GB2396023A

GB2396023A - Imaging system with purging device to prevent adhesion of particles

Info

Publication number: GB2396023A
Application number: GB0223162A
Authority: GB
Inventors: Martyn Richard Huw Knowles; Adam Whybrew; Heather Booth; Edward Butcher
Original assignee: Oxford Lasers Ltd
Current assignee: Oxford Lasers Ltd
Priority date: 2002-10-05
Filing date: 2002-10-05
Publication date: 2004-06-09
Also published as: AU2003269226A1; GB0223162D0; WO2004031742A1

Abstract

An imaging system for producing images of particles in a fluid, has a light source 113 and/or an imaging device such as camera 101 and an optical component assembly arranged to be situated, at least in use, between the light source or the imaging device and the particles 106 to be imaged. The optical component assembly consists of a light transmissive optical component such as transparent window 103, 110 and a fluid purging device 104, 105, 108, 109 arranged to supply a flow of purging fluid adjacent to the optical component to prevent the particles adhering to it. The purging fluid may be a gas which may be supplied through radial slots. A swirl plate (6, Fig 2) may function as a second purging device.

Description

Imaging system The present invention relates to an imaging system for

producing images of particles, for example for measuring the sizes of particles and/or the velocities of particles in a fluid.

It is frequently necessary, or at least desirable, to obtain information concerning the size and/or shape of particles present in fluid. For example, many industrial chemical processes involve the precipitation of solids from liquid solutions, or the bubbling of gases in liquids. Other processes involve the presence of solid particles in liquids or gases, or liquid droplets in gases, for example. The correct functioning of many of these, and other, processes is dependent upon a particular size and/or shape distribution of the "particles" (whether the "particles" are solid, liquid or gaseous) in the fluid.

For example, in the manufacture of many pharmaceuticals, the drug is crystallized out of solution, and the structure of the crystals may vary depending upon conditions such as the temperature and the concentration of the solution.

In other processes, the drug is formulated from powders, or ground down from solids, for example. It would be highly desirable to many pharmaceutical companies to be able accurately to determine, as the process progresses, whether or not the correct crystal structure is being formed, since the discovery at the end of the process that the process has failed can waste an enormous amount of time and money. This is also true of many other processes, for example the production of cement, since the particle size distribution of the cement particles needs to be within a certain defined range in order for the cement to have the required physical and chemical characteristics. Numerous other processes would also benefit from an accurate method of determining the size and/or shape of particles in a fluid.

Imaged based sizing is an important technique for measuring particle sizes. Particles from (for example) 1 micron in diameter and larger (up to hundreds of millimetres) can be measured. The only other technique routinely

used for sizing of solids in this size range in industrial processes is laser diffraction. Compared with laser diffraction, image based sizing can measure larger particles and/or more dense distributions and/or give shape information. It can be non-invasive and generally produces unambiguous, easily verifiable results. A typical image based system uses a camera (or other imaging device) fitted with one or more suitable lenses to capture images of the individual particles. These images are then analysed by computer software to produce parameters such as the number of particles, particle diameters and distributions.

Illumination of the particles can be front or back-illumination and can be from a laser or non-laser light source. The camera is often a CCD (charge coupled device) or CMOS (complementary metal oxide semiconductor) camera to enable easy interface with the computer.

As already indicated, one particular application of image based sizing is the analysis of particle size distribution during the production of the particles.

This is of relevance to the producer of any product which is a particle or powder, or is made from a particle or powder (for example the pharmaceutical industry and the cement industry). Typically the powder is produced in one machine, perhaps a mill, and then conveyed to a subsequent machine for packaging or further processing. In many cases the size and the size distribution of the particles is an important aspect of the quality of the product. Techniques exist for measuring samples of the particles or powders but these are off-line or at-

line techniques (rather than in-line) and so are inconvenient and can not generate data fast enough to form part of a feedback system to control the machine that makes the powder. Another problem with such techniques is that the sample may be unrepresentative of the particles produced, as a whole. By imaging the particles as they exit the producing machine (for example), image based sizing can generate immediate data that can be used to monitor the process in real time or to control the producing machine. There is a problem, however, when the powders produced adhere to the pipe or window through which the imaging is being performed. When sufficient particles have adhered to the pipe or window then the image quality is degraded and eventually the pipe or window becomes opaque or translucent.

The present invention seeks to provide a practicable and efficient solution to this problem.

Accordingly, a first aspect of the present invention provides an imaging system for producing images of particles in a fluid, comprising: (a) a light source and/or an imaging device; and (b) an optical component assembly situated, at least in use, between the light source or the imaging device and the particles to be imaged, the optical component assembly comprising a light transmissive optical component and a fluid purging device arranged to supply a flow of purging fluid adjacent to the optical component to prevent the particles adhering thereto. Preferably the purging fluid is a purging gas, for example air or nitrogen.

(In principle any fluid could be used, but preferably the fluid is inert, and more preferably it is inert gas.) Systems for keeping lenses or optical windows free from contaminants are known for other applications. United States Patent No. 4,784,491 discloses an optical sensor head for guiding an arc welding process, in which long cylindrical housings extending beyond optical lens and window arrangements are provided with spiral grooves which are intended to provide a swirling motion to a gas which flows towards the welding process, in order to shield the optical arrangement from weld spatter and smoke. United States Patent No. 4,836, 689 discloses a radiation pyrometer for monitoring the temperature of turbine blades of a gas turbine. The pyrometer includes a gas purge system for keeping an optical lens of the pyrometer free from contaminants from the gas turbine.

These known systems are designed specifically for their particular applications and are suitable only for these applications. For example, the system disclosed in US 4,784,491 directs a gas towards a welding process via long cylindrical housings which are oriented directly at the welding process. The system disclosed in US 4,836,689 is enclosed in a long sight tube and the purging gas

flows through the tube past the pyrometer lens towards the turbine blades.

Neither system would be suitable for use with a light source and/or imaging device of an imaging system, and neither system would be modifiable by the skilled person (and the skilled person would not attempt such modification) for use with such an imaging system.

A second aspect of the present invention provides a method of producing images of particles in a fluid by means of the imaging system according to the first aspect of the invention, the method comprising supplying a flow of purging fluid adjacent to the optical component by means of the fluid purging device, thereby preventing the particles adhering to the optical component.

As indicated above, "particles", in the present specification, may be solid,

liquid, or gaseous (or they may even be of intermediate phase, for example gelatinous particles). For example, the particles may be solid particles in a liquid or a gas, or they may be liquid droplets in a gas, or they may be gas bubbles in a liquid. The particles may even have the same phase as the fluid in which they are present, for example they may comprise immiscible liquid droplets in a liquid.

Most commonly, however, the particles will be solid particles in a gas.

The light source emits electromagnetic radiation preferably in the visible region of the spectrum; however, at least in the broadest aspects of the invention, "light" (and the associated term "optical") may refer to non-visible wavelengths of the electromagnetic spectrum, for example infrared, microwave, or ultraviolet wavelengths. Consequently, although the image generated by the imaging device will normally be a visible image (or at least an image formed from the detection of visible light), in other embodiments of the invention, the image generated by the imaging device may be invisible to the human eye until it has been processed and presented as an image in the visible region of the spectrum. Furthermore, the expression "illuminated" will normally mean that the particles are illuminated with visible light, but in other embodiments they may be "illuminated with invisible electromagnetic radiation.

Illumination is required in the present imaging system (as with any imaging system) and the subject can be front or back illuminated. Either technique may be used, but back illumination is generally preferred so that the particles are viewed in shadow; this improves the contrast and therefore improves the accuracy of the imaging. The light source may be a laser of any type, fluorescence produced from laser light, a lamp or LED, or other light sources such as arcs and discharges or natural or ambient light. However, for small moving particles then it is important to avoid blurring of the particle image owing to its motion during the exposure time of the camera. This blurring can be eliminated by either using a short exposure camera (or other imaging device) or a short intense illumination. Very short exposures are possible using gated image intensified cameras, for example. Alternatively, highly intense, short duration illumination can be used as from a pulsed laser or flashlamp or LED. It is preferable to use a diffuse and/or incoherent light source to avoid speckle coherence and diffraction effects especially when imaging small particles at high magnification. The duration of the exposure or illumination to prevent significant blurring depends upon the velocity and size of the particle, but is generally less than 1 microsecond.

Advantageously, the imaging device may comprise a camera, especially an electronic camera. Preferred imaging devices include CCD and CMOS cameras, for example. The imaging device may comprise at least one lens (or lens assembly), preferably a high magnification lens having a long working distance.

Preferred and optional features of the invention are specified in the dependent claims, and reference should be made thereto.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a schematic illustration of the main components and the arrangement of a preferred embodiment of an imaging system according to the invention; and

Figure 2 is an exploded illustration of part of a preferred embodiment of an imaging system according to the invention, showing in detail the components thereof. Figure 1 shows, schematically, a length of steel pipe 107 which forms part of the imaging system of the invention and which has replaced an identical length of pipe in an industrial process. The industrial process may, for example, be the production of a pharmaceutical or the production of cement, or substantially any process in which particles 106 flowing through a process pipe (in the direction indicated by the arrow) need to be monitored by the imaging systems. At diametrically opposed locations and extending radially outwardly from the pipe are a pair of flanged short pipes 114 and 115. The outwardmost end of each short pipe is sealed and provided with a transparent window 103, 110 (i.e. a transmissive optical component). A line of sight (i.e. an optical transmission path) therefore extends through window 110, along short pipe 115, across the process pipe 107 (through which the particles 106 flow), along the opposite short pipe 114 and through the opposite window 103. The windows 103 and 110 preferably are optically flat, but they could instead provide substantially any optical manipulation, e.g. one or both of them could be lenses, prisms beamsplitters, of the like. Preferably the windows 103 and 110 are formed from glass, and preferably the short pipes 114 and 115 are formed from steel (and are welded to the pipe 107).

Adjacent to window 103, and arranged in the optical transmission path extending through both windows, is an imaging device 101 in the form of a camera. The camera 101, which preferably is a digital camera (preferably having a high number and density of pixels, e.g. an array of 1000 x 1000 pixels, each of 9 microns square), includes a lens 102 (preferably a high magnification, long working distance lens). Adjacent to the opposite window 110, and also arranged in the optical transmission path extending through both windows, is a light

source 113, preferably in the form of a laser. The laser 113 preferably comprises a Nd: YAG laser which preferably is frequency doubled to operate at 532 nm and preferably also is Q-switched to produce a pulse duration of the order of nanoseconds or microseconds (e.g. 5 nanoseconds). The laser light beam is guided by means of beam delivery optics 112, and passes through a fluorescing and diffusing device 111 (for example as describe in co-pending UK patent application no. 0117989.4 from Oxford Lasers Limited) which destroys the coherence of the beam and thereby prevents the occurrence of laser speckle on the image.

Each window 103,110 is part of an optical component assembly comprising the window (i.e. the optical component) and first and second fluid purging devices adjacent thereto. First and second fluid purging devices 104 and 105 are adjacent to window 103, and first and second fluid purging devices 109 and 108 are adjacent to window 110. Each fluid purging device preferably comprises a gas purging device which supplies a flow of purging gas adjacent to its respective window to prevent the particles 106 adhering thereto.

Figure 2 shows in detail the various components of one of the optical component assemblies shown only schematically in Figure 1. (For simplicity only one optical component assembly is shown.) The components are:- 1 tube, 2 fixing plate, 3 cross tube, 4 large gasket, 5 plate gasket, 6 swirl plate (i.e. the second fluid purging device), 7 air distribution plate unit, 8 dowel, 9 air distribution plate (i.e. the first fluid purging device), 10 purging gas supply tube, 11 washer, 12 gasket, 13, bolt, 14 gland, 15 window gasket, 16 ferrule, 17 window housing, 18 nut and 19 window (i.e. the optical component).

Of particular note are the air distribution plate 9 and the swirl plate 6. The air distribution plate 9 comprises the first fluid purging device, and it has a plurality of radially directed slots through which the purging gas is supplied radially across the window 19, thereby providing a "cushion" of the purging gas adjacent to the window, to repel particles from the window. The swirl plate 6, however, is provided with a plurality of non-radial slots (i.e. the slots have a

tangential component, as well as a radial component, to their directivity) through the purging gas is supplied, and which cause the purging gas to swirl or revolve (and/or which create turbulence in the supplied purging gas) in order to trap and fling the particles away from the window. The swirl plate 6 comprises the second fluid purging device.

One advantage of the invention is that the amount of gas flow required to keep the windows clean is normally quite low. For example, with cornstarch flowing through a 50 mm internal diameter pipe at a rate of 100 kg per hour, gas flow rates of 8 litres per minute per cushion injector and 6 litres per minute per swirl injector are sufficient to keep the windows clean. This is of great significance because a gas transportation system used to convey powder products from one machine to the next can not always tolerate the addition of significant amounts of extra gas. That is, it is not possible to simply use extremely high purging gas flow rates to keep the windows clean.

An additional feature of the invention is that should the windows become contaminated over a long period of time or if a gas feed to an injector fails, then it is possible to clean the windows and flange area by briefly increasing the gas flow rate. In the case of cornstarch, for example, a flow rate of 15 litres per minute is sufficient to clean the windows and flange. This is an advantageous feature of the invention compared to having to disassemble the system for cleaning. A further feature of the system according to the invention is that by comparing the position of a particle in one camera frame with its position in the next frame or by double exposing a single camera frame, it is possible to calculate velocity data about the particles.

Claims

1. An imaging system for producing images of particles in a fluid, comprising: (a) a light source and/or an imaging device; and (b) an optical component assembly arranged to be situated, at least in use, between the light source or the imaging device and the particles to be imaged, the optical component assembly comprising a light transmissive optical component and a fluid purging device arranged to supply a flow of purging fluid adjacent to the optical component to prevent the particles adhering thereto.

2. A method of producing images of particles in a fluid, by means of the imaging system according to claim 1, the method comprising supplying a flow of purging fluid adjacent to the optical component by means of the fluid purging device, thereby preventing the particles adhering to the optical component.

3. A system according to claim 1 or a method according to claim 2, in which the optical component assembly comprises first and second fluid purging devices, each of which is arranged to supply a flow of purging fluid adjacent to the optical component to prevent the particles adhering thereto.

4. A system or method according to claim 3, in which the first fluid purging device supplies purging fluid closer to the optical component than does the second fluid purging device.

5. A system or method according to any preceding claim, in which the (or each) fluid purging device supplies purging fluid substantially across the optical component and substantially parallel thereto.

6. A system or method according to any preceding claim, in which the (or each) fluid purging device comprises a plurality of fluid outlets arranged generally around the periphery of the optical component, substantially in a plane substantially parallel to the optical component.

7. A system or method according to claim 6, when dependent upon claim 3 or any claim dependent thereon, in which the fluid outlets of the first fluid purging device are arranged such that they direct the purging fluid substantially radially inwardly from the periphery of the optical component.

8. A system or method according to claim 7, in which the first fluid purging device provides a cushion of purging fluid immediately adjacent to the optical component, to repel the particles from the optical component before they can impinge thereon.

9. A system or method according to any one of claims 6 to 8 when dependent upon claim 3 or any claim dependent thereon, in which the fluid outlets of the second fluid purging device are arranged such that they direct the purging fluid inwardly from the periphery of the optical component non-radially so that the purging fluid supplied by the second fluid purging device has a swirling motion.

10. A system or method according to claim 6 or any claim dependent thereon, in which the fluid outlets of the (or each) fluid purging device are arranged substantially uniformly around the periphery of the optical component.

11. A system or method according to any preceding claim, in which the (or each) fluid purging device includes purging fluid supply and control means to supply purging fluid in a controlled manner to the (or each) fluid purging device.

12. A system or method according to any preceding claim, in which the purging fluid is a gas, preferably an inert gas.

13. A system or method according to any preceding claim, further comprising a vessel to which the optical component assembly is attached, for containing the particle-laden fluid.

14. A system or method according to claim 13, in which the vessel comprises a pipe through which the particle-laden fluid is arranged to flow.

15. A system or method according to any preceding claim, in which the optical component comprises a window, a lens, a beam-splitter or a prism.

16. A system or method according to any preceding claim, comprising both the light source and the imaging device.

17. A system or method according to claim 16, comprising two said optical component assemblies, one of which is situated, at least in use, between the light source and the particles, and the other of which is situated, at least in use, between the imaging device and the particles, the light source and the imaging device being arranged, at least in use, such that light emitted by the light source passes through both optical components and the particle-laden fluid, and is received by the imaging device.

18. A system or method according to any preceding claim, in which the light source comprises at least one of: a laser; a flashlamp; an electric arc; an incandescent or fluorescent lamp; a flashcube; a light emitting diode.

19. A system or method according to any preceding claim, in which the light source includes a diffuser and/or a fluorescing device.

20. A system or method according to any preceding claim, in which the light source includes beam delivering optics.

21. A system or method according to any preceding claim, in which the light source comprises a pulsed laser, preferably a pulsed Nd: YAG laser.

22. A system or method according to any preceding claim, in which the imaging device comprises a camera, preferably an electronic camera.

23. A system or method according to any preceding claim, in which the imaging device comprises at least one lens.

24. A system or method according to any preceding claim, further comprising feedback control means which controls the intensity of the light emitted by the light source dependent upon the intensity of the light received by the imaging device.

25. A system or method according to any preceding claim, in which the particles comprise pharmaceutical or cement powder.

26. A system or method according to any preceding claim, comprising a system or method for determining the sizes and/or velocities of the particles.

27. A system or method according to Claim 26, in which the size and/or velocity information determined is used to control at least part of a production line.

28. A system or method according to Claim 27, in which the production line (or part thereof) comprises a granulator, a grinder, a drier, a roller compactor, a powder mill, or the like.

29. A system or method according to any preceding claim, in which the purging gas is supplied at a flow rate in the range 1 - 100 litres per minute (per purging fluid delivery device).

30. A system or method according to any preceding claim, in which the particles have a size in the range 1- 1000 microns.

31. A system or method according to any preceding claim, in which the particles have a velocity in the range O - 100 metres per second.