In this paper, we try to show the state of the art on the formation control of multiple quadrotors. From the literature, we summarize some most interested issues such as potential applications, representation of multiple quadrotor formation, consensus, formation control, formation configurations, localization, etc. According to these research, we point out the difficulties and the tendency of research in the field of multiple quadrotors control.

In this paper, an interactive leader-follower configuration of multiple quadrotors is considered, which means that the leaders can react with respect to the followers. In order to analyze the consensus of such a multiquadrotor system, an interaction matrix is proposed. The convergence condition of the trajectory tracking error is then proved to be related to the smallest eigenvalue of the interaction matrix. In order to achieve an aggressive formation, we develop a bounded composite nonlinear feedback-based formation controller with hyperbolic functions as a main contribution. Simulation and real-time experimental results show that the formation task is achieved with small overshoot and rapid response speed. Real-time experiments show four quadrotors following a circular reference trajectory with high linear velocity up to 2.7 m/s.

Aiming at the problems of “local minimum” and “unreachable target” existing in the traditional artificial potential field method in path planning, an improved artificial potential field method was proposed after analyzing the fundamental causes of the above problems. The method solved the problem of local minimum by modifying the direction and influence range of the gravitational field, increasing the virtual target and evaluation function, and the problem of unreachable targets is solved by increasing gravity. In view of the change of motion state of robot fish in amphibious environments, the improved artificial potential field method was fused with a dynamic window algorithm, and a dynamic window evaluation function of the optimal path was designed on the basis of establishing the dynamic equations of land and underwater. Then, the simulation experiment was designed under the environment of Matlab2019a. Firstly, the improved and traditional artificial potential field methods were compared. The results showed that the improved artificial potential field method could solve the above two problems well, shorten the operation time and path length, and have high efficiency. Secondly, the influence of different motion modes on path planning is verified, and the result also reflects that the amphibious robot can avoid obstacles flexibly and reach the target point accurately according to its own motion ability. This paper provides a new way of path planning for the amphibious robot.

The seam tracking operation is essential for extracting welding seam characteristics which can instruct the motion of a welding robot along the welding seam path. The chief tasks for seam tracking would be divided into three partitions. First, starting and ending points detection, then, weld edge detection, followed by joint width measurement, and, lastly, welding path position determination with respect to welding robot co-ordinate frame. A novel seam tracking technique with a four-step method is introduced. A laser sensor is used to scan grooves to obtain profile data, and the data are processed by a filtering algorithm to smooth the noise. The second derivative algorithm is proposed to initially position the feature points, and then linear fitting is performed to achieve precise positioning. The groove data are transformed into the robot’s welding path through sensor pose calibration, which could realize real-time seam tracking. Experimental demonstration was carried out to verify the tracking effect of both straight and curved welding seams. Results show that the average deviations in the X direction are about 0.628 mm and 0.736 mm during the initial positioning of feature points. After precise positioning, the average deviations are reduced to 0.387 mm and 0.429 mm. These promising results show that the tracking errors are decreased by up to 38.38% and 41.71%, respectively. Moreover, the average deviations in both X and Z direction of both straight and curved welding seams are no more than 0.5 mm, after precise positioning. Therefore, the proposed seam tracking method with four steps is feasible and effective, and provides a reference for future seam tracking research.

This paper addresses the problem of controlling a leader-follower formation of quadrotors (UAVs), which can be considered as a System of Systems. A distributed control scheme for the motion of the formation is proposed, ensuring consensus of the UAVs and collision avoidance. Each UAV has local and limited neighbors and uses weighted relative positions and velocities of its neighbors. In the simulation section, a comparison of using the formation controllers with weighted and unweighted topology is given. The results show that the proposed control strategy can keep the formation with some initial conditions, unlike the strategy with unweighted topology. The simulations also show that our proposed control strategy can be applied for both one leader and multiple leaders formation.