Inertial Tail-like Appendage Use in Quadrupeds Improves Stability in Diagonal Sequence Walking Gaits

There are two main sequences of footfall patterns for quadrupedal walking: lateral, adopted by most quadrupedal animals, and diagonal, preferred by quadrupedal primates. We observe that, compared with the lateral sequence (LS) gait, the diagonal sequence (DS) gait produces a larger stride displacement (i.e., higher average speed) but at the cost of decreased body stability. This work aims to increase the stability of the DS gait by investigating the use of an inertial tail-like appendage. We model this actuated tail as a multi-link manipulator whose dynamics describe how the system moves in response to joint torques. Employing a Lagrangian analysis, we derive tail oscillations that can resist the gravitational torque that drives the system off-balance. We validate our approach on a servo-based quadruped robot with an actuated tail, and compare the stride displacements of the LS and stabilized DS gaits. There, we experimentally show that with the help of an actuated tail, such a quadruped can take advantage of the higher stride displacement (larger) of the DS gait while maintaining stability. Future work will consider the use of an actuated tail to stabilize locomotion on unstructured terrain, as well as to balance dynamic trotting gaits.