Calibration-free continuous bin level sensor

Abstract

A sensor assembly is described herein that can automatically calibrate itself upon installation into an empty bin, eliminating the need to actually fill the bin to calibrate the level reading. The sensor will provide consistent measurement regardless of material properties (permittivity, density, temperature or moisture content). The capacitive nature of the sensor means that in some circumstances, it will sense the material through plastic/glass/fiber glass thereby allowing the sensor assembly to be mountable on the outside of a bin or container. The electrodes of the sensor system are designed to provide a continuous level reading.

Claims

1. A sensor assembly for measuring the level of a material or fluid in a container or vessel comprising: a sensing capacitive element configured from two parallel sensing electrodes positioned adjacent the container or vessel such that changes in a material level cause a proportionate change in a first capacitance of the sensing electrodes, the sensing capacitive element including a first ground electrode and a driven electrode coupled to a first oscillator, wherein said sensing first ground and driven electrodes have a length L.sub.L and a nominal capacitance per unit length C.sub.0; a reference capacitive element configured from two parallel reference electrodes positioned adjacent to a bottom of the container or vessel, the reference capacitive element including a second ground electrode and a second driven electrode coupled to a second oscillator, said reference second ground and driven electrodes having a length L.sub.R and a capacitance per unit length C.sub.0, wherein said reference electrodes are in contact with the material or fluid within the container; and an electronics module configured to measure a material or fluid level of the container, said electronics module having at least first and second oscillators, each of said oscillators coupled to a respective amplifier, said electronics module including a processor to process frequency signals received from each of the sensing and reference capacitive elements, each signal of which is a function of electrode capacitance, to be processed by said processor to determine a capacitance for each of said sensing and reference electrodes in each of an empty container and the container having the material or fluid therein, the processor multiplying a ratio of a sensing capacitance to a reference capacitance to a ratio of a length of the reference electrode to the sensing electrode to thereby calculate a level measurement of the material or fluid in the container.

2. The sensor assembly of claim 1, wherein the oscillators that drive the sensor electrode and reference electrode are configured to operate at a plurality of frequencies as determined by said processor.

3. The sensor assembly of claim 1, wherein at least one of the sensing electrodes and at least one of the reference electrodes is held at constant electrical potential and wherein the other sensing electrode and reference electrode are driven at a frequency and potential defined by the electronics module.

4. The sensor assembly of claim 3, wherein said sensing electrodes mounted external to the container or vessel include a metallic shield driven at the same frequency and potential as the driven electrode.

5. The sensor assembly of claim 1, wherein the sensing electrodes, reference electrodes or both sets of electrodes are configured to be positioned externally on a non-metallic container or vessel.

6. The sensor assembly of claim 1, wherein said two parallel sensing electrodes are vertically positioned either internal to or external to the container or vessel.

7. The sensor assembly of claim 1, wherein said two parallel reference electrodes are positioned submersed at or near the bottom of container or vessel.

8. A sensor assembly for measuring the level of a material or fluid in a container or vessel comprising: a sensing capacitive element configured from two parallel sensing electrodes positioned adjacent the container or vessel such that changes in a material level cause a proportionate change in a first capacitance of the sensing electrodes, wherein said sensing electrodes have a length L.sub.L and a nominal capacitance per unit length C.sub.0; a reference capacitive element configured from two parallel reference electrodes positioned adjacent to a bottom of the container or vessel, said reference electrodes having a length L.sub.R and a capacitance per unit length C.sub.0, wherein said reference electrodes are in contact with the material or fluid within the container; and an electronics module configured to measure a material or fluid level of the container, said electronics module having a plurality of oscillators, a plurality of amplifiers and a processor to process signals received from the sensing and reference capacitive elements, wherein said oscillators are configured to be electrically coupled to said sensing and reference electrodes so as to generate a frequency signal to be processed by said processor thereby determining a capacitance of said sensing and reference electrodes to thereby calculate a level measurement of the material or fluid in the container, wherein each of the sensing capacitive and reference capacitive elements using multi-frequency measurements are used to determine an empty container capacitance; and wherein a total capacitance is determined of the sensing capacitive element and a total reference capacitance of the reference capacitive elements when the reference capacitive element is immersed in the fluid or material; and wherein the level of the fluid or material within the container is generated from a product of: a ratio of L.sub.R (reference electrode length) and L.sub.L (sensing electrode length) and a ratio of: a. a difference of the total capacitance and the empty capacitance of the sensing element as a numerator; b. a difference of the total reference capacitance and the empty capacitance of the reference element as a denominator.

9. A sensor assembly for measuring the level of a material or fluid in a container or vessel comprising: a sensing capacitive element configured from two parallel sensing electrodes positioned adjacent the container or vessel such that changes in a material level cause a proportionate change in a first capacitance of the sensing electrodes, wherein said sensing electrodes have a length L.sub.L and a nominal capacitance per unit length C.sub.0; a reference capacitive element configured from two parallel reference electrodes positioned adjacent to a bottom of the container or vessel, said reference electrodes having a length L.sub.R and a capacitance per unit length C.sub.0, wherein said reference electrodes are in contact with the material or fluid within the container; and an electronics module configured to measure a material or fluid level of the container, said electronics module having a plurality of oscillators, a plurality of amplifiers and a processor to process signals received from the sensing and reference capacitive elements, wherein said oscillators are configured to be electrically coupled to said sensing and reference electrodes so as to generate a frequency signal to be processed by said processor thereby determining a capacitance of said sensing and reference electrodes to thereby calculate a level measurement of the material or fluid in the container, wherein the sensor assembly also determines an empty container capacitance wherein a level measurement (H.sub.1/L.sub.R) is determined for a non-empty fill level wherein the sensing and reference capacitance at frequency (f1) are determined; and wherein a second level measurement (H.sub.2/L.sub.R) for some non-empty fill level differing from (H.sub.1/L.sub.R) by at least 10% using the multi-frequency method which includes determination of sensing and reference capacitance at frequency (f2) are determined; and wherein the empty container reference electrode capacitance is generated from a ratio of: a. a difference of the measured capacitance of the sensing element at the first frequency and the measured capacitance of the sensing element at the second frequency as a numerator added to the product of H.sub.2/L.sub.R and the measured capacitance of the reference element at the second frequency less the product of H.sub.1/L.sub.R and the measured capacitance of the reference element at the first frequency as a numerator; b. a difference of H.sub.2/L.sub.R and H.sub.1/L.sub.R as a denominator; thereby generating the empty container sensing electrode capacitance from a sum of the total capacitance at the first frequency and the product of H.sub.1/L.sub.R and the difference of the empty container reference electrode capacitance and the reference capacitance at the first frequency.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other important objects and advantages of the present invention will be apparent from the following detailed description of the invention taken in connection with the accompanying drawings in which;

(2) FIG. 1 illustrates an example embodiment of a two sensing element system for bin level measurement according to the teachings herein;

(3) FIG. 2 illustrates in an example embodiment of an active shield for a bin or tank level sensor as taught herein;

(4) FIG. 3 illustrates a chart of an example bin level measurement of a PVC pipe arrangement using soybean and wheat as taught herein;

(5) FIG. 4 illustrates a chart of various example bin level measurements using various example sensor arrangements as taught herein;

(6) FIG. 5 illustrates a schematic a frequency generating circuit and processing means for a multi-frequency sensor as taught herein;

(7) FIG. 6 illustrates a chart of oil type sunflower seeds and plastic 6 mm air gun pellets using a measured capacitance example sensor arrangement as taught herein.

(8) FIG. 7 illustrates a chart of the sunflower seeds and air gun pellets of FIG. 6 and an estimated fill level versus an actual level when a permittivity independent method is used as taught herein; and

(9) FIG. 8 illustrates a photograph of an example bin level measurement apparatus as taught herein.

DETAILED DESCRIPTION OF THE INVENTION

(10) Following are more detailed descriptions of various related concepts related to, and embodiments of, methods and apparatus according to the present disclosure. It should be appreciated that various aspects of the subject matter introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the subject matter is not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.

(11) Referring now to the Figures, in particular to FIGS. 1, 2 and 5, in one example embodiment, a sensor system or assembly 100 for use on a bin or container 10 consists of two sensing elements 112 and 114, an electronics module 120 and an optional shield material 130 (in this example, a metal tape). A first sensing element 112 is a bin level element that is configured to be long enough to span from a top 12 to a bottom 14 of bin 10. A second sensing element 114 is a reference element that is for a reference measurement and should preferably be mounted at bottom 14 of bin 10 so that it will normally be fully exposed to the material being measured. In this example embodiment, sensing elements 112 and 114 each consist of a length of jacketed parallel conductors (such as twin lead radio wire).

(12) In other related embodiments, the sensing elements are mountable internally or, in the case of plastic, glass, or fiberglass bins, the sensing elements or leads or electrodes may be mounted externally. External mounting for outdoor applications suggests that the sensing elements be covered by metal tape, which is driven by electronics module 120, to serve as an active shield.

(13) In this example embodiment, electronics module 120 measures the capacitance of the two sensing elements 112 and 114. The electrodes serving as a relaxation oscillator timing capacitor circuit that operates such that the oscillator frequency is a function of the electrode capacitance. In a related embodiment, a circuit drives the electrodes with a constant frequency and uses the output of an impedance divider or bridge to estimate the capacitance. A schematic diagram of an example embodiment of a frequency generating circuit 500 and processing means for a multi-frequency sensor is shown in FIG. 5. The reference and bin level sense electrodes are each driven by a Schmitt Trigger based astable oscillator. The frequency of oscillation is inversely proportional to the product of the feedback resistance and the electrode capacitance. The frequency of the oscillators can be controlled by changing the feedback resistance in the Schmitt Trigger which can be accomplished with a processor controller. For the oscillators shown in the schematic,

(14) $f\left(\mathrm{kHz}\right)=\frac{9.5592.Math.{10}^8}{R_{\mathrm{feedback}}\left(\mathrm{kOhm}\right).Math.C_{\mathrm{electode}}\left(\mathrm{pF}\right)}$
where R.sub.feedback is the total feedback resistance (the parallel resistance when the frequency select switch is closed). C.sub.electode includes capacitance associated with the material in the bin or container plus any constant parasitic capacitance. The frequency select switches decrease the resistance by about a factor of 10 over the open switch resistance which in turn increases the oscillation frequency by a factor of 10. The values shown lead to oscillation frequency of 400 kHz when the switch is open and 4 MHz when the switch is closed when the electrode capacitance is about 20 pF. The processor monitors the frequency of oscillation and can thereby calculate the electrode capacitance. The processor performs the appropriate algebraic steps to determine the bin level and outputs that result in the form of PWM duty cycle, frequency, analog voltage, or digital value.

(15) In situations where electrodes are mounted on the exterior of the bin, active shielding may be required to prevent unwanted bias in the capacitance measurements by external influences such as rain and nearby material. A partial electric circuit 200 describing the active shielding of sensor system 100 is shown in FIG. 2. In this example electric circuit 200, one electrode from each sensing element 112A (or 114A (not shown)) is held at electrical ground 210 and the other active electrode 112B (or 114B (not shown)) is driven at some frequency by the measurement circuit, which in this example is an oscillator 220 and amplifier combination 230. Included in the electronics is an amplifier for each active electrode that follows the signal driving that particular active electrode. This signal is applied to a shield tape member 240 that is placed over the sensing electrodes in external bin outdoor mounting applications.

(16) Referring again to FIG. 1, in normal operation, bin level sensing element 114 is partially buried (or covered) in some bulk material or partially submerged in a liquid. For FIG. 1, the following parameters in this example embodiment are defined as follows:

(17) C.sub.0 is sensing element capacitance per unit length (11.8 pF/m for 300 twin lead)

(18) L is the length of the sensing element in meters (m)

(19) R is the length of the reference element in meters

(20) H is the height of the material in the bin in meters (unknown)

(21) The bin level can be expressed as a percentage of the total electrode length L:

(22) $\mathrm{bin}\mathrm{level}=\frac{\left(C_{\mathrm{tot}}-C_{{\mathrm{tot}}_{\mathrm{empty}}}\right)}{\left(C_{\mathrm{ref}}-C_{{\mathrm{ref}}_{\mathrm{empty}}}\right)}.Math.\frac{R}{L}.Math.100\%$
C.sub.tot is the total capacitance of the bin level sensing element (measured)

(23) C.sub.ref is the total capacitance of the reference sensing element (measured)

(24) C.sub.tot.sub.empty is the capacitance of the bin level sensing element when the bin is completely empty (measured during installation)

(25) C.sub.ref.sub.empty is the capacitance of the reference sensing element when the bin is completely empty (measured during installation)

(26) Significant in this expression for calculating or measuring the bin level is that all of the physical parameters are eliminated: 1) the bin sensing element length L can be arbitrary from installation to installation; 2) the reference sensing element length R can be arbitrary; 3) the nominal capacitance of the sensing elements C.sub.0 can be unknown; and 4) the material permittivity ∈′ can be unknown. The fact that the material permittivity can be unknown further implies that: a) the material density can be unknown; b) the material temperature can be unknown; c) the material moisture level can be unknown; and d) the measurement frequency can be unknown.

(27) In various embodiments, some of the sensor systems described herein are configured to determine the bin level as long as an empty bin measurement of the capacitance is made of the two sensing elements during sensor installation. The empty measurements can be made without any material in the bin, but this only needs be done once. In normal operation, the reference sensing element is assumed to be fully exposed to the material in the bin, meaning it is best mounted at the bottom of the bin. The electrical measurements used to determine bin level can easily be biased by parasitic contributions to capacitance. Hence, additions to the overall capacitance measurement due to wiring, circuitry, installation, etc., are not influenced by the permittivity of the material in the bin. The various measurement methods developed for bin level minimize the parasitic contributions through the differences of the measured capacitance in the numerator and denominator, effectively canceling out parasitic contributions to the measurement.

(28) In a related embodiment, the need for calibration measurements of any type (including the empty bin measurement during installation) can be eliminated entirely, making this a calibration free sensor. This is accomplished by making capacitance measurements of the level sensing and reference electrodes at two frequencies, nominally about 100 kHz and about 1 MHz, resulting in the following expression for the bin level:

(29) $\mathrm{bin}\mathrm{level}=\frac{\left(C_{{\mathrm{tot}}_{f1}}-C_{{\mathrm{tot}}_{f2}}\right)}{\left(C_{{\mathrm{ref}}_{f1}}-C_{{\mathrm{ref}}_{f2}}\right)}.Math.\frac{R}{L}.Math.100\%$

(30) C.sub.tot.sub.f1 is the measured capacitance of the bin level sensing element taken at frequency f1

(31) C.sub.tot.sub.f2 is the measured capacitance of the bin level sensing element taken at frequency f2

(32) C.sub.ref.sub.f1 is the measured capacitance of the reference level sensing element taken at frequency f1

(33) C.sub.ref.sub.f2 is the measured capacitance of the reference level sensing element taken at frequency f2

(34) All four measurements are taken at the time of the level measurement meaning there is no separate set of calibration measurements. Furthermore, all of the properties of the single frequency bin level method (and mathematical expression) remain intact: 1) the level sensor requires no calibration; 2) the level sensor is material independent, requiring no knowledge of material permittivity; 3) the level sensor is frequency independent, requiring no knowledge of the measurement frequencies which in turn means that the frequencies can drift over time without affecting the level measurement; 4) no knowledge of the electrode capacitance C.sub.0 is required, the only requirement being that the capacitance per unit length be constant; and 5) the electrode lengths R and L can be arbitrary in size but their values must be known.

(35) Substantially accurate application of the two frequency bin level method as taught herein depends on the permittivity of the sensed material varying with frequency. This generally is not a problem for hygroscopic materials such as grain but for some materials, like glass, this will present a challenge. For constant permittivity materials, it is preferable to utilize one of the single frequency methods as taught herein for bin level with its requirement of a single empty bin calibration measurement.

(36) The need for calibration can also be eliminated for the single frequency constant permittivity scenario if the multi-frequency approach can first be applied to material that has frequency dependent permittivity. In this case, the empty container calibration constants C.sub.tot.sub.empty and C.sub.ref.sub.empty can be determined algebraically from bin level measurements computed using the multi-frequency method. These levels, H1/L and H2/L can then be combined with the capacitance measurements at the two levels to determine the empty container capacitance values as follows:

(37) $C_{{\mathrm{ref}}_{\mathrm{empty}}}=\frac{\left(C_{\mathrm{tot}@H1}-c_{\mathrm{tot}@H2}\right)-\frac{H_1}{R}.Math.C_{\mathrm{ref}@H1}+\frac{H_2}{R}.Math.C_{\mathrm{ref}@H2}}{\left(\frac{H_2-H_1}{R}\right)}$$C_{{\mathrm{tot}}_{\mathrm{empty}}}=C_{\mathrm{tot}@H1}+\frac{H_1}{R}\left(C_{{\mathrm{ref}}_{\mathrm{empty}}}-C_{\mathrm{ref}@H1}\right)$

(38) In both expressions, the ‘@H1’ and ‘@H2’ refer to previously defined capacitance measurements C.sub.tot and C.sub.ref made at fill levels H1 and H2 respectively. The levels will preferably differ from one another by about 10% to about 25%. The frequencies of measurement for each fill level are not required to be the same.

(39) The benefits of using two frequencies to determine the single frequency calibration constants are that it is not necessary to perform a physical calibration measurement and the resulting calibration constants can be updated continuously, thus accounting for shifts in parasitic capacitance. When constant permittivity material is encountered, the sensor assembly can automatically shift to a single frequency calculation using the most recent calibration values. During single frequency operation, the calibration values cannot be updated.

(40) Referring now to FIG. 3 there is illustrated a graph 300 of data gathered of an example bin level measurement of a PVC pipe arrangement using soybean and wheat as taught herein. The sensor system was also tested on salt. The grain level test consisted of taping the twin lead to the outside of a 34 inch long and one inch diameter PVC (plastic) pipe. The pipe was filled with either soybean or wheat grain and the effect of the capacitance change on an oscillator frequency was recorded, with the oscillator frequencies ranging from about 97 KHz to about 100.5 KHz. The plot shows the performance of sensor assembly 100 on both wheat and soybeans. The horizontal axis is approximately 10ths of an inch of grain height. In this example embodiment, the grain level was sensed through the plastic pipe; hence a sensor attached to the outside (or inside) of a plastic grain tank is operative. As shown, the different grains have different slopes on the curves. Because this is a capacitance sensor, different materials will read differently, but the bin level calculation will still be “measured material” independent. Further, the moisture level of a fill material may normally affect the reading, however the permittivity independence of the sensor assembly taught herein means this should not affect the bin level measurement.

(41) Referring now to FIG. 4 there is illustrated a graph 400 of various example bin level measurements using various example sensor arrangements as taught herein. In these example embodiments, a Clapp oscillator with a 300 ohm twin lead was used. Frequency (vertical axis) was plotted against the container height (in inches—horizontal axis) for: 1) an aluminum cylinder; 2) inside a plastic bucket: 3) 2 revolutions around the outside of the container; 4) and electrodes located on both walls of a container. In one embodiment, the twin lead was wrapped around a bucket containing salt and the sensor was able to detect the salt level. In another related embodiment, the sensor was attached to the support structure inside a tank and the level reading was still generated. In these example embodiments, the oscillator frequencies ranged from about 4.47 MHz to about 4.54 MHz.

(42) Referring now to FIGS. 6-8, there are shown data plots for oil-type sunflower seeds versus airgun pellets using an apparatus shown in FIG. 8 according to the teachings herein. In particular, FIG. 8 illustrates a photograph of an example bin level measurement apparatus used to make capacitance measurements as the material level is varied in a 36″ long acrylic tube with a 0.1″ wall thickness. The electrodes, consisting of 300 Ohm twinlead transmission line, are attached to the outside of the tube. This “through the wall” measurement is intended to demonstrate the versatility of a capacitance measurement.

(43) FIG. 6 illustrates a chart 600 of oil type sunflower seeds and plastic 6 mm air gun pellets using a measured capacitance example sensor arrangement as taught herein. The plotted data shows the measured capacitance from this apparatus versus the fill level for two different fill materials: oil type sunflower seeds and plastic 6 mm air gun pellets. Also shown in chart 600 are the linear fit equations for the capacitance measurements for both materials. These equations are the calibration expressions that would be used to estimate fill level from capacitance measurement. A well-known weakness of prior art single electrode level measurements is that the calibration curves are dependent on the fill material.

(44) FIG. 7 illustrates a chart 700 of the sunflower seeds and air gun pellets of FIG. 6 and an estimated fill level versus an actual level when a permittivity independent method is used. In particular, chart 700 shows estimated fill level versus the actual level when a permittivity independent method is used from the ratio of level and reference capacitance measurements. As chart 700 shows, the single permittivity independent method exhibits highly accurate results for both the plastic pellets and the sunflower seeds. Further, with this method no material dependent calibration is required.

(45) In one example embodiment, a level measurement system as taught herein the electrodes can be mounted either internal to any material container or external to a non-metallic material container with external mounting utilizing an active electrical shield to prevent biasing of the capacitance measurement by external influences such as rain or proximity of other materials.

(46) In a related embodiment, a level measurement system as taught herein uses the multi-frequency capacitance measurements collected at different fill levels and can be mathematically manipulated so as to estimate the totally empty container capacitance enabling the sensor to be utilized in a single frequency mode for materials with permittivity that is independent of frequency. This single frequency mode taught herein circumvents the need for physical calibration measurements by using the multi-frequency level estimate as the known calibration point for the single frequency measurement.

(47) The advantages of the various embodiments described herein include but are not limited to providing continuous level information in the form of a frequency output, an analog output, PWM signal, or digital; and providing discrete signals for particular levels (half full, ¾ full, etc.) and the output is linear. Because continuous level sensing is available, the system can also provide flow rate information by differentiating changes in the bin level. In a related embodiment, an active shield is included which removes unwanted external influences from the measurement.

(48) The following patents that relate to capacitive sensors are herein incorporated by reference in their entirety and constitute part of the disclosure herein: U.S. Pat. No. 6,539,797 and 2006/0236275 to Breed.

(49) Having thus described several illustrative embodiments, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of this disclosure. While some examples presented herein involve specific combinations of functions or structural elements, it should be understood that those functions and elements may be combined in other ways according to the present invention to accomplish the same or different objectives. In particular, acts, elements, and features discussed in connection with one embodiment are not intended to be excluded from similar or other roles in other embodiments. Accordingly, the foregoing description and attached drawings are by way of example only, and are not intended to be limiting.