismatic joints of the mechanism intersect at some point of the moving plate of the mechanism and three arbitrary positions connected to those three prismatic joints are specified. In other words, real-time computation for all forward kinematics solutions is feasible without any additional 6-axis absolute position sensor. It is expected that by using results of this study, the computation burden in real time control of this type of system will be reduced and calibration procedure in implementing the real system can be easily performed.
Translational and rotational workspace determinations of the manipulator are carried out through the computation of displacements in the active prismatic joints. It has a relatively small but effective functional workspace in which the volume is proportional to the range of linear displacement of the active prismatic joints. The workspace size of the mechanism is investigated when the magnitude of the maximum rotational displacement of each of three spherical joints and the maximum linear displacement of each of three passive prismatic joints are varied.
A Jacobian matrix for the proposed structure is derived so the relationship between actuator forces and output forces/moments of the mechanism can be analyzed. A series of simulations were performed to verify the results of the kinematics analyses and to evaluate the load characteristics of the system. The results showed us a relatively homogeneous load distribution characteristic with respect to any external forces exerted to the platform and the configurational changes of the manipulator.
A kinematic and dynamic optimal design procedure of 3-PPSP parallel manipulator that involves various local and multi-criteria performance optimizations are performed. The composite design index using fuzzy logic is employed to deal with the multi-criteria based design problems. Dynamic analyses are also performed using Lagrangian method and kinematic influence coefficients approach. The dynamic characteristics of the mechanism is analyzed by investigating the isotropic characteristics of the output effective inertia matrix with respect to the input joint vectors. A prototype manipulator system based on the proposed concept is built to show the feasibility of the mechanism.
