Application of high-speed videography

for in-flight deformation measurements of aircraft propellers

Boleslaw Stasicki and Fritz Boden

DLR / German Aerospace Center Institute of Aerodynamics and Flow Technology

Bunsenstr. 10, 37073 Göttingen / Germany

ABSTRACT

A combination of high-speed stroboscopic imaging with the Image Pattern Correlation Technique (IPCT) enables for non-intrusive measurement of surface deformation of fast vibrating or rotating objects. In this paper the dedicated instrumentation for the measurement of the deformation of aircraft propellers as well as first results of its application will be described.

Keywords: High-speed imaging, video stroboscope, deformation measurement, aircraft, flight-test

1. INTRODUCTION The in-flight investigation of the deformation of a fast spinning propeller is a demanding task. It is obvious, that conventional methods like strain gauges and accelerometers are difficult to use because of problems with their installation, their wiring, the data transmission, and the balancing of the propeller. Furthermore such sensors provide their data only at the location where they have been installed.

The need for advanced, non-intrusive e.g. optical measurement techniques has been defined and goal-oriented scheduled as a task of the European Specific Targeted Research Project (STReP) AIM - Advanced In-flight Measurements Techniques. This project, which was launched on 1st of November 2006, intends to make advanced, non-intrusive measurement techniques applicable for time and cost effective industrial flight testing as well as in-flight testing for research. The international AIM-Partners representing both the research institutes and aircraft industries in Europe working closely together are: DLR (D), Piaggio Aero Industries (I), Eurocopter Deutschland (D), Eurocopter SAS (F), Airbus France (F), ONERA (F), NLR (NL), EVEKTOR (CZ), Cranfield University (GB), MPEI-Technical University (RUS) and Flughafen Braunschweig-Wolfsburg. All achievements will be performed in full-scale flight test on five aircrafts (VfW614 ATTAS, Dornier Do 228, Piaggio P.180, Fairchild Metro II, and Airbus A 380) and three helicopters (Eurocopter EC 135 ACT/FHS, Eurocopter Superpuma, MBB Bo 105).

Although several sophisticated investigations for quite different applications have been performed by means of the described instrumentation [1, 2, 6, 8], in this paper only one of AIM´s tasks will be discussed because of high interest to the High-Speed Imaging Community: the optical measurement of deformation of rotating propeller blades..

2. THE INVESTIGATED OBJECTS 2.1 Small turbo-propeller aircraft Piaggio P.180

The Piaggio P.180 Avanti (Fig. 1) is a remarkably efficient twin-engine business aircraft with an unusual three lift surfaces configuration. It seats up to nine passengers in a pressurized cabin, and may be flown by one or two pilots. The Avanti has two turboprop Pratt & Whitney engines PT6A-66 performing up to 850 HP each placed on the wing in a pusher configuration. Due to the laminar flow design of the complete aircraft its max. speed is 732 km/h, but fuel consumption is reduced by 40 % relative to the jet aircrafts of this size.

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Fig.1. The Piaggio P.180 Avanti. Notice its unusual three lifting surface pusher configuration

The P.180 makes a distinctive square wave noise when passing overhead, due to wing wake and engine exhaust interaction effects on the propeller which causes some additional deformation of the propeller blades passing this stream. This deformation is an interesting phenomena for both the researchers and industrial engineers. Therefore the AIM-Partners made an effort and designed and manufactured a special instrumentation capable for non-intrusive measurement of the blade deformation in-flight.

2.2 Helicopter EC 135

Besides the investigation of the deformation of propeller blades during free flight (like done at the P.180) the measurement of the deformation of the main rotor blades of a flying helicopter is of high interest as well. The rotor blades are heavy loaded due to strong centrifugal forces and periodically changing lift. Especially in fast forward flight of the helicopter a rotor blade passes several load states while rotating. The knowledge about varying loads on the blades is required by helicopter manufacturers who want to design rotor blades in an optimal way.

The EC 135 (Fig.2) is a small and modern rotorcraft with a take off weight of approximately 2.6 tons. Due to its flight performance and its low noise level it is commonly used for civil rescue and business trans-portation tasks. The DLR owned EC 135 ACT / FHS has an integrated fly-by-wire fly-by-light control sys-tem which can be used to simulate the flight perform-ance of other rotorcrafts. It is equipped with a highly sophisticated flight data recording system too. These properties make the EC 135 ACT / FHS a perfect test platform to apply new advanced measurement techniques to flight testing, thus it is used to test the applicability of optical deformation measurement techniques like the image pattern correlation technique IPCT.

Fig.2. The Eurocopter EC 135 ACT / FHS

3. IMAGE PATTERN CORRELATION TECHNIQUE 3.1 Principle of IPCT

The Image Pattern Correlation Technique (IPCT) is an optical, non-intrusive measurement technique. It is based on photogrammetry in combination with modern correlation algorithms developed for Particle Image Velocimetry (PIV).

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* a b c

Fig. 3. Principle of the IPCT a) reference image b) deformed image c) displacement vectors

The simplest IPCT setup consists of one monochrome camera observing an object to be investigated covered with a random pattern. After a reference image of a latent object has been acquired (Fig. 3a) the object will be set into a load condition which causes its deformation. The second image (Fig. 3b) or a sequence of images will be recorded under such deformed states. The image(s) of the deformed object will be cross-correlated with the reference image. A 2D displace-ment vector field (Fig. 3c) will be obtained as the result. Using image pairs of the randomly patterned object acquired by a stereoscopic camera system, its 3D position and shape can be obtained.

The basic principle of this stereoscopic IPCT can briefly be explained by referring to its similarity to the spatial vision in humans. Objects in the field of view are simultaneously observed under two different viewing angles. Similar patterns in both images are detected. If the viewing positions and the mapping functions of both optical systems are known, the 3D coordinates of the similar pattern identified in both images can be calculated by means of triangulation.

Figure 4 schematically shows the functionality of stereoscopic IPCT. Next a more detailed description shall be given.

Fig. 4. Procedure of stereoscopic IPCT

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During the first step the investigated surface H(X;Y) is recorded by two cameras (camera 1 and camera 2). Both cameras are looking at the same field of view, but under different viewing angles. A cross correlation algorithm identifies similar regions in the recorded images of both cameras. For best results a random dot pattern is applied onto the surface. The correlation procedure delivers the coordinates of areas with similar dot pattern in image 1 (coordinates x1, y1) and in image 2 (coordinates x2, y2). With known intrinsic parameters (e.g. focal length, distortion, principal point) and extrinsic parameters (position and orientation) of both cameras, the 3D coordinates of the recognized areas with the same dot pattern are determined by means of central projection and triangulation.

The application of the described algorithm to all areas in the image depicting the same dot pattern regions on the surface finally yields to a high accurate reconstruction of the complete 3D surface. By comparing the measured 3D surface under an unstressed reference state (e.g. a still propeller blade of the aircraft standing on ground) to the surface under load con-ditions (e.g. the same blade of spinning propeller), the displacement vectors and thus deformations can be deduced with a high accuracy. If the material characteristics of the observed object are known the local stress can be calculated as well.

3.2 Random dot pattern for IPCT

Although IPCT is a robust method, for good results the used pattern must be optimized. It must provide a high contrast and sufficient density of information. The experiments have shown that circular dots of equal diameter are advantageous. The diameter of the dots has to be chosen dependent on the camera resolution and its field of view. The dot size and the dot density (number of dots per surface unit) have to be optimized for sufficient spatial resolution of the result. Both, black dots on white surfaces (Fig. 5) and white dots on black surfaces (Fig. 6) lead to similar accuracy and spatial reso-lution of the results.

In case of high inclination of the cameras optical axis to the surface of the investigated object for large fields of view (e.g. IPCT imaging of the aircraft wing from the fuselage) the standard dot pattern can not provide satisfactory results. The dots on the areas near to the camera appear too large providing too low spatial resolution whereas the dots situated far away will not be resolved because of limited resolution and depth of focus of the camera and hence provide no information. For this case a special dot pattern called "cauliflower pattern" shown in Fig. 7 has been designed [7]. This pattern is advantageous also for IPCT measurements of moving objects whose distance from the camera position changes in a significant range.

Fig. 5. Positive random dot pattern

Fig. 6. Negative random dot pattern Fig. 7. Universal "cauliflower" dot pattern

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4. THE IMAGING SYSTEM 4.1 Principle of operation

The IPCT images of the rotating propeller blade as well as the images of the helicopter main rotor blade have been acquired by means of the existing High-Speed Video-Stroboscope [3, 8]. It can control simultaneously up to four cameras of CV-M10, CV-A1 or CV-A2 type manufactured by JAI company.

The principle of this apparatus is shown in Fig. 8 and the apparatus itself in Fig. 9. The shutter of the electronic camera(s) is controlled by the trigger signal received from the rotating object. The position of the object on the frame can be varied by means of the phase shift between the trigger pulses coming from the object and the camera shutter control pulses.

The components of the system are designed as PCI / PCIe plug-in cards and placed within a 19" industrial computer case. For in-flight testing the system has been extended by a 28 V DC power supply and by an internal uninterruptible power supply (UPS) maintaining power for 10 min. Moreover a shockproof solid state flash memory drive has been installed to prevent data loss. For the subsequent data transfer (for evaluation) the system comprises two removable hard disk drives (ATA / S-ATA).

Fig. 8. Block circuit of the video stroboscope Fig. 9. Video stroboscope

4.2 Selection of the camera

For static deformation or for very slow object movement any standard camera can be applied. Special requirements appear for imaging of fast moving objects. In this case the shutter of the camera must be precisely synchronized with the movement of the object and the image integration time must be short enough to prevent the motion smear (Fig. 10).

At the nominal propeller speed of 2000 rpm the linear velocity of the dot pattern on the tip of the blade is as high as 250 ms-1. i.e. 0.25 mm µs-1. To keep the motion smear of the dots lower than 1 mm (related to the pattern plane) the image integration time should not exceed 4 µs. Moreover, the camera shutter must be precisely synchronized with the rotation of the propeller to record the propeller always in the same position.

low smear

high smear

There are only a few CCD cameras available on the market providing both, short shutter time and an asynchronous reset / restart capability. One of the most suitable cameras available since several years is the cost effective camera type CV-M10. It provides a very

dot pattern

Fig. 10. The dot smear caused by the object motion

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short integration time of 1.25 µs and immediate shutter function i.e. response on the external trigger pulse with nearly no delay. However the spatial resolution of this camera is 760 x 580 pixels² which can be insufficient in many applications.

The JAI company offers also pin-compatible cameras CV-A1 and CV-A2 with significantly higher resolution of 1380 x 1035 pixels² and 1620 x 1220 pixels² respectively. Although both cameras have been released much later than mentioned CV-M10, their shutter function is antediluvian and not applicable for fast phase-locked imaging. The perma-nent internal combination of the shutter function with the line scan is a heavy drawback in the triggered imaging of high-speed events, because it causes a time jitter which results in a random distribution of the object position on the recorded images. Moreover, the shutter time in the range of microseconds is not available in the asynchronous, i.e. triggered mode of camera operation.

b c a d

Fig. 11. Modification of cameras CV-A1 and CV-A2 a) original camera, b) PCB with new timing generator and CCD controller,

c) additional housing, d) improved camera

However, the high resolution of these cameras and their compatibility with CV-M10 encouraged us to upgrade their electronics ourselves. Both cameras have received an additional PCB with improved timing generator and with a new circuit controlling their CCD-sensors (Fig. 11).

Furthermore a new library file (dll) has been written enabling for the integration of the new functions and parameters of these cameras into the existing imaging hard- and software. After this upgrade the cameras provide very short integration time of about 1 µs and no jitter of the shutter function. Thus they can be now used for triggered, e.g. stroboscopic imaging of very fast objects. Besides these improvements the cameras are now ready for PIV measurements as well since they can be operated in the double frame mode acquiring two images with very short inter-frame time of two microseconds only.

4.3 The frame grabber

The frame grabber of PCIe type (Fig. 12) used in the video stroboscope can simultaneously acquire images from up to four synchronized or non-synchronized cameras of type JAI CV-M10, JAI CV-A1 or JAI CV-A2 supporting their reset / restart functions. Both upgraded and non-upgraded cameras can be used; the camera type will be detected automatically. An additional module has been designed for fast frame grabber configuration including a double frame PIV function. The on-board memory of 256 MB DRAM enables the frame grabber to take full advantage of the PCIe bandwidth.

4.4 The phase shifter and GPS module

The digital phase shifter [4] (Fig. 13) has been designed for the video stroboscope system [3, 6] to display the fast moving objects in real time in slow motion. As the programmed phase shift does not depend on the object frequency, the position of the selected blade does not move within the viewing area when the trigger frequency, i.e. the speed of the

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FG extension module

Fig. 12. A four channel PCIe frame grabber with extension module

propeller, changes. The function of the phase shifter is fully controlled by the video stroboscope software. Therefore the number of investigated blades as well as the blade position on the screen can be changed by software very easily even during the recording of the images.

Besides the phase shifter circuit, the board contains a programmable digital frequency synthesizer (DDS) providing synchronized sine and square signals. These signals can be used for the control of electro-mechanical actuators e.g. for the ground vibration test of an aircraft wing with simultaneous stroboscopic image acquisition.

Moreover, a universal counter has been implemented on this card. This counter can, for instance, be used to measure and record the object trigger frequency. In our case the propeller speed can be simultaneously measured enabling for a speed stamp (label) on each acquired frame.

The phase shifter has been extended recently by a specially developed GPS / IRIG-B receiver module. This module delivers the absolute time with a resolution of 1 millisecond and the aircraft position indications which are recorded simultaneously with the acquisition of images. This data ensures precise synchronization with other measurements carried out in the aircraft.

GPS-module

Fig. 13. Digital phase shifter and a GPS-module

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4.5 Object illumination

As mentioned previously the image integration time must be short enough to prevent the motion blur. To ensure a sufficient exposure a high illumination intensity must be provided. A xenon lamp properly synchronized with the camera shutter is a satisfactory choice. For the ground test an EG&G Model 2029 has been used. The light was output by means of a fiber optic (Fig. 15) and aligned for the optimum blade illumination. The image integration time was determined by the shutter time (selected in the range of 1.25 µs to 10 µs) which was shorter than the Xenon light pulse width of about 20 µs. For even shorter exposures the technique of frequency conversion of pulsed laser light by means of a dye as de-scribed in [5] seems to be a good solution. The application of this technique however, was not necessary in the case of the P.180 aircraft.

5. EXPERIMENTAL SET-UP For the IPCT measurement the blade must be covered with a random pattern. Therefore an adhesive foil sheet with a dot pattern has been affixed on each blade of the propeller (Fig. 14). All optical components i.e. two cameras on adjustable heads that can be turned and tilted, a laser sensor and a xenon lamp with fiber optic have been integrated to a rigid support by means of a stable frame made of Linos X-95 profiles. Both, the complete optical unit and the industrial computer have been mounted within the luggage compartment of the P.180 (Fig. 15). The optical components have been precisely positioned and adjusted to ensure free optical paths through the windows incorporated into the compartment cover and a sufficient view of the investigated part of the propeller blade.

Fig. 16 gives an overview of the setup used for stereoscopic IPCT measurements on the P.180.

As mentioned before, the recording system has to be precisely synchronized with the rotation of the propeller. This is realized by using a laser sensor pointing onto a reflecting

Fig. 16. The experimental set-up installed into the aircraft

Fig. 14. The aircraft ready for the test. The dot pattern can be seen on the propeller blades.

Fig. 15. The image acquisition equipment installed in the luggage compartment

camera 1 camera 2

laser sensor

fiber opticpattern

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pad on the propeller spinner. The proper phase shift between the trigger pulse and the corresponding image shot is being controlled by the phase shifter which triggers both cameras and the xenon lamp. As the phase shift does not depend on the signal frequency the selected blade will be imaged in the same position at each propeller speed. During the tests the propeller speed was changed from about 1000 up to 2000 rpm.

The images taken by both cameras are simultaneously digitized and stored within the frame grabber. Then they are transmitted to the computer memory. The acquired images can be viewed in real time on the screen of a notebook connected to the industrial computer via Ethernet. The same notebook enables to control and set all system parameters, as the direct access to the computer built into the luggage compartment is not possible.

The measurement setup for the EC 135 helicopter, which is shown in Fig. 17, is done in a similar way. A stripe of the above mentioned dot patterned adhesive foil is affixed on two of the main rotor blades (Fig. 18). Two cameras, one Alluris SMS 300 digital high-end stroboscope (both pointing to the patterned area) and one laser sensor (pointing to a reflecting pad on the rotor hub) are mounted on the helicopter's winch hard points by using a Linos X 95 supporting beam. The controlling of the cameras and the recording of the pictures is performed by the video stroboscope.

Fig. 17. The experimental set-up installed at the helicopter 1, 2 – mountings; 3 – support; 4, 5 – cameras; 6 – strobe light; 7 – laser sensor

Fig. 18. The pattern on the rotor blade

6. RESULTS AND DATA EVALUATION The images recorded during the testing at the P.180 and at the EC 135 had been processed with the stereoscopic IPCT mentioned above. The adopted commercial software DaVis distributed by LaVision had been used for this purpose. In addition to the measurement images the software needs a calibration of the stereoscopic camera setup which is done by recording a calibration target with a regular dot grid placed in front of both cameras.

With the extrinsic and intrinsic camera system parameters obtained by the calibration DaVis can easily calculate the 3D position and shape of the investigated propeller or rotor blade. Some recorded images and processed preliminary results are depicted in Fig. 19. It can be seen clearly that the propeller blade is recorded exactly in the same position for each revolution and at each rotational speed. Also the optical quality of the picture is quite good – no motion blur occurs and the illumination is sufficient. So the triggering works excellently without any jitter and the illumination by the xenon flash lamp in combination with the short shutter times is chosen in an optimal way. The results of the DaVis post processing accurately give the right surface of the propeller blade for every load state. The change of the blades angle of attack with increasing spin can be exactly observed from these obtained results.

After calculating the 3D surfaces of the observed object by using the stereoscopic image pair different recorded load states can be compared to an unloaded reference state and thus the deformation of the structure can be determined.

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Fig. 19. Samples of recorded images and processed results of the P.180 propeller blades

This procedure is shown in Fig. 20 where a sample of an image pair recorded at the EC 135 helicopter and a processed result are depicted. The 3D surface and its 3D position is calculated with DaVis by using the recorded image pair. To obtain the deformation both the recorded images and the calculated surface are used. In principle the software first transforms the dot pattern projected on the camera chip to the calculated 3D surface. This is done for the reference surface as well as for the loaded surface. Then the shift between the same patterned regions on the reference surface and on the loaded surface is calculated. The result of this process is the local deformation on the complete surface. The color code on the depicted surface shows the deformation of the blade surface obtained from the images of the loaded state with respect to the reference surface calculated from former images without any load. So IPCT can be used as a non-intrusive strain gauge.

Fig. 20. Sample of a recorded image pair and processed deformation results of the EC 135 rotor blade

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7. CONCLUSION The presented ground tests demonstrated the good performance of the described measurement system and the robustness of the IPCT. The obtained results give a preview to the potential of stereoscopic IPCT to measure planar deformations non-intrusively with a high accuracy within a short time. Even though the cameras for propeller imaging were situated in an unfavorable position in the Piaggio aircraft the acquired images are of good quality which is sufficient for the IPCT evaluation. The same applies to the images recorded at the helicopter. The camera resolution of 760 x 580 pixels² matched well the dot size of 2-3 mm at full blade view. A shutter time of 5 µs ensures motion blur free images, however the exposure is slightly too low (too dark images). The usage of higher illumination energy will help. An increased shutter time of 12.5 µs ensures a good image exposure at negligible motion blur. The further increase of the shutter time over 20 µs, e.g. to 50 µs leads to a full light energy exploitation of the 20-25 µs long strobe. Nevertheless, the acquired images show a motion blur clearly visible to the naked eye. However, such blur can be neglected in the relative IPC technique, where the reference image is taken at the same propeller speed i.e. provides equal blur.

Because of the limited time available for the feasibility tests only a rough preview recording could be done. Nevertheless, the collected data confirm the applicability of both the imaging system and the pattern used for the described task. As the next step both flight-tests at aircraft and helicopter are scheduled in Genova and Braunschweig respectively.

Due to their higher resolution the application of upgraded cameras JAI CV-A1 or JAI CV-A2 can be advantageous for the quantitative IPCT evaluation. These cameras will replace the CV-M10 in the future measurements.

Synchronization of image acquisition with other instrumentation installed on board of a research aircraft will be performed by means of GPS technique.

ACKNOWLEDGMENTS

The performed and presented feasibility study was part of the AIM project funded by the European Commission within the 6th framework (contract No. AST5-CT-2006-030827-AIM). The measurement technique as well as the applied in-strumentation had been developed by DLR using its own resources.

The authors would like to thank:

• Dr. Claudio Lanari and his co-workers (Piaggio Aero Industries, Italy) for providing the Aircraft P.180 for the tests, for intensive assistance and excellent cooperation

• Eurocopter and the DLR flight operations department in Braunschweig to provide all help we needed to perform the EC 135 test.

• our colleagues of the DLR department of Experimental Methods at the Institute of Aerodynamics and Flow Technology in Göttingen for all the theoretical, practical and administrative support.

REFERENCES

[1] Michaelis, D.; Frahnert, H., Stasicki, B. (2004): "Accuracy of Combined 3D Surface Deformation Measurement and 3D Position Tracking in a Wind Tunnel," ICEM12 - 12th International Conference on Experimental Mechanics, Politecnico di Bari, Italy

[2] Kirmse,T., Wagner, A.: "Advanced Methods for In-Flight Flap Gap and Wing Deformation Measurements in the Project AWIATOR," Conference Proceedings, CD-ROM (ceas 2007-206),S. 1 - 6 First CEAS European Air and Space Conference, Berlin (Germany), 10.-13. 09. 2007, ISSN 0700-4083 (2007).

[3] "Verwendung einer Videokamera und Vorrichtung zur stroboskopischen Aufzeichnung von Vorgängen," DLR-Patent DE 4309353.1

[4] "Digitaler Phasenschieber," DLR-Patent DE 195 44 642 [5] " Verfahren zum Abbilden eines Objekts auf einem Bildsensor und Lichtquelle zum Beleuchten eines Objekts mit

ultrakurzen Pulsen aus nicht kohärentem Licht," DLR patent application DE 10 2006 007 687.7-51

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[6] Stasicki, B.: "Investigation of fast, repetitive events by means of non-standard video techniques," 7th Int. Symposium on Fluid Control, Measurement and Visualization (FLUCOME), Sorrento, Italy, 25-28 August 2003, paper No.231, 7 pages, CD ROM, ISBN 0-9533991-4-1 (2003)

[7] "Messverfahren und Messanordnung mit einem stochastischen Punktmuster sowie stochastisches Punktmuster zur Verwendung dabei," DLR patent application DE 2007 056 777.6

[8] Stasicki, B.; Kirmse, T.; Frahnert, H.: "A multi-camera image acquisition system and its application for the investigation of flow related events," Rom Proc. (ISFV12-5.6), Optimage Ltd., Edinburgh, UK, S. 1-10 12th International Symposium on Flow Visualization, Göttingen, Germany, 10-14.09.2006, ISBN 0-9533991-8-4 (2006)

[9] Boden, F.; Kirmse, T.; Stasicki, B.; Lanari, C.: "Advanced optical in-flight measurements on deformation of wings and propeller blades, "19th EU SFTE Symposium, Manching, Germany, 22.-24.09.2008

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