Design Methodologies of the Hybrid Actuation Approach for Human-Friendly Robot Dongjun Shin 1 , Oussama Khatib 1 , Mark Cutkosky 2 1 Artificial Intelligence Laboratory, Stanford University, Stanford, CA 94305, USA {djshin, ok}@robotics.stanford.edu 2 Biomimetic and Dexterous Manipulation Laboratory, Stanford University, Stanford, CA 94305, USA cutkosky@stanford.edu Abstract— Safety is a critical characteristic for robots de- signed to operate in human environments. Presenting the ana- lytical model of the hybrid actuation for human-friendly robot development, this paper has been proposed design methodolo- gies to improve performance factors such as range of motion, payload, and acceleration while maintaining the safety factor of effective inertia. The optimized parameters with various design requirements have been provided with 1DOF and 2DOF applications. Comparison between current design parameters and the optimized parameters for a current platform shows the performance improvement. Several directions for future work into generalization and application for manipulator design and control are discussed. I. INTRODUCTION A. Background of Human-Friendly Robot In these days, most commercial robots are deployed in restricted environment where close physical interaction between robot and human is strictly regulated. However, there is growing demand for human-friendly robots allowing operation in close proximity with humans. The hardest challenge in human-friendly robots supporting a variety of commercial uses is how to achieve the competing objectives of safety and performance. High performance robots employ- ing powerful yet heavy motors with stiff and high-gear-ratio transmissions adversely increase reflected inertia and impact load in unexpected collisions between robot and human. Furthermore, since conventional robots display unexpectedly high impedance outside their bandwidth, they are not able to guarantee the safety with their active collision avoidance controller. On the other hand, safety robots designed with compliant drive trains significantly limit the robots’ perfor- mance in terms of payload and control bandwidth. Previous efforts to increase the safety of robot arms while maintaining control performance have included relocating the actuators to the base and powering the joints with cables (PaCMMA)[11], [8] and employing a series elastic actuator (SEA) [10]. Zinn took both advantages of PaCMMA and SEA [19]. Other works have employed variable compliance [2], links with high-strength composite materials to minimize inertia [1], and compliant, energy-absorbing layers, and proximity sensors to detect impending collisions [9]. Most commercial robots exploit high stiffness to achieve high performance. They utilize high gear reduction ratio to Fig. 1. Stanford Human Safety Robot (S2ρ) compensate for the lack of the power of electrical motors. Unfortunately, this results in robots that have high effective inertia, since the inertia is proportional to the square of the gear reduction ratio. High stiffness and inertia can generate large impact force in a collision. While conventional robots are able to deal with external impact forces within their control bandwidth, they can display unexpectedly high impedance outside their bandwidth. Although safety can be achieved by the strict limitation of the power and velocity of high performance manipulators, as is done in medical devices, an innovative scheme must be developed to make general-purpose robots safe in human environments. B. Hybrid Actuation Approach Robots have traditionally relied on electromagnetic actua- tors, which offer excellent controllability but poor power-to- weight ratios compared to pneumatic muscles. Even more limiting is their inability to exert large sustained forces without high transmission ratios between the motor and the load. The high transmission ratios result in robot arms with high mechanical impedance, which are inherently less safe than their biological counterparts when unexpected contacts occur. To address these design issues, the Stanford Safety