PULSED FLUORESCENCE VELOCIMETER AND METHOD FOR FLUORESCENCE MEASUREMENT OF VELOCITY

Abstract: A noninvasive pulsed fluorescence gas velocimeter (10) that determines dynamic parameters of a gas flow from detection of fluorescence re-radiation of excited molecules (20, 26, 28) of the gas. The present invention has higher altitude capability than devices that utilize particulate scattering. The velocimeter requires no probes and utilizes a laser or other light source (50) that does not perturb the gas flow being measured.

Description: Background of the Invention The present invention pertains to fluorescence gas velocimeters and to a method for fluorescence measurement of velocity. Present techniques for air data measurement mainly rely on external probes, such as in aircraft, to measure total and static pressures and temperatures which are used to derive required air data parameters. In certain environments, such as high speed aircraft or spacecraft, such probes are impractical where the invasive nature of the probe results in severe heating of the probe and ultimate destruction of the probe. In advanced supersonic and hypersonic flights of aircraft, the energy of the gas or air flow is sufficient to result in a prohibitively high rate of heat deposition on the probe. Military missions often require aircraft having minimum radar and electro-optic scattering cross sections. These cross sections are significantly increased by the use of external probes. Other art, such as advanced optical devices, has been developed to attain noninvasive measurement of gas flow parameters. Such a device is e.g. known from EP-A-227 911. This known device uses a light projection means and beam splitting means to form several beams from the emitted single light beam. Those several beams are directed to a sample air volume in order both to form linearly independent sets of radiation fringes at the sample location as well as to focus the radiation at the sample location so as to cause selected air molecules to fluorescense. The emitted single light beam is shortly switched-off in predetermined intervals in order to focus backscattered radiation and fluorescent emissions by the same optical components from the sample volume onto one or more photo-detectors, the signals of which can be used to determine the velocity of the carrier of the device. Departing from this prior art device it is therefore the object of the present invention to devise a fluorescence gas velocimeter and a method for velocity measurement which operates reliably in great altitudes. This object is achieved by the features of the independent claims. Further embodiments of the inventive velocimeter may be taken from the dependent claims. The present invention is much less sensitive to such density decrease of particles, since it relies on inelastic scattering from molecular constituents within the flow. Summary of the Invention Effective noninvasive sensing of gas flow external to aircraft spacecraft and other vehicles, for obtaining information such as air data, is achieved by the present invention. Air data include dynamic pressure, Mach number, free stream temperature and flow angles, among other characteristics. Such air data are required by an aircraft flight control system for accurate determination of surface forces and other aerodynamic properties. Additionally, the invention may used for many other gas sensing applications. The present invention is based on laser-induced fluorescence in the gas flow. Laser or other radiation is focused into the volume of gas being measured to induce fluorescence. The invention depends on fluorescence scattering from naturally occurring atmospheric molecular constituents, rather than particulate constituents, which have a large density decrease with altitude, thus resulting in higher operational altitudes than other velocimeters of the related art. The density cross section product for fluorescence processes on molecules is significantly higher than those that depend on particulate scattering. The higher product results in a greater sensor signal-to-noise ratio. Another advantage of the invention is that it does not require a coherent laser source for operation. Although lasers are an excellent source, any source having sufficient power at the chosen wavelength and a capability for focusing to a sufficiently small spot size, may be used. The wide latitude in the kind of power sources usable in the invention results in low cost construction. Brief Description of the Drawings Figure 1a is a profile of the output of the light source of the present invention. Figure 1b shows a diagram of an embodiment of the present invention plus a graph of the fluorescence output of excited molecules of the measured atmospheric gas flow. Figure 2a illustrates fluorescence re-radiation of molecules after excitation by the light source. Figure 2b is an example of an image on the sensor. Figure 2c is a processed image illustrating some of the measured parameters. Figures 3a and 3b show embodiments of a three-dimensional velocimeter. Description of the Preferred Embodiment Figures 1a, 1b, 2a, 2b and 2c illustrate the preferred embodiment 10 of the present invention. In Figures 1a and 1b, laser signals 12, 14, 16 and 18, provided by light projection means 64, having light source 50 and optical system 46, and each having a short pulse width and high peak power, occur at a period of time (Δt) apart at a high repetition rate and illuminate a gas flow at areas 20, 26 and 28, focused at 22. At focused region 22, molecular constituents 20, 26 and 28 (for examples, NO₂, OH, etc.) of the atmospheric gas flow absorb illuminating radiation and are raised to an excited molecular state. Laser 24, which may be a pump laser, is a short wavelength laser (i.e., 193 to 600 nanometers (nm)). Each irradiated molecule re-emits photons at a typically lower energy (with a longer wavelength), having a time delay determined by the type of molecular scatterer. The operational pressure and temperature determine the amount of collational deactivization of the excited state. The delayed molecular emission results in a sequence of fluorescence-emitting light sources 20, 26 and 28, which move across field of view 32 of imaging sensor 30 which operates at a wavelength suitable for sensing the selected molecular fluorescence re-radiation. Sensor 30 incorporates photo array 36 having sensors in two dimensions, which is sensitive to the emitted wavelength of the excited molecules 20, 26 and 28 which are within field of view 32, which emit a longer wavelength than that of the wavelength of laser beam 24. For instance, a 390 nm laser may result in emissions from the molecules at 410-420 nm. Yet, there may be 390 nm scattering from focused laser spot 22 in the gas. However, so that array 36 does not pick up signals from laser beam 24, a Rayleigh filter 38 is utilized to prevent transmittance of signals having laser beam 24 wavelength and to allow transmittance of emitted radiation from excited molecules 20, 26 and 28. In lieu of the Rayleigh filter, sensor 30 may be time-gated so as not to be detecting at the time of the laser pulse. Windows 40 and 42 are physically matched to the surface 44 of the craft or platform so as to avoid any protusion beyond the craft. Windows 40 and 42 allow transmittance of laser beam 24 and receipt of light from molecules 20, 26 and 28. Windows 40 and 42 may be made from glass, quartz or other appropriate materials. Appropriate focusing of laser beam 24 is provided by optical system 46 and the received signal is focused on to array 36 by optical system 48. Optical systems 46 and 48 need not be co-planar but merely have fields-of-view that cover, though not necessarily simultaneously, the same area of the observed gas flow. The velocity of spots 20, 26 and 28 is the relative velocity between the gas flow and measurement system 34. The output from imaging sensor 30 is processed by processor 60 to calculate the two-dimensional projected angular relationship (i.e., the angle of attack or angle of sideslip) between the sensor or platform axes x and y and the gas flow vector V by measuring angle ø of the best fit linear approximation, shown as a processed image on display-indicator 58 in Figure 2c, to spots 20, 26 and 28, as shown in Figure 2a, on sensor 36 image as shown in Figure 2b. The two-dimensional projected velocity of the flow is determined by distance between spots 20 and 26 or between spots 26 and 28. The distance is proportional to the velocity-pulse separation product (V.Δt). Three-dimensional sensing systems are illustrated in Figures 3a and 3b. The three-dimensional nature or properties of the gas or air flow can be characterized by a pair of two-dimensional imaging sensors 30, as shown in Figure 3a, having optical or focal plane axes 62 not alined with each other (through preferably orthogonal to each other and both projected in a direction parallel to the host vehicle cross-section). Thus, the pair of sensors 30 can determine angle of attack, angle of sideslip and total flow velocity. Only one light projection means 64 is necessary whether there is one or two sensors 30. However, two light projection means 64 may be paired with two sensors 30 sensing in quite diverse directions or at different locations. Also, the three-dimensional sensing can be characterized by a pair of arrays 36 offset from each other but utilizing the same optical system 48, as depicted in Figure 3b. Array 36 may be constructed of photo-diodes, charge-coupled detectors or other light detectors. The detectors may be combined with micro-channel intensifiers or other photo-multiplication means if desired or needed as application of the velocimeter warrants. The above-noted embodiment 10 uses a two-dimensional array 36; however, array 36 may be a one-dimensional or a linear array together with a timing mechanism that coordinates with the times of the laser pulses 12, 14 16 and 18 that excite areas of molecules 20, 26 and 28. Graph 11 of figure 1b shows the magnitude of the fluorescence output (1(r)) versus the distance (V·Δt or r) between amplitude peaks 52, 54 and 56 of illumination of the excited molecules 20, 26 and 28 in a plane incorporating peaks 52, 54 and 56. V·Δt or r is in the direction of flow of the observed gas. The "x" of the x-y coordinates in figure 1b is an axis parallel to the longitudinal axis of the host vehicle or platform. The "y" of the same coordinates is an axis parallel to a cross-section plane of the host vehicle or platform and is in the "vertical" direction. The "r" is determined by the focal plane coordinates and the optical expansion ratio (i.e. , field of view) of optical system 48 and array 36. The distance of the measured gas flow from the host vehicle is indicated by the focus point of optics system 48 of velocimeter 10. Angle ø is the angle between the x axis and the direction of the gas flow molecules 20, 26 and 28. Angle ø may be in any quadrant of the x-y coordinates. Angle ø on a linear or one-dimensional array is determined from the location of the highest level detected signal on the array. V is a vector of the gas flow that is calculated in processor 60 and displayed on display-indictor 58. Δt is a scalar indicating the measured time period from the centers of illumination of molecules 20, 26 and 28. The product V·Δt is a position vector from one point to another in the observed gas flow. The basis of noninvasiveness of gas measurement outside a vehicle carrying velocimeter 10 is peculiarly characteristic to the present invention. The obvious noninvasive feature is the lack of probes and is a feature of some related art systems. The significant, but unobvious, noninvasive feature is the effective nondisturbance of the measured flow to the extent that reasonably accurate measurements of the gas flow can be attained, even at high altitudes (i.e., greater than 200 thousand feet above sea level). If a very high-powered laser is pulsed into a small spot in the gas flow, the high power density heats the gas and the gas becomes more rarified resulting in a change of the index of refraction of the gas. Also the heat of the high-powered laser causes flow disruption. For more useful and reliable accuracy, measurements of gas flow should be made without perturbing the flow and changing the index of refraction. The perturbance of gas flow and rarification of the gas are invasive effects which reduce the signal-to-noise ratio and accuracy of the measuring system, although a more concentrated or smaller spot should increase the signal-to-noise ratio. The size of the spot downstream where it is detected determines the possible accuracy with which the measuring system can measure the gas flow (i.e., measure distance V·Δt between spots). The spot would more greatly increase in size due to diffusion and mixing caused by perturbation of the gas flow or rarification of the gas. Another factor contributing to spot diffusion of the gas is turbulence of the host vehicle of the velocimeter. Thus, the laser spot needs to be focused at a distance sufficient enough to maintain laminar or free stream flow of the gas measured. The light source may be a laser, not a high powered laser, but medium powered, or may be a flash lamp, having a short pulse width and an appropriate focus. If the source is a laser, coherence is not critical, that is, a narrow line width is not mandatory.

Claims: A noninvasive pulsed fluorescence gas velocimeter (10), comprising: pulsed light projection means (64) mounted on a platform, for projecting a light beam to periodically excite molecules at time intervals Δt at a predetermined spot in a gas flow; two-dimensional sensor means (30) mounted on the platform, and having a field of view arranged to detect radiation emitted from a plurality of excited spots; and processing means (60), connected to said sensor means (30), for determining the velocity v from the distance r between detected fluorescence light spots according to the relation r = v·Δt and for indicating velocity and direction of the molecules in the gas flow. Velocimeter according to claim 1, characterized in that : said pulsed light projection means (64) comprises a light source means (50) and a first optical means (46), said first optical means (46) positioned between said light source means (50) and the molecules in the gas flow, for generating and focusing the light beam on the molecules; and said sensor means (30) comprises a detector means (36) for fluorescence detecting and a second optical means (48), said second optical means (48) positioned between said detector means (36) and the molecules in the gas flow, for focusing the light of the fluorescence scattering from the molecules onto said detector means (36). Velocimeter according to claim 2, characterized in that second optical means (48) comprises a Rayleigh filter means (38) for absorbing light of the same frequency of said light source means (50) and transmitting the light of the fluorescence scattering. Velocimeter according to claim 3, characterized in that said processing means (60) comprises: a processor (60), connected to said detector means (36), for processing fluorescence light data from said detector means (36) into measurement parameter data; and display-indicator means (50), connected to said processor (60), for indicating gas flow parameters such as velocity including direction of gas flow. Velocimeter according to claim 4, characterized in that said detector means (36) has a two-dimensional array. Velocimeter according to claim 4, characterized in that : said processor (60) is connected to said light source means (50). Velocimeter according to claim 4, characterized in that said light source means (50) is a laser light source (50). Velocimeter according to claim 4, characterized in that said light source is a non-laser flash unit. Velocimeter according to one of the claims 1 to 8, characterized by : a first optical window (40) between said first optical means (46) and the gas flow; a second optical window (42) between said second optical means (48) and the gas flow, wherein said first (40) and second (42) optical windows may isolate said velocimeter (10) within a vehicle (44) from the gas flow. Velocimeter according to claim 9, characterized in that said sensor means comprises: a first detector (30) having a first field of view (32), for detecting fluorescence from the molecules; and a second detector (30) having a second field of view (32), for detecting fluorescence from the molecules; whereat said processor (60) is connected to said first and second detectors (30), for processing three-dimensional measurement data from said first and second detectors (30). A method for noninvasive fluorescence measurement of velocity, including direction, of flow of a gas, relative to a platform, comprising the steps of: periodically exciting molecules at time intervals Δt with non-medium perturbing light at a fixed spot in said flow of gas; two-dimensionally detecting said spots of fluorescence of the molecules within a fixed field of view relative to said platform; and calculating the velocity v and direction of the gas flow from the distance r between detected fluorescence light spots according to the relation r = v·Δt and from the positions of the detected spots.

Inventors: BENSER, EARL T.
Applicants: HONEYWELL INC.
Application year: 1989
Main classification: G01P5
IPC codes: G01P13, G01P5