Measuring and testing – Liquid level or depth gauge – Immersible electrode type
Reexamination Certificate
1999-04-13
2001-09-25
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
Immersible electrode type
C073S29000R, C340S450000, C340S612000
Reexamination Certificate
active
06293145
ABSTRACT:
FIELD OF INVENTION
The invention relates to a sensor for measuring the filling level of a fluid in a vessel, in particular a fuel tank. The sensor has an electrode group which extends vertically over the fillable vessel height and dips into the fluid and which forms electrical capacitors whose capacitances change in a measurable fashion when there are changes in the filling level. The capacitances are determined by a connected evaluation circuit and represented as a signal which describes the filling level.
BACKGROUND OF THE INVENTION
Various arrangements and systems are known for measuring the filling level, in particular, in fuel vessels. For example, in the case of capacitive filling level sensors such as are known from U.S. Pat. No. 2,700,901, a rod-shaped capacitor is dipped vertically into the fluid. The fluid and the gas located above it act as dielectrics with different dielectric constants of this capacitor. When the filling level changes, the capacitance of the capacitor also changes, and it is consequently used as a measurement variable for the filling level in the vessel.
By means of various influences, such as fabrication tolerances, long-term change and the change in the composition of the fluid in the vessel, the characteristic curve of such a filling level sensor changes. In addition, after manufacture, adjustment is necessary in order to compensate for tolerances. A large number of fluids also separate out when standing over a relatively long period or they combine with other substances such as water, and thus change their physical properties, including their dielectric constants, independently.
For these reasons, when capacitive sensors which operate continuously are used, measuring errors occur, since, in addition to the filling level, the height-dependent dielectric constants also affect the filling level measurement in a nondeterministic way.
In order to avoid these disadvantages, discretely operating filling level sensors have been disclosed. Such capacitively operating sensors have a large number of individual capacitors distributed over the entire measured height. The greater the number of measurement points, the greater the resolution of the sensor. This arrangement is independent of changes in the dielectric constant of the fluid and is thus immune to the abovementioned sources of error. Unfortunately, it is extremely complex and expensive. In addition, the resolution which can be achieved with such sensors is often too low, which is a further disadvantage particularly in situations where there are small filling levels.
The object of the present invention is to specify a filling level sensor which can be manufactured cost-effectively and simply, which operates precisely and without maintenance and which is largely independent of changes in the dielectric constant of the filling material.
SUMMARY OF THE INVENTION
The object is achieved with a first variant of the invention by virtue of a sensor having at least one measuring electrode which extends over the entire fillable vessel height, and a plurality of reference elements which are arranged at different reference heights within the fillable vessel height.
In a second variant of the invention, the solution is achieved by virtue of a sensor having a plurality of measuring electrodes which are arranged in such a way that each measuring electrode has significant change in width at a reference height assigned to it, and that the entire fillable vessel height is passed over by the measuring electrodes.
A continuous measurement of the filling level takes place in each case between the reference heights. By virtue of the geometric characteristics which the electrodes of the sensor have at the reference heights, discontinuities arise in the course of the electric characteristic variables of the sensor when the surface of the fluid passes over the reference heights. These discontinuities serve as reference marks for the measurement and are used by the connected evaluation electronics to correct the display.
A first refinement of the invention provides for an opposing electrode which forms electric capacitors with all the measuring electrodes. In addition, in a first development of the invention there is provision for the reference elements to be reference electrodes and also for the one opposing electrode to form electrical capacitors with all the reference electrodes. As a result, the number of electric connections necessary is reduced to a minimum and the design of the sensor becomes very simple.
In another refinement of the sensor according to invention it is provided that the reference elements are thermistors. Such components have proven their value and are therefore provided as an alternative to the capacitive technique.
Another development provides for the distances between the reference heights at small filling levels to be smaller than at large filling levels. In this way, the residual amount of fluid which is still present when the tank is almost empty can be determined relatively precisely.
So that a clear division of the filling volume can be represented on the display of the filling level with little expenditure, in a subsequent development of the invention the reference heights are selected such that even if the vessel is irregularly shaped, a regular division of the filling volume by the reference heights is produced.
The fabrication of the sensor according to the invention may be made very simple if the electrode group, as is provided in other refinements of the invention, is arranged on a bent carrier, and if the evaluation circuit is arranged on the same carrier as the electrode group. The carrier may be provided with the circuit and the electrodes before the bending operation, when it still has an easy-to-handle shape.
Because this material is very robust and durable, there is provision, in another refinement according to the invention, that the carrier for the electrode group is composed of enameled sheet steel.
In order to simplify the fabrication further, there is provision that the electrode group is implemented on its carrier as a printed circuit.
A method according to the invention for measuring the filling level is achieved by means of a sensor according to the invention. The filling level display is corrected with the discontinuities in the electrical behavior of the sensor. These results rely on the arrangement of the measuring electrodes and reference elements when the filling level changes beyond a reference height.
As a further possible means of correcting the filling level measurement, there is a provision in one development of the method according to the invention that the measurement signal of the filling level sensor is corrected with a signal relating to the removal of fluid, in particular fuel consumption.
This is achieved according to the invention that the total value of the fluid removal of which is converted into a height value is subtracted from the last measured value of the filling level, at least between two reference heights, and the current measured value is corrected with the result.
It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention.
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Cygan Michael
Mannesmann VDO
Mayer Brown & Platt
Williams Hezron
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