Experimental Considerations on an Attitude Control Systemwith Measurement Noises for an X4-Flyer

Keigo Watanabe, Hirotaro Iwabe, Yusuke Ouchi, and Isaku NagaiGraduate School of Natural Science and Technology

Okayama University, 3-1-1 Tsushima-nakaKita-ku, Okayama 700-8530, Japan{watanabe, in}@sys.okayama-u.ac.jp

Abstract— This paper investigates about the influence ofsensor noises to the attitude control system of X4-Flyer, wheresuch noises come from vibrations due to the rotation of a rotor.It is pointed out that such measurement noises are serious inthe gyroscope sensor which measures an attitude, and then weconsider some noise counterplans from hardware and software.The validity of the proposed vibration-proof method or noiseprocessing is verified through actual system experiment.

Index Terms— Aerial robot, Measurement noises, Vibration-proof technique, Attitude control.

I. INTRODUCTIN

In recent years, the rescue operations by the robot in adisaster site attract attention from people. Among them, thesearch of disaster victims at the spot which man cannot comeinto, the activity in aerial photography, etc. are expectedby using the vertical-take-off-and-landing type aerial robot(VTOL type aerial robot) [1][2]. Moreover, it is considered tobe used not only in rescue operations but also in broad fieldssuch as agricultural-chemicals spraying, the surveillance andinspection of equipments for distributing electricity, etc.

Since VTOL-type aerial robots can perform a VTOL anddoes not need to use a runway, it can also be employed ina narrow space. Especially, the “X4-Flyer” has four rotors,and has the feature of controlling three attitude angles andthe center of gravity of the airframe in the inertia systemsby changing those number of revolutions. Therefore, there isan advantage that the mobility and maneuverability are highcompared to other VTOL type aerial robots [3].

The X4-Flyer is controlled by an operator through aremote-control system, for example in outdoor environments.In order to avoid a reversal of the airframe and unexpectedmovement during the operation, the airframe needs to stabi-lize its posture autonomously based on the information frommounted sensors. However, a controller design is not easyfor the X4-Flyer to realize an underactuated system whichcontrols six generalized coordinates with four inputs [4].

Moreover, when performing any dynamical model basedcontrol, we need some parameters, such as the weight of theairframe and the moment produced on the airframe and thethrust generated, etc. Furthermore, such parameters tend to

be affected by the influence of the structure of the airframe,or the performance of a rotor. The weight of the airframe andthe produced moment change with the kind and structure ofmaterial used, and the size of the airframe. Moreover, it isdifficult to estimate the generated thrust correctly, because thenumber of revolutions of a rotor is practically less than thetheoretical one, due to the drag produced on the blade and therotational efficiency of a motor. Thus, after manufacturing theX4-Flyer actually, it needs to obtain the desired parametersthrough the measurement experiments and then to design adesirable controller.

In this research, an X4-Flyer is actually designed andmanufactured, and it aims at the construction of an attitudecontrol system of the airframe to assist an operator duringremote control. This paper mainly describes attitude controlexperiments of X4-Flyer, and it points out that there areserious noises at the measurements of the attitude usinggyroscope sensors. From this fact, we discuss the use ofa filter to process the measurement noises that come fromthe rotor vibration and a vibration-proof action from theviewpoint of hardware.

II. MANUFACTURED X4-FLYER

A. Overview of X4-Flyer

The X4-Flyer [5], [6] is one name of the VTOL type aerialrobot which has four rotors, and is more generally called aquad-rotor type aerial robot [7]. The appearance of X4-Flyeris shown in Fig. 1. The X4-Flyer carries a circuit and a batteryin the center of the airframe, and has a total of four rotorsall around. While carrying out an advance flight by the thrustgenerated with each rotor, the attitude control of the airframeis also performed by adjusting the number of revolutions ofeach rotor.

The attitude angle and the movement direction of X4-Flyerare shown in Fig. 1. The X4-Flyer at the time of hoveringkeeps the airframe stable by adjusting the thrust balance ofeach rotor.

Viewing the airframe from the top, anti-torques are mu-tually generated by rotating the rotors 1 and 3 in the coun-terclockwise direction, and rotating the rotors 2 and 4 in theclockwise direction, to prevent the rotation of the airframe.

Fig. 1. Schematic drawing of X4-Flyer

Fig. 2. Control of pitch

Moreover, when flying, three attitude angles (φ, θ, ψ) andthe center of gravity (x, y, z) of the airframe in the inertiasystem are controlled by changing the number of revolutionsof the rotor of front and rear, right and left. In what follows,we describe how the number of revolutions of each rotor ischanged.

• X-directional translational and pitch motionThe X-directional translational motion and the pitch (θangle) motion are shown in Fig. 2. Under the conditionthat the number of revolutions of the rotors 2 and 4 is inan equal state, these motion are performed by decreasing(or increasing) the number of revolutions of the rotor 1,and by making the number of revolutions of the rotor 3increase (or decrease).

• Y-directional translational and roll motionThe Y-directional translational motion and the roll (φangle) motion are performed, under the condition thatthe number of revolutions of the rotors 1 and 3 is equal,by decreasing (or increasing) the rotation of the rotor 2and by making the number of revolutions of the rotor 4increase (or decrease).

• Z-directional translational motionThe Z-directional upward (or downward) motion canbe realized by increasing (or decreasing) the numberof revolutions of four rotors equally.

• Yaw motionThe yaw (ψ angle) motion is shown in Fig. 3. Keepingthe total thrust of the rotors constant, this motion is per-formed by increasing equally (or decreasing) the numberof revolutions of the rotors 1 and 3, while decreasing

Fig. 3. Control of yaw

Fig. 4. Photograph of an X4-Flyer

(or increasing) equally the number of revolutions of therotors 2 and 4.

B. Overview of Manufactured X4-Flyer

The appearance and size of X4-Flyer currently manufac-tured by this research are shown in Fig. 4. The present X4-Flyer carries the microcomputer circuit for control, and themotor driver in the center of the airframe. The battery wasnot carried yet but has obtained electric power from theexternal power supply with the cable. The weight of thepresent airframe is 0.9 kgf.

The rotor of X4-Flyer has the structure of transmitting therotation of a motor to a shaft through a one-step gear, asshown in Fig. 5.

As a result of measuring the thrust of the rotor consistingof the blade made from firing styrene of the X.R.B seriesof HIROBO Ltd., the maximum thrust obtained from onerotor was 0.49 kgf. Since the ratio of the weight of thewhole airframe to a thrust must generally be 3:4 or morein a vertical climb, a thrust that is about 1.3 times the totalweight is needed to perform a VTOL. From this fact, theairframe weight that can carry out a vertical takeoff usingthis rotor is

0.49× 4

1.3≈ 1.5 [kgf] (1)

Therefore, a vertical takeoff is possible for X4-Flyer whoseairframe weight is 0.9 kgf.

Three gyroscope sensors of one axis are used to measureφ, θ and ψ angles of the airframe. As shown in Fig. 6, thegyroscope sensor is set in the center of the airframe so that

Fig. 5. Rotor

Fig. 6. Gyroscope sensors and an accelerometer

each detection direction may correspond to a φ angle, a θangle, and a ψ angle. KRG-4 produced by Kondo Science isused for a gyroscope sensor. The range of detection of thissensor is +100 deg from −100 deg, and so it is said to beenough for detecting the attitude angle of the airframe.

We tried to detect the attitude angle of the body usinggyroscope sensors. However, errors occurred in measuredvalues and exact attitude angles were not able to be detectedby using only gyroscope sensors. It can be correctly detectedusing a gyroscope sensor if the angular velocity to be detectedis large, but a detection error may arise if not the case.

In addition to the use of gyroscopes, we tried to detect theattitude angles of the airframe by mounting an accelerometerto the airframe and using it. Since an accelerometer detectsthe attitude angle of the airframe by the change of thegravitational acceleration, it does not produce a detectionerror easily, even if the angular velocity of the airframe issmall. Therefore, accumulation errors can be suppressed byusing gyroscope sensors and an accelerometer properly, andan improvement in accuracy can be achieved.

As shown in Fig. 6, in addition to three one-axis gyroscopesensors, one three-axis acceleration sensor was mounted onthe airframe to detect the attitude angle of X4-Flyer. Theattitude angles of the airframe, φ, θ and ψ angles are detectedusing these sensors. KXM1050 produced by Kionix Inc. isused for an accelerometer. The measurement range of thissensor is +2 G from −2 G, so that it can be said to beenough for detecting the attitude angles of the airframe.

Fig. 7. Attitude control experiment

The blade made from firing styrene in the X.R.B seriesof HIROBO Ltd. is used for an old rotor blade. As statedabove, in the thrust experiment with the simple substance ofthe rotor that uses this blade, we obtained the thrust per rotorrequired in order that the airframe may take off.

It is safe when the blade made from firing styrene contactsan object during rotation, but there exists a problem that ablade will bend backward when the number of revolutions isincreased. A blade will be deteriorated due to such bendingbackward, and it causes the thrust reduction of a rotor andthe individual difference between rotors.

Therefore, the blade for LotusT580 by LotusRC Technol-ogy Company was adopted as a new blade. Since this bladeis a product made from a plastic, it does not bend backwardat the time of rotor rotation. Moreover, LotusT580 for whichthis blade is used is radio controlled X4-Flyer type, and grossweight is set to 1.8 kg at the time of the maximum flight. Itseems that it can obtain a thrust required for a flight usingthis blade because the gross weight of X4-Flyer planned nowis 1.3 kg.

Moreover, an O ring type propeller saver was used forthe attachment of a blade, where such a propeller saver isused also for the fixation of the propeller for radio controlledairplanes and helicopters. The O ring is extended and absorbsa shock, even if the blade under rotation contacts any object,so that damages to the contacted object and blade itself areprevented.

III. ATTITUDE CONTROL OF X4-FLYER

An attitude control method is realized by feed-backing theattitude angles detected by sensors mounted on the airframe.

A. Controller Design

When controlling the attitude and position of an X4-Flyer,there is much research which uses PID controllers [8], [9],[10], [11]. However, it is said that the PID controller isunsuitable for controlled objects with a high responsibilitylike an X4-Flyer, instead there is a study being performed byusing a PD controller [12]. So, a PD controller is used forthe attitude control of an X4-Flyer.

Fig. 8. The result of PD control Fig. 9. The result of yaw control

Let the desired values of attitude angles for the roll, pitch,and yaw direction be rdφ, rdθ and rdψ , and let their currentvalues be given by rφ and rθ and rψ , respectively. Moreover,letting the desired values of angular velocities be rdφ, rdθ andrdψ , and letting their current values be given by rφ, rθ and rψrespectively, the control input ui of each axis in PD control,ui [Nm], i = φ, θ and ψ are as follows:

ui = Kpi(rdi − ri) +Kdi(r

di − ri), i = φ, θ, ψ (2)

where Kpi and Kdi denote the proportional and derivativegains. The proportional gains were calculated from the useof the principal moment of inertia [12], whereas the derivativegains were determined through trial and error, which aregiven by

Kpφ = 0.05915, Kpθ = 0.05915, Kpψ = 0.02331

Kdφ = 3, Kdθ = 3, Kdψ = 0.02

Furthermore, using four inputs composed of force u1 forthe translational motion in z-direction and of torques u2 =uφ, u3 = uθ, u4 = uψ for other rotational motion yields eachangular velocity of the rotor. Then, they are transformed intothose in voltage and further transformed into suitable valuesfor the PWM, where it was assumed that u1 = 22 [N] wasfixed during the experiment. Note that it was greater than16.6 [N] that was a minimum force to lift the weight of 1.3kgf, because normally the force of being 1.3 times much asthe weight is required to lift an arbitrary airframe.

B. Attitude Control Experiments

An attitude control experiment of the X4-Flyer is con-ducted using the PD controller. The number of revolutionsof rotors is changed to make the airframe hover based onthe attitude angles detected by the sensors mounted on theX4-Flyer.

The experimental setup for this experiment is shown inFig. 7. The airframe is taken off by enlarging the thrust ofrotors gradually by a controller connected with the X4-Flyer,where a stabilized power supply is used for a power supply.The number of revolutions of the rotors is changed basedon the attitude angles detected by the sensors mounted onthe airframe, and it is checked whether it can hover without

Fig. 10. Experiment of vibration examination

Fig. 11. The detection result of rollangle

Fig. 12. The detection result of yawangle

losing balance. Since there is a danger that the airframe willcollide with the surrounding thing, the moving range of thebody is restricted with several vinyl strings extended in theupward direction of the airframe.

As a result of the experiment, the attitude angle of theairframe after a takeoff was changed sharply, and the airframelost balance. The φ, θ, and ψ angles changed violently, asshown in Fig. 8 and Fig. 9, and it did not converge to 0deg. Moreover, when the experimental situation viewed by anexperimenter was compared with the detected result, it turnedout that the detected attitude angle changed more violentlythan the angle of the actual airframe.

Since the detected attitude angle rather than the actualattitude angle of the airframe was changed violently, noisesoccurred due to any disturbances when the sensors detectedthe attitude angles, so that it seems that the correct angleswere not detectable. Vibrations generated by the rotation ofrotors can be considered as one of the causes of generatingnoises. In order to reduce the effect of vibrations, it needsto remove noises by relaxing mechanically the vibrationstransmitted to the sensors, or by using a filter when usingthe attitude angles.

IV. VIBRATION-PROOF STRATEGY OF AIRFRAME

A. The Effect of Vibration due to Rotor Rotation

An experiment was conducted to check whether the vibra-tion of the airframe by which it is generated by the rotor

Fig. 13. Vibration-proof structure

Fig. 14. Frequency analysis of agyro sensor

Fig. 15. Frequency analysis of anaccelerometer

rotation affects the attitude angle detection. As shown inFig. 10, the X4-Flyer was attached to the upper part of a fixedstand, and we investigated the change of the attitude angle atthe time of rotating rotors. The detected attitude angle wassent and recorded on a notebook PC by serial communication.Although the fixed stand had a structure that was able to beinclined to the directions of φ and θ angles in the range of±30 deg, it was fixed in the level state at the time of theexperiment. The voltage was set to 10 V at the power supplyusing the stabilized power supply.

When the attitude angles acquired by the experiment werechecked, each of the φ, θ and ψ angles included the error.The time variations of the detected φ and ψ angles are shownin Fig. 11 and Fig. 12, respectively. Since the airframe wasfixed in the level state and it was also not moved in thedirection of the ψ angle, the ideal values of the φ, θ, andψ angles should be 0 deg. However, the detected φ and θangles included noises, in which the RMS values of the noisegenerated for the φ and the θ angles were 25.01 deg and 26.28deg, respectively. Moreover, the ψ angle was greatly driftedwith the progress of time.

B. Vibration-proof for Sensors

In order to reduce the influence of vibrations generatedon the airframe, vibration-proof materials were preparedbetween a sensor circuit and the airframe. As shown inFig. 13, vibrations transmitted to the sensor circuit areabsorbed and reduced by attaching the sensor circuit to the

Fig. 16. The detection result of rollangle

Fig. 17. The detection result of yawangle

airframe through sponge rubbers and extruded polystyrenefoams.

The use of a low pass filter is mentioned as an action froma software side to the vibration of the airframe. The anglevalue obtained from the gyroscope sensor pass is presentlypassed to the low pass filter, setting-up a cutoff frequency tobe 10 Hz. Note however that this value is referred to that of alow-pass filter currently used for commercial radio-controlledhelicopters etc., so that it cannot declare that it is optimal.

Therefore, a frequency analysis is conducted about theangle obtained from the gyroscope sensor, and a suitablecutoff frequency is selected. Moreover, the same analysis isalso conducted for the angle obtained from the accelerometer,and a low pass filter is applied by using the selected cutofffrequency. The results of having performed a frequencyanalysis for the angles obtained from the gyroscope sensorand the accelerometer are shown in Fig. 14 and Fig. 15,respectively. It is found from the obtained analysis resultsthat the spectra of the angles obtained from the gyroscopesensor and the accelerometer have both shown some highvalues over 10 Hz. Therefore, a cutoff frequency of the lowpass filter was set to 10 Hz.

C. Results of Vibration-proof Action

After having the two vibration-proof actions, when thesame experiment as performed before was conducted, thedetected angles φ and ψ were like Fig. 16 and Fig. 17,respectively. It was seen that the RMS values of the noisesmeasured for the φ and θ angles were 5.12 deg and 6.38deg, respectively, so the noises became quite small. Moreover,although a sensor drift occurred as usual on the ψ angle likeFig. 17, the amount of change per time decreased about to1/3 compared to Fig. 12 before taking such actions.

D. Attitude Control Experiment of the Airframe

As mentioned above, as a result of taking actions from bothsides of hardware and software against vibrations generatedon the airframe, the influence by vibrations was reduced.

In this stage, an attitude control experiment of the airframewas conducted again to check what attitude control is possi-ble.

The experimental setup used the same one as Fig. 10.However, the airframe was not fixed in the level state andthe experiment was conducted, allowing it to be inclined inthe directions of φ and θ in the range of ±30 deg. Thevoltage was set to 10 V at the power supply using thestabilized power supply. After supporting by hand the bodyin the state where the rotor was rotated and lifting a hand, itwas checked whether a posture could be maintained withoutthe body inclining. After the rotor’s having supported thebody of a rotation state by hand and lifting a hand, it waschecked whether a posture could be maintained without thebody inclining. After supporting the airframe, whose rotorswere rotating, by hand and releasing it, it was confirmedwhether the X4-Flyer was able to maintain the attitudewithout showing inclinations. Note that the ψ angle was notcontrolled in this experiment, but only the φ and θ angleswere controlled.

Out of 16 experiments, we succeeded two times in main-taining an attitude from several seconds to several minutes.The succeeded φ and θ angles are shown in Fig. 18 andFig. 19, respectively.

Although the airframe maintained the level mostly duringthe experiment, the noises occurred at the φ and θ angles, asshown in Fig. 18 and Fig. 19, where the RMS values of suchnoises were 8.07 deg and 8.99 deg, respectively. Moreover,as discussed in the previous subsection, it is confirmed thatthe noises related to the φ and θ angles generated duringthe experiment came from the vibration of the airframe, ifwe remember that the previous RMS values for their noiseswere 5.12 deg and 6.38 deg. Thus, it is thought from theexperimental results that it is possible to maintain the attitudeof the airframe, even if there is a noise whose RMS value isabout 9 deg.

When the attitude of the airframe was not maintained, ofcourse, the attitude was broken down greatly and changedinto the state which has inclined without the ability to correctan attitude. From this fact, a possibility that the thrust ofa rotor is still insufficient is also considered. Moreover, itseems that it can perform stable attitude control by usingmore suitable gains, because the D gain of PD controller usedfor attitude control was the value calculated by experiment.Investigation of increase of a rotor thrust, the weight savingof the airframe, and constant gains, etc. is mentioned as anaction for these problems.

V. CONCLUSIONS

This paper has considered on some actions against thenoises of sensor measurements for the X4-Flyer through theattitude control experiment from both sides of hardware andsoftware. Although the airframe was not always able to bekept level as a result of attitude control, we succeeded in theattitude angle detection of the airframe with few measurementerrors and the attitude control of the X4-Flyer in a gradewhich does not lose balance greatly.

Fig. 18. The result of roll angle Fig. 19. The result of pitch angle

However, the increase in a rotor thrust, the weight savingof the body, and the method of determining PD constantgains which guarantees the stability (when position control isincorporated) need to be reexamined to perform more stableattitude control. For example, there is a design method [12]by using the Lyapunov stability theory.

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