Patent ID: 6539797
Filing Date: 2003-04-01
Classification: G01F

Abstract:
A capacitive level sensor for measuring the level of fluid or material contained within a reservoir comprising:a capacitive sensor array adapted to be disposed in the reservoir along an axis of measurement of the material or fluid contained within the reservoir, the sensor array comprising a plurality of reference electrodes wherein the capacitance of each of the electrodes varies in accordance both with the extent of the array's immersion in the fluid or material and the dielectric constant of the fluid or material, the plurality of reference electrodes comprising: a lower electrode adapted to be positioned near the bottom of the reservoir and whose capacitance provides an estimate of the dielectric constant of the fluid or material contained within the reservoir; and a middle electrode that extends along the length of the array and whose capacitance varies from a calibrated initial value to a value that is dependent on the level of and the dielectric constant of the fluid or material contained within the reservoir as determined from the capacitance of the lower electrode; a circuit capable of measuring the capacitance of each reference electrode and generating a digitized sensor voltage representing the capacitance of each electrode; and a microprocessor for receiving and processing the digitized sensor voltages to determine the fluid or material level, wherein the level is determined as a linear function of the capacitance of the middle electrode and is compensated by the amount and rate of change of the capacitance of the lower electrode, and wherein the microprocessor determines the level of the fluid or material in accordance with: level=K&CenterDot;VMcorrected&af;(l)-A&CenterDot;VLcorrected&af;(l)VLcorrected&af;(l)-B&CenterDot;VMcorrected&af;(l),whereA=VM&it;â€ƒ&it;Î”0VL&it;â€ƒ&it;Î”0B=VL&it;â€ƒ&it;Î”maxVM&it;â€ƒ&it;Î”maxK=levelmaxLmax&CenterDot;VL&it;â€ƒ&it;Î”0VM&it;â€ƒ&it;Î”maxâ€ƒVMcorrected(l)=VM(l)âˆ’VMcal VLcorrected(l)=VL(l)âˆ’VLcal Lmax represents the fluid or material level when the fluid or material is at the top of the middle reference electrode; levelmax represents a calibrated full fluid level; VM(l)=VMcalâˆ’(&egr;râˆ’1)Â·(VM&Dgr;0+VM&Dgr;maxÂ·l) VL(l)=VLcalâˆ’(&egr;râˆ’1)Â·(VL&Dgr;0+VL&Dgr;maxÂ·l) VL&Dgr;0, VM&Dgr;0 represent the change in the digitized sensor voltages from the lower and middle reference electrodes from the calibrated initial values to values obtained when the level of the fluid or material is at the bottom of each electrode; VL&Dgr;max, VM&Dgr;max represent the change in digitized sensor voltages from the lower and middle reference electrodes from the calibrated values to values obtained when the electrodes are each fully immersed in the fluid or material; where: VL(l) and VM(l) represent the digitized sensor voltages from the lower and middle reference electrodes as a function of the level of the fluid or material; VLcal and VMcal represent the calibrated initial values for the digitized sensor voltages from the lower and middle reference electrodes, calibrated when the sensor array is not immersed into the fluid or material; and &egr;r is the relative dielectric constant of the fluid or material.