1 Introduction
The underwater world has been considered by humans to be the largest potential strategic resource base in the future. In recent years, many countries and multinational consortia have invested heavily in the development of competition, in order to obtain and occupy more seabed resources at the right time. The most popular among countries is the exploitation of polymetallic nodules on the seabed. Polymetallic nodules are recognized as the most valuable in mining technology and mining costs. However, polymetallic nodules are only found on seabed sediments at depths of 4000 to 6000 m underwater. Due to the influence and limitation of seawater pressure, corrosion, oceanic environment, tides, seabed topography, deposit location and marine meteorology, various types of subsea mining robots have been studied in various countries. In order to achieve the required output and efficiency, subsea mining robots must travel on a predetermined mining path, and control of subsea mining robots has been a key technology. The submarine mining robot should cooperate closely with the surface vessel and the mining system to transport the collected minerals to the surface vessel through the mining pipeline. In this process, the submarine mining robot will certainly be affected by many factors such as underwater currents, lifting hoses, umbilicals, hull movements and seafloor sediment bearing capacity and shearing forces generated by itself. The direction of travel of the subsea mining robot changes and deviates from the predetermined trajectory. In order to solve this problem, the direction closed-loop control is generally adopted to ensure the direction is unchanged, but the walking track cannot be guaranteed to return to the predetermined trajectory, which has been a major technical problem in the control of the subsea mining robot.
2 Technical problems in the control of subsea mining robots - translational error
The structure of the subsea mining robot can be simplified into three major parts: the traveling mechanism, the mining mechanism, and the control mechanism.
The running mechanism consists of two tracks, which can control the robot to advance, retreat and shift, turn, and play the role of the robot's two mechanical legs. The mining agency is the robot, which is dedicated to the collection of nodule mines on the seabed. The control mechanism is the command center of the robot and is the brain of the robot. The working mode of the submarine mining robot is to mine while walking according to the established route according to the state of the minerals detected in advance, and to transport the collected ore to the sea through the pipeline.
When the robot is working in seabed mining, it is affected by many unknown and random factors such as seabed geological environment, current, pipeline and cable. The walking route will often deviate significantly from the original direction, as shown in Figure 1:
The predetermined route of the robot walking is as shown by the broken line op in Fig. 1, from the o point coordinate (0, 0) to the p point coordinate (xp, yp), and the angle with the horizontal axis is q0. When the robot walks to point a, it is disturbed by external force, and the original forward direction is changed along the angle of qa. In order not to make the robot deviate from the forward direction, the direction control mechanism passes the deviation adjustment link to return to the original point when it reaches point b. The q0 direction continues to advance. At this time, although the robot returns to the original initial angle, the robot cannot reach the predetermined p point. As can be seen from Fig. 1, the robot is now walking in a bp line parallel to the predetermined route. The difference between two parallel lines is defined as the translation error.
3 Working principle of path setting and trajectory tracking control for submarine mining robots
The key to how to eliminate the translation error is that simple direction control cannot be used. We have to give the subsea mining robot an artificial intelligence function, in addition to being able to discriminate the deviation of the forward direction, it is also necessary to determine whether to walk on the predetermined route. In this way, the robot can not only know the running target, but also know which way to go to reach the target in the journey from A to B.
(1) Path setting
Set the movement path of the work vehicle according to the location and range of the work area, including the starting point, azimuth angle, calculation speed, end point and swing travel.
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