Apparatus and Method for Analysing a Surface

Abstract

Apparatus for analysing a surface which, in use, is subject to drag, the apparatus comprising, a light source for generating light of at least one predetermined wavelength, a light source holder for holding and positioning the light source so as to direct it at the surface, a light detector for detecting reflected light from the surface and generating a signal in response thereto, a light detector holder for holding the light detector and positioning it so as to detect the reflected light, and a connector for connecting the light detector to a microprocessor to analyse the signal. Also disclosed is a method of analysing a surface which, in use, is subject to drag.

Claims

1. Apparatus for analysing a surface which, in use, is subject to drag, the apparatus comprising; a) a light source for generating light of at least one predetermined wavelength, b) a light source holder for holding and positioning the light source so as to direct it at the surface, c) a light detector for detecting reflected light from the surface and generating a signal in response thereto, d) a light detector holder for holding the light detector and positioning it so as to detect the reflected light, and e) a connector for connecting the light detector to a microprocessor to analyse the signal.

2. Apparatus as claimed in claim 1, wherein the light source detector and/or the light detector holder is/are independently movable, and preferably are independently movable so as to be directable at substantially the same portion of the surface to be analysed.

3. Apparatus as claimed in claim 2, wherein the light source detector and/or the light detector holder are independently pivotable, preferably to position the light source and/or the light detector at independent predetermined angles with respect to the surface normal.

4. Apparatus as claimed in claim 1, wherein the reflected light is diffuse reflected light.

5. Apparatus as claimed in claim 1, wherein the reflected light is specular reflected light.

6. Apparatus as claimed in claim 1, wherein the light source may be positioned at an incident beam angle of 0° to 90°, preferably 10° to 80° with respect to the surface normal.

7. Apparatus as claimed in claim 1, wherein the light detector may be positioned at a reflected beam angle of 0° to 80° with respect to the surface normal.

8. Apparatus as claimed in claim 1, wherein the light source comprises a light emitting diode.

9. Apparatus as claimed in claim 1, wherein the light source is adapted to generate collimated light.

10. Apparatus as claimed in claim 1, wherein the light source is a patterned light source, preferably a patterned collimated light source.

11. Apparatus as claimed in claim 10, wherein the patterned light source comprises a curved or angled pattern.

12. Apparatus as claimed in claim 10, wherein the patterned light source comprises a plurality of light sources distributed in a predetermined pattern.

13. Apparatus as claimed in claim 1, wherein the light source comprises a laser.

14. Apparatus as claimed in claim 1, wherein the light source comprises a lens, preferably an adjustable lens.

15. Apparatus as claimed in claim 1, wherein the light detector comprises an image sensor, preferably a charge-coupled device (CCD) camera or a complementary metal-oxide-semiconductor (CMOS) sensor camera.

16. Apparatus as claimed in claim 1, wherein the light detector comprises a lens, preferably a magnifying lens.

17. Apparatus as claimed in claim 1, wherein the light is visible light.

18. Apparatus as claimed in claim 17, wherein the visible light is selected from one or more of: a) blue light, preferably having a peak wavelength in the range 450 nm to 495 nm, b) green light, preferably having a peak wavelength in the range 495 nm to 570 nm, and/or c) red light, preferably having a peak wavelength in the range 620 nm to 750 nm.

19. Apparatus as claimed in claim 1, wherein the light source and/or the light detector comprise optical filters, preferably polarising optical filters.

20. Apparatus as claimed in claim 1, wherein the light source generates a light beam having a diameter in the range 0.5 mm to 5 mm, preferably in the range 1 mm to 3 mm.

21. Apparatus as claimed in claim 1, further comprising a microprocessor connected to the light detector.

22. Apparatus as claimed in claim 1, further comprising a power supply, preferably a battery pack.

23. Apparatus as claimed in claim 1, wherein the light source, the light source holder, the light detector and the light detector holder, and optionally, the microprocessor and power supply, are contained within a housing.

24. Apparatus as claimed in claim 1, wherein the light source and light source holder are contained in a first housing and the light detector and light detector holder are contained in a second housing.

25. Apparatus as claimed in claim 1, further comprising at least one Global Positioning System (GPS) navigation device.

26. Apparatus as claimed in claim 1, wherein the apparatus is for determining the contamination of a surface and/or for determining the roughness of a surface.

27. Apparatus for determining the contamination on a surface which, in use, is subject to drag, the apparatus comprising; a) a light source for generating light of at least one predetermined wavelength, b) a light source holder for holding and positioning the light source so as to direct it at the surface, c) a light detector for detecting reflected light from the surface and generating a signal in response thereto, d) a light detector holder for holding the light detector and positioning it so as to detect the reflected light, and e) a connector for connecting the light detector to a microprocessor to analyse the signal.

28. A method of analysing a surface which, in use, is subject to drag, the method comprising; a) generating a light beam of at least one predetermined wavelength, b) directing the light beam on to a portion of the surface to form an illuminated area, c) detecting the intensity of the reflected light across the illuminated area, d) comparing the intensity of the reflected light at positions across the illuminated area, thereby analysing the surface.

29. A method as claimed in claim 28, wherein the surface is a surface of a propeller blade or a turbine blade or a vehicle.

30. A method as claimed in claim 29, wherein the vehicle is an aircraft, a water vessel or a land vehicle.

31. A method as claimed in claim 28, wherein the method is for determining the contamination of a surface and the contamination comprises ice, dirt, dust, oil, grit, other particulates and/or organic matter.

32. A method as claimed in claim 28, wherein the method is for determining the roughness of a surface.

33. A method as claimed in claim 28, further comprising measuring the incident beam angle and/or the angle of reflection with respect to the surface normal.

34. A method as claimed in claim 28, further comprising stabilising the position of the illuminated area.

35. A method as claimed in claim 28, further comprising adjusting the intensity of the illuminated area.

36. A method as claimed in claim 28, wherein detecting the intensity of the reflected light comprises identifying the location of the illuminated area, and acquiring the image of the illuminated area.

37. A method as claimed in claim 28, further comprising c1) producing a profile of intensity against a distance axis across the illuminated area.

38. A method as claimed in claim 37, further comprising e) generating a threshold value that is higher than an intensity that can be recorded from a comparative surface and f) summing the light intensity values greater than the threshold value for all points along the distance axis.

39. A method for determining surface related drag of a surface, the method comprising; a) analysing at least a portion of the surface as claimed in claim 28, b) determining the surface structure on the portion of the surface c) optionally, determining the surface energy of the portion of the surface, d) selecting a drag value model, e) applying the drag value model to the surface structure and, optionally, the surface energy, and f) generating a drag factor associated with the surface.

40. A method as claimed in claim 39, further comprising g) relating the drag factor to likely reduction in fuel consumption for the vehicle.

41. A method as claimed in claim 39, wherein determining the surface energy of the portion of the surface comprises measuring the contact angle of a liquid droplet, preferably a water droplet, on the portion of the surface.

Description

[0074] In the accompanying drawings,

[0075] FIG. 1(a) illustrates a bottom view of apparatus according to an embodiment of the present invention.

[0076] FIG. 1(b) illustrates a top view of the apparatus illustrated in FIG. 1(a).

[0077] FIG. 1(c) illustrates a side cross sectional view of the apparatus illustrated in FIG. 1(a), with the cover removed to show the working parts.

[0078] FIG. 2 is a graph of drag as a function of mean particle diameter for particles on the surface.

[0079] FIG. 3 is a diagram illustrating the protocol of Experiment 1, below.

[0080] FIG. 4(a) is a 3D plot of intensity of the image at each pixel for the dirty surface of Experiment 1.

[0081] FIG. 4(b) is a graph of grey scale (illumination from 120 mm) as a function of distance (pixels) of a line scan of the intensity across regions of the image for the clean area (Experiment 1).

[0082] FIG. 4(c) is a graph of grey scale (illumination from 120 mm) as a function of distance (pixels) of a line scan of the intensity across regions of the image for the dirty area (Experiment 1).

[0083] FIG. 4(d) is a graph of variance as a function of distance (pixels) for FIG. 4(b).

[0084] FIG. 4(e) is a graph of variance as a function of distance (pixels) for FIG. 4(c).

[0085] FIG. 5 is a 3D plot of intensity of the image at each pixel for the clean and dirty surfaces of Experiment 2.

[0086] FIG. 6 is a flow diagram showing the method for determining drag associated with a surface.

[0087] FIG. 7 is a flow diagram showing the method.

[0088] FIG. 8 a) to e) show histograms of grey scale against pixel distance for light spots of abrasive sheets as discussed below, where the mean particle size of the abrasive particles is (a) 162 μm, (b) 59 μm, (c) 35 (d) 22 μm, (e) mirror surface.

[0089] FIG. 9 shows schematically the use of a patterned light source to determine relative angle and distance of light source and surface.

[0090] FIG. 10 is graph showing correlation between the sum of the light intensities under the curve and the average particle diameter (measurements all done in triplicate) as used in Example 10.

[0091] FIG. 1(a) shows the bottom view of an embodiment of the apparatus 2 for determining surface contamination. The apparatus 2 is designed with a housing 4 having in its bottom portion a sensor aperture 6 through which the light source 12 (in this case a blue laser of peak wavelength 445 nm) which is mounted in the housing 4, may shine through a polarising source filter 14. Light reflected from the surface (not shown) to be investigated is detected by light detector 10 (in this case a CCD camera), also mounted in the housing 4, after passing through a detector polarising filter 16.

[0092] The apparatus 2 has three adjustable legs 8, arranged so that the height and orientation of the sensor aperture 6 (and hence light source 12 and light detector 10) may be controlled when the apparatus is situated on a surface to be investigated.

[0093] The top view of the apparatus 2 is shown in FIG. 1(b). The apparatus has a light source slider 26 and a light detector slider 24 connected to the light source holder (not visible in FIG. 1(b)) and light detector holder (also not visible in FIG. 1(b)) respectively. The sliders 24, 26 enable the angle of the light detector 10 and light source 12 to be adjusted independently. The detector angle scale 25 and source angle scale 27 connected to the light detector holder and light source holder respectively, enable an investigator to determine and manually set the required angle for each.

[0094] A microprocessor holder 18 forms part of one side of the housing 4 and contains a microprocessor to analyse the detector signal. The microprocessor holder 18 contains USB connector 20 and SD connector 22 for connecting the microprocessor of the apparatus 2 to other equipment. At the other side of the housing 4 to the microprocessor holder 18 there is a power supply holder 30 which contains the battery pack to power the apparatus 2. A start button 28 is also situated on the housing 4.

[0095] FIG. 1(c) shows a cross sectional side view through the apparatus 2. The detector holder 34 is connected to a detector lever 38 which pivots on movement of the detector slider 24, thereby changing the angle of the light detector 10 (not visible in FIG. 1(c)). Similarly, the source holder 32 is connected to a source lever 40 which pivots on movement of the source slider 26, thereby changing the angle of the light source 12 (not visible in FIG. 1(c)). Thus, the angle of the beam generated by the light source 12 and passing through the sensor aperture 6 can be varied independently of the angle of the light detector 10.

[0096] FIG. 3 illustrates the arrangement of equipment to conduct Experiment 1, described below. An LED light source (no additional collimation) situated on an optics rail generates a light beam incident on a portion of the surface of aircraft at an angle φ with respect to surface normal N. A detector is situated normal N to the surface. This angle of incidence is effectively constant over the small area of the surface which is imaged by the camera.

[0097] FIG. 9 shows schematically the use of a patterned light source to determine relative angle and distance of light source and surface. The patterned light source is a pattern of two arcs (twin arc pattern) on either side of the lens of the light source. Other patterns may, of course, be used. For example the pattern could be circular, oval, or any polygon shape such as a triangle, square or rectangle, pentagon or hexagon. The pattern may also or alternatively comprise a plurality of light sources (e.g. point light sources) at predetermined distances from each other, the distance apart in the image relative to the sensor size may be calibrated to give distance for a given lens. A more complex pattern may also be used such as interlocking circles of light. Generally, the light pattern may be formed though a collimated pattern plate placed in front of a high intensity light source, such as a ring flash or regular stroboscopic light source. The pattern could also be projected onto the surface using a focusing lens.

[0098] The surface being studied 101 perpendicular to the light source 103 produces, in the detector, a non-distorted image 105 of the twin arc pattern. When the light source 107 is at an angle relative to the surface 101, a distorted image 109 of the twin arc pattern is produced indicative of the angle. Similarly, when the surface 111 is at an angle relative to the light source a different distorted image 113 of the twin arc pattern is produced, again indicative of the angle. The size of the pattern image relative to the size of the source pattern indicates the distance between the surface 101 and the light source 103 for a known distance using a focusing lens of a known focal length.

EXPERIMENT 1

Sample: Clean Painted Planar Section of an Aircraft

[0099] There were two regions on the sample:
Region 1—clean
Region 2—an area with an oily film with representative ‘dirt’.
Illumination—a green LED (532 nm) at different distances, d, with no additional focussing optics.

[0100] The data produced is thus an image of part of the aircraft under green illumination.

[0101] The specific conditions of measurement were as follows:

Camera at 90 degrees to surface
Laser at 30 degrees

Camera Make—Canon

Model—Canon EOS 7D

Resolution Unit—Inch

[0102] Exposure Time— 1/50 seconds

F-number—11

ISO Speed Ratings—200

Exposure Bias Value—0

[0103] Focal Length of lens—50 mm
Color Space—sRGB

Exif Image Width—5184

Exif Image Height—3456

White Balance—Auto

[0104] Camera to surface distance: approx 30 cm.

[0105] A 2D intensity map, i.e. the reflected green LED intensity at different pixels is the output as shown in FIG. 4(a). The dirt spot is in the centre of the plot labelled A in FIG. 4(a).

[0106] The larger peaks to the left of the plot are due to reflections of the window light—the plane is quite reflective.

[0107] Other results—for illumination from 120 mm—the closest distance used as shown in FIGS. 4(b) and 4(c) and are line scans of the intensity across regions of the image of the plane, FIG. 4b) for the clean region, FIG. 4(c) the dirty region.

[0108] The smooth variation in intensity for both clean and dirty regions is due to the non-uniform illumination from the LED.

[0109] In addition to the smooth intensity variation seen for the clean surface, there is a dip in intensity in the region corresponding to the dirt spot. The scan for the dirty area (indicated as A in FIG. 4(c)) being through the centre of the dirt spot.

[0110] The intensity changes rapidly locally where there is granularity in the surface, e.g. due to particles or variation in the dirt. Thus one approach to analysis is to look at the change in intensity between neighbouring pixels, as shown in the FIGS. 4(d) and 4(e) graphs of variance in grey scale as a function of (pixel) distance.

[0111] In the clean region in FIG. 4(d), there is some noise in change between adjacent pixels for a line scan. In the dirty region as shown in FIG. 4(e) the increased noise around 1000 pixels reflects the granularity in the image due to the structure within the dirty area.

EXPERIMENT 2

[0112] Experiment 2 was generally similar to Experiment 1—but here there is a lens in front of the LED. The lens is placed to give a collimated beam. Thus, if the beam has low divergence it would be suitable to give stand-off illumination.

Sample: Clean Painted Planar Section of an Aircraft

[0113] Region 1—clean
Region 2—an area with an oily film with representative ‘dirt’.
Illumination—a green LED (532 nm) at different distances, d, with a focussing lens which is at a fixed distance from the led to give a collimated beam.
Camera—focussed on the surface at a distance of 20 cm.

[0114] FIG. 5 illustrates a 2D intensity map. The dirty area (indicated by B in FIG. 5) and granularity in the image can be seen as in Experiment 1.

EXPERIMENT 3

[0115] This experiment used abrasive paper as these provide rough surfaces with particles of generally known dimensions—both lateral and in height. A range of abrasive papers is available offering a wide range of characteristic dimensions. Images were recorded with laser illumination. No lens was used for collimation, offering an approximately parallel beam over the small area of the image.

[0116] The camera focussed well at this short distance. The spacing between pixels was measured for each camera distance and zoom setting. This allows us to relate the distance measured in pixels to spatial dimensions on the imaged surface. This allowed correlation of the image with the grit size of abrasive paper.

[0117] FIG. 8 a) to d) shows the results of imaging the abrasive paper samples and consists of histograms of grey scale as a function of pixel distance along the line of measurement. FIG. 8 e) shows the result of imaging a mirror surface as a comparator.

[0118] This experiment shows that the apparatus according to the invention is useful for determining particle size of contaminants or surface structures which (as discussed below) may be related to drag.

EXAMPLE 1

[0119] For remote monitoring according to the invention there is generally a need to capture an image of the light spot on the surface of the aeroplane or other structure to be analysed. The light will generally be aimed at the surface at an angle between 0 or 1 and 90 degrees from normal. This light source may be ground based or situated on a drone. A detector will generally capture an image of the light spot or pattern on the surface being investigated. A telephoto lens system will generally be used to focus the image on the sensor in such a way that the image of the light spot or pattern will fill a proportion, for example 60%, of the sensor capture area. The sensor may be ground mounted or mounted on a drone. The light source and detector may be in separate units, which can be moved independently. Preferably, the light source is collimated and may be one or more monochromatic light sources (e.g. a laser) or white light.

Method

[0120] A rig was constructed consisting of two optical rails, post holders, lens holders, a light source and a camera. A red laser (650 nm±10 nm, <1 mW) or blue LED were used as the light source. In both cases the light source was approximately 50 cm from the surface to be measured. The light source was placed adjacent to a lens holder with a matched pair of achromatic lenses (100 mm; 100 mm). For the laser this produced a diffuse circular spot of 25 mm diameter. For the blue LED this set up produced a circular spot of light approximately 30 mm in diameter. The camera (Infinity 2.1 Lumenera, with 18-108 mm macro zoom lens) was mounted behind and above the light source and focussed on the light spot. A low exposure time of typically 1-10 ms was used for the camera to avoid saturation of the light density in the relevant channels.

[0121] The test surfaces were three samples with low, medium and high levels of synthetic dirt applied to the surface. These surfaces were illuminated with a red laser at normal incidence and the image of the illuminated area recorded with the camera at an angle of 30° to the surface normal. A line scan of the intensity across the image of the illuminated spot was recorded. The area under the curve of the line scan across the illuminated spot region was integrated for both the red and green channels for the different levels of synthetic dirt. The results are indicated in Table 1 below showing integrated intensities for red and green channels.

TABLE-US-00001 TABLE 1 Low Medium High Contamination Contamination Contamination Red Channel Intensity 2437 1770 543 (arb units) Green Channel 132 36 0 Intensity (arb units)

Threshold Method

[0122] A number of methods may be used to determine surface structure, one important method of analysis uses a threshold method.

[0123] There is a relationship between surface roughness and the intensity of reflected light as shown at least by Experiment 3 above. The relationship may be quantified by taking the intensity above a threshold level which has the effect of reducing the background scatter from a smooth surface and measuring increasing light scatted by increasing roughness on a surface. Granularity can be seen in the image which relates to particle sizes. Generally, dirt or other contamination on the surface reduces the amount of light scatter.

[0124] The threshold may be determined for particular surfaces according to the following protocol:

1) Capture spot image
2) Resize image
3) Normalise pixel intensities of image
4) Map spot shape
5) Acquire pixel intensity for a cross section of spot image (different cross sections can be used)
6) Produce profile of pixel intensity against measure of distance along spot
7) Create a threshold (cut off value) that is higher than an intensity that can be recorded from a mirrored surface (see for example FIG. 8, in particular the mirror surface of FIG. 8e). The threshold value chosen will depend on colour of surface.
8) Sum the light intensity values greater than the threshold value for all points along the distance axis.

[0125] The final value may be used to correlate with roughness and/or dirtiness based on previous calibrations. This enables a determination to be made of the surface structure and hence an analysis of the surface.

Method Using a Patterned Light Source

[0126] A stand-off measurement was made using a camera system with an attached ring flash and patterned aperture covering the front of the ring flash and camera. Lenses of differing focal lengths could be used to give different working distances for the stand-off measurement.

EXAMPLE 2

[0127] Results of reflected light from a rough surface and a reflective surface pre and post cleaning are shown in Table 2 below. The stand-off distance was 1.5 m using a 135 mm lens on a cropped sensor which equates to about 202 mm.

[0128] The light source was a xenon ring flash attached to the font of the 135 mm lens with a patterned aperture mounted over the ring flash in such a way that the lens could capture images through a central aperture of 50 mm.

[0129] This was surrounded by another two apertures to form a pattern of light on the surface. In this case, the apertures were two opposing segments of a 75 mm circle, each 5 mm wide.

[0130] The flash was triggered and an image was taken of the surface. The camera was carefully positioned so to capture an image of the reflection of the patterned light on the surface. The exposure was adjusted to capture light from the ring flash and not the ambient light. The image was converted to a grey scale and normalised and then the average pixel intensity of the central disc between the reflected arcs of light was recorded.

TABLE-US-00002 TABLE 2 Standard Dev. Gray Mean Grey Value Value Nature of Surface (arb. Units) (arb. Units) Rough 132 41 Dirty 61 22 Clean 20 8

EXAMPLE 3

[0131] Results of reflected light from a side panel of a black car pre and post cleaning are shown in Table 3. Measurement were taken at midday with an overcast sky. The stand-off distance was 1.5 m using a 135 mm lens on a cropped sensor which equates to about 202 mm.

[0132] The system used was the same as described in example 2 above.

TABLE-US-00003 TABLE 3 Standard Dev. Gray Mean Grey Value Value Nature of Surface (arb. Units) (arb. Units) Dirty 74 8 Clean 12 3

EXAMPLE 4

Procedure for Determining the Drag of an Aircraft Surface of Unknown Mean Particle Size

[0133] Two portions of aerofoil surface were evenly coated with dirt of undefined mean particle size—one with larger and one with smaller particles. Both surfaces were measured using the protocol of Experiment 3 to provide plots of grey scales against pixel distance across the light spot and then the threshold method was used to determine the light intensity for each case. Using the calibration plot of FIG. 10 the corresponding mean particle size was calculated. Subsequently the plot of FIG. 2 was used to relate the particle size to provide a figure for the drag associated with each surface. Table 4 below shows light intensity and mean particle size and drag determined using this method.

TABLE-US-00004 TABLE 4 Mean Particle Light intensity diameter Drag (N) Surface A 7085 43 microns 50.6 Surface B 5374 89 microns 50.85

Relating Surface Cleanness to Drag

[0134] Dirt or increases in roughness of a surface may result in an increase in drag for given surface size and shape and air speed. This is a result of increased turbulence of the boundary layer. This is demonstrated using wind tunnel experiments.

[0135] FIG. 2 shows that for a given number of particles as the number of particles increase the drag increases. In addition different dirt particle distribution may result in different drag values.

Relating Surface Polish to Drag

[0136] A polished surface can provide reduced drag. So in a similar manner to that described above the inventive apparatus (optionally with contact angle measurements) may be used to determine the quality of the polish on a surface. The quality of polish on a surface relates to the drag thus enabling determination of the drag value for a particular surface in an air flow.