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 identify suitable parameters for stable locomotion. However, most existing stability criteria are tailored to robots driven solely 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 propose a method to increase the robot's stability margin by adjusting the value of thrusts. This process is called stability enhancement. The criterion 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 straightforward to implement because it only needs to adjust the thrusts. These provide insights into how multiclass forces such as inertia force, fluid force, thrust, gravity, and buoyancy affect the robot's stability.