Dynamic activities such as running are usually considered difficult. Most robots trying to do dynamic activities have complicated control systems which depend on sensory data to combat the percieved instability of the activity. We believe running is easy, and can be sustained on a flat surface without any sensors if the mechanism is properly constructed. As long as the runner continues making the same set of motions, it will keep running even in the presence of slight disturbances. We have created a simulated quadrupedal robot which runs using a variety of gaits, including trotting and pacing, without any sensing. The simulations we used are physically realistic rigid-body simulations created using the Creature Library. The joints are controlled by an actuator in series with a spring; the spring constant is unchanged during a run and the actuator length is controlled.
We created a running quadruped in stages. First, we created a planar monopod; we stabilized the hopping height through careful choice of desired leg lengths, and we stabilized the pitch through a specially curved foot (it turns out a circular foot will work, but is not optimal). Second, we created a planar biped, treating it as a pair of monopods connected at the hips. Finally, we created the quadruped as a pair of bipeds.
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