Abstract:
A compartment in a housing has a wall serving as a pressure diaphragm in communication with the contained fluid to be measured. A measuring element with a spherical cap is arranged in the compartment for the uniform transmission of pressure to electrically responsive pressure sensitive means on the measuring element. The measuring element contacts the pressure diaphragm at virtually only a single point. The diameter of the measuring element and, accordingly, of the compartment which includes the measuring element, can be considerably reduced, so that the mechanical strength of the pressure sensor is increased.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to fluid pressure measuring, more specifically to an assembly including a protective separator and pressure sensitive element on support means, for transmitting pressure to the element accurately over broad manufacturing and assembly tolerances. 
     2. Description of the Prior Art 
     Sensors for detecting pressures in diesel and gasoline engines, ranging from the relatively high pressure of combustion to the lower pressure of fuel delivery pumps, generally have a measuring element which includes a pressure-sensitive element on support means, in a transmitter housing that includes a compartment which extends sealingly through an opening in the pressure vessel wall of the engine or pump system and into the pressure vessel. The support means is screwed into the compartment, with the sensitive portion of the element pressed flatly against the inner surface of the compartment at a diaphragm wall which extends into the vessel wherein the diaphragm yields to the fluid pressure. 
     For this purpose, however, a relatively large inner diameter of the compartment is required, which reduces the strength of the transmitter housing. In addition, the pressure responsive diaphragm of the compartment wall must be against the pressure sensitive element as flat as possible. Even the smallest tolerance differences and angle errors can lead to measurement errors because of the relatively small film thickness of the pressure-sensitive element. 
     SUMMARY OF THE INVENTION 
     The pressure sensor, according to the invention, provides accurate transmission of pressure to the pressure-sensitive element with small overall dimensioning and relatively large assembly tolerances. In addition, no thread is needed for installing the support means. Accordingly, the diameter of the compartment can be relatively small, so that the mechanical strength of the housing is improved. The mechanical fastening of the measuring element of the transmitter housing is ensured in a simple manner, e.g. by means of a weld connection to the support means which may comprise a steel rod. Since no installation thread is located on the support in the area of the pressure-sensitive element and the diaphragm within the compartment, this area is free of mechanical vibrations or deformation brought about by means of the installation of the transmitter housing in the pressure vessel wall. The pressure values transmitted by the diaphragm are accordingly not falsified by means of mechanical vibrations from the pressure vessel wall. The pressure-sensitive area of the pressure sensor is decoupled to a great extent from the twisting of the transmitter housing when it is screwed into the wall. 
     Conductor stripes, printed on the measuring element in a simple manner make contact with a pressure sensitive resistive film outside the area of the pressure sensor that is acted upon by pressure. The conductor strips and the resistive film can overlap. Since no feed wires are needed in the critical transmitter area and no measures for insulation are needed, a simple electrical feed line to the resistive film results. 
     Accordingly, there is provided a pressure sensor for detecting the pressure of a fluid confined behind a wall of a pressure vessel. The pressure sensor includes pressure sensitive means, electrically responsive to mechanical pressure, located in a compartment of a housing. The compartment, which is closed relative to the fluid, includes a diaphragm in communication with the fluid, which is moveable in response to changes in the pressure of the fluid. The pressure sensitive means is mounted in the compartment for receiving mechanical pressure exerted by movement of the diaphragm. A cap is mounted between the pressure sensitive means and the diaphragm for transmitting the mechanical pressure between them. 
     A support body fixedly joined with the housing, positions the pressure sensitive means so that it receives the mechanical pressure from the diaphragm. In one embodiment the support body is fixedly joined to the housing by a weld. The cap may include a curved surface and further, a hemispheric surface, at point of contact with the diaphragm. 
     The pressure sensor comprises an insulating layer on the support body, electrical conductor strips located on the insulating layer, and a layer of material that is electrically responsive to pressure, located on the insulating layer and overlapping with the conductor strips. 
     The pressure sensitive means may be manufactured by thick-film technique. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention be more fully comprehended, it will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a longitudinal sectional view through a pressure sensor. 
     FIG. 1a is a partial sectional view of the pressure sensor of FIG. 1, taken along lines 1a--1a. 
     FIG. 2 is a longitudinal sectional view through the measuring element of the pressure sensor. 
     FIG. 3 is a longitudinal partial sectional view through the measuring element of FIG. 2, taken along lines 3--3. FIG. 4 is a sectional view of FIG. 2 taken along lines 4--4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before explaining the invention in detail, it is to be understood that the invention is not limited in its application to the detail of construction and arrangement of parts illustrated in the drawings since the invention is capable of other embodiments and of being practiced or carried out in various ways. It is also to be understood that the phraseology or terminology employed is for the purpose of description only and not of limitation. 
     Referring to FIG. 1, housing 10 of pressure sensor 11 is inserted in bore hole 13 of the wall 14 of a pressure vessel and fastened therein by thread 12. As shown, it projects through wall 14 into the pressure vessel for communication with the fluid of which the pressure or variation in pressure, is to be measured. Housing 10 can also end flush with the wall or be set back and protected in bore hole 13 so long as housing 10 is in communication with the fluid under measurement. The housing 10 comprises an approximately central compartment 15, having a base constructed as a pressure diaphragm 16. Measuring element 17 of pressure sensor 11 locates in compartment 15 against pressure diaphragm 16. External thread 12 of housing 10 does not extend as far as the end of the housing 10 which faces the pressure medium, so that the pressure diaphragm 16 is free from mechanical stresses and vibrations. In the area of this threadless portion 18, housing 10 is connected with measuring element 17 by laser and electron beam weld 22. Measuring element 17 is accordingly fastened so as to rest against pressure diaphragm 16. The strength of housing 10 is not decreased by means of the weld 22. 
     Alternatively, as may be more clearly seen in FIG. 1a, a continuous transverse bore hole 21 in portion 18 contributes to a mechanical connection produced between housing 10 and measuring element 17 with the aid of a welding or soldering connection. Instead of a welding connection, another mechanical connection, e.g. screws, can also be used. In order not to reduce the strength of the housing 10, the transverse bore hole should be of a very small diameter. The weld 22 serves both for fastening the measuring element 17 in the compartment 15 and for producing a counterforce or pre-tensioning for pressure diaphragm 16. Electrical lines 23 lead from measuring element 17 to an electrical connection, or to an evaluating circuit, not shown. 
     The measuring element 17 comprises, as shown in more detail in FIGS. 2-4, a support body 27 of cylindrical high-grade steel, which has two diametrically opposed flattened portions 28 at the longitudinal side. An insulating layer 30 of enamel work material covers flattened portions 28 and extends to end face 29 of support body 27. Body 27 has rounded edges 31, so that a good adherence of the insulating layer 30 is achieved. 
     Electrical conductor strips 32, e.g. of silver palladium, are applied by any thick or thin film conventionally known technique on the insulating layer 30 at the flattened portions 28 and approximately along the edges 31. A pressure-sensitive resistor 33 is located on end face 29 of body 27 on the insulating layer 30. Resistor 33 and conductor strips 32 overlap in the area of the edges 31. The resistor 33 can work on the basis of known piezo-resistive effect and is printed on, particularly by thick-film technique. A cover layer 34, which extends over the point of overlapping between resistor 33 and conductor strips 32, is located on resistor 33. The cover layer 34 serves as protection for the resistor and also as a cementing connection for a cap 35 which is placed on the end face 29 of support body 27. This cap 35 is produced from a hard material, preferably of hardened steel or ceramic. Cap 35 can be designed as a hemisphere or with a curved surface. It can take the form of a half ball on end face 29 as shown in FIGS. 2, 3 and 4. 
     It is important that the contact of the cap 35 on the pressure diaphragm 16 be in the shape of the radius of a point or dot, i.e. that the support be punctiform. 
     The welding connections 22 are arranged on body 27 to avoid damage to resistor 33. 
     The mode of operation of pressure sensor 11 is sufficiently known and is therefore only explained briefly here. The bending produced by the pressure of the fluid against pressure diaphragm 16 is transmitted to resistor 33 in a uniform manner by means of the cap 35. Since cap 35 contacts the pressure diaphragm 16 in a virtually punctiform manner, the latter need not itself be constructed so as to be exactly flat and parallel to resistor 33. Maximum displacement of the pressure diaphragm 16 is transmitted uniformly and with minimum error to resistive film 33 by means of the planar side of cap 35. Non-linearities of the measurement value due to asymmetrical pressing on resistor 33 can be prevented by means of the punctiform contact between cap 35 and the diaphragm. The flat surface (the side facing the resistive film) of the cap can more easily be constructed with substantially less roughness than the inner surface of the diaphragm. Multiple pressure points against resistor 33 from a rough diaphragm surface, leading to a non-linear characteristic data line is thereby avoided. Cap 35 can also have a curved surface instead of a hemispherical one. The smaller the mass of cap 35, the better the dynamics of the pressure sensor 11. 
     In an alternate embodiment, not shown, the base of the compartment comprising the diaphragm, is curved. The radius of the base and the cap must be different, from one another, however, in order to avoid jamming. 
     The pressure sensors of the present invention can be used in an advantageous manner in pressure measurement in distributor pumps of fuel injection systems and in combustion pressure measurement in diesel or gasoline engines, because they can be assembled with looser tolerances suitable for low cost mass production 
     Although the present invention has been described with respect to details of certain embodiments thereof, it is not intended that such details be limitations upon the scope of the invention. It will be obvious to those skilled in the art that various modifications and substitutions may be made without departing from the spirit and scope of the invention as set forth in the following claims.