Ultrasonic assisted protective coating removal

A paint or other protective coating removal arrangement involving the use of reciprocal motion ultrasonic frequency mechanical energy applied to the coating by a variety of tool and abrasive substrate members in the company of surface preparation agents such as coolant, heating, softening, and/or abrasive agents. The invention is particularly applicable and disclosed in terms of protective coating removal from aircraft, such as is often necessary for replacement or in the reutilization of aircraft with different identification markings. The coating removal arrangement is environmentally and human operator safe in comparison with presently used coating removal arrangements such as abrasive blasting and chemical solvent removal.

BACKGROUND OF THE INVENTION 
This invention relates to the field of paint or other protective coating 
removal from structures such as aircraft. 
Protective coatings are used for a variety of functions on vehicles such as 
aircraft. In such service, the protective coating provides immunity to 
oxidation or corrosion, provides thermal insulation and shielding, and is 
a major tool for appearance enhancement and the provision of camouflage 
and identification, as well as providing optical and electrical signature 
control. 
During the life of a painted or coating protected object, hereinafter 
referred to typically as an aircraft, the applied coating often requires 
removal for a variety of reasons, including replacement of worn and 
weathered coating materials repair (local), and changes in the appearance, 
camouflage or identification of the aircraft--such as might occur in the 
sale of an operational U. S. Air Force aircraft to a friendly foreign 
nation as part of an arms agreement. The removal of present-day coatings 
from weapons systems is, however, quite labor intensive and often requires 
the use of highly activated physical and chemical materials. 
Coating removal technology has, at the present time, lagged the development 
of new polymeric resins in the protective coating art. In the past when 
alkyd primers, alkyd topcoats and acrylic nitrocellulose topcoats or 
earlier developed substances were used as aircraft coating materials, 
their removal was readily accomplished with solvent-based strippers which 
employed, for example, methylene chloride as a major component. However, 
as coatings have changed from alkyds and nitrocellyloses to epoxies, 
polyurethanes, and fluoropolymers, such traditional solvent-based 
strippers have become inefficient or ineffective in coating removal, as 
well as being on the OSHA/EPA toxic materials listing. 
Presently used coatings moreover have a useful life expectancy of 5-7 years 
as a result of their environmental, erosion, and fluid resistance 
characteristics. Such life is in notable contrast with a functional life 
of about two years for the alkyd and acrylic nitrocellulose coatings 
previously used. The continued polymerization and aging of these newer 
coatings throughout their life and their resulting increased resistance to 
chemical stripping materially adds to the difficulty of coating removal. 
These coatings therefore are often capable of enduring beyond the first 
usage period of a weapon system. 
The chemical industry has provided improved strippers for use with the 
presently-used coatings by adding activating agents to the traditional 
solvent stripper solutions. Commonly used activators include phenols, 
chlorinated phenols, and amine compounds. However, in addition to being 
unable to effectively and economically remove epoxy and polyurethane 
coatings, such compounds are found to pose human health risk and have 
therefore become substances that are regulated by environmental protection 
agencies and occupational safety and health agencies of the federal and 
state governments. Phenol-activated strippers are the most effective of 
these groups, but often require as many as five stripping applications. 
Such strippers are particularly undesirable in that phenol compounds are 
biodegradable only with a difficulty and therefore can cause especially 
difficult environmental pollution when used in significant quantities. The 
addition of hexavalent chromium compounds to these strippers as a 
corrosion inhibiting agent further restricts the use of such strippers 
from an environmental viewpoint. 
Chemical paint strippers are also inappropriate for the removal of 
protective coatings from the non-metallic organic matrix composite 
materials how being used in aircraft structures--materials such as epoxy 
impregnated woven graphite filament fabric. Chemical paint strippers 
cannot be used for paint removal from such composite materials because of 
the high risk of the stripper chemically attaching organic components of 
the material. 
Mechanical coating removal by abrasive blasting is one current alternative 
to the ue of chenical stripping. Such abrasive media as crushed corn cobs, 
glass beads, plastic beads, walnut shells, synthetic diamond dust, garnet 
particles, and dry ice carbon dioxide pellets have been employed in 
abrasive blasting removal rocesses. High pressure fluids such as water 
have also been used in this type of coating removal. All of these 
techniques have, however, met with such limited success, that a 
cost-effective and safe arrangement for removing protective coatings, 
particularly from aircraft structures is yet a pressing present day need. 
The use of plastic beads in abrasive blasting coating removal from aircraft 
structures and the status of coating removal technology in general is 
described in a technical report titled "Evaluation of the Effects of a 
Plastic Bead Paint Removal Process on Properties of Aircraft Structural 
Materials" published by the Materials Laboratory, Air Force Wright 
Aeronautical Laboratories, Air Force Systems Command, Wright-Patterson Air 
Force Base, Ohio, 45433, and identified as report number AFWAL-TR-85-4138 
dated December 1985. Copies of this report are available from the 
publishing organization and also from the National Technical Information 
Service. The contents of the December 1985 AFWAL report is hereby 
incorporated by reference herein. 
As described in the AFWAL December 1985 report, the use of abrasive 
blasting techniques as an alternate to chemical stripping in metal-skinned 
and organic matrix composite skinned aircraft raises a number of concerns 
as to possible undesired side effects of abrasive blasting on the 
airframe, including the following: 
a. Surface roughness and its potential effects on aerodynamic drag; 
b. Fatigue properties of cleaned metal alloys as a result of the induced 
surface roughness; 
c. Removal of protective metal coatings such as aluminum alloy layers and 
cadmiun plating from steel structure; 
d. Effects on the bond strength of aluminum honeycomb and thin skin 
aluminum metal-to-metal bonded structure. 
e. Effects on the physical properties of graphite/epoxy composite 
materials; 
f. Intrusion of the particulate matter on the wear properties of lubricated 
bearings in the airframe and consequent effects; 
g. Thin skin warpage as a result of surface cold working; 
h. Effects on fatigue crack growth rate as a result of compressive residual 
stress on the surface and tensile residual stress in subsurface material; 
i. Effects on dye penetrant inspection techniques; and 
j. Intrusion of blast particles into avionic compartments. 
The patent art also discloses the attention of inventors to arrangements 
for removing paint and other protective coating materials. This attention 
is evidenced by the patent of J. V. Jones, U.S. 3,623,909, which concerns 
an electrically heated tool and a method for using the tool in paint 
removal. Also included in this art are the patents of H. F. Fairbairn, 
U.S. 4,182,000 which concerns a hand held scraper-sander, B. K. Hoffman, 
U.S. 4,466,851 which concerns a hand held scraper that is especially 
suited for removing fragments of a gasket from automobile engine 
components and P. Toth, U.S. 3,195,232 which concerns a stripping device 
suitable for wall paper removal. 
Additionally included in this art is the patent of R. R. Mason, U.S. 
4,398,961, which concerns a fuel combustion heated device and method of 
use thereof for removing old paint. Also included in this art is the 
patent of W. G. Goerss, U.S. 4,443,271, which concerns an apparatus and 
method used for cleaning floor grates employing high-pressure water jets. 
Further included in this art is the IBM Technical Disclosure Bulletin Vol. 
21, No. 7, dated December 1978, entitled "Stripping Procedure for 
High-Energy and Ion-Bombarded Resists", authored by L. H. Kaplan and S. M. 
Zimmerman which concerns the removal of resist material layers that have 
become hard and glossy after high-energy implantation processes and 
wherein a combination of hot concentrated nitric acid at a temprature of 
80.degree. to 120.degree. C., and ultrasonic agitation are employed. The 
Kaplan and Zimmerman disclosure bulletin includes a possible inference 
that stripping is accomplished in an ultrasonic agitated bath of nitric 
and phosphoric acids. 
In addition, the use of vibrational energy is well known in the patent art 
as is evidenced by the patents of E. J. Murray, U.S. 3,584,327 which 
concerns an ultrasonic energy transmission system, L. Balamuth et al, U.S. 
3,809,977 concerning an ultrasonic tool kit and motor, A. G. Bodine, U.S. 
3,342,076 which concerns a sonic frequency resonator of the pressurized 
fluid energized type. In addition, the patents of E. C. McDaniel, U.S. 
2,651,148; W. T. Harris, U.S. 2,848,672; R. D. McGunigle, U.S. 2,947,886; 
L. Balamuth et al, 2,990,616; C. M. Friedman, U.S. 3,368,280; A. Shah, 
U.S. 3,619,671; R. C. McDaniel, U.S. 3,754,448; Akuris et al, U.S. 
3,980,906, G. Bradfield, U.K. 758,631, and A. E. Crawford, U.K. 2,032,221; 
show a variety of sonic and ultrasonic tools that are uable in dental 
settings for example. 
It is, of course, also well known in the art to employ ultrasonic agitation 
of a container filled with a solvent or chemical reagent for cleaning 
purposes. Apparatus of this type has been commercially available and used, 
for example, in the cleaning of jewelry and in the cleaning of electronic 
parts. Ultrasonic energy has also been used for welding and industrial 
melting fusion arrangements such as in the fabrication of built-up 
assemblies from plastic component parts. 
It may be noted that none of these examples is concerned with the use of 
ultrasonic energy for the removal of paint or protective coatings from 
damage-susceptible surfaces such as the exterior of an aircraft. 
SUMMARY OF THE INVENTION 
In the present invention, mechanical energy of a reciprocating or vibratory 
nature, with the vibrations occurring in the ultrasonic frequency range, 
is employed to assist in the removal of protective coatings from aircraft 
and other objects. The invention contemplates both the use of an excited 
scraping tool and energized abrasive particles as a delivery means for the 
ultrasonic energy. The disclosed ultrasonic energy apparatus has been 
found to be significantly improved in coating removal ability with respect 
to previous vibrating tool apparatus. 
An object of the invention is therefore to provide an ultrasonic energy 
assisted protective coating removal arrangement. 
It is another object of the invention to provide coating removal apparatus 
which operates with significantly lower energy input--energy levels an 
order of magnitude decreased from that of comparable lower frequency 
apparatus. 
It is another object of the invention to provide a viscoclastic coating 
removal apparatus which achieves increased apparent hardness in the 
removed coating material. 
It is another object of the invention to provide a coating removal 
apparatus which operates with significantly reduced displacement amplitude 
with respect to normally used removal apparatus. 
It is another object of the invention to provide an ultrasonic coating 
removal arrangement wherein assisting media such as temperature change 
fluids or chemical softening agents can be employed. 
It is another object of the invention to provide a protective coating 
removal arrangement which is subject to use in both small scale and large 
scale environments. 
It is another object of the invention to provide a protective coating 
removal arrangement which is suitable for use in combustible or other 
hazardous environments. 
It is another object of the invention to provide a coating removal 
arrangement which is safe for use with respect to the environment and with 
respect to human operators. 
Additional objects and features of the invention will be understood from 
the following description and the accompanying drawings. 
These and other objects of the invention are achieved by a protective 
coating removal apparatus for a physcial damage susceptible aircraft 
surface covered with a coating layer to be removed comprising: transducer 
means for generating reciprocal motion mechanical energy of at least 
twenty kilohertz ultrasonic movement frequency; a coating engagement tool 
physically connectable with a mechanical energy output portion of the 
energy transducer means at one tool end and receivable at the opposite 
tool end on the damage susceptible aircraft surface in ultrasonic energy 
transferring mechanical engagement with the coating layer and in sliding 
relationship with the aircraft surface; and moving means responsive to one 
of the coating layer presence indicators of scraping tool resistance force 
and optical energy reflection difference between the paint coating and the 
aircraft surface for moving the ultrasonic frequency mechanical energy 
excited coating engagement tool over the surface of the aircraft in 
engagement with successive portions of the coating layer.

DETAILED DESCRIPTION 
Concern for the effects of a paint or protective coating removal sequence 
on the structural integrity and other functional aspects of modern-day 
aircraft are very real. In the case of both the F-15 aircraft shown in 
FIG. 1 and the proposed organic matrix composite elements to be 
increasingly employed in future aircraft such as B-2 and ATF, the abrasive 
blast coating removal concerns recited above and other aspects of coating 
removal are, for example, the subject of ongoing formal technical 
investigations seeking an optimum coating removal arrangement. 
When aircraft that employ conventional metallic surface materials such as 
the popular Alclad 7075-T6 clad aluminum are subjected to plastic bead 
coating removal in accordance with present day coating removal practices, 
it is not unusual to have the aircraft surface incur a significant degree 
of physical damage. This damage may include erosion of the cladding layer 
to a severe degree, with pitting, thinning and cracking effects attending 
the erosion. In the high speed and high structural loading environment of 
a modern military aircraft, surface which have been damaged to this degree 
are unacceptable. Moreover, when the newer organic matrix composite 
materials are employed in aircraft surfaces an abrasive blast coating 
removal sequence can result in the cutting of matrix filaments and heavy 
disruption of the epoxy filling between filaments; damage of this type is 
also too severe to be acceptable. 
The prospect of surface damage from abrasive blasting and the unsuitability 
of chemial stripping agents for use in modern-day aircraft coating removal 
operations clearly indicates the need for an improved stripping 
arrangement, an arrangement as shown in FIG. 1 of the drawings for 
example. 
In FIG. 1, one aircraft currently used by the U.S. Air Force, an F-15 
fighter, is shown undergoing a small area protective coating removal 
procedure wherein one of the aircraft markers, a cockpit adjacent insignia 
102 is being removed. Such removal would be accomplished, for example, if 
the aircraft were being transferred to a friendly nation, or being 
refurbished and is exemplary of a removal arrangement that is usable on a 
larger scale over the entire aircraft. In the FIG. 1 drawing, a human 
operator 104 is shown using an ultrasonic kinetic energy tool 110 for 
removing the insignia 102, as is indicated by the removed area 114. 
In the FIG. 1 coating removal arrangement, the tool 110 is excited with 
ultrasonic reciprocating motion by a transducer 106 held in the operator's 
hand 112. The tool 110 is energized by an energy source that is not shown 
in FIG. 1, but is tethered to the transducer 106 by the flexible conduit 
108. Preferably, the transucer 106 is of the electrical energy to 
mechanical energy type and may be of the of the transducer type disclosed 
in or or more the above referred to U.S. Patents 3,980,906; 3,809,977; 
3,754,448; 3,619,671; 3,584,327; 3,368,280; 2,990,616; 2,947,886; 
2,848,672; 2,651,148, and U.K. 758,631 and 2,032,221 which are 
incorporated by reference herein. The transducer 106 may also operate in 
conjunction with a transistorized or solid-state electronic power 
converter apparatus connected to the transducer 106 by way of an 
electrical cable embodiment of the flexible conduit 108. 
Electrically operated transducers of the FIG. 1 type are also commercially 
available in embodiments having input energy levels ranging upward from 
400 watts. One apparatus of this type is the Sonicator Heat Systems Inc. 
ultrasonic generator and transducer which is manufactured by Sonicator 
Systems, Inc. of Newark, New Jersey. The Sonicator transducer is of the 
barium type and operates at a power level of about 750 watts delivered to 
the transducer. The Sonicator apparatus operates at an ultrasonic 
frequency of 50 kHz. Larger ultrasonic systems, systems operating in the 
range of 5 to 10 kilowatts of input energy or more, are commercially 
available and are, of course, desirable for large surfaces of an aircraft 
or other extended area structures. Generally, transducers which provide 
mechanical energy output at a frequency of twenty kilohertz and above are 
considered to be ultrasonic a nature. Ultrasonic transducers which are 
energized by compressed air, pressurized hydraulic fluid or other 
pressurized fluid sources of energy are disclosed in the above referred to 
U.S. Patent 3,742,076 and are considered to be within the scope of the 
invention. With such larger transducers, mechanically-supported and 
machine-guided arrangements such as robotic devices which can be 
programmed for the stripping of a predetermined shape and area may be 
desirable. 
FIG. 4 in the drawings provides additional details of a hand-held 
arrangement of the invention. In FIG. 4, an aluminum exterior surface 
portion of an aircraft 400 is shown in the process of having a protective 
coating 402 removed. In the FIG. 4 arrangement, a tool 404 may have a 
square or blunt edge 414 that is disposed at an angle enabling energy 
transferring engagement of the coating 402. 
The tool 404 in FIG. 4 is energized in the reciprocal or vibratory axial 
motion fashion indicated at 412. Such motion is intended to achieve both 
sliding, non-engaging and non-damaging tool movement over the aircraft 
surface 416, along with energy-transferring compression, impacting, 
shearing, and other destructive engagement with the coating 402 in a 
contact region 414. The square or blunt edge embodiment of the tool 404 as 
shown in FIGS. 4 and 5 of the drawings is one plausable arrangement for a 
coating engagement tool for the instant invention. As is illustrated, for 
example, by the end portion of a mill file that has been ground clean and 
square on a grinding wheel or by the square edge of a broken plane of 
glass, such square edge tool arrangements can, indeed, be effective as 
coating engagement and removing tool devices. The very fine or even 
microscopic feather edge which often results from a grinding or glass 
breaking act often, in fact, enhances the coating removal capability of 
such square edge tools and can also provide an effective cutting device - 
as is often painfully apparent to perious working with such materials. 
When used in the present invention, apparatus, such tools are to be held 
at a small angle with respect to the metal surface in order that the tool 
edge slide freely and without energy loss over the metal surface but 
engage the coating material in a substantially head on arrangement that 
imparts ultrasonic energy to the coating material. 
Another tool embodiment usable in the FIGS. 4 and 5 coating removal 
sequences, in fact, an embodiment that is to be preferred, is shown in 
FIG. 2 of the drawings. In the FIG. 2 drawing, the tool 204 is shown to 
include pointed and sharpened working edge portion 212 which subtends an 
angle 206 that is in the order of twenty degrees in size. The body of the 
FIG. 2 tool may be in the range of 0.050 inch in thickness as is indicated 
at 210. The relatively thin body portion and the twenty degree taper to 
the tool working edge 212, in fact, give the FIG. 2 tool a razor blade 
like appearance. During use, the tool 204 is energized with vibrational 
energy motion as is indicated at 214 in FIG. 2 and is preferably disposed 
at an angle 208 with respect to the coated surface; the angle 208 is in 
the range of five to twenty-five degrees in size. An angle in the middle 
of this range, i.e. an angle of fifteen degrees is shown in FIG. 2. 
Displacement amplitudes of one thousandth of an inch or even less are found 
to be satisfactory for the ultrasonic energy motion 214; this motion 
amplitude is notably smaller than the ten thousandths of an inch to one 
hundred thousandths of an inch amplitude usually needed with sonic 
frequency or lower frequency removal tool energizations. The low amplitude 
ultrasonic energization is also conducive to non engagement sliding of the 
tool working edge over the surface 200 that is being cleaned. 
It is notable that the coating material 200 being removed in the FIG. 2 
arrangement of the invention, is frequently found to be responsive to 
ultrasonic energy tool energization in an unexpectedly favorable manner. 
Even though the material being removed is often an intentionally tenacious 
substance such as polyurethane or the above-identified epoxy or 
fluoropolymer coating, it is often noted that in the presence of 
ultrasonic frequency coating removal techniques, such materials display a 
surprising brittle behavior. An increased brittle behavior is, of course, 
found to be decidedly better for removal purposes than is the viscoelastic 
response normally displayed by these and other coating materials. In 
particular, viscoelastic materials are rate sensitive so that the higher 
rates of loading as achieved with the ultrasonic energy removal procedures 
described herein causes these materials to act in a brittle manner. 
A uniquely effective energy transfer is also achieved between the working 
edge portion 212 of an ultrasonic energy excited tool 204 and the coating 
202. This increased energy transfer is demonstrated by the increased rate 
of loading--a loading increase observed when similar tools that are 
energized with subsonic or sonic frequency energy are contrasted with the 
present ultrasonic frequency energy excited tools. This enhanced energy 
transfer is also manifest in thermal darkening of the removed coating and 
thermal dulling of the tool working edge 212 in the case of ultrasonic 
energy excitation. The duration of the elevated temperature is found to be 
relatively short--on the order of one millisecond, however, tool working 
edges made of carbide or diamond materials are desirable with the 
ultrasonic frequency energization in order to achieve practical tool life 
in a working environment in the presence of expected elevated tool 
temperatures. Infrared motion pictures or video camera images as are known 
in the imaging art, can be used to quantify the times, temperatures, and 
precise nature of the tool and coating heating and optimize its utility in 
the coating removal process. 
In view of the more effective energy transfer to the removed coating by the 
tool 204 when ultrasonic energy energization is used, it is found that 
significantly lower total energy input to the removal process will yet 
provide desirable coating removal action. Energy input levels decreased by 
an order of magnitude from those required with sonic or subsonic frequency 
energized removal apparatus are, in fact, found to be satisfactory in the 
case of the described ultrasonic energy energization. 
In the case of ultrasonic frequency tool energization, it is also found 
that relatively little force is required for urging the energized tool 204 
or 404 into contact with the receding edge of the coating being removed. 
In most instances this urging requires no more than simple maintenance of 
physical contact between the ultrasonic frequency vibrating tool and the 
receding coating edge. In the case of robotic or automatic feeding of the 
tool or workpiece as described below and in FIG. 5 of the drawings, these 
low urging forces enable a desirable simplification and downsizing of the 
feed apparatus used. 
The urging force applied to the transducer in FIG. 5 is of course, to be 
distinguished from the vibrational force at ultrasonic frequency that is 
generated by the transducer. The urging or travel force is a 
unidirectional force applied to the transducer and is opposed in F.dbd.MA 
fashion by the combined mass of the tool and transducer and also by the 
tool working edge meeting the edge of the coating 402 or 522--i.e., when 
travel movement is stopped by the tool encountering the coating edge. The 
vibrational force applied to the coating 402 or 522, that is, the 
ultrasonic frequency force, can be much larger than the urging force--in 
the same manner that the well-known air impact hammer used for concrete 
pavement breaking and the like, exerts much larger forces on the concrete 
being broken than are exerted by the human operator or by gravity acting 
on the air hammer. 
According to the present invention, the reciprocal or vibratory axial 
motion 412 in FIG. 4, is provided at ultrasonic vibration frequency, by 
the mechanical energy transducer 406 which may be of the piezoelectric 
crystal or alternatively of the magnetic flux (e.g., moving coil in a 
magnetic field) type, or of the pressurized fluid type. The transducer 406 
in FIG. 4 and the tethering conductor 408 may be considered a generic 
representatious of any of these transducer types, however, an electrical 
transducer is to be preferred for convenience and control. In the case of 
an electrical to mechanical transducer 406, electrical energy of a 
suitable type is supplied from an energy conversion circuit apparatus 410 
by way of a tethering flexible electrical conductor array 408 that 
connects the conversion circuit apparatus with the transducer 406. 
The energy conversion circuit apparatus 410 in the case of an 
electrical-to-mechanical energy transducer at 406, may be of the type 
which employs an electronic oscillator circuit coupled to power amplifier 
transistors that are energized by an AC to DC conversion power supply. 
The apparatus 410 is therefore an energy conversion circuit which in the 
electrical case rearranges the typical 60 Hz or 400 Hz electrical supply 
energy into the voltage, current and waveform desired for operating the 
selected transducer 406. In the case of a fluid-powered transducer at 406, 
the conversion apparatus 410 could, for example, include an air 
compressor, valves, modulators and other fluid flow control devices. 
The square or blunt edge 414 and the sharpened edge 212 are, of course, two 
of the many possible shapes which may be employed is conveying the 
mechanical energy of the transducer to the protective coating. Among the 
desired properties for the tool and the edges 212 and 414 are the 
following: positive engagement with the protective coating being removed; 
sufficient mechanical strength and thermal resistance to withstand long 
periods of use; shape convenient for sharpening and reuse; minimal mass to 
be accelerated by the transducer 406; shaped as needed for compatibility 
with the surface being cleaned; compatibility with a sliding nominal 
energy transfer engagement with the aircraft surface 416--an engagement 
providing minimal friction, galling cutting, or other energy transfer. 
High carbon steels such as tool steel, carbide steel, or stainless steel 
or as indicated above, diamond, are preferred materials for use in the 
tools 204 and 404. 
FIG. 5 in the drawings shows an arrangement of the invention varied from 
the FIG. 1 and FIG. 4 arrangements in several respects. In FIG. 5, the 
aircraft skin segment 500 is shown to be of an organic composition, such 
as the above-mentioned organic matrix composite which may include a woven 
fabric incorporating graphite and epoxy resin as major components. The 
protective coating used with this matrix composite skin surface, the 
coating 522, can be of a type similar to that used with the aluminum skin 
surface in FIG. 4. The coating in FIG. 5 is, however, presumed to be of a 
material or a physical state which results in ultrasonic energy removal of 
coating in pieces. This precisive removal is shown by the coating pieces 
at 536 and 538 and by the coating voided area 534. The coating types 
identified earlier herein are applicable to both FIG. 4 and FIG. 5 skin 
surfaces. 
The tool 404 and the reciprocal or vibratory axial motion indication 412 in 
FIG. 5 are similar to the corresponding portions of FIG. 4. A transducer 
of the type described at 406 in FIG. 4 is also presumed in FIG. 5, but is 
not shown for the sake of drawing simplicity. The transducer employed in 
FIG. 5 may, of course, be of a different physical and energy output size 
than the transducer 406 in FIG. 4, in keeping with the machine feed and 
other differences in FIG. 5. 
The FIG. 5 arrangement of the invention also includes a tool and work 
surface enclosure 524 which serves to provide a controlled atmosphere, 
indicated at 526, that is conducive to and assisting in removal of the 
protective coating 522. Communicating with the atmosphere 526, by way of a 
pair of ports 502 and 506 in the housing 524, is a flow of material 504 
capable of assisting the tool 404 in removing the coating 522. The flow 
504 may, for example, include a coolant fluid such as a refrigerant gas, 
e.g., nitrogen or carbon dioxide that has been changed from a liquid to a 
gas, a heating fluid such as hot air or steam, and/or a supply of abrasive 
material such as silicon carbide granules. A coating softening agent such 
as a water-based softener or a chemical solvent softener, may also be used 
in the flow 504. The residue from the flow 504, together with the removed 
portions of the coating 522 are intended to depart the enclosure 524 by 
way of the port 506, as is indicated by the exit flow 508. The flows 504 
and 508 may, of course, be assisted by the addition of a pump or other 
flow-inducing apparatus known in the art. 
The size of the enclosure 524 can be used to determine the lead time or 
soaking time access of the material supplied in the flow 504 to the 
coating 522 prior to coating engagement by the tool 540. Alternately, it 
may be desirable to pre-apply some materials of the flow 504 in a separate 
step or a separate enclosure from that used for the tool 540. Sealing of 
the enclosure 524 against leakage of the materials of the flow 504 is 
provided by the resilient members 518 attending the tool 404 and the 
resilient members 520 located at the junction of the enclosure 524 and the 
coating 522 and the aircraft surface 528. These resilient members allow 
movement of the tool 540 and movement of the enclosure 524 to occur while 
maintaining an effective seal of the enclosure 524. 
Also included in the FIG. 5 apparatus is a pair of tension members 510 and 
512, and a pair of rotatable reels 514 and 516 by which the tool 540 and 
the enclosure 524 can be moved over the surface 528 of the aircraft as 
removal of the protective coating 522 ensues. The reels and tension 
members 514, 516, 510 and 512 may, of course, be motor driven and may 
comprise part of a machine or automatic feed system which can also be 
closed-loop in nature and can thereby move the tool 404 in response to the 
progression of the coating removal process. 
The reels and tension members may alternately be embodied in the form of a 
robotic device of the type used, for example, in the automative industry. 
With such a robotic system, wherein movement of the tool 540 and the 
enlosure 524 is accomplished by an extended multiply pivoted manipulative 
arm, as is represented by the arm end portion 542 and its attachment 
header and fastener 544 and 546 in FIG. 5. Robotic arms of this type are 
shown in the U.S. Patents of Flick, 3,618,786; Kiryu et al. 4,546,724; and 
Toutant et al, 4,604,715; which are hereby incorporated by reference 
herein. 
Such arms can, of course, be arranged to respond to changes in the force 
urging the tool 404 into contact with the coating 522 in FIG. 5 and 
thereby maintains the tool in contact with the receding edge of the 
coating. The generated tool to coating urging force may be sensed using 
force sensor located in the arm mechanism, the transducer 406 or in the 
connection between transducer 406 and tool 504. A sensor capable of 
responding to this urging force is, for example, included in the Flick 
patent, see, for instance, the abstract and column 1, lines 6-7. 
The desired robotic arm could also be arranged to respond to optical or 
infrared signal differences between reflections from the coating 402 and 
reflections from the coated surface in the voided area 534, as is shown in 
FIG. 5. In this instance, the arm is driven or programmed to close the 
void area 534 by moving the tool into contact with the coating 522 
whenever the existence of a void area is detected. Detectors of this 
optical type are disclosed in the patent of J. Cornu et al, U.S. 
4,413,910, which is hereby incorporated by reference herein and also in 
the above-identified Kiryu et al and Toutant patents. The fiber optic and 
reflected signal arrangement shown in the Toutant el al patent is 
especially adaptable to the sensing and movement needs of the FIG. 5 
apparatus. An illumination source of either the visible or infrared type 
and a companion sensor are shown at 530 and 532 in FIG. 5; such devices 
may be mounted in a convenient location that is connected to the enclosure 
524 or located remotely and connected optically to the enclosure 524 by 
fiber optic devices as taught in the Toutent patent. The FIG. 5 apparatus, 
of course, implies that the transducer which energizes the tool 540 is in 
some not shown way connected with the housing 524 and moved along with the 
housing 524 by the robotic arm 542 or the tension members 510 and 512. 
The use of coolant or heating fluids in the material flow 504, of course, 
implies a temperature sensitive response by the coating 522, such a 
response is commonly encountered in the coating art. Many of the 
present-day coatings, for example, also become brittle and subject to 
ready fracture from energy received from a tool such as the tools 404 or 
540 upon being chilled to below room temperature; such response is 
desirable and conducive to the coating removal-in-pieces arrangement shown 
in FIG. 5. Liquid nitrogen, cooled hydrocarbon solutions, or cooled 
liquids of the fluorinated hydrocarbon solvent type may therefore also be 
desirable for use in the flow 504, in addition to the previously recited 
refrigerant gases. Additionally, heating or chemical reactant fluids may 
provide a more removal-susceptible characteristic to the coating 522. 
Two arrangements for the coating engagement tool are disclosed herein in 
FIG. 2 and in FIGS. 4 and 5; in each of these instances the tool is shown 
in cross-section or in a side view. A top or plan view of a tool suitable 
for use in the invention is also shown at 302 in FIG. 3 of the drawings 
with the direction of ultrasonic energization being indicated at 308. A 
tool width compatible with with the hundreds of watts of ultrasonic energy 
excitation described herein is indicated at 306 and a transducer 
engagement portion indicated at 300 in FIG. 3. The tool 302 may be 
connected to a transducer of the type shown at 406 in FIG. 4 by a gripping 
of the tool engagement portion 300 in a mating socket portion of the 
transducer with positive retention of the tool in the socket being 
accomplished by spring force or threaded arrangements that are known in 
the art. The coating engagement edge 304 of the tool 302 may be of either 
the FIG. 2 or FIGS. 4-5 type. 
The shape of the working end of the tool 302 in FIG. 2 may be varied in 
accordance with the woven fabric nature of the aircraft skin segment 500 
in order to achieve optimum coating removal with minimal skin surface 
damage. The movement frequency of the tool 302 in FIG. 3, the angle of 
tool application to the aircraft surface, the tool feeding and other 
similar variables are factors which can affect coating removal efficiency. 
Such variables can be finally fixed after a period of experience with a 
particular coating removal environment. Persons skilled in the coating 
removal art will appreciate that the fixation of all variables in advance 
of practical experience with a particular coating removal situation is 
undesirable, in other words, some flexibility is desired in arrangments 
such as shown in FIGS. 2, 4 and 5 to allow for individual conditions. 
FIG. 6 in the drawings shows additional aspects of the invention including 
use of the coating removal apparatus in a hazardous atmosphere--as 
represented by the proximity of the aircraft fuel 610 and the fuel vent 
port 612 and vent port cover 614 to the coating removal site. In the FIG. 
6 arrangement of the invention, the aircraft skin segment 600 may be a 
portion of the aircraft wing, for example, wherein the fuel tanks and tank 
venting arrangements normally reside. Since the described ultrasonic 
energy tranducers may be made free of the opening and closing of 
electrical contacts and electrical arcing, the FIG. 6 illustrated 
protective coating removal as well as the removal arrangement shown in 
FIGS. 1, 2, 4 and 5 herein may be practiced in hazardous 
combustion-susceptible atmospheres without danger of igniting fuel vapors 
or other flammable materials. 
The FIG. 6 arrangement of the invention also employs reciprocating 
ultrasonic energy having lateral movement parallel to the surface 618 of 
the aircraft, as is indicated at 608. In the FIG. 6 arrangement of the 
invention, the tool 404 in FIGS. 4 and 5 is replaced with a substrate 
member 604 on which is disposed an abrasive coating 606. Ultrasonic 
transducers for use at 602 in FIG. 6 and capable of providing the lateral 
motion indicated at 608 are, of course, available in the commercial 
marketplace, and may also be of the piezoelectric crystal or magnetic or 
pressurized fluid type, as described above for the transducer 406. The 
substrate member 604 may be mated with the transducer 602 using a spring 
loaded or threaded attachment arrangement as are known in the art. 
In the FIG. 6 arrangement of the invention, protective coating removal is 
accomplished by a rubbing, abrading or grinding action. In such a coating 
removal arrangement the addition of new abrasive material and the flushing 
of coating materials and other spent materials as described for the flow 
504 in FIG. 5, and as indicated by the arrows 620 and 622 in FIG. 6 may be 
desirable. 
The FIG. 6 arrangement of the invention may also be used as a supplement to 
the FIGS. 1, 2, 4 and 5 representations of the invention in order to 
achieve either polishing or smoothing of the underlying aircraft surface 
or final small quantity protective coating removal or initial 
pre-treatment of the coating to be removed. The FIG. 6 arrangement of the 
invention may also include an enclosure of the type shown at 524 in FIG. 5 
in order to provide either a desired atmosphere 526 or a containment for 
spent materials. 
The described invention therefore comprises the bringing together on a 
coated surface of ultrasonic energy agitation of a tool member, in 
combination with possible solvent or other coating conditioning agents 
abrasive materials and. Such a combination is a possible alternative to 
the abrasive blasting and chemical removal techniques which are currently 
employed on aircraft. The described invention may, of course, be used with 
other than aircraft equipment, and may be scaled upward and downward as to 
energy levels, tool sizes, and utilization times, as is appropriate to the 
coating material and area involved. The frequency of the ultrasonic energy 
used in the invention may be varied in the range of 20 kHz and upward, 
including presently available commercial equipment which operates in the 
50 kHz range. The described protective coating removal arrangements are 
inherently environmentally and human-operator safe, a marked improvement 
over the presently-used chemical and abrasive blasting removal techniques. 
It will be understood that the terms protective coating, coating, paint, 
and the like are used interchangeably herein without limitation of the 
invention. 
While the apparatus and method herein described constitute a preferred 
embodiment of the invention, it is to be understood that the invention is 
not limited to this precise form of apparatus or method, and that changes 
may be made therein without departing from the scope of the invention, 
which is defined in the appended claims.