Integral Sliding Mode Flight Controller Design for A Quadrotor and The Application in A Heterogeneous Multi-Agent System
This paper investigates a novel integral sliding mode control (ISMC) strategy for the waypoint tracking control of a quadrotor in the presence of model uncertainties and external disturbances. The proposed controller has the inner-outer loop structure: The outer loop is to generate the reference signals of the roll and pitch angles, while the inner loop is designed by using the ISMC technique for the quadrotor to track the desired x, y positions, roll and pitch angles. The Lyapunov stability analysis is provided to show that the negative effects of the bounded model uncertainties and external disturbances can be significantly decreased. The designed controller is then applied to a heterogeneous multi-agent system (MAS) consisting of quadrotors and two-wheeled mobile robots (2WMRs) to solve the consensus problem. We present the control algorithms for the 2WMRs and quadrotors. Consensus of the heterogeneous MAS can be reached if the switching graphs always have a spanning tree. Finally, the experimental tests are conducted to verify the effectiveness of the proposed control methods.
The aforementioned control methods need the complete knowledge of the quadrotor dynamics, indicating that the exact model parameters are known in the controller design. However, the errors in the parameters such as the mass, the center of mass and inertia can deteriorate the performance of the controller. In addition, the external disturbances also inevitably affect the performance of the flight controller. One of the most common methods for dealing with the model uncertainties and disturbances is the adaptive control approach . In , the disturbances are quantized by using the designed quantum logic, and then the adaptive controller is employed to stabilize the quadrotor. Some other methods for correcting parameter errors or estimating the states of the quadrotor can be found in , , , , . Some experimental studies are carried out on the flight controller design for quadrotors with the extra payload, e.g., , . Sliding mode control (SMC) is a well developed technique to deal with uncertainties and disturbances such that the system can reach the desired states in finite time . One of the major advantages of using the SMC is that the matched uncertainties (the disturbances in the control input channel) can be effectively rejected. Later, the integral sliding mode control (ISMC) technique has been deeply investigated . By using the ISMC approach, the reaching phase is eliminated and the system trajectory starts in the designed sliding surface. The ISMC technique is used for the altitude control for a small helicopter with ground effect compensation in . However, the work in  only considers the altitude information, which is not suitable for the flight controller design of a quadrotor. Besides controlling a single quadrotor, many researchers study the control of a group of robots , , , , . One of the major advantages of employing the multiagent systems (MASs) is that it is able to conduct the complex tasks that are beyond the capability of an individual agent. For the control of the MAS involving quadrotors, designing a robust flight controller that can reject the model uncertainties and external disturbances plays an important role in guaranteeing the cooperative tasks to be successful. In practice, teams of robots can also involve various dynamics, for example, the agents may have different model orders or versatile maneuvers, which result in the differences of agents’ ability and function. This belongs to the cooperative control of the heterogeneous MAS , . As an example, the task of search and rescue can be performed with higher efficiency if both unmanned aerial vehicles and ground robots are employed.
The main contribution of this paper is three-fold: _ The ISMC strategy for the quadrotor flight control is proposed. We present the inner loop ISMC-based controller incorporating the reference angle signals and the desired position information. Accordingly, the stability analysis by using the Lyapunov approach is provided. _ We implement the LQR-based  and the proposed ISMC-based controllers on a commercially-available quadrotor. The performances of the controllers are illustrated and compared in terms of mean square error (MSE). To emphasize the effectiveness of the designed ISMC-based flight controller, we further enlarge the model uncertainties and external disturbances by attaching an unknown weight to the quadrotor, and then the experimental comparisons between the two controllers are conducted. . _ The consensus control methods for a group of MAS involving 2WMRs and quadrotors are proposed. The sufficient condition for the MAS to reach consensus is that the switching graphs always have a spanning tree. The designed consensus control approaches are realized in practical applications.
In this paper, an ISMC-based flight controller for the quadrotor waypoint tracking task has been discussed. We first presented the modelings of the quadrotor and the actuators, and then introduced the inner-outer loop structured LQRbased control strategy. In order to decrease the negative effects caused by the model uncertainties and the external disturbances, we proposed the ISMC-based flight controller incorporating the reference angular signals and the desired position information into the inner loop of the controller. By using the ISMC technique, we eliminated both the reaching phase to the sliding surface and the chattering in the control signals. The detailed stability analysis was provided. The designed controller was then applied to solve the consensus problem for a heterogeneous multi-agent system. From the experimental testings, it is shown that the effects of the bounded model uncertainties and external disturbances are significantly rejected in conducting the waypoint tracking task, and the consensus algorithms for the 2WMRs and quadrotors work effectively. Future research will be focused on the quadrotor hovering control at the precise position and the flight controller design in the discrete-time domain. Interesting extensions of this work can also be pursued in considering actuator faults , time-delays  and designing the novel flight controller using the model predictive control (MPC) technique , . A video of the experiments is posted on the following URL: https://youtu.be/u1qqB166O-8
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