3.1. Application to Aerial Vehicles
Jesimar et al.
[87] applied a Genetic Algorithm to path planning for UAVs during the emergency landing situation. Path planning uses a mathematical function, which caters to all constraints. Three different path planning approaches are used: the Genetic Algorithm, greedy heuristic approach, and multi-population algorithm. The greedy approach helps determine the quick solution, whereas the genetic algorithm returns a better quality solution, which helps mitigate the computational time. The methods were validated on a huge dataset with different levels of difficulty. Simulations were carried on the FlightGear simulator, where the behavior of UAVs was checked under different wind directions and velocity directions. The overall statistical analysis demonstrates that combining genetic algorithms with a greedy approach is beneficial for the path planning of UAVs.
3.2. Application to Ground Vehicles
Farhad et al.
[88] attempted to investigate the neural network technique with statistical dimension reduction techniques to execute the navigation task and obstacle avoidance for the robot. The proposed method uses two feed-forward neural networks with a back-propagation learning algorithm. Laser (SICK) is used with a 180
∘
range to visualize the surrounding environment. The algorithm checks on real-world and experimental results to prove the efficacy of the proposed algorithm. Lingyan Ran et al.
[89] worked on vision-based lightweight robot navigation based on uncalibrated spherical images. To improve the trajectory formation, the navigation problem is divided into sub-classification tasks. A 360
∘ fisheye camera is introduced for acquiring spherical images in different heading directions. The classification is accomplished using a Convolutional Neural Network (CNN). The CNN tends to predict paths in different directions with high efficiency. Experimental results prove the validity of the proposed method. Ngangbam Herojit et al.
[90] presented the problem related to the navigation of ground robotics and obstacle avoidance with an Artificial Neural Network. The entire area is divided into five segments, then MLP (Multilayer Perceptron) is activated to find the collision-free path. The simulation proves that the said algorithm gives optimal results for reaching the goal position.
3.3. Application to Underwater Vehicles
Carlos Miguel et al.
[91] worked on underwater glider (UWG) vehicles to ensure their mission success and safety. UWG is considered energy-efficient vehicles, and for performing journeys, they are equipped with sensors that collect data from their surroundings. For a safe journey underwater, the vehicles need to maneuver with a low speed and cater to strong ocean waves, which require extensive path planning. Gliders are often involved in multi-objective functions, e.g., shortest path, obstacle avoidance, energy efficiency, etc. The proposed method involves the non-dominated sorting genetic algorithm II (NSGA-II) to support the motion of gliders in a 3D environment. Glider kinematic simulators coupled with NSGA-II were used to perform experiments for controlling multiple control parameters to perform trajectory optimization. The authors were able to configure the parameters for the desired trajectory and proved to perform a real-time experiment in the ocean.
4. Hybrid Algorithms
4.1. Application to Aerial Vehicle
Author Li et al.
[92] discussed the engineering problems with the help of the Improved Moth Flame Optimization. The proposed algorithm is implemented on a Levy flight trajectory formation. Harun Ilango et al.
[93] presented the comparison of the Moth Flame Optimization, Bats Optimization Algorithm, and Artificial Bee Colony Algorithm for the landing stage involved in UAV. The objective lies in determining the optimal landing path for UAVs in a minimum amount of time. The empirical results obtained from the Moth Flame Optimization Algorithm take less time to find the optimal path than the other algorithms. Rehan Tariq et al.
[94] presented the Intelligent Moth Flame Optimization-Based Clustering (IMOC) for drone assistance. The technique is used for maximum coverage using the cluster head approach, which helps find the optimal route. The comparison was made with Ant Colony Optimization (ACO), Grey Wolf Optimization (GWO), and Comprehensive Learning Particle Swarm Optimization (CLPSO). When compared with these algorithms, the proposed algorithm IMOC outperforms the other algorithms in improving the path criterion for UAVs.
4.2. Application to Ground Vehicles
Hao Wang et al.
[95] presented the integration of the Artificial Neural Network with Fuzzy Logic for path planning. The fuzzy neural network can process data in parallel form and can process fuzzy inference functions according to the need for planning the mobile robotics trajectory. The simulations were performed in an unknown environment with static obstacles. The results obtained from simulations show a fast convergence and high efficiency for finding the optimal path. Pengchao Zhang et al.
[96] proposed that the integration of the traditional algorithm with the heuristic programming algorithm based on AI has beneficial results. The traditional rapidly exploring random tree algorithm is incorporated with a neural network to tune path smoothness and path planning functions. The simulations were performed in real-time to check the feasibility of the improved algorithm, which shows the improved results for handling navigation problems. Buddhadeb Pradhan et al.
[97] investigates the problem of multi-robots for finding the goal point. Each robot’s motion is controlled by the Particle Swarm Optimization, which the Feeds Forward Neural Network tunes. The coordination algorithm is implemented by a coordinated, cooperative algorithm that maintains the step count of all robots. In the first step, the Artificial Neural Network (ANN) is hybridized with PSO to find the easiest path using acceleration and velocity constraints; the second step, the first and second-order stability analysis, is employed to carry out the convergence. The experiments performed with the proposed algorithm show efficacy and demonstrated promising results. Same as ground vehicles, researchers have also worked on autonomous underwater vehicles.
4.3. Application to Underwater Vehicles
Daqi Zhu et al.
[98] have demonstrated the autonomous underwater vehicles (AUV) in two steps by proposing Glasius Bio-inspired Neural Network (GBNN): (i) the construction of a grid map is done via discretization of 2D environment, (ii) then the neural network is constructed on the grid map. At the final stage, a full path is converged using GBNN, and obstacles are avoided using path templates. The simulations show that AUV can fully cover the environment and shows exceptional maneuverability when stuck in a deadlock situation without delay. The results also demonstrate a low overlapping rate with minimum path planning time when using the proposed algorithm.
5. Challenges Involved in Path Planning Methods
Though many researchers have studied the path planning for ground, aerial, and underwater vehicles, no algorithm/technique can guarantee 100% results; moreover, the tendency to get stuck in local/global optima or the incapability to judge the obstacle in front may lead to numerous challenges involved with these techniques. These drawbacks significantly affect the performance of the autonomous guided vehicle. Some challenges are mentioned below:
The most used approach for detecting obstacles or planning a path is the deployment of sensors or cameras around any vehicle
[99][100]. However, these sensors’ readings are neither accurate nor reliable as they are integrated with noise, temperature, and system oscillations, etc. This leads to uncertainty in the system output, which causes unintentional error in the output of the algorithm
[101]. The vehicle may produce oscillations and noise, which affect the real-time efficiency related to the data acquired from the environment
[102]. Plenty of research has been performed to mitigate and cater the noise occurrence in the vehicle system; however, this is still a challenge. These problems and plenty others widely disturb the implementation of any algorithm in real-time
[103]. In vision-based algorithms, the problem lies in identifying pairs of points in the same dimension
[104]. This causes ambiguity in identifying points, which results in inconsistent interpretation of any image
[105].
Another problem lies in some algorithms that rely on the surrounding environment map for the vehicle to make any decision for navigation. This leads to unnecessary halts in the motion of the vehicle. Baldoni et al.
[106] demonstrated this challenge through simulations and shows that the generation of the optimal path for any vehicle is complex, and even if the vehicle reaches the desired destination point, it does not produce smooth navigation.
ANN may have a lot of advantages, as stated earlier, but they require an extensive data set of the surrounding area for the adjustment of hidden layers
[107]. The famous backpropagation algorithm has its disadvantages, as it quickly converges to the local minima problem
[108].
Table 2 depicts the challenges involved in path planning.
Table 2. Challenges involved in Path Planning.
Cause |
Challenges |
Source |
Sensors/camera |
The readings form these sensors are not accurate nor reliable as they are integrated with noise, temperature, and system oscillations, etc. This arises uncertainty in the system output, which causes unintentional error in the output of the algorithm. |
[99][100][101] |
Noise occurrence |
Plenty of research has been performed to mitigate and cater the noise occurrence in the vehicle system; however, this is still a challenge. These problems and plenty others widely disturb the implementation of any algorithm in real-time. |
[103] |
Vision-Based |
The problem lies in identifying pairs of points in the same dimension. This causes ambiguity in identifying points, which results in inconsistent interpretation of any image. |
[104][105] |
ANN |
This algorithm has numerous advantages, but they require a large data set of the surrounding area for the adjustment of hidden layers. The famous backpropagation algorithm has its own disadvantages, as it easily converges to the local minima problem. |
[107][108] |
56. Conclusions
Trajectory planning is often required in autonomous vehicles. Over the last decade, a lot of research has been performed to address the strengths and challenges involved in autonomous vehicles. This paper comprehensively discussed and summarized the numerical techniques and optimization techniques involved in ground, aerial, and underwater vehicles. Some strengths and challenges are mentioned in
Table 3.
Table 3. Strengths and Challenges Involved in Hybrid Methods for Ground & Aerial Vehicles.
Algorithms |
Strengths |
Challenges |
Implementation |
Time Complexity |
Fuzzy Logic |
(a) The fuzzy rules can be tuned for desirable requirement [3] |
(a) Difficult to create membership functions |
Real-time and simulation |
T≥0(n2)T≥0(n2) |
|
(b) Control logic implementation is easy [109] |
|
|
|
(c) Can be easily integrated with bio-inspired algorithms [3] |
|
|
|
Neural Network |
(a) Works best in real-time |
(a) Difficult to handle buried neuron layers in the network [107] |
Real-time and simulation |
T≥0(n2) |
|
(b) Imitate human control logic easily |
(b) Increase in layers increases complexity [107] |
|
|
(c) Use of backpropagation results in a local minimum problem [110] |
|
|
|
(d) Acquiring a large data set in real-time is difficult [107] |
|
|
|
Genetic Algorithm |
(a) Faster convergence rate and optimization capability [111] |
(a) Get stuck in local minima problem when environment complexity increase [112] |
Simulation |
T≥0(n2) |
|
(b) Combine well with other algorithms [111] |
(b) Produce oscillations in system [113] |
|
|
(c) Because of easy implementation integrate well with other algorithms [114] |
|
|
|
ABC |
(a) Requires fewer control parameters [68] |
(a) Slow convergence rate [115] |
Simulation |
T≥0(n2) |
(c) Because of easy implementation integrate well with other algorithms [114] |
|
|
|
Simulated Annealing |
(a) Good at approximating global optimum [78] |
(a) Slow convergence rate [78] |
Simulation |
T≥0(n2) |
|
GWO |
(a) Fast convergence rate [116] |
(a) Implementation gets tricky when complex scenarios arise [117] |
Simulation |
AntLion |
(a) Produces good results in complex environment [122] |
(a) Involvement of a lot of variables makes it difficult to handle when integrated with different algorithms [123] [124] |
Simulation |
T≥0(n2) |
The most pertinent conclusion points are summarized as follows:
-
Consolidation of available information: A detailed review of the trajectory planning and optimization is presented from the application point of view on ground, aerial, and underwater vehicles. The DARPA challenge 2007 related to robotics, Lord Rayleigh work related to dynamic soaring in 1883, and some extensions related to the underwater vehicle are elaborated. Algorithms, i.e., numerical techniques for implementing the path planning, are discussed.
-
Survey of trajectory optimization techniques: A comprehensive overview related to optimization algorithms and numerical techniques that have been utilized for performing trajectory formation and its optimization.
-
Problem formulation and generation of optimal trajectories: An explanation of how different algorithms can be integrated to build a mathematical model for planning and the formation of trajectory components can be achieved presented with a literature survey.
-
Limitations and a way forward: Though numerous works review robotics, aerial and underwater vehicle systems have been presented together with optimization techniques and numerical methods, and it has been observed no single algorithm produces desired results or accurate output; therefore, a hybridization of different algorithms has been used by researchers. Two optimization algorithms or two numerical methods together can be integrated, or a mix and match of techniques can be achieved for obtaining the desired characteristics results.
|
(b) Requires less computational time [114] |
|
|
|
|
(b) Lesser variable involvement [116] (c) Easily integrated with other algorithms [118] |
|
|
|
Moth Flame |
(a) Compared to other algorithms, it produces good solutions in complex scenarios [119] |
(a) Has premature convergence rate [120] |
Simulation |
T≥0(n2) |
|
WOA |
(a) Easy implementation with fast convergence rate [121] |
(a) Difficult to handle in a complex environment [58] |
Simulation |
T≥0(n2) |