Abstract:
A method and apparatus for correcting for the temperature sensitivity of pressure sensitive paint using two layers of paint: one that is sensitive to both pressure and temperature and one that is sensitive to only temperature changes. A temperature sensitive paint layer or film is placed on a model to be tested in a wind tunnel for pressure changes over the surface of the model. The first film is responsive to temperature changes and has a magnitude of luminosity depending upon the temperature of the ambient air. Placed over the temperature film is a pressure sensitive paint or film that is sensitive primarily to pressure changes, but also to temperature changes, and produces a magnitude of luminosity depending upon the change in both temperature and pressure. The pressure paint layer is transparent to the luminescence of the temperature layer and a measurement of the luminosity of the temperature layer and the pressure layer is taken. The pressure layer luminosity is corrected for the temperature layer luminosity, thereby enabling a true pressure reading to be determined by comparing the magnitude of luminescence of both the temperature layer and the pressure layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to pressure sensitive paints, and more particularly to a method and apparatus of correcting for the temperature sensitivity of pressure sensitive paints. 
     BACKGROUND OF THE INVENTION 
     Many vehicles, such as automobiles and aircraft, have to operate in an atmosphere of dense gas. Therefore, it is highly desirable to optimize the aerodynamics of these vehicles. Furthermore, with certain vehicles such as aircraft, certain aerodynamic interactions must be known to assure that the aircraft will perform properly and be capable of being flown in a controlled manner. To this end, several methods have been used to measure pressure on the surface of models of different vehicles to assure that they will be able to operate efficiently and properly during travel. Wind tunnels are often used to simulate a vehicle traveling at a particular speed through the atmosphere. Once the model is placed in the wind tunnel, air is moved at a particular speed over the model to test how the model reacts to the wind speed. One particular measurement is the pressure produced over the various surfaces of the model during the tests in the wind tunnel. To measure these different pressure points, many techniques have been developed. 
     Generally, wind tunnels use several mechanical devices to measure the pressure changes along the surfaces of the models placed in the wind tunnel. Each of these numerous mechanical devices are affixed to different electrical leads, which electrical leads are coupled to a computer to produce a representation of the pressure changes produced on the model by the wind in the wind tunnel. These systems are often cumbersome, time consuming, and hard to set up and take down for each model being tested. 
     Other methods of detecting pressure changes include pressure sensitive paints (PSP). These PSPs are able to luminesce at a particular wavelength when pressure is applied thereto. Generally, PSPs luminesce when a particular type of light energy is applied to them at a particular wavelength. Also, the PSPs are oxygen permeable. In the wind tunnel, the PSP, being oxygen permeable, receives a particular amount of oxygen depending upon the pressure being applied to an area. When a particular wavelength of light is applied to the PSP, it luminesces. If oxygen is in the matrix of the PSP, the oxygen absorbs a certain amount of the luminescent energy that would otherwise be emitted, thereby changing the luminescence of the PSP depending upon the amount of oxygen that is absorbed into the PSP layer. Therefore, a pressure of oxygen is determined and interpreted therefrom. A general pressure of the air around the particular area of the model being measured is determined from the known concentration of oxygen. Often the PSPs are sensitive not only to pressure, but also temperature. Therefore, temperature can effect the degree of luminescence from the PSP. Due to this temperature sensitivity, pressure cannot be accurately determined from the luminescence of the PSP alone. 
     Several methods have been attempted to correct for the temperature sensitivity of PSPs with only marginal success. One method is to simply shorten the amount of time between when measurements are taken. That is, the time between a zero or initial reference, that being when no wind or pressure is being applied to the model, and when the maximum amount of wind pressure is being applied to the model. However, this does not actually correct for temperature, but rather simply reduces the amount of temperature change that occurs and reduces the error of the PSP to one that is acceptable for the tests. 
     Other methods have attempted to mix several different sensors into a single film wherein each is affected by temperature, pressure, or other factors. The mixing of all of the phosphorescence species into a singular film, however, has reduced the temperature error of the PSP only marginally. Furthermore, it is difficult to provide each of the particular phosphorescent species in a singular film since it increases the effort and cost necessary to produce such a film. 
     Additionally, attempting to correct for the temperature change through data conversion after the test data has been taken during a test has been tried. Again, this method is long and arduous and only corrects for a certain amount of the temperature related error. Additionally, this method is only marginally helpful in correcting for the temperature change in the PSPs. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a system that can adjust or take account of the temperature sensitivity of a pressure sensitive paint to produce a measurement that is nearly error free due to temperature sensitivity of the pressure sensitive paint. 
     A first preferred embodiment of the present invention includes a system for detecting at least two physical characteristics near a surface being tested. A first luminescent film, capable of emitting light having a wavelength in a first discrete range, is placed on the surface. A second luminescent film, capable of emitting light having a second wavelength in a second discrete range, is placed over the first film. A radiation source which emits radiation able to excite the first luminescent film and the second luminescent film is focused on the films. An analysis system detects the brightness of the light emitted by the first luminescent film and the brightness of the light emitted by the second luminescent film. The first luminescent film and the second luminescent film are placed on the surface substantially coplanar and the first luminescent film is substantially transparent to the light emitted from the second luminescent film. 
     A second preferred embodiment of the present invention comprises a method of more accurately determining a pressure on a surface being tested. The method comprises measuring the brightness of light emitted by a first film sensitive to temperature, which covers the surface being tested. Additionally, the brightness of light emitted by a second film, which is sensitive to both pressure change and temperature, and which covers the first film, is measured. At least a first measurement of the brightness of light emitted by each of the films is taken. Then the surface is made to experience a pressure and temperature change. At least a second measurement of the brightness of light emitted by each of the films is taken. Finally, an accurate determination of the pressure experienced by the surface is obtained by comparing the first measurements and the second measurements. 
     A third preferred embodiment of the present invention includes a system to determine the pressure over an area of a surface. A member under test has a first film applied so as to surround the member, wherein the first film comprises a sensor that is adapted to emit light at a first wavelength. A second film is placed over the first film to surround the first film. The second film comprises a second sensor adapted to emit light at a second wavelength. The first film is transparent to the second wavelength. A test is performed and each sensor in each film emits light at a particular brightness which is measured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Still other advantages of the present invention will become apparent to those skilled in the art after reading the following specification and by reference to the drawings in which: 
     FIG. 1 shows a schematic representation of a preferred embodiment of a system of the present invention to measure the brightness of light emitted by two films placed on a model in a wind tunnel; and 
     FIG. 2 an enlarged view of the area represented by  2  in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a pressure sensing system (PSS)  8  in accordance with the present invention is generally shown. The PSS  8  is used to analyze a model  10  placed in a wind tunnel  11 . Model  10  may form any structural member or other component, however, in one example it comprises a portion of a wing of an aircraft. A fan  12  or similar device produces an airflow in the direction of arrow A in the wind tunnel  11 . The PSS  8  makes use of layers of film, generally paints or other materials suspended in a polymer binder or matrix, which are coated on the model  10 . With specific reference to FIG. 2, the layers generally include a primer layer  14  which assists in the adhesion of the other layers to the model  10  itself. A commonly used primer is WAL-2 produced by Boeing, Inc. It will be understood that any suitable primer may be used. Also, if the remaining films or layers provide sufficient adhesion between the layers and the model  10  itself, a primer layer  14  would not be necessary. Next, a layer of temperature sensitive paint (TSP)  16  is placed over the primer layer  14 . The TSP layer  16  changes luminescence or brightness depending upon the temperature of the ambient air surrounding the model  10 . Finally, a layer of pressure sensitive paint (PSP)  18  is layered on top of the TSP layer  16 . The PSP layer  18  changes luminescence or brightness primarily depending upon the air pressure placed on the PSP layer  18 , however, the PSP layer  18  changes luminescence depending upon temperature. 
     Various PSPs and TSPs or films are generally known in the art, such as those disclosed in Woodmansee, M. A., and J. C. Dutton,  Methods for Treating Temperature Sensitivity Effects of Pressure - Sensitive Paints,  35th Aerospace Sciences Meeting &amp; Exhibit, 1997, hereby incorporated by reference. Generally, the PSP layer  18  must be permeable to the surrounding atmosphere and able to absorb oxygen. As a general description, the PSP layer  18  luminesces when it is illuminated or irradiated with a particular wavelength light. The lumninescence of the PSP layer  18  is known or measured in this initial state when the PSP layer  18  is only subjected to irradiation. The active material or sensor in the PSP layer  18  is excited and emits a photon of light, creating a light signal, centered at a particular wavelength different than the wavelength of the illumination source. The sensor may comprise any chemical species which is able to be excited and emits radiation upon return to a ground or unexcited state. Preferably, the radiation emitted is a photon. 
     When oxygen is present in the PSP layer  18 , then the oxygen molecules absorb a certain amount of the energy being emitted by the sensor in the PSP layer  18 . The change in luminescence correlates to the concentration of oxygen present in the PSP layer  18 . The higher the concentration of oxygen, the lower the luminescence of the sensor in the PSP layer  18 . As pressure increases in the surrounding atmosphere, more and more oxygen is forced into the PSP layer  18 . Therefore, pressure may be measured by determining the lack of luminescence of the sensor from the PSP layer  18  which depends on the concentration of oxygen. 
     The TSP layer  16  also includes a sensor which is able to emit a photon at a wavelength, and produce a light signal, different than the wavelength emitted from the sensor in the PSP layer  18 . The TSP layer  16  also emits the photon when the sensor in the TSP layer  16  is excited by a radiation source. The amount of luminescence in the TSP layer  16  depends upon the ambient temperature. 
     The Pressure Sensing System (PSS)  8  is implemented by first placing the model  10  in the wind tunnel  11  after the appropriate layers have been placed on the model  10 . The PSS  8  includes an illuminating or irradiating source  20  that can illuminate the model  10  and provides the optical energy that excites the sensors in both the TSP layer  16  and the PSP layer  18 . The illuminating source  20 , for example, may be a light that appears generally blue and has a wavelength centered at about 450 nanometers. It will be understood that any suitable illuminating source, depending upon the sensors in the TSP layer  16  or the PSP layer  18 , may be used. Additionally, the illuminating source  20  need not emit visible wavelengths depending upon the sensors of the particular TSP layer  16  or the PSP layer  18 . 
     Referring further to FIG. 1, a PSP camera  22  and a TSP camera  26  are both aimed at the model  10 . The PSP camera  22  includes a PSP filter  24 , while the TSP camera  26  includes a TSP filter  28 . The wavelengths of the photons emitted from the TSP layer  16  differ from those emitted by the PSP layer  18 . Therefore, the PSP filter  24  filters all wavelengths except for the wavelengths being emitted by the PSP layer  18 , while the TSP filter  28  filters all wavelengths except for the wavelengths being emitted by the TSP layer  16 . Therefore, the PSP camera  22  only registers the luminescence or brightness of the light emitted from the PSP layer  18 , while the TSP camera  26  only measures the luminescence or brightness of the light emitted by the TSP layer  16 . 
     The PSP camera  22  produces a signal based upon the magnitude of the luminescence of the PSP layer  18  and transmits the signal along lead  30  to a PSP processor  32 . The TSP camera  26  produces a signal depending upon the magnitude of the luminescence of the TSP layer  16  and transmits the TSP signal along lead  34  to the TSP processor  36 . The PSP processor  32  processes the signal transmitted along lead  30  and determines the magnitude of the luminescence being emitted by the PSP layer  18  and calculates a pressure related to the luminescence being emitted by the PSP layer  18 . The pressure calculated based upon the luminescence is then displayed on a PSP output device  38 , such as a suitable display system. Similarly, a TSP processor  36  determines a temperature based upon the luminescence of the TSP layer  16  and then displays a temperature based upon the luminescence on a TSP output device  40 . Output device  40  may also comprise a suitable display system. 
     Since the luminescence of the PSP layer  18  also depends upon the temperature change of the atmosphere or the model  10  in the wind tunnel  11  not all of the luminescence being emitted by the PSP layer  18  correlates to the actual pressure being experienced by that particular area of the model  10 . The TSP layer  16 , however, is not affected by pressure and its luminescence only relates to the actual temperature change that is affecting the sensors in the TSP layer  16 . Therefore, the output displayed on the TSP output device  40  is used to correct the output displayed on the PSP output device  38  to insure an accurate depiction of the pressure being experienced by the PSP layer  18 . 
     A preferred method of the present invention for determining the absolute or real pressure being experienced by the model  10  may proceed as generally described herein. When there is no wind being produced by the fan  12  in the wind tunnel  11 , an initial reference illuminescence is determined using the PSP camera  22  and the TSP camera  26 . The illuminating source  20  illuminates the TSP layer  16  and the PSP layer  18  such that the particular wavelengths associated with the respective sensors are emitted. The TSP layer  16  and PSP layer  18  both emit photons which are detected by the TSP camera  26  and the PSP camera  22 , respectively. The cameras  22 ,  26  transmit the signals to the respective processors  32 ,  36  to determine the initial pressure and temperature measurements. These measurements are then output to the output devices  38 ,  40 . In this way, an initial or reference illuminescence can be determined for both the PSP layer  18  and the TSP layer  16 . 
     Hence, the initial reference point for the PSP layer  18  is known as I ref, PSP  or I ref, fl,  and represents the luminescence recorded by the PSP camera  22  when no wind or pressure is being produced on the model  11  by the fan  12 . Similarly, an initial reading for the TSP layer  16  is made and produces a point or reading I ref, TSP  or I ref, fl . 
     Then, once a pressure or wind is being produced on the model  10  by the fan  12 , at least a second measurement is taken by the PSP camera  22 . The on or test reading of the PSP camera  22  outputs a second value, designated I on, PSP  or I on, sl . Similarly, the TSP camera  26  produces at least a second signal when the fan  12  is on, wherein the corresponding second measurement is designated as I on, TSP  or I on, fl . 
     Therefore, an initial reference illumination signal and at least a second test or on signal so that the difference between the two can be determined. The change in luminescence of the TSP layer  16  is designated I Δ, TSP  or I Δ, fl.  The change in luminescence of the PSP layer  18  is designated I Δ, PSP or  I Δ, sl . The difference in luminescence of the TSP layer  16  and the PSP layer  18  is compared to known illumination magnitudes to determine the respective temperature and pressure. Also, the PSP layer  18  has a known degree of luminescence due to the temperature it experiences. Since a temperature is recorded on the TSP output device  40 , the illuminescence output recorded on the PSP output device  38  is corrected for the temperature recorded on the TSP output device  40  producing a corrected luminescence for the PSP layer  18  designated I corr, PSP . 
     The cameras  22 ,  26  generally take a picture or scan the model  10  when readings are being taken. Therefore, the respective I ref  and I on  readings refer to a plurality of points or pixels in the picture taken. Each pixel in the picture is given a particular identifier and value which makes up the I ref  and I on  readings. Also, the correction for temperature change in the I on, PSP  is corrected, or can be corrected, pixel by pixel with the corresponding I on, TSP  readings. Therefore, a highly accurate pressure map indicating the pressure change can be created for the model  10 , using a PSP layer  18  since the luminescence of the PSP layer  18  can be corrected for temperature. The pressure map allows one to determine a pressure at any point on the model  10 . 
     The present invention is particularly useful for a low wind speed or pressure change experiment. In particular, in a low wind speed experiment, the pressure change may produce a degree of luminescence change in the PSP layer  18  that is similar to the luminescence change in the PSP layer  18  due to a temperature change. Therefore, being able to accurately correct for the temperature influence on the pressure reading is particularly useful in such a test. However, it will also be understood that the present invention is useful in high pressure or high wind speed experiments as well as to more accurately measure pressure changes. 
     Furthermore, it is the ability of the PSS  8  to provide knowledge of the discrete temperature change and discrete pressure change at each particular point on the model  10  that is particularly useful. The PSP layer  18  allows a measurement of a pressure change over the entire surface area of the model  10  being tested, since the pressure experienced by the model  10  may vary at different areas thereon. Similarly, the temperature may also change from one discrete area to another discrete area of model  10 . Therefore, using the present invention allows both pressure and temperature readings to be made for each discrete area of the model  10 . Furthermore, both temperature and pressure readings are made at the same time; thus there is no time difference error. 
     One skilled in the art will also understand that several modifications may be made to the PSS  8  of the present invention without departing from the spirit thereof. In particular, a single processing unit may be used to process the signal from both the PSP camera  22  and the TSP camera  26 , which may similarly be displayed on a single output device. Additionally, more than one illuminating source may be used depending upon the wavelengths that would excite the active materials in either the PSP layer  18  or the TSP layer  16 . One skilled in the art will also realize that a compound camera may include two lenses to receive both signals into a single compound unit, which may then transmit to either two separate processors or a single processor as mentioned above. 
     The description of the invention is merely exemplary embodiments in the present invention. One skilled in the art would readily recognize from such discussion and from accompanying drawings and claims that various changes, modifications, variations may be made therein without the spirit and scope of the invention