Content area
Abstract
A mechanism is said to be reactionless if, for any motion of the mechanism, there is no reaction force and moment at its base or supporting structure at all times. A systematic study of the synthesis and kinematic analysis of reactionless spatial multi-degree-of-freedom parallel mechanisms is performed in this thesis.
Firstly, a new kind of 3-DOF parallel mechanism referred to as a parallelepiped mechanism is proposed and a practical design is implemented. The 3-DOF parallelepiped mechanisms are dynamically balanced using counterweights and counter-rotations and are used as legs to synthesize multi-degree-of-freedom parallel mechanisms. Reactionless spatial 6-DOF parallel mechanisms are obtained by dynamically balancing each detached leg mechanism independently based on an algorithm using point masses to replace a moving platform. The dynamic simulation software ADAMS was used to simulate the motion of the mechanisms and to verify that the mechanisms are reactionless at all times and for arbitrary trajectories.
Next, the synthesis of novel reactionless spatial 3-DOF and 6-DOF mechanisms without separate counter-rotations, using four-bar linkages is addressed in this thesis. Based on the conditions of dynamic balancing of a single planar four-bar linkage moving in the plane, the spatial problem is shown to be equivalent to ensuring that the inertia tensor of reactionless four-bar linkages remains constant when the planar mechanism(s) is(are) moving. The reactionless conditions for planar four-bar linkages undergoing spatial motion are first given. A mechanism composed of a pair of connected reactionless four-bar linkages with constant inertia tensor is constructed. Then, a reactionless spatial 3-DOF mechanism is synthesized using four-bar linkages and further serves as a leg to synthesize a reactionless 6-DOF parallel mechanism.
Finally, the kinematic analyses including the inverse and direct kinematics as well as the determination of singularity loci and workspace of both the 3-DOF leg mechanisms and the 6-DOF parallel mechanisms proposed in this thesis are solved. The Jacobian matrices of the mechanisms associated with different actuation schemes are derived. Geometrical algorithms, discretization methods or analytic methods are proposed for the determination of the workspace and singularity analysis for the mechanisms. Finally, the graphical representations that show the relationship between the singularity loci and the constant-orientation workspace of the proposed mechanisms are given.
Reactionless spatial multi-degree-of-freedom mechanisms have great potential applications such as space robots, telescope mirror mechanisms and some industrial high speed devices. All the results of kinematic analysis will be of great help during the design process and for the control of these mechanisms.
