Lately, a novel multirotor aerial vehicle capable of handling single rotor failures was presented. When a rotor fails, physically reconfiguring one of the remaining rotors of an hexarotor allows to compensate for maneuverability limitations. In this work, experimental results show the per- formance of the vehicle in a trajectory-following task in both nominal and fault conditions.Fil: Pose, Claudio Daniel. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Electronica; ArgentinaFil: Francisco Presenza. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Electronica; ArgentinaFil: Mas, Ignacio Agustin. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Instituto Tecnológico de Buenos Aires; ArgentinaFil: Giribet, Juan Ignacio. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Electronica; Argentina. Instituto Tecnológico de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Saavedra 15. Instituto Argentino de Matemática Alberto Calderón; Argentina11th International Micro Air Vehicle Competition and ConferenceMadridEspañaUniversidad Politécnica de MadridConsejo Superior de Investigaciones Científica

Francisco Presenza

Giribet, Juan Ignacio

Mas, Ignacio Agustin

Pose, Claudio Daniel

English

CONICET Digital

IMAV2019-7 11th INTERNATIONAL MICRO AIR VEHICLE COMPETITION AND CONFERENCETrajectory Following with a MAVUnder Rotor Fault ConditionsClaudio D. Pose∗1, Francisco Presenza1, Ignacio Mas3,4, and Juan I. Giribet1,2,31Facultad de Ingenierı́a, Universidad de Buenos Aires, Argentina2Instituto Argentino de Matemática “Alberto Calderón” (IAM)3Consejo Nacional de Investigaciones Cientı́ficas y Técnicas (CONICET), Argentina4Instituto Tecnologico de Buenos Aires, ArgentinaABSTRACTLately, a novel multirotor aerial vehicle capableof handling single rotor failures was presented.When a rotor fails, physically reconfiguring oneof the remaining rotors of an hexarotor allowsto compensate for maneuverability limitations.In this work, experimental results show the per-formance of the vehicle in a trajectory-followingtask in both nominal and fault conditions.1 INTRODUCTIONMultirotor aerial vehicles have become very popular inrecent years, due to the fact that the electronic systemsneeded to fly them have increased their availability and use-fulness, decreasing their cost and weight. Simplicity andcost-effectiveness have turned out to be very appealing and,as a consequence, an increasing number of applications haverisen in many fields, such as agriculture, surveillance, andphotography, among others. Fault tolerance has been ad-dressed in the literature as a matter of high importance, in par-ticular for multirotor vehicles, see for instance [1, 2, 3, 4, 5, 6]and references therein.In particular, in [7] is studied the capability of compen-sating for a rotor failure without losing the ability to exerttorques in all directions, and therefore keeping full attitudecontrol in case of failure. For this, at least six rotors areneeded, and have to be tilted with respect to the vertical axisof the vehicle. The proposed solution in [7], was tilting therotor (or arms) of the hexarotor inwards. Experimental re-sults for the proposed solution can be found in [8], wherethe vehicle takes off, performs different maneuvers and landssuccessfully with one motor in total failure, maintaining fullattitude and altitude control. While the system proved to workcorrectly, there was a direction that, when exerted torque in,performed noticeably worse with respect to the rest.To overcome this limitation, in [9] a slight modificationwas proposed for the vehicle, where, besides tilting the rotorsinwards, servomotors were added in two of them to recon-figure their position in case of a failure. Simple experiments∗Email address: cldpose@fi.uba.arwere performed with the vehicles in cases with and withoutfailure, in a hovering state and with simple maneuvers, andit was concluded that the new fault tolerant design performedmuch better than the one proposed and evaluated in [7, 8].This work presents a more extensive performance evalua-tion to compare the maneuverability of the vehicle proposedin [9] in cases with and without failure, by means of a trajec-tory following experiment in an indoor environment.The manuscript is organized as follows. First, a shortdescription of the proposed vehicle is presented. Then, thecharacteristics of the vehicle used as a platform for the ex-periments are described, as well as the setup of the indoorenvironment where the flights were carried out. Finally, theresults obtained are shown and compared for the flights of thevehicle with and without a total failure in one rotor.2 PROPOSED FAULT-TOLERANT HEXAROTORWhen dealing with total rotor failures in hexarotors, it hasbeen proved that a standard hexarotor configuration (one withthe rotors spaced evenly in a plane, pointing upwards, with al-ternated spinning direction, as in Fig. 1) is not fault tolerant inthe event of a failure of this type, in the sense of maintainingcontrol over its four degrees of freedom (rotation around itsthree axis, and vertical speed). One degree of freedom willbe lost, being generally the yaw axis the one chosen to belost control of, as it allows the possibility to land the vehiclesafely. From this point on, fault tolerance will be meant in thesense that the system maintains complete altitude and attitudecontrol.Suppose a standard hexarotor configuration with γ = 90o(see Fig. 2), which, with the vehicle in hovering mode, suf-fers a total loss of rotor number 3 (M3), a counter-clockwise(CCW) rotating motor. Then, this rotor no longer generatesthrust to produce torque on the x-axis, and neither does itgenerate torque on the z-axis due to the spinning propeller.Then, turning off the opposite rotor (M6), which generatesexactly the opposite torque, is an adequate solution. In thiscase, the system is not fault tolerant, as there will exist atorque qw = (Mx,My,Mz) (worst case direction torque)that will require a negative speed from M6 (see [7]), whichcannot be achieved. The solution using the inward-tilted ro-tors with γ > 90o, allows M6 to hold the hovering state with aSEPTEMBER 29th TO OCTOBER 4th 2019, MADRID, SPAIN 55IMAV2019-7 11th INTERNATIONAL MICRO AIR VEHICLE COMPETITION AND CONFERENCEFigure 1: Top view of the proposed vehicle.Figure 2: Side view of the proposed vehicle. γ denotes theinward/outward tilt and δ the side tilt.small positive speed, which in turn allows the vehicle to exerttorque in the direction qw. However, the maximum achiev-able magnitude of this torque is small, as the maneuver islimited by the saturation of M6. Rotation in the yaw axisis the most stressful maneuver, as they require higher speedvariations from the motors with respect to similar maneuversin pitch or roll.The work done in [9] proposed to add servomotors in twoof the vehicle’s arms, in order to tilt the rotors at an angleδ 6= 0 (see Fig. 2), in case of a failure of one of the rotors.By doing this, part of the vertical thrust produced by the rotoris used to generate torque in yaw, allowing to compensate thelow maneuverability in that axis. Which rotor will be tilted,and the magnitude and direction of the tilt angle will dependin which of the six rotors is under failure.3 EXPERIMENTAL SETUPTo provide a comparison of flight performance betweenthe hexa-rotor in a nominal and a failure state, two identicalexperiments were carried out. An identical fixed trajectory tofollow is given both for the case of the vehicle without failure,and for a case where rotor 3 is under total failure.The vehicle used for the experiments is based on a com-mercial model. The frame is the DJI-F550, with a distancebetween rotors of 550mm. The actuators installed on thisframe are T-Motor 2212-920KV motors, with 9545 plasticself-tightening propellers, driven by 20A electronic speedcontrollers (ESC). The battery used is a 4S 5000mAh 20CLiPo that allows approximately 15 minutes of hovering flight(without failures). The flight computer used is a custom-designed board [10] developed by the GPSIC Lab [11] tosupport experiments that are usually carried out on this kindof vehicles. It is based on the LPC-1769 microcontroller, anARM Cortex M3 that runs at 120MHz, and several sensorssuch as the MPU-6000 IMU, the HMC5883L digital com-pass and the BMP180 barometer, sending flight informationto MATLAB (for data analysis) through a 57600bps XBeewireless connection. The control loop runs at 200Hz, wherethe pitch, roll, and yaw angles are estimated and a PID con-trol algorithm calculates the torque for vehicle stabilization.Then, the allocation algorithm gives the force of each motorin order to achieve the desired torque, and a simple functionconverts this value into the PWM signals commanded to theESC. Two additional PID control loops are used for positioncontrol in the XY plane, where the input is the error in po-sition, and the output actuates over the pitch and roll com-mands. One last PID control loop is used for height control,actuating directly on the vertical thrust command.To switch between the different configuration of the rotorsfor the nominal and failure case, a servomotor is added inrotor 1, that tilts it over the arm’s axis at δ1 = 0o for thevehicle without failures, and at δ1 = 10o in the case of afailure in rotor 3, as shown in Figure 3.Figure 3: Servomotor in rotor 1 in the case without failure(left) and in the case of a failure in rotor 3 (right).In order to provide position information, an ultrasonic-based indoor navigation system from Marvelmind was used.This system consists of a network of stationary ultrasonicbeacons interconnected via radio interface, one mobile bea-con installed on the vehicle to be tracked, and one central mo-dem that calculates the position of the mobile beacon. For theexperiments, four stationary beacons were placed in a squarewith a side length of 8m, 40cm above the floor, as shown inFig. 4.SEPTEMBER 29th TO OCTOBER 4th 2019, MADRID, SPAIN 56IMAV2019-7 11th INTERNATIONAL MICRO AIR VEHICLE COMPETITION AND CONFERENCEFigure 4: Environment setup for the experiments. The sta-tionary beacons are placed on chairs at a height of 40cm, in asquare of 8x8m.Figure 5: Time between consecutive position estimationsfrom the indoor positioning system, during one of the flights.(Inset) Histogram of the time plot, using 10ms intervals.The system was configured to provide position estima-tion at a 12Hz rate, but may not provide data (or provide datawith low accuracy) in cases where the line of sight betweenthe mobile beacon and the stationary beacons is obstructed.In Figure 5, the time between consecutive samples of posi-tion information (accurate or not) is shown, during one of theflights of the experiments. The data rate of the positioningsystem is mostly stable at 85ms (around 12Hz), but it can beobserved that there are several occasions where this time isdoubled, corresponding to a failure to obtain position infor-mation. An inset of axis shows the histogram of the sameexperiment, where around 90% of the samples correspond toa time interval of 85ms±10ms.The chosen path for the experiments was the Gerono tra-jectory (or ”infinity” trajectory) in the XY plane. The yawdirection was fixed at zero during the entirety of the experi-ments, so that a maneuver in pitch moves the vehicle alongthe X axis, and a maneuver in roll moves it in the Y axis. Thevertical thrust remained manually controlled by the pilot forsafety reasons. The vehicle takes off from the ground, is po-sitioned around the center of the flight area, and the positioncontrol is activated. In the moment of activation, the currentposition is taken as the center of the Gerono trajectory.4 RESULTSThe flight trajectory for the vehicle without failures isshown in Figure 6. The vehicle takes off at t = 0s, and theposition control is activated at t = 33s, where the current po-sition is taken as reference. The vehicle performs three anda half full Gerono trajectories, before the position control isdeactivated at t = 190s, where it lands safely. It can be ob-served that there are several outliers in the measured positionat t = 72s, t = 109s, t = 126s and t = 184s, that corre-spond to errors in the position calculation of the Marvelmindtracking system, to which the vehicle reacts accordingly, butrecovers quickly and remains on path.In Figure 7, the PWM values commanded to the six ro-tors are shown. As expected for a nominal case of a hexarotorin a near-hovering situation, all the PWM values are almostequal, driving the rotors at around 50% of their maximumspeed, which provides a wide margin for performing maneu-vers without saturating any rotor.The flight trajectory for the vehicle with a failure in rotor3 is shown in Figure 8. The failure is activated before take-off, and is present during the full flight. The vehicle takes offat t = 0s, and the position control is activated at t = 18s,where the current position is, again, taken as reference. Thevehicle again performs three and a half full Gerono trajecto-ries, before the position control is deactivated at t = 176s,and is returned to the take-off point to land. In this test, therewere no occurrences of glitches in the position estimation.It can be noticed that in the Y-axis sinusoidal trajectory,there is some overshoot in the positive direction, while thereis no significant overshoot in the negative direction. This isbecause the rotor in failure state is positioned over the Y axisof the vehicle, and the roll maneuver in one direction seemsto be less responsive than in the other.In Figure 9, the PWM values commanded to the six rotorsare presented for the case with failure. While the PWM valuefor rotor 3 is zero during the flight, the commands to the restof the rotors do not significantly differ for the previous case.SEPTEMBER 29th TO OCTOBER 4th 2019, MADRID, SPAIN 57IMAV2019-7 11th INTERNATIONAL MICRO AIR VEHICLE COMPETITION AND CONFERENCEFigure 6: Gerono trajectory for an hexarotor without failure.Figure 7: PWM values during the flight without failureRotors 2 and 4 increase its speed (and thus its thrust), tocompensate for the lack of thrust of the motor located be-tween them. All the maneuvers performed during the trajec-tory do not require a great variation of speed (the PWM val-ues for all the rotors only vary around ±5%). This suggeststhat the vehicle with failure also is able to perform aggressivemaneuvers without saturating the rotors.Figure 8: Gerono trajectory for an hexarotor with a failure inrotor 3.Figure 9: PWM values during the flight with a failure in rotor3.Both cases performed satisfactorily, as the trajectory wascorrectly followed. Moreover, during manual take off, land-ing, and the diverse maneuvers made to position the vehiclefor the experiments, it was not noticeable any difference inmaneuverability between both cases, even while performingvery aggressive movements to test the system. A video of theSEPTEMBER 29th TO OCTOBER 4th 2019, MADRID, SPAIN 58IMAV2019-7 11th INTERNATIONAL MICRO AIR VEHICLE COMPETITION AND CONFERENCEpreliminar test of the fault tolerant vehicle can be sen in [12],and another of one of the experiments is attached to this work.5 CONCLUSIONThe proposed hexarotor vehicle was able to follow withgood performance a given trajectory, both in a nominal case,and with a total failure in one rotor. Moreover, there is noappreciable difference in the behaviour between both cases,as all the rotors operate at a speed pretty far away from theirsaturation limits, giving plenty of margin for different maneu-vers.Still, the failure case shows slight asymmetries in the tra-jectory, indicating that there are some maneuvers that are per-formed better than others. This may be caused either by therotors working at different average speeds, or by the maneu-ver requiring different speed variations from each rotor.ACKNOWLEDGEMENTSThis work has been sponsored through the UBA-PDE-18-2019 grant from Universidad de Buenos Aires, Argentina,Instituto Tecnologico de Buenos Aires ITBACyT Interdici-plinario DT13/2018, and PICT 2016-2016 grant from Agen-cia Nacional de Promoción Cientı́fica y Tecnológica (Ar-gentina). Claudio Pose would like to thank the Peruilh Foun-dation, whose grant made this research possible.REFERENCES[1] M. Saied, B. Lussier, I. Fantoni, C. Francis, H. Shraim,and G. Sanahuja. Fault diagnosis and fault-tolerant con-trol strategy for rotor failure in an octorotor. IEEEInternational Conference on Robotics and Automation,pages 5266–5271, 2015.[2] D. Vey and J. Lunze. Structural reconfigurability anal-ysis of multirotor UAVs after actuator failures. 54thConference on Decision and Control, pages 5097–5104,2015.[3] Duc-Tien Nguyen, David Saussie, and Lahcen Saydy.Fault-tolerant control of a hexacopter uav based on self-scheduled control allocation. In 2018 InternationalConference on Unmanned Aircraft Systems (ICUAS),pages 385–393, 2018.[4] D. Vey and J. Lunze. Experimental evaluation of an ac-tive fault-tolerant control scheme for multirotor UAVs.3rd International Conference on Control and Fault-Tolerant Systems, pages 119–126, 2016.[5] G. P. Falcon, V. A. Marvakov, and F. Holzapfel. Faulttolerant control for a hexarotor system using incremen-tal backstepping. IEEE Conference on Control Applica-tions (CCA), pages 237–242, 2016.[6] M. W. Mueller and R. D’Andrea. Stability and controlof a quadrocopter despite the complete loss of one, two,or three propellers. In 2014 IEEE International Confer-ence on Robotics and Automation (ICRA), pages 45–52,May 2014.[7] Juan I Giribet, Ricardo S Sanchez-Peña, and Alejan-dro S Ghersin. Analysis and design of a tilted rotorhexacopter for fault tolerance. IEEE Transactions onAerospace and Electronic Systems, 52(4):1555–1567,2016.[8] Juan I Giribet, Claudio D Pose, Alejandro S Ghersin,and Ignacio Mas. Experimental validation of a faulttolerant hexacopter with tilted rotors. InternationalJournal of Electrical and Electronic Engineering andTelecommunications, 7(2):1203–1218, 2018.[9] C. Pose and J. Giribet. Fault tolerance analysis of anhexarotor with reconfigurable tilted rotors (preprint).58th Conference on Decision and Control, 2019.[10] L. Garberoglio, M. Meraviglia, C. D. Pose, J. I. Giribet,and I. Mas. Choriboard III: A Small and Powerful FlightController for Autonomous Vehicles. In 2018 ArgentineConference on Automatic Control (AADECA), pages 1–6, Nov 2018.[11] Grupo de Procesamiento de Señales, Identificación yControl, Facultad de Ingenieria, Universidad de BuenosAires. http://psic.fi.uba.ar.[12] Fault tolerant control of a double-tilted hexacopter.https://www.youtube.com/watch?v=der3QUWz4Vg.SEPTEMBER 29th TO OCTOBER 4th 2019, MADRID, SPAIN 59

Trajectory Following with a MAV under Rotor Fault Conditions

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Trajectory Following with a MAV under Rotor Fault Conditions

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