1. Introduction
With the constant decrease of mineral resources on land, many countries and organizations are attracted to the rich mineral resources contained in the seafloor. At present, the promising commercial marine mining system is the type of hydraulic lifting pipeline, including a marine mining vehicle as a core component [
1]. The potential market of deep sea mining vehicle is large. Due to the influence of complicated hydrodynamic effects and particular mechanical interaction between vehicle and sediment, it is difficult to attain satisfactory path-tracking performance for the mining vehicle.
Before exploration of the path-tracking control strategy of deep-sea mining vehicles, a dynamic model is needed. Hong et al. [
2] conducted dynamic simulations of a tracked vehicle through a simplified dynamic model and investigated steering performance. Kim et al. [
3,
4] compared the advantages and disadvantages of two model types on tracked vehicles and used a large number of numerical simulations to analyze the hydrodynamic effects. For a seafloor-tracked vehicle, a single-body model was conducted to achieve a fast analysis of the moving process [
5]. The above dynamic model greatly simplified the actual situation to facilitate the numerical calculation. In a vertical plane, the forces and moments that validated the Reynolds Average Navier–Stokes equations were measured for an autonomous underwater vehicle (AUV) [
6]. Phillips et al. [
7] utilized an analysis of CFD to predict hydrodynamic forces and moments for its accurate prediction. The hydrodynamic damping was acquired by the computational fluid dynamic approach in an open-frame remotely operated vehicle (ROV) [
8]. The CFD method has more accuracy than the empirical formula for the calculation of hydrodynamic effects. For an underwater tracked vehicle with a ladder trencher and a rock-crushing tool, the influence between the mining tool part and the tracked vehicle part was studied for optimized design of the underwater track vehicle system [
9,
10]. Li et al. [
11] studied a single track shoe to improve the tractive performance and optimize the structural parameters of the track. For a saturated soft-plastic soil, Wang et al. [
12] proposed a shear stress-displacement empirical model validated by a track segment shear test. Baek et al. [
13] studied the mechanical properties of the soil thrust by the limit equilibrium analysis technique on clayey soil. Owing to the particular environment of the seafloor, the research studies of deep-sea soil are essential. Vu et al. [
14] utilized a number of numerical simulations of an underwater construction robot in up-cutting mode to analyze the change of forces, moments, energy, and power in different conditions. For an AUV, Sun et al. [
15] built a dynamic model to study the performance of path tracking. Dai et al. [
16,
17] extended a new simulation to obtain relationships of the seabed sediment by the discrete element method and used the Reynolds stress model to complete CFD simulation. Many researchers have done outstanding work in the field of underwater dynamics modeling, but most of them have conducted deep research in local areas such as rapid modeling, hydrodynamic effects, and deep-sea soil mechanics. Few people combine the above achievements to establish a more comprehensive dynamic model. However, a dynamic model that fully considers the influence of various factors is essential in the study of path tracking for deep-sea mining vehicles.
For an excellent accuracy of path tracking, a suitable control strategy is essential. Yeu et al. [
18] constructed a vector pursuit algorithm for the generation of a specified motion path and proposed a new control method through traction force and track slip; this control strategy was effective but also complex. Hong et al. [
19] formulated an algorithm that contained the control of forwarding velocity and heading angle for the path tracking of a tracked vehicle; this control scheme was widely accepted. Zhang et al. [
20] investigated a hybrid fuzzy PID controller for an ROV and used the small gain theorem to analyze the stability. Londhe et al. [
21] proposed the robust nonlinear PID-like fuzzy controller for trajectory tracking and validated its better and robust control performance. Lamraoui and Qidan [
22] proposed two path-following controllers based on active disturbances rejecter control to obtain a high tracking accuracy. The fuzzy logic control was widely applied in path tracking for its outstanding performance in nonlinear systems. Based on a particle swarm optimization algorithm, a fuzzy logic controller was designed to control the ROV vertical trajectory [
23]. For a dynamic positioning system of vessels, an adaptive fuzzy controller was developed to eliminate the influence of environmental disturbances and develop the control accuracy [
24]. Chen et al. [
25] developed the fuzzy controller for path tracking and heading tracking of an ROV, which was optimized by a genetic algorithm. Londhe and Patre [
26] proposed an adaptive fuzzy sliding mode control scheme for an AUV and eliminated the problem of chattering. Dai et al. [
17,
27] researched an adaptive neural-fuzzy control strategy and validated its better performance with collaborative simulations, and they proposed a fuzzy adaptive PID algorithm as the motion control strategy for an underwater operating vehicle. Based on the superiority of fuzzy control, many scholars have conducted in-depth research on the optimization methods of fuzzy controllers, which mainly focus on the optimization of fuzzy rules and affiliation functions. It is valuable to use genetic algorithms to optimize fuzzy controllers to achieve more accurate trajectory tracking of deep-sea mining vehicles.
From the previous research, it can be found that the establishment of the dynamic model of a deep-sea mining vehicle was seldom considered with the spatial hydrodynamic effects and mechanical interaction between vehicle and sediment. Simultaneously, the rules of fuzzy control are easily affected by the experience limitations of the designer, which may lead to reduce control performance. So, in this paper, a multi-body dynamic (MBD) model of a deep-sea mining vehicle, which considers the mechanical interaction between vehicle and sediment and spatial hydrodynamic effects, was developed first. Next, a fuzzy controller that contained double input and output was proposed for path tracking, and a genetic algorithm optimized the controller through the collaborative motion simulation. Finally, the performance of the optimized fuzzy controller was analyzed.
3. Exploration of the Path-Tracking Controller
Fuzzy logic control is widely used in marine robotic fields [
34]; its performance is mainly affected by fuzzy rules. The design of fuzzy rules is often influenced by the experience limitations of the designer, which may lead to reduced control performance. The introduction of a genetic algorithm to optimize fuzzy rules can significantly eliminate the influence of subjective factors on the control performance.
3.1. Design of Fuzzy Controller
A Mamdani-style fuzzy controller [
35] was designed for path-tracking control of the mining vehicle, where the input parameters are the path-tracking error and path-angle error, and the output parameters are the base speed and additional speed ratio. The domains of
,
,
, and
are [−600 mm, 600 mm], [−6°, 6°], [0.5 m/s, 1.1 m/s], and [−0.3, 0.3], respectively; all of them have seven language variable values defined as NB, NM, NS, Z, PS, PM, and PB. As is shown in
Figure 12, the type of z and s membership functions were used in NB and PB respectively, and the triangular membership function was applied in other language variable values. The other parameters used the same design form of the degree of membership function in
Figure 12.
When both types of error are small, the base speed of the mining vehicle can be increased, and vice versa, it needs to be reduced. At the same time, the path-tracking error has more influence than the angle error for an additional speed ratio; when the absolute value of the path-tracking error is large, the additional speed ratio should be set to a large value and have the same language direction as the former. Similarly, when the absolute value of the path-tracking error is small, the additional speed ratio is also small. Based on the above empirical experience, the fuzzy rules demonstrated in
Table 3 and
Table 4 were created, and the centroid strategy was applied in defuzzification.
.
3.2. Optimization of Fuzzy Controller
As for a global search algorithm, a genetic algorithm can obtain the global optimal solution of an optimization problem. First of all, it is essential to decide the decision variables and constraints according to the optimization problem and then establish an optimization model and objective function. Second, the encoding and decoding methods need to be determined. Finally, the individual evaluation method and relevant operating parameters should be defined.
In this study, a total of 98 parameters need to be optimized in
Table 3 and
Table 4; and all the parameters are integers. The value range is [
1,
7], which is equivalent to [NB, NM, NS, Z, PS, PM, PB] in the optimization process. The genetic algorithm named MI-LXPM was applied to resolve this optimized problem for its efficiency in integer-constrained optimization problems [
36,
37], and real encoding was adopted. In the movement of the mining vehicle, the smallest path-tracking error is expected, and the objective function is given by:
The optimization function in the genetic algorithm always minimizes the fitness function, so in the minimum objective function, the fitness function is given by:
where
is the bounds of the objective function with conservative estimation, and its value is set to 3.
The crossover operator and mutation operator are the most critical parameters of the genetic optimization algorithm, which work together to perform a global search and a local search of the search space. The values of the crossover operator and mutation operator were taken as 0.8 and 0.05 to ensure that the genetic algorithm has good search performance.
The fuzzy controller was established in MATLAB/Simulink, so it could be optimized by a genetic algorithm through the co-simulation between the controller and the MBD model. The co-simulation process of Simulink and Recurdyn is depicted in
Figure 13.
When the co-simulation was running, the genetic algorithm would update the parameters according to the fitness function values in each iteration until it met the iteration conditions and given the best parameter value. The optimization flow chart is manifested in
Figure 14.
After 239 generations, the optimal parameters were acquired, and the rules of the optimized fuzzy controller are shown in
Table 5 and
Table 6.
3.3. Simulation Results of Two Controllers
The fuzzy control optimized by the genetic algorithm was obtained. Then, the collaborative motion simulation of the path tracking was carried out. The curve of the expected path in the co-simulation was defined as (Unit:m):
The change of the Z-direction position was ignored, and the mining vehicle forwarded along the positive direction of the
X-axis. We found that both controllers were capable of path tracking from the simulation results displayed in
Figure 15. In linear motion, the trajectory of the actual motion almost coincides with the expected path. In contrast, in the steering motion, the turning radius of the actual motion trajectory will be larger than the radius of the expected path, and the path error was larger than when driving in a straight line. However, compared with the original fuzzy controller, the optimized fuzzy controller has better control accuracy.
In the simulation process, the left track speed
and right track speed
of MBD model in
Figure 16 were continuously changed by the optimized fuzzy controller to adjust the path error and obtain a better control performance. During straight and steering motion, the
and
were maintained near a more stable value; while in the transition period of the motion state, there would be a quick adjustment. It can be found that the optimized fuzzy controller has excellent ability to control the track speed in the straight and steering motion in a complex seabed environment.
Figure 17 indicated the velocity of the mining vehicle. In the beginning of the motion, the responding speed of the optimized fuzzy control performed better than the original one, and there was less fluctuation with the optimized fuzzy controller in case of the large sudden interference. It was evident that the stability of the velocity using the original fuzzy controller was not as good as that of the optimized fuzzy controller.
Figure 18 demonstrated the path-tracking error of the simulation process. The largest path-tracking error was 214 mm with the optimized fuzzy controller, while without optimization, the largest path-tracking error was 598 mm. The latter path-tracking error varied more sharply throughout the motion.
Figure 19 indicated the path-angle error. The path-angle error was small in the straight motion and large in the steering motion. It was clear that the optimized fuzzy controller has better control performance, with a smaller maximum angle error and fluctuation.
The path-tracking error and the path-angle error were changed continuously in the motion process of the mining vehicle, so the reference velocities that were used to control the left track and right track would change at the same time according to the control rules of the fuzzy controller. The slip rate of the left and right tracks in the movement process is shown in
Figure 20. At the beginning of the straight motion, the slip rate fluctuations were small, and the vehicle was in a relatively stable state; in the subsequent steering motion, the slip rate fluctuated more, which meant that the stability of the steering motion was not as good as the straight process, and more frequent and greater adjustments were required to reduce the path error; when the car completed the steering motion and entered the straight motion again, the slip rate of the track was also maintained in a smaller fluctuation range. In the whole motion process, the slip rate of the crawler was between 0 and 0.03 most of the time. The slip rate in this interval could provide the vehicle with greater traction and obtained a higher energy efficiency.
From the above simulation results, there was no doubt that the optimized fuzzy controller has achieved the path-tracking control of the deep-sea mining vehicle. Furthermore, its control accuracy and stability, which fully met the design requirements, had been significantly improved compared to the controller designed by human experience. The genetic algorithm could develop a satisfactory fuzzy controller for the mining vehicle, as long as a suitable dynamic model was established ahead.