Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1999-11-01
2001-08-14
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S663000, C324S686000
Reexamination Certificate
active
06275048
ABSTRACT:
PRIORITY CLAIM
This application is based on and claims the priority under 35 U.S.C. §119 of German Patent Application 198 50 245.1-52, filed on Oct. 31, 1998, the entire disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
The invention relates to capacitive sensors for sensing a characteristic feature of a liquid or gaseous medium capable of entering into a hollow space in the sensor. A method for producing such capacitive sensors is also disclosed.
BACKGROUND INFORMATION
Sensors operating by using the sensing ability of a plate capacitor are known. The medium, a characteristic of which is to be sensed or measured, forms a dielectric between the plates of the plate capacitor. Such capacitive sensors are particularly suitable for sensing the filling level of a liquid such as oil in a container. Basically medium characteristics or parameters other than the filling level can also be derived from the dielectric characteristics of the medium. Tubular capacitors with cylindrical “plates” are also known for such sensing purposes. For example, German Patent Publication DE 196 44 777 C1 describes, in addition to the general prior art, a filling level sensor arrangement with a plurality of capacitive sensors which are electrically interconnected in several groups. The capacitive sensors forming the groups are arranged in stacks on one side of a carrier. The evaluation of the measured signals takes place in a quasi-digital mode for each group of capacitive sensors, whereby the effort and expense for the electrical wiring and for the evaluation circuit arrangement are increased.
European Patent Publication EP 0,401,111 B1, published in French, as translated in German Patent Publication DE 690 01 151 T2 discloses a capacitive sensor arrangement of the type to which the present invention relates. Two carrier plates carry metal capacitor coatings or claddings on plate surfaces facing each other. The carrier plates are interconnected by several rivets. Spacer rings are positioned on the rivet shafts between the plates. The rivets and the spacer rings are necessarily positioned outside the areas of the metal capacitor coatings or claddings. Further, the rivets and spacer rings must be electrically insulated from the metal capacitor coatings or cladding. These features of the known sensor increase its costs.
The carrier plates referred to hereafter as circuit boards or simply as boards are made of printed circuit board material and etching methods are used to structure the metal capacitor coatings or claddings and the required conductor leads. The boards carry as part thereof several coatings forming capacitor plates spaced from one another by the rings on the rivets and include for example a main electrode for sensing a filling level and a dielectric measuring electrode for measuring a possibly changing dielectric between the capacitor plates. A further metal coating may be provided to form an electric screen. These various coatings or cladding elements are individually connected by leads to respective connector areas on the boards. These connector areas and possibly further elements on the boards are connected to a separately arranged electronic measuring device or signal evaluating circuit not shown in any detail. The electric conductors required for connecting the sensor to a remote signal evaluating circuit are trouble prone and costly.
Besides, the measuring accuracy of the known sensor leaves room for improvement. In the known sensor the measuring accuracy depends much on the structural stability of the sensor namely on the number of rivets and the spacing of the rivets from each other. The rivets cannot assure in all instances that the boards will not bend or warp. Such warping is undesirable because it varies the spacing between the capacitor plates or rather metal coatings on the boards. A non-constant spacing varies the measuring accuracy.
OBJECTS OF THE INVENTION
In view of the foregoing it is the aim of the invention to achieve the following objects singly or in combination:
to avoid the above drawbacks of conventional capacitive sensors by assuring a constant, uniform spacing between the capacitor plates formed by the respective metal coatings on the carrier boards;
to substantially simplify the construction of such sensors while improving their measuring accuracy;
to properly screen capacitive sensors against external, adverse field influences;
to adapt the production techniques of multiboard printed circuit arrangements to the economic, simultaneous production of a multitude of capacitive sensors; and
to improve the dimensional and structural stability of capacitive sensors made of printed circuit boards and spacer plates.
SUMMARY OF THE INVENTION
According to the invention it has been recognized that printed circuit techniques known for producing multiboard or multilevel printed circuit panels are adaptable to the production of capacitive sensors. According to the invention a spacer plate is inserted or sandwiched between two carrier boards to form a capacitor. The spacer plate is provided with a central opening forming a hollow space between the carrier boards. The hollow space is so positioned in the spacer plate that in the sensor the hollow space will be flanked on both sides by the capacitor plates formed by metal coatings on the two carrier boards. The spacer plate provides a margin area around the hollow space and this margin area keeps the carrier boards properly spaced with a constant spacing from each other even in the area of the hollow space. This spacing is exactly determined by the thickness of the spacer plate including its margin areas. Openings or ports are positioned to permit the fluid in the form of a liquid or gas to enter into the hollow space of the capacitive sensor. These openings include at least one first port at the bottom end of the sensor for entry of the fluid into the sensor and at least one pressure equalization second port above the bottom to permit pressure equalization between the hollow space inside the sensor and outside the sensor in the fluid container. The first and second ports may pass through the margin area of the intermediate spacer plate and/or directly through one or both of the outer carrier boards. Metallized lead-through contacts are provided for electrically interconnecting the carrier boards and the spacer plate. The just described sensor structure forms in its assembled state a multilevel printed circuit panel with hollow spaces. The medium to be measured enters into these hollow spaces through said first port. The margin area of the intermediate spacer plate provides a distinctly better dimensional stability for the present sensor compared to rivets with spacer rings on their shafts. As a result the present sensor achieves a better measuring accuracy.
Another advantage of the invention is seen in that the individual carrier boards can be electrically connected with each other through the intermediate spacer plate by metallized lead-through contacts. Such lead-through contacts may be formed by multi-board printed circuit techniques. Moreover, any electronic signal evaluation circuit for processing sensed signals can be integrated into the present sensors on an outer surface of an outer carrier board, whereby complicated trouble-prone electrical conductors and connections, for example by bonding wires, are avoided.
According to the method of the invention multitudes of the present sensors are produced simultaneously in an especially advantageous, particularly economic manner, by sandwiching a large sheet provided with a plurality of intermediate segmented spacer plates having hollow spaces in the spacer plates between two correspondingly large sheets comprising segmented carrier boards with metal coatings, whereby a sandwich panel is produced in which the segments form individual sensors which are interconnected by lands or strips. The individual boards and spacer plates forming a stack are adhesively bonded to one another to form the sandwich panels. The individual sensors are then cut out of the
Brown Glenn W.
Daimlerchrsyler AG
Fasse W. F.
Fasse W. G.
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