Photoacoustic control of a pulsed light material removal process

The present invention provides an automated system and method for removing one or more layers of a material from a substrate. The system and method include irradiating a structure comprising at least one layer of material formed on a substrate with a light beam having an intensity sufficient to ablate the materials in order to expose selected regions of the substrate, where the ablated material generates photoacoustic signature signals; scanning the structure with the light beam along a predetermined path at a scan speed; detecting the photoacoustic signature signals; determining an updated scan speed functionally related to the detected photoacoustic signals; and directing the scan speed to be equal to the updated scan speed. Another embodiment exposes a selected layer of a multilayered structure in a process which includes irradiating the surface of multilayered structure at a first location with a light beam having sufficient intensity to ablate the irradiated layer and generate photoacoustic pressure wave signals; detecting the photoacoustic pressure wave signals generated at the irradiated surface; comparing representations of the photoacoustic pressure wave signals with a reference value corresponding to a photoacoustic signature signal of a layer of the structure selected to be exposed; and directing the light source to scan the surface of the structure at a scan speed functionally related to the difference between the photoacoustic pressure wave signals and the reference value.

The present invention relates to a material removal system, and more 
particularly, to a system that directs pulsed light (radiant energy) at a 
material to be removed from a substrate and uses the resulting 
photoacoustic effect to control the amount of material removed. 
BACKGROUND OF THE INVENTION 
Material coatings are pervasive in our energy-intensive, 
consumption-oriented society. Coatings provide: immunity to corrosion, 
thermal insulation, shielding, as well as appearance enhancement, and an 
aid in identification. 
During the life of many manufactured products, such as aircraft and ships, 
painted coatings require removal and replacement for a variety of reasons. 
For example, refurbishment of the paint on aircraft is a regular 
maintenance item. Commercial airlines repaint their aircraft about every 
4-5 years of service, depending on the age and operating hours of the 
craft. The United States military typically repaints its aircraft after 
three years of service, or less. 
The removal of paint from the surfaces of aircraft presents special 
problems, in part, due to their large and irregularly shaped surfaces. 
Another difficulty is that, because the surfaces of aircraft are 
principally lightweight aluminum or organically based composite materials, 
they are particularly susceptible to damage which could compromise their 
structural integrity. 
Many different methods have been used to remove painted coatings from 
aircraft. One type, the "particle medium blast" (PMB) method involves 
impinging the surface to be stripped with particles such as BB's, steel 
shot, wheat, starch, and/or sand. However, this method generates unwanted 
dust, requires large quantities of bulk blast materials and is noisy. PMB 
is also difficult to control. If the impinging particles dwell too long at 
one location, the surface of the aircraft may become pitted or stress 
hardened which can change the loading on that portion of the aircraft. PMB 
also damages putty joints often found on aircraft between surface plates. 
Another major problem is when paint materials, which are generally toxic, 
are impinged by the blast particles, the waste produced is toxic. Toxic 
waste requires special handling in order to dispose of it in a manner 
which minimizes damage to the environment. Another problem with PMB is 
that it generates a lot of dust which obscures the area being stripped. 
This impairs visual feedback necessary to control the process, resulting 
in damaged surfaces. 
Some airlines have used water jets to remove paint from aircraft. However, 
friction caused as a water jet impacts a surface such as aluminum 
generates heat which can damage the aluminum, especially if it is thin, 
like that found on aircraft. Another problem with this method is that the 
high pressure water can penetrate into the internal regions of the 
aircraft which are susceptible to water damage. 
It is also known in the art to apply chemical compounds to painted surfaces 
in order to chemically breakdown the layers of paint, thereby stripping 
the paint away from the surface to be exposed. Certain of such chemical 
compounds have been used to remove paint from aircraft. However, such 
compounds may pose a risk to human health, are usually toxic, and often 
not biodegradable. Overall, these types of compounds are difficult and 
costly to dispose of because they present serious environmental problems. 
Mechanical paint removal techniques have also been employed. For example, 
U.S. Pat. No. 4,836,858, entitled "Ultrasonic Assisted Paint Removal 
Method" discloses a hand held tool which uses an ultrasonic reciprocating 
edge placed in contact with the surface to be stripped. Unfortunately, 
employment of this tool is labor intensive and relies upon the skill of a 
human operator to use it effectively. Further, control of this tool is a 
problem because the aircraft surface may still be damaged if there is 
excessive tool dwell at one location. 
Radiant energy paint removal techniques are also known in the art. One such 
system uses a laser and video frame grabber in a video controlled paint 
removal system in which paint is stripped from a surface using particle 
medium blast methods of the type discussed above while a video camera 
converts images of the surface being stripped into electronic data 
signals. The data signals are used to control the particle blast. A 
processor compares the data signals with parameters stored in a memory to 
determine whether sufficient paint has been removed from the surface being 
stripped. If an insufficient amount of paint has been removed, then the 
surface continues being irradiated. If the optically irradiated area has 
been adequately stripped, the processor directs the particles to strip 
another area. 
A significant problem with the video controlled paint removal system is 
that the amount of data which is generated and which must be processed is 
enormous. Hence, real time control of video controlled paint removal 
systems is extremely difficult. Another problem with a video controlled 
paint removal system is that large amounts of dust are generated from the 
effect of the particle blast impacting the surface being stripped. The 
dust can impair the scene being observed by the video camera, also making 
real time control of the process difficult. 
Thus, it can be appreciated that coating removal, and particularly, the 
removal of paint from large, delicate surfaces such as found on aircraft 
is a problem which has not yet been satisfactorily solved. 
The practice of photoacoustic spectroscopy (PAS) for analyzing a given 
solid material is also known. In PAS, light energy is absorbed by a solid 
material, converted into an acoustic wave or pressure pulse which is 
characteristic of the solid material, and then converted into an 
electrical signal for analysis purposes. In such PAS systems, a laser is 
employed to direct light energy at the solid material, although other 
types of light sources may also be used. The material absorbs the light 
energy in a way characteristic of the particular solid material being 
irradiated. Any light absorbed by the material is converted, in part or in 
whole, to heat. An acoustic signal results from the time dependent heat 
flow from the solid material to the surrounding gas. The heat flow causes 
oscillatory time dependent pressure in a small volume of gas at the 
solid-gas interface. An additional source of time dependent pressure in 
the gas can arise when the absorbing solid ablates and subsequently burns 
to release its heat of combustion. It is this motion of the gas which 
produces the acoustic signal that is characteristic of the solid, also 
referred to as the photoacoustic characteristic of the solid. 
The strength of the photoacoustic wave is approximated by a theory 
attributable to Taylor and Sedov (D. A. Freiwald and R. A. Axford, J. 
Appl. Phys., Vol. 46, p. 1171, 1975), which predicts the pressure behind 
the shock for a spherical blast wave to be: 
EQU P=[2/(.gamma.+1)](4/25)(.xi..sub.o.sup.5)(E/R.sup.3) 
where .gamma. is the heat capacity ratio for the ambient air, .xi. is a 
constant, (.xi.=1.03 for air), E is the energy absorbed by the surface of 
the material being irradiated, and R is the distance of the pressure wave 
from the surface. The Taylor, et al. theory predicts that the strength of 
the wave is directly proportional to the energy absorbed by the surface, 
which in turn depends on the absorptivity of the surface at the wavelength 
of the light source. 
An example of a system which detects and measures a photoacoustically 
generated pressure pulse to control a material removal process is 
described in U.S. Pat. No. 4,504,727, "LASER DRILLING SYSTEM UTILIZING 
PHOTOACOUSTIC FEEDBACK." The system described in the '727 patent uses 
photoacoustic feedback to control laser drilling of a multilayered printed 
circuit board. The system analyzes the photoacoustic feedback signals by 
comparing the photoacoustic outputs from the different layers of the 
circuit board with reference signals stored in a memory, and adjusts the 
laser parameters such as pulse duration, wavelength, and energy output, 
pulse repetition rate, and the number of pulses for each successive layer 
accordingly. The circuit board is mounted to an X-Y moving table under the 
direction of the control system which positions selected hole sites on the 
circuit board under the laser beam. However, the system described in the 
'727 patent is not suitable for removing selected layers of material from 
large surfaces in predetermined patterns. 
The types of lasers described in the '727 patent are very long wavelength 
devices, therefore, every surface looks black to such lasers, with little 
or no reflected light energy. Because of this, it would be difficult to 
distinguish material coatings based on their albedo. Therefore, the use of 
a far infrared optical energy source to ablate material from a structure 
likely results in generation of photoacoustic pulses that depend more on 
the mechanical damping properties and resonance of the structure than on 
the characteristics of the ablating material. 
Furthermore, the system described in the '727 patent modulates the output 
of the light source as a function of the amplitude of the photoacoustic 
signals. Such modulation disadvantageously shortens the useful life of the 
laser. 
Thus, there is great need for a system and method which can easily and 
inexpensively remove coatings and which does not present the environmental 
problems of some of the systems and methods described above. A need also 
exists for a coating removal system and method that can be controlled to 
avoid subjecting a surface which is to be exposed to an excessive amount 
of energy which would damage delicate structures. A further need exists 
for a coating removal systems which can be automated. Still further, a 
need exists for a system and method which promotes a long service life of 
the light source. 
SUMMARY OF THE INVENTION 
The present invention provides an automated system and method for removing 
material which includes (1) irradiating a structure comprising at least 
one layer of material formed on a substrate with a light beam having an 
intensity sufficient to ablate the materials in order to expose selected 
regions of the substrate, where the ablated material generates 
photoacoustic pressure wave signals; (2) scanning the structure with the 
light beam along a predetermined path at a scan speed; (3) detecting the 
photoacoustic pressure wave; (4) determining an updated scan speed 
functionally related to the detected photoacoustic pressure wave signals; 
and (5) directing the scan speed to be equal to the updated scan speed. 
Another embodiment of the present invention exposes a selected layer of a 
multilayered structure in a process which includes (1) irradiating the 
surface of multilayered structure at a first location with a light beam 
having sufficient intensity to ablate the irradiated layer; (2) detecting 
photoacoustic pressure wave signals generated at the irradiated surface; 
(3) comparing representations of the photoacoustic pressure wave signals 
with a reference value corresponding to a photoacoustic pressure wave 
signal of a layer of the structure selected to be exposed; and (4) 
directing the light source to irradiate a second selected location on the 
structure if the result of step (3) indicates the photoacoustic pressure 
wave signals are within a predetermined limit of the reference value. 
An advantage of the present invention is that it provides an automated 
system for removing coatings from a substrate. Another advantage of the 
invention is that it is able to remove layers of coatings without damaging 
the layer to be exposed. A further advantage is that the invention may be 
employed to expose a selected layer of a multilayered structure. The 
invention also overcomes noise, environmental, and material handling 
problems associated with prior art material removing systems. A still 
further advantage is that the invention may be employed to remove coatings 
having varying, non-uniform thicknesses. These and other advantages of the 
present invention will become more readily appreciated upon review of the 
specification, drawings, and claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following description is of the best mode presently contemplated for 
carrying out the invention. This description is not to be taken in a 
limiting sense, but is made for the purpose of describing the general 
principles of the invention. The scope of the invention should be 
determined with reference to the claims. 
The present invention provides an automated system and method for exposing 
a selected surface of a layer of material of a multilayered structure 
formed of two or more layers of material, as for example, two layers of 
paint formed on an aluminum plate. The operation of the invention relies 
upon the phenomenon that different types of materials generate 
photoacoustic pressure wave signals when they ablate, and that these 
signals may be distinguished and used to control the process. Ablation is 
the rapid decomposition and vaporization of a material resulting from the 
absorption of energy by the material and is associated with the generation 
of pressure waves radiating from the surface of the material. The 
intensity of the pressure waves is characteristic of the ablating 
material. 
An example of a system and method that uses photoacoustic feedback to 
control a material removal process embodying various features of the 
present invention directs an intense light beam to irradiate the surface 
of a layer of material of a multilayered structure which may be formed one 
or more layers of material formed on substrate. The intensity of the light 
is sufficient to cause the irradiated surface to absorb enough light 
energy in the form of heat so that the material comprising the layer 
ablates. This generates a photoacoustic pressure wave signals 
characteristic of the amount of material. As this process continues, 
additional material from the successive layers ablate. By detecting and 
discriminating the photoacoustic pressure wave signals, it can be 
determined when a particular layer of the substrate has been exposed. The 
light is then directed to irradiate another region of the structure, 
thereby preventing damage to the substrate from excessive heat absorption. 
Thus, the photoacoustic pressure wave can be used to control a material 
removal process. 
The present invention is particularly well suited for removing layers of 
paint and epoxy from metallic surfaces. This is because the photoacoustic 
pressure wave signal characteristic of bare metal is virtually 
non-existent. The invention may also be used to strip corrosion from metal 
substrates. One principal advantage of the invention is that selected 
layers of a structure can be exposed without regard to the thickness of 
any of the layers. The above applications are provided by way of example 
only, and are not to be construed as limiting application of the 
invention. 
A general overview of the present invention is described with reference to 
FIG. 1 where there is shown pulsed light source 10 controlled by light 
control o circuit 11. Light source 10 generates light beam 12 which is 
emitted through window 13 so that it irradiates a selected region of 
multilayered structure 14. For purposes of illustration, structure 14 is 
described herein as having layers 16 and 18 formed on substrate 20, where 
in this example, it is desired to expose substrate 20. However, it is to 
be understood, that structure 14 may be comprised of any number of layers 
formed on a substrate to be exposed. Light source 10 provides the energy 
used to remove layers 16 and 18 through a process known as ablation. In 
typical applications of the invention, layers such as 16 and 18 are 
comprised of organic compounds. 
An important feature of the invention is that it limits the amount of light 
exposure to which substrate 20 is subjected so that it is not damaged from 
absorbing too much energy. Damage caused by heat includes changes in the 
desired mechanical properties of the material, as for example, the modulus 
of elasticity, tensile strength, and/or shear strength. Heat damage could 
also cause the material to become distorted from its intended shape. 
Layer 16 is ablated upon exposure to pulsed light beam 12, causing 
photoacoustic pressure wave signals 22 to be generated from irradiated 
surface 17. Pressure wave signals 22 which are detected by fast pressure 
transducer 24. After removal of layer 16 in the area of where light beam 
12 impinges structure 14, continued irradiation results in ablation of 
layer 18, accompanied by generation of its own photoacoustic pressure wave 
signals. 
During this time, light source 10 scans structure 14 in a predetermined 
path at a speed related to the average peak intensity of photoacoustic 
pressure wave signals 22. A computer controlled robotic positioning 
system, comprising robotic positioner 28 to which light source 10 is 
mounted, and robotic controller 29, is employed to move the light source 
along a predetermined path. Robotic positioner 28 is controlled by robotic 
controller 29 in accordance with instructions provided by data processor 
30. Fast pressure transducer 24, also mounted to robotic positioner 28, is 
positioned to detect photoacoustic pressure wave signals 22 generated from 
structure 14. By way of example, data processor 30 may be an IBM AT 
compatible personal computer, although the scope of the invention includes 
the use of data processors other than that specifically identified above. 
Robot positioner 28 may be a CIMROC 4000 Robot Controller manufactured by 
CIMCORP Precision Systems, Inc., Shoreview, Minn., although other 
commercially available industrial robots may also be employed in the 
implementation of the present invention. Techniques for controlling 
robotic positioner 28 so that it travels a predetermined path are well 
known by those of ordinary skill in the robotic field of technology. 
Transducer 24 transforms photoacoustic pressure wave signals 22 into an 
analogous series of electrical pulses having pulse amplitudes 
corresponding to the intensity of the photoacoustic pressure wave signals 
22. Transducer 24 may be realized using a PCP Piezotronics Model 106B50 
piezoelectric sensor. Signal conditioner 26 receives the pulsed output 
signal of pressure transducer 24 and transforms it into a DC analog 
electrical output signal having a DC value proportional to the peak pulse 
amplitude of the signal received by signal conditioner 26. The output 
signal of signal conditioner 26 is received by data processor 30 as a 
variable control input signal. Based on the value of the control input 
signal, processor 30, using an appropriate processing routine, calculates 
(or otherwise determines) an appropriate robotic speed value between 
minimum and maximum values for the scan speed of robotic positioner 28, as 
described below. Data processor 30 then generates a speed control output 
signal to robot controller 29 that corresponds to the determined robotic 
scan speed value. Robotic controller 29 then directs robotic positioner 28 
to travel at a speed corresponding to the speed control output signal. 
In lieu of calculating a robotic speed value, for example, the output 
signal from signal conditioner 26 may be used as an address in a look-up 
table stored or generated by data processor 30 using appropriate 
processing software. Such software may retrieve a robotic speed value 
stored in particular address where the stored robotic speed value 
corresponds to a speed between minimum and maximum values. Data processor 
30 then provides a scan speed control output signal based on that value to 
robot controller 28. 
The speed value determined by processor 30 may be related to the peak pulse 
amplitude of the output signal of pressure transducer 24 by a decreasing 
function, as for example, linear with a negative slope, as shown in FIG. 
2. Such function may be bounded within upper and lower speed limits. 
Examples of the relation between the speed value and the peak pulse 
amplitude are described below. In FIG. 2, if the peak pulse amplitude of 
the output signal 24a of pressure transducer 24 is equal to or less than a 
minimum average peak pulse threshold value, Threshold.sub.min, then the 
speed value is determined to be a maximum scan speed value, Scan 
Speed.sub.max. If the peak pulse amplitude of the output signal of 
pressure transducer 24 is equal to or greater than the maximum peak pulse 
threshold value, Threshold.sub.max, then the speed value is determined to 
be the minimum scan speed, Scan Speed.sub.min. If the peak pulse amplitude 
of the output signal 24a is equal to some amplitude P.sub.2, then the 
speed value is determined to be S.sub.2, where Threshold.sub.Min &lt; P.sub.2 
&lt;Threshold.sub.Max and ScanSpeed.sub.Max &gt;S.sub.2 ScanSpeed.sub.Min. 
However, it is to be understood that there may be applications where it is 
desirable for the scan speed to be related to the output signal of 
pressure transducer 24 by an increasing function. Techniques for 
generating such an output signal from a digital data processor 
corresponding to a value determined by the processor are well known by 
those of ordinary skill in the art. 
The values for Threshold.sub.min and Threshold.sub.max may be determined 
empirically as described below. A number of test scans are made using a 
light source such as light source 10 to irradiate a fresh sample structure 
representative of the structure that is to be processed by the method and 
system of the present invention. The operating parameters of the light 
source, such as modulation frequency and duty cycle may be varied for each 
test. Also, the distance between the light source and the test sample, 
referred to as the standoff distance, may be varied. A typical standoff 
distance is 1.0 inch. The photoacoustic pressure wave signals generated at 
the ablating surfaces of the test samples are detected by a pressure 
transducer such as pressure transducer 24 and recorded using suitable 
recording means. For example, photographs of an oscilloscope screen of an 
oscilloscope, or equivalent device, connected to receive and display the 
output signals from the pressure detector, may be taken. An individual 
(hereafter, "operator") then examines each of the test samples and 
determines, based on personal discretion, which ones have suitable 
finishes. Such discretion may consider the color and/or texture of the 
irradiated surface of the test sample. The operator then identifies the 
test sample having the most material removed, but still having an 
acceptable surface finish. The minimum threshold value, Threshold.sub.min, 
is obtained by approximating the average value of the peak amplitudes of 
the photoacoustic pressure wave signals associated with that test sample. 
The operator may also identify the test sample having the least amount of 
material removed, but still having an acceptable surface finish. The 
maximum threshold value, Threshold.sub.max, is obtained by approximating 
the average value of the amplitudes of the photoacoustic pressure wave 
signals associated with this latter test sample. 
Another way to determine the minimum and maximum threshold values is to 
perform the steps described in the preceding paragraph, but to also scan 
the test sample at different speeds while the test sample is being 
irradiated. Then, the test sample having the least amount of material 
removed, but still having an acceptable finish is identified. The maximum 
threshold value is obtained from the amplitude of the photoacoustic 
pressure wave signals generated from the surface of that test sample in 
accordance with the methods described in the preceding paragraph. The scan 
rate at which this test sample was run then becomes the minimum scan 
speed, Scan Speed.sub.min. The test sample having the most material 
removed, but still having an acceptable surface finish is identified. The 
minimum threshold value is obtained from the amplitude of the 
photoacoustic pressure wave signals generated from the surface of that 
test sample. The rate at which this test sample was run becomes the 
maximum scan speed, Scan Speed.sub.max. Determination of the minimum and 
maximum scan speeds are described in greater detail below. 
The maximum scan speed is determined, for example, by first observing the 
maximum scan rate at which beam 12 irradiates a "footprint" of the surface 
and still remove sufficient material. The footprint is that area ablated 
by light source 10 while the scan speed of the light beam is zero. The 
maximum scan rate may then be established at a rate somewhat less than the 
maximum observed scan rate in order to provide for a margin of error. A 
high scan rate is desirable because it reduces the time required to expose 
the desired material. However, if the scan rate is too high, insufficient 
material will be removed to fully expose the desired material. It is 
preferable to provide the maximum scan rate with a safety factor to 
account for experimental error and variations in the characteristics of 
the structure having the surface to be exposed. A maximum scan rate may be 
established which is slightly less than the maximum acceptable observed 
scan rate that resulted in exposure of the selected surface having an 
acceptable surface finish. For example, if the maximum observed acceptable 
scan rate is 2.0 inches/second and a safety factor of 10 percent is 
desired, the maximum scan rate may be established at 1.8 inches/second. 
The minimum scan rate of robotic positioner 28 is established by first 
observing the slowest speed at which light beam 12 can scan the surface 
without damaging the surface to be exposed. Damage occurs if the area 
being irradiated is exposed to too much light energy, which when absorbed, 
is transformed into heat. As with the maximum scan rate, it is preferable 
to incorporate example, assuming the slowest acceptable observed scan rate 
is 2.0 inches/second and a safety factor of 10 percent is desired, the 
minimum scan rate may be established at 1.8 inches/second. 
Data processor 30 may be suitably programmed so as to determine a scan 
speed, between minimum and maximum scan speeds, which is functionally 
related to a digital representation of the peak intensities transducer 24. 
Such function may be increasing or decreasing, depending upon the 
particular application. 
Again referring to FIG. 1, nozzle 34, mounted to robotic positioner 28, 
ejects a particle stream, as for example, carbon dioxide pellets 32 which 
are directed to impinge, and thereby cool, structure 14 at an area that 
has just been heated as a result of being irradiated by light beam 12. 
Pellets are supplied to nozzle 34 from a carbon dioxide pellet source 36 
which may be of the type commercially available from Cold Jet, Inc., 
Loveland, Ohio. Pellets 32 also sweep away ablated materials and prevent 
their condensation on emitting window 13 or structure 14. The ablated 
materials and expended particle stream materials 80a are sucked away from 
the area being ablated through nozzle 80 and duct 82, and are collected by 
vacuum system 84. However, it is to be understood that the particle stream 
may also be comprised of dry gas, liquid, or other solid particles 
entrained in a gas. 
Referring to FIGS. 3A and 3B, light source 10 preferably includes broadband 
xenon flashlamp 548 mounted in housing 512. Broadband optical energy 
generally refers to optical energy that includes spectral components with 
wavelengths that may range from 170 nm to 5000 nm. A flashlamp or 
flashtube is a gas filled device which converts electrical energy to 
optical energy by passing current through a plasma typically contained in 
a transparent tube through which the optical energy is transmitted. 
Housing 512 includes upper housing 550 attached to lower housing 552 by 
fasteners 554. Reflector 516 is mounted in lower housing 570 so that 
portions of light generated by flashlamp 548 are reflected out of housing 
512 through quartz window 13. Housing 512 may be fabricated from black, 
hard anodized aluminum. Gasket 556 is interposed between upper and lower 
housings 550 and 552 to keep moist air from penetrating chamber 551 in 
upper housing 550. Electrical connectors 567 at the ends of optical energy 
source 514 are supported in and extend through apertures 562 in walls 563 
of lower housing 552. Flashlamp 548 is positioned within fused quartz 
water jacket 551 mounted between walls 563 of lower housing 552. The 
position of flashlamp 548 is maintained by "O"-ring compression fittings 
558 that fit over electrical connectors 567a and 567b, and are fastened to 
walls 563 by threaded fasteners, not shown. "O"-rings 559 interposed 
between compression fittings 558 and walls 563 provided a water tight seal 
therebetween. By way of example, reflector 516 may have an elliptical 
cross-section as shown in FIG. 3B, having a major axis of 7.00 cm, a minor 
axis of 2.80 cm, and a length of about 15.00 cm. In such case, the 
longitudinal axis of flashlamp 548 is generally coincident with a focus of 
reflector 516. However, it is to be understood that the cross-section of 
reflector 516 may be shaped in a variety of ways, preferably for example, 
as a keyhole or cusp. 
Referring to FIG. 3A, access to flashlamp 548 is obtained through removable 
access plates 570 and 572 releasably mounted to lower housing 552 by 
means, not shown, as would be known by those skilled in the art. "O"-ring 
571 provides a watertight seal between access plate 570 and lower housing 
552. Likewise, "O"-ring 573 provides a watertight seal between access 
plate 572 and lower housing 552. 
Electrical power to energize flashlamp 548 is conventionally provided by 
high voltage coaxial cable 579 that penetrates upper housing 550 through 
cable fitting 568 and includes center conductor 566a and braided conductor 
strap 566b. Center conductor 566 is conventionally connected to high 
voltage terminal post 569a with a lug 565 soldered or brazed to the center 
conductor. Terminal post 569a is electrically connected to flashlamp 548 
via braided cable 575a brazed to high voltage electrical connector 567a. 
Electrical return is provided by braided cable 575b brazed or soldered to 
low voltage electrical connector 576b and to terminal post 569b. The end 
of braided conductor strap 566b is terminated with lug 565b which is 
connected to terminal post 569b. 
Flashlamp 548 may be removed from lower housing 552 as follows: First, 
electrical power must be disconnected from housing 512. Then, quick 
connect fittings 555 are disconnected from inlet and outlet tubes 544 and 
546, respectively. Fasteners 554 are removed from stantions 557 connected 
to lower housing 552 so that the lower housing may be separated from upper 
housing 550. Then, access plates 570 and 572 are removed from lower 
housing 552. Braided cables 575a and 575b are unbolted from terminal posts 
569a and 569b, respectively. Compression fittings 558 are unfastened from 
walls 563 and slipped out over their respective braided cables 575a and 
575b. Then, flashlamp 548 may be carefully slipped out of water jacket 547 
through either of apertures 562 and out of lower housing 552. Replacement 
of flashlamp 548 is accomplished by performing in reverse order, the steps 
recited above for removing the flashlamp. 
Light generated by flashlamp 548 is emitted through quartz water jacket 551 
and exits lower housing 552 through window 13 either directly, or by 
reflecting off of reflective surface 517 of reflector 516. Window 13 is 
preferably manufactured of fused quartz because such material has 
excellent transparency and high resistance to heat. Further, the 
transparency of quartz does not degrade from exposure to ultraviolet 
light. Gasket 581 is interposed between window 13 and window frame 580 so 
that the window is held in a watertight arrangement to lower housing 552 
by bolts 582. 
Flashlamp 548 and reflector 516 are preferably cooled with deionized water 
having a temperature, for example, of about 50.degree. F. supplied at a 
rate of about 2 gpm from a water supply (not shown) to housing 512 through 
inlet tube 544 and returned through outlet tube 546. The deionized water 
preferably has an electrical resistance of at least 1 megohm. Inlet tube 
544 penetrates upper housing 512 and is connected to manifold 574, mounted 
in lower housing 552, having multiple outlets 576 which penetrate 
reflector cavity 564 to distribute water over the length of flashlamp 548 
and fill the reflector cavity. Water also penetrates the tapered ends 588 
of quartz water jacket 551 to cool electrical connectors 567a and 567b, 
and flashlamp 548. Heat resulting from the generation of radiant energy 
from flashlamp 548 is absorbed by the water and transported out of chamber 
564 through port 578 in fluid communication with outlet tube 546. 
It is well known that in order to maximize the service life of a flashlamp, 
the operation of the flashlamp should be critically damped, that is, it 
should be operated with a dampening coefficient of about 0.77. Factors 
that determine the dampening coefficient of a flashlamp include: 
inductance of a single mesh pulse forming network ("PFN") typically 
employed in a flashlamp power circuit, capacitance, C, of the PFN, arc 
length of the flashlamp, and operating voltage, V, across the terminals of 
the flashlamp. The energy output, E, of a flashlamp is characterized by 
the relation E=CV.sup.2. However, V should only be varied by no more than 
about .+-. 5 percent of the optimum voltage in order to maximize service 
life. Further, it is not practical to vary C because of the expense of 
additional capacitors required to implement such a circuit and because of 
the life limiting character of this type of circuit. Therefore, in order 
to maximize the useful life of flashlamp 548, it is preferably operated at 
a constant repetition rate with a fixed pulse width. 
By way of example only, flashlamp 548 may be configured as having a tube 
filled with xenon gas at a pressure of 60.0 KPa, an overall length of 28 
cm, a 7 mm inside diameter, 9 mm outside diameter, and 15 cm arc length. 
This particular flashlamp is preferably operated at a repetition rate of 
4-5 Hz with a full-width, half-maximum ("FWHM") fixed pulse width in the 
range of 1200-1800 microseconds and an input energy of about 100-120 
joules/cm of arc length. As is characteristic, the useful output energy of 
a flashlamp available to irradiate the surface of structure 14 is 
approximately 20-25 percent of the input energy to the flashlamp. The 
flashlamp is powered by a suitable power supply, not shown, as would be 
known by those of ordinary skill in the art. 
Because flashlamp 548 is operated with a damping coefficient of about 0.77, 
the preferred method of controlling the energy flux (joules/second) at the 
surface of structure 14 is to establish an appropriate distance between 
the flashlamp and the surface of the structure since the incident energy 
intensity at the surface of the structure is generally inversely 
proportional to the distance between the surface and the flashlamp. The 
distance between the flashlamp and the surface of structure 14 is more 
conveniently discussed with reference to the standoff distance, d, between 
the surface of the structure and window 13, since the window and the 
flashlamp are a fixed distance apart. 
Although the preferred embodiment utilizes a broadband flashlamp to 
generate light beam 12, the invention may also employ other types of 
pulsed light sources that generate visible or ultraviolet light. Such 
light source may also include a laser. The output of a broadband flashlamp 
offers the advantage of providing electromagnetic spectrum components 
which may have high probabilities of being absorbed by the different types 
of materials to be subjected to the method and process of the present 
invention. 
Light control circuit 11 may be of the type described in U.S. patent 
application Ser. No. 07/645,372, entitled "Ruggedized Flashlamp Exhibiting 
High Average Power and Long Life," by Richard G. Morton and William J. 
Connally, filed Jan. 24, 1991, assigned to the same assignee as the 
present application, and incorporated herein by reference. 
FIG. 4A shows a more detailed view of the signal conditioning circuit 26. 
As seen in FIG. 4A, such conditioning circuit 26 is comprised of several 
elements. A line power unit 60 maintains the pulse train signal received 
via signal line 24a from pressure transducer 24 ground-based and at 
positive polarity. (Hereinafter, it is noted that the signal appearing or 
present on a given signal line may be referred to using the same reference 
number as the signal line. That is, e.g., the pulse train signal provided 
by acoustic transducer 24 on signal line 24a may also be referred to as 
"signal 24a".) Line power unit 60 thus provides a positive signal 60a to 
sample-and-hold unit 62. Track-and-hold unit 62 holds the peak value of 
output signal 60a until the next pulse arrives and provides an output 
signal 62a representing the peak amplitude of each pulse of output signal 
60a, as illustrated in FIGS. 4B and 4C. By way of example, FIG. 4B 
presents a typical output signal 60a of line power unit 60 consisting of 
pulse 100 having a peak amplitude of 30 at time t=7, pulse 104 having a 
peak amplitude of 50 at t=20, pulse 106 having a peak amplitude of 60 at 
t=36, and pulse 108 having a peak amplitude of 15 at t= 50, where t 
represents time and the units of amplitude and time are arbitrary. (For 
example, the amplitude units may be millivolts,and the time units may be 
milliseconds). The output signal 62a of track-and-hold circuit 62 is 
directed to A/D converter 68 in response to receiving a reset signal from 
processor 30. In accordance with the example above described with 
reference to FIG. 4B, and now also referring to FIG. 4C, signal 62a has 
amplitude 30 over the time interval 7-20, amplitude 50 over the time 
interval 20-36, amplitude 60 over the time interval 36-50, and amplitude 
15 from the time interval beginning at t=50. 
The output signal 62a of track-and-hold circuit 62 is scaled and converted 
into a digital output signal 68a by A/D converter 68. Digital output 
signal 68a is provided as the variable input to data processor 30, as 
discussed above. Data processor 30, in turn, outputs a speed control 
signal to robotic controller 29 in order to drive robotic positioner 28 at 
the appropriate speed along a predetermined path. Data processor 30 also 
generates a clocked reset signal 30a which triggers track-and-hold circuit 
62 to provide signal 62a to A/D converter 68. 
FIG. 5 illustrates the photoacoustic pressure wave signal of yellow primer 
when ablated from an aluminum substrate. using a series of radiant energy 
pulses. The amplitude of the photoacoustic signature corresponding to 
pulse numbers 0-150 had a shock strength that averaged about 9.3 
lbs/in.sup.2 which corresponds to the photoacoustic pressure waves of 
yellow primer (MIL-P-23377) being ablated by an excimer laser. The output 
of the laser was modulated at a frequency of 10 Hz and had a duty of cycle 
of 2.times.10.sup.-7. The photoacoustic pressure waves were detected with 
a PCP Piezotronics Model 106B50 piezoelectric sensor and provided to an 
oscilloscope. Each pulse number represents a laser pulse impinging the 
primer with an intensity of 3.8 joules/cm.sup.2. As seen in FIG. 5, the 
shock strength of the primer rapidly diminished between pulse numbers 150 
and 180, indicating that a decreasing amount of material was being 
ablated. From about pulse number 180 and higher, it can be seen that the 
pulse amplitudes had a strength of approximately 7 mv, indicating that the 
laser was irradiating a different material than the one previously 
ablated, which in this example was aluminum. The difference in the 
photoacoustic pressure wave signals between the primer and the aluminum is 
thus clearly distinguishable. 
FIG. 6 illustrates the electrical analog of the photoacoustic pressure wave 
signals resulting from the ablation of white paint on an aluminum surface 
irradiated with a single pulse of a Xenon flashlamp at an intensity of 15 
J/cm.sup.2. The photoacoustic pressure wave signals were detected using a 
PCP Piezotronics Model 106B50 piezoelectric sensor having an output 
directed to an oscilloscope. In FIG. 6, the pulse amplitude is represented 
on the vertical axis by 50 mv/division, and time is represented on the 
horizontal axis by 1 seconds/division. FIG. 7 similarly illustrates the 
electrical analog of the photoacoustic pressure wave signals resulting 
from the ablation of black paint from an aluminum surface, where the 
vertical axis is scaled at 100 mv/division and the horizontal axis is 
scaled at 1 millisecond /division. The photoacoustic pressure wave signals 
of the black paint were obtained in the same manner as the photoacoustic 
pressure wave signals of the white paint. Accounting for the difference in 
the scales along the ordinate axes between FIGS. 6 and 7, the maximum 
pulse amplitude for the white paint is about 130 mv and the maximum pulse 
amplitude for the black paint is about 650 mv. 
Hence, it can be seen that the differences in the photoacoustic pressure 
wave signals between black and white paint is readily distinguishable by 
the electrical circuitry described with reference to FIG. 4A. 
Thus, the present invention may be employed to remove layers of coatings of 
a structure to expose a selected substrate in an automated process 
controlled by the photoacoustic signature pressure pulses generated by the 
irradiated layers. For this embodiment, it is preferable that the 
structure be sufficiently irradiated to ablate all of the materials that 
are to be removed, but not to an extent which damages the layer to be 
exposed. 
The operation of the method and system of the above described embodiment of 
the present invention may be more fully appreciated with reference to the 
flowchart presented in FIG. 8 and the following discussion. The operating 
parameters for the minimum and maximum scan speeds of robotic positioner 
28 (Scan Speed.sub.min and Scan Speed.sub.max, respectively), as well as 
the modulation frequency and duty cycle for light source 10 are 
initialized at step 100. Path instructions are input into and read by data 
processor 30 at step 102. The path instructions define the predetermined 
path of robotic positioner 28. Then, based on the path instructions, 
values corresponding to the initial position, P.sub.o, and the end 
position, P.sub.end, of robotic positioner 28 are set at step 104. Next, 
the initial scan speed of robotic positioner 28 is set equal to the 
minimum scan speed, Scan Speed.sub.min at step 106. Robotic positioner 28 
is enabled at step 108 and moved to its initial position, P.sub.o at step 
110. At steps 112 and 114, light source 10 and carbon dioxide pellet 
source 36 are enabled. At this stage, light source 10 is irradiating 
structure 14 at an appropriate location and the system is ready to analyze 
data used to control the process. 
Pressure transducer 24 detects photoacoustic pressure wave signals 
generated by the ablated materials of structure 14 which are processed by 
signal conditioning circuit 26 and provided to data processor 30 for 
analysis at step 116. At step 118, data processor 30 determines if the 
intensity of the photoacoustic pressure wave signals ("PWI") are equal to 
or less than a minimum threshold value, Threshold.sub.min. If that 
determination is YES, then the process proceeds to step 122 where data 
processor 30 defines the value for the scan speed, Scan Speed, to be equal 
to the maximum scan speed, Scan Speed.sub.max. If the determination at 
step 118 is NO, then data processor 30 determines if the intensity of the 
photoacoustic pressure wave signal is equal to or greater than the maximum 
threshold value, Threshold.sub.max. If the determination at step 120 is 
YES, data processor 30 defines the scan speed to be equal to the minimum 
scan speed, Scan Speed.sub.min. If the determination at step 120 is NO, 
then data processor 30 determines the scan speed, as previously described 
herein, at step 126 and sets the variable corresponding to the scan speed 
equal to the determined scan speed at step 127. 
Next, data processor 30 provides a scan speed control output signal to 
robotic controller 29 at step 128 which directs robotic positioner 28 to 
move at the appropriate scan speed. 
Next, data processor 30 reads data representative of the position of 
robotic positioner 28 at step 130 and determines the position of robotic 
positioner 28 at step 131. One method by which the position of robotic may 
be determined uses a feedback signal 29a provided from robotic controller 
29 to data processor 30. Feedback signal 29a may include the output 
signals of one or more rotary shaft encoders, not shown, having 
phase-quadrature output signals. Feedback signal 29a typically includes 
data from one rotary shaft encoder for each axis of motion necessary to 
define the position of robotic positioner 28, as for example, by 
coordinates of the X, Y, and Z axes, as well as any rotational axes. 
Techniques for providing data necessary to define the position of a robot 
actuator and for interpreting such data are well known by those of 
ordinary skill in the art. 
After the position of robotic positioner 28 has been determined, a decision 
is made at step 132 as to whether the position, P.sub.1, of robotic 
positioner 28 is the position at the end of the predetermined path, 
P.sub.end, defined by the path instructions at step 102. If the 
determination at step 132 is YES, then the processing of structure 14 is 
complete, since light source 10 has scanned the entire predetermined path. 
Then, at step 136, data processor 30 provides output signals to disable 
carbon dioxide pellet source 36, robotic controller 29 disable robotic 
positioner 28, and light control circuit 11 to disable light source 10. 
However, if the determination at step 132 is NO, then light source 10 has 
not scanned the entire predetermined path along structure 14, indicating 
structure 14 has not been completely processed. Then, the process 
continues at step 116, described above. 
A major feature of this invention is that the process is controlled by 
varying the scan speed as a function of the peak pulse amplitude of the 
photoacoustic pressure wave signals. This control technique promotes a 
long and useful service life of the light source as opposed to systems in 
which the photoacoustic pressure wave signals are used to control 
modulation of the output of the PAS generating light source. 
In the example of the preferred embodiment described above, the amplitude 
of the pressure wave signal pulses is used as an input to control the 
speed of a robotic positioner. By way of example, it has been found that 
when removing painted coatings such as polyurethanes from aluminum 
substrates, such as aircraft surfaces, the speed at which light source 10 
scans structure 14 ranges from about 0.5 in./sec. for 6-8 mil highly 
reflective paint (nominal) to about 1.0 in./sec for 6-8 mil, highly 
absorptive paint. 
In one experiment, a broadband xenon flashlamp was used to expose an 
aluminum surface of a structure comprising an aluminum substrate (2024-T3, 
bare) having a thickness of 0.032 inches coated with a 0.001-0.002 inch 
layer of epoxy primer (MIL-P-23377) and a 0.006 inch polyurethane topcoat 
(MIL-C-83286) formed on the primer. The flashlamp was modulated to have an 
output of 4 Hz with a 1500 microsecond pulse width. The standoff distance 
between the light source and the structure was 0.38 inches, whereby the 
structure was irradiated at a nominal intensity of 9-10 joules/cm.sup.2 
with an effective footprint about 0.125 inches wide. The light source 
scanned the structure at a nominal speed of 0.06 inches/second, providing 
a surface exposure rate of 0.36 in.sup.2 /second. Using these values, both 
the topcoat and primer were removed from the structure. However, it is to 
be understood that in the implementation of the present invention, 
structures, such as structure 14, may be irradiated with an incident 
intensity that may range from 1-30 joules/cm.sup.2, where incident 
intensity refers to the intensity of the optical energy at the surface of 
the structure. The appropriate incident intensity depends on such factors 
as the thermal and reflective characteristics of the particular material 
being processed. 
It is to be understood that the parameters and data presented in the 
preceding paragraph are provided by way of example only, and are not to be 
construed as limiting the scope of the invention. For example, the pulse 
widths of the output of the light source and the scan rate may be 
determined empirically in accordance with the requirements of a particular 
application. For example, pulse widths of 2400 microseconds are well 
suited for exposing the surfaces of metal substrates, whereas pulse widths 
of 1200 microseconds are more appropriate for exposing the surfaces of 
epoxy composites which are less tolerant of heat. Furthermore, the scan 
speed of the light source may be varied depending on factors such as the 
type and thickness of the materials to be removed. 
While the present invention has been described with reference to removal of 
paint from a metal substrate, the invention may be more generally 
employed. For example, the invention could be used to remove oxidation 
from a metal substrate, such as ferrous oxide (rust) from steel. The 
invention may also be employed to remove coatings from non-metallic 
substrates. 
In a second embodiment of the present invention, the photoacoustic 
signature signals from different coatings may be used as a basis to 
control a system for exposing a predetermined layer of a multilayered 
structure. The embodiment takes advantage of the fact that materials may 
be identified by their photoacoustic pressure wave peak intensities. 
Referring to FIG. 1, the second embodiment differs from the first 
embodiment in the way data processor 30 utilizes information corresponding 
to the photoacoustic pressure wave signals received from signal 
conditioning circuit 26. 
Referring to FIG. 1, the second embodiment is described with reference to 
structure 14, where for purposes of illustration, structure 14 may have an 
aluminum substrate 20, a middle layer 18 of black paint, and an outer 
layer 16 of white paint, and it is desired to remove the white paint to 
expose the black paint. First, a small discrete area of layer 16 is 
removed to expose the desired material at layer 18. Then, the human 
operator manually controls light source 10 to irradiate a sample area of 
layer 18 in order to generate photoacoustic pressure wave signals 
characteristic of that layer as it ablates. The peak amplitude of these 
reference signals are used to establish a reference value which is stored 
in data processor 30 and which will be compared to the amplitudes of 
photoacoustic signals generated by ablation of structure 14. From FIGS. 6 
and 7, it can be seen that the maximum pulse amplitude for the white paint 
is represented by about 130 mv and the maximum pulse amplitude for the 
black paint is about 650 mv. So a reference value is selected to represent 
ablation of the black layer, i.e., at 650.+-..delta., where .delta. 
represents a tolerance limit. 
Next, structure 14 is irradiated by light source 10 while the light source 
scans the structure at a maximum scan speed to prevent damaging the layer 
to be exposed, i.e, layer 18 of black paint. Such maximum scan speed may 
be determined as previously discussed herein. While the surface of the 
structure is being ablated, processor 30 continuously compares the 
amplitude of the photoacoustic pressure wave ("PAS") generated by the 
ablating materials. Initially, the PAS signals generated by the white 
paint (layer 16) have an amplitude of about 130 mv. The difference between 
the reference value and the amplitude of the white paint pressure wave 
signals [(650.+-..delta.)-130] indicates that layer 16 has not been fully 
removed. At first, the difference between the PAS signals from the white 
paint and the reference value is relatively high. Therefore, the scan 
speed of the robotic positioner is modified to be relatively slow, as for 
example, a minimum scan speed, to assure removal of the white paint. 
However, as the amount of white paint remaining in the irradiated area 
diminishes, the black paint becomes exposed, and generates its own PAS 
signals having amplitudes of about 650 mv. The difference between the 
amplitude of the detected PAS signals and the reference value becomes 
prgressively less as more black paint is exposed. Therefore, the 
difference between the reference value and the PAS signals generated at 
the surface of the structure diminishes. Continued exposure of black paint 
results in a progressive increase in the scan speed of the robotic 
positioner to prevent damaging the black paint. Then, when the difference 
between the reference value and the PAS signals is less than some 
threshold value, the scan speed is controlled to be a maximum scan speed 
to prevent ablating too much black paint. The scan speed is maintained at 
the maximum speed until the difference between the PAS signals and the 
reference become greater than a minimum threshold value. 
Data processor 30 outputs speed control signals functionally related to the 
difference between the amplitude of the detected PAS signals and the 
reference value. For example, if the difference between the value of the 
amplitude of photoacoustic pressure wave ("PAS") signals and the reference 
value is equal to or less than some minimum threshold value, then the scan 
speed of the robotic positioner may be controlled to be a maximum scan 
speed. If the difference between the value of the amplitude of the PAS 
signals and the reference value is equal to or greater than a maximum 
threshold value, the scan speed may be controlled to be a minimum scan 
speed. However, if the value of the amplitude of the PAS signals is 
greater than the minimum threshold value, but less than the maximum 
threshold value, then the scan speed is controlled such that the value of 
the scan speed is functionally related to such difference by a decreasing 
function, which may for example, be linear, as shown in FIG. 9. 
This embodiment operates most effectively where, as in the example above, 
the photoacoustic pressure wave signals generated by the different 
materials in layers 16 and 18 are distinguishable. Although the second 
embodiment has been described with reference to a specific example, it is 
to be understood that the scope of the invention can be generalized to 
expose a selected layer of material of a structure having any number of 
layers comprised of material combinations other than those identified 
above. 
While the present invention has been described in terms of preferred 
embodiments, it is to be understood that the invention is not to be 
limited to the exact form of the apparatus or processes disclosed. 
Therefore, it is to be understood that the invention may be practiced 
other than as specifically described without departing from the scope of 
the claims.