Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Patent
1993-06-22
1995-05-23
Wieder, Kenneth A.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
324 711, 324664, 324686, 324690, 73 6143, G01R 2726, G01N 2722
Patent
active
054184657
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a sensor for measuring the mixture ratio of liquids and in particular to a small-scale sensor for measuring the mixture ratio of fuel mixtures.
BACKGROUND OF THE INVENTION
European Patent Application No. 0 335 168 A2 describes a sensor which determines the alcoholic concentration of a fuel mixture to allow an internal combustion engine to be optimally operated. The sensor's measuring cell has an inlet and an outlet to allow the fuel mixture to flow through the measuring cell. The outer walls of the measuring cell and a central cylinder consist, at least partially, of conductive material and form a cylindrical capacitor, which is a component of an evaluation circuit. The capacitance of the cylindrical capacitor is influenced by the composition of the fuel mixture which constitutes the dielectric. Not only does the capacitance change along with the composition, but the conductance of the fuel mixture changes as well. This conductance is likewise detected by means of the capacitor's electrodes. Up until now, this arrangement had been realized in macroscopic dimensions using conventional manufacturing methods.
SUMMARY OF THE INVENTION
The present invention provides a sensor for measuring the mixture ratio of liquids, in which first and second electrodes are applied to first and second ceramic carriers, respectively. At least one of the first and second electrodes is applied using thick-film technology. The first and second ceramic carriers define a cavity therebetween for carrying the liquids.
An advantage of the sensor according to the present invention is that it is able to be realized on a very small scale, so that its application does not require any significant amount of additional space. It is also advantageous that the ceramic housing of the measuring cell demonstrates very good media compatibility, particularly for fuel mixtures. Furthermore, it is advantageous that the sensor is able to be assembled using known methods that are easily mastered, and using materials from thick-film technology. The housing can be manufactured and the circuit elements, such as the electrodes of the measuring capacitor, can be applied in one process step.
It is particularly advantageous for the measuring cell to have another sealed cavity, on whose inner walls electrodes are applied. Together with a reference medium filling the additional cavity, these electrodes form a reference capacitor, which likewise constitutes a component of the evaluation circuit. Long-term drifts and the temperature sensitivity of the sensor can be reduced by incorporating a reference capacitor in the measuring process. This is because, given the same operating conditions, the characteristic of the reference capacitor changes similarly to the characteristic of the first capacitor, whose dielectric is the liquid to be measured.
Since the reference medium is supposed to be occluded in the additional cavity, but is subject to thermal expansion, it is advantageous for the additional cavity to communicate via a passage with a compensating volume. One advantageous refinement of the compensating volume, which is likewise filled with the reference medium, consists in developing one boundary wall of the compensating volume as a flexible ceramic membrane. Another advantageous method for compensating for the temperature sensitivity of the sensor consists in measuring the temperature of the liquid or of the reference medium directly. This can be achieved advantageously by applying temperature-dependent resistors, using thick-film technology, to the inner walls of a cavity or of the compensating wall, to place them in direct contact with the liquid or with the reference medium. The structure of the measuring cell in thick-film technology makes it possible for parts of or for the entire evaluation circuit to be applied to the measuring cell itself using thick-film hybrid technology. This increases the interference immunity of the signal evaluation. Furthermore, the measuring cell structure makes it
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patent: 4710757 (1987-12-01), Haase
patent: 4905655 (1990-03-01), Maekawa
patent: 5151660 (1992-09-01), Powers et al.
Maihoeffer Bernd
Seipler Dieter
Ziegenbein Botho
Do Diep
Robert & Bosch GmbH
Wieder Kenneth A.
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