Instantaneous stiffness effects on impact forces in human-friendly robots

Joint stiffness plays an important role in both safety and control performance, particularly in human-friendly robots using artificial pneumatic muscles. Due to the limited control bandwidth of pneumatic muscles, stiffness characteristics and their effects on safety in the frequency domain should be taken into account. This paper introduces the concept of instantaneous stiffness and validates its model with the Stanford Safety Robot (S2ρ. The potential effects of instantaneous stiffness on safety is explored through experimental comparison of peak impact accelerations under various impact conditions. Instantaneous stiffness demonstrates different effects on the impact acceleration depending on impact velocity and controller gain. Finally, the paper discusses the stiffness characteristics as a guideline for design and control to improve the robot safety while maintaining the control performance.