A Variable Impedance Knee Mechanism for Controlled Stance Flexion During Pathological Gait

A variable impedance knee mechanism (VIKM) has been developed as an orthotic intervention for individuals with weakened or paralyzed knee extensors. The purpose of the VIKM is to substitute for the function of eccentric quadriceps contraction to allow controlled levels of knee flexion during stance phase of gait and stair descent. The design concept, mechanism optimization, fabrication, bench testing, and initial results from walking tests with an able-bodied subject are reported. The VIKM utilizes a linear magnetorheological (MR) fluid damper with a four-bar linkage transmission to provide controllable resistance to knee motion. The design of the linkage enables the VIKM to provide large torques to resist motion at any knee angle. The prototype VIKM and full leg orthosis weigh 3.50 kg. The VIKM can provide a maximum of 64.5 N·m of torque in 35 ms. The average passive resistance is less than 4 N·m at an angular velocity of 210°/s. The ability of the VIKM to lock against knee flexion and allow knee motion under high loading is also demonstrated. Future work will focus on the development of a closed loop control system to automatically adjust the resistance level of the VIKM during walking and on clinical evaluation of the VIKM in pathological gait.