Transactions on Modelling and Simulation vol 10, 1995 WIT Press, ISSN X

Transcription

1 Accuracy evaluation of a laser interferometry measurement system in long distance dynamic measurements A. Cunha, A. Laje, A. Gomes, E. Caetano Department of Civil Engineering, Porto University, R. Bragas, 4000 Porto, Portugal Abstract This paper, apart from stressing the interest of the use of laser interferometry techniques in Experimental Structural Dynamics, refers the evaluation of the level of precision provided by a laser sensor based on that kind of technique, in the development of not only short, but specially relatively long distance dynamic measurements. Simultaneously, an attempt to get further knowledge concerning the degree of importance of the nature and type of surfaces observed by the laser beam has also been made, using several steel and concrete sample surfaces, with different characteristics, in terms of rugosity and painting. The main results presented, specially expressed in terms of comparisons of frequency response and coherence function estimates obtained either with the laser sensor or with conventional piezoelectric accelerometers, and using a distance of observation of about 5 and 50 meters, clearly evidence the feasibility of application of this new laser system, in order to get accurate information about the dynamic structural response, either in time or in frequency domain, placing the laser sensor at a relatively long distance from the structure, although the use of simple polished steel sample surfaces is strongly recommended. 1 Introduction The recent development of laser equipments has contributed to remarkably amplify the capacity of capture of information concerning the structural motions, in comparison with the use of mechanical transducers, making feasible to obtain in real time high resolution images characterizing the dynamic behaviour of structures of Civil Engineering. Therefore, the Faculty of Engineering of Porto has created a new research project in order to introduce in Portugal this modern technology for the

2 284 Computational Methods and Experimental Measurements experimental observation of models and prototypes of civil engineering structures, like buildings, bridges or dams, permitting in particular a high resolution and accurate characterization of the structural motions, without direct contact with the structure, and allowing the obtainment of appropriate graphical images and the easy conjugation of the laser technology with modern data acquisition and processing systems available. In the context of this project, a new laser sensor has been acquired and several preliminary tests have been performed, based on a steel laboratory model, in order to evaluate the level of accuracy provided by this laser sensor in comparison with conventional piezoelectric accelerometers, either for short or relatively long distances of observation, and directing the laser beam onto different types of surfaces (using concrete or steel samples). It is then objective of this paper, apart from stressing the interest of the use of laser interferometry techniques in Experimental Structural Dynamics, to present some of the main results obtained, that evidence the possibility of getting accurate dynamic measurements with this new laser system, in the context of the experimental analysis and identification of civil engineering structures, even when relatively long distances of observation are considered. 2 The use of laser interferometry techniques for dynamic measurements in Civil Engineering Optical techniques are presently widely used for the measurement of displacements in full scale structures, particularly in the context of nondestructive testing (NDT). Laser Doppler Velocimetry (LDV) is a well known technique of this type, commonly applied in Fluid Dynamics for the determination of particle velocities, that may also be used for the development of dynamic measurements of large structures like bridges, buildings or aircrafts. This technique offers specific advantages over the established methods of experimental vibration analysis and modal testing, using discrete contacting accelerometers, namely: it permits non-contact measurements, avoiding either tedious attachment procedures, or the loading effect of attached transducers that may affect the true dynamic response of the tested structure; it is a generally high sensitive technique, allowing full-field measurements by laser scanning with high spatial resolution, producing multipoint data that can be input into existing structural analysis programs and improve the quality of the results; it can still be used in environments where the access to the tested structure may be difficult or dangerous. The principles behind the LDV technique are very simple. When a beam of coherent light is reflected from a moving surface, its frequency changes according to the well-known Doppler effect. Although thefractionalchange of the frequency of the light wave is very small (1 part in 10* or less), it can be measured very accurately using optical interferometry in conjunction with electronic frequency measurement equipments. The velocity of the moving surface is derived directly from the frequency changes.

3 Computational Methods and Experimental Measurements 285 The sensor used in this work (VPI laser sensor, system A, from OMETRON) is based on a Michelson interferometer, in which a laser beam is divided into reference and signal beams. The signal beam is directed onto the moving test surface, and the back-reflected light is combined with the internal reference beam. The frequency of the reflected beam is shifted by an amount Fd = 2v / X in accordance with the Doppler effect, where v is the velocity of the moving surface and X is the wavelength of the laser radiation. The intensity of the interference between the reference and signal beams is a signal with a frequency fy proportional to the velocity absolute value \v\. To also assess the sense of the surface motion, two independent detection channels of the interference intensity are used. The two channels are configured so that the output signals obtained are phase shifted by ±90, depending on the sense of the movement of the test surface. In practice, this is accomplished imposing an interferometer path difference presented to one channel, one quarter of a wavelength longer than the presented to the other. These signals are electronically mixed with a frequency carrier and the resulting frequency shifted Doppler signal is converted to an analog voltage directly proportional to the instantaneous value of the surface velocity. 3 Accuracy evaluation of the laser system 3.1 Case study; Definition and test objectives The evaluation of the accuracy provided by the VPI laser sensor has been done observing the dynamic behaviour of a laboratory model, schematically represented in Figure 1, consisting of a single storey plane frame with 1.5 m height and a span of 1.5 m, formed by steel HEB 120 profiles. The tests developed with that purpose have been performed in several phases, in order to achieve the following main objectives: - Preliminary evaluation of the level of precision provided by the laser system in short distance dynamic measurements; - Application of the laser system to the identification of modal parameters and comparison of the results with those obtained numerically; - Accuracy evaluation in relatively long distance dynamic measurements, getting some sensitivity with regard to the importance of the type and characteristics of the surfaces observed Accuracy evaluation in short distance dynamic measurements At a first phase of work, a preliminary evaluation of the level of precision provided by the new laser system was performed, using a distance of observation of 5m and analysing the dynamic behaviour of the model simultaneously with the VPI laser sensor and with piezoelectric accelerometers from Briiel & Kjaer, model 4379S.

4 286 Computational Methods and Experimental Measurements VW jcnsor X ]f I5JU 1 1 Loser beam ^ 3~* BAK Accct. Y m Fig. 1 - Schematic representation of the tested structure. The comparison of the results obtained through those two alternative procedures was made in terms of frequency response functions (FRF's) and coherence functions estimated using an impulsive load applied on a given point of the structure, assumed as a reference point, and measuring the corresponding structural response on several other points. The estimation of the frequency response functions was developed taking into consideration the structural discretization indicated on Figure 1 (dividing one column in 12 elements of equal length), and applying horizontally successive impulsive loads on node 1, with a PCB impulse hammer, model 086B50. The measurement of the structural response, in terms of horizontal velocities of vibration, was done successively on each of the 12 nodes considered on the column, using simultaneously the VPI laser sensor and the piezoelectric accelerometers from B&K, working as velocity transducers. Similar analog filtering conditions were adopted in both cases. The frequency content of the time signals captured was analysed in the frequency range Hz, with afrequencyresolution of 2 Hz Figure 2 presents an example of frequency response and coherence functions obtained, measuring the structural response at node 4. The observation of the FRF's and coherence functions estimates corresponding to the 12 points of measurement considered reveals a very close agreement between the results provided by the new laser system and by the conventional equipment, particularly in the frequency range Hz The higher discrepancies observed above the upper value of that interval, associated to the decrease of the coherence, are essentially due to a low frequency content of the input, out of that range. These results were already previously extensively presented [1,2] and were the basis for the application of this laser technique to the identification of modal parameters (natural frequencies and modal shapes) of the tested structure, a very close agreement between identified and numerical modal parameters having also been registred in this case [3].

5 Computational Methods and Experimental Measurements 287 FRFW ; C 12Q: 60 I 0 ft E -120; '^ B 0.8 uj tr FREQUENCY (Hz) Fig. 2 - Comparison of frequency response and coherence functions obtained using the VPI laser system () and the conventional equipment ( ) Accuracy evaluation in long distance dynamic measurements After getting confidence about the excellent level of precision provided by the VPI laser sensor in short distance dynamic measurements, new tests were performed in order to get sensitivity with regard to the influence of the distance of observation, as well as of the type and characteristics of the surfaces observed. For that purpose, the VPI laser sensor was placed at a distance of around 50 meters from the steel plane frame, and the comparison between results achieved simultaneously with the laser system and with the B&K accelerometers was always made based on the smoothed FRF estimates obtained applying the impulsive load horizontally at node 1 and measuring the corresponding structural response, in terms of horizontal velocity of vibration, at node 5. The frequency content of the time signals captured was analysed, in this case, in the frequency range Hz, with a frequency resolution of 1 Hz, and similar analogfilteringconditions were also adopted. In order to get information concerning the importance of the type and characteristics of the surface observed by the laser sensor, several surface samples were produced and applied at node 5 of the column with a magnet. Table 1 summarizes the main characetristics of some of those samples.

6 288 Computational Methods and Experimental Measurements Table 1 - Main characteristics of the sample surfaces used. Sample Material Steel Concrete Polished * * * Rugosity Soft Rough Without Painting Red White Reflecting From the observation of the whole set of smoothed FRF estimates, obtained with those 10 sample surfaces, making averages over 5 raw power spectra and cross spectra estimates in each case, it was possible to draw the following main conclusions: - The VPI laser sensor can effectively provide a good level of accuracy even for relatively long distances of observation, when using steel sample surfaces, permitting then to obtain reliable FRF estimates in rather close agreement with those achieved using the B&K piezoelectric accelerometers, although the corresponding coherence functions present relatively lower values due to a higher level of noise; - In this case, the best results were undoubtably obtained using the simple polished steel sample surface (Figure 3), and the use of white or reflecting ink didn't contribute to improve the results. However, the use of red ink seems to be an acceptable alternative; - The concrete surfaces are clearly inappropriate for the use of accurate vibration velovity measurements in structures with natural frequencies of interest in the considered range Hz, using the VPI laser sensor placed at great distances, as one can see observing Figure 4, in which FRF 1-5 estimates obtained with this sensor at distances of 5 and 50 m are compared; - On the other hand, the results corresponding to the 6 concrete sample surfaces reveal the inexistance of a strong sensitivity with regard either to the rugosity characteristics of the concrete surfaces, or to the type of ink used, as all the FRF estimates seem quite similar to those presented in Figure 4.

7 Freq Resgonst Computational Methods and Experimental Measurements 289 Fig. 3 - Smoothed FRF 1-5 and coherence function estimates corresponding to sample 1, obtained with the VPI laser sensor (at the bottom) and with the B&K accelerometers (at the top). Fig. 4 - Smoothed FRF 1-5 and coherence function estimates corresponding to sample 7, obtained with the VPI laser sensor (distance of observation equal to 5m, at the top, and to 50m, at the bottom).

8 290 Computational Methods and Experimental Measurements 4 Final remarks and trends for the conclusion of the present study The main objective of the present study has been the evaluation of the level of precision provided by a laser sensor, based on a laser interferometry technique, in the development of not only short, but specially relatively long distance dynamic measurements. Simultaneously, an attempt to get further knowledge concerning the degree of importance of the type and nature of surface observed by the laser beam has also been made, based on the use of different steel and concrete sample surfaces. The main results previously presented, specially expressed in terms of comparisons of FRF estimates obtained either with the VPI laser sensor or with B&K piezoelectric accelerometers, and using a distance of observation of 5 and 50 m, clearly evidence the feasibility of application of this new laser system, in order to get accurate information about the dynamic structural response, either in time or in frequency domain, placing the laser sensor at a relatively long distance from the structure, although the use of simple polished steel sample surfaces is strongly recommended. However, it was also possible to conclude that the level of accuracy provided by this sensor is also dependent on the range of frequencies analysed, decreasing at higher frequencies. Therefore, the study presented in this paper is being now completed with complementary tests, with two main objectives: - To develop an extensive analysis of sensitivity of the level of accuracy provided by the VPI laser sensor with regard to the frequency content of the structural response. For that purpose, an electro dynamic shaker has been used in order to excite the structure harmonically with a certain frequency, and appropriate software to control the laser scanning has been developed, permitting to obtain high resolution images characterizing the distribution of velocities of vibration in a large number of points of the structure, as well as the corresponding modal shapes; - To analyse the precision provided by the laser sensor when making dynamic measurements in structures with lower natural frequencies, typically in the range 0-50 Hz, as it usually happens in the structures of Civil Engineering. For that purpose, the present study is being completed with the analysis of the dynamic behaviour of a slender reinforced concrete tower, around 30m height. REFERENCES 1. Cunha, A. Accuracy Evaluation of a Laser Interferometry Measurement System in Long Distance Dynamic Measurements, Research Report, Porto University, Portugal, Cunha, A ; Caetano, E.; Laje, A. & Gomes, A. A Laser System for Dynamic Measurements on Civil Engineering Structures, Proceedings of the 10*h Int.

9 Computational Methods and Experimental Measurements 291 Conf. on Experimental Mechanics, pp , Lisbon, Portugal, Balkema Cunha, A; Caetano, E ; Laje, A & Gomes, A A Laser System for the Identification of Dynamic Parameters of Civil Engineering Structures, in ERCAD Berlin94, pp , Proceedings of the 2 Int. Conf. on Earthquake Resistant Construction and Design, Berlin, Germany, Balkema Drain, L.E. The Laser Doppler Technique, John Wiley & Sons, Ltd., ACKNOWLEDGEMENTS The present work has been done in the context of the development of the Laboratory of Structures of the Faculty of Engineering of Porto University, supported by the Portuguese institution JNICT on the basis of Programs CIENCIA and STRIDE, and involving the collaboration of other research centers, namely: Bundesanstalt fur Materialforschung und Priifung (BAM, Berlin), Virginia Polytechnic Institute and State University (VATECH, Blacksburg, USA), Joint Research Centre (Ispra) and Laboratorio Nacional de Engenharia Civil (LNEC, Lisbon). Thanks are due to those institutions and, particularly, to Prof Alfred Wicks, from VATECH, who encouraged the beginning of the Laser Project.