FI69372B - MEASUREMENT METHOD FOR APPARATUS FOR MAINTENANCE WITH FASTA CORNECT AEMNENS MASSFLOEDE OCH FUKTIGHET ELLER NAOGON ANNAN EGENSKAP - Google Patents

Description

69372

MEASUREMENT METHOD AND APPARATUS FOR DETERMINING THE MASS FLOW AND HUMIDITY OF A SOLID, GRANULAR MATERIAL OR ANY OTHER PROPERTIES

The present invention relates to a method and apparatus for measuring at radio frequencies, by means of which the mass flow and moisture or some other quantity of a solid, granular substance can be determined simultaneously. The material can be, for example, wood chips, grain, coal, ash or some other poorly electrically conductive substance.

At present, there are no devices that can absorb the mass flow and moisture of a solid, granular material flowing in a tube, passing through a conveyor belt, or falling freely. Flow sensors are generally based on differential pressure or vortex formation or contain moving propeller-like parts, making them suitable only for liquids or gases flowing in pipes. The flow of conductive fluids can be measured magnetically. Magnetic methods are only suitable for solid, granular materials when a liquid is used as the conveyor. This is rarely possible. The flow of solids, granules 15 can be measured with an oblique plate. It is a plate mounted obliquely in the flow tube against which the substance falls and thus causes a force to be measured. The bevel plate only measures the mass flow and assumes that the falling velocity is constant.

The measurement method and sensor of the present invention seeks to eliminate the weaknesses and error sources of the above measurement methods. The method presented here is characterized in that the measuring sensor operates at radio frequencies. The measurement method is particularly suitable for measuring the mass flow of solids, granules. The measurement does not change the flow of the substance. With the sensor, it is possible to measure two things at the same time, for example, the average instantaneous density and the humidity of the substance. If the substance flows at a constant velocity, the average density 27 is directly proportional to the mass flow of the substance.

2 69372 28 In cases where the mass flow rate varies, the rate can be measured in a known manner, e.g. with a Doppler radar. Another method is to use two measuring sensors in succession and to determine the delay with which the measurement results correlate, whereby the flow rate can be determined on the basis of the delay and the distance between the sensors. There are usually small variations in the mass flow so that correlation can be performed.

The measuring method according to the invention is further characterized in that the measurement takes place as described in claims 1, 2 and 3.

For each application, a calibration curve must be generated to determine the complex dielectric constant and the density of the substance to be measured and another measurand, for example moisture. The measuring sensor according to the invention is characterized in that it has a structure as set out in claims 4, 5, 6 and 7.

In the following detailed description, reference is made to the following figures.

Figure 1 Schematic diagram of a measurement system consisting of a resonator and a Doppler radar.

45 Figure 2 Schematic diagram of a measurement system consisting of two resonators and a correlator.

Figure 3 Side and end coaxial resonator consisting of a strip and a tube.

Figure 4 Side and end of a coaxial-50 li resonator consisting of a diagonally mounted strip and a tube.

Figure 5 Side and end of a coaxial resonator consisting of a longitudinal strip and a tube.

Figure 6 Side (resonator) onetel resonator made of metal tube extension and non-conductive tube.

55 Figure 7 Dielectric constant of wood chips as a function of density and moisture.

Figure 8 Measurement of the mass flow on the conveyor belt with the sensor of Figure 6.

The measuring system according to the invention can be used, for example, in the particle board industry, where the raw material, wood chips, is generally transported in metal pipes between different manufacturing steps. In this case, it is often necessary to know how many kilograms of raw material flow at any point in a minute. It is also important to know what the moisture content of the chips is. It is important to perform both measurements, for example, at the point where the chips 65 flow into the drying oven and at the point where they flow into the gluing machine.

As a solution to the measurement problem, it is possible to use the measurement system according to Figure 1 or 2. In practice, the rate of fall of the chips is almost constant, as they fall through the sensor from a low constant height, so a measurement of the flow rate 70 is not necessary.

The measuring sensor may be according to Figures 3, 4, 5 or 6 or a similar resonator structure. The resonators of Figures 3, 4 and 5 are coaxial resonators. The metal tube 1 forms the outer conductor and the metal strip or tube 5 the inner conductor. The high-frequency power for measuring the resonant frequency 75 and the goodness number is connected to the resonator by two switching loops 6, to which the supply cables are connected. The supports 13 made of insulating material (Fig. 6) hold the strip in place. The resonator of Figure 3 resonates at a frequency at which the strip is about a quarter wavelength long. The boundary wavelength of the tube 1 is twice the widest 80 sides of the tube for a rectangular tube and 1.7 times the diameter for a round tube, so if the strip extends longer than the central axis of the tube, the radio wave cannot propagate in the tube at resonant frequency. When the ends of the tube extend far enough on both sides of the resonator, all power remains in the resonator and has a very high goodness-85 figure. Then it is also possible to measure the losses of relatively low-loss substances, i.e. the moisture of relatively dry wood chips.

Figure 4 of the resonator has a resonance frequency close to the frequency at which the strip 5 is a half-wave mittatnen. In order for the cut-off wavelength of the tube 1 to be considerably higher than the resonant frequency, the strip must be placed in a sufficiently oblique tube to increase the length of the strip and thus reduce the resonant frequency.

The resonant frequency of the resonator of Figure 5 is close to the frequency at which the average length of the conductor 5 is half a wave. The central conductor must therefore be considerably longer than the diameter of the tube in order for the cut-off frequency to be sufficiently high.

4 69372 96 The resonator of Figure 6 is a hollow resonator. It consists of an extension 7 of the metal tube 1 with its ends 8. A tube 9 made of insulation (e.g. plastic) with the same diameter as the tube 1 passes through the resonator, so that no agglomerations 100 or vortices are formed in the material flow to be measured. The dimensions of the resonator must be chosen so that the resonant frequency is lower than the lower limit frequency of the tube 1. This condition is easy to meet. Then the power cannot escape into the tube 1 and the resonator has a high quality factor.

The main advantage of the resonators of Figures 3, 4 and 5 is easy installation, 105 if the mass flow is in a metal tube, in which case the flow tube can be used as an external conductor. The advantage of the resonator of Figure 6 is the unobstructed flow of the substance to be measured.

As the substance to be measured flows through the resonator, it also passes through the electric field of the resonator. Since the relative dielectric constant 110 er of the substance differs from the relative dielectric constant of air, the substance affects the wavelength and thus the resonant frequency. The imaginary part of the electroelectric constant, which is proportional to the losses of matter, affects the goodness of the resonator. Thus, by measuring the resonant frequency and the quality factor of the resonator in a manner known from radio technology, an effective 115 complex dielectric constant 115 is obtained. From this it is possible to deduce the average density of the substance to be measured in the tube and another quantity using the calibration curve. In the case of wood chips, an experimental plot for density and moisture is shown in Figure 7.

The largest source of error for resonators is the inhomogeneous field distribution on the cross-sectional area of 120 tubes 1. In the case of a coaxial resonator (Figures 3, 4 and 5), the electric field is strongest on the surface of the strip 5. If the chip current varies unevenly across the cross-section of the pipe, it will cause a certain error if it cannot be taken into account. If necessary, an insulating tube can be placed around the central conductor, which prevents the substance from penetrating right onto the surface of the strip and thus obtaining a mass flow into a more uniform electric field. It is possible to make the field of the resonator of Fig. 6 almost constant by choosing the waveform and the dimensions of the resonator correctly. A suitable form of resonance is, for example, TEm with a circular waveguide resonator or TEm with a rectangular waveguide resonator, whereby the electric-130 field is close to its maximum value in the whole measuring range.

5 69372 131 The mass flow to be measured can also be in the free state, in which case the measuring sensor according to Figures 4, 5 or 6 is positioned so that the mass falls through the sensor. The mass flow can also be on a conveyor belt, whereby the conveyor belt 14 with its mass falls through the sensor, Fig. 8. For example, the sensor according to Fig. 6 135 is suitable for the purpose. The conveyor belt must be an insulating material and its proportion must be calibrated away from the measured values of the sensor.

Figure 2 shows a measurement method in which the determination of the flow rate is performed using correlation. The measuring method comprises two measuring sensors 2, 140 in the flow tube reproducing at a distance d, the interpretation electronics 10, the adjustable delay 11 and the correlation 12. The adjustable delay 11 and the correlator 12 are implemented in a manner known from electronics, for example digital. Due to small variations in the mass flow, the measurement result of sensor A varies, as does that of sensor B. The correlator 12 retrieves the delay t which gives the best correlation 145 between the measurement results. The flow rate is then d / t.

The field of the resonator of Figure 5 is strongest at the ends of the center conductor 5, so that each particle flowing past generates a two-peak signal, the delay between the peaks of which depends on the flow rate. By performing autocorrelation on the signal, a delay is obtained, and the flow rate can be determined in the same way as in the case of two resonators.

The measuring method and sensors according to the invention can in some cases also be applied to flow measurements of liquids and gases. The liquids to be measured must be poorly electrically conductive. If there are irregularities in the liquid, for example air bubbles, the flow rate is obtained by clear Doppler and correlation measurements.

Claims (7)

1. A method for measuring the mass flow and humidity of the solid, granular or equivalent substance or any other property can be characterized in that the mass flow is fed into a metal tube (1) into which is formed an electromagnetic resonator (2), whose resonant frequency is lower than the boundary frequency of the tube (1), and through whose field the substance flows and thereby influences the resonant frequency and quality factor of the resonator, which is measured and from which the temporal mean density and humidity or any other property of the mass flow can be derived. Measurement method according to claim 1, characterized in that the flow rate of the blank is measured with a Doppler radar (3) positioned before or after the transducer (2) directed obliquely through the flow, whereby the flow rate causes an offset of the echo frequency, which is measured and dropped, together with the average density. from the resonator (2) the mass flow can be derived. Apparatus for realizing a method according to claim 1 or 2, characterized in that one end of a metal tab or tube (5) is attached to the wall of the metal tube (1) perpendicular to the axis si of the tube (1), that its length is about two thirds of The diameter of the tube (1), together with the walls of the tube (1), forms a coaxial resonator (2) whose resonant frequency is lower than the limit frequency of the tube (1). Apparatus for the realization of a method according to claim 1 or 2, characterized in that both ends of a metal tab or tube (5) are fixed obliquely to the walls of the metal tube (1) relative to the axis sl of the tube (1), so that its length is greater. than the diameter of the tube (1) and together with the walls of the tube (1) it forms a coaxial resonator (2) whose resonant frequency is lower than the threshold frequency of the tube (1). Il