PETRA: a pedestal exoskeletal testbed with real-time architecture for upper limb rehabilitation robotics

More than 85% of stroke survivors present motor impairment in the upper limb, yet access to rehabilitation robotics remains concentrated in high-income countries due to costs exceeding 10,000 USD and architectures dependent on external computers. This economic barrier limits the ability of researchers in low- and middle-income countries (LMICs) to develop and validate new therapeutic strategies. This article presents PETRA (Pedestal Exoskeletal Testbed with Real-time Architecture), an open-source, 3-degree-of-freedom kinematic validation testbed manufactured in PETG for upper limb rehabilitation research at a cost under 25 USD per joint. PETRA adopts a fixed pedestal configuration that decouples the robot's dynamics from the user's biomechanical disturbances, providing a controlled environment for characterizing parametric cycloidal transmissions and deterministic dual-core firmware without PC/ROS dependency. Experimental validation with over 300 trials confirms positioning accuracies within clinical requirements while identifying polymer friction as the primary technical constraint. By providing a low-cost, replicable methodological standard, PETRA facilitates the transition from conceptual design to robust assistive technology in institutions with limited resources.