Gait

A gait is a pattern of coordinated limb motion and contact used in locomotion. In a periodic gait, the order of foot contacts, their relative timing and the duration of each contact characterize a cycle. A quadrupedal robot can change gait without changing the number of legs or their basic mechanical arrangement.

For a repeated foot motion with period $T$, the stance phase is the interval during which that foot contacts the ground; the swing phase moves it between contacts. If stance lasts $T_s$, its contact duty cycle is $\beta =T_s/T$. Relative phase specifies how one leg's cycle is shifted from another's. Equal duty cycles do not determine a unique gait because those phase shifts can differ.

For an original four-leg schedule, take front-left, front-right, hind-left and hind-right phases $(0,1/2,3/4,1/4)$ with $\beta =3/4$. Apart from transition instants, three feet contact the floor. A second constructed schedule takes phases $(0,1/2,1/2,0)$ and $\beta =1/2$: diagonal pairs contact together and alternate every half-cycle, illustrating a trot. The first schedule has total contact-time sum $4\beta T=3T$ per cycle; the second has $2T$. Summing leg contact times does not mean a stride itself lasts three or two periods.

A real dynamic apparatus illustrates why timing must connect to forces. The 2015 MIT Cheetah demonstration adjusted approach strides and motor forces to execute a running jump; its account describes bounding as successive small jumps.^[ref] The numerical schedules above are not measurements from that experiment.

A support polygon is useful for a stipulated static contact model, but a contact schedule alone does not establish static or dynamic feasibility. Friction, joint motion, actuator capability and angular momentum can matter. Measuring foot-contact times alongside body motion and ground reactions tests whether a planned gait is actually realized. Nonperiodic transitions, starts and obstacle maneuvers additionally require more than one fixed repeating cycle.