RU2366909C1 - Multichannel device for measurement of pyrometric characteristics - Google Patents

Abstract

FIELD: physics, measurements.

SUBSTANCE: invention is related to metering equipment. Device represents a polychromator with fiber optic input, concave diffraction grid and multi-element row of photodetectors at the inlet. Photodetectors are connected via signal amplifiers to programmable commutator. From the commutator outlet signals are supplied via ADC to personal computer. Fiber light conductor serves for transmission of signal to a safe distance, and also for averaging of spatial heterogeneity of investigated object. Deliberately large amount of spectral channels makes it possible to select wavelengths by programmable method, which will be optimal for investigation of this object, and also by grouping of row elements to form band filters of rectangular shape with specified width in set wave lengths.

EFFECT: improved accuracy, information value, expansion of dynamic range of measurements.

2 cl, 6 dwg

Description

The present invention relates to measuring technique, in particular to spectroscopic ratio pyrometry, and can be used to create instruments for analyzing and controlling high-temperature technological processes and in solving research problems related to measuring the characteristics of processes that are primarily fast-moving in inhomogeneous media, for example, an explosion type .

In the field of color pyrometry in recent years, the greatest attention has been paid to the development of temperature control in technological processes. Monitoring devices must give a result in real time in order to use it to control the process. Much attention was paid to increasing the accuracy of measurements, for example, by simultaneously determining the emissivity and calculating the true temperature. At the same time, there is comparatively little information on the problem of measuring temperature for research purposes, especially for the diagnosis of fast processes, for example, an explosion. Unfortunately, under these conditions, it is practically impossible to take into account the spectral and temperature dependence of the emissivity of the object, therefore, calculations are usually done in the approximation of constant emissivity, that is, in the approximation of an absolutely gray body. Here, it is essential to exclude the influence of geometric factors, since relative measurements are sufficient to calculate the color temperature. Color temperature is used as a generalizing parameter characterizing the efficiency of ongoing processes. Such measurements are usually expensive, and often simply unique. Therefore, the main task of the measuring complex is to record in real time the largest possible amount of information, as well as the creation of hardware and advanced programs for the subsequent processing of this information.

A multi-channel spectral light measuring device is known [see Pat. USA No. 4909633, IPC G01J 01/04, G01J 05/60, publ. 1990.03.20.], Comprising a lens, a fiber optic bundle, divided into three channels, individually positioned to three light-receiving elements having different spectral sensitivities. The three light components are simultaneously converted into three electrical signals and then into digital quantities by means of an analog-to-digital converter (ADC). This device does not allow with sufficient accuracy and reliability in a wide dynamic range to make absolute measurements of pyrometric characteristics. Since the tow is drawn from a finite number of fibers, it has a discrete structure. With uniform illumination this circumstance does not matter, but the heterogeneity of the object leads to different illumination of different fibers of the bundle and to errors in the distribution of light energy over the spectral channels of the analyzer. To reduce this error, the authors of the device remove the end of the bundle from the focal plane of the lens, as a result of which its discrete structure appears to be “smeared”, but the photometric region is also blurred.

A known device for measuring the pyrometric characteristics of a process that we have selected as a prototype is [see Pat. RF to the floor. Maud. No. 32594, IPC G01J 05/60, prior. 2003.04.21.], Including an optical circuit element that decomposes radiation into quasimonochromatic components, and photosensitive elements following it in the optical path, which use silicon photodiodes, the connection circuit of which corresponds to the photodiode mode, and an optical circuit element that decomposes radiation into components represents a series consisting of at least two filters that transmit radiation in different regions of the spectrum.

Using this device, the optical characteristics of the explosion are investigated over time. To collect spectral information, a set of photodiodes with bandpass filters is used, forming a high-speed multichannel spectral analyzer. The signals of the photodiodes via coaxial cables are transmitted to digital oscilloscopes, where they are stored for further processing. Signals proportional to the illumination at the analyzer installation point are recorded. The recorded information using a computer according to a well-known program is processed in order to obtain pyrometric characteristics, such as the absolute values of illumination at the point of installation of the receivers, the light intensity of the source and color temperature, the average volume of the luminous body.

This device also does not allow to measure absolute pyrometric characteristics with sufficient accuracy and reliability; moreover, the signals in it are transmitted through long cables subject to electromagnetic interference. The device is not reliable enough, because a block of photodetectors with light filters is located near the object, that is, in the explosion zone there are fragile optical elements that must be protected.

To measure the brightness of a certain zone, it is necessary to place identical lenses and field apertures in front of the receivers and strictly reduce their field of view.

Interference filters have a characteristic feature. Their strip width and center position depend on the angles of incidence, which leads to significant errors for different angular dimensions of the object or its asymmetric development during the explosion.

We have proposed a high-precision high-speed reliable instrument for measuring in a wide dynamic range the absolute pyrometric characteristics of inhomogeneous objects, such as an explosion.

The claimed technical result was obtained by us when, in a multichannel device for measuring pyrometric characteristics, mainly fast processes in heterogeneous media, such as an explosion, which includes an element that decomposes the radiation from the measured object into spectral components located in the optical path, a photodetector assembly and an information recording and processing unit in in the form of ADC and computer, new is that in the optical path in front of the element that decomposes the radiation, an additional fiber of lights is installed ode, the output end of which serves as the input diaphragm of the radiation decomposing element, the radiation decomposing element itself is made in the form of a concave diffraction grating, in the output plane of which there is a photodetector assembly in the form of a multi-element photodetector array equipped with amplifiers, each element of which receives radiation in a narrow spectrum band , while the number of line elements is selected larger than the number of ADC channels, and the computer is equipped with a function for calculating absolute illumination, absolute brightness, and e color temperature.

The term “multichannel” means the ability to receive the more spectral information, the more spectral channels, and also means the ability to select the optimal wavelengths for calculating the temperature. The number of registration channels is limited by the number of ADC channels. To limit the number of registration channels, that is, ADC channels, means to limit the information content of the device. Therefore, in this case, the choice implies the number of spectral channels is greater than the registration channels, that is, the number of line elements is greater than the number of ADC channels

In order to increase the sensitivity and speed of registration of the device, the neighboring elements of the multi-element photodetector line are combined into groups.

The device allows two measurement modes, namely: measuring the illumination created by the entire object as a whole, and measuring the brightness of the selected limited area. Accordingly, either the average color temperature over the object or the local temperature of the selected zone is calculated. In the first case, the fiber is fixed at such a distance that the entire radiation region is inside the aperture angle of the fiber, in the second case, a lens is mounted between the object and the fiber so that a given part of the image of the object, for example, the central part, falls on the input end of the fiber. Approaches to solving these problems are known.

The optical fiber makes it possible to carry out measurements directly inside the object. Approaches to solving this problem are known.

Figure 1 shows a diagram of the claimed device, which shows: the input end 1 of the optical fiber 2, its output end 3, dispersing * (* Dispersing is an element that decomposes radiation into monochromatic components. [I.V. Peysakhson. "Optics of spectral devices" , Engineering, Leningrad., 1975, p. 34 et seq.]) Element 4, multi-element photodetector 5, programmable switch 6, ADC 7, computer 8 and object 9.

Fig. 2 is a diagram illustrating the conversion of a light or light intensity meter (Fig. 2a) to a local brightness meter (Fig. 2b), where the input end 1 of the optical fiber 2, the selected area 1 'of the object 9, corresponding to the projection of the input end 1 of the optical fiber, are shown 2, and lens 10.

Figure 3 shows the diagrams of the formation of a band-pass optoelectronic filter when combining several adjacent elements of the photodetector line: Fig. 3a is the hardware function of one element of the line, Fig. 3b is the hardware function of the group of connected elements of the line, where S _OTH is the relative sensitivity of the registration circuit, λ is the wavelength and at the same time the x coordinate of the element of the ruler, because the ruler is installed in the focal plane of the lattice along the dispersion direction.

Figure 4 presents graphs of spectral brightnesses over time and the calculation of the color temperature of the test object ** (** By test object we mean the secondary standard with known pyrometric characteristics) when testing the layout of the device.

Figure 5 and 6 are an enlarged view of the graphs of the characteristics of the test object, shown in figure 4.

The claimed device operates as follows (see Figure 1). The radiation from an inhomogeneous object that is rapidly changing in space 9 enters the input end 1 of the optical fiber 2. The input end of the fiber is installed at such a distance that the luminous zone does not extend beyond its angular aperture. The output end 3 of the fiber 2 acts as the input diaphragm of the dispersing element 4, made in our device in the form of a concave diffraction grating, which creates color images of the diaphragm in its focal plane, in which a multi-element photodetector 5 with electric signal amplifiers (not shown) is installed. The outputs of the amplifiers through a programmable switch 6 are connected to the ADC 7. The information is transmitted in digital form via a USB channel (not shown) to computer 8, in which it is registered, then processed and stored in numerical and graphical form.

The dispersing element of the claimed design also performs another function necessary in such measurements, namely, the reduction of the field of view of all radiation receivers in a strictly specified direction.

Switch 6 is controlled programmatically via a slow channel of communication with computer 8. Approaches to solving this problem are known. This part of the program allows you to select the desired wavelengths for signal registration by ADC channels 7, and also to form bands of the desired width at these wavelengths.

A greater number of spectral channels, significantly more than what is required for calculating the color temperature, makes it possible to choose, for example, by software, the optimal wavelengths depending on the properties of the object. This makes it possible to exclude lines of nonequilibrium origin or lines of self-absorption, as well as those not related to the processes under study, and to take into account only a priori reliable intervals. In favorable circumstances, an increase in the number of channels increases the accuracy of approximation to the Planck distribution and, accordingly, the accuracy of calculating color temperature.

To ensure the maximum number of spectral channels, we selected a diffraction grating. The diffraction grating decomposes the radiation of the object into a continuous spectrum, which makes it possible to obtain any number of spectral channels.

It is possible to register the illumination created by an object or its part by measuring the illumination directly of the analyzer's input diaphragm. In this case, the device should be close enough to the object. Only in this case the entire input aperture is filled and it is possible to photometer an extended object with virtually no signal loss. In addition, the input diaphragm, like a pinhole camera, builds an image of the object on the input optics, i.e. creates spatially inhomogeneous illumination, which leads to spectral analysis errors and temperature calculation errors. The latter circumstance is especially pronounced on a concave lattice.

The use of a fiber light guide eliminates both of the noted drawbacks of the pinhole camera circuit.

First, the fiber aperture is close to the input aperture of the diffraction grating, and the transmission loss in the spectral region traditional for color pyrometry is small for the fiber, so that the signal will be preserved if the device is significantly removed by tens or hundreds of meters.

Secondly, one of the properties of a fiber is its ability to not only transmit light, but also to mix light rays coming from different directions. As applied to the problem of explosion photometry, this means the ability to obtain spatial average values of the radiation force or illumination, as well as average color temperatures for any configuration of the cloud of expansion of hot particles. Strictly speaking, the heterogeneity is not eliminated, but converted to axisymmetric, which is enough to suppress the error. Using the property of the fiber to integrate spatial inhomogeneities in addition to the typical application as a signal transmission line and, in combination with other features of the device, allowed to achieve the claimed result.

The width of the spectral band is determined by the dispersion of the lattice and the size of the element itself. The more elements of the line in the operating range of the spectrum, the narrower the bandwidth in each spectral channel. Calibration of photodetectors in a narrow band of the spectrum allows you to measure temperature in a wider range. For a device with broadband filters, as, for example, in the prototype, maximum accuracy is achieved at temperatures close to that at which the calibration was done. At other temperatures, the error increases. This is explained by the influence on the measurement result of the shape of the radiation curve, which is determined by the temperature of the body. The narrower the strip of the measurement channel, the smaller the reduction errors, the smaller the influence of the shape, the wider the measurement range.

Favorable circumstances can also be considered as the presence of sufficient time for recording the signal of many spectral channels. This is important, for example, for serial-type ADCs. When photographing short pulses, it is necessary to reduce the number of recording channels. The program allows you to determine the color temperature at least three wavelengths. Approaches to solving such problems are known.

Neighboring elements of a multi-element line can be combined, for example, in a software way in order to increase sensitivity or increase the speed of registration. In this case, band-pass filters with steep symmetrical fronts are formed (see Figure 3), the slope of which is determined by the hardware function of one element, and the bandwidth is determined by the number of combined elements and the magnitude of the dispersion of the lattice. Thus, it is possible to organize optical-electronic bandpass filters for the desired wavelength intervals with identical characteristics in width and shape of the transmission curve, in contrast to a set of glass or interference filters, the identity of which is impossible for technological reasons.

The identity of the bands simplifies and speeds up the procedure for calculating the desired values, that is, the process of processing the results of the experiment, and also increases the accuracy of the measurements.

The combination of elements is also useful for the hardware integration of strongly nonuniform spectra, which, as a rule, is also observed in explosion spectra.

In the inventive device, the analyzed radiation of a potentially hazardous object is transmitted through an optical fiber to the analytical part of the device, which may be in a shelter. The use of a fiber allows not only to solve the photometric problems of an inhomogeneous object, but also to ensure the safety of equipment.

The implementation of the dispersing element in the form of a concave diffraction grating instead of a flat one reduces the number of optical elements susceptible to vibrations, thermal expansion and other destabilizing factors, which is especially important for working in the field, and also reduces the weight and dimensions of the device.

An example of a specific implementation.

We have made and tested a device based on the claimed device.

The device showed good metrological and operational properties.

A quartz fiber with a core diameter of 0.8 mm and a length of 20 m was used as a fiber. The concave diffraction grating 40 × 40 mm in size has a radius of 100 mm and curved strokes with a density of 400 lines / mm. The operating wavelength range is 400-800 nm. The Hamamatsu photodetector line is silicon integrated, contains 16 photodiodes 0.9 × 1.45 mm in size. It is possible to use a ruler with the number of elements 35 of the same company. The amplifier block is made on AD8034 chips from Analog Devices. To convert analog signals, a serial USB-3000 ADC with a channel polling rate of 3 MHz was used. Communication with a Note-Book computer is via a USB channel. Arbitrary selection of channels is carried out programmatically. The optical and electronic circuits together with the ADC are placed in a protective case measuring 100 × 120 × 250 mm.

The performance of the device is practically limited by the speed of the photodiode array and the speed of digital conversion. In this implementation, a temporal resolution of the color temperature meter of about 1 μs was achieved. Using a commercially available PIN-photodiode array and modern broadband amplifiers, you can get a speed of 0.1 μs. Such speeds can be realized using parallel-type ADCs or multi-channel digital oscilloscopes installed directly next to the analyzer.

Figure 4 shows the test results of the device.

As a test object, we chose a pulsed electric discharge in a dielectric capillary.

The instrument was calibrated using a reference lamp with a temperature of about 2800 K. The reproducibility of temperature measurements of the reference lamp itself was 0.3%. The color temperature of the discharge plasma was 28,000 K, which is 10 times higher than during calibration. The measured temperature in the discharge turned out to be 10% higher than the value obtained by the certified brightness temperature meter, which is comparable with the error of the brightness pyrometer itself. This indicates a wide dynamic range of measurements of the claimed device.

The absolute brightness of the object was 4 orders of magnitude higher than when calibrating the device, so a neutral attenuator was installed in front of the entrance. With calibrated neutral attenuators, such as mesh, it is possible to measure the absolute spectral brightness and illumination with the same accuracy.

The software, specially developed for this device based on well-known algorithms, allows you to calculate the absolute or relative illumination or brightness in the selected channels, as well as calculate the color temperature by fitting to the nearest Planck curve to minimize the standard deviation, as well as spectral brightness or illumination at arbitrary wavelengths by calculating in the black body approximation.

Data processing and output of the results of calculating the time course of the temperature is performed within a few seconds, which allows you to get the results directly at the test site and quickly use them to prepare the next experiment.

Claims (2)

1. A multichannel device for measuring pyrometric characteristics, mainly fast processes in heterogeneous media, such as an explosion, including an element that decomposes the radiation from the measured object into spectral components, a photodetector assembly, an information recording and processing unit in the form of ADCs and computers, different the fact that in the optical path in front of the element that decomposes the radiation, an additional optical fiber is installed, the output end of which serves as the input diaphragm of a radiation decomposing element, the radiation decomposing element itself is made in the form of a concave diffraction grating, in the output plane of which there is a photodetector in the form of a multi-element array equipped with amplifiers, each element of which receives radiation in a narrow band of the spectrum, while the number of elements of the array is large, than the number of ADC channels, and the computer is equipped with a function for calculating absolute illumination, absolute brightness, and color temperature. 2. The device according to claim 1, characterized in that the adjacent elements of the multi-element photodetector line are combined into groups.

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