We have developed a service robot that supports daily life of humans by traveling in human living spaces. This paper aimed to develop a mechanism that permits safe and smooth traveling of a robot by changing the traveling performance according to the environment. A traveling robot was proposed, which has a parallel form based on inverted pendulum and a series one like a bike and is able to switch from one form to another. Three intermediate forms between the parallel and series as well as their dynamics and control laws were described. The transformation sequence consists of a parallel form, three intermediate forms and a series form, and the planned control inputs to change the form while traveling were also detailed. A control system that switches three control laws for each parallel, series and mixed forms according to the robot form was constructed. The transformation from the parallel to the series form, when the robot opens its legs, as well as the transformation from the series to the parallel form, when the robot closes its legs, were verified in the experiments.
