DesignandImplementationofaHardware-in-the-Loop … · 2020. 10. 21. · e tilt trirotor unmanned...

Research ArticleDesign and Implementation of a Hardware-in-the-LoopSimulation System for a Tilt Trirotor UAV

Li Yu , Guang He , Shulong Zhao , Xiangke Wang , and Lincheng Shen

College of Intelligence Science and Technology, National University of Defense Technology, Changsha 410073, China

Correspondence should be addressed to Guang He; [email protected]

Received 3 January 2020; Accepted 9 October 2020; Published 21 October 2020

Academic Editor: Maria Vittoria Corazza

Copyright © 2020 Li Yu et al. -is is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

-e tilt trirotor unmanned aerial vehicle (UAV) is a novel aircraft that has broad application prospects in transportation.However, the development progress of the aircraft is slow due to the complicated control system and the high cost of the flightexperiment. -is work attempts to overcome the problem by developing a hardware-in-the-loop (HIL) simulation system basedon a heavily developed and commercially available flight simulator X-Plane. First, the tilt trirotor UAV configuration and dynamicmodel are presented, and the parameters are obtained by conducting identification experiments. Second, taking the configurationof the aircraft into account, a control scheme composed of the mode transition strategy, hierarchical controller, and controlallocation is proposed. -ird, a full-scale flight model of the prototype is designed in X-Plane, and an interface program iscompleted for connecting the autopilot and X-Plane. -en, the HIL simulation system that consists of the autopilot, groundcontrol station, and X-Plane is developed. Finally, the results of the HIL simulation and flight experiments are presented andcompared. -e results show that the HIL simulation system can be an efficient tool for verifying the performance of the proposedcontrol scheme for the tilt trirotor UAV.-e work contributes to narrowing the gap between theory and practice and provides analternative verification method for the tilt trirotor UAV.

1. Introduction

Tiltrotor UAV is an aircraft that has three flight modesincluding hover, transition, and forward.-erefore, it enjoysmany advantages, such as long endurance, high mobility,and less site limitation [1, 2]. In the hover mode, it canvertically takeoff and land (VTOL), so that there is no needfor a runway. In the forward mode, long-distance trans-portation can be achieved due to its long endurance and highcruise speed. Due to these advantages, a tiltrotor UAV hasreceived considerable attention in recent decades [3, 4]. Itcan be applied to aerial photography, target identificationand localization, environmental protection, and so on. It isworthwhile to mention that the tiltrotor UAV may performan impressive role in the field of urban air traffic [5]. Airbushas proposed a tilt-wing aircraft named Vahana for theurban air mobility passenger transportation mission [6].Besides, Uber has taken an interest in the urban air trafficbased on the VTOL aircraft [7]. However, the development

progress of tiltrotor UAV is slow because of the high costand risk of flight experiments. Few mature platforms can beapplied for engineering applications [8].

At present, many countries such as the United States,Korea, Israel, and China are devoted to developing thetiltrotor UAV for its outstanding advantages. -e Eagle Eyedeveloped by the United States is one of the successfultiltrotor UAVs in operation. However, there are severalserious accidents caused by the complexity of the controlsystem and aerodynamic model, and the aircraft has notcome into extensive use [9]. Korea designed a tiltrotor UAVnamed Smart UAV, which is configured similar to the EagleEye. Although many experiments with the Smart UAV havebeen carried out, the aircraft is still in the test stage [10, 11].Israel Aircraft Industries designed a tilt trirotor UAV namedPanther.-e rear rotor can tilt for the control of yawmotion.Note that Panther is the first tilt trirotor UAV, which hasbeen delivered to the army as equipment [12]. -e VTOLaircraft FireFLY6 developed by Birds Eye View Aerobotics

HindawiJournal of Advanced TransportationVolume 2020, Article ID 4305742, 17 pageshttps://doi.org/10.1155/2020/4305742

mailto:[email protected]://orcid.org/0000-0001-6878-7127https://orcid.org/0000-0002-0010-6185https://orcid.org/0000-0002-9610-962Xhttps://orcid.org/0000-0002-5074-7052https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.1155/2020/4305742

has six propellers. -e two rotors fixed at the rear of theaircraft are only for hovering and remain off during for-warding flight.-e other rotors in the front are driven by thesame servo and can synchronously tilt 90 degrees for modetransition [13]. Panther and FireFLY6 are two typical air-crafts that have been applied in practice; however, the cruiseefficiency and flight stability are still needed to be improved.

-e development of the tiltrotor UAV suffers fromvarious difficulties including the control scheme and veri-fication method. More and more research works are pub-lished to solve the two problems. Kang et al. built themathematical model of the tiltrotor UAV, and a neuralnetwork controller was designed for stable flight control.-econtrol performance under turbulent wind conditions wasvalidated through the nonlinear simulation [14]. Papach-ristos et al. proposed an explicit model predictive controlscheme relying on constrained multiparametric optimiza-tion, and the effectiveness of the scheme was demonstratedbased on a tri-tiltrotor equipped with rotor-tilting mecha-nisms [15]. Yucel et al. designed a tiltrotor UAV namedTURAC using a cheap, rapid, and easily reproducibleprototyping methodology. Mathematical and CFD analyseswere performed to optimize the design. -e low-cost pro-totyping methodology was verified by ground and flightexperiments [16, 17]. Many control algorithms are proposedfor a tiltrotor UAV, while few efficient verification methodsare developed.

Due to the cost of designing a prototype and high risk forconducting a flight experiment, most of the advanced al-gorithms proposed for the tiltrotor UAV are validated bysoftware simulation. It is well known that the effectiveness ofsimulation relies on the accurate mathematical model, whichis difficult to be obtained. -e HIL simulation with a highdegree of accuracy as an actual flight contributes to verifyingthe control scheme and improving development efficiency.-e most realistic flight simulator X-Plane is widely used fordeveloping and testing flight control scheme [18]. Adrianoet al. developed a HIL simulation system that consists of anacademic autopilot and X-Plane to verify and optimize thehardware. -e fixed-wing attitude control scheme wasproposed and verified by the HIL simulation, in whichX-Plane was used to simulate the aircraft dynamics, sensors,and actuators [19, 20]. Sergio et al. designed a quadrotorusing Plane Maker provided by the X-Plane flight simulatorand proposed a novel approach to design an attitude con-troller for the quadrotor according to the learning algorithm.-e simulation system composed of Simulink and X-Planewas built to investigate and verify the control algorithm [21].Zhang et al. developed a test system, including autopilot andX-Plane, to validate the control structure and narrow the gapbetween the theory and practice. -e results show that theautopilot that passed the validation in HIL simulation can bedirectly applied to the real flight [22]. Due to the variablestructure of the tilt trirotor UAV during flight, the aero-dynamic model is too complex to be acquired. However, thetilt trirotor UAV with unique configuration can be designedwith the help of Plane Maker, and the aircraft model im-ported into the X-Plane flight simulator is capable of sim-ulating real flight with a high degree of accuracy. It is evident

that HIL simulation based on X-Plane is an effective, rapid,and low-cost method to develop and verify control schemesfor a tilt trirotor UAV.

In this paper, the motivation is to provide an efficientmethod for verifying the performance of the hardware andsoftware designs of the control scheme for a tilt trirotor UAV.-e X-Plane-based HIL simulation system that contributes todeveloping and verifying the control scheme of the aircraft ispresented. First, we design a tilt trirotor UAVwith three rotors,two servos, elevator, and aileron. For the designed aircraft, thecontrol principle and mathematical model are presented. -ecritical parameters as the moment of inertia, mass, and coef-ficients of the rotor are obtained through the identificationexperiments. Second, in order to achieve stable control of theaircraft in different modes, a complete control scheme thatconsists of the mode transition strategy, hierarchical controller,and control allocation is developed. -e control scheme iscoded using C programming language and embedded in theautopilot designed for HIL simulation and flight experiment.-ird, the advantages of the X-Plane flight simulator are il-lustrated in detail. According to the tilt trirotor UAV pa-rameters, the 3-dimension (3D) full-scale model used tosimulate actual flight dynamics is developed using PlaneMaker. And then, the HIL simulation system that consists ofthe autopilot, ground control station, andX-Plane is developed.Finally, the HIL simulation and flight experiment results arepresented and compared to illustrate the reliability of the HILsimulation system. -e control scheme validated by the HILsimulation can be directly moved on to the flight experiment,and only control gains need to be adjusted. Based on the HILsimulation system, the risk and cost of the flight experimentcan be reduced; meanwhile, the development efficiency isimproved.

-e remaining sections are arranged as follows. InSection 2, the configuration and control principle of the tilttrirotor UAV are demonstrated, and then, a serial ofidentification experiments is conducted. Section 3 details thecontrol scheme of the aircraft. In Section 4, the accurate full-scale 3D flight model based on the prototype is designedusing Plane Maker, and then, the HIL simulation system isdeveloped. In Section 5, the results of the HIL simulationand flight experiment are presented. In Section 6, theconclusion and future research work are outlined.

2. Tilt Trirotor UAV

-e designed tilt trirotor UAV is a novel aircraft with aunique structure. To further understand the aircraft, webriefly describe the configuration and dynamic model of thetilt trirotor UAV. -en, the parameter identification iscompleted by conducting different experiments.

2.1.ModelDescription. As shown in Figure 1, the tilt trirotorUAV is equipped with three rotors, two servos, elevator, andaileron. -e tilt trirotor UAV has three flight modes, in-cluding hover, transition, and forward modes. Two frontrotors are able to be tilted by using servos and poles, so thatthe aircraft can fly under different modes. Note that the two

2 Journal of Advanced Transportation

tilting servos are embedded in the nacelles. -e two frontrotors are able to tilt from 30∘ to −90∘ with respect to thevertical axis, and the rear rotor is installed vertically. -eright rotor and rear rotor rotate in counter-clockwise whilethe left in clockwise.

Taking the hierarchical control structure into account,position control is achieved by adjusting the attitude of theaircraft. In the hover mode, three rotors and two servos areused to control attitude. -e different thrusts between the leftand right rotors are used to control the roll motion. -e rearrotor can compensate for the moment generated by two frontrotors to stabilize the pitch. -e yaw moment is created by thedifference of tilting angles between the two front rotors. In theforward mode, the elevator and aileron are used for pitch androll control, respectively. -e aircraft is designed without therudder, so the yawmotion is achieved by adjusting the thrust oftwo front rotors, and the rear rotor remains off in the forwardmode. -e principle of flight control in the hover and forwardmodes are shown in Figure 2.

-e tilting angles of two rotors are denoted by α1 and α2,respectively. -e thrust Fi of the rotor i is influenced by therotation speed ωi. -e symbol δa represents the angle ofaileron deflection, and the angle of elevator deflection isrepresented by δe.-emathematical model of the tilt trirotorUAV that contains kinematic equations, navigation equa-tions, force equations, andmoment equations is expressed asfollows [23, 24]:

_χ � RBET · V,

_V � g · Δ +Fbm

− ωb × V,

_Θ � RBER · ωb,

_ωb � −I−1b ωb × Ib · ωb( − τb( ,

⎧⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨

⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩

(1)

where Δ is defined as

Δ �

−sin θ

sinϕ cos θ

cosϕ cos θ

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦. (2)

In these expressions, the position of the center of gravityin the world frame is expressed as χ � [X, Y, Z]T,V � [u, v, w]T denotes the linear velocity of the aircraft in thebody frame, m denotes the mass, ωb � [p, q, r]

T representsthe rotational angular velocity, Θ � ⌈ϕ, θ,ψ⌉T is the Eulerangle, and Ib is the inertial matrix with respect to the bodyframe.-e rotation matrix from the body frame to the worldframe is RBER, and RBET denotes the transformation matrixfrom the body frame to the world frame. -e force andmoment imposed on the aircraft are denoted using Fb andτb, and these force and moment are generated by the rotors,wings, and surfaces. It should be pointed out that theaerodynamics of the tilt trirotor UAV is difficult to beobtained; however, the mathematical model is an importantpart of the simulation. To efficiently tackle this issue, theX-Plane flight simulator is introduced to provide dynamicsof the aircraft; to this end, parameters of the prototypeshould be acquired for designing a 3D flight model.

2.2. Parameter Identification. In this section, the parameteridentification and related work of the tilt trirotor UAV arepresented. On the one hand, constant parameters includingthe moment of inertia and coefficients of the rotor areobtained. On the other hand, the method used for estimatingthe tilting angle is demonstrated.

-e compound-pendulum method and bifilar torsionpendulum method are used for measuring the moment ofinertia of the aircraft [25, 26]. According to the methodsdescribed above, the measurement of the oscillation period isthe most important part. To obtain the oscillation period, theautopilot with sensors is introduced to the experiment. -e

Rotor 1

Rotor 3

Rotor 2

Servo 1

Aileron

Elevator

Figure 1: Airframe and actuators of the tilt trirotor UAV.

Journal of Advanced Transportation 3

oscillation period can be obtained by recording the change ofattitude. -e partial results are shown in Figure 3.

-ree groups of experiments are conducted for differentaxes, respectively, and the accurate period is obtained bycalculating the average of three measurements. According tothe value of the oscillation period, the moment of inertiawith respect to three axes can be acquired as follows:

Ib �

0.2196 0 0

0 0.2291 0

0 0 0.4781

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦. (3)

-e rotor is one of the most important actuators for thetilt trirotor UAV. In our work, the rotor of the aircraft iscomposed of a 13 × 6 inch propeller and a motor. A novelintelligent ergometer that can record the value of force,moment, and rotation speed is applied to test the rotor. -etest system that contains a rotor, ergometer, lithium battery,and computer is shown in Figure 4.

Assuming that the air density is constant, there is a linearrelationship between the thrust and the square of rotationspeed. -e thrust and torque can be calculated by the fol-lowing equation [27].

Fr � kfω2,

Mr � kdω2.

⎧⎨

⎩ (4)

Data are acquired after the test of the rotor, and thrustcoefficient kf and torque coefficient kd can be obtained byusing the least square method. According to the specificationof the motor and propeller, the maximum thrust that a rotorcan provide is around 53.3N. -e curve-fitting results areshown in Figure 5, and then, the value of rotor coefficientsare given as follows:

kf � 4.410 × 10−5

,

kd � 9.309 × 10−7

.

⎧⎨

⎩ (5)

0.05

0

–0.05

0 10Time (s)

20 30

ϕ (r

ad)

0.05

0

–0.05

0 10Time (s)

20 30

θ (r

ad)

0 10Time (s)

20 30

0.5

–0.5

–1

0

–1.5

–2

–2.5

–3

ψ (r

ad)

Figure 3: Test results.

Roll

F2

ϕ

θ

+δa

–δa

–δe

α1α2

F3 F1

F2F1

Pitch

Yaw

Hover mode Forward mode

Figure 2: Control principle.


-e yaw control is achieved by adjusting the tilting angleof the rotor. Taking the structure of the tilting mechanisminto account, it is difficult to install a sensor for measuringthe tilting angle. -erefore, a utility approach should beproposed to estimate the tilting angle for flight control. Inthe practical application, the function relation between theangle and command should be built first, and then, we canestimate the tilting angle online according to the command.-e calibration is divided into two steps. In the first step, anangle measuring instrument is fixed in the rotor, the tiltingangle is changed by adjusting the output command of theautopilot, and the value of tilting angle corresponding todifferent commands is recorded. Due to the existence ofinstallation error, two rotors are calibrated separately. -emeasurement process is presented in Figure 6.

In the second step, by fitting the recorded data, thefunction expression of the tilting angle and command isacquired. -e fitting results are shown in Figure 7, and thefunction can be acquired as in equation (6). It is evidentthat the two rotors have a different tilting angle whengiven the same command. -is problem is caused by the

installation error and can be solved based on the result ofcalibration. -e contributions of equation (6) contain twoaspects. On the one side, the decoupled control allocationalgorithm can be designed based on the estimation of thetilting angle. On the other side, the precise control of thetilting angle is achieved without the angle sensor. It isnoteworthy that the commands η1 and η2 are solved fromthe desired tilting angles, and then, the two front rotorscan be driven by the servos to the desired angles, re-spectively. In the real flight, the tilting angle error can belimited to no more than 2 degrees by using the estimationmethod:

α1 � −1.980 × η1 + 0.255,

α2 � −1.917 × η2 + 0.117. (6)

It should be mentioned that, for building the accuratemathematical model in the X-Plane, parameters such aswingspan and rotor position are also obtained. According tothe model described above, the control scheme and a full-scale 3D flight model are developed.

Propeller

Motor

Ergometer Battery

Figure 4: Rotor test.

Thru

st (N

)

25

20

15

10

5

0

ω2 (rad/s)2 ×1050 1 2 3 4 5

Torq

ue (N

·m)

0.5

0.4

0.3

0.2

0.1

0

ω2 (rad/s)2 ×1050 1 2 3 4 5

Figure 5: Test results of the rotor.

Figure 6: Calibration of the tilting angle.


3. Control Design

-e control scheme consists of the mode transition strategy,hierarchical controller, and control allocation. Consideringthe existence of the hover, transition, and forward modes,the mode transition strategy is designed for achieving safetymode conversion. -e tilt trirotor UAV is an underactuatedsystem, and a hierarchical controller using PID is developedto control position and attitude. -e control allocation isproposed to provide the mapping from the virtual controlcommands to the manipulated inputs of the aircraft. For thetilt trirotor UAV, the three parts described above are nec-essary for flight control.

3.1. Mode Transition Strategy. -e altitude and attitude arethe key factors for stable mode transition.-e control systemaims to stabilize the altitude and attitude in flight. As amatter of fact, it is difficult to obtain the accurate aerody-namic model in the transition mode, so that the airspeedshould be increased as quickly as possible to ensure flightsafety. A phased mode transition strategy is designedaccording to the mode transition command and flight air-speed. To illustrate the transition process clearly, the tran-sition from the hover mode to the forward mode is called theconversion phase, and the transition from the forward modeto the hover mode is called the reconversion phase.

-e conversion phase is completed with the increase ofthe airspeed. First, after receiving the conversion command,the two front rotors tilt forward to an angle Pf within Tf1seconds. With the tilting angle increasing, the airspeed in-creases, and the wings start generating lift. Second, if theairspeed achieves Vf in which the aerodynamic can com-pensate a part of the gravity, all rotors are shut down;meanwhile, two front rotors tilt forward to the horizontalposition within Tf2 seconds. Finally, the aircraft enters intothe forward mode and cruises at Vc. In this work, thereconversion phase is a relatively simple process. After re-ceiving the reconversion command, the two front rotors areshut down and tilted backward to the vertical position withinTb seconds. And then, the aircraft enters into the hovermode and begins to decelerate flight. Note that Tf2 and Tbshould be set small enough to ensure the stable of thetransition flight. Conversion and reconversion phases areshown in Figure 8.

3.2. Hierarchical Controller. Due to the tilt trirotor UAV isan underactuated system, a PID-based hierarchical con-troller is introduced to achieve the position and attitudecontrol. In terms of hover and forward modes, two con-trollers are applied, respectively. Note that the hover con-troller is also used in the transition mode. Figure 9 shows ablock diagram of the flight control system.

For the hover controller, the outer loop is used for theposition control, and it receives the desired position from thenavigator and outputs the desired thrust vector[Ux, Uy, Uz]

T in the world frame. According to the inversetransformation and desired yaw angle ψd, the desired pitchangle θd and roll angle ϕd can be solved; besides, the virtualthrust TH in the body frame is obtained [28]. -e inner loopattitude controller receives the desired angles and providesthe virtual control torque [R, P, Y]T. -e virtual thrust andcontrol torque are inputs of the allocator, and then, thecontrol inputs of actuators can be obtained based on thecontrol allocation algorithm. For the forward controller, theouter loop position controller is composed of the longitudecontrol and lateral control. L1 navigation logic is used for thelateral control; then, the desired yaw angle ψd and roll angleϕd can be obtained based on the desired position and currentposition [29]. -e longitude control is completed based onthe total energy control method; the desired pitch angle θdand virtual thrust TF are acquired according to the altitudeand airspeed [30]. -e attitude controller in different flightmodes has two loops including an angular loop and anangular rate loop. -e angular loop uses the P controller toproduce the desired angular rate for the angular rate loop.And then, the virtual control torque [R, P, Y]T is provided bythe angular rate loop, which is based on the PID controller.

3.3.ControlAllocation. In this section, the control allocationalgorithm that provides the mapping from the control inputsto the manipulated inputs of the aircraft is presented [31]. Itshould be pointed out that the control allocation under theforward mode is the same as the traditional fixed-wing. Sothat we focus only on the control allocation for the hovercontroller. -e tilt trirotor UAV has four virtual controlinputs R, P, Y, and TH, where R is directly linked to rollcontrol, P is used for pitch control, Y is related to yawcontrol, and the altitude is controlled by TH. -ere are fiveactuators including three rotors and two servos that can be

0

–0.5α 1

(rad

)

–1

–1.5

0 0.2 0.4Command

0.6 0.8 1 0 0.2 0.4Command

0.6 0.8 1

0

–0.5

α 2 (r

ad)

–1

–1.5

Figure 7: Fitting results of the tilting angles.


used for flight control. It is known that the change of ro-tation speed is much faster than the change of the tiltingangle. According to the response speed of actuators, thecalculation of actual outputs can be divided into two parts.

For the first part, the rotation speeds of three rotors areacquired by R, P, and TH. -e right rotor, the left rotor, andthe rear rotor are labeled by 1, 2, and 3, respectively. Tocalculate rotation speeds from the virtual commands, thetilting angles are determined based on the equation (6).

R

P

TH

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦ � H

ω21ω22ω23

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦, (7)

where

H �

−kfr1y cos α1 + kd sin α1 −kfr2y cos α2 − kd sin α2 0

kfr1x cos α1 kfr2x cos α2 kfr3xkf cos α1 kf cos α2 kf

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦.

(8)

-e vector [rix, riy, riz] denotes the position of the rotor iin the body frame. Since the matrix H is a square matrix, thekeypoint to obtain the rotation speed ⌈ω21,ω

22,ω

23⌉

T is to

verify the reversibility of the matrix H. Taking the symmetryof the aircraft into account, we have r2y � −r1y andr2x � r1x. In this manner, the determinant of matrix H canbe written as

|H| � −2k3fr1y cos α1 cos α2 r1x − r3x(

+ k2fkd sin α1 + α2( r1x − r3x( .

(9)

For the hover mode and transition mode, the tiltingangles are limited to (−90∘, 30∘). Considering the differentialcontrol of two front rotors, we have α1 + α2 ≤ 0.-en, we canconclude that

|H|≠ 0. (10)

According to the above derivation, the reversibility of His proved, so that the rotation speed can be obtained by

ω21ω22ω23

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦� H

− 1

R

P

TH

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦. (11)

For the second part, there are two tilting angles that needto be determined based on Y. -e yaw control of the aircraftis achieved by the differential tilting angles generated by two

Conversioncommand

Reconversioncommand

Hover mode

Hover mode

Power off

Forward mode

Cruiseflight

Forward mode

Accelerateflight

Decelerateflight

Tilting angle = 0°

Tilting angle = Pf Tilting angle = Pf

if V ≥ Vf

Figure 8: Mode transition strategy.

Navigator

Referenceposition

Referenceattitude

Outer loop Inner loop

Longitude control(L1 guidance law)

Position control(P controller)

Speed control(PID controller)

Lateral control(total energy control)

Attitude control(P controller)

Attitude control(P controller)

Rate control(PID controller)

Rate control(PI controller)

Controltorque

Controlallocator

δe, δa

α1, α2ω1, ω2, ω3

Virtualthrust

Modetransitioncommand Current

positionCurrentattitude

Figure 9: Block diagram of the control system.


front servos. Considering the fact that the yawmotion can beadjusted effectively by tilting the two front rotors in oppositedirections. From this point of view, the tilting angles aregiven as follows:

α1 � α0 + δ · Y,

α2 � α0 − δ · Y, (12)

where δ is a small constant that depends on the maximumtilting angle and the corresponding yaw moment, α0 is anoriginal angle, which is relevant to flight modes, and α0 � 0in the hover mode. -e proposed control allocation is veryuseful for the aircraft without the tilting angle sensor; be-sides, it is easy to be applied in real flight.

4. HIL Simulation System

X-Plane is a commercially available flight simulator writtenby Laminar Research, that can be installed on computersrunning Windows, Linux, or Mac OS. In this work, X-Planeis applied to provide the model of the tilt trirotor UAV andsimulate its dynamics. -e advantages and communicationinterface are illustrated, and then, a full-scale 3D flightmodel is designed.

4.1.X-PlaneFlight Simulator. -e flight simulator X-Plane ischosen because of its ability to predict the aircraft’s flyingqualities with high accuracy. -is is performed by using theblade element theory.-e aircraft is divided into many smallelements, and the forces acting on each element are cal-culated several times per second. Compared to the otherflight simulators such as Flight Gear and FSX, X-Plane ismore flexible and advanced. Moreover, it is the only flightsimulator that has obtained certification from the FederalAviation Administration (FAA), which makes the simula-tion results more credible [32]. Numerous companies asCessna, Cirrus, and Boeing have purchased X-Plane as anengineering tool; in addition, many researchers have appliedX-Plane to develop and test control schemes. X-Planeprovides Plane Maker and Airfoil Maker to enable users tocreate the aircraft and airfoils as easily as possible. -ebenefits of using X-Plane are

(a) Realistic simulation environment: Laminar Researchclaims that using the blade element theory to cal-culate the forces on the aircraft is more accurate thanthe stability derivativemethod.-e cloud cover, rain,wind, thermals, microburst, and fog can be simu-lated in X-Plane. -e certification from the FAAallows researchers to achieve high levels of confi-dence in simulation results. -e results of manypapers have verified that the simulation based onX-Plane has a high degree of accuracy as an actualflight.

(b) Database of aircraft models: thousands of mannedand unmanned vehicle models are freely available fordownload at the X-Plane forum. Although notnecessarily certified, these models provide quickstarting points for testing. Most important of all,

Plane Maker makes it possible for researchers todesign their aircraft model, so that the particularaircraft such as the tilt trirotor UAV can be devel-oped and tested.

(c) Communication: X-Plane can communicate withexternal processes via User Datagram Protocol(UDP). -e UDP protocol is well suited for simu-lating because there is no check as delivery, detec-tion, or error correction; it is possible to achievehigh-speed data traffic. X-Plane accepts controlsignals to drive actuators and outputs flight infor-mation. Note that X-Plane is capable of outputtingall the navigation data necessary to perform asimulation and allows users to adjust the update ratefrom 1 to 99 packets per second.

-e data packet of X-Plane that has 41 bytes follows astandard format [33]. -e first four bytes denote thecharacters “DATA,” which indicates that this is a datapackage.-e fifth byte is a code “I,” and it is used for internalpolicy. -e remaining 36 bytes are divided into nine groups:the first group of four bytes presents a label to identify the setof data, and the other eight groups denote the data need to besent. -e first byte of the group is the sign bit, which tellswhether the number is positive or negative. To further il-lustrate the data packet, a packet that contains Euler angles isshown in Figure 10. -e packet is labeled 17 in X-Plane(version 10.42), and the values of the Euler angles are se-lected. Not all the groups are filled with data in practice, andthe detailed information can be acquired from the manual.

4.2. Design of the Flight Model. -e 3D model of the tilttrirotor UAV is performed using Plane Maker, which allowsthe user to create nearly any type of aircraft [34]. -eprogram provides a graphical interface to design an aircraftaccording to the physical specifications (weight, enginepower, wingspan, wing area, control surfaces, and the centerof gravity). And then, X-Plane is capable of predicting howthat aircraft will fly. Based on Plane Maker, we can edit thefuselage cross section at a maximum of 20 locations alongthe fuselage length, with up to nine points at each crosssection. -is allows for much more accurate modeling of thefuselage than simply stating the radius and length, as is thecase in most software which can only model simple bodies ofrevolution.

To build an accurate X-Plane model, useful informationsuch as dimensions and weight of the aircraft is measured asshown in Table 1, and then, the designing process is dividedinto three parts. First, the fuselage is the basic frame foradding other parts, and it is relatively simple to be modeledbased on the prototype. Second, wings and control surfacesare designed according to the airfoil and actual parameters.Finally, the rotor consists of a motor, and a 13 × 6 propelleris modeled. For the motor, the value of input such as +1provides 1500 watts of power to the aircraft, and the motorturns off when given 0. Figure 11 shows the prototype andX-Plane model. It should be mentioned that the X-Planemodel has three rotors, two servos, elevator, and aileron.


-erefore, it is able to drive actuators as the prototype doeswhen given the same command.

4.3. HIL Simulation System Setup. -e HIL simulationsystem consists of a dedicated autopilot, X-Plane simulator,and ground control station. -e diagram of the HIL sim-ulation system for the tilt trirotor UAV is shown in Fig-ure 12. An open-source ground control station namedQGroundControl that can parse and package the UDPpacket is applied. We have completed the secondary de-velopment of the QGroundControl, so that it can be used toconnect with the tilt trirotor UAV in the X-Plane envi-ronment. -e control scheme is embedded and realized inthe autopilot using the C programming language. -e au-topilot hardware is placed inside the simulation loop inorder to test and validate both the hardware and controlscheme, and it receives the states of the aircraft simulated byX-Plane and outputs control commands. -e controlcommands are sent to the X-Plane and used to controlrotation speeds, surfaces, and tilting angles of the flightmodel. Note that all state information necessary for flightcontrol can be provided by X-Plane.

-e autopilot shown in Figure 12 is developed for thecontrol of the tilt trirotor UAV.-e autopilot embedded twoARM Cortex-M4 microcontrollers, working up to 168MHzof clock rate. In addition, a triaxial gyro, triaxial accelerator,triaxial magnetic field meter, barometric altimeter, andairspeed meter are integrated into the autopilot. -e auto-pilot linked with an external GPS module can be applied toperform the predefinedmission in the flight experiment.-einterface resource of the autopilot contains 1 serial pe-ripheral interface (SPI), 1 S-bus (RC receiver), 2 CAN BUS, 4serial ports, and 16 PWMoutputs. In HIL simulation, a serialport is used to communicate with the ground control station,and the RC receiver transmits manual control signals to theautopilot through the S-bus port.

It should be pointed out that the X-Plane is capable ofproviding a realistic simulation environment. Moreover, theautopilot and ground control station used in HIL simulation

is the same as the flight experiment. Based on HIL simu-lation, we can test both the autopilot and control scheme.-e autopilot that passed the validation in HIL simulationcan be directly moved on to the prototype, and the flightexperiment can be conducted with minimum modification.

5. Results

In this section, the mode transition flight is conducted in theHIL simulation environment to verify and improve thecontrol scheme. And then, the flight experiment using thesame autopilot and control scheme is carried out. -e ef-fectiveness of the control scheme and HIL simulation systemare demonstrated by comparing the results of the simulationand experiment.

5.1. HIL Simulation Results. In order to verify the controlscheme and test the HIL simulation system, we set an oblongairline flight mission, in which the mode transition processcan be demonstrated clearly. After receiving the takeoffcommand, the aircraft is capable of carrying out the missionautomatically. -e predefined oblong airline contains fifteenwaypoints.-e aircraft switches the mode and completes themission as shown in Figure 8. In the first waypoint, theaircraft receives the conversion command and switches theflight mode by tilting two front rotors. And then, the aircraftconducts the mission under the forward mode. When theaircraft arrives at the fourteenth waypoint, it receives thereconversion command and enters into the hover mode.Finally, the aircraft flies to the fifteenth waypoint and landsunder the hover mode.

Mode transition flight is the most important part of theflight mission. -e key parameters for mode transition inHIL simulation are shown in Table 2. Based on the modetransition strategy mentioned earlier, the conversion phaseis completed by tilting two front rotors as shown in Fig-ure 13. It is noteworthy that we mainly focus on the con-version phase due to the reconversion phase is a relativelysimple process.

To further present the flight mission, the trajectory of theaircraft in HIL simulation is shown in Figure 14. -e flightmission consists of fifteen waypoints, in which waypoint 1and waypoint 14 are introduced for mode transition. -eaircraft takes off and flies to waypoint 1 under the hovermode, and then, it enters into the conversion phase based onthe conversion strategy. It enters into the forward modewhile the airspeed reaches Vf and flies from waypoint 2 towaypoint 13 under the forward mode. After arriving atwaypoint 14, the altitude decreases to 25m, and the aircraftenters into the hover mode according to the reconversionstrategy. In the last stage of the flight mission, the flight

Byte 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 – 40

D A T A I L1 L2 L3 L4 P1 P2 P3 P4 R1 R2 R3 R4 Y1 Y2 Y3 Y4 T1 – Z4

Header Group 1 Group 2 Group 3 Group 4

YawRollPitchLabel 17

Group 5 – group 9

Value

Figure 10: Packet format.

Table 1: Specifications of the prototype.

Parameters Value UnitLength 1.6 mWingspan 1.8 mChord 0.24 mWing area 0.4 m2Weight 6.9 kgRight rotor position (0.23, 0.256, 0) mLeft rotor position (0.23, −0.256, 0) mRear rotor position (−0.437, 0, 0) m


performance of the hover mode is demonstrated. Due to theairspeed is still large while entering into the hover mode, theaircraft begins to decelerate flight under the influence of theaerodynamic. After the airspeed decreases to less than5(m/s), the aircraft flies to the last waypoint and lands. Itshould be mentioned that the whole flight is conductedautomatically within 365 s, and we just provide the takeoffcommand by remote control.

-e airspeed and altitude are shown in Figure 15. -eyellow shaded areas denote the conversion and reconversionphases. Note that the cruise airspeed in the forward mode isset to Vc � 20(m/s), which is obtained based on the aero-dynamic analysis and flight test of the prototype. It should bepointed out that the conversion phase costs about 5 s from20.5 s to 25.5 s, the time used for tilting to anglePf � −0.46 rad is Tf1 � 3.0 s, the aircraft uses about 1.8 s toaccelerate to Vf � 10(m/s) with tilting angle Pf, and then,

the two rotors tilt to the horizontal position withinTf2 � 0.2 s. -e reconversion phase is completed withinTb � 0.4 s. Due to the low airspeed when entering into theforward mode, the altitude of the aircraft decreases about 5meters at first, and then, it climbs and accelerates withenough thrust under the forward mode. -ere are two peaksin the airspeed curve between 200 s and 260 s. -e aircraftneeds to turn left and reduce altitude significantly whengetting through waypoints 11 and 12, so the curve of theairspeed and altitude has fluctuations. Around 284 s, thereconversion phase is completed, and then, the aircraftbegins to decelerate by increasing the pitch angle under thehover mode, and the increasing of the altitude around 285 sis caused by the aerodynamics. After the airspeed decreasesto 0(m/s), the aircraft flies to the last waypoint with velocityless than 5(m/s).

-e roll angle, pitch angle, and yaw angle of the aircraftin HIL simulation are shown in Figure 16. -e aircraft hasachieved a good attitude control performance with the hi-erarchical controller. It is obvious that the desired pitchangle between 284 s and 289 s is about 0.52 rad, which is themaximum desired pitch angle in the hover mode. -erefore,the aircraft is capable of decelerating with a positive pitchangle.-e control input signals of the two front tilting servosare shown in Figure 17.-e pulse width modulation (PWM)signals are used to drive actuators, and the range is1000 ∼ 2000 (duty cycle 0% ∼ 100%). -e differential

Figure 11: -e prototype and X-plane model.

Universal serialbus

2 ARM Cortex-M4 (168MHz)

1 S-bus

2 CAN BUS

4 Serial ports

16 PWM

1 Serial peripheral interface

Autopilot

RCreceiver

Ground control station X–Plane model

States

Control States

ControlUDP

Figure 12: HIL simulation system.

Table 2: Parameters for mode transition.

Symbol Value UnitTf1 3.0 sPf −0.46 radVf 10 m/sTf2 0.2 sTb 0.4 sVc 20 m/s


control of the tilting angles for yaw motion in the hovermode and transitionmode is demonstrated clearly. Note thatthe two rotors tilt forward simultaneously in the conversionphase. However, the two rotors tilt backward to the verticalposition directly in the reconversion phase. In the hovermode, there exists a constant differential angle for dealingwith the torque generated by the rear rotor.

-e HIL simulation results illustrate that the proposedcontrol scheme is useful for the control of the tilt trirotorUAV. -e attitude and altitude of the aircraft in the tran-sition mode are two key factors to describe the control

performance. Figure 16 shows that the attitude is able totrack the reference angle accurately in the transition mode.Moreover, the altitude fluctuation after the reconversionphase is limited to 10m. -e proposed HIL simulationsystem contributes to testing and validating both the au-topilot and control scheme.

5.2. Flight Experiment Results. Based on the autopilot andcontrol scheme that passed validation in HIL simulation, aflight experiment is conducted to further demonstrate the

Hover mode Transition mode Forward mode

Figure 13: Mode transition in HIL simulation.

Z (m

)

100

50

0

–600–500

–400–300

–200–100

0100

200100

0

–100 X (m

)Y (m)

–200

–300

–400

11

4

5

10

Land Takeoff

8

9

3

12

7

6

2

13

Waypoint 1

Waypoint 2

Waypoint 13

Waypoint 14

Waypoint 15

Figure 14: Flight trajectory of the aircraft in HIL simulation.


effectiveness of the HIL simulation system and controlscheme. -e autopilot and the ground control station usedin HIL simulation are directly applied for the flight ex-periment. -e parameters for the mode transition in theflight experiment are the same as HIL simulation shown inTable 2, and only some gains of the hierarchical controllerare adjusted to improve the control performance. Topresent the realistic of the HIL simulation with X-Plane, a

flight mission similar to the one used in HIL simulation isdesigned for the flight experiment.

Considering the importance of the mode transition inthe whole flight, the mode transition process of the tilttrirotor UAV in the air is recorded by a camera fixed on theaircraft. By comparing Figures 18 and 13, we can concludethat the mode transition strategy can be carried out effec-tively both in the HIL simulation and flight experiment.

AirspeedAltitude

30

25

20

Airs

peed

(m/s

)

15

10

5

–5

0

90

70

80

60

Alti

tude

(m)

40

50

10

20

30

0

–100 50 100 150 200

Time (s)250 300 350

Figure 15: Airspeed and altitude of the aircraft in HIL simulation.

Reference angleEuler angle

0.5

0

ϕ (r

ad)

θ (r

ad)

ψ (r

ad)

–0.5

0 50 100 150 200 250 300 350

0 50 100 150 200 250 300

0.40.2

0–0.2

280 285 290 295

350

0 50 100 150Time (s)

200 250 300 350

0.4

0.2

0

–0.2

2

0

–2

Figure 16: Attitude of the aircraft in HIL simulation.


Figure 19 shows the 3D trajectory of the aircraft in the flightexperiment. It is obvious that the trajectory is similar to theone generated by HIL simulation because of the similar flightmission.-e airspeed and altitude of the aircraft in the flightexperiment are shown in Figure 20. -e whole flight iscompleted within 362 s. -e conversion and reconversionphases are covered by the yellow shaded area. -e con-version phase consumes about 4.2 s from 30 s to 34.2 s; 3 s isused for tilting two front rotors from the vertical position tothe angle Pf � −0.46 rad. Under the tilting angle, the aircraftaccelerates to Vf � 10(m/s) within about 1 s, and then, thetwo rotors tilt forward to the horizontal position withinTf2 � 0.2 s. Around 285 s, the reconversion phase is com-pleted withinTb � 0.4 s.-e conversion phase consumes lesstime in real flight than in HIL simulation. -e varyingtendencies of airspeed and altitude are similar to the curvesdepicted in Figure 15. However, two main differences can bepointed out clearly. On the one side, due to the increasing ofthe airspeed, the altitude increases significantly around 40 s.On the other side, the airspeed is generally bigger than5(m/s) during the hover mode. It is noteworthy that theairspeed meter used in the experiment has an error whenworking in the low airspeed case. Meanwhile, the value of the

airspeed meter is also influenced by the gust in the flightexperiment.

-e attitude of the aircraft in the flight experiment isshown in Figure 21. It should be mentioned that the pitchangle is not tracked well during the conversion phase;however, the changing of the pitch is relatively smooth. Fordecelerating flight between 284.5 s and 288.5 s, the desiredpitch angle is 0.2 rad provided by the outer loop positioncontroller. -e control input signals of the tilting servos areshown in Figure 22; note that the curves match the resultsdepicted in Figure 17.

As shown in the results of the flight experiment, thepredefined flight mission can be conducted well using theproposed mode transition strategy, hierarchical controller,and control allocation. Besides, the results of the flightexperiment are remarkably similar to the results of HILsimulation. -e flight experiment not only demonstrates theeffectiveness of the control scheme but also verifies thereliability of the developed HIL simulation system. -econtrol scheme and autopilot that passed the validation inHIL simulation can be easily applied to the flight experi-ment. -e extra work required is to adjust the control gainsof the controller based on the platform.

Le� rotorRight rotor

2000

2000

1800

1600

0 10 20 30 280 290 300270

2000

1800

1600

1900

1800

1700

1600

PWM

(μs)

1500

1400

1300

1200

1100

10000 50 100 150 200

Time (s)250 300 350

Figure 17: Tilting commands of two front rotors in HIL simulation.

Hover mode Transition mode Forward mode

Figure 18: Mode transition in the flight experiment.


Z (m

)

100

50

0

–300 –200 –100 0 100 200 300 200X (m)

–100

0

100

Y (m

)

200

300

400

500

600

Land

10

5

9

12

7

3

6

213

114

8

Takeoff

Waypoint 1

Waypoint 2

Waypoint 13

Waypoint 14

Waypoint 15

Figure 19: Flight trajectory of the aircraft in the flight experiment.

AirspeedAltitude

30

25

20

Airs

peed

(m/s

)

15

10

5

0

90

70

80

60A

ltitu

de (m

)

40

50

10

20

30

0

–100 50 100 150 200

Time (s)250 300 350

Figure 20: Airspeed and altitude of the aircraft in the flight experiment.


6. Conclusions

-is paper presents the design and implementation of theHIL simulation system for the tilt trirotor UAV that containsadvantages of helicopter and fixed-wing. In this paper,several parameter identification experiments are completedbased on the developed prototype to obtain the parametersof the aircraft. -e control scheme that consists of the modetransition strategy, hierarchical controller, and control

allocation is proposed for the control of the tilt trirotorUAV. And then, a full-scale 3D flight model that cansimulate actual flight dynamics is designed according to theacquired parameters. In order to improve efficiency andreduce the risk of the flight experiment, the HIL simulationsystem including the autopilot, ground control station, andX-Plane is developed. -e HIL simulation and flight ex-periment results are presented and compared to demon-strate the performance of the control scheme. -e HIL

Reference angleEuler angle

0.5

0

ϕ (r

ad)

θ (r

ad)

ψ (r

ad)

–0.5

0 50 100 150 200 250 300 350

0 50 100 150 200 250 300

0.40.2

0–0.2

280 285 290 295

350

0 50 100 150Time (s)

200 250 300 350

1

0.5

0

–0.5

2

0

–2

Figure 21: Attitude of the aircraft in the flight experiment.

Le� rotorRight rotor

2000

2000

1800

1600

10 20 30 40 280 290 300270

2000

1800

1600

1900

1800

1700

1600

PWM

(μs)

1500

1400

1300

1200

1100

0 50 100 150 200Time (s)

250 300 350

Figure 22: Tilting commands of two front rotors in the flight experiment.


simulation system has a high degree of accuracy as an actualflight. Based on the developed HIL simulation system, de-fects or problems can be found andmodified before the flightexperiment, so that the workload and risk of the flightexperiment are reduced. -e work provides an alternativeverification method for the development of the tilt trirotorUAV and can be used as a guide for those who want toresearch the novel aircraft with a special configuration. Inthe future work, we will pay more attention on the im-provement of the control system based on HIL simulation.

Data Availability

-e data used to support the findings of this study areavailable from the corresponding author upon request.

Conflicts of Interest

-e authors declare that there are no conflicts of interestregarding the publication of this paper.

Acknowledgments

-is work was supported by the National Natural ScienceFoundation of China (grant nos. 62003356 and 61973309).

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