In 2001at Cornell University, Steve Collins built a passive dynamic walking robot based on the ideas of Tad McGeer who pioneered the field. The robot could walk down a plank without power, sensors, or a control system. It was built from metal rods, springs, and weights in just the right configuration such that the legs and arms would swing in a coordinated way as it ambled down. The robot was also able to walk efficiently on a flat surface by giving it a small push. McGeer had previously noticed that adding knees made passive walking more stable for bipedal machines.