This study proposes a lightweight long-reach outdoor robot arm equipped with highly backdrivable actuators, named “ParaLambda,” for terrain-contact tasks on uneven ground. The robot employs a parallel-link structure and carbon-fiber reinforced polymer links to achieve both long-reach and lightweight construction, while low-gain PD control with quasi-direct-drive actuators provides passive compliance against external forces without requiring additional force/torque sensors. As a representative application, outdoor mowing work in rough terrain is considered. The developed prototype has a maximum reach of 4.15 m and an arm weight of 8.35 kg. To evaluate the feasibility of the proposed concept, preliminary indoor experiments and outdoor grass-cutting experiments were conducted. The experimental results confirmed stable repetitive motion, terrain-contact capability through passive compliance, and practical outdoor mowing operation while maintaining joint torque within the allowable actuator range. These results demonstrate the feasibility of lightweight compliant long-reach manipulators for outdoor terrain-contact applications.

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