Stability Criterion and Stability Enhancement for a Thruster-assisted Underwater Hexapod Robot

The stability criterion is critical for the design of legged robots’ motion planning and control algorithms. If these algorithms cannot theoretically ensure legged robots’ stability, we need many trials to find suitable parameters for stable locomotion. However, most existing stability criteria are with robots only driven by legs and cannot be applied to thruster-assisted legged robots. Here, we propose a stability criterion for a thruster-assisted underwater hexapod robot by finding maximum and minimum allowable thruster forces and comparing them with the current thrusts to check its stability. On this basis, we increase the robot’s stability margin by adjusting the value of thrusts. This process is called stability enhancement. The criterion has a clear physical meaning because it uses the optimization method to transform multiple variables such as attitude, velocity, acceleration of the robot body, and the angle and angular velocity of leg joints into one kind of variable (thrust) to judge the stability directly. In addition, the stability enhancement method is easy to implement because it only needs to adjust the thrusts. These provide insight into the mechanisms underlying how multi-class forces such as inertia force, fluid force, thrust, gravity, and buoyancy affect the robot’s stability.