Control of a Quadrotor Equipped with a Fixed-wing by Tilting Some of Four Rotors

—Unmanned aerial vehicles (UAVs) are being expected to be used for the vegetational observation and the information collection of disaster sites. Especially, rotorcrafts typified by helicopters are attractive, because they are able to hover and achieve vertical take-off and landing (VTOL). However, rotorcrafts have a disadvantage that it cannot have a long-distance flight, because they fly by the thrust of upward direction. Aircrafts with tilt rotors are developed in order to overcome such disadvantages. Such aircrafts can be hovering and take a VTOL and also a long-distance flight by changing the angle of the rotor. In this research, it is aimed at proposing a VTOL-type UAV with a fixed-wing and four tiltable rotors and controlling it.


I. INTRODUCTION
Unmanned aerial vehicles (UAVs) are being expected to be used for the vegetational observation and the information collection of disaster sites [1]. Especially, rotorcrafts typified by helicopters are attractive, because they are able to hover and achieve vertical take-off and landing (VTOL) [2]. This feature facilitates the operation of the UAVs. However, rotorcrafts have a disadvantage that it cannot have a long-distance flight, because they fly by the thrust of upward direction. Aircrafts with tilt rotors are developed in order to overcome such disadvantages. Such aircrafts can be hovering and take a VTOL and also a long-distance flight by changing the angle of the rotor [3] [4] [5]. In other words, aircrafts with tilt rotors have several advantages, compared to both rotorcrafts and fixedwing aircrafts.
In this research, it aims to propose a VTOL-type UAV with a fixed-wing and four tiltable rotors and to control it. Four tiltable rotors are placed on the front and rear of the airframe and the both sides of a fixed wing. When this UAV takes a hovering and a VTOL, each rotor is upward like a quadrotor. When the UAV shifts to a level flight, the thrust generated from the front and rear rotors maintains the airframe altitude, so that this UAV can move forward by obtaining driving force from the right and left rotor thrust. Since this type of UAV with a fixed wing can obtain the lift force by moving forward, it is possible of achieving an efficient level flight, compared to conventional quadrotor-like UAVs. Thus, the present objective is to perform the position and attitude control by switching a rotorcraft mode, which is able to take a VTOL and hovering, and a fixed-wing aircraft mode, which is able to have a fast and long-distance flight. Thus, this paper proposes a VTOL-type UAV with a fixed-wing and four tiltable rotors. A controller is designed to control a position and attitude by changing tilt angle of rotors placed on the both side of fixed-wing. The effectiveness of the control method is verified by some simulations. The UAV with four tiltable rotors and a fixed-wing proposed in this reserch is shown in Fig. 1. This airframe has a fixed wing and four tiltable rotors, where each rotor can be tilted back and forward against the airframe. Fig. 2 shows the case that the airframe is viewed from the plus y-axis, where each rotor is tilted backward. The angle of each rotor is referred to from a vertically upward axis. Note here that the rotational direction of the rotor 2 and the rotor 4 is mutually reverse to cancel a counter torque generated by their revolutions during a level flight.

A. Structure of a UAV with Four Tiltable Rotors and a Fixedwing
x z y Fixed-wing

B. Definition of the UAV with a Fixed-wing and Tiltable Rotors
The definition of coordinates is shown in fig. 2, and the robot coordinate C is defined such that the origin is the center of the airframe, positive x-axis is set as the forward direction of the airframe, positive y-axis is set as the right direction of the airframe, and positive z-axis is set to be downward perpendicular to the airframe. Similarly, the world coordinate E is a right-handed coordinate where positive Z-axis is set to vertically downward. The center position of the airframe is represented by in the world coordinate, and the rotational angles for roll, pitch and yaw in the robot coordinate system are represented as  ,  and  respectively, then the attitude of the airframe is represented by

III. DERIVATION OF DYNAMICAL MODEL
A dynamical model of the UAV with four tiltable rotors and a fixed-wing is derived by the Lagrangian method. Here, the mass of the airframe is m and the moment of inertia around each axis is represented by x I , y I and z I respectively, the lift force and the drag force are L and D respectively, and the moment of inertia of the rotor is r J . Letting x u be the input of translational motion of x direction, z u be the input of translational motion of z direction, 2 u be the input of roll motion, 3 u be the input of pitch motion, and 4 u be the input of yaw motion, the dynamical model of the UAV with a fixedwing and four tiltable rotors is derived as follows:  is the sum of x-directional components of rotation of rotors, z  is the sum of z-directional components of rotation of rotors, which are calculated by In this paper, a PD controller is used for controlling the UAV with four tiltable rotors and a fixed-wing. Letting   A rotational direction of rotors of the UAV with a fixed wing and four tiltable rotors is different from a conventional quadrotor. In other words, it cannot use a conventional control method for  by applying the difference of torques between rotors. Therefore, the present airframe controls  by changing k is a P gain, 6 k is a D gain and d  is the target value of  . In Fig. 3(a)  Control of the position in z-direction is affected by the gravity, the input of x-direction, the lift force and the drag force. Therefore, the position controller for z-direction is represented by     9 10 9 9 7 9 7 9 k is a P gain and 10 k is a D gain.

D. Controller for the x-directional Position by Tiltable Rotors
k is a P gain, 12 k is a D gain and d x is the target value of x-direction. In Fig. 4(a) Acording to the relationship among these variables, a logical switching rule is represented as follows: This simulation is intended to verify that the x-directional position of the airframe is controlled by controlling the magnitude of the thrust and tilt angle, keeping a constant height. The physical parameters used for the simulation are shown in Table 1. The initial state of the UAV with four tiltable rotors and a fixed-wing is . The feedback gains are decided such as , through trials and errors.
It is found from Fig. 5 that the position, i.e., the states x, y and z converge from the initial position to the goal position.
Then, the attitudes  ,  and  are not changed as shown in Fig. 6. Fig. 7 shows that the tilt angles for the second and fourth rotors changed so as to approach the desired position of x-axis, keeping a desired height (i.e. 1 m).    It is found from Fig. 8 that the position z converges from the initial position to the goal position, keeping x and y at the initial state. Then, the attitude  converges from the initial attitude to the goal attitude, while  and  are not changed as shown in Fig. 9. Fig. 10 shows that the tilt angles for the second and fourth rotors change so as to approach the desired attitude of  .

VI. CONCLUSION
In this paper, a VTOL-type UAV that has a fixed-wing and four tiltable rotors has been introduced. A PD controller was designed to control the position and attitude by controlling tiltable rotors. It was found from some simulations that the airframe position in X-direction and attitude of  was controlled. For future work, it needs to design a controller for a level flight and verify the effectiveness of such a controller, together with the present controller, through experiments for an actual machine.