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Optical inspection system for the inner surface of a pipe using detection of circular images projected by a laser source
Measurement ELSEVIER
Measurement 13 (1994) 99 106
Optical inspection system for the inner surface of a pipe using detection of circular images projected by a laser source Takahiko Inari ,,a Kazuo Takashima ", Masaru Watanabe b, Junji Fujimoto b "Industrial Electronics and Systems Development Laboratory, Mitsubishi Electric Corporation, Amagasaki, Hyogo, 661, Japan b Corporate Research and Development Laboratory, Tonen Corporation, 1-3-1 Nishi-Turugaoka Ohi-machi, Irumagun, Saitama, 354, Japan
Abstract
An optical inspection system for the inner wall of a pipe has been developed. This system uses the projection of a circular pattern formed by a conically reflecting mirror on the inner wall, and detection of circular images from the illuminated circumferential inner surface. Irregularities of the images due to corrosion or accumulation of deposits on the inner surface are inspected by this system. Inspection of the shape of the inner wall has been impossible by current methods. The experimental system reported in this paper has the following practical specifications: resolution of detection of ___0.1 ram, and inspection speed of 30 mm/s at an interval of 1 mm along the pipe. The system is now being used for practical inspection.
Key words. Inspection; Optical sensing device; Inner wall of pipe; Projection of optical image; Image processing; Transputer; CCD image sensor; Laser diode
1. Introduction
The inner walls of pipes that are used for piping or for heat exchangers of installations in manufacturing or power plants should be inspected periodically, because corrosion and accumulation of various deposits develop on the inner walls. Performing an automatic and high-speed inspection requires new instruments. Currently, methods using eddy current sensors or rotating ultrasonic sensors are generally known for defect inspection. However, these methods suffer from such problems as the restricted range of materials of pipes to be inspected in case of the eddy current method, and, for the ultrasonic * Corresponding author. 0263-2241/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSD1 0263-2241 (93)E0036-R
method, the need of a transmission medium such as water. An optical fiber scope is unsuitable for automatic inspection, because this method needs human observation during inspection. As these methods, moreover, cannot detect the shapes of inner walls of pipes, quantitative inspection of defects such as corrosion and deposits appearing on inner walls is impossible. In previous works [1,2], systems using optical distance sensors inserted into pipes were developed. The distance sensor used in these works is known as the triangulation principle method, and consists of a laser diode as the illumination light source, a lens for forming a spot image of the illuminated surface point, and a one- or twodimensional photosensor to detect the position of the spot image. From the position of the spot image and the dimensions of the optical system of
100
T. Inari et al./Measurement 13 (1994) 99-106
the sensor, the distance of the illuminated point from the base line of the sensor is calculated. As this sensor detects the distance of only one point on a surface, rotating the sensor around the central axis of the pipe and moving it in the axial direction of the pipe are required for inspection of the overall surface. Therefore, a complicated precision mechanism is required to rotate and drive the optical sensor, which restricts the realization of high-speed and real-time inspection in practice. The system we developed uses a method in which a circular optical pattern is projected by a special light projector on the inner wall and a circular image reflected from the circumferential surface of the wall is detected by a small-size image sensor. This system requires only a mechanism to drive the sensing device in the axial direction. Testing in practical application proved the possibility of real-time inspection with a detection resolution of _+0.1 mm. This system is now being used in practical inspection. The pipes to be inspected, in many practical cases, have inner surfaces coated with iron or brass. For high-resolution inspection, frequent washing of the inner surfaces is required.
2. Principle The principal structure of the optical sensing device developed is shown in Fig. 1(a). A circular pattern is projected on the inner wall of a pipe by the projector. Reflected light from the illuminated circumferential surface is collected by the lens, which forms a circular image on the twodimensional detector, as shown in Figs. 1 (b) and (c). This circular image shows the shape of the cross section of the inner wall. Figure 1(b) shows a smooth circular pattern from a normal or regular surface of the inner wall, whereas Fig. 1(c) shows an abnormal or irregular pattern resulting from an irregular surface. When the optical sensing device travels in the axial direction of the pipe, an overall shape image of the inner wall can be obtained. Figure 2 shows the optical configuration explaining the principle of the inner shape detection. The ray emanating from point t is projected onto a surface point P. Point q is the image point on the
Circular Pattern Window ~ / ~ /
Circular pattern projector
J.
................. .......
/
............................
/
~ /
Lens two dimensional detector
......./. ..........F..,
,'~,f
/
I
....................
Wall of pipe
(a)
0 (b) (c) Fig. 1. (a) Principal structure of optical sensing device; (b) conceptual circular image formed by reflection from circumferential regular inner surface; (c) from irregular inner surface.
////////~J/P///////
Inner wall of pipe
Lens
0
Fig. 2. Optical configuration explaining the principle of detection.
two-dimensional detector reached by a scattered ray after passing through the center of the lens r. The scattered rays from point P form an image at point q. As P is one of the points of the circular optical pattern projected on the inner wall, q is the point on the circular image corresponding to it. In this figure, if Yp, which is the inner radius of the pipe, is the distance from the center line of the optical sensing device to point P, it can be calculated by using the following equation. tan A tan B "'xP=d tan A + t a n B '
(1)
where A is the angle of the direction of the ray originated from the light source, B is the angle of the reflected ray, and d is the distance between the points t and r. The angle B can be calculated by
T. Inari et al./Measurement 13 (1994) 99-106
using the distances l and y, where l is the distance between the lens and the center point s of the detector, and y is the distance between point q and point s. Point q is on the circular image, detected by the two-dimensional detector. Then, the angle B is given by, (2)
tan B = y/l.
The values of the radii calculated as Yp from Eq. ( 1) give the profile of the inner wall.
3. Construction of the experimental sensing device A sketch of the sensing device for the experiment is shown in Fig. 3. The diameter of the device is 13 mm and its length is 60 mm. These dimensions are suitable for a pipe of 19 mm inner diameter, which is called a 1-inch pipe. The light source of the projector is a 780 nm laser diode having an output power of 30 mW. The circular optical pattern is produced by the optical apparatus included in the sensing device, which consists of a lens for focusing and projecting a light beam from the laser diode and a conical mirror for spreading the light beam conically. The rays from the conical mirror are projected through a transparent glass window onto the inner wall of the pipe. A charge sweep device (CSD) image sensor is used as the two-dimensional detector. This device has 30%
101
superior sensitivity to the usual charge coupled device (CCD) by improving the sweep circuit for charge induced by the illuminating light, so it is advantageous for small sizing. The number of pixels of this device is 0.16 millions (355 x 450).
4. Estimation of the performance of the sensing device The targets of the system are 25 mm for the diameter detection range, and + 0.1 mm or below for depth resolution of the defects. System performance evaluation is described by using a test piece in the experiment. On the inner wall of a 19 mm inner diameter test pipe concave and convex defects ranging from 0.2 to 0.8 mm in depth were made, as shown in Fig. 4(a). The circular image formed by illuminating the inner wall with the laser light was detected first by the image sensor. The center of the circle of the image was derived and distances between the center and the sampling points on the circumference of the circle were calculated in an image processing system. The real inner radii of the pipe were calculated successively using the distance data. Figure 4 (b) shows the results of both the processed image data and the calculated radii. This experiment proves that it is possible to realize a detection resolution of + 0.1 mm or less.
5. Configuration of a practical system Laser diode ~
m
Circular pattern projected /
Lens age sensor device
Pipe
Conical mirror
~" Data output
Image processing system
Fig. 3. Sketch of the structure of the experimental optical sensing device.
The inspection system we developed consists of three blocks as shown in Fig. 5, that is, an optical sensing device, a high-speed image processing system, and a driving mechanism. In the following subsections, each of the system components will be detailed. 5.1. Optical sensing device
In practical use the optical sensing device is housed in an inspection pig. Construction of the device is basically the same as that described in
102
T. Inari et aL/Measurement 13 (1994) 99-106
Optical sensing device \
(a)
Driving mechanism
\
-
o"
IPo°s;r d ;UPePSY mf~ge sensor Is~stegmpr°cessngt ~
CRT
CRT
Fig. 5. Configuration of the inspection system.
(ram) PIXEL
-
(b} ......... ~,,r,~ .
+ .
.
.
.
.
-I.0
(rnm) 1
.
÷1.O
>
.,~¢°" IItlIII
~..
,.'j~
.
.
.
.
.
.
FT+'. .
.
- " .....
f "'j~
,.: c-.,,
:;
, n i, , ,,
h i~{'~ ........
........
l
........
Fig. 4. Results of fundamental experiments: (a) dimensions of defect examples formed on the test piece; (b) results of detection of defects on the test piece.
the previous section. The pig is mounted on a supporting mechanism in order to travel smoothly inside a pipe. The appearance of an example of the pigs is shown in Fig. 6.
5.2. Image processing system During the movement of the optical sensing device in the axial direction of the pipe, the circular
images are read into the image sensor and the image processing system at finite intervals. The required resolution in the axial direction and the required moving speed decide the number of image frames to be detected per second. The necessary image processing time is decided by the number of frames. If the moving speed is 30 mm/s or less and a resolution of 1 mm along the axis is required, the rate of reading images should be 30 frames per second. In our processing system, the number of the sampling points to be detected on the circumference of a circular image is 60 at regular intervals. The positions of the sampling points are derived by detection of peak positions in directions of radii of the circle, in small searching regions set around the positions expected to be found. If the searching region has an area of 100 pixels, the number of pixels that needs to be read from an image memory is 6000 per frame. The necessary processing time is, therefore, less than 5.6 ~ts per pixel in order to process 30 frames per second. Such high-speed processing is impossible with a general-purpose microprocessor. For high-speed processing, we developed a parallel processing system composed of four transputer processors, one of which reads image data from the image memory, and data processing of one frame is done through cooperation of the four transputers. The construction of the image processing system is shown in Fig. 7. By using this system, a processing time of 30 ms per frame or less has been realized. The language used for the processing is OCCAM.
103
T. Inari et al./Measurement 13 (1994) 99-106
Fig. 6. Appearanceof the inspectionpig includingoptical sensingdevice. Processing
Motor Encorder P~Deto be inspected
r=>> Transputer t - 7 Read
out
i
Transputer
I image memory
I~ ----~
i
Air
i
=J
Processing
LINK
Cable ~ . ~
[ Transputer
F-->>I
Fig. 7. Block diagram of the image processingsystem.
~ Air~sel ~t ~M..~jJl
1.~\
'~'
Pig for [~ulling
Optical sensing device
Image processingsystem
Fig. 8. Principleof the driving mechanismconstruction. special mechanism made to keep it at the center of the pipe's interior. Figure 9 shows a pistol-type driving mechanism for use in practical inspection. This is one type of the driving mechanism.
5.3. Driving mechanism The principle of the driving mechanism is explained in Fig. 8. The sensing device is pushed into the opposite end of the pipe by applying air pressure. After reaching the end, the supply of air is stopped, and the sensing device is pulled back at constant speed by using a cable connected to a roller. Inspection is done while the sensing device is retracted. The sensing device is supported by a
6. Results of testing The trial system was used to inspect the interior of a 1-inch diameter pipe for examination ol~ its performance. The speed of inspection was 15 mm/s at intervals of 1 mm along the axis of the pipe. An example of the results of the test is shown in Fig. 10. The figure displays the surface develop-
104
Z Inari et al./Measurement 13 (1994) 99-106
Fig. 9. Appearance of the pistol-type driving mechanism.
Fig. 10. Display showing the result of detection of a test pipe's inner surface.
ment of the inner wall of the pipe, where the longitudinal axis corresponds to the direction of the axis of the pipe, and the horizontal axis to the direction of the circumference of the pipe. On the display, we can find a concave defect of about 0.5 mm in depth on the inner surface, which was formed by corrosion. This system has been used in practical inspec-
tion. Figure 11 shows an example of inspection work using this system. The apparatus under inspection in the figure is a heat exchanger. The many types to be inspected have inner surfaces coated with iron or brass, contaminated by practical use. If high-resolution inspection is required, in some cases, washing the inner surface is needed. Examples of displays in our system are shown
T. Inari et al./Measurement 13 (1994) 99 106
105
Fig. 11. An example of inspection work using the practical system developed. in Figs 12 and 13. Figure 12 shows an example of the results of practical inspection, which is similar to Fig. 10. Figure 13 is another practical display.
In the figure, the left picture shows the developed inner surface, in which the degree of the defects are shown by variation of the colors, the central
Fig. 12. An example of the displays of the practical system (similar to Fig. 10).
106
T. Inari et al./Measurement 13 (1994) 99-106
Fig. 13. Another example of the displays of the practical system which shows the developed inner surface including defects, the profile of the defect sampled, and the cross section of the pipe including the sampled defect.
the projection light ray. The circular images are processed by a high-speed processor developed using transputers. By using this system, the ability of inspecting the interior of a pipe with a detection resolution of _+0.1 mm at a speed of 15 ram/s, 30 mm/s, at intervals of 1 mm along a pipe has been proved. However, some problems were found in practice; one of them is the influence of surface conditions of the inner wall on the system performance. The practical system developed has been applied to the inspection of pipes, e.g. heat exchangers. The principle of this system may be applied to the measurements of various objects such as middle- or large-scale pipes, both inside and outside, plane objects and others.
picture shows the profile of the defect sampled selectively from the left picture, and the fight picture shows the cross section of the pipe including the defect shown in the central picture, in which the part of the defect is shown by different color from the normal part. This system can be applied to 1-inch pipes with 18-25 mm inner diameter, and 3/4-inch pipes, with 14.5-19 mm inner diameter.
This study and development have been performed as one of the research projects of PEC (Petroleum Energy Center). The authors are grateful for PEC's support of this work.
7. Conclusion
References
We have developed an inspection system for the interior of a pipe using detection of circular images produced by reflection of a circular light pattern projected on the inner wall from a light source, and proved the ability of application in practice. The circular projection pattern is formed by an optical apparatus using a conical mirror spreading
[1] D.L. Cunningham, J.L. Doyle and D. Hoffman, Optical nondestructive evaluation of pipe inner wall condition, Rev. Prog. Quant. Nondestr. Eval. 4B (1985) 789-797. [2] Optical technique for internal diametrical measurement of steam generator tubes, Anon. Siga'na Res. Inc., Richland, WA; Electric Power Res. Inst. Rep., EPRI NP 1244, November 1979.
Acknowledgement
Measurement 13 (1994) 99 106
Optical inspection system for the inner surface of a pipe using detection of circular images projected by a laser source Takahiko Inari ,,a Kazuo Takashima ", Masaru Watanabe b, Junji Fujimoto b "Industrial Electronics and Systems Development Laboratory, Mitsubishi Electric Corporation, Amagasaki, Hyogo, 661, Japan b Corporate Research and Development Laboratory, Tonen Corporation, 1-3-1 Nishi-Turugaoka Ohi-machi, Irumagun, Saitama, 354, Japan
Abstract
An optical inspection system for the inner wall of a pipe has been developed. This system uses the projection of a circular pattern formed by a conically reflecting mirror on the inner wall, and detection of circular images from the illuminated circumferential inner surface. Irregularities of the images due to corrosion or accumulation of deposits on the inner surface are inspected by this system. Inspection of the shape of the inner wall has been impossible by current methods. The experimental system reported in this paper has the following practical specifications: resolution of detection of ___0.1 ram, and inspection speed of 30 mm/s at an interval of 1 mm along the pipe. The system is now being used for practical inspection.
Key words. Inspection; Optical sensing device; Inner wall of pipe; Projection of optical image; Image processing; Transputer; CCD image sensor; Laser diode
1. Introduction
The inner walls of pipes that are used for piping or for heat exchangers of installations in manufacturing or power plants should be inspected periodically, because corrosion and accumulation of various deposits develop on the inner walls. Performing an automatic and high-speed inspection requires new instruments. Currently, methods using eddy current sensors or rotating ultrasonic sensors are generally known for defect inspection. However, these methods suffer from such problems as the restricted range of materials of pipes to be inspected in case of the eddy current method, and, for the ultrasonic * Corresponding author. 0263-2241/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSD1 0263-2241 (93)E0036-R
method, the need of a transmission medium such as water. An optical fiber scope is unsuitable for automatic inspection, because this method needs human observation during inspection. As these methods, moreover, cannot detect the shapes of inner walls of pipes, quantitative inspection of defects such as corrosion and deposits appearing on inner walls is impossible. In previous works [1,2], systems using optical distance sensors inserted into pipes were developed. The distance sensor used in these works is known as the triangulation principle method, and consists of a laser diode as the illumination light source, a lens for forming a spot image of the illuminated surface point, and a one- or twodimensional photosensor to detect the position of the spot image. From the position of the spot image and the dimensions of the optical system of
100
T. Inari et al./Measurement 13 (1994) 99-106
the sensor, the distance of the illuminated point from the base line of the sensor is calculated. As this sensor detects the distance of only one point on a surface, rotating the sensor around the central axis of the pipe and moving it in the axial direction of the pipe are required for inspection of the overall surface. Therefore, a complicated precision mechanism is required to rotate and drive the optical sensor, which restricts the realization of high-speed and real-time inspection in practice. The system we developed uses a method in which a circular optical pattern is projected by a special light projector on the inner wall and a circular image reflected from the circumferential surface of the wall is detected by a small-size image sensor. This system requires only a mechanism to drive the sensing device in the axial direction. Testing in practical application proved the possibility of real-time inspection with a detection resolution of _+0.1 mm. This system is now being used in practical inspection. The pipes to be inspected, in many practical cases, have inner surfaces coated with iron or brass. For high-resolution inspection, frequent washing of the inner surfaces is required.
2. Principle The principal structure of the optical sensing device developed is shown in Fig. 1(a). A circular pattern is projected on the inner wall of a pipe by the projector. Reflected light from the illuminated circumferential surface is collected by the lens, which forms a circular image on the twodimensional detector, as shown in Figs. 1 (b) and (c). This circular image shows the shape of the cross section of the inner wall. Figure 1(b) shows a smooth circular pattern from a normal or regular surface of the inner wall, whereas Fig. 1(c) shows an abnormal or irregular pattern resulting from an irregular surface. When the optical sensing device travels in the axial direction of the pipe, an overall shape image of the inner wall can be obtained. Figure 2 shows the optical configuration explaining the principle of the inner shape detection. The ray emanating from point t is projected onto a surface point P. Point q is the image point on the
Circular Pattern Window ~ / ~ /
Circular pattern projector
J.
................. .......
/
............................
/
~ /
Lens two dimensional detector
......./. ..........F..,
,'~,f
/
I
....................
Wall of pipe
(a)
0 (b) (c) Fig. 1. (a) Principal structure of optical sensing device; (b) conceptual circular image formed by reflection from circumferential regular inner surface; (c) from irregular inner surface.
////////~J/P///////
Inner wall of pipe
Lens
0
Fig. 2. Optical configuration explaining the principle of detection.
two-dimensional detector reached by a scattered ray after passing through the center of the lens r. The scattered rays from point P form an image at point q. As P is one of the points of the circular optical pattern projected on the inner wall, q is the point on the circular image corresponding to it. In this figure, if Yp, which is the inner radius of the pipe, is the distance from the center line of the optical sensing device to point P, it can be calculated by using the following equation. tan A tan B "'xP=d tan A + t a n B '
(1)
where A is the angle of the direction of the ray originated from the light source, B is the angle of the reflected ray, and d is the distance between the points t and r. The angle B can be calculated by
T. Inari et al./Measurement 13 (1994) 99-106
using the distances l and y, where l is the distance between the lens and the center point s of the detector, and y is the distance between point q and point s. Point q is on the circular image, detected by the two-dimensional detector. Then, the angle B is given by, (2)
tan B = y/l.
The values of the radii calculated as Yp from Eq. ( 1) give the profile of the inner wall.
3. Construction of the experimental sensing device A sketch of the sensing device for the experiment is shown in Fig. 3. The diameter of the device is 13 mm and its length is 60 mm. These dimensions are suitable for a pipe of 19 mm inner diameter, which is called a 1-inch pipe. The light source of the projector is a 780 nm laser diode having an output power of 30 mW. The circular optical pattern is produced by the optical apparatus included in the sensing device, which consists of a lens for focusing and projecting a light beam from the laser diode and a conical mirror for spreading the light beam conically. The rays from the conical mirror are projected through a transparent glass window onto the inner wall of the pipe. A charge sweep device (CSD) image sensor is used as the two-dimensional detector. This device has 30%
101
superior sensitivity to the usual charge coupled device (CCD) by improving the sweep circuit for charge induced by the illuminating light, so it is advantageous for small sizing. The number of pixels of this device is 0.16 millions (355 x 450).
4. Estimation of the performance of the sensing device The targets of the system are 25 mm for the diameter detection range, and + 0.1 mm or below for depth resolution of the defects. System performance evaluation is described by using a test piece in the experiment. On the inner wall of a 19 mm inner diameter test pipe concave and convex defects ranging from 0.2 to 0.8 mm in depth were made, as shown in Fig. 4(a). The circular image formed by illuminating the inner wall with the laser light was detected first by the image sensor. The center of the circle of the image was derived and distances between the center and the sampling points on the circumference of the circle were calculated in an image processing system. The real inner radii of the pipe were calculated successively using the distance data. Figure 4 (b) shows the results of both the processed image data and the calculated radii. This experiment proves that it is possible to realize a detection resolution of + 0.1 mm or less.
5. Configuration of a practical system Laser diode ~
m
Circular pattern projected /
Lens age sensor device
Pipe
Conical mirror
~" Data output
Image processing system
Fig. 3. Sketch of the structure of the experimental optical sensing device.
The inspection system we developed consists of three blocks as shown in Fig. 5, that is, an optical sensing device, a high-speed image processing system, and a driving mechanism. In the following subsections, each of the system components will be detailed. 5.1. Optical sensing device
In practical use the optical sensing device is housed in an inspection pig. Construction of the device is basically the same as that described in
102
T. Inari et aL/Measurement 13 (1994) 99-106
Optical sensing device \
(a)
Driving mechanism
\
-
o"
IPo°s;r d ;UPePSY mf~ge sensor Is~stegmpr°cessngt ~
CRT
CRT
Fig. 5. Configuration of the inspection system.
(ram) PIXEL
-
(b} ......... ~,,r,~ .
+ .
.
.
.
.
-I.0
(rnm) 1
.
÷1.O
>
.,~¢°" IItlIII
~..
,.'j~
.
.
.
.
.
.
FT+'. .
.
- " .....
f "'j~
,.: c-.,,
:;
, n i, , ,,
h i~{'~ ........
........
l
........
Fig. 4. Results of fundamental experiments: (a) dimensions of defect examples formed on the test piece; (b) results of detection of defects on the test piece.
the previous section. The pig is mounted on a supporting mechanism in order to travel smoothly inside a pipe. The appearance of an example of the pigs is shown in Fig. 6.
5.2. Image processing system During the movement of the optical sensing device in the axial direction of the pipe, the circular
images are read into the image sensor and the image processing system at finite intervals. The required resolution in the axial direction and the required moving speed decide the number of image frames to be detected per second. The necessary image processing time is decided by the number of frames. If the moving speed is 30 mm/s or less and a resolution of 1 mm along the axis is required, the rate of reading images should be 30 frames per second. In our processing system, the number of the sampling points to be detected on the circumference of a circular image is 60 at regular intervals. The positions of the sampling points are derived by detection of peak positions in directions of radii of the circle, in small searching regions set around the positions expected to be found. If the searching region has an area of 100 pixels, the number of pixels that needs to be read from an image memory is 6000 per frame. The necessary processing time is, therefore, less than 5.6 ~ts per pixel in order to process 30 frames per second. Such high-speed processing is impossible with a general-purpose microprocessor. For high-speed processing, we developed a parallel processing system composed of four transputer processors, one of which reads image data from the image memory, and data processing of one frame is done through cooperation of the four transputers. The construction of the image processing system is shown in Fig. 7. By using this system, a processing time of 30 ms per frame or less has been realized. The language used for the processing is OCCAM.
103
T. Inari et al./Measurement 13 (1994) 99-106
Fig. 6. Appearanceof the inspectionpig includingoptical sensingdevice. Processing
Motor Encorder P~Deto be inspected
r=>> Transputer t - 7 Read
out
i
Transputer
I image memory
I~ ----~
i
Air
i
=J
Processing
LINK
Cable ~ . ~
[ Transputer
F-->>I
Fig. 7. Block diagram of the image processingsystem.
~ Air~sel ~t ~M..~jJl
1.~\
'~'
Pig for [~ulling
Optical sensing device
Image processingsystem
Fig. 8. Principleof the driving mechanismconstruction. special mechanism made to keep it at the center of the pipe's interior. Figure 9 shows a pistol-type driving mechanism for use in practical inspection. This is one type of the driving mechanism.
5.3. Driving mechanism The principle of the driving mechanism is explained in Fig. 8. The sensing device is pushed into the opposite end of the pipe by applying air pressure. After reaching the end, the supply of air is stopped, and the sensing device is pulled back at constant speed by using a cable connected to a roller. Inspection is done while the sensing device is retracted. The sensing device is supported by a
6. Results of testing The trial system was used to inspect the interior of a 1-inch diameter pipe for examination ol~ its performance. The speed of inspection was 15 mm/s at intervals of 1 mm along the axis of the pipe. An example of the results of the test is shown in Fig. 10. The figure displays the surface develop-
104
Z Inari et al./Measurement 13 (1994) 99-106
Fig. 9. Appearance of the pistol-type driving mechanism.
Fig. 10. Display showing the result of detection of a test pipe's inner surface.
ment of the inner wall of the pipe, where the longitudinal axis corresponds to the direction of the axis of the pipe, and the horizontal axis to the direction of the circumference of the pipe. On the display, we can find a concave defect of about 0.5 mm in depth on the inner surface, which was formed by corrosion. This system has been used in practical inspec-
tion. Figure 11 shows an example of inspection work using this system. The apparatus under inspection in the figure is a heat exchanger. The many types to be inspected have inner surfaces coated with iron or brass, contaminated by practical use. If high-resolution inspection is required, in some cases, washing the inner surface is needed. Examples of displays in our system are shown
T. Inari et al./Measurement 13 (1994) 99 106
105
Fig. 11. An example of inspection work using the practical system developed. in Figs 12 and 13. Figure 12 shows an example of the results of practical inspection, which is similar to Fig. 10. Figure 13 is another practical display.
In the figure, the left picture shows the developed inner surface, in which the degree of the defects are shown by variation of the colors, the central
Fig. 12. An example of the displays of the practical system (similar to Fig. 10).
106
T. Inari et al./Measurement 13 (1994) 99-106
Fig. 13. Another example of the displays of the practical system which shows the developed inner surface including defects, the profile of the defect sampled, and the cross section of the pipe including the sampled defect.
the projection light ray. The circular images are processed by a high-speed processor developed using transputers. By using this system, the ability of inspecting the interior of a pipe with a detection resolution of _+0.1 mm at a speed of 15 ram/s, 30 mm/s, at intervals of 1 mm along a pipe has been proved. However, some problems were found in practice; one of them is the influence of surface conditions of the inner wall on the system performance. The practical system developed has been applied to the inspection of pipes, e.g. heat exchangers. The principle of this system may be applied to the measurements of various objects such as middle- or large-scale pipes, both inside and outside, plane objects and others.
picture shows the profile of the defect sampled selectively from the left picture, and the fight picture shows the cross section of the pipe including the defect shown in the central picture, in which the part of the defect is shown by different color from the normal part. This system can be applied to 1-inch pipes with 18-25 mm inner diameter, and 3/4-inch pipes, with 14.5-19 mm inner diameter.
This study and development have been performed as one of the research projects of PEC (Petroleum Energy Center). The authors are grateful for PEC's support of this work.
7. Conclusion
References
We have developed an inspection system for the interior of a pipe using detection of circular images produced by reflection of a circular light pattern projected on the inner wall from a light source, and proved the ability of application in practice. The circular projection pattern is formed by an optical apparatus using a conical mirror spreading
[1] D.L. Cunningham, J.L. Doyle and D. Hoffman, Optical nondestructive evaluation of pipe inner wall condition, Rev. Prog. Quant. Nondestr. Eval. 4B (1985) 789-797. [2] Optical technique for internal diametrical measurement of steam generator tubes, Anon. Siga'na Res. Inc., Richland, WA; Electric Power Res. Inst. Rep., EPRI NP 1244, November 1979.
Acknowledgement