The Design and Control of Manipulators with Decoupled and Configuration-Invariant Inertia Tensors

Design methods are presented for achieving decoupled and invariant inertia tensors for planar and spatial open-loop kinematic chain manipulators. Appropriate actuator locations as well as appropriate mass distributions are utilized in order to achieve the decoupled and invariant arm inertia tensors. It is found that a key to achieving the decoupled inertia tensor for more than two degree-of-freedom arms is to mount the actuators remotely and transmit the drive torques using a transmission. This approach eliminates the reaction torques transmitted between adjacent links. In order to examine how the relocation of actuators modifies the manipulator inertia tensor, a model of the transmission is presented. The manipulator inertia tensor is then derived in terms of the actuator displacements. The results of the analysis are stated as design guidelines. First, a design guideline is given for achieving decoupled spatial open-loop kinematic chain mechanisms. Second, sufficient conditions are provided to achieve decoupled and invariant inertia tensors for n degree-of-freedom planar open loop kinematic chains. These designs lead to dynamic behaviors that are particularly desirable for manipulators performing high-speed and high-accuracy trajectory control tasks. These guidelines are used in designing the 3 d.o.f articulated M.I.T direct-drive arm. The manipualtor, with a decoupled and nearly invariant inertia tensor, shows a substantial improvement in dynamic behavior and control performance.