Mechanism Design and Kinematics Analysis of Spider-like Octopod Robot

Based on the study of spider motion, a spider-like Octopod robot is proposed and its forward and inverse kinematics are analyzed. The position of the end of the mechanical leg in space at certain joint angles and the rotation angles of each joint when the end of the mechanical leg is positioned in space can be obtained.


Introduction
As the most important branch of robotics, mobile robots are composed of tracked robots, wheeled robots and foot robots [1]. Tracked robots and wheeled robots have the advantages of stable structure and fast speed, but they have high requirements for the ground, poor flexibility, and can not adapt to rough roads and muddy marshes. In some specific working environments, people urgently need a kind of robot which can move stably, flexibly and theoretically reach any point on the ground. From this point of view, people put their research focus on bionics, hoping to inspire design inspiration from animals in nature and develop intelligent, stable and reasonably structured robotic systems by imitating animal motion and control mechanism, thus replacing human beings in various dangerous, complex and unpredictable environments to complete tasks [2]. Foot robots were born and quickly became the focus of academic research.
Octopod robots have great research value. At present, the research on quadruped and hexapod robots by scholars at home and abroad also provides important reference for the research of Octopod robots.

Mechanism Design
Mechanism design is the most basic design of the robot. The mechanism design of the spider-like Octopod robot is mainly divided into the overall mechanism design and the foot mechanism design.

The overall mechanism design
Based on the analysis and simplification of the spider body structure, a spider-like Octopod robot shown in Figure 1 is proposed. The trunk of the robot is a regular octagon, and the eight legs of the robot are located on the eight vertices of the regular octagon. The structure of each foot is exactly the same.

The foot mechanism design
The structure of the mechanism legs is shown in Figure 2. Each leg consists of three steering engines, three steering gear brackets and an imitation tibia connecting rod. One end of the first steering gear bracket is fixed on the trunk, and the other end is fixed with the first steering engine. The rotation axis of the first steering engine is perpendicular to the plane of the trunk.The rotation axis of the first steering engine is connected with one end of the second steering gear bracket to form a rotating pair, which forms the root joint of the mechanical leg.The other end of the second steering gear bracket is fixed with the second steering gear. The rotation axis of the second steering gear is parallel to the plane of the trunk. The rotation axis of the second steering engine is connected with one end of the third steering gear bracket to form a rotating pair, which forms the hip joint of the mechanical leg. The other end of the third steering gear bracket is fixed to the third steering engine, and the rotation axis of the third steering engine is parallel to the plane of the trunk. The rotation axis of the third steering gear is connected with the imitation tibia connecting rod to form a rotating pair, which forms the knee joint of the mechanical leg. The second steering gear bracket is the root of the mechanical leg. The third steering gear bracket is the femur of the mechanical leg. The imitation tibia connecting rod is the tibia of the mechanical leg.
The degree of freedom of an open kinematic chain is equal to the sum of the degrees of freedom of all the kinematic pairs in the kinematic chain. Each of the legs mentioned above has three joints composed of rotating pairs and is driven by three steering engines. The degree of freedom of the kinematic chain is equal to the number of driving parts, so each of the legs has a definite movement in space. is fixed on the tibia.The origins of the coordinate systems are at the heel joint, hip joint, knee joint and foot end, respectively. The X-axis points to the extension direction of the rod, the Z-axis points to the rotation direction of the joint, and the Y-axis is perpendicular to the corresponding X-axis and Z-axis, which constitute the right-hand coordinate system.

Inverse Kinematics Analysis
Inverse kinematics analysis refers to solving three joint rotation angles and obtaining inverse kinematics equations when the spatial position of the foot end of the mechanical leg is known.