Design of a pneumatic variable stiffness ankle joint

The foot-ground impact is a vital problem for biped robots walking stability. Various works have been done to reduce the impact and enhance the walking stability. This paper investigates the foot-ground impact using mathematic models and introduces the design of a pneumatic variable stiffness ankle joint. First, the torque and energy consumption of the ankle joint during double support phase are derived from a seven-link idealized model. Then, in order to find the optimal range of ankle stiffness, dynamic simulations are conducted on two virtual prototypes with variable stiffness ankle and rigid ankle respectively. After that, a pneumatic unit is designed based on the simulation results and a test is performed to measure its range of stiffness. Finally, combining the pneumatic unit with a parallelogram mechanism, an integrated variable stiffness ankle joint is developed.