By 1991, Honda’s engineers knew that completing the basic functions of bipedal walking would require better stabilization, and developed three control techniques. Target ZMP Control is a posture controlling strategy to readjust the robot after an unfavorable movement that almost leads to a fall. This technique had already been developed by Prof. Ichiro Kato and his colleagues at Waseda University. Floor Reaction Control eases landing impact, and is unaffected by bumps on the walking surface. Finally, Foot Planting Location Control is a variable and adaptive controlling strategy that exactly places the foot/leg on a point of landing that is in accordance to the circumstance. These were developed separately before being combined to ensure individual robustness of each strategy.
Knee length was increased to 40cm to simulate the quick human step speed of 4.7km/h.
The first autonomous locomotion model had a large head cover. The stabilized walking mechanism was established with the E5, which achieved stable, two-legged walking even on steps or sloping surfaces.
Experimental Model 6 had autonomous control of balancing when going up and down stairs and slopes, and when stepping over an obstacle.
Of course, Honda was still keeping everything top secret. Nobody knew what was going on in their robotics lab, but that would soon change. The next step was to attach the legs to a body and create a humanoid robot.