Level measurement for storage silos.

BACKGROUND OF INVENTION

Field of the Invention

0001The invention relates to an apparatus for determining the level of a material in a storage silo. More specifically, the invention relates to such an apparatus which includes an AC signal generator whose output frequency is automatically and continuously swept through a predetermined frequency range.

Description of Prior Art

0002Existing devices for automatically measuring the level of material, for example, bulk powdered or granular materials, in storage silos, include capacitance probes, ultrasonic echo detectors, pressure transducers and mechanical range finders. An example of such a device is illustrated in U. S. Patent No. 4,043,199, Greer, issued August 23, 1977, which is a mechanical device and which detects pressure on a collapsible sleeve 14, which is disposed in a storage silo storing granular material, when the granular material surrounds portions of the collapsible sleeve. U. S. Patent No. 4,495,807, Field et al, issued January 29, 1985, teaches an electronic device for sensing liquid levels. Specifically, it utilizes a balanced R.F. bridge.

0003Capacitance probes utilize a conducting rod or cable vertically suspended inside the silo and immersed in the stored material. The electrical capacitance of the probe varies, depending on the level and dielectric properties of the stored material. This variation alters the AC or DC char­acteristics of an electrical circuit connected to the probe. Since the dielectric properties of the stored material depend on factors other than level, including moisture content and density, this type of device does not provide an absolute measure of level, and requires recalibration when used for material with nonconstant dielectric properties.

0004Ultrasonic echo detectors utilize the pulse echo technique to determine the distance from the ultrasonic transducer to the surface of the material whose level is to be measured. This device suffers severe limitations when used with granular materials in a dusty environment, due to high scattering loss at the surface of the material and high attenuation of the ultrasonic signal by dust-laden air.

0005Pressure transducers placed at the bottom of the storage silo produce an electrical signal propor­tional to the pressure in the material at that point. For most granular and powdered materials the level of the material cannot be deduced from this pressure, since pressure is a nonlinear function of depth, due to friction within the material.

0006Mechanical ranging devices consist of many variations of a weight suspended on a cable which is lowered until contact with the material is detected by loss of tension in the cable or other means. This device cannot provide continuous measurement due to its slow sampling rate and is unsuitable for use in environments containing abrasive dust, because of its susceptibility to mechanical breakdown.

0007A further approach for measuring the level of material in a storage silo or tank is illustrated in U. S. Patent No. 3,695,107, Hertz et al, issued October 3, 1972. In this patent, a delay line, consisting of an outer tubular conductor 2 and a central inner conductor 3, is disposed in the storage silo along the complete length thereof. A pulse is transmitted into the delay line from the top end thereof, and the pulse is reflected from the air material border in the silo. The time taken for the pulse to travel to the air/material border and back is measured to thereby determine the distance from the top of the storage silo to the top of the material.

0008Impedance 11 is included in the bottom end of the delay line to prohibit reflections from the bottom of the delay line.

0009U. S. Patent No. 4,135,397, Krake, issued January 23, 1979, teaches a refinement of the system taught in the '107 patent and also includes an auxiliary transmission line 34 which is used to calibrate the measuring system.

0010Although the '107 and '397 patents use terminating impedances, the impedances are matched to the characteristic impedance of their respective lines presumably in open air. However, as is well known, the impedance of the line will be altered when the line is disposed in the material of the silo. Accordingly, the terminating impedances in the '107 and '397 patents will not completely eliminate reflections from the bottom ends of their respective lines.

0011The above described time domain devices are difficult to build because of the need to resolve sub-nanosecond signals for useful level resolution.

0012In addition, none of the patents above dis­cussed utilize a swept frequency generator to generate a signal swept in frequency along a transmission line disposed in a silo.

SUMMARY OF INVENTION

0013In accordance with the invention, an apparatus for determining the level of material in a storage bin or silo includes a cable means disposed in the silo along the full length thereof. A swept frequency generator applies a swept frequency signal to the top end of the cable means. Reflections of the transmitted signal from the air-material boundary will produce peak voltages at frequency intervals. The frequency intervals are a function of the level of the material. The apparatus includes means for measuring the frequency intervals and further means for deter­mining the material level using the detected frequency intervals.

0014The cable means include terminating impedances which are variable in magnitude, and means for varying the magnitude of the terminating impedances during the operation of the apparatus whereby to eliminate reflections from the bottom end of the cable means.

BRIEF DESCRIPTION OF DRAWINGS

0015The invention will be better understood by an examination of the following description, together with the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Figure 1 illustrates one embodiment of the invention wherein the cable means comprises a single pair of parallel uniform conductors;</li><li>Figure 2 is a section through II-II of Figure 1;</li><li>Figure 3 illustrates an alternate embodiment of the invention including two pairs of uniform parallel conductors; and</li><li>Figure 4 is an alternate arrangement of the embodiment illustrated in FIgure 3.</li></ul>

DESCRIPTION OF PREFERRED EMBODIMENTS

0016Referring to Figure 1, a storage bin or silo 1, having a height H, stores a material 3 whose top surface is at a level ℓ above the bottom of the silo 1 and distance D below the top of the silo. The material 3 can comprise a liquid or a granular or particulate material.

0017A cable means, illustrated generally at 5, and having a length equal to the height H of the silo, extends from the top 7 to the bottom 8 of the silo.

0018Disposed at the top of the silo is electronic circuit means illustrated generally at 9. The electronic circuit menas includes a swept frequency generator G, a frequency interval detector Dt, a microprocessor MP and a variable DC generator D.C.

0019The cable means 5 includes a pair of parallel uniform conductors 11 and 13 which are maintained in the same plane by dielectric cover 15 which covers both conductors and forms a rigid center piece between the conductors. The conductors are terminated by a variable impedance means 17, for example, a PIN diode.

0020The conductors 11 and 13 form an electrical transmission line which approximates closely the theo­retical "distortionless" transmission line. The characteristic impedance of such a line is determined by the intrinsic impedance of the medium surrounding the conductors, and by the geometric shape of the cable cross-section. When the cable is suspended in the air, its characteristic impedance is relatively constant over a wide range of ambient conditions and can be considered to be truly resistive since the transmission line id "distortionless".

0021When the transmission line is immersed in a medium whose intrinsic impedance is different from that of air, the characteristic impedance of the transmission line is accordingly different from the characteristic impedance of the transmission line in air. When the medium has a relatively low dielectric loss angle, the impedance of the transmission line is essentially resistive and the transmission line approximates the "distortionless" condition. In this condition, th eline can be effectively matched by a terminating resistive impedance.

0022In operation, the frequency of the generator G is automatically and continuously swept through a predetermined frequency range. The generator impresses an AC electrical signal, preferable an electrical sinusoidal signal, onto the transmission line. This signal is propogated along that portion of the transmission line suspended in air above the material 3 stored in the silo 1. At the air-material boundary 4, the impedance of the line changes abruptly causing the electrical signal to be mismatched to the impedance of the transmission line within the material. This results in a portion of the signal being reflected back towards the sending end of the transmission line from the surface of the material, and the remainder of the surface of the material, and the remainder of the signal being transmitted into the portion of the transmission line immersed in the material. This transmitted signal is propagated with negligible distortion in materials having a low dielectric loss angle.

0023The signal reflected back from the surface of the material combines with the forward wave to form a standing wave pattern along the      portion of the transmission line suspended in air. This signal is absorbed by the matched impedance of the electronic circuit, for example, a matched impedance in the generator G, and no further reflection occurs.

0024When the variable impedance device 17 is not adjusted to be a match with the submerged part of the transmission line, a portion of the signal propagated along the submersed part of the transmission line is reflected back towards the sending end of the mismatch which occurs at the junction of the transmission line and the unmatched impedance of the variable impedance matching device 17. The remainder of the signal is absorbed by the device. A portion of the reflected signal is transmitted through the material-air boundary and a portion is rereflected back towards the variable impedance device from the material-air boundary. The transmitted portion modifies the standing wave pattern along the portion of the transmission line in air and is absorbed by the matched impedance of the electronic circuit. The rereflected portion sets up a multiple echo which further modifies the standing wave pattern on the portion of the line in air. This multiple echo is eventually totally absorbed and attentuated and a stead state condition prevails.

0025On the portion of the transmission line sus­pended in air, the standing wave pattern which is com­prised only of the signal impressed on the trans­mission line by the generator G and the first echo from the air-material boundary has a voltage dis­tribution, V, along the transmission line described by the following mathematical expression: V = Ae^xd Cos (ωt + βx) + ρAe^xd Cos (ωt - βx) For a line approximating the "distortionless" condition, α, the attenuation factor may be considered zero. The expression now becomes: V = A Cos(ωt + βx) + ρACos (ωt -βx), where ACosωt is the voltage expression for the signal impressed onto the transmission line by the electronic circuit: x is the positive distance along the part of the transmission line in air, with x = o at the air material boundary: β = ^ω/ν (ν = velocity of propogation for the transmission line in air),ω and t denote angular frequency and time respectively: ρ = the voltage reflection coefficient in the transmissioin line at the air - material boundary. When the dielectric loss angle for the material is small,ρ is essentially a real number, independent of frequency.

0026From the above, it is evident that, for the standing wave pattern comprised of the signal impressed on the transmission line by a fixed frequency generator and the first echo from the air -­material boundary, the voltage varies along the portion of the transmission line suspended in air, with successive maxima (or minima) spaced by the diostance equal to half the4 wavelength of the impressed signal. This wavelength depends only on the velocity of propagation in the transmission line suspended in air and is essentially constant for a given frequency over a wide range of ambient conditions.

0027In accordance with the invention, the frequency of the sinusoidal signal impressed onto the transmission line by the generator G is automatically and continuously swept through a predetermined range of values. This causes the voltage at any fixed point on the transmission line to vary. Considering only the standing wave pattern comprised of the signal impressed onto the line and the first echo from the air - material boundary, this variation is in such a fashion that successive peak or null values of voltage occur at frequencies separated by a fixed frequency interval Δf=ν/2x, where ν and x are as previously defined. When the fixed point at which the voltage is monitored is the point at which the electronic circuit is connected to the transmission line, (the top end of the transmission line) the quantity Δf is equal to ^ν/2d, where D is the distance along the transmission line from the point of interconnection of the electronic circuit to the air - material boundary as shown in Figure 1. The detector Dt automatically determines several successive values for the variable quantity Δf as the frequency is swept over the entire measuring range.

0028The voltage standing wave pattern on the portion of the cable suspended in air above the stored material is modified by the multiple signal echoes caused by the impedance mismatch at the junction of the transmission line and the variable impedance device 17. This modification causes a modulation of successive values of the quantity Δf as the frequency is swept over the entire measuring range. The micro­processor MP automatically detects and quantizes this modulation. The microprocessor then controls the DC current generator D.C. to provide as biasing current to the variable impedance device 17 which cuases the resistance of the PIN diode to change. The AC resistance of the PIN is inversely proportional to the DC current flowing through it. The value of the DC current is controlled such that, by successive approximations, the resistance of the PIN diode is made to equal the impedance of the transmission line submerged in the stored material.

0029This causes the AC signal energy transmitted into the submersed portion of the transmissioin line to be effectively completely absorbed by the PIN diode. This eliminates the multiple echo interference on the standing wave pattern on the portion of the cable suspended in air and the modulation of successive values of the quantity Δ f is removed.

0030The microprocessor Mp automatically detects the lack of modulation of the quantity f. The circuit measures and stores this quantity and, from the relationship D = v/2 Δf, the variable quantity D, the distance from the point of intersection of the top of the cable or silo to the top surface of the storage material, is automatically calculated in the micro­processor which receives the quantity Δf from the detector Dt.

0031For any given silo, the height H is constant. This value is electrically encoded and permanently stored in the microprocessor. When a calculation of the quantity D has been completed, the value of D is automatically subtracted from H and the result is encoded into an electrical signal which is available at output terminals of the microprocessor. This signal is directly analogous to the level ℓ of the stored material in the silo and may be used locally, or electrically transmitted to a remote point to actuate an automatic visual display or recording decide, (not shown) or to feed a data processing machine.

0032It is evident that the measurement derived by the foregoing method for the level of bulk granular or powdered materials stored in a silo is independent of the actual value of the dielectric properties of the stored material provided that the dielectric loss angle of the material is relatively small. From the published data this is found to be true of many types of granular and powdered material. It is especially true for agricultural grain and seed materials, for the measurement of which the apparatus is particularly suited. The measurement of level in materials with a low dielectric loss angle is a function only of the velocity of propagation in the cable when the surrounding medium is the air in the empty section of the silo above the stored material. The velocity variations caused by changes in temperature, humidity, and dust content of the air are insignificant. The conductors are jacketed in a low friction plastic to prevent coating.

0033A second embodiment of the invention is illustrated in Figure 3. As can be seen in Figure 3, the cable means comprises two pairs of parallel uniform conductors or transmission lines 19 and 21. The transmission line 19 is terminated with a short circuit 23, while the transmission line 21 is terminated with an open circuit 25.

0034The electronics unit 9 in this situation consists only of the swept generator G, the frequency interval detector Dt and a combiner splitter C/S. The output of the generator is fed to the combiner splitter wherein the signal is split so that half of the signal is fed to 19 and the other half is fed to 21. The combiner splitter is also fed to the detector so that signals at the top end of the transmission lines 19 and 21 are combined and thence are combined in the combiner splitter then fed to the detector Dt. The combiner splitter preferably has a high side-to-side isolation.

0035As the generator signal impressed on both 19 and 21 are equal in amplitude and phase, echoes returned from the air-material surface, are in phase and will therefore add in the combiner. Echoes returned from the lower end of the pairs 19 and 21 are in anti-phase and therefore cancel in the combiner. It is pointed out that the signals are in anti-phase since the reflection coefficient for a short circuited transmission line is equal in magnitude but in anti-­phase with the reflection coefficient for an open circuited transmission line when both cables are in the same intrinsic medium. Thus, the natural phase cancellation of the arrangement eliminates the interfering effects of multiple echoes and enables the quantity Δf and, consequently the level of the material, to be determined as already described with respect to the Figure 1 embodiment.

0036A variant of the Figure 3 embodiment is illustrated in Figure 4. In Figure 4, once again, there are two transmission lines formed from three uniform parallel conductors 27, 29 and 31.

0037One of the transmission lines is formed from the conductors 27 and 29 while the other of the trans­mission lines is formed from the conductors 29 and 31. Thus, one of the conductors is common to both of the transmission lines.

0038The transmission line formed of the conductors 27 and 29 is terminated in a short circuit 33, while the transmission line formed in the conductors 29 and 31 is terminated in an open circuit 35. In all respects, the Figure 4 embodiment operates in the same manner as the Figure 3 embodiment.

0039Although several embodiments have been des­cribed, this was for the purpose of illustrating, but not limiting, the invention. Various modification, which will come readily to the mind of one skilled in the art, are within the scope of the invention as defined in the appended claims.