This article presents experimental test results for joints used in a biomimetic bipedal robot. In this work, magnetic resonance imaging (MRI) and computed tomography (CT) scans are utilized to inform the design of joints of similar size and function to the biological counterparts. Three lower body joints, to be actuated by artificial muscles, were designed and constructed. Then the range of motion and passive stiffness were tested. The knee joint consists of a four-bar mechanism that provides increased extensor moment arm as the joint becomes more flexed, a “screw home” locking mechanism analog, and large contact surfaces for force distribution. The hip, ankle, and foot are hybrid hard-soft joints, consisting of a ball and socket held together with an outer, inflatable sleeve made from a braided pneumatic actuator (BPA) material. These joints provide a novel way for real-time stiffness adjustments and energy storage during the gait cycle. Results show that the physical knee prototype matches the previous simulation of joint movement (Steele, A., Hunt, A., and Etoundi, A., 2018, “Biomimetic Knee Design to Improve Joint Torque and Life for Bipedal Robotics,” Bristol, UK.). A linear relationship exists between the increase in angle and the force required to bend the hybrid joints. First, this article documents a process that others may use to develop their own joints. Second, the range of motion and passive forces in the hybrid hard-soft joints is characterized, which will enable improved control of the joints and inform other researchers to whether a hybrid joint design is appropriate for their applications. This process has several applications in prosthetic designs and robotics.
Journal of Mechanisms and Robotics Open Issues