Sensor set stabilization system for miniature UAV Wojciech Komorniczak 1, Tomasz Górski, Adam Kawalec, Jerzy Pietrasiński Military University of Technology, Institute of Radioelectronics, Warsaw, POLAND ABSTACT In this paper we present the concept of multiple sensors data acquisition from onboard of an Unmanned Air Vehicule (UAV). Because of flight instabilities caused by atmosferic movements (winds, thermals etc..) it is necessary to apply active stabilization in order to obtain reliable readings from observation sensors. The most stabilization-demanding sensor is Synthetic Aperture Radar (SAR) and in this paper two methods of stabilization are presented: hybrid (electromechanical) and electronic. sensors, unmanned air vehicule, stabilization 1. INTRODUCTION UAVs gain more and more attention last years especially in military applications. This is caused by an obvious profit that using UAVs it is possible to reduce human loss since soldier s life is not jeoperdised during reconnaissance missions. Since startegic UAVs (eg. Global Hawk) are of the same size and weight as manned aircrafts, their exploatation costs are also comparable to regular planes. Therefore is is particulary interesting to consider using very small UAVs, not havier than tens of kilogramms. Platforms of this type are relatively cheap in exploatation and need very little logistic support. In most cases, small UAVs are sufficient for reconnaissance missions 1. Their drawback is related to instabilities of the flight caused by atmosferic movements. More precisely, small wing load and narrow flight speed envelope (around 70 to 100 mk/h) cause a strong relation between platform stability and atmosferic movements (Fig. 1). In the figure, it is possible to see aircraft orientation along longitudinal axis and lateral axis during circular flight around point of interest. It is possible to notice, that apart from constant bank (around longitudinal axis typical for circular flight), considerable oscialtions (15 to 20 degrees) are present. This behaviour considerably deteriorates surveillance process 1. Fig. 1. UAV s instabilities along longitudinal axis and along lateral axis as a function of time during circular flight. 1 email: wojciech.komorniczak@wel.wat.edu.pl Photonics Applications in Astronomy, Communications, Industry, and High-Energy Physics Experiments 2009, edited by Ryszard S. Romaniuk, Krzysztof S. Kulpa, Proc. of SPIE Vol. 7502, 750217 2009 SPIE CCC code: 0277-786X/09/$18 doi: 10.1117/12.837312 Proc. of SPIE Vol. 7502 750217-1
2. ELECTRO-MECHANICAL STABILIZASTION UNIT Instabilities can be partially eliminated using electro-mechanical stabilization. Unit of this type can be placed in the center of mass of an aircraft (Fig. 2) and serves as the mounting point of observation sensors. This unit can compensate instabilities of high amplitude (tens of degrees around longitudinal and lateral axis, and up to around 10 degrees around vertical axis) and low angular velocity (up to 60 degrees per second). This solution is sufficient for operation of optical sensors like daylight camera or thermal camera. Elcetro-mechanical stabilization module consists of three electronic circuits (Fig. 3) of the same structure. In each of them, pizoelectric gyroscope (MEMS) is used as an angular velocity meter (rate gyro). Processing unit is using Proportional Integrational Differential (PID) loop to achieve stabilization effect. PID loop coefficients are chosen independently for each axis. Mechanical design of the unit is presented in figure 4. Distinctive feature of this solution is the fact, that all rotation axes have single midpoint in the center of mass of the aircraft. This allows to avoid coupling between stabilization action of the electro-mechanical unit and platform movements. Fig. 2. UAV platform with electro-mechanical stabilization unit. Fig. 3. Electronic circuit for electro-mechanical stabilization (3 stabilization channels). Proc. of SPIE Vol. 7502 750217-2
Fig. 4. Mechanical design of the electro-mechanical unit. High angular-velocity oscilations (above 60 degrees per second) can not be eliminated by electro-mechanical unit described earlier. This is caused by sensors high momentum of inertia (comparable to momentum of inertia of the aircraft itself). Therefore it is necessary to find non-mechanical stabilization method. 3. ELECTRONIC COMPENSTATION OF INSTABILITIES Low amplitude and high frequency oscillations can be compensated electronically. Optical sensors used in presented solution, have build-in electronic stabilization. This allows to compensate movements up to a few degrees. Synthetic Aperture Radar, considered as potential sensor, doesn t have such stabilization. Idea of SAR stabilization is based on Attitude and Heading Reference Systems (AHRS) in combination with appropriate SAR processing algorithms (Fig. 5). Fig. 5. Electronic SAR instabilities compensation. Proc. of SPIE Vol. 7502 750217-3
Fig. 6. AHRS unit for electronic instabilities compensation for SAR. Designed AHRS unit uses 13 sensors (Fig. 6). Data fusion from these sensors is based on Kalman filter and gives Euler angles for all three axes with update frequency of 500Hz [3]. Unit presents no time drift and high short-time acuracy (necessary for SAR). SAR imaging is a technique, that produces high resolution images by processing multiple radar pulses in order to obtain a single image pixel. To obtain this effect a relatively small antenna is used (low angular resolution). Received echoes are sampled in the receiver and recorded in the computer memory for batch processing. This processing (so called focusing) can be divided into two phases: range compression and azimuth compression. Additionally, between range compression and azimuth compression, it is necessary to perform range migration processing. In the ideal case, several assumptions are fulfilled: straight-line flight path, constant velocity of the platform and constant orientation of the system. Unfortunately in practice these assumptions can be fullfilled (to certain extent) only for spaceborne SAR systems. For airborne systems (eg. UAV), platform oscillations have strong negative effect on SAR performance. In fact, without platform movement compenstation, focusing is not possible. Compensation can be achieved in two ways: autofocusing algorithms or AHRS corrections incorporated by focusing algorithm. There are three types of errors caused by flight instabilities. These are: trajectory deviations, velocity error and attitude error. Trajectory deviations from straight line cause phase error to the received signal. This type of errors can be compensated using AHRS system by applying appropriate phase corrections to focusing algorithm. In order to achieve this compensation it is necessary to know trajectory with accuracy of order of fractions of wavelength. Trajectory update frequency must by hight enough to obtain reliable trajectory approximation. Because of these constraints, using only GPS data is insufficient, since it s accuracy is of order of meters and it s update frequency is too low (around 10 Hz). The next problem is platform changing velocity. It may be caused by atmospheric movements or by changes in the engine performance. Compensation of this effect can be put in the focusing algorithm (based on registered ground speed). Moreover it is possible to compensate this effect by dynamically changing Pulse Repetition Frequency (PRF) according to actual ground speed. Last error is related to changes in platform attitude. This causes angular movement of the antenna and the antenna beam. This results in shortening of synthetic aperture and causes problems with focusing algorithm. This effect occures for rotations around vertical axis and longitudinal axis. This error should be compensated by electro-mechanical unit by rotating the antenna. In more advanced systems the same compensation can be achieved using array antenna and beamforming. In such a case there is no need for mechanical unit. 4. CONCLUSIONS Presented concept of active stabilization for UAV sensors is in the final implementation stage (both hardware and software). Preliminary tests support theoretical discussion. Complete system should be ready in the autumn 2009. It will be then possible to make a whole range of tests with data registration for off-line ground analysis. Proc. of SPIE Vol. 7502 750217-4
ACKNOWLEDGEMENTS This work is supported by Polish government (grant PBZ-MNiSW-DBO-04/I/2007). BIBLIOGRAPHY [1] Kawalec A., Komorniczak W., Pietrasiński J., Czarnecki W.: Evaluation of low cost micro UAV platform for sensor suite, International Radar Symposium IRS 2006, Kraków, Poland, 24-26.05.2006, pp. 605-608. [2] Kawalec A., Komorniczak W., Pietrasiński J.: The Instability Measurements for the Unmanned Aerial Platform as a Radar Carrier, International Radar Symposium IRS 2008, Wrocław, Poland, 21-23 May 2008, pp. 71-74. [3] Liggins M., Hall D., Llinas J.: Multisensor Data Fusion. Theory and Practice. CRC Press 2008. Proc. of SPIE Vol. 7502 750217-5