Backlush Clutch is adopted for the knee joint of UT-theta and UT-theta2. "How to use the knee joint" is the very key phrase in not a few athletics. It generates a large torque to go over a step, bends as if it's a cushion at the touch down, and swings like a pendulum by the gravity. Backlush clutch shows such a variety of functions.
The above illustrates the basic idea. A and B are the thigh and the shank, respectively. Motor torque is transmitted first to the movable part(a), and then to the fixed part(b) through the contact. The name of "backlush clutch" comes from a large backlush(d) between a and b. Torque transmission phase and torque insulation phase are switched by controlling d by utilizing both the motor output and the gravity without any optional actuators.
Torque transmission phase
Torque insulation phase
The torque generated by the knee actuator is unilaterally transmitted to the shank by making d in the above figure zero for the sake of supporting the body and kicking the ground, while it is insulated by keeping d to be a certain constant d0 to realize a natural-looking and energy-saving swing and not to transmit the shock at the moment of landing to the upper body.