| Most snake robots are mainly used to perform exploration tasks,but this type of robot can only passively adapt to the environment and lacks autonomous operation capability,and its application in exploration is limited.In addition,the snake robot is a serial robot with multiple joints,and too many degrees of freedom make the process of implementing its motion controller too complicated.Therefore,in this paper,a snake robot with a two-finger gripper was designed to endow the snake robot with the ability to grasp and carry objects,and then,in order to simplify its motion control,a control method based on a double-layer central pattern generator(CPG)with Hopf oscillator was designed and experimentally verified,which is divided into the following aspects.(1)The influence of different connection methods on the motion of the snake robot was analyzed.In order to make the snake robot realize the motion in 3-D space and at the same time have a strong capability of carrying load,a snake robot with orthogonal connection between joints was designed.A mechanical gripper was installed in the head joints of the robot,and the first four joints were selected as grasping modules to enable it to complete the grasping and handling tasks,and the maximum mass of the grasped object was 1.57 kg under the hardware performance was calculated according to the selected motor parameters.The structure of the snake robot was abstracted as a linkage mechanism,and its kinematic model was established by the D-H method,and the corresponding D-H parameter table was given;on the basis of the kinematic model,the kinetic model of the snake robot was established by the Lagrangian method,and the joint rotation angle was selected to describe the system state,and the corresponding expression of the kinetic equation was obtained.(2)The working space range of the grasping module without considering the rollover problem was obtained according to the kinematic model of the snake robot,which provided a theoretical basis for the subsequent implementation of the grasping function.To simplify the control complexity of the robot,a double-layer CPG method based on Hopf oscillator was introduced in the controller for generating the robot’s gait control signals,dividing the motion process into two layers of processing,with layer one used to generate the basic motion signals and layer two used for signal decoupling and optimizing the control curve so that the two signals could work independently while maintaining certain coupling characteristics.After that,a grasping operation flow was designed to achieve data fusion between robot motion commands and grasping operation commands.(3)A joint simulation environment of Coppelia Sim and Simulink software was built based on the snake robot dynamics model.In the simulation environment,the causes affecting the snake robot to produce rollover problems were analyzed,and a circular posture was designed to improve its stability in the process of grasping objects.After that,the effectiveness of the double-layer CPG control method based on Hopf oscillator was verified in the simulation environment,and the driven snake robot was able to complete five motion gaits of meandering,side-shifting,side-rolling,creeping and turning motion,and their motion speeds were 22.5cm/s,17.0cm/s,13.7cm/s,4.9cm/s and 27.6°/s under the simulation environment with static friction coefficient of 0.3 and dynamic friction coefficient of 0.1,respectively.(4)The prototype platform of the proposed snake robot was built,and the basic motion performance experiment,the gripping and handling function experiment and comprehensive test experiment were designed respectively.The experimental results show that the actual motion speeds of the meandering,side-shifting,side-rolling,creeping and turning motions are17.0 cm/s,13.9 cm/s,9.1 cm/s,3.3 cm/s and 20°/s,respectively,and the maximum mass of the grasped target is 284.2 g under the premise of ensuring the speed of motion,and 635.3 g without considering the speed of motion.It further proves that the designed snake robot has strong motion and active operation capability. |
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