Modular BLDC Actuator Design for Upper-Extremity Rehabilitation Devices

Upper-extremity rehabilitation devices require compact actuators capable of slow, controllable motion. This study presents a modular actuator integrating a brushless direct-current motor, a custom 7.2:1 planetary gearbox, a motor-shaft incremental encoder, a commercial RC-type electronic speed controller, and an external triac-based phase reconfiguration circuit. The switching circuit is an implementation element that separates drive-command magnitude from direction selection; it is not presented as a novel drive topology, and its electrical behaviour was not characterised. The mechanical components were manufactured by fused-deposition modeling using polylactic acid for low-cost rapid prototyping. Encoder feedback was converted through the gearbox ratio to estimate the output angle response. The actuator was tested with suspended masses of 250 g, 500 g, and 1 kg on a 25 cm moment arm, corresponding to nominal maximum gravitational torques of approximately 0.61, 1.23, and 2.45 Nm in the horizontal arm configuration. A sinusoidal reference of 90° amplitude at 0.5 Hz was tracked in all three recorded tests. The root-mean-square tracking error increased monotonically with load, from 12.4° to 13.3° and 15.0°, and the phase lag increased from 53 ms to 58 ms and 66 ms, indicating a systematic load-dependent degradation under a single fixed gain set. A load capacity assessment of the printed transmission indicates that surface durability, rather than tooth bending, governs the usable torque range. The results demonstrate proof-of-concept closed-loop and bidirectional bench operation. Direct output-shaft sensing and repeated trial validation remain necessary before rehabilitation device integration.