Design of a Humanoid Robot Leg with Ankle-Knee Parallel Mechanism

Leg design is key to the dynamic locomotion of humanoid robots. To increase a humanoid robot's locomotion performance, the inertia of its leg needs to be reduced. One of the effective solutions is to move the joint actuators as close to the robot body as possible and to transmit motion through a comparatively lighter mechanical mechanism. In this paper, we propose a leg design with a novel mechanism, called the ankle-knee parallel mechanism, for the six-degrees-of-freedom full-scale humanoid robot leg design. The ankle-knee parallel mechanism consists of a set of parallelogram mechanisms on the thigh of the robot and a parallel mechanism with 2 revolute-spherical-spherical parallel links in the ankle joint. The parallelogram mechanisms transmit motion to the parallel mechanism to drive the ankle joint through two cranks that are coaxial to the knee joint. It makes placing the knee and ankle actuators up to the thigh possible, which significantly reduces the inertia of the legs and improves the robot's locomotion capability. In this design, the mechanism resulted in a 30.3% reduction in overall leg inertia. Knee and ankle joints and their 3 actuators are involved in the parallel mechanism. Their coupling relationship is modeled and analyzed to make decoupled control possible on the robot leg. Experiments indicate the effectiveness of the mechanism and modeling.