| The ocean is of great significance to the survival of human beings.In the 14 th Five-Year Plan,deep-sea exploration has been listed as one of the ten key research directions in the field of scientific and technological innovation.With the rapid development of deep-sea exploration,there is an urgent need of subsea exploration and operation equipment with outstanding performance.As for traditional exploration equipment such as thruster-based type,tracked type and bionic type,their performanc is affected by water flow.What’s more,there are some disadvantages such as difficulty in fixed-point operation and serious disturbance to environment.Luckily,underwater legged robot can solve these problems.However,subsea walking for legged rbobot is more likely to overturning and slipping instability due to the influence of water flow,slope,mud and other factors.Therefore,how to keep the stability when subsea walking is the key to the successful application of legged walking in the seabed.In this paper,the research on how to improve subsea walking stability by the upper layer cooperative control strategy and the lower layer cooperative control algorithm is studied,which is based on a leg-propeller hybrid hexapod robot.Mechanical modeling is the basis of subsea motion performance analysis and control of robot.Based on the analysis of the environmental effect such as hydrodynamic force,foot-ground force and thrust force,the dynamics models of the single leg and the body are derived.Furthermore,analytical relationship between the whole body’s acceleration and joint driving torque,hydrodynamic force and thrust force is established through the coupling relationship.Using the constraints of joint torque,the spatical envelope surface of the acceleration of the robot under eight different conditions is analyzed.It is found that the water environment has a serious influence on subsea motion performance and walking stability,which provides the theoretical basis for how to improve the the stability when subsea walking through cooperative control.Cooperative control is an effective method to improve the subsea walking stability.To improve stability,a gait switching method and foot trajectory adapted to subsea environment are designed.According to the actual working environment,the subsea working condition is defined and the instability is analyzed.Based on the established stability criteria for subsea slope environment,the upper layer cooperative control strategy is established from the point of ensuring stability and saving energy,which is based on the walking mode of foot walking and mixed walking.Based on the complete dynamic model of the hexapod robot,the lower layer feedforward composite cooperative control algorithm is designed,in which propellers actively work.The two-layer cooperative control designed will effectively improve the stability when hexapod robot subsea walking.Simulation analysis is an effectively method to study the stability of robot subsea walking.To analyze the influence of different factors and verify the effectiveness of cooperative control,a simulation platform based on Vortex,a high-precision and real-time software,is developed using C++.The effects of walking gait,flow velocity,propeller control algorithm and slope on stability are analyzed in detail.It is found that among Gait 2,Gait 3 and Gait6,the walking stability increases gradually under different working conditions.Furthermore,the walking stability can be improved further when using propeller thrust in mixed walking.With the help of cooperative control stategy and cooperative control algorithm,the hexapod robot can walk up the 20°slope autonomically at the maximum flow velocity of 1.5m/s which is real-time changing.The success of walking up the 20°slope verifies the effectiveness of the cooperative control to improve stability.This paper establishes the complete dynamic model of a leg-propeller hexapod robot and design the two-layer cooperative control,which is based on the upper layer cooperative control stategy and the lower layer cooperative control algorithm.Through simulation platform designed,the influence of various factors on walking stability is analyzed,and the effectiveness of the designed cooperative control to improve stability is verified.The results of research provide a solution for legged robot instability in the subsea. |
