This article introduces a model-based strategy for a quadruped robot with differentiated fore- and hind-leg ground reaction force patterns to generate animal-like running behavior. The proposed model comprises a rigid body and two eccentric spring-loaded inverted pendulum (eSLIP) legs with dampers. The eSLIP model extends the traditional SLIP model by adding a bar to offset the spring direction. The proposed two-leg eSLIP (TL-eSLIP) model's fore- and hind legs were designed to have the same offset magnitude but in opposite offset directions, producing different braking and thrusting force patterns. The TL-eSLIP model's reference leg trajectories were designed based on the fixed-point motion of the eSLIP model. Additionally, the legs were clock torque-controlled to modulate leg motion and stabilize the model to follow its natural dynamics. The model's equations for motion were derived, and the model's dynamic behavior was simulated and analyzed. The simulation results indicate that the model with leg offsets and in either trotting or pronking has differentiated leg force patterns, and it is more stable and has larger basins of attraction than the model without leg offsets. A quadruped robot was built for experimental validation. The experimental results demonstrate that the robot with differentiated legs ran with differentiated ground reaction force patterns and ran more stably than another robot with the same leg morphology.
Keywords: SLIP; foreleg; ground reaction force; hind leg; modeling; quadruped robot; running.
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