Investigating cascaded control with constraints with application to unmanned aerial vehicle and induction motor drive
Creators: name: ZHU, Yuankang
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400607
subject: Signal processing
classificationCode: 400702
subject: Automation engineering

description: Over the last several decades, unmanned aerial vehicle and induction motor drives have gained extensive attention in various applications. Due to their nature nonlinear and coupled dynamics, there are always challenges for developing a robust control system to achieve accurate tracking performance. Since most of the physical systems are operated in a closed-loop system structure with a controller, it is important to investigate a suitable controller for the entire closed-loop system. This thesis is focused on the cascaded control system design with the application to the hexacopter and three-phase AC induction motor drive. Five algorithms are introduced in this research: (1) the conventional model-based control method; (2) automatic tuning process of the PID controller; (3) DOBC method; (4) first-order SMC plus the disturbance observer design; (5) continuous second-order SMC design.

The major contribution of this thesis is the design of the cascaded control structure with the application to the hexacopter and induction motor drive under operational constraints. Rather than considering the entire second-order dynamics, we consider two controllers separately by dividing the whole dynamics into two first-order systems. This action can reduce the design complexity and achieve better control capability.

Various problems have been investigated with the model-based method. Firstly, due to the existence of non-linearity inside the system, a linearisation procedure is required while designing the model-based controller. Besides, some physical parameters can be tough to obtain, especially for the UAV platform with very small parameter values. Lastly, some of the time-varying parameters have variations in different operating environments. These existing problems in the model-based method have motivated the development of other control algorithms, like automatic tuning method or the SMC in this thesis. 

The first investigated linear control method is auto-tuner design. The auto-tuner's main idea is to fit the physical plant's dominant dynamics into an integrator plus a time delay model, which is used to select the PID controller gains. Another advantage of the automatic tuning process is that it automatically calculates all the parameters with a minimized human intervention. The disturbance observer-based control method is introduced together with proportional control. The estimated disturbance observer is designed by assuming a constant input disturbance and incorporating it into the control signal in order to achieve the integral action. It ensures the anti-windup mechanism naturally proceeding when the control signal reaches its operational limits. The experimental tests have shown that the disturbance observer-based control method is more robust than the traditional PI while achieving the disturbance rejection.

The final part of this research is investigating the nonlinear SMC methods, including the first-order SMC and the continuous second-order SMC. Firstly, the first-order SMC is designed with the disturbance observer design. This is because the nonlinear SMC is more powerful to deal with the nonlinear dynamics, and the observer's existence reduces the SMC gain. Therefore, the chattering phenomenon can be reduced so that the control performance can be improved. However, the first-order SMC controls the system in the discontinuous-time. To address this problem, a continuous second-order SMC is introduced in this thesis. The chattering phenomenon is further reduced by its continuous nature. The second-order SMC is implemented on the induction motor platform with a fix-time convergence. The comparison studies are performed between the proposed four algorithms and the model-based method to Demonstrate closed-loop performance.