Title of Invention

PRESSURE SENSOR

Abstract

Pressure sensor for sensing the pressure of a medium which is bounded by a wall, in particular for sensing the pressure in the pump working space of a fuel injection pump, having a housing which is arranged in a through-hole (2) of the central locking screw (4) of the fuel injection pump. The housing has a separate front part (8) which is welded to a measuring element carrier (16) and together with the latter, a measuring element (14), a pressure transmission element (12) and a dia- phragm (10) forming the base of the front part (8) of the housing forms a measuring head (6) which is prestressed in the pressure transmission direction in a defined fashion and cannot be disassembled. The measuring head (6) makes contact with other components of the pressure sensor (1) and with the through-hole (2) exclusively by means of an outer flange (20) which is arranged on the outer circumference of the measuring head (6).

Full Text

ROBERT BOSCH GmbH, 70442 Stuttgart Pressure Sensor Prior Art The invention is based on a pressure sensor of the generic type of the main claim. In such a pressure sensor, known for example from DE-A-43 03 Oil, the base of the sensor housing is designed as a pressure trans¬mission element. The sensor housing has an internal thread into which a plunger is screwed, which presses the measuring element onto the base with prestress in order to transmit pressure. The level of this prestress is decisive for the function of the pressure sensor and is set in advance. When there is excessively high or exces¬sively low prestress of the measuring element, there is the risk of the measurement curve not being linear. In addition an excessively high or excessively low prestress has a negative effect on the" fatigue strength of the measuring element and of the base. If the screwed plunger turns intentionally or unintentionally out of its defined set position, the prestress which acts on the measuring element and the base also changes. In addition, at the high pressures in the injection system, settling occurs in the thread turns, as a result of which the prestress diminishes over time. Finally, the European Patent EP 0 4 00 060 dis¬closes a measuring device for sensing a pressure, in which the prestress on the measuring element is also generated by means of disconnectable, positively engaging connections. As in the case of the screwed connection, settling occurs at joints of these connections over time, which reduces the prestressing force on the measuring element. Advantages of the invention The pressure sensor according to the invention having the features of the independent Claim 1 has, in contrast, the advantage that the welding of the measuring element carrier to the housing avoids changes to the prestress once it has been set. As a result, it is, on the one hand, ensured that the measuring element always operates in the linear range. On the other hand, the service life of the pressure sensor is increased while a level of prestress which is optimum for the fatigue strength is retained. Since the measuring head is mounted in the through-hole of the wall exclusively by means of its outer flange, the force flux of the clamping-in forces runs mainly via the outer flange, as a result of which no additional clamping-in forces are superimposed on the prestress forces acting on the diaphragm and on the measuring element, with the result that this also makes it possible to avoid, in a targeted fashion, any change to the prestress. In addition, the measuring head constitutes a separate assembly which can be manufactured cost-effectively as a pre-assembled unit. Advantageous developments of the pressure sensor specified in the independent Claim 1 are possible by virtue of the measures disclosed in the dependent patent claims. Drawings An exemplary embodiment of the invention is illustrated in the drawing and explained in more detail in the following description. Fig. 1 shows a longitudinal section through a preferred exemplary embodiment in accordance with the invention. Description of the Exemplary Embodiment The preferred embodiment (illustrated in Fig. 1) of a pressure sensor 1 in accordance with the invention is installed in a through-hole 2 of a central locking screw 4 of a known distributor injection pump, which is not illustrated. The side of the pressure sensor 1 pointing towards the pressure medium has a measuring head 6 which is arranged in a separate front part .8 of the housing of the pressure sensor Land contains, viewed in the pressure-transmission direction, a diaphragm 10, a pressure bolt 12, a measuring element 14 and a measuring element carrier 16. The measuring element carrier is preferably a cylindrical metal plate 16 whose radially outer circumferential face is welded to the radially inner circumferential face of the front part 8 of the housing in such a way that the measuring element 14, which is arranged on the side of the measuring element carrier 16 pointing towards the pressure bolt 12, exerts a defined prestress force on the diaphragm 10 via the pressure bolt 12, as a result of which the diaphragm 10 is deflected towards the pressure medium by approximately 5 µm. From the order of magnitude of the deflection of the diaphragm it is apparent that even very small geo¬metric changes in the measuring head 6 can substantially change the preset prestress force, especially since the diaphragm 10 is of relatively stiff design owing to the high fatigue strength. The diaphragm 10 is preferably welded to the front part 8 of the housing, the wall thickness of the diaphragm 10 being small with respect to the wall thick¬ness of the front part 8 of the housing. The front part 8 of the housing is thin-walled in the region of the welded connection to the measuring element carrier 16, with the result that the two parts can be connected to one another by means of a laser welded connection 18. Owing to the welded connections, the measuring head 6 forms a unit which cannot be disassembled, with the result that the prestress on the diaphragm 10 cannot be changed, either intentionally or unintentionally. An outer flange 20 is arranged on the outer circumference of the front part 8 of the housing and, in order to mount the measuring head 6 in the through-hole 2, it is clamped in axially between a shoulder 22 which reduces the diameter of the through-hole 2 and a rear part 24 of the housing which is screwed into the through- hole 2. For this purpose, the rear part 24 of the housing has, in a rear region viewed in the pressure transmission direction, an external thread 26_which_is screwed into an internal thread of the through-holeT2 and which maintains a relatively large axial distance from the measuring head 6 arranged at the front end of the pressure sensor 1. This axial distance ensures that clamping-in forces which are transmitted along the external thread 26 do not act on the measuring head 6. One end of the wall of the rear part 24 of the housing which points towards the measuring head 6 is of a step-shaped design such that a relatively long stepped part 28 is clamped against the outer flange 20 of the front part 8 of the housing and is connected to it, at its radially outer circumferential edge, by means of welded connection 30, the rear part 24 of the housing and the front part 8 of the housing overlapping one another in the radial direction with radial spacing. Here, the longer stepped part 28 surrounds the wall part, project¬ing from the outer flange 20 to the rear part 24 of the housing, of the front part of the housing. A relatively short stepped part 32 of the end of the wall of the rear part 24 of the housing lies opposite the end face of the front part 8 of the housing, with axial spacing. In accordance with the preferred embodiment, the outer flange 20 is circumferential, but it may also comprise one or more outer-flange sections which are arranged on the outer circumference of the front part 8 of the housing with the circumferential spacing from one another. The shoulder 22 of the through-hole 2 has a circumferential sealing lip 34 which points in the pressure transmission direction and against which the outer flange 20 of the front part 8 of the housing is clamped. Since the pressures occurring in the injection system are very high, the sealing lip 34 is composed of metal and deforms elastically and/or plastically under the clamping-in force. As a result, the deformations owing to the clamping-in forces take place at the sealing lip 34 and not at the outer flange 20 of the front part 8 of the housing. The sealing lip 34 is preferably integral with the central locking screw 4, but separate sealing elements such as sealing washers or spacer rings may also be used. As is clear in Fig. 1, the measuring head 6 makes contact with other components of the pressure sensor 1, and with the through-hole 2, exclusively via the faces of the outer flange 2 0 which point in the pressure trans¬mission direction, as a result of which the measuring head 6 is mounted clamped-in in the through-hole 2 of the central locking screw 4. In contrast, all the other outer faces of the measuring head 6 are at a distance from the adjoining faces of other components. This also ensures that the measuring head 6 and, in particular, the pres tress of the diaphragm 10 not influenced by any clamping-in or bearing forces. This is because the force flux of the clamping-in forces runs from the external thread 26 via the relatively long stepped part 28 of the rear part 28 of the housing and via the outer flange 20 into the shoulder 22 of the through-hole 2 and thus around the prestressed diaphragm-pressure bolt-measuring element arrangement. Since the relatively short stepped part 32 of the rear part 24 of the housing lies opposite the end face of the front part 8 of the housing with axial spacing, it is likewise impossible for any clamp¬ing- in forces to be transmitted via these faces. The same also applies to the radial circumferential faces in the region of the overlap of the rear part 24 of the housing with the front part 8 of the housing, since these faces are arranged with radial spacing from one another. As is clear from the previous embodiments, the housing of the pressure sensor is, in accordance with the preferred embodiment, in two parts and is composed of the front part 8 of the housing forming the measuring head 6 and of the rear part 24 of the housing which clamps the latter in. As a result, the choice of material and the dimensioning of the two housing parts 8, 24 can be adapted separately to the different requirements. Thus, the wall thickness of the rear part 24 of the housing is relatively large in comparison with that of the front part 8 of the housing in order, on the one hand, to be able to apply a high screwing-in torque to the rear part 24 of the housing and, on the other hand, to be able to weld through the front part 8 of the housing to the measuring element carrier 16 from the outside. On the other hand, the division of the housing into two parts permits the separate measuring head 6 to be made as short as required. As a result, the pressure bolt 12 is also short with a correspondingly low mass, which, in view of the high pressure accelerations to which it is subjected, leads to lower inertial forces and to a lower vibration excitation of the measuring system. In addition, a short pressure bolt 12 brings about a relatively rigid trans¬mission of the pressure surges from the diaphragm 10 to the measuring element 14. Such pressure sensors 1 can preferably be used for measuring pressure in distributor injection pumps of fuel injection systems and for measuring combustion pressure in diesel engines or spark-ignition engines. When they are used in distributor injection pumps, the instantaneous pressure in the pump working space is sensed by means of the pressure, sensor 1 and the start of delivery and the end of delivery of the pump are deter¬mined therefrom. These parameters can then be used to calculate the delivery period and the delivery quantity, which serve as input parameters for the adjustment and control functions of the injection system. The method of operation of the pressure sensor 1 is adequately known and therefore described here only briefly. The sensing of the pressure in the pump working space of the fuel injection pump is effected by means of the axial deflection which is generated by the pressure of the pressure medium in the diaphragm 10 and which is transmitted uniformly to the measuring element 14 with the aid of the pressure bolt 12. The measuring element 14 contains a chip in which, on the one hand, the measuring principle and, on the other hand, the evaluation of the measurement data are implemented. In accordance with the preferred embodiment, measurement is piezo-resistive, the resistance values within a full-bridge circuit changing when pressure is applied to the diaphragm 10, and the measurement signals being derived by means of measuring head pins 36 projecting from the measuring element carrier 16. Since the measuring head pins 36 are easily accessible from the outside through the rear opening of the rear part 24 of the housing, the measuring head 6 can also be adjusted in the installed state. CLAIMS Pressure sensor (1) for sensing the pressure of a medium bounded by a wall, in particular for sensing the pressure in the pump working space of a fuel injection pump, having a housing (8, 24) which is arranged in a through-hole (2) of the wall and which is terminated, with respect to the medium whose pressure is to be measured, by means of a base (10) against which a pres¬sure transmission element (12) bears on a side facing away from the medium, which pressure transmission element (12) transmits the pressure, acting from the medium onto the base (10) , to a measuring element (14) which is arranged in the housing (8) , characterized in that the housing contains a separate front part (8) which is connected to a measuring element carrier (16) by means of a first welded connection (18) and forms, together with said carrier (16), the measuring element (14), the pressure transmission element (4) and a diaphragm (10) forming the base of the front part (8) of the housing> a measuring head (6) which is prestressed in the pressure transmission direction in a defined fashion, cannot be disassembled and makes contact with other components of the pressure sensor (1) and with the through-hole (2) exclusively via an outer flange (20) which is arranged on the outer circumference of * the measuring head (6). 2. Pressure sensor according to Claim 1, charac¬terized in that the outer flange (20) is arranged on the outer circumference of the front part (8) of the housing and, in order to mount the measuring head (6) in the through-hole (2), is clamped in between a shoulder (22) which reduces the diameter of the through-hole (2) and a rear part (24) of the housing which is screwed into the through-hole (2) of the wall. 3. Pressure sensor according to Claim 2, charac¬terized in that the rear part (24) of the housing has an external thread (26) which is screwed into an internal thread of the through-hole (2) and which is arranged with axial spacing from the measuring head (6). 4 m Pressure sensor according to one of Claims 2 or 3, characterized in that an end of the rear part (24) of the housing which points t6wards the measuring head (6) is of step-shaped design such that a relatively long stepped part (28) is clamped against the outer flange (20) of the front part (8) of the housing and is con¬nected to the outer flange (20) at its radially outer circumferential edge by means of a second welded con¬nection (30) , the rear part (24) of the housing and the front part (8) of the housing overlapping one another in the radial direction with radial spacing, and a rela¬tively short stepped part (32) of the end, pointing towards the measuring head (6) , of the rear part (24) of the housing lying opposite the end face of the front part (8) of the housing with axial spacing. 5. Pressure sensor according to one of Claims 2 to 4, characterized in that the shoulder (22) of the through-hole (2) has a circumferential sealing lip (34) , against which the outer flange (20) of the front part (8) of the housing is clamped. 6. Pressure sensor according to one of the preceding claims, characterized in that the measuring element carrier is formed by a metal plate (16) whose outer circumferential face is connected to the inner circum¬ ferential face of the front part (8) of the housing by means of the first welded connection (18) and on whose side pointing towards the pressure transmission element (12) there is arranged the measuring element (14), which preferably operates according to the piezo-resistive measuring principle. 7. Pressure sensor according to one of the preceding claims, characterized in that the front part (8) of the housing is thin-walled in the region of the first welded connection (18) to the measuring element carrier (16) , and in that the first welded connection (18) is manufac¬tured by means of a laser welded connection. 8. Pressure sensor according to one of the preceding claims, characterized in that the wall contains a central locking screw (4) , having the through-hole (2) , of a fuel injection pump. 9. Pressure sensor according to one of the preceding claims, characterized in that^the_rdiaphragm (10) is welded to the front part (8) of the housing, and the wall thickness of the diaphragm (10) is small in comparison with the wall thickness of the front part (8) of the housing. 10. Pressure sensor, substantially as herein described, with reference to the accompanying drawings.

Documents:

1569-mas-1998-abstract.pdf

1569-mas-1998-claims filed.pdf

1569-mas-1998-claims granted.pdf

1569-mas-1998-correspondnece-others.pdf

1569-mas-1998-correspondnece-po.pdf

1569-mas-1998-description(complete)filed.pdf

1569-mas-1998-description(complete)granted.pdf

1569-mas-1998-drawings.pdf

1569-mas-1998-form 1.pdf

1569-mas-1998-form 26.pdf

1569-mas-1998-form 3.pdf

1569-mas-1998-other documents.pdf

abs-1569-mas-1998.jpg