- Email: [email protected]
Design and use of an electronic speckle pattern interferometer for testing of turbine parts
Opacs and La3ers in Engineering 2 (1981) 1—12
DESIGN AND USE OF AN ELECTRONIC SPECKLE PATFERN INTERFEROMETER FOR TESTING OF TURBINE PARTS OLE J. LØKBERG
Physics Department, Norwegian Institute of Technology, N—7034 NTH, Trondheirn, Norway and PER SVENKE
Gas Turbine Division, Kongsberg Vaapenfabnk, N—3600 Kongsberg, Norway ABSTRACT
The construction and use of an electronic speckle pattern interferometer for vibration testing of critical gas turbine parts in the factory, is described.
LNTRODUCFION
In gas turbines and other rotating machinery it is of great importance to know the resonance frequencies of critical components. Due to the high temperatures and stresses, the rotor components constitute the most critical parts in a gas turbine. This means that at an early stage in the design and development process the natural frequencies of the turbine blades must be determined. Both frequency and nodal pattern must be known to avoid excitation of dangerous resonance frequencies within the turbine’s operating range. These vibration tests were originally done at the Gas Turbine Division, Kongsberg Vaapenfabrik by point measurements with piezoelectric transducers, while the final certification was performed with strain gauge measurements as the turbine was actually working. Apart from the usual uncertainty one gets from using point measurements, the procedure was time consuming and costly. It was therefore highly desirable to develop better and faster ways to do the initial testing of the wheels. The final qualification would still be done by strain gauge testing under running conditions. The optics group at the Norwegian Institute of Technology had considerable experience in vibration testing by different optical methods. This group was therefore to decide what optical method would be most suitable and afterwards construct a system for testing of the different turbine parts. 1 Optics and Lasers in Engineering 0143-8166/81/0002-0001/$0250---© Publishers Ltd, England, 1981 Printed in Northern Ireland
Applied
Science
2
OLE J. LØKBERG, PER SVENKE
In this paper we will describe the system chosen, the practical problems encountered and some of its specific uses.
SYSTEM REQUIREMENT AND METHODS CONSIDERED
The system should preferably meet the following requirements: (1) the fringe patterns depicting the vibrational modes should be easy to interpret, (2) the system should be rather insensitive to external disturbances as it is to be operated in industrial environments, (3) the measuring speed should be sufficiently high so that routine measurements could be performed if necessary without introducing delay in production, (4) the system’s operation and adjustment should be simple enough to be performed by people without optical training, and (5) the price tag, both in initial cost and running expenses, should be acceptable. A setup based upon conventional holographic interferometry was first considered, but rejected due to lack of operational speed and stability. Although the use of monobath developers cut the exposure—development time down to approximately 90 seconds for time averaged recordings, the procedure was too slow and cumbersome for the multiple parameters to be varied. The real-time holographic method using monobath developing in a liquid gate was fast enough, but the fringe quality was too low to make the vibration patterns easily interpreted by inexperienced workers. (The same drawback, by the way, ruled out the visual speckle interferometer.) In addition, to ensure a high rate of successful holograms in the industrial surrounding, the cost of anti-vibrational precautions would be rather high. A possible solution could be to use ordinary laser interferometry, which certainly would be faster in use than the transducer measurements. However, we would then be back to the point measurement again, and scanning procedures would be necessary for a sufficiently complete picture of the vibration patterns. The method considered to be best suited for the purpose and which was chosen, was Electronic Speckle Pattern Interferometry, usually shortened to ESPI. ESPI is a successful marriage between holography and videotechnique, which works in real-time and is rather insensitive to external disturbances. In its basic mode of operation, the ESPI system is easy to operate and adjust if properly designed. The vibration patterns are easily viewed as they are displayed on a TV monitor. This large size display is a valuable feature when a group of people want to simultaneously observe and discuss the interferograms. The only negative property of the method (at that time) was the rather low resolution of the fringe patterns compared to ordinary holographic interferometry. This was not considered to be a serious drawback as the system was
ELECTRONIC SPECKLE PATI’ERN INTERFEROMETRY IN TESTING OF TURBINE PARTS
3
to be used mainly for frequency mapping purposes. However, if high quality fringe patterns were desired either for stress calculations or displays, it should also be possible to make holographic recordings in the setup. ESPI PRINCIPLE
We will not plunge deeply into a description of the ESPI’s working principle here. The interested reader may consult references 1—3, for example. It suffices to say that ESPI in its vibrational mode of operation is wholly equivalent to (image) holography where the reference and image waves impinge on the TV
~~EIi::
Fig. 1(a).
Vibration of turbine blade as recorded by holographic interferometry.
4
OLE J. LØKBERG, PER SVEN’KE
7~.
flr~W
Fig. 1(h).
Vibration of turbine blade as recorded by ESPI.
target in the same direction. The two waves interfere and create an image interferogram, whose intensity is converted into a videosignal by the TV scanning process. This videosignal is subsequently high-pass filtered and rectified—a process which can be considered equivalent to reconstruction in holography. The processed videosignal is then fed into a TV monitor where we will see the image of a vibrating object covered with a fringe pattern, from which we can determine the distribution of vibration amplitudes across the object. The equivalence between an ESPI and a holographic recording is seen in Figs. 1(a) and (b), which show a turbine blade vibrating in its flapping mode. The brightest fringe or the zero order fringe at the base indicates the node and the nth bright fringe thereafter represents approximately a vibration amplitude
ELECTRONIC SPECKLE PATIERN INTERFEROMETRY IN TESTING OF TURBINE PARTS
5
of n (A/4), where A is the laser wavelength and we have assumed normal illumination and observation. Even if the quality of this particular holographic recording is rather low, we still see that the ESPI recording is more noisy and speckled. This is caused by the much lower resolution of the TV camera compared to the film emulsion (approximately 20 lines/mm v. 3000 lines/mm). ESPI’s tolerance towards an object’s instability and external disturbances, is brought about by the combination of short exposure (40 ms by European TV standard) and high repetition rate (25 Hz—European TV). The last statement really means that a new ‘hologram’ is recorded and displayed every 40 ms. The exposure freezes most unwanted movements, while the bad TV frames due to gross movements are usually masked by the subsequently good frames. These two features are also the basis for the real-time presentation which makes ESPI so ideally suited for vibration analysis where we want to vary frequency, excitation level and maybe phase, and immediately observe how these parameters affect the vibration pattern.
THE ESPI SETUP AND ITS OPERATION
Hardware The short delivery time did not permit construction of professional-looking, compact ESPI system. We choose to use standard commercially available optical and mechanical components whenever possible. In this way the system could be delivered almost immediately, based upon components primarily borrowed from the Optics Group. The lay-out for the system is shown in Fig. 2. The light from the 15 mW He—Ne laser is split into two parts by a variable beam-splitter, VB. The reflected beam is spread by a microscope lens, MB, and reflected by a small mirror, M1, to illuminate the object. Two simple lenses, L1 and L2, image the object on to the TV target through a slightly wedged glass plate, P. Lens L1 gives a real, demagnified image of the object, which is relayed by L2. By adjusting the position of the lenses and the camera, we could obtain an optimal image size on the monitor. Additional lenses were also furnished to obtain different magnifications if required. The beam transmitted through beam-splitter VB, is reflected by three mirrors M2—M3—M4 before being spread and filtered by a lens—pinhole combination, LP. The resulting spherical wave is then reflected by the glass wedge, P, on to the TV target to constitute the reference wave in the interferometer. (P is wedged 50 to avoid the back reflection of the reference wave hitting the target.) P is mounted on a small table which can be tilted and rotated to facilitate the alignment of the object and reference waves.
6
OLE J. LØKBERG, PER SVENKE
U
w 0
1)
0 •0
4 ~
~
141 -J
I.-~-
II
4 L&W
I
~uJ >~
I
I
U
ELECTRONIC SPECKLE PATUERN INTERFEROMETRY IN TESTING OF TURBINE PARTS
7
Conventionally, holographic recordings can be made in the setup by replacing P with a film holder, removing the imaging lenses and rearranging the illumination. Most of the components are mounted on triangular optical benches which are fastened by double tape to the table. The remaining components are mounted on magnetic holders. The table consists of a 3-mm-thick steel plate lying on top of a 30-mm-thick stone plate, size 1 X 2 m2. The steel plate with the optical setup on top can be rolled on the bigger stone plate by putting long, thin steel cylinders between the plates. This moving operation is occasionally used to make room for big objects. The table is supported by a wooden frame isolated from ground vibrations by 6 small inner tubes. Total weight of the setup is approximately 400 kg, where part of the load can be moved within the frame to level the table surface when objects of different weights are to be examined. As the TV registration medium we chose an RCA 4322 Si-tube mounted in a Telemation camera. The tube was picked mainly because of its high tolerance towards high light intensities, as we feared that direct laser light might hit the target by careless operation of the system. This tube is also rather sensitive to the red He—Ne laser light. Its greatest disadvantages are low resolution and internal reflections leading to fringes across the monitor. Even when a glass wedge was cemented on the front surface, very disturbing patterns could still be observed on most tubes. Fortunately, the magnitude of this effect varied from tube to tube and after testing 15 tubes (which represented the national storage), we picked the most acceptable tube. The camera had a separate sync—signal output. It could also be used with manual video—amplification which is useful at low light levels. The videosignal processor consisted of a bandpass filter with low—high frequencies 05/5 MHIz, followed by a double rectifier. As the sync—signal would be destroyed in the filter, the sync—signal from the camera is fed directly into the external sync—input of the TV monitor. The 9 inch Conrac TV monitor has high resolution, contrast and brilliance, features we have found necessary to obtain interferograms of high quality. At optimum conditions 5 fringe orders could be detected on simple movements like the flapping movement shown in Fig. 1. (The ESPI recording in Fig. 1 is obtained by use of an improved system with a TV target of higher resolution.) More typical recordings are shown in Fig. 3 (a) and (b) representing vibrations of the turbine blade most extensively investigated. If more accurate amplitude mapping and phase measurements are desired, this can be achieved by phase modulation techniques. Mirror M 3 in Fig. 2 is then replaced by a small, lightweight mirror mounted on the centre of a high frequency loudspeaker, and the amplitude—phase measurement techniques described in References. 4 and 5 are used.
8
OLE J LOKRERG. PER SVFNKE
(a)
—~ *
Fig. 3.
(h) Typical recordings of turbine blade vibrations in the ESPI system.
ELECTRONIC SPECKLE PATtERN INTERFEROMETRY IN TESTING OF TURBINE PARTS
9
System operation The ESPI system has been extensively used since 1976 without any malfunctioning occurring (except for a clogged up pinhole). It has been adjusted and operated by technicians and mechanics with no formal optical training, this being outweighed by their enthusiasm in working with a laser instrument. The operators were instructed to treat the laser light with non-hysterical respect. The setup was placed in a room in a position where it would be very difficult to look directly into the direct laser beam. The system can be operated in subdued room light which adds to the laser safety. Usually the lamp on the operator’s desk is on. The greatest practical problem is caused by the unexpected high magnitudes of the ground vibrations, especially the mini-earthquakes caused by turning of ship axles in a nearby building. In a few instances the measurements have had to be postponed until the turning stopped. Under normal working conditions the stability of the optical setup is fully satisfactory, as shown in Fig. 4, which shows one of the authors sitting in the setup without disturbing the vibration pattern on the TV monitor. The objects to be investigated are usually rather heavy, 20—70 kg, and have to be hoisted on to the table where they are fastened to an identical fixture to that of the turbine. Before that, the object’s surface has been lightly coated with
II
__I~!w
‘S..
,-.-~-~-,---
Fig. 4.
i Demonstration of the system’s stability.
10
OLE J. LØKBERG, PER SVENKE
3M ‘Codit’ retro-reflective paint. The coating is used to get high uniform reflections from the curved surfaces. This is especially important for some turbine wheels where the blades have to be illuminated and observed at steep angles to get a full view of one blade without obscuration from its neighbours. Approximately 30 x 30 cm2 can be recorded simultaneously with the laser power available. This corresponds to 4 blades on most wheels, but as mentioned above only one blade is usually examined at a time. The object is excited by a single piezoelectric crystal fastened by beeswax. Considerable testing was done to determine the position of the exciter which gave optimum excitation of the interesting resonances. The operator’s adjustment of set-up starts with equalising the object and reference wave pathlengths and focusing the object image (these steps are necessary only when the object’s size requires a different magnification, usually the object’s position is kept fixed). If the weight of the new object is different, the reference wave pinhole might need realignment due to a different bending of the rather thin optical table. Finally the operator checks, and if necessary adjusts, the reference wave’s in-line position with the object wave. The object—reference ratio is then set by rotating the variable beam-splitter until the interferograms on the monitor have optimum contrast. It should be noted that when the objects have the same size and weight and care is observed in the mounting—demounting stages, all these adjustments are unnecessary. In the beginning, the frequency mapping procedure was thorough with video recording and picture taking of all resonances. However, the amount of information acquired soon outgrew the analysing capacity. A simple procedure was then adopted. The operator varies the frequency until the first resonance is detected, the frequency is carefully adjusted at constant excitation level until maximum amplitude is obtained. This frequency value is registered as fres. The excitation is then lowered until the fringe pattern shows only a black fringe. This excitation voltage is registered as Vres. The operator then proceeds to the next resonance and repeats the procedure. Only new unexpected vibration patterns are photographed or sketched. This procedure maps the resonance frequencies quite accurately (±1Hz with care), the excitation value is more uncertain, in the order of 10—15%. It could be considerably improved by also measuring the excitation of higher fringe orders using phase modulation. However, variation due to differences in excitator position was considered to be a greater source of error. In addition, these excitation values are used only as a guide to which resonances are easily excited. Under working conditions, the turbine blades are subject to high temperatures (+ 700°C) and centrifugal forces. These two factors will change the resonance frequencies in opposite ways. A temperature increase will decrease the frequencies while the stiffening effect of the centrifugal forces results in
ELECTRONIC SPECKLE PATFERN INTERFEROMETRY IN TESTING OF TURBINE PARTS
11
frequency increase (somewhat dependent on the mode shape). The combined effect gives a net increase in frequency value which has been determined by theory and experiment for the frequencies of interest. Using this correction factor, the resonance frequencies as determined by ESPI and by strain gauge measurements under working conditions are comparable within 1—2%. The long range goal is to use ESPI while the turbine is running so as to avoid the costly and cumbersome strain—gauge instrumentation. The high temperature and turbulence should not be a serious hindrance as we have heated objects to well above + 700°Cwith a blowtorch and still observed the vibration pattern using chopped exposure ESPI.6 However, the interior construction of the gas turbine makes it extremely difficult if not impossible to stabiise the rotating image by derotator techniques.
SPECIFIC APPLICATIONS OF THE SYSTEM
In a certain period of the development of a new gas turbine at Kongsberg Vaapenfabrik, the ESPI system was extensively used to check the vibration characteristics. All initial rotating and static parts were examined at the prototype stage in order to determine potential resonant natural frequencies. In some cases it was necessary to modify the prototype design in order to avoid resonance conditions and in this work the ESPI system appeared especially useful. The effect of minor changes in geometry such as cutting off material and increasing local stiffnesses could be seen immediately on the TV monitor making the process of modification very effective. In one case it was necessary to increase the overall thickness of a part to obtain the required stiffness and in this case a simple Al-model was made to check if the increased thickness had given the desired effect. The result was seen to be consistent with the later actual parts. In the initial development period all critical parts were checked in order to establish a baseline for production tolerances, etc., but after this only sample checks are made. This use of the ESPI system in the design and development of the new KG5 gas turbine has proved very successful as there has been no vibration failure in any gas turbine after more than 15 000 hours of accumulated experience.
CONCLUDING REMARKS
In this paper we have described how a simple ESPI system has been constructed and used for testing and development of gas turbine components. The theme of the project itself, vibration measurements of, for example turbine
12
OLE J. LØKBERG, PER SVENKE
blades, can hardly be considered new as it is a rather classical holographic study. The basic use of the system has also been rather simple, like using Chiadni figures for vibration testing but with the added ability of finding the excitation sensitivity of the antinodes. However, we find the easy handling and use of the equipment to be excellent proof of how well ESPI can be used for testing in industrial environments, if necessary on a routine basis. The system in its present form has two weak features; it is not sufficiently stable and its light economy and resolution are not adequate to inspect an entire wheel which is sometimes desired. This will be corrected by the delivery of a new ESPI system based upon the speckle reference principle suggested by Slettemoen.7 This system will, in addition, be compact and extremely easy to adjust.
ACKNOWLEDGEMENTS
Alnong the many persons supporting this project, we want especially to thank 0. M. Holje and B. Stâlaker for help and advice during the construction of the apparatus. One of us (0. J. L.) wants to thank the Electrical Engineering Department, University of Minnesota and Optical Science Centre, University of Arizona for support as most of the writing was done during his stay at these institutions.
REFERENCES
1.
J. N. BUTLERS, Proc. Engineering Uses of Coherent Optics, Ed. E. R. Robertson, Cambridge Univ. Press, London, 1976, pp. 155. 2. J. N. BUTtERS, R. Jo~s and C. Wyi~s,Speckle Metrology, Ed. R. K. Erf, Academic Press, London, 1978, pp. 111. 3. 0. J. LOKBERG, Phys. Technol, 11 (1980) 16. 4. 0. J. LØKBERG and K. HOGMOEN, J. Phys. E. 9 (1976) 847. 5. 0. J. LØKBERG and K. HØGMOEN, Appi. Opt., 15 (1976) 2701. 6. 0. J. LØKBERG, App). Opt., 18 (1979) 2377. 7. G. A. SLETrEMOEN, Appi. Opt., 19 (1980) 616.
DESIGN AND USE OF AN ELECTRONIC SPECKLE PATFERN INTERFEROMETER FOR TESTING OF TURBINE PARTS OLE J. LØKBERG
Physics Department, Norwegian Institute of Technology, N—7034 NTH, Trondheirn, Norway and PER SVENKE
Gas Turbine Division, Kongsberg Vaapenfabnk, N—3600 Kongsberg, Norway ABSTRACT
The construction and use of an electronic speckle pattern interferometer for vibration testing of critical gas turbine parts in the factory, is described.
LNTRODUCFION
In gas turbines and other rotating machinery it is of great importance to know the resonance frequencies of critical components. Due to the high temperatures and stresses, the rotor components constitute the most critical parts in a gas turbine. This means that at an early stage in the design and development process the natural frequencies of the turbine blades must be determined. Both frequency and nodal pattern must be known to avoid excitation of dangerous resonance frequencies within the turbine’s operating range. These vibration tests were originally done at the Gas Turbine Division, Kongsberg Vaapenfabrik by point measurements with piezoelectric transducers, while the final certification was performed with strain gauge measurements as the turbine was actually working. Apart from the usual uncertainty one gets from using point measurements, the procedure was time consuming and costly. It was therefore highly desirable to develop better and faster ways to do the initial testing of the wheels. The final qualification would still be done by strain gauge testing under running conditions. The optics group at the Norwegian Institute of Technology had considerable experience in vibration testing by different optical methods. This group was therefore to decide what optical method would be most suitable and afterwards construct a system for testing of the different turbine parts. 1 Optics and Lasers in Engineering 0143-8166/81/0002-0001/$0250---© Publishers Ltd, England, 1981 Printed in Northern Ireland
Applied
Science
2
OLE J. LØKBERG, PER SVENKE
In this paper we will describe the system chosen, the practical problems encountered and some of its specific uses.
SYSTEM REQUIREMENT AND METHODS CONSIDERED
The system should preferably meet the following requirements: (1) the fringe patterns depicting the vibrational modes should be easy to interpret, (2) the system should be rather insensitive to external disturbances as it is to be operated in industrial environments, (3) the measuring speed should be sufficiently high so that routine measurements could be performed if necessary without introducing delay in production, (4) the system’s operation and adjustment should be simple enough to be performed by people without optical training, and (5) the price tag, both in initial cost and running expenses, should be acceptable. A setup based upon conventional holographic interferometry was first considered, but rejected due to lack of operational speed and stability. Although the use of monobath developers cut the exposure—development time down to approximately 90 seconds for time averaged recordings, the procedure was too slow and cumbersome for the multiple parameters to be varied. The real-time holographic method using monobath developing in a liquid gate was fast enough, but the fringe quality was too low to make the vibration patterns easily interpreted by inexperienced workers. (The same drawback, by the way, ruled out the visual speckle interferometer.) In addition, to ensure a high rate of successful holograms in the industrial surrounding, the cost of anti-vibrational precautions would be rather high. A possible solution could be to use ordinary laser interferometry, which certainly would be faster in use than the transducer measurements. However, we would then be back to the point measurement again, and scanning procedures would be necessary for a sufficiently complete picture of the vibration patterns. The method considered to be best suited for the purpose and which was chosen, was Electronic Speckle Pattern Interferometry, usually shortened to ESPI. ESPI is a successful marriage between holography and videotechnique, which works in real-time and is rather insensitive to external disturbances. In its basic mode of operation, the ESPI system is easy to operate and adjust if properly designed. The vibration patterns are easily viewed as they are displayed on a TV monitor. This large size display is a valuable feature when a group of people want to simultaneously observe and discuss the interferograms. The only negative property of the method (at that time) was the rather low resolution of the fringe patterns compared to ordinary holographic interferometry. This was not considered to be a serious drawback as the system was
ELECTRONIC SPECKLE PATI’ERN INTERFEROMETRY IN TESTING OF TURBINE PARTS
3
to be used mainly for frequency mapping purposes. However, if high quality fringe patterns were desired either for stress calculations or displays, it should also be possible to make holographic recordings in the setup. ESPI PRINCIPLE
We will not plunge deeply into a description of the ESPI’s working principle here. The interested reader may consult references 1—3, for example. It suffices to say that ESPI in its vibrational mode of operation is wholly equivalent to (image) holography where the reference and image waves impinge on the TV
~~EIi::
Fig. 1(a).
Vibration of turbine blade as recorded by holographic interferometry.
4
OLE J. LØKBERG, PER SVEN’KE
7~.
flr~W
Fig. 1(h).
Vibration of turbine blade as recorded by ESPI.
target in the same direction. The two waves interfere and create an image interferogram, whose intensity is converted into a videosignal by the TV scanning process. This videosignal is subsequently high-pass filtered and rectified—a process which can be considered equivalent to reconstruction in holography. The processed videosignal is then fed into a TV monitor where we will see the image of a vibrating object covered with a fringe pattern, from which we can determine the distribution of vibration amplitudes across the object. The equivalence between an ESPI and a holographic recording is seen in Figs. 1(a) and (b), which show a turbine blade vibrating in its flapping mode. The brightest fringe or the zero order fringe at the base indicates the node and the nth bright fringe thereafter represents approximately a vibration amplitude
ELECTRONIC SPECKLE PATIERN INTERFEROMETRY IN TESTING OF TURBINE PARTS
5
of n (A/4), where A is the laser wavelength and we have assumed normal illumination and observation. Even if the quality of this particular holographic recording is rather low, we still see that the ESPI recording is more noisy and speckled. This is caused by the much lower resolution of the TV camera compared to the film emulsion (approximately 20 lines/mm v. 3000 lines/mm). ESPI’s tolerance towards an object’s instability and external disturbances, is brought about by the combination of short exposure (40 ms by European TV standard) and high repetition rate (25 Hz—European TV). The last statement really means that a new ‘hologram’ is recorded and displayed every 40 ms. The exposure freezes most unwanted movements, while the bad TV frames due to gross movements are usually masked by the subsequently good frames. These two features are also the basis for the real-time presentation which makes ESPI so ideally suited for vibration analysis where we want to vary frequency, excitation level and maybe phase, and immediately observe how these parameters affect the vibration pattern.
THE ESPI SETUP AND ITS OPERATION
Hardware The short delivery time did not permit construction of professional-looking, compact ESPI system. We choose to use standard commercially available optical and mechanical components whenever possible. In this way the system could be delivered almost immediately, based upon components primarily borrowed from the Optics Group. The lay-out for the system is shown in Fig. 2. The light from the 15 mW He—Ne laser is split into two parts by a variable beam-splitter, VB. The reflected beam is spread by a microscope lens, MB, and reflected by a small mirror, M1, to illuminate the object. Two simple lenses, L1 and L2, image the object on to the TV target through a slightly wedged glass plate, P. Lens L1 gives a real, demagnified image of the object, which is relayed by L2. By adjusting the position of the lenses and the camera, we could obtain an optimal image size on the monitor. Additional lenses were also furnished to obtain different magnifications if required. The beam transmitted through beam-splitter VB, is reflected by three mirrors M2—M3—M4 before being spread and filtered by a lens—pinhole combination, LP. The resulting spherical wave is then reflected by the glass wedge, P, on to the TV target to constitute the reference wave in the interferometer. (P is wedged 50 to avoid the back reflection of the reference wave hitting the target.) P is mounted on a small table which can be tilted and rotated to facilitate the alignment of the object and reference waves.
6
OLE J. LØKBERG, PER SVENKE
U
w 0
1)
0 •0
4 ~
~
141 -J
I.-~-
II
4 L&W
I
~uJ >~
I
I
U
ELECTRONIC SPECKLE PATUERN INTERFEROMETRY IN TESTING OF TURBINE PARTS
7
Conventionally, holographic recordings can be made in the setup by replacing P with a film holder, removing the imaging lenses and rearranging the illumination. Most of the components are mounted on triangular optical benches which are fastened by double tape to the table. The remaining components are mounted on magnetic holders. The table consists of a 3-mm-thick steel plate lying on top of a 30-mm-thick stone plate, size 1 X 2 m2. The steel plate with the optical setup on top can be rolled on the bigger stone plate by putting long, thin steel cylinders between the plates. This moving operation is occasionally used to make room for big objects. The table is supported by a wooden frame isolated from ground vibrations by 6 small inner tubes. Total weight of the setup is approximately 400 kg, where part of the load can be moved within the frame to level the table surface when objects of different weights are to be examined. As the TV registration medium we chose an RCA 4322 Si-tube mounted in a Telemation camera. The tube was picked mainly because of its high tolerance towards high light intensities, as we feared that direct laser light might hit the target by careless operation of the system. This tube is also rather sensitive to the red He—Ne laser light. Its greatest disadvantages are low resolution and internal reflections leading to fringes across the monitor. Even when a glass wedge was cemented on the front surface, very disturbing patterns could still be observed on most tubes. Fortunately, the magnitude of this effect varied from tube to tube and after testing 15 tubes (which represented the national storage), we picked the most acceptable tube. The camera had a separate sync—signal output. It could also be used with manual video—amplification which is useful at low light levels. The videosignal processor consisted of a bandpass filter with low—high frequencies 05/5 MHIz, followed by a double rectifier. As the sync—signal would be destroyed in the filter, the sync—signal from the camera is fed directly into the external sync—input of the TV monitor. The 9 inch Conrac TV monitor has high resolution, contrast and brilliance, features we have found necessary to obtain interferograms of high quality. At optimum conditions 5 fringe orders could be detected on simple movements like the flapping movement shown in Fig. 1. (The ESPI recording in Fig. 1 is obtained by use of an improved system with a TV target of higher resolution.) More typical recordings are shown in Fig. 3 (a) and (b) representing vibrations of the turbine blade most extensively investigated. If more accurate amplitude mapping and phase measurements are desired, this can be achieved by phase modulation techniques. Mirror M 3 in Fig. 2 is then replaced by a small, lightweight mirror mounted on the centre of a high frequency loudspeaker, and the amplitude—phase measurement techniques described in References. 4 and 5 are used.
8
OLE J LOKRERG. PER SVFNKE
(a)
—~ *
Fig. 3.
(h) Typical recordings of turbine blade vibrations in the ESPI system.
ELECTRONIC SPECKLE PATtERN INTERFEROMETRY IN TESTING OF TURBINE PARTS
9
System operation The ESPI system has been extensively used since 1976 without any malfunctioning occurring (except for a clogged up pinhole). It has been adjusted and operated by technicians and mechanics with no formal optical training, this being outweighed by their enthusiasm in working with a laser instrument. The operators were instructed to treat the laser light with non-hysterical respect. The setup was placed in a room in a position where it would be very difficult to look directly into the direct laser beam. The system can be operated in subdued room light which adds to the laser safety. Usually the lamp on the operator’s desk is on. The greatest practical problem is caused by the unexpected high magnitudes of the ground vibrations, especially the mini-earthquakes caused by turning of ship axles in a nearby building. In a few instances the measurements have had to be postponed until the turning stopped. Under normal working conditions the stability of the optical setup is fully satisfactory, as shown in Fig. 4, which shows one of the authors sitting in the setup without disturbing the vibration pattern on the TV monitor. The objects to be investigated are usually rather heavy, 20—70 kg, and have to be hoisted on to the table where they are fastened to an identical fixture to that of the turbine. Before that, the object’s surface has been lightly coated with
II
__I~!w
‘S..
,-.-~-~-,---
Fig. 4.
i Demonstration of the system’s stability.
10
OLE J. LØKBERG, PER SVENKE
3M ‘Codit’ retro-reflective paint. The coating is used to get high uniform reflections from the curved surfaces. This is especially important for some turbine wheels where the blades have to be illuminated and observed at steep angles to get a full view of one blade without obscuration from its neighbours. Approximately 30 x 30 cm2 can be recorded simultaneously with the laser power available. This corresponds to 4 blades on most wheels, but as mentioned above only one blade is usually examined at a time. The object is excited by a single piezoelectric crystal fastened by beeswax. Considerable testing was done to determine the position of the exciter which gave optimum excitation of the interesting resonances. The operator’s adjustment of set-up starts with equalising the object and reference wave pathlengths and focusing the object image (these steps are necessary only when the object’s size requires a different magnification, usually the object’s position is kept fixed). If the weight of the new object is different, the reference wave pinhole might need realignment due to a different bending of the rather thin optical table. Finally the operator checks, and if necessary adjusts, the reference wave’s in-line position with the object wave. The object—reference ratio is then set by rotating the variable beam-splitter until the interferograms on the monitor have optimum contrast. It should be noted that when the objects have the same size and weight and care is observed in the mounting—demounting stages, all these adjustments are unnecessary. In the beginning, the frequency mapping procedure was thorough with video recording and picture taking of all resonances. However, the amount of information acquired soon outgrew the analysing capacity. A simple procedure was then adopted. The operator varies the frequency until the first resonance is detected, the frequency is carefully adjusted at constant excitation level until maximum amplitude is obtained. This frequency value is registered as fres. The excitation is then lowered until the fringe pattern shows only a black fringe. This excitation voltage is registered as Vres. The operator then proceeds to the next resonance and repeats the procedure. Only new unexpected vibration patterns are photographed or sketched. This procedure maps the resonance frequencies quite accurately (±1Hz with care), the excitation value is more uncertain, in the order of 10—15%. It could be considerably improved by also measuring the excitation of higher fringe orders using phase modulation. However, variation due to differences in excitator position was considered to be a greater source of error. In addition, these excitation values are used only as a guide to which resonances are easily excited. Under working conditions, the turbine blades are subject to high temperatures (+ 700°C) and centrifugal forces. These two factors will change the resonance frequencies in opposite ways. A temperature increase will decrease the frequencies while the stiffening effect of the centrifugal forces results in
ELECTRONIC SPECKLE PATFERN INTERFEROMETRY IN TESTING OF TURBINE PARTS
11
frequency increase (somewhat dependent on the mode shape). The combined effect gives a net increase in frequency value which has been determined by theory and experiment for the frequencies of interest. Using this correction factor, the resonance frequencies as determined by ESPI and by strain gauge measurements under working conditions are comparable within 1—2%. The long range goal is to use ESPI while the turbine is running so as to avoid the costly and cumbersome strain—gauge instrumentation. The high temperature and turbulence should not be a serious hindrance as we have heated objects to well above + 700°Cwith a blowtorch and still observed the vibration pattern using chopped exposure ESPI.6 However, the interior construction of the gas turbine makes it extremely difficult if not impossible to stabiise the rotating image by derotator techniques.
SPECIFIC APPLICATIONS OF THE SYSTEM
In a certain period of the development of a new gas turbine at Kongsberg Vaapenfabrik, the ESPI system was extensively used to check the vibration characteristics. All initial rotating and static parts were examined at the prototype stage in order to determine potential resonant natural frequencies. In some cases it was necessary to modify the prototype design in order to avoid resonance conditions and in this work the ESPI system appeared especially useful. The effect of minor changes in geometry such as cutting off material and increasing local stiffnesses could be seen immediately on the TV monitor making the process of modification very effective. In one case it was necessary to increase the overall thickness of a part to obtain the required stiffness and in this case a simple Al-model was made to check if the increased thickness had given the desired effect. The result was seen to be consistent with the later actual parts. In the initial development period all critical parts were checked in order to establish a baseline for production tolerances, etc., but after this only sample checks are made. This use of the ESPI system in the design and development of the new KG5 gas turbine has proved very successful as there has been no vibration failure in any gas turbine after more than 15 000 hours of accumulated experience.
CONCLUDING REMARKS
In this paper we have described how a simple ESPI system has been constructed and used for testing and development of gas turbine components. The theme of the project itself, vibration measurements of, for example turbine
12
OLE J. LØKBERG, PER SVENKE
blades, can hardly be considered new as it is a rather classical holographic study. The basic use of the system has also been rather simple, like using Chiadni figures for vibration testing but with the added ability of finding the excitation sensitivity of the antinodes. However, we find the easy handling and use of the equipment to be excellent proof of how well ESPI can be used for testing in industrial environments, if necessary on a routine basis. The system in its present form has two weak features; it is not sufficiently stable and its light economy and resolution are not adequate to inspect an entire wheel which is sometimes desired. This will be corrected by the delivery of a new ESPI system based upon the speckle reference principle suggested by Slettemoen.7 This system will, in addition, be compact and extremely easy to adjust.
ACKNOWLEDGEMENTS
Alnong the many persons supporting this project, we want especially to thank 0. M. Holje and B. Stâlaker for help and advice during the construction of the apparatus. One of us (0. J. L.) wants to thank the Electrical Engineering Department, University of Minnesota and Optical Science Centre, University of Arizona for support as most of the writing was done during his stay at these institutions.
REFERENCES
1.
J. N. BUTLERS, Proc. Engineering Uses of Coherent Optics, Ed. E. R. Robertson, Cambridge Univ. Press, London, 1976, pp. 155. 2. J. N. BUTtERS, R. Jo~s and C. Wyi~s,Speckle Metrology, Ed. R. K. Erf, Academic Press, London, 1978, pp. 111. 3. 0. J. LOKBERG, Phys. Technol, 11 (1980) 16. 4. 0. J. LØKBERG and K. HOGMOEN, J. Phys. E. 9 (1976) 847. 5. 0. J. LØKBERG and K. HØGMOEN, Appi. Opt., 15 (1976) 2701. 6. 0. J. LØKBERG, App). Opt., 18 (1979) 2377. 7. G. A. SLETrEMOEN, Appi. Opt., 19 (1980) 616.