In order to improve the stability of the trotting quadruped robot and to decouple the control of the robot torso motion along six directions, an approach based on virtual model is presented for trot gait control. The controller mainly consists of two main modules: the virtual model control at support phase and the virtual model control at flight phase. During the support phase, the mathematical relationship are mapped between the joint torques of diagonal support legs and the virtual forces acted on the center-of-mass of the torso. And the values of virtual torso forces are regulated to control the torso attitude and height, as well as the forward velocity and the yaw angular velocity of the robot. During the flight phase, lateral velocity is introduced to plan the toe trajectory. And virtual spring-damper sections are implemented to drive the flight toes to track the planned trajectories. In addition, while designing the controller, a state machine is introduced to monitor the legs' states and output phase switching commands for trot gait regulation. The simulations show that the robot is able to trot omni-directionally on flat ground as well as uneven terrains, even suffering from external impacts. And thus the effectiveness and robustness of the controller are proved.
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