A Design Configuration and Optimization for a Multi Rotor

Multi rotor UAVs offer great potential wide range of challenging applications due to the high manoeuvrability and to the potential to hover, take off and fly in small areas. Nevertheless, their design is in some way critical. The main concern is their inadequate level of handling qualities due to the intrinsic instabilities of this type of small size vehicles. A specific hardware with a control matrix is also required to stabilize and manoeuvre the aircraft. A non-marginal trust-to-weight ratio is mandatory that implies adequate sizing of the power output provided by the propulsion system, generally compromising their endurance and their payload capabilities. In the last years, in the attempt to overcome these issues, several multi rotor unmanned vehicles have been developed. The aim of the present project is to create a compact, robust and highly manoeuvrable autonomous UAV. The quad-rotor ELISA (intEgrated muLtIrotor for Surveillance Applications) is controlled by changing the rotation speed of the motors. The torque in the yaw direction is cancelled by spinning two of the propellers clockwise and the other two anticlockwise. The optimal configuration has been chosen in order to increase the aircraft structural stiffness and to enhance the stability and the controllability of the vehicle. The mini-rotors are supported by a set of equally spaced composite radial bars, each of them linked with a central payload case. The research steps presented in this paper concern the analysis of the vehicle configuration and the definition of the mathematical model describing the dynamic behaviour.. Main results of this work will be reported and widely discussed in the paper. 1.0 INTRODUCTION Rotary-wing UAVs can have capabilities to perform missions that can not be achieved with fixed-wing UAVs. Rotary-wing vehicles have the potential to be very useful in territorial monitoring and, especially in the last years, R/C helicopters are used for aerial survey. Many University research groups have developed rotary-wing systems, even if today the widespread research and use is almost reserved to military organizations. Recently, the University of Maryland has also developed two rotarywing micro UAVs [1]. This micro vehicle has two counter-rotating coaxial rotors and weights 140 grams. Small rotary-wing UAVs with VTOL and hover capabilities can have many applications; these UAVs could be especially useful for indoor flight or for urban missions. Using UAVs for reconnaissance in these situations is also challenging because of the short line of sight and many obstacles. They are able to fly in areas with obstacles and poor quality GPS signals. Incorporating a reliable semi-autonomous or autonomous control system in these small vehicles, the operator does not have to constantly monitor the platform flight parameters or location. However the on board software will have to be very compact to fit in the available memory of the small microprocessors, RTO-MP-SCI-202 31 1 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED A Design Configuration and Optimization for a Multi Rotor UAV but powerful enough to provide control with sensor data of limited quality. These vehicles can be very challenging. Recently, a quad-rotor is a rotary wing UAV that has been the subject of several recent research projects. Small quad-rotors have many exciting potential missions including flight indoors and in urban areas. However, the development of the control systems needed to fly. The most known quad-rotor is the Draganflyer [2], a commercial product from RC Toys; this vehicle is flown using an R/C transmitter and its onboard electronics. The pilot can control the throttle setting for the four motors and the yaw, pitch and roll rates of the platform. The recent version of Draganflyer includes four infrared heat sensors to allow the quad-rotor to level itself while it is being flown outdoors. Another platform is the EADS Quattrocopter, used as a testbed for developing micro air vehicle flight control [3]. The Quattrocopter is capable of 20 minute flight with a single charge of its lithium batteries. The vehicle is length 65 cm, weights about half a kilogram and its detectable fuselage can be stored in a backpack. The electric motors allow this UAV to operate quietly. Under development in Australia the X-4 flyer [4] has a frame length of 70 cm and weights 2 kg with almost 20 cm diameter rotors. The first flight testing was conducted using a truck battery and tether chord to provide power to the platform. However, these tests were not successful and the trust margin of the X-4 flyer was not large enough to allow controllable flight. The next goals for improvement are to design a new X-4 flyer that would be capable to produce more thrust with a wireless serial link and a camera system. Research teams in some Universities are developing quad-rotor system control, starting from commercial available model. For example, a research team from France employed the commercial Draganflyer to study its stabilization [5]. In the same direction, a research group at the University of Pennsylvania is developing a quad-rotor using the commercial model HMX-4. Due to the weight limitations, no GPS or additional accelerometers could be placed on the platform. At the Cornel University two quad-rotor projects have been performed. The goal of the first project was to develop a method to estimate the attitude of a vehicle by using an offboard vision system and three onboard gyroscopes [6]. The second project was concentrated on the four thrust producing units and structure of a quad-rotor. These two areas were especially important since this quad-rotor was heavier (6.2 kg) than the previously mentioned designs. At the Aerospace Engineering Department (DIASP) of Politecnico di Torino, the Flight Mechanics Group is working on the development of a quad-rotor for the territorial monitoring and the map project for a dead zone and/or a grey area. The research is oriented to analyze how work and how maneuver this kind of platform. In order to better understand this problem, considering the dynamic equations and the performance, trying to improve the configuration, we have to realize the aircraft. The research principle scope is to optimize the automatic control of a multi quad-rotors in formation flight, providing these platforms with a mini autopilot. 2.0 MODEL DESCRIPTION This paper is focused on the design and the mathematical model of a four-rotor flying vehicle. A quadrotor is mechanically simple and is controlled only changing the speed of rotation of the four driving motors. The torque in the yaw direction is cancelled by spinning two of the rotors clockwise and the other two anti-clockwise. The attitude control in roll, pitch and yaw direction is obtained by varying the rotational A Design Configuration and Optimization for a Multi Rotor UAV 31 2 RTO-MP-SCI-202 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED speed, which eliminates the mechanical complexity of a pitch linkage. So the four rotor units do not require cyclic and collective pitch commands. The total thrust is controlled with the simultaneous variation of all rotor speed. The optimal configuration has been chosen according to minimize the aircraft weight and to optimize the control strategy of the platform, taking into account the aircraft structural stiffness. A symmetric cruciform layout with peripheral propulsion units was selected in order to simplify the balance of the separate thrusters. We can pick out between two options; one configuration has the four rotors connected to the central fuselage by four separate composite bars. In the second case, the mini-rotors can be supported by a set of equally spaced composite radial bars, reinforced by a square frame, each of them linked with the central payload case (Fig.1). Figure 1: Second quad-rotor configuration The second configuration is penalized by the structural weight, but it is definitely less prone to vibrations and bending, providing higher stiffness. In fact, the torsional stiffness of a closed section is substantially bigger than this of an opened frame section. To avoid small structure flexibility, the second configuration is chosen. The structure was required to be simple, rugged and demountable. The goals for a MAVs has to make a system that can be used by a single operator and can stay in a backpack [7]. In order to minimize the weight, graphite bars are considered for the structure construction and a sandwich of fiber glass and Airex for the central body. The hub has to be designed to ensure the correct location and orientation of the struts on assembly and could be a simple under-over clamping system to provide a rugged demountable part. For the prototype project the electric motors and the rotors was sourced from commercial RC equipment, for reasons of simplicity and practicality. The use or rigid rotors simplifies the aerodynamic modelling, even if it is possible that the external disturbances are increased. In order to increase the safety vehicle, taking into account that the propellers are not protected, an external carbon fibre with a rectangular profile is adopted. The characteristics of the reference aircraft are presented in Tab.1. Table 1: Aircraft characteristics A Design Configuration and Optimization for a Multi Rotor UAV RTO-MP-SCI-202 31 3 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED Dimension 650x650 mm Propeller size 254x120 mm Structural weight 800 gr Total weight 1300 gr Payload weight 300/350 gr Brushless engine Size 30x 37mm Weight 55 gr Speed controller Maximum 20 A Weight 14 gr Two propellers rotate anti-clockwise and the other two clockwise; in order to manoeuvre the aircraft is necessary to control the rotor speed. In particular, a control matrix must be designed to opportunely obtained the desired combination of attitude. In hovering, all propellers must rotate with the same rotational speed. As a consequence, the thrust of all rotors must be equal and the torques have the same modulus but the opposite direction. Figure 2: Hover control For the forward flight, considering the first rotor has the displacement direction, the rotation is made around the pitch axis. The thrust on the rotor 3 is increased instead the thrust on the rotor 1 is decreased to maintain the equilibrium of the total torque. To move the platform in the right lateral side, the rotation is made around the roll axis and the thrust on rotor 2 is increased contemporarily the thrust on rotor 4 is decreased. The yaw is obtained increasing the thrust on the even rotors and decreasing the thrust on the odd ones, in order to maintain constant the altitude. 3.0 MATHEMATICAL MODEL In order to analyze the dynamic behaviour of a multi rotor UAV and to evaluate the flight stability and quality, we consider a rigid blade model, neglecting the dynamic coupling between the rigid body and the structure flexibilities. This is due to the small vehicle dimensions and because we don’t study the aeromechanics stability. Taking into account the aerodynamics behaviour, each blade is dipped in an air field due to the combination of the rotational speed, the dragging speed and the inflow (due to the lift blade action). A Design Configuration and Optimization for a Multi Rotor UAV 31 4 RTO-MP-SCI-202 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED Considering the model of a small four rotor, the inflow dynamics can be modelled as a uniform one, in this way: ( ) 1 . 3 1 a 3 64 1 16 a 75 . 0         − θ σ + σ = λ where σ is the solidity ratio, a is the lift slope (usually 5.7) and 75 . 0 θ is the blade pitch at 75% radius [8]. The twist is regarded linear, taking into account the simple mechanism of this quad-rotor: ( ) 2 . 3 r tw 0 θ + θ = θ where r is the radial location on the blade, measured from the centre of rotation to the blade tip, 0 θ is the command twist in which we consider the reliance on the angular velocity variation and tw θ is the linear twist rate. The mathematical model of the four rotor system includes the fuselage (the central body), the four principal rotors and the inflow of the principal rotors. The fuselage dynamics is modelled as a six d.o.f. rigid body, considering the Euler non linear equations. The blades and fuselage aerodynamics is considered linear. The forces and moments of inertia for the blade infinitesimal elements are: ( ) 3 . 3 dm a r F d r M d a dm F d