Design of Active/Passive Hybrid Compliance in the Frequency Domain - Shaping Dynamic Compliance of Humanoid Shoulder Mechanism

1 Introduct ion Humanoid robots which share the space and environments with human, should have compliance. There are two approaches to realize the robot compliance. One is active compliance based on control theories on which many researches have heen reported [1],,~[6], the other is passive compliance adopting the mechanical elasticity. The former is known to have a. limited performance in high frequency due to the small resolution of sensors, the finite sampling time and the limited actuator power, and the noise in signals. The latter is effective in all frequency while it is difficult to design the parameters, and impossible to change them adap0 -7803 -5886 -4 /00 /$ 10 .00© 2000 IEEE 225 ance in the frequency domain from the following two standpoints. (1) Frequency dependent design and (2) Hybrid implementation of active and passive compliance. Compliance in high frequency is concerned to ensure safety requirements, while that in low frequency must be carefully tuned to satisfy stationary requirements such as positioning accuracy. Thus, the requirements on compliance would be better fulfilled if it is designed in frequency domain. The first standpoint represents this consideration. The second standpoint, on the other hand, is from the robustness consideration. Implementation of compliance in frequency domain would be significantly difficult in high frequency. To overcome this, it is proposed in this paper to use both active and passive compliance. The H a control theory [8] plays an important roll, in sharing compliance in frequency domain between active compliance and passive one. We designed and fabricated the 'cybernetic shoulder' [7], a three degrees-of-freedom shoulder mechanism for humanoids. This mechanism has advantages such as (1) large mobile area, (2) singularityfree, (3) human-like motion of the center of rotation and (4) ease to introduce passive compliance. The active/passive hybrid implementation of (:ompliance is applied to and experimentally verified with the torso robot with the cybernetic shoulder. 2 Passive compliance 2 .1 T h e c y b e r n e t i c s h o u l d e r a n d i t s c o m p l i a n c e Figure 1 shows the model of the cybernetic shoulder, where B and ~ are two degrees-of-freedom gim-