Task-specific manipulability metrics for redundancy optimization in cooperative manipulation

Purpose This paper aims to investigate how redundant degrees of freedom can improve task-relevant dexterity and force capability in cooperative manipulation without disturbing commanded object motion. It compares task-specific manipulability metrics, particularly when the dominant external-load direction differs from the commanded motion direction. Design/methodology/approach A closed-loop inverse-kinematics and null-space gradient-projection framework is used to compare velocity, force and directional-force manipulability. The objectives are evaluated on a planar dual-arm hardware platform using external loads and actuator-current-based effort measurement. A complementary MuJoCo study with two 7-degrees of freedom Franka Panda manipulators extends the comparison to static holding, translational pick-and-place and six-dimensional pick-and-place from six initial object configurations. Findings Hardware experiments show that the objectives produce distinct internal configurations while preserving object motion. Velocity manipulability improves kinematic dexterity, while directional-force manipulability yields lower effort than force-only optimization when the commanded motion and external-load directions differ. The spatial simulations further show that all three objectives improve or maintain their respective measures across different initial configurations while maintaining low object-tracking errors. Originality/value The paper provides an implementation-oriented comparison of velocity, force and directional-force manipulability objectives and offers practical guidance for selecting redundancy objectives according to task and external-load direction.