The Servoleli-20 Rotorcraft UAV Project - IEEE Xplore

15th International conference on Mechatronics and Machine Vision in Practice (M2VIP08),2-4 Dec 2008,Auckland,New-Zealand

The Servoleli-20 Rotorcraft UAV Project Juntong Qi, Dalei Song, Lei Dai, Jianda Han

State Key Laboratory of Robotics, Shenyang Institute of Automation, CAS Nanta Street 1 14#, Shenyang, China Graduate School, Chinese Academy of Sciences Beijing, China Email: qijtgsia.cn

Abstract - This paper describes recent research on the design, implement and testing of a small-scaled rotorcraft Unmanned Aerial Vehicle (RUAV) system - ServoHeli-20. A turbine-powered UAV weighted less than 10kg was designed, and its major

components tested by a group of undergraduate students at Shenyang Institute of Automation, Chinese Academy of Sciences in Shenyang, China. The aircraft was designed to reach a top speed of more than 20mps and fly a distance of more than 10 kilometers and then is going to be used as a testbed for experimentally evaluating advanced control methodologies dedicated on improving the maneuverability, reliability as well as autonomy of RUAV. Sensors and controller are implemented onboard. The full system has been tested successfully in the autonomous mode using the multi-channel decoupling PID controller. The results show that the rotorcraft UAV can follow the trajectory which assigned by the ground control station exactly in the real windy environment.

I.

INTRODUCTION

Unmanned aerial vehicles (UAV) are useful for many

applications where human intervention is considered difficult or dangerous. Traditionally, the fixed-wing UAV has been served as the unit for these dangerous tasks because the control is easy. Rotary-wing UAV, on the other hand, can operate in many different flight modes which the fixed-wing one is unable to achieve, such as vertical take-off/landing, hovering, lateral flight, pirouette, and bank-to-turn. Due to the versatility in maneuverability, helicopters are capable to fly in and out of restricted areas and hover efficiently for long periods of time. These characteristics make RUAV applicable for many military and civil applications.

However, the control of RUAV is difficult. Although some control algorithms have been proposed [I]-[6], most of them were verified by simulation instead of real experiments. One reason for this is due to the complicate, nonlinear and inherently unstable dynamics, which has cross coupling between main and tail rotor, and lots of time-varying aerodynamic parameters. Another reason is that the flight test is in high risk. If a RUAV lost its control, it would never be stabilized. This paper details the development of an unmanned helicopter (UJAV) testBed-9ServoHei-20 [7] (Fig2re 1), and the

platform is introduced in Section II. The introduction of sensor package is in Section III. The modeling of the UAV helicopter system is presented in the Section IV. In Section V, we introduce an independent-channel control scheme as a baseline control of the platform. In the end, we conclude our work and discuss some future research issues. II. SERVOHELI2O PLATFORMDESCRIPTION

As the basic aircraft of the RUAV system, we chose the small-scaled model helicopter which is available in the market. Such a choice is easy for us to exchange the accessories and P

ServoHeli-20 aerial vehicle is a high quality helicopter which is changed by us using a RC model helicopter operating with a remote controller. The modified system allows the payload of more than 5 kilograms, which is sufficient to take

the whole airborne avionics box and the communication units. The fuselage of the helicopter is constructed with sturdy ABS composite body and the main rotor blades are replaced with heavy-duty carbon fiber reinforced ones to accommodate extra payloads. The vehicle is powered by a 90-class glow plug engine which generates 3.Ohp at about 15000 rpm, a displacement of 14.95cc and practical angular rate ranging 2,000 to 16,000 rpm. The full length of the fuselage is 1260mm as well as the full width of it is 160mm. The total height of the helicopter is 410mm, the main rotor is 1600mm and the tail rotor is 260mm.


In the actual flight environment, the weight and the size of responsive and reliable. strictare limited. box avionicsthe Our airborne control .. . . . . . . . . . . . . . . whichishownbox,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~........... inFigure2,isacompactaaluminum l oy~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~... ~ ~ ~HEH~ ~ ~ ~H.H~ ~ ~ ~....................... ~ ~ ~ ~ ~. ~. .~.~. ~. .~.~..... ..... .. ..... ....... packagemountedonthel gear.andingThe center of gravity of~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...... ... theboxliesontheIMUd where evicenotis the geometry~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~................... center of the system that ensure the navigation data form IMU~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ accurate. The digitalcompass and the IMU which are taken as~~~~~~~~~~~~~~w.......... thehorizontalcenteroavionics fthegravityothef system to~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~............... . . . . . . . . . .......

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numerical model, and then other serial of input data was tested

using the proposed model. The blue line is the measurement of

B. Passive and Active Vibrations Isolation

In our avionics box, we use rate gyros and accelerometers to measure rates about three axes,ran d to alongc3 axes; an independent ARM processor is used to extract absolute roll and pitch. However, in the real flight environment, the sensors will be subjected to rotor frequency vibrations; both the rate and acceleration readings are grossly inaccurate; consequential, so to is the attitude estimation. In order to isolate the unit from these frequencies, we use

the passive and active isolation method. The passive method is that the sensors are spring mounted inside the main avionics box. With the foam damping included, the isolation can act as a passive effect. However, the active method is the Kalman filter way to isolate the vibrations and biases. A typical plot of the forward and lateral velocities before and after isolation is given in Figure 6. 4 T

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V. INDEPENDENT-CHANNEL CONTROL SCHEME

Simulation studies have shown that a better strategy for the control of a small-scaled helicopter is to use the flight controller as consisting of two cascaded controllers: an inner loop and an outer loop. The inner loop, that has the faster dynamics, is designed as the attitude controller which takes desired attitude angles as inputs and generates the actuator commands that will result in the desired attitude. The outerloop controller, which controls the slower translational rate variables, takes desired velocity or position as input and generates desired angels to the inner loop. The overall flight control scheme is shown in Figure 8, while five linear controllers is designed to control the engine speed, height, yaw, lateral and longitudinal motion. ± Position / Velocity+ Attitude_ Controller Controller s

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Fig. 7 Rotorcraft UAV modeling verification

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the yaw rate from the real flight as well as the red line is calculated by the numerical model and the real flight input. As is shown in the Figure 7, the estimation output is similar to the rea flgtdt-n ecncnld httepooe modeling method is useful to the rotorcraft UAV.

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Fig. 6 Velocities before and after isolation IV. ROTORCRAFT UAV MODELING

For effective hovering identification, the original model in reference [8] is decomposed into three groups (longitudinal, lateral, and yaw-heave coupling), and a semi-decoupled model is obtained. Each group has a decoupled system matrix, and the coupling characteristics is presented only in the control matrix. Thus, the number of unknown parameters and control inputs are reduced and the control loops are semi-decoupled. Then, to identify the unknown parameters in the MIMO semi-decoupled Fig. 8 Overall fight control scheme model, a new cost function is proposed to make the traditional method of SISO system frequency estimation [9] applicable to A. Engine Speed Control the MIMO state-space models. The proposed cost function is The engine speed during the hover-envelope flights is p s presented in the addition form of the frequency error of every rpm as a rsl of t input-output pair for transfer matrix, and the parameters are idnife by miiizn th cos fnto. Th simplified..manual mode. To get a steady rotary speed of the engine, a PID moe an prose idniicto meho free th selectiono controller iS used in feedback control which from the speed sensor to the throttle demands. When collective pitch changing, stmto an cosran is no rqie. the power of the engine will change as a result of it. A feed Take the yaw -heave model for example. We have got the forward term from the collective pitch is introduced to

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compensate for the extra loading experienced. The engine control scheme is shown in Figure 9.

Innerloop:RollRateStabilizationloop

Fig. 12 Roll control scheme

Fig. 9 Engine speed control scheme

B.

Height Confrol

E. Autonomous FlightoResult

A two-loop control scheme for the rotorcraft UAV system was design and tested using the ServoHeli-20 platform. We design some specified trajectories to be flown. These trajectories were selected in order to evaluate the inner loop and outer loop response over several different sequences of inputs. We selected a tunnel way to be followed, as is shown in the Figure 15. The proposed controller handled this flight trajectory with minimal error, Figure 16. Figure 14 and Figure 15 show that the angles and velocities which controlled by inner loops are also get a stable response.

The height control is a one loop scheme which a PI controller using feedback from the height sensor generates collective pitch demands in Figure 10. h =remllote

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Fig. 10 Height control scheme

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The yaw control two loop structure is presented in Figure 11. As is shown in this figure, the inner loop is a yaw rate stabilization loop which proportional control using yaw rate feedback from the IMU output demands to the rudder servo

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Fig. 13 Forward and lateral velocity during the flight

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Fig. 11 Yaw control scheme

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Similar to the yaw control scheme, IMU, digital compass the and GPS are used as the feedback sensors. .to . maintain . . and lateral and longitudinal position with simple proportional PI controllers. The inner loop is pitch / roll rate stabilization component as well as the outer loop serves as the position feedback unit which is shown in Figure 12.

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a simple but useful control law in unmanned aerial vehicle experiments. The rotorcraft UAV system has been tested successfully for full autonomous flight including autonomous take off and landing. The next step is to integrate the visual and IMU estimation into a unified sensor suite and to develop advantage autonomous flight control algorithm for maneuverable fight.

helicopter system and is

AcKNOWLEDGMENT

This paper is supported by National Natural Science Foundation of China "Control of Mobile Robots Based on Study of Hand Tele-operation" (60705028). The authors gratefully acknowledge the contribution of Shenyang Institute of Automation, Chinese Academy of Sciences and reviewers' comments. Fig. 15 Trajectory in the Google Map

REFERENCES

[1] P. Sanders, A. DeBietto, Eric Feron, "Hierarchical Control of Small Autonomous Helicopters," Proceedings of the 37th IEEE Conference on Decision & Control, pp. 3629-3634, Tampa, Florida USA, December

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[5] T. J. Koo, D. H. Shim, 0, Shakernia, "Hierarchical Hybird System Design on1 Berkeley UAV," International Aerial Robotics Competition, August 1998.

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[6] Zhe Jiang, Jianda Han, "Enhanced LQR Control for Unmanned Helicopter in Hover," 1 st International Symposium on Systems and Control in Aerospace and Astronautics, pp, 1438-1344, Harbin, China, January 2006.

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[7] Juntong Zhe Jiang, Jlanda Han, Design and implement Qi, Xingang of a rotorcraft UAVZhao, in Conf Rec. 2006 IEEE international testbed, conference robotics and biomimetics, Kunming,, China, Decemeber,

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pp.lO9 114,2006 [8] Mettler B., Dever C. and Feron E., "Scaling Effects and Dynamic

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Longitute (deg)

10 Longitute (deg) X 109 Fig. 16 Desired and real trajectory

Characteristics of Miniature Rotorcraft," Journal of Guidance Control and Dynamics, Vol. 27, (3), pp.466-478, 2004. [9] Bendat J.S. and Piersol A.G., "Engineering Application of Correlation and Spectral Analysis," John Wiley & Sons, USA, 1993.

VI. CONCLUSIONS

This paper describes the current status of the ServoHeli-20 autonomous helicopter. We have introduced the system implementation of the rotorcraft UAV and control scheme for model scaled helicopter. A remote-controlled model helicopter is selected as the basic helicopter, which is changed to adapt to the heavy load in future. We also introduce the sensors and algorithm for attitude and position estimation. The two loop linear control scheme is presented in this paper for UAV

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