Development and validation of a novel balanced robot manipulator

Conventional robot manipulator designs have significant reaction forces, reaction moments, and joint torques during operation. These are a result of static effects, such as the gravity force acting at the center of mass, and dynamic effects from the motion of the link mechanisms. A large proportion of joint torque and power consumption is expended for gravity compensation, with the remaining required for motion of the links. At higher accelerations, the dynamic forces and moments become more dominant. Large reaction forces can cause vibrations or instability of the base, which reduces precision, causes joint fatigue over time, and increases energy expenditure. These reactions can be reduced through mechanical design approaches to balance the manipulator linkage mechanism. The balancing types range from static balancing of the gravity force to fully reactionless mechanisms with zero reaction forces and reaction moments. This field of balanced robot manipulation has seen considerable interest in recent years, as these manipulators have increased payload capacity and improved precision for many industrial tasks. This thesis presents the development of a novel force balanced robot manipulator, where the center of mass position is constant, and the net dynamic forces about the center of mass are zero. Starting from a literature review of the mechanical design of balanced robot manipulators, four broad approaches are identified: \textit{Passive Component Addition, Active Component Addition, Inherent Balancing, and Synthesis}. Through a comparison of these approaches in terms of suitability for a robot manipulation task, inherent balancing is chosen as the approach to explore the scope of a 2 degree of freedom (DOF), planar, force balanced manipulator. Several concepts and multiple combinations of design parameters of closed-chain manipulator designs within this scope are analyzed and compared using a mechanical and control co-design approach for the workspace, total mass, reaction forces/moments and joint torques before and after force balancing modifications. From this activity, a four bar linkage with two actuated joints co-located in the base was identified as the chosen design for prototyping based on the considered metrics. From this analysis, the Forbal manipulator is mechanically designed, prototyped and validated to investigate the improvements in carrying out trajectory tasks from force balanced robot manipulation. This manipulator has a modular design with 2 variants: \textit{Forbal-2}, with 2-DOF planar motion as determined by the concept comparison, and \textit{Forbal-5} its extension to a spatial 5-DOF manipulator which retains force balancing properties. The design considerations in terms of geometric, kinematic, and dynamic design that fulfill the force balance conditions while maximizing workspace are discussed. Then, the inverse kinematics of both variants are derived from geometric principles. The improvements from force balancing both manipulator variants are validated through comparative experiments with fully force balanced with counter masses and unbalanced configurations. The results demonstrate that the balanced configuration yields a reduction in the average reaction moments of up to 66\%, a reduction of average joint torques of up to 79%, as well as a noticeable reduction in position error for Forbal-2. For Forbal-5, which has a higher end effector payload mass, the joint torques are reduced up to 84% for the balanced configuration. The experimental results validate that the balanced manipulator design is suitable for applications where the reduction of joint torques and reaction forces/moments helps achieve millimeter level precision.