U.S. patent application number 10/707630 was filed with the patent office on 2004-09-30 for a method for reformer tube in situ inspection radius calculation.
This patent application is currently assigned to QUEST INTEGRATED INC.. Invention is credited to Bondurant, Phillip D., Cowling, Thomas J., Lilley, Ronald C., Roberts, Richard D..
Application Number | 20040189987 10/707630 |
Document ID | / |
Family ID | 32508436 |
Filed Date | 2004-09-30 |
United States Patent
Application |
20040189987 |
Kind Code |
A1 |
Bondurant, Phillip D. ; et
al. |
September 30, 2004 |
A METHOD FOR REFORMER TUBE IN SITU INSPECTION RADIUS
CALCULATION
Abstract
A method for inspecting a reformer tube for chemical processing
for damage such as creep and metal dusting. The method includes the
steps of focusing a coherent light beam onto an interior of a tube
or piping and detecting at least a portion of a reflection of the
light beam from the tubing by converting the detected light beam
into an electrical signal and the processing of the electrical
signal to determine a radius of tube under-going in situ
inspection.
Inventors: |
Bondurant, Phillip D.;
(KENT, WA) ; Lilley, Ronald C.; (FEDERAL WAY,
WA) ; Roberts, Richard D.; (FEDERAL WAY, WA) ;
Cowling, Thomas J.; (FEDERAL WAY, WA) |
Correspondence
Address: |
HAYWARD A. VERDUN
609 ALDER AVE. NE.
BAIN BRIDGE ISLAND
WA
98110
US
|
Assignee: |
QUEST INTEGRATED INC.
1012 Central Avenue South
KENT
WA
|
Family ID: |
32508436 |
Appl. No.: |
10/707630 |
Filed: |
December 25, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10707630 |
Dec 25, 2003 |
|
|
|
09713415 |
Nov 15, 2000 |
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Current U.S.
Class: |
356/241.1 |
Current CPC
Class: |
G01N 21/954 20130101;
G06T 2207/30164 20130101; G06T 7/0004 20130101 |
Class at
Publication: |
356/241.1 |
International
Class: |
G01N 021/00 |
Claims
1. A method for forming a profile of a radius of the interior of a
reformer tube comprising the steps of; projecting a light beam on
an interior surface of said reformer tube; collimating said light
beam to focus on the surface of said reformer tube; forming said
collimated beam into a ring on the surface of said reformer tube;
projecting an image of said ring onto a surface of a light detector
and moving said ring along an axis of said reformer tube; detecting
light reflected from the surface of said reformer tube; processing
reflected light data collected by the detector; and forming a
radius profile.
2. The method for forming a profile of the radius of the interior
of a reformer tube according to claim 1, further comprising the
step of reflecting the light beam off of a conical mirror
surface.
3. The method for forming a profile of the radius the interior of a
reformer tube according to claim 2, wherein the conical mirror
further includes a parabolic surface for maintaining focus of the
light beam on expected reformer tube diameter.
4. The method for forming a profile of the radius of the interior
of a reformer tube according to claim 3, further comprising the
steps of, reflecting the light beam off a mirror, and rotating said
mirror to produce a ring of light on the surface of said tube.
5. The method for forming a profile of the radius of the interior
of a reformer tube according to claim 3, further comprising the
steps of, reflecting the light beam off the surface of said
reformer tube, and rotating a light source to produce a ring of
light on the surface of said reformer tube.
6. The method for forming a profile of the radius of the interior
of a reformer of a reformer tube according to claim 4, wherein the
light source is at least one of an LED, and a laser.
7. The method for forming a profile of the radius of the interior
of a reformer of a reformer tube according to claim 5, wherein the
light source is at least one of an LED, and a laser.
8. The method for forming a profile of the radius of the interior
of a reformer tube according to claim 1, further comprising the
steps of, reflecting said beam off the surface said reformer tube,
and rotating an LED to produce a ring of light on the surface of
said reformer tube.
9. The method for forming a profile of the radius of the interior
of a reformer tube according to claim 1, wherein the light beam is
focused on an interior axis of said reformer tube.
10. The method for forming a profile of the radius of the interior
of a reformer tube according to claim 9, further comprising the
step of reflecting said beam off the surface of a conical
mirror.
11. The method for forming a profile of the radius the interior of
a reformer tube according to claim 9, further comprising the step
of reflecting said beam off the surface of the conical mirror with
a parabolic surface for maintaining focus of said beam on the
expected reformer tube diameter.
12. The method for forming a profile of the radius of the interior
of a reformer tube according to claim 9, further comprising the
steps of, reflecting said beam off a mirror, and rotating said
mirror to produce a ring of light on the surface of said tube.
13. The method for forming a profile of the radius of the interior
of a reformer tube according to claim 9, further comprising the
steps of, reflecting said beam off the surface said reformer tube,
and rotating an LED to produce a ring of light on the surface of
said reformer tube.
14. The method for forming a profile of the radius of the interior
of a reformer tube according to claim 9, further comprising the
steps of, reflecting said beam off a mirror at the surface said
tube, and rotating an LED to produce a ring of light on the surface
of said reformer tube.
15. The method for forming a profile of the radius of the interior
of a reformer tube according to claim 9, further comprising the
steps of, reflecting said light beam off the surface said reformer
tube, and rotating an LED to produce a ring of light on the surface
of said reformer tube.
16. A method for forming a profile of a radius of the interior of a
reformer tube comprising the steps of; projecting a light beam on
an interior surface of said reformer tube; collimating said light
beam to focus on the surface of said reformer tube; rotating at
least one of the light source and a mirror to scan the interior
surface of the reformer tube; detecting light reflected from the
surface of said reformer tube; processing reflected light data
collected by the detector; and forming a radius profile.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present patent application is related to our copending
application entitled "A Method For Reformer Tube In Situ Inspection
Radius Calculation," U.S. application Ser. No. 10, ###,### filed
12/25/2003, which is incorporated by reference as if fully set
forth herein.
BACKGROUND OF INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to inspection of materials,
and more particularly to the inspection of the surface of materials
for creep, metal dusting, irregularities, and manufacturing flaws.
With still greater particularity the invention pertains to the
inspection of the interior of cylindrical surfaces such as reformer
tubes used in chemical processing.
[0004] 2. Description of the Related Art
[0005] Reformer tubes are used in many chemical processes. Examples
include tubes used to produce ammonia, methanol, hydrogen, nitric
and sulfuric acids, and cracking of petroleum. Reformer tubes, also
called catalyst tubes, are one of the highest cost components of
such plants both in capital and maintenance. A typical installation
consists of several hundred vertical tubes. These tubes represent a
significant cost for replacement and can be a major source of plant
unavailability if unplanned failures occur.
[0006] Such tubes are frequently subjected to pressure changes and
contact with corrosive substances. Under such situations creep,
metal dusting, and surface irregularities frequently develop. If
left untreated, creep will develop into cracks that will propagate
leading to failure of the tube.
[0007] The plant operator is faced with balancing production needs
against tube life and risk of tube failure. The Inner Diameter (ID)
of these reformer tubes is generally between 76 mm (3.0 inches) and
127 mm (5.0 inches). During plant operation the catalyst filled
tubes are externally heated to allow the reforming reaction to
occur. One of the major concerns in plant operation is that the
reformer tubes operate at an elevated temperature such that they
are susceptible to a failure mechanism referred to as "creep". This
condition exists due to the elevated temperatures and stresses
imposed by internal pressure, thermal gradients, and mechanical
loading cycles. Being able to identify and locate such damage in
its early stages is essential for optimizing plant operation.
[0008] Conventional Nondestructive Examination (NDE) inspection
techniques currently applied to reformer tubes are geared to
finding creep damage in the form of internal cracking. However,
with the trend towards larger tube diameters and longer intervals
between turnarounds, the detection of such defects may not allow
for sufficient time for forward planning of tube replacements.
Also, such "end of life" techniques do not allow any
differentiation between the "good" tubes. Early detection of
underutilized tube life can prevent the lost opportunity on both
unrealized production through running them too cool and tube life
"giveaway" if good tubes are discarded prematurely.
[0009] Typically, destructive testing is used on a small number of
tubes removed from the reformer to try and determine the absolute
life remaining. Whatever the method is used, the results are used
on a sample size that is not statistically valid. It is preferable
that all the tubes be surveyed with a NDE technique to characterize
their relative condition in order to make sense of the absolute
condition assessment provided by the destructive testing.
[0010] Reformer tubes undergo creep strain, in the form of
diametrical growth, on the first day that they are fired. The
ability to accurately measure and record this growth means that the
tubes' condition can be monitored on day one. Therefore, not only
can individual tubes be retired from service at an appropriate
time, but also the reformer as a whole can be assessed for
performance.
[0011] Another problem that can occur in reformer tubes is metal
dusting. Metal dusting is a condition where the process stream
attacks the interior of the reformer tube with subsequent,
significant metal loss. This can be severe enough to be the life
limiting condition for the tube. Typically, the metal dusting
damage is limited to a 360.degree. circumferential band around the
catalyst tube's interior surface where the critical temperature
range exists.
[0012] External diameter measurements have been used but they are
limited as the automated devices only measure across one diameter
and are often access-restricted by tube bowing. Manual measurements
are too time consuming to provide more than a few readings per
tube. No external measurement method can provide diameter growth
data at or through the reformer refractory. External measurements
are inherently less precise as they are based on a cast surface
rather than the internal machined surface and do not take into
account the effects of oxide shedding. The most accurate growth
measurements are obtained when `as new` baseline data has been
taken prior to the tube being fired for the first time. However, if
this is not available by using the top portion of the tube that is
operating outside the creep temperature as a reference diameter,
the growth profile of the tube can be determined at any stage in
its life.
[0013] Accordingly, there is a need for an automated method and
apparatus capable of examining the internal surfaces of reformer
tubes. The method should be nondestructive and provide both
absolute and relative information on tube profile.
SUMMARY OF INVENTION
[0014] The present invention has been made in view of the above
circumstances and has as an aspect a method for inspecting a
reformer tube for chemical processing for damage such as creep and
metal dusting. The method includes the steps of focusing a coherent
light beam onto an interior of a tube or piping and detecting at
least a portion of a reflection of the light beam from the tubing
by converting the detected light beam into an electrical signal and
the processing of the electrical signal to determine a radius of
tube under-going in situ inspection.
[0015] An embodiment of the invention employs a solid-state laser
diode. A focusing lens is located in front of the diode to focus
the laser at a spot on the surface of the tube to be inspected. The
diode and focusing lens are rotatable within the tube to allow the
spot to form a ring as they are rotated. As the probe moves through
the tube the spot scans the entire surface. A photo detector is
arranged behind an imaging lens to detect the intensity of the
spot. Both the detector and the imaging lens rotate in the same
fashion as the laser diode and its focusing lens. The optical paths
are selected so that the diode, photo detector and surface of the
tube form a triangle. The distance between the detector and diode
is fixed. This results in the reflected spot moving on the surface
of the photo detector in proportion to the distance to the internal
surface of the tube. Signal processing means can then use that
information to reconstruct a three dimensional image of the
internal surface of the tube. The image may either be displayed on
a monitor or printed for later review.
[0016] With present technology a 15-meter tube can be inspected
within three minutes. An inspection such as this will provide over
1,000,000 radius readings. The method provides means to compress
this information to allow easy manipulation and analysis.
[0017] A further aspect of the present invention employs a laser or
light emitting diode (LED) and a cone shaped reflector to project a
ring of illumination on the interior of the tube to be inspected. A
charge-coupled device (CCD) is arrayed so as to scan the ring and
report the reflectivity and profile. Signal processing circuitry
reconstructs an image of the interior of the tube.
[0018] The use of the internal laser mapping technique is not only
useful in preventing tube failure but has huge potential in
optimizing production from the whole tube set without sacrificing
reliability.
[0019] Additional aspects and advantages of the invention will be
set forth in part in the description which follows, and in part
will be obvious from the description, or may be learned by practice
of the invention. The aspects and ad vantages of the invention will
be realized and attained by means of the elements and combinations
particularly pointed out in the appended claims.
[0020] To achieve these and other advantages and in accordance with
the purpose of the present invention, as embodied and broadly
described, the present invention can be characterized according to
one aspect of the invention as comprising a method for the in situ
inspection of a reformer tube or similar type tube or piping used
in chemical processing for damage such as creep and metal dusting.
The method includes the steps of focusing a coherent light beam
onto an interior of the tube and detecting at least a portion of a
reflected light beam and converts the detected light beam into an
electrical signal and further processes the electrical signal to
determine a radius of the tube.
[0021] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only are not restrictive of the invention, as
claimed.
BRIEF DESCRIPTION OF DRAWINGS
[0022] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate several
embodiments of the invention and together with the description,
serve to explain the principles of the invention.
[0023] FIG. 1 is a block diagram of an embodiment of the
Invention;
[0024] FIG. 2 is diagram of the optical components of the FIG. 1
embodiment;
[0025] FIG. 3 is a perspective detail view of the probe of the
invention;
[0026] FIG. 4 is a block diagram of an alternate embodiment of the
Invention;
[0027] FIG. 5 is diagram of the optical components of the FIG. 4
embodiment.
DETAILED DESCRIPTION
[0028] Reference will now be made in detail to the present
embodiments of the invention, examples of which are illustrated in
the accompanying drawings. Wherever possible, the same reference
numbers will be used throughout the drawings to refer to the same
or like parts (elements).
[0029] FIG. 1 is a block diagram of an embodiment of the invention.
Components may be either hardware components or software modules as
described below. The probe includes a light source 1.
[0030] The purpose of the light source 1 is to project a spot of
light on the surface (not shown) to be inspected. Light source 1 is
typically a laser diode or light emitting diode (LED). A light
source controller 2 is connected to light source 1. Controller 2
sets and controls the optical power level of light source 1. The
power level may either be fixed or varied to produce a preset
signal level.
[0031] A position sensitive photo detector (PSD) 3 is situated so
as to detect the spot made by light source 1. Photo detector 3 may
be a lateral effect photodiode, photodiode array, or charge coupled
device (CCD). This description assumes the device chosen for 3 is a
lateral effect photodiode. Detector 3 may have either one or two
axes, dependant upon the specific measurement geometry. In this
description only a single axis detector and associated lens (not
shown) is used. Similarly a 2D detector may be used for 3 if the
light beam from light source 1 is rotated, by means of a rotating
mirror, and detector 3 is fixed. An amplifier filter 4 is connected
to detector 3. Amplifier/filter 4 converts the small signal
currents generated by detector 3 into voltages. A filter sets the
bandwidth of the system.
[0032] The required bandwidth is determined by a combination of the
speed of probe spin and the required sample rate. Amplifier and
filter may be combined into a single unit by inserting active
components into a differential amplifier to produce an active
filter. A suitable amplifier may be an integrated circuit
differential amplifier such as a AD712. A feedback path is provided
between amplifier/filter 4 and light source controller 2 to allow
control of light source 1 dependent upon the intensity of signal
received at detector 3. One or more gain systems 6 may be connected
to the output of amplifier/filter 4.
[0033] This embodiment shows 2 gain systems (5 and 6) each
providing a different level of gain. The addition of one or more
additional gain systems increases the dynamic range of the system
and allows rapid changes of surface reflectivity to be measured.
The outputs of the several gain stages are recorded simultaneously,
and the largest value that is below saturation is used. Several
gain systems may be connected in parallel to provide usable signals
in all situations.
[0034] A normalization and synchronization processing circuit 7 is
connected to the output of gain system 6. The first function of
system 7 is to select the signal from multiple gain systems 6 with
the maximum dynamic range without saturation. The second function
is to convert the individual detector readings to a calibrated
measurement of the distance between light source 1 and the surface
to be scanned. For a ED detector the calibration may be found by
solving equation 1 as the follows: 1 Measurement = g v 1 - v 2 v 1
+ v 2 EQ . 1
[0035] In the equation 1, v.sub.1 and v.sub.2 are the current
readings from either end of detector 3. (g) is a calibration
function used to remove non-linearity in the detector and optics.
(g) may be determined empirically by scanning calibrated tube
samples of various diameters and using the resultant data points to
find coefficients of the function (g). As an alternative to the
calculation using (g) in the above equation, a lookup table with
some form of interpolation may be used. In order to eliminate
electronic drift the system periodically turns light source 1 off
and measures electronic offset voltages. The offset voltages are
then subtracted from subsequent readings made with light source 1
on. Normalization and synchronization processing circuit 8 also
measures the surface reflectivity. The reflectivity of the surface
is computed from the detector signal level, gain, and light source
power level. Variations in surface reflectivity can provide useful
information about the surface. Normalization and synchronization
processing circuit 7 also collects data from a rotational encoder
in 8. The correlation of the signals from encoder in 8 and from
photo detector 3 assures equally spaced samples around the
circumference of a tube.
[0036] Two motors 11 and 12 move the probe. The first motor 11
rotates the probe within a tube. A motor drive circuit 13 controls
motor 11. Encoder 8 is connected to motor 11 allowing determination
of the absolute rotational position of the probe. The output of
encoder 8 is connected to normalization and synchronization
processing circuit 7. A second motor 12 provide axial positioning
of the probe. A second motor drive circuit 14 controls motor 12. An
axial incremental encoder 15 connected to motor 12 provides
information on the axial position of the probe in the tube.
[0037] A system data storage display and control module 16 provides
overall control of the probe. Module 16 receives information on the
distance between light source 1 and the surface sought to be
inspected and surface reflectivity from normalization and
synchronization processing circuit 7. Module 16 also receives
information about the axial position of the probe from encoder 15.
Module 16 controls the rotational position of the probe in the tube
by sending an on and off signal to rotational motor drive circuit
13. Module 16 also controls the axial position by sending on/off
and forward/backward signals to axial motor control circuit 14.
[0038] FIG. 2 is diagram of the optical components of the FIG. 1
embodiment. A light source 1 produces beam of light. Light source 1
is usually a diode laser but could be a light emitting diode in
special applications. A collimating lens 21 downstream of light
source 1 converts the light emitted by light source 1 into a
collimated beam which does not spread to any appreciable extent
over the inspection range 22"-22". The beam produces a target spot
22 where it impacts the target surface 23. A photo detector 27 is
mounted at an angle to the optical train of components 1, 21, and
22. The angle is detected such that the imaged spot at 26 is in
focus over the measurement range 22" to 22". Photo detector 27 is
usually a lateral effect photodiode array or charge coupled device
(CCD) photo detector. Photo detector 27 may either be a one
dimensional (1D) or two dimensional (2D) device. If photo detector
27 is a 2D device additional information may be generated at the
expense of greater bandwidth. An imaging lens 24 is mounted in
front of photo detector 27. Imaging lens 24 projects an image of
the surface of target 23 onto photo detector 27. The image
projected onto photo detector 27 includes an image 26 of the spot
22 where the collimated beam impacts the target. The position of
imaged spot 26 varies with the distance from light source 1 to
target 23 due to parallax. If, for example, the surface of target
23 is at 22" the imaged spot is at 26". Similarly, if the target is
at 22"the imaged spot will be at 26". The result is that imaged
spot 26 moves back and forth across the surface of photo detector
27 in an amount relative to the distance between target surface 23
and light source 1. Photo detector 27 thus generates an electrical
signal containing information about the distance between 1 and
23.
[0039] The complete optical system, as described, is rotated around
the central axis of the probe in order to scan the complete
circumference of the tube. Other embodiments modify the basic
optical system to reduce the need for slip rings for the electrical
signals required by the laser diode and detector. By locating the
laser diode source and collimating lens on axis with the probe
longitudinal axis and using a rotating 45.degree. mirror to deflect
the light beam at a right angle to the probe longitudinal axis, the
beam can be made to scan the circumference of the tube. A second
mirror and imaging lenses are also rotated with the first mirror,
to form an image of the light spot onto a stationary 2D PSD. This
approach has the advantage of eliminating slip rings from the
probe. The disadvantage is that the 2D PSD provides 4 outputs
instead of 2, and requires more processing to compute the radius
data. Another disadvantage of this method is that a transparent
tube or windows in the housing are required to support the laser
diode and detector, while still allowing a light path. The window
or transparent tube are subject to scratches or dirt which provide
reflection paths for light leakage between the laser diode source
and the detector. These light leakage paths cause errors in the
radius measurement. Also, the electrical wires necessary to connect
to either the diode or detector must cross the path of the light
beam at some point in the rotation of the lens/mirror assembly.
Other variations of the optical arrangement are possible. The
embodiments described herein are not intended to limit the scope of
the invention, but rather are for illustrative purposes.
[0040] FIG. 3 shows a detailed profile view of the probe of an
embodiment in this invention. The probe consists of a rotating
optical head 30 mounted to a body 31. The relative size of the
reformer tubes present other challenges to laser-optical probe
design. Previous probes of this type have been for smaller diameter
tubing, up to 2-inches in diameter. For larger diameter tubes, such
as reformer tubes, the weight and inertia of the probe and its
rotating components must be reduced to make the approach practical.
In one embodiment of the system, the probe head 30 is spinning at
1800 rpm. Replacing a metal spinning head with one made of
Delrin.TM., an engineering plastic made by Dupont, provides weight
and inertia reduction while maintaining structural strength,
thermal stability, and impact resistance.
[0041] The probe body 31 is made relatively small compared to the
diameter of the reformer tube. This allows weight reduction, and
has the benefit of allowing the same probe body to be used in
several different tube sizes by changing the centering spring
assemblies, and the probe head. Weight reduction is important in
the reformer tube application because the probe is drawn through
the tube by a motorized positioning system. Reducing the weight
reduces the size and cost of the positioning system.
[0042] A tether and electrical connections are made to the probe
through connector 32. Electrical connections between the connector
and the optical head are made through the probe body by means of an
internal slip-ring. Probe rotation is by means of a motor inside
probe body 31. Centering rings 35 are spring loaded arms with
non-metallic wheels to hold the probe in the center of the tube,
and allow for axial motion through the tube.
[0043] Reformer tubes are made of special alloys to withstand the
temperature and pressure regimes to which they are subjected.
During normal operation, portions of reformer tubes operate at or
near the structural stability limit of the tube metallurgy. If the
probe leaves any traces of other metals on the inner tube surfaces,
such as aluminum or lead, the traces of these metals will enter the
pores of the tube wall and cause rapid cracking and failure of the
tube.
[0044] Therefore, it is important that only non-metallic components
be used where the probe is in contact with the tube surface. The
probe's centering spring mechanism 35 contacts the sides of the
tube with wheels 36 made from Delrin.TM., an engineering plastic
with metal-like properties.
[0045] One of the problems that affects accuracy in PSD based laser
triangulation systems is when unintended reflections cause
additional light to impinge on the sensor. These reflections arise
when light reflects from the surface being measured, bounces off
other surfaces and enters the detector from various angles. Because
the PSD sensor measures the centroid of the light imaged on its
surface, reflections cause a skewing of the image centroid. The
present invention minimizes reflections by placing the laser 37 at
the front of the probe head and in front of the detector 38. In
contrast to designs with the laser behind the detector, this
reduces the exposure of the detector to light reflections off the
probe head 30, body 31, and centering spring assemblies 35.
[0046] FIG. 4 is a block diagram of an alternate embodiment of the
invention. This embodiment projects a ring of light onto the
interior surface of the reformer tube. The ring is then scanned
with an array of light sensors to produce and reconstruct an image
of the interior of the tube.
[0047] A light source 40 is connected to a power controller to
maintain a light level sufficient to be sensed by an image
detector. Typically light source 41 is a laser diode with output in
the infrared or visible portion of the spectrum. A light detector
42 is positioned in such a manner as to view an image of the ring
of light. Light detector 42 is a two dimensional array of
photosensitive cells. Most commonly detector 42 is a Charge Coupled
Device (CCD) array of photocells. Such arrays are commonly used in
video cameras. Array 42 is divided into individual pixels each
represented by an X coordinate and a Y coordinate. The signal from
each pixel is proportional to the intensity of light falling on
that pixel. The detector is controlled by a timing and control
module 43. In general, module 43 scans the array in a line-by-line
fashion. Each individual line is moved to an output register. The
individual pixels are then shifted out in a serial operation. The
CCD array is of the Frame Transfer type which uses two identical
CCD arrays, one for scanning and one for storage. The second array
is shielded from light and acts a buffer to allow reading an image
while a second image is being formed on the first array. A typical
sensor is a 512.times.512 array having 262,144 elements or pixels.
Typically, there are additional light shielded pixels in the array
which provide buffering of the active pixels. This increases the
number of total pixels which must be read.
[0048] A typical 512.times.512 array requires a clock speed of 80
Mhz to read out with time for the buffer pixels, transferring the
frame, etc., for a 120 Hz frame rate. Some arrays have a split
output structure, so that two pixels are read at a time. This
reduces the clock rate to 40 Mhz, but requires two parallel output
processing channels. In tested prototypes the ring image has a
thickness of 3-5 pixels. At the maximum radius the number of pixels
actually used is provided by equation 2 as follows:
2.multidot..pi..multidot.r.multidot.5.congruent.8 k pixels EQ.2
[0049] Where r, at the maximum radius is 256 for a 512.times.512
array. In other words each frame will hold about 8k pixels of
useful information.
[0050] During use, array 42 converts the image to an array of
intensity values. The Analog to Digital Converter 44 connected to
Array 42 converts this analog signal to a digital one. An eight-bit
A to D converter has been found suitable for 44. The signal next
goes to a Look-Up-Table (LUT) 45 to convert the Cartesian X, Y
coordinates received from the Timing and Control Module 45 into
radial coordinates r,.sub.t. LUT 45 may be a logical array or an
addressable device such that when an X, and Y address is input, a
value for r and .phi. is provided. This may be done with
non-volatile memory devices such as ROM, PROM, EPROM, EEPROM, flash
memory, or a volatile memory such as static or dynamic RAM. The
speed of operation required for this embodiment dictates the use of
volatile memory, such as synchronous RAM. The access time must be
under 30 nanoseconds for a 512.times.512 array. A 256 k.times.18
device provides 9 bits each for r and .phi.. For a 10-bit .phi.,
the MSB of the Y address value can be added to .phi. to form 10
bits. A 9-bit .phi. is used for 360 points per rotation, 10 bits
for 720 points per rotation, for 1/2.degree. resolution.
[0051] The X and Y address of each pixel is fed to the address
lines of the LUT at the same time that the value of intensity for
the pixel is available from the A to D converter 44.
[0052] Because LUT 45 is a volatile memory device, it must be
loaded with the lookup values before use. This may be done on power
up of DSP 50 or host PC 51. LUT 45 is programmed with the
corresponding r and .phi. for each X and Y address of sensor 42
according to equation 3 as follows:
r=SQRT (X.sup.2+Y.sup.2), .phi.=tan.sup.-1(Y/X) EQ. 3
[0053] The intensity value from array 42 and the r, .phi. values
from LUT 45 are sent to a Data Range Control module (DRC) 46. DRC
46 reduces the amount of data used in further processing steps. The
actual image from array 42 includes only about 8 k of pixels out of
a typical 256 k pixels in a 512.times.512 array. The pixels of
interest are in a circular area in the outer third of the array.
The definition of the pixels of interest t is r>r.sub.t where
r.sub.t is the radius threshold value and is the minimum r value of
interest. No image data ever occurs at r-values less than r.sub.t.
DRC 46 includes logic circuitry which only passes to FIFO 47 the
information of interest i.e. r>r.sub.t.
[0054] Even with the use of DAC 47 there are still too many pixels
without useful information for easy processing. For example in a
3-in ID pipe with a probe measuring range from 2.25 in to 2.75 in.
radius and a 512.times.512 array the measuring range will cover 85
pixels with r having a range of from 171 to 256. If r.sub.t is set
to 170 there are still over 170,000 pixels per frame to be
processed. Only the pixels that are illuminated by the light source
40 provide useful information. Since only pixels with intensity
over a certain threshold are of interest that fact may be used to
eliminate surplus pixels. A high-speed comparator 48 is used to
compare the intensity value of each pixel with a threshold value
I.sub.t. The output of comparator 48 triggers the data selection to
pass only the data including r, .phi., and I, where I is greater
than the threshold I.sub.t. This reduces the data sent to FIFO to
about 8k pixels. The threshold value I.sub.t is set with a DAC
device 49. DAC 49 is set from the host system via the DSP
controller 50 during calibration.
[0055] Since the actual data rate is greater than 40 MHz
accordingly; a FIFO 47 is used to buffer the data at that rate.
Data entering FIFO 47 clocks at the scan rate of the array 42 but
is sporadic and has many gaps due to the action of the data
selection effect of DRC 46 and comparator 48. FIFO 47 buffers the
data for DSP 50, which is thus able to read the data at a slower
rate. DSP 50 must still process all 8 k of data in under 8.3
milliseconds in order to process 120 frames/sec. FIFO 47 will
buffer one frame of data for an 8 k FIFO or two frames for a 16 k
FIFO.
[0056] The Digital Signal Processor (DSP) 50 performs the actual
computation of the radius of the tube by finding the centroid of
the imaged light on each radial spoke .phi..sub.k where k indexes
the angle through 360 or 720 increments depending on the
resolution. The centroid for each radial spoke .phi..sub.k is
computed according to equation 4 as follows: 2 i r i I i i I i EQ .
4
[0057] Where r is the radius value from 0 to 255 for a
512.times.512 array, and I is the corresponding intensity value for
that pixel, i is the index value for the array of points comprising
the radial spoke. In practice, i starts at a radius much larger
than 0 since the image is set in the outer third of array 42. A DSP
50 of sufficient speed can perform the division and provide the
radius value directly. If a lower speed DSP 50 is used it can
compute the numerator and denominator of the above equation and
provide the values as separate outputs. A post processing operation
in host PC 51 computes the actual radius value and converts the
output to engineering units such as inches or millimeters.
[0058] The processed data is sent to host PC 51 via a high-speed
interface; suitable methods include serial interfaces such as
RS232, RS485, USB, or IEEE-1394 (Firewire) or parallel interfaces
such as a PC parallel port or EPP.
[0059] Host PC 51 receives the data from the probe processing
system, and does any post processing of the data and formats it for
storage on hard disk or removable media storage and displays the
data on a graphics screen.
[0060] FIG. 4 illustrates an implementation having a single
processing channel. For an image sensor with a dual output
structure, another channel of processing is added with another A/D
converter, LUT, Data Range Controller, comparator, and FIFO.
[0061] FIG. 5 is a front elevation view of the optical system of
the FIG. 4 embodiment. The components are located in a tube 64 to
be inspected. A laser diode 61 is located at the center of tube 64
in such a manner that the light emitted by diode 61 is parallel to
the axis of tube 64. A collimation lens 62 is located on said axes
to focus the light on the interior surface of tube 64. A cone
mirror 63 is located coaxial to diode 61 and lens 62 to form the
light emitted from diode 61 and collimated by lens 62 into a ring
65 on the surface of tube 64. Mirror 63 is preferably a parabolic
conical mirror to aid in focusing the beam on the interior surface
of the tube. An imaging lens 66 coaxial with diode 61 lens 63 and
mirror 63 is situated in such a manner as to project an image of
ring 65 onto the surface of an imagining array 67. Imaging array 67
senses the image projected onto its surface and converts the image
into electrical signals.
[0062] The above examples and embodiments are exemplary only the
invention being defined solely by the attached claims.
[0063] It will be apparent to those skilled in the ar.sub.t that
various modifications and variations can be made in the "A Method
For Reformer Tube In Situ Inspection Radius Calculation" of the
present invention and in construction of this invention without
departing from the scope or intent of the invention.
[0064] Other embodiments of the invention will be apparent to those
skilled in the art from consideration of the specification and
practice of the invention disclosed herein. It is intended that the
specification and examples be considered as exemplary only, with a
true scope and spirit of the invention being indicated by the
following claims.
[0065] Reference will now be made in detail to the present
embodiments of the invention, examples of which are illustrated in
the accompanying drawings. Wherever possible, the same reference
numbers will be used throughout the drawings to refer to the same
or like parts (elements).
* * * * *