## 1 Introduction
[PERSON] adopted a dynamic auxiliary system in the controller of a missile-target interception guidance system for compensating the effects of constrained inputs [29], and the simulation results demonstrate that the robustness of the proposed method is effectively improved. In view of this, a SMC method with auxiliary system can be an effective solution to the control design of the tilt rotor UAVs with input constraints. Based on the above analysis, this paper aims to design a control scheme consisting of a sliding mode control and auxiliary dynamic (SMC-AD) for a tilt tri-rotor UAV with constrained inputs. The control system adopts cascade control strategy, which is composed of position loop and attitude loop. In the position loop, as is the same for the practical problem of input constraints, a SMC controller with an auxiliary dynamic system is designed. In the attitude loop, the SMC approach is chosen to achieve stable control performance, and a disturbance observer is applied to alleviate the chattering and the influence caused by parameter perturbation and unknown external disturbances. Besides, the control allocation mapping the inputs of the actuators to the outputs of the designed cascade controller, is completed by transforming it into a constrained optimization problem. The main contributions are enumerated as follows. 1. A nonlinear sliding mode control scheme, composed of a SMC with auxiliary dynamic (SMC-AD) in the position-loop and a SMC with disturbance observer in the attitude loop, is proposed for a tilt tri-rotor UAV. This method is the early application in solving the flight control of the tilt tri-rotor UAV with constrained inputs. Meanwhile, the nonlinearity, parameter perturbations and other unknown external disturbances are also considered in the control scheme. The stability of the overall system is proven mathematically and the effectiveness is verified by simulations and experiment. 2. In order to solve the inconsistency between the number of the virtual control quantity and the actual actuators, a control allocation method based on optimization algorithm is developed for the tilt tri-rotor UAV to obtain high control precision. The tilt tri-rotor UAV has three flight modes, including the hover mode, the fixed-wing mode, and the transition mode. In the stage of vertical take-off and landing, it is necessary to design a good hover mode controller to ensure the stability of the flight process and the accuracy of take-off and landing. In the transition mode, a hybrid controller is commonly used, which is realized by the weight distribution of the hover mode controller and the fixed wing controller. Thus, the flight control of the hover mode is the premise of realizing the whole process of flight. In this paper, we will focus on the flight control of the tilt tri-rotor UAV in the hover mode. The remaining sections are arranged as follows. Section 2 describes the prototype and mathematical model of the tilt tri-rotor UAV. Section 3 states the main results in detail, including the design of the cascade controller, control allocation, and the stability analysis. Section 4 performs some simulations and experiments to verify the theoretical results. Section 5 draws the final conclusions.