Calibration-free continuous bin level sensor
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.
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 L and a nominal capacitance per unit length C 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 R and a capacitance per unit length C 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 L and a nominal capacitance per unit length C 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 R and a capacitance per unit length C 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 R (reference electrode length) and L 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 L and a nominal capacitance per unit length C 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 R and a capacitance per unit length C 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 1 /L 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 2 /L R ) for some non-empty fill level differing from (H 1 /L 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 2 /L R and the measured capacitance of the reference element at the second frequency less the product of H 1 /L R and the measured capacitance of the reference element at the first frequency as a numerator;
b. a difference of H 2 /L R and H 1 /L 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 1 /L R and the difference of the empty container reference electrode capacitance and the reference capacitance at the first frequency.