Para-Engineer kit brings parallel robots to ROS with new URDF extension

Fabian Finkbeiner and his lab have developed PARA-ENGINEER, an educational kit that can build a wide variety of parallel robots for teaching and demonstrations. Initially, the entire software stack ran as a single Python script handling everything from user input to stepper control. To make the system more modular and scalable, they are migrating to a ROS-based architecture.

This migration revealed a critical gap: URDF cannot natively describe closed kinematic loops present in parallel robots. While alternatives like SDFormat exist, they were too complex for the team's need for a simple structure-only description. Their solution introduces a new <constraint> element derived from standard joint descriptions, specifying origins, axes, and joint types for cut frames. This element is ignored by existing ROS tools, maintaining full compatibility. Loop-closure equations are solved using a Newton–Raphson algorithm in a separate package. The team is validating the approach in Rviz and has prepared a pull request for URDFDOM and URDFDOM-Headers. Future plans include integrating constraints into robot-state-publisher and adding kinematic solvers for MoveIt, with validation on industrial-grade parallel robots.