MONITORING OF OBJECTS IN AN OPTICALLY DENSE ATMOSPHERE

Abstract: A method of locating an object in an optically dense atmosphere (15) includes transmitting a pulsed wave optical signal (16) towards the object through the atmosphere adjacent the object. A particular application is the location of the surface (14) of the contents of furnace (10), for example the burden (12) in a blast furnace. A received signal including at least a portion of the signal reflected by the object is monitored and analyzed in relation to the transmitted signal to determine the location of the object. This analysis comprises identifying the trailing edge of the received pulse signal to ascertain the time elapsed between the detection of the transmitted and received signals.

Description: "MONITORING OF OBJECTS IN AN OPTICALLY DENSE ATMOSPHERE" Field of the Invention This invention relates to the location of an object in an optically dense atmosphere, and has particular application to the location of the surface of the contents of a furnace, for example the burden in a blast furnace, and more generally to surface profile measurement. Optically dense atmospheres of interest include foggy or dusty atmospheres, chemical fogs and smogs, and battlefield conditions. The "atmosphere" may be liquid, gaseous, vapour or a mixture of these. Background Art Several techniques have been proposed for locating blast furnace burden and/or determining burden profile, with a view to facilitating improved control of the profile of the burden during blast furnace operation. Mechanical systems for stock depth measurement in a blast furnace were developed along with charging techniques last century and are an integral part of current blast furnace technology. The original mechanical system consists of a steel rod which is inserted vertically into the furnace until it strikes the burden surface. The length of the rod is measured to give the burden depth. Disadvantages include restricted operating range, low availability (since the rod must be withdrawn during charging) , questionable accuracy (as the rod can penetrate the burden surface) , and high capital and maintenance costs. In order to improve performance, the mechanical approach has been extended to measurement-of burden profiles by replacing the rigid stock rod with a wire and weight, as disclosed in U.S. patent 4,574,494 to Mailliet et al. Despite these problems mechanical stock rods are obviously adequate for the task, and represent well established technology. The major impetus for development of alternative non-contact burden location techniques has come from the need for burden surface profiling, as opposed to simple depth measurement at a point. A measurement approach based on ultrasonic acoustic energy is the only reported non-contact method known to the present applicant which does not use electromagnetic radiation as the probe beam. The technique relies on measuring the transit time of acoustic energy to determine the distance, but suffers from the fact that the velocity of the acoustic wavefront is very dependent on the temperature, pressure and moisture content of the atmosphere. Since these factors vary widely in a blast furnace, this technique has not been used in practice. The other non-contact techniques which have been reported make use of electromagnetic radiation 4 at wavelengths ranging from microwaves (10 μm) through to γ-rays (10 ~ μm) . The γ-ray system uses large quantities of Co to generate the 10~ μm wavelength radiation which is used as the probing beam for ranging. A system operating on this principle was built in the USSR but is considered impractical for reasons of personnel safety and low precision. Microwave-based burden location has been proposed for depth measurement and profiling purposes, for example in Trans. Iron Steel Inst. Japan (1984) 24, No. 5, 420. A continuous wave modulated microwave source is used in either frequency modulated or amplitude modulated form. The signal reflected from the burden is detected and its phase compared with the source, the difference being directly related to source-target separation. The major reported disadvantages of the technique are interference due to multiple reflections and low spatial resolution due to wide beam divergence. An optical method is described by Noel in CNRM No. 4 (July 1965) at page 7. There is substantial attraction in an optical technique in that it requires relatively simple and inexpensive equipment and does not entail physical access to the interior of the blast furnace. As originally proposed, the arrangement employed, an optical altimeter and the location of the burden was calculated on the optical triangulation principle. The optical technique was developed for blast furnace burden profile measurement by employing a giant pulse Nd.YAG laser and continuing to rely upon optical triangulation. This profile approach is described in Iron and Steel Engineer, January 1984, 28 by Inazaki et al. As noted by Inazaki et al, the original optical burden location technique did not achieve widespread use because of the loss of light in the dust laden atmosphere present in blast furnaces. A scanning mechanism for simultaneous profiling of burden depth and temperature is disclosed in U.S. patent 4322627 to Pirlet. Summary of the Invention While confirming some advantages over traditional burden location techniques, the initial work by the present applicant has indeed confirmed that substantial difficulties are met in signal analysis arising from the optically dense aerosol atmosphere and from the substantial variation in burden position which can arise during blast furnace operation, especially when the furnace is being run down for maintenance or otherwise. These effects produce a dynamic range of several orders of magnitude in the return signal level, and the problem is essentially the reliable detection of the return signal in the face of this signal variation. It is an object of the invention to at least in part alleviate these difficulties and so achieve a satisfactory optical burden location technique which is preferably capable of ready adaptation to surface profile measurement. The invention therefore affords a method of locating an object in an optically dense atmosphere, for example the surface of the contents of a furnace such as the burden of a blast furnace, comprising transmitting a pulsed wave optical signal, preferably of laser light, towards the object through the atmosphere adjacent the object and monitoring a received signal including at least a portion of the signal reflected by the object, and analyzing the received signal in relation to the transmitted signal to determine the location of the object, wherein said analysis comprises ascertaining the time elapsed between detection of the transmitted and received signals and 'utilising this time in said determination. The invention further provides a method of locating an object in an optically dense atmosphere, for example the surface of the contents of a furnace such as the burden of a blast furnace, comprising transmitting a pulsed wave optical signal, preferably of laser light, towards the object through the atmosphere adjacent the object and monitoring a received signal including at least a portion of the signal reflected by the object, and analyzing the received signal in relation to the transmitted signal to determine the location of the object, wherein said analysis comprises identifying the trailing edge of the received pulse signal and utilizing this trailing edge in said determination. Advantageously, said analysis is a time-of-flight analysis relying upon the time delay between the transmission of the initial signal and the receipt of the trailing edge of the received pulse signal. The identification of the trailing edge arises from a realization that the leading edge of the received pulse from the burden is significantly masked by signals received from said atmosphere adjacent the object whereas the trailing edge is not so masked and largely represents the reflected light which has travelled for the longest time and is therefore substantially free of light reflected fromthe intervening atmosphere. Preferably, the method utilizes an optical arrangement designed to produce a return signal of amplitude which is independent of the distance to the object. More particularly, there is included receiving optics arranged to produce an image in the focal plane which increases in size (i.e. goes out of focus) as the distance of the reflecting burden surface reduces. Most preferably, the detector is located in the image plane where the image size at the maximum desired range, that is the maximum distance of the burden surface, coincides with the detection area. The invention further provides apparatus for locating an object in any optically dense atmosphere, for example the surface of the contents of a furnace such as the burden in a blast furnace, comprising means for transmitting a pulsed wave optical signal, preferably of laser light, towards the object through the atmosphere adjacent the object, means for monitoring a received signal including at least a portion of the signal reflected by the object, and means for analyzing the received signal in relation to the transmitted signal to determine the location of the object, wherein said analysis comprises identifying the trailing edge of the received pulse signal and utilizing this trailing edge in said determination. The invention still further provides apparatus for locating an object in any optically dense atmosphere, for example the surface of the contents of a furnace such as the burden in a blast furnace, comprising means for transmitting a pulsed wave optical signal, preferably of laser light, towards the object through the atmosphere adjacent the object, means for monitoring a received signal including at least a portion of the signal reflected by the object, and means for analyzing the received signal in relation to the transmitted signal to determine the location of the object, wherein said analysis comprises ascertaining the time elapsed between detection of the transmitted and received- signals and utilising this time in said determination. The apparatus advantageously further includes means for varying the angular directions of the transmitted and/or received signals. Such means may, e.g., comprise a pair of co-axial inversely opposed and relatively rotatable disc lens means, e.g. disc prisms, lenses or Fresnel ' lenses. This feature may be employed to adjust the mutual inclination of the signals so as to vary the overlap region and thereby optimise the received signal according to the approximate position of said surface. However, the feature also has wider application as a means of scanning the beams over a surface, e.g., the burden surface is a blast furnace, and so permitting the measurement of the surface profile. To this end, the invention still further provides apparatus for scanning a surface, e.g., the surface of the contents of a furnace such as the burden in a blast furnace, comprising means for transmitting a pulsed wave optical signal, preferably of laser light, towards the surface through the atmosphere adjacent the surface, means for monitoring a received signal including at least a portion of the signal reflected by the surface, and means for analyzing the received signal in relation to the transmitted signal to determine " the location of the surface, characterized by respective optical means for varying the angular directions of the transmitted and received signals each comprising a pair of relatively rotatable disc lens means e.g. disc prisms or lenses arranged in the respective optical paths of the signals. These optical means are preferably co-operatively controlled so as to execute overlapping matching scans of the surface profile to be measured. The invention may extend to a blast furnace or other furnace or burner incorporating apparatus according to any one or more of the above described aspects of the invention, such apparatus being operatively mounted with respect to one or more optical windows in a wall of the furnace, wherein the surface of the contents of the furnace comprises said object to be located. Brief Description of the Drawings The invention will be further described, by way of example only, with reference -to the accompanying drawings, in which: Figure 1 is a schematic sectional diagram of part of a blast furnace modified to carry out the method of the invention; Figure 2 is a schematic sectional view of an adjustable optical arrangement for varying the angular direction of the transmitted or received signal; Figure 3 is a plot of several .output signals from the apparatus, calibrated on the X-axis to show distance in metres to the burden surface; Figure 4 is a circuit diagram for a signal processing circuit for analyzing the return signal at the detector; Figure 5A is a plot, logarithmic on the Y-axis, of the amplitude of the received pulse as a function of the distance to the burden surface, i.e. the range, for different aerosol conditions, without the optical arrangement according to the preferred practice of the invention; Figure 5B is a plot similar to Figure 3A but indicating results from the preferred optical arrangement; and Figure 6 is a diagram illustrating the principle of the preferred optical arrangement. Best Modes of Carrying out the Invention The illustrated blast furnace 10 (Figure 1), which in operation contains a burden 12 with a variable surface profile 14 and an overlying optically dense aerosol atmosphere 15, is modified for the practice of the invention by mounting a transmitter 16 and a detector 18 to opposite optically isolated sides of a beam 19, outside respective adjacent circular transparent windows 20, 21 in a wall 11 of the furnace. Detector 18 is conveniently a silicon photodiode detector, while the transmitter is typically a laser, for example a sealed Nd:YAG laser emitting a 4nS pulse with a peak power of about 5MW. This laser was found to have the best combination of the properties required for the burden location task: it has a wavelength which enables the use of conventional optical materials while maintaining minimal aerosol attenuation, its efficiency is relatively high, cost is low and it is very well established technology. Burden reflectance measurements and blast furnace aerosol attenuation/scattering observations have not revealed any particularly strong wavelength dependence, apart from gas phase absorption lines observed in the aerosol tests for wavelengths below 1 micron. The wavelength of the selected laser is 1.064μm. Transmitter 16 is associated with optics 24 to collimate the output pulse and matching optics 26 images the reflected signal onto the detector. The transmitter and detector optical axes are separated and " relatively inclined so that the transmitted beam 6 and detector field of view 8 overlap only near and below the expected highest position of the burden surface. This substantially reduces light scattered from the nearer aerosol in the furnace. Indeed, one or both of optics 24, 26 may include provision for varying ^ the mutual inclination of the transmitted and received signals so as to optimise the received signal according to the approximate position of the burden surface. Figure 2 is a schematic sectional representation of a suitable arrangement for achieving this adjustability at either optics site and for allowing wider and controlled matching variation of the angular directions of the two signals so that the apparatus may be employed to scan and measure the surface profile of the burden. A pair of inversely opposed, co-axial disc prisms 50, 51 of similar dimensions is supported as shown in respective annular end plugs 54, 55 of co-axial sleeves 56, 57. Discs 50, 51 have flat faces 58, 59 respectively normal to and inclined to axis 52. Faces 58 face each other so that faces 59 are exposed outwardly. Sleeve 56 is rotatably supported by bearings 60, 61 in an outer housing 62 and sleeve 57 is similarly rotatably supported by bearings 63, 65 within sleeve 56. Motors 64 are provided to separately rotate the sleeves, and therefore the disc prisms, via transmissions 66, under servo control from respective sensor discs 68 associated with the sleeves. By rotating disc prisms 50, 51 relative to each other, the relative angular positions of inclined prism faces 59 change and a beam of -light 6a incident on one of these faces 59 emerges from the prism set at 6b at an angle to axis 52 which depends on the relative angular positions of the disc prisms. Instead of disc prisms 50, 51, one might employ a pair of inversely opposed, co-axial, mutually rotatable Fresnel lenses. In the most satisfactory arrangement, a disc prism is used in transmission optics 24 and a Fresnel lens in receiver optics 26. Windows 20, 21 are typically around 50mm in diameter and 150mm apart centre-to-centre. Where rotatable disc prisms 50, 51 are employed in the transmission optics 24, the window may be, say, about 25 or 26mm in diameter. The receiver window is suitably about 70mm in diameter with disc prisms and about -40mm with rotatable Fresnel lenses. Windows 20, 21 are provided on a disc-like base plate 28 which also supports beam 19 and is a removable part of the furnace wall. Plate 28 is in two parts separated by a heat insulating gasket and carries two depending tubes 30, 31 about- the respective windows. An inert gas such as nitrogen is flowed from pipe 35 through passages in plate 28 across the inner faces of windows 20, 21 and down the interior of tubes 30, 31. This gas is effective to continuously clean the windows during use of the facility. The detected signal reflected from the burden surface is processed in relation to the transmitted signal on a time-of-flight basis, by being analysed in a suitably programmed microprocessor or microcomputer coupled to receive the output of detector 18. Figure 3 shows the- resultant output in a simple arrangement in which an A/D convertor is used to record the output from both a start detector viewing a portion of the laser pulse signal directly, and from detector 18. The burden return signals D occur about 50nS after the laser is fired, indicated by pulse A, and the distance to the burden surface can be determined by accurately measuring the separation of the two peaks A and D. A noticable feature of Figure 3 is the slowly rising hump C on the left (i.e. leading) side of the burden return signal D. It is believed by the inventors that this hump is due to atmospheric scattering by the aerosol atmosphere of the furnace interior. It is further considered by the inventors that the presence of this hump in general precludes the application of leading edge detection, which is the usual method employed in other applications of time-of-flight distance measurement. Time-of-flight measurement has not been used in burden surface location and optical triangulation techniques have not proven entirely satisfactory. The inventors consider that the solution lies in the novel step of identifying and monitoring the trailing edge E of each of the burden return pulses D. It will be noted that the trailing edge E is in general a sharp and clean line of conventional Gaussian shape, unmasked by any scattering hump akin to hump C. The electronics coupled to the receiver/detector and transmitter may be largely conventional for time-of-flight measurement except that, in accordance with the invention, they are adapted to detect the trailing edge of the return pulse. For example, the electronics may conveniently comprise a constant fraction discriminator and a diagram of such a processing circuit is presented in Figure 4. In this analogue technique, the incoming •signal is differentiated, by a coaxial cable with a shorted branch, and the negative part of the result- is split into two parts, one of which is delayed for a fraction of the width of the peak to be detected. This delayed signal is subtracted from the remaining input signal and the zero-crossing or null point of the resultant output is detected. As implied by the name, this null point represents a constant fraction of the input pulse, more or less independent of its amplitude. As the constant fraction discriminator works only the negative part of the derivative of the detected pulse, the circuit is identifying and working on the trailing edge of the pulse. It is found that a fraction of 20% peak amplitude is satisfactory: it is sufficiently as low a value as possible while still avoiding difficulties with noise and other fluctuations. According to one embodiment of the invention, the aforementioned null point can be obtained by sensing the output of detector 18, or a signal derived from this output, "upstream" of one end of a terminal length of co-axial cable which is impedance matched at its other end so as to reflect a portion of the detector output. The reflected signal is delayed and inverted on reaching the sensor and thus sums with the detector output akin to the constant fraction discriminator. Digital analysis may alternatively be employed. Figure 5A highlights an aspect of known optical techniques for locating the burden surface. This figure comprises plots of the amplitude of the burden return signal, on a logarithmic Y-axis, against the range, i.e. the distance of the burden surface from the transmitter and detector. The designation "90% aerosol- condition" indicates the worst aerosol 90% level (i.e. most dense aerosol for 90% of furnace operation) . It will be observed that the dynamic range of the instrumentation is required to be about 8 orders of magnitude if the burden distance range is to be from 4 to 18 metres. Two factors contribute to return signal dynamic range; the first is the variation due to changes in spatial range. For an isotropically 2 scattering target there is a 1/R fall off in signal level. The second and major contribution to return signal fluctuation is aerosol attenuation variations. Equation (1) below is the basic laser radar ("lidar") equation where P (R) is received power, P is transmitted power, t is pulse length, c is the velocity of light, A is receiver area, B(R) is aerosol backscatter coefficient, Al(r) is aerosol attenuation coefficient and R is spatial range: P (R) - P (ct/2) B(R)A r R j2 exp[-2 2 The 1/R factor in equation (1) leads to signal varations for different operating conditions as follows: basic operation (spatial range of burden depth 7 - 9m) : signal variation = 1.65 orders of magnitude charging delay/overfill condition (6 - 12m) signal variation = 4.0 orders of magnitude shutdown/maintenance condition (7 - 22m) : signal variation = 10 orders of magnitude Figure 5 depicts the principle of a preferred optical arrangement for the detector. Shown here is a- simple imaging system (i.e. a lens) producing different sized images at a given distance from the lens (detector position) for different positions of the same object. The return signal is of amplitude which is independent of the distance to the burden surface. The arrangement of the invention is a short range laser radar system, and so the limited depth of field of the optical detector can be used to produce an image in the focal plane which increases in size (goes out of focus) as the target range reduces. Variations in image size as a function of range R are described by: 4A D Rm 2 R 2 F= where F is the fraction of energy received, A_ is detector area, Rm is the maximum range, f is the focal length of the imaging system and I is the object (spot) size. This principle is adapted in the optics of the embodiment illustrated in Figure 1. Specifically, the detector is located in the image plane where the image size at the maximum desired range, that is the maximum distance of the burden surface, coincides with the detection area. The result of these measures for compressing the dynamic range, plotted as a function of range for typical dimensions of the transmitter/detector equipment, is shown in -Figure 5B. This figure is based on designing the instrument for an optimum range of 9m, and show s that the signal variation between average aerosol conditions and the worst aerosol 90% level (i.e. most dense aerosol for 90% of furnace operation) is just over one order of magnitude (i.e. the span AB on the logarithmic y-axis) . The corresponding variation for delay/overfill operation is just over two orders of magnitude. In the normal operating condition the 2 system is seen to compensate fully for the 1/R variation and for the aerosol-related range variations which are predicted to occur for at least 90% of the time. An advantage of the optical burden surface location in accordance with the described preferred embodiment of the invention is a significant increase in the spatial range relative to conventional mechanical stock rods, for example from a range less than a few metres to a range of about 20m. The optical system is also available for measurement much sooner after charging, and the capital cost, is reduced by an order of magnitude. A significant advantage of the inventive system is that it is readily adaptable to both bell-charged and chute-charged furnaces. The former class of blast furnace is especially crowded in the head and there is little space between the bell(s) and the armour. The space is sufficient, however, to pass the tubes " 30, 31. It will be appreciated that the simple arrangement described above in connection with burden location could be extended to provide a complete profile of a burden surface utilizing only a single transmitter/detector installation, compared with the dual spaced installations of prior triangulation- based non-contact arrangements. Mechanisms of the type shown in Figure 2, provided for each of the transmitted and received signals, would be co-operatively controlled, e.g., by a computer or microprocessor, to cause the optical signals to execute an overlapping two-dimensional scan across the interior of the blast furnace. The laser transmitter would need to be capable of a suitably high pulse transmission rate. In a more advanced arrangement, provision may be made for simultaneously producing a thermal profile of the burden surface. In this arrangement, the received light beam is divided, by a wavelength sensitive beamsplitter, into the reflected laser signal and the infra-red radiance component from the burden surface. The reflected laser signal is analysed as before and the infra-red component mapped in two dimensions to output the desired thermal profile of the burden surface.

Claims: CLAIMS : 1. A method of locating an object in an optically dense atmosphere, comprising transmitting a pulsed wave optical signal towards the object through the atmosphere adjacent the object and monitoring a received signal including at least a portion of the signal reflected by the object, and analyzing the received signal in relation to the transmitted signal to determine the location of the object, wherein said analysis comprises identifying the trailing edge of the received pulse signal. 2. A method according to claim 1 wherein said analysis is a time-of-flight analysis relying upon the time delay between the transmission of the initial signal and the receipt of the trailing edge of the received pulse signal. 3. A method according to claim 1 or 2 wherein the object to be located is the surface of the burden of a blast furnace. 4. A method according to claim 1, 2 or 3.. wherein said monitoring of the received signal includes utilizing return signal receiving optics arranged to produce an image in the focal plane which increases in size (i.e. goes out of focus) as the distance of the reflecting object reduces. 5. A method according to any preceding claim wherein the transmitted signal comprises a laser - derived signal. 6. Apparatus for locating an object in an optically dense atmosphere, comprising means for transmitting a pulsed wave optical signal towards the object through the atmosphere adjacent the object, means for monitoring a received signal including at least a portion of the signal reflected by the object, and means for analyzing the received signal in relation to the transmitted signal to determine the location of the object, wherein said analysis comprises identifying the trailing edge of the received pulse signal. 7. Apparatus according to claim 6 wherein said analysis is a time-of-flight analysis relying upon the time delay between the transmission of the initial signal and the receipt of the trailing edge of the received pulse signal. 8. Apparatus according to claim 6 or 7 further comprising signal receiving optics arranged to produce an image in .the focal plane which increases in size (i.e. goes out of focus) as the distance of the reflecting object reduces. 9. Apparatus according to claim 8 wherein the monitoring means is located in the image plane where the image size at the maximum desired range, that is the maximum distance of said object, coincides with the detection area. 10. Apparatus according to any one of claims 6 to 9 wherein said analyzing means comprises means to differentiate the received signal and a constant fraction discriminator configured to work on the negative part of the derivative of the received signal. 11. Apparatus according to any one of claims 6 to 10 further comprising a pair of coaxial inversely opposed and relatively rotatable disc lens means for varying the angular directions of the transmitted and/or received signals. 12. A method of locating an object in an optically dense atmosphere, comprising transmitting a pulsed wave optical signal towards the object through the atmosphere adjacent the object and monitoring a received signal including at least a portion of the signal reflected by the object, and analyzing the received signal in relation to the transmitted signal to determine the location of the object, wherein said analysis comprises ascertaining the time elapsed between detection of the transmitted and received signals. 13. Apparatus for locating an object in any optically dense atmosphere, comprising means for transmitting a pulsed wave optical signal towards the object through the atmosphere adjacent the object, means for monitoring a received signal including at least a portion, of the signal reflected by the object, and means ' for analyzing the received signal in relation to the transmitted signal to determine the location of the object, wherein said analysis comprises ascertaining the time elapsed between detection of the transmitted and received signals. 14. Apparatus for scanning a surface, comprising means for transmitting a pulsed wave optical signsl towards the surface through the atmosphere adjacent the surface, means for monitoring a received signal including at least a portion of the signal reflected by the surface, and means for analyzing the received signal in relation to the transmitted signal to deteremine the location of the surface, characterized by respective optical means for varying the angular directions of the transmitted and received signals each comprising a pair of relatively rotatable disc lens means arranged in the respective optical paths of the signals. • 15. A furnace incorporating apparatus according to any one of claims 6 to 11, 13 and 14, which apparatus is operatively mounted with respect to one or more optical windows in a wall of the furnace, wherein the surface of the contents of the furnace comprises said object to be located. 16. A furnace according to claim 15 comprising a blast furnace in which the surface of the burden therein comprises said object to be located.

Inventors: FYSH STUART ALFRED, SCOTT JOHN CHRISTOPHER, FYSH, STUART, ALFRED, SCOTT, JOHN, CHRISTOPHER
Applicants: BROKEN HILL PTY CO LTD, THE BROKEN HILL PROPRIETARY COMPANY LIMITED
Application year: 1988
Main classification: G01S17
IPC codes: C21B7, G01S7, G01S17