  The legged robots with variable stiffness actuators (VSAs) can achieve
energy-efficient and versatile locomotion. However, equipping legged robots
with VSAs in real-world application is usually restricted by (i) the redundant
mechanical structure design, (ii) limited stiffness variation range and speed,
(iii) high energy consumption in stiffness modulation, and (iv) the lack of
online stiffness control structure with high performance. In this paper, we
present a novel Variable-Length Leaf-Spring Actuator (VLLSA) designed for
legged robots that aims to address the aforementioned limitations. The design
is based on leaf-spring mechanism and we improve the structural design to make
the proposed VSA (i) compact and lightweight in mechanical structure, (ii)
precise in theoretical modeling, and (iii) capable of modulating stiffness with
wide range, fast speed, low energy consumption and high control performance.
Hardware experiments including in-place and forward hopping validate advantages
of the proposed VLLSA.
