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 multi-frequency method for calculating a level of fluid or material contained within a container or vessel comprising the steps of:
providing 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 LL and a nominal capacitance per unit length C0, wherein the capacitance of said sensing capacitive element varies in accordance both with the extent of the immersion of the parallel electrode in the fluid or material and a dielectric constant of the fluid or material;
providing 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 LR and a capacitance per unit length C0, wherein said reference electrodes are configured to be in contact with the material or fluid within the container, and wherein the capacitance of said reference electrodes is a function of the dielectric constant of the fluid or material; and
providing 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 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, by:
determining a measured capacitance of the sensing capacitive element at a first frequency (f1) and at a second frequency (f2);
determining a measured capacitance of the reference capacitive element at the first frequency (f1) and at the second frequency (f2); and
generating the level of the fluid or material within the container from a product of: a ratio of LR (reference electrode length) and LL (sensing electrode length) and
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;
b. a difference of the measured capacitance of the reference element at the first frequency and the measured capacitance of the reference element at the second frequency as a denominator; and
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, thereby avoiding the need to perform a physical calibration measurement of the container.
2. The method according to claim 1 further comprising the step of providing a metallic shield on said sensing electrodes mounted external to the container and driving said metallic shield at the same frequency and potential as a driven electrode.
3. A single frequency method for calculating the level of fluid or material contained within a container or vessel comprising the steps of:
providing 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 LL and a nominal capacitance per unit length C0 and wherein the capacitance of said sensing capacitive element varies in accordance both with the extent of the immersion of the parallel electrode in the fluid or material and a dielectric constant of the fluid or material;
providing 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 LR and a capacitance per unit length C0, wherein said reference electrodes are configured to be in contact with the material or fluid within the container, and wherein the capacitance of said reference electrodes is a function of the dielectric constant of the fluid or material; and
providing 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 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, by:
determining an empty container capacitance of each of the sensing capacitive and reference capacitive elements using multi-frequency measurements;
determining a total capacitance 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
generating the level of the fluid or material within the container from a product of:
a ratio of LR (reference electrode length) and LL (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;
thereby avoiding the need to perform a physical calibration measurement of the container and accounting for shifts in parasitic capacitance values.