HyRoDyn

Parallel mechanisms are increasingly being used as modular subsystem units in various robots and man-machine interfaces for their superior stiffness, payload-to-weight ratio, and dynamic properties. This leads to series-parallel hybrid robotic systems, which are difficult to model and control due to the presence of various closed loops. Most model-based kinematic and dynamic modeling tools resolve loop closure constraints numerically and hence may suffer from inefficiency and accuracy issues. Also, they do not exploit the modularity of robot design. Further, such systems can have variable mobility, different assembly modes, and impose redundant constraints on the equations of motion. Hence, it is interesting for kinematics researchers to study the analytical solutions to geometric problems associated with a specific type of PM, and their importance over numerical solutions is irrefutable. But this domain-specific knowledge is often underrepresented in the design of model-based kinematics and dynamics software frameworks.

Hybrid Robot Dynamics (HyRoDyn) is a modular software workbench written in C++ for solving the kinematics and dynamics of highly complex series-parallel hybrid robots. The main idea behind HyRoDyn is to store the closed-form solutions to the loop closure constraints in a configurable mechanism library that is identified by its type (e.g., 1-RRPR, 2SPU+1U, 2SPRR+1U, 6-UPS). Based on submechanisms defined in a hybrid robot, HyRoDyn can modularly compose the loop closure function of the overall system in an automated way. The resulting loop closure Jacobian has a block diagonal structure that can be exploited in the computation of various forward and inverse kinematics and dynamics algorithms. HyRoDyn is implemented in C++ and utilizes recursive O(n) multi-body dynamics algorithms for tree-type systems from the Rigid Body Dynamics Library (RBDL) based on Featherstone's algorithms. Presently, closed-form solutions to mechanisms such as 1-RRPR, 2-SPU+1U, 2-SPRR+1U, 6-RUS, 6-UPS, and parallelogram chains are available in its submechanism libraries. To save time and human expertise, the tool is extended with a numerical approach for parallel mechanisms that are not available in submechanism libraries. HyRoDyn can be used to analytically solve the kinematics and dynamics of any arbitrary series-parallel hybrid robot. The action of the robot can be arbitrarily selected.

The input to HyRoDyn is a SMURF file, which can be generated using a Blender-based visual editor called Phobos. The use of a visual editor makes it easy to annotate the parallel submechanism modules in a robot and assemble them to build highly complex robot models. Further, HyRoDyn has been integrated into the Robot Construction Kit (RoCK) middleware as an orogen component that provides both forward and inverse mappings between the independent joint space of the robot and its actuation space at position, velocity, acceleration, and force levels. The HyRoDyn-orogen component can be used for simulation as well as real-time control of complex series-parallel hybrid robotic systems.