A VERY-HIGH-PRESSURE TRANSDUCER, PARTICULARLY FOR DETECTING THE PRESSURE OF A HYDRAULIC FLUID

Abstract: The transducer comprises a support structure (1,10,15) carrying piezoresistive sensors (8), particularly of the thick-film type, which are intended to be resiliently deformed by the fluid pressure to be measured and to provide an electrical signal indicative of the value of the pressure. The support structure comprises a substantially mushroom-shaped metal body (1) with a head (2) and a shank (3). An axial blind hole (4) is formed in the shank (3) and is intended to contain the fluid whose pressure is to be measured. The hole (4) also extends into the head (2) and, with the end surface of ti′e head, defines a wall (6) which is resiliently deformable by the pressure to be measured. The piezoresistive sensors (8) are connected to the end surface (2a) of the head (2) of the mushroom-shaped body (1) in correspondence with or adjacent the deformable wall (6) so that they can in turn be deformed as a result of the deformation of the wall. The sensors (8) are deposited on a thin plate (7) fixed to the upper surface of the head (2) of the mushroom-shaped body (1). The plate also carries the components (9) of the circuits for amplifying and processing the signals generated by the sensors (B). A metal structure (10,17) is also connected to the mushroom-shaped body (1) to screen it from radio-frequency interference.

Description: The present invention relates to a high pressure transducer, particularly for use in motor vehicles, of the kind defined in the first part of Claim 1. A transducer of this kind is disclosed in US-A-4 712 082. This known transducer comprises a tubular housing having an upper portion which surrounds with radial play the head of a mushroom-shaped body carrying a strain-sensitive silicon chip, and an externally threaded lower portion which extends around and along the entire length of the shank of said body. The structure of this transducer is such that the pressure under measure acts on both the housing and said body tending to displace one with respect to the other. Another transducer having a substantially mushroom-shaped body with a threaded shank is disclosed in WO-A-8 602 999. The object of the present invention is to produce an improved high-pressure transducer of the type indicated above, which enables the detection of very high pressures of the order of or greater than 1000 bars, for example, for measuring the fuel pressure in a direct-injection diesel engine. According to the invention, this object is achieved by means of a high-pressure transducer of the type specified above, whose main novel features are defined in the second part of Claim 1. The transducer preferably includes a plurality of piezoresistive sensors which are interconnected so as to form a bridge circuit. Moreover, the sensors are conveniently deposited by a silk-screen printing technique on a thin plate fixed to the upper surface of the head portion of the mushroom-shaped body by a layer of vitreous glue. To advantage, the plate also carries the components of a circuit for amplifying and processing the signals generated by the piezoresistive sensors in operation. A metal screening structure is also conveniently connected to the mushroom-shaped body and surrounds the piezoresistive sensors and the circuits for amplifying and processing the signals in order to eliminate the effects of radio-frequency interference. Further characteristics and advantages of the high-pressure transducer according to the invention will become clear from the detailed description which follows with reference to the appended drawings, provided by way of non-limiting example, in which: Figure 1 is a perspective view of one embodiment of the transducer according to the invention, and Figure 2 is an axial section taken on the line II-II of Figure 1. The very-high-pressure transducer shown in the drawings comprises a substantially mushroom-shaped metal body 1 with a circular head 2 and a shank 3. As can be seen in Figure 2, an axial blind hole 4 is formed in the shank 3 and also extends partly into the head 2, ending a predetermined distance d from the flat, upper end surface 2a of the head. The lower end portion of the shank 3 has external threading 5 which enables it to be coupled to a tube or an opening of a chamber containing the hydraulic fluid whose pressure is to be measured. A wall 6 is defined between the end surface of the hole 4 which is intended to contain the fluid and the upper surface 2a of the head 2 (Figure 2) and is resiliently deformable by the pressure to be measured. The upper face of a plate of electrically insulating material, for example ceramics (alumina), indicated 7, carries thick-film resistors 8 which are deposited by known silk-screen printing techniques. The thick-film resistors are conveniently interconnected electrically so as to form a bridge circuit, and are deposited on a central portion of the plate 7. On the same face,the plate 7 carries the components 9 of a circuit for supplying, amplifying and processing the signals generated by the thick-film resistors 8 in operation. The plate 7 is fixed to the upper surface 2a of the head 2 of the body 1 by the interposition of a layer of vitreous glue, so that the thick-film resistors are located in correspondence with or adjacent the wall 6 (Figure 2). In operation, the wall 6 is deformed to an extent dependent on the pressure of the fluid introduced into the hole 4. This deformation causes a corresponding deformation of the plate 7 and the thick-film resistors 8. The corresponding variation in the resistance of the resistors enables the generation of an electrical signal indicative of the value of the pressure of the fluid introduced into the hole 4. A substantially cup-shaped body is indicated 10 and has a base wall with a central aperture, indicated 11 in Figure 2. The body is conveniently made of metal, particularly aluminium, and is arranged around the head portion 2 of the body 1 so that the shank 3 extends through its aperture 11. The body 10 is fixed to the body 1 by means of screws 12 (only one of which is visible in Figure 2). A sealing ring 13 is clamped between the body 10 and an annular circumferential groove in the head portion 2 of the body 1. In the embodiment illustrated, the side wall of the body 10 extends beyond the upper end surface 2a of the head 2 of the body 1. The upper end edge of the side wall of the body 10 has a groove 14, in the bottom of which a sealing ring 16 is positioned and in which the circumferential edge of a cover element, indicated 15, is then engaged. A metal diaphragm 17 is gripped between the cover element 15 and the body 10 and extends over the plate 7 and the components fitted thereto. The diaphragm acts as a screen against radio-frequency interference: together with the body 10 and the body 1, it forms a closed structure which encloses the piezoresistive sensors 8 and the associated circuits for supplying, amplifying and processing the signals. The cover element 15 is connected to the body 10 by the upsetting of the outer edge part 10a of the top of the side wall of the body, as can be seen in particular in Figure 2. The electrical supply of the piezoresistive sensors 8 and the associated circuits carried by the plate 7 and the picking up of the signals generated thereby take place through rod-shaped conductor terminals 18 (Figure 2) having respective bent lower ends connected to the circuits carried by the plate 7. The conductive terminals 18 extend through insulating elements 19 located in apertures in the diaphragm 17 and then pass through the cover element 15 so as to be accessible from the outside. The elements 19 insulate the terminals 18 from the diaphragm 17, forming circular coupling capacitors therewith. In the embodiment illustrated, shaped conductor blades 20 connected to the conductive terminals 18 extend through the cover element 15 and have respective arms 20a which extend within a quadrangular annular projection 21 integral with the cover element 15, forming therewith an electrical connector. Naturally, the connecting terminals of the transducer may be made in many different ways from that illustrated by way of non-limiting example in the appended drawings. As stated above, in operation, the pressure of the fluid introduced into the hole 4 causes the deformation of the wall 6 and a corresponding deformation of the piezoresistive sensors 8 which cause the generation of an electrical signal indicative of the pressure of the fluid. The wall 6 conveniently has a thickness d which is somewhat less than the general thickness of the head 2 so that, in practice, the deformation due to the pressure is localised just in correspondence with the portion 6 of the wall. The high-pressure transducer according to the invention can easily be made so that it is usable for measuring pressures in various ranges of values. In particular, a different diameter of the hole 4 and a different thickness d of the wall 6 will correspond to each range of pressure values measurable. In particular, the transducer enables the measurement of pressures of the order of or greater than 1000 bars. It can therefore be used, as already stated above, for measuring the fuel pressure in a direct-injection diesel engine. The pressure transducer according to the invention may also conveniently be used in systems for the automatic control of motor vehicle suspensions.

Claims: A high-pressure transducer comprising a support structure (1, 10, 15) including: a substantially mushroom-shaped metal body (1) having a head (2) and a shank (3) which is adapted to be inserted into a tube or a chamber containing the fluid whose pressure is to be measured; an axial blind hole (4) being formed in the shank (3) for receiving said fluid; the hole (4) extending into the head (2) and defining, with the upper surface (2a) of the head (2), a wall (6) which is resiliently deformable by the pressure to be measured; a hollow housing (10) arranged around the head (2) of said metal body (1) and fixedly coupled thereto; a cover member (15) coupled to said housing (10) and carrying electrical connection means (20, 20a); said support structure (1, 10, 15) being provided with a threading (5) for connection with a corresponding threading of said tube or chamber; and at least one piezoresistive sensor (8) connected to the upper surface (2a) of the head of said body (1) in correspondence with or adjacent the deformable wall (6) so as to be deformed as a result of the deformation of said wall (6), and coupled to said electrical connection means (20, 20a); characterized in that the housing (10) and the mushroom-shaped body (1) are detachably connected with each other, the head (2) of said body (1) being radially in contact with the housing (10), the shank (3) of said body (1) having a terminal portion which protrudes outside the housing (10) and which is provided with the said threading (5). A transducer according to Claim 1, characterised in that the at least one piezoresistive sensor (8) is deposited on a thin plate (7) glued to the upper surface (2a) of the head portion (2) of the mushroom-shaped body (1). A transducer according to Claim 1 or Claim 2, characterised in that it includes a plurality of piezoresistive sensors (8). A transducer according to Claim 2 or Claim 3, characterised in that the plate (7) also carries the components (9) of circuits for amplifying and processing the signals generated by the piezoresistive sensor or sensors (8) in operation. A transducer according to Claim 4, characterised in that a metal screening structure (10, 17) is connected to the mushroom-shaped body (1) and surrounds the piezoresistive sensor or sensors (8) and the circuits (9) for amplifying and processing the signals.

Inventors: DELL'ORTO, GIUSEPPE, ROSSI, GIUSEPPINA, TIMOSSI, GIAN MARIA
Applicants: MARELLI AUTRONICA S.P.A.
Application year: 1989
Main classification: G01L9
IPC codes: G01L9