Basic considerations of the degrees of freedom of multi-legged locomotion machines

All multi-legged locomotion machines that do not need any dynamic balance control can be classified functionally into several levels. We define the minimum walking functions of multilegged locomotion machines as follows: (i) two-dimensional walking; (ii) keeping the body horizontal on irregular terrains; (iii) keeping the absolute height of main body constant. Our main interest is in how many active degrees of freedom are necessary and sufficient to realize the above functions. Although consideration of the degrees of freedom seems to be fundamental in developing multi-legged locomotion machines, this problem has not yet been studied. The active degrees of freedom are examined in this paper using a four-legged machine which offers the minimum number of legs necessary to maintain static stability. It is shown that six active degrees of freedom are necessary and sufficient to realize the above functions.