Pressure-detecting device and method of manufacturing the same

One end of a rod-like pressure-conveying member is disposed in a sensing unit, and the other end extends into and through an insertion hole of an engine. A combustion pressure, to which the other end of the rod-like member is exposed, is conveyed to the sensing unit through the pressure-conveying member for the detection of the combustion pressure. The pressure-conveying member resonates at a knocking frequency fn of the engine and the knocking frequency fn is detected based on the resonance of the pressure-conveying member.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Applications No. 2004-173708 filed on Jun. 11, 2004, and No. 2004-191244 filed on Jun. 29, 2004, the disclosure of which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a pressure-detecting device wherein the combustion pressure of an engine is conveyed through a rod-like pressure-conveying member to a sensing unit which outputs a signal in accordance with the conveyed pressure. The pressure-detecting device can be applied to sensors of combustion pressure.

RELATED ART OF THE INVENTION

Pressure-detecting devices, each comprising a sensing unit, which outputs signals in accordance with pressure, and a rod-like pressure-conveying member, of which one end is disposed on the side of the sensing unit and of which the other end extends out of the sensing unit, have already been proposed (for example, JP-5-34231A). A strain gauge, for example, is used as the sensing unit.

The pressure-detecting devices with a rod-like pressure-conveying member are applied to sensors of combustion pressure (so-called cylinder pressure) of engines and the like.

Specifically, the end of the rod-like pressure-conveying member extending out of the sensing unit is inserted into a hole of an engine. The combustion pressure of the engine working on the end of the rod-like pressure-conveying member is conveyed to the sensing unit through the pressure-conveying member for the detection of the combustion pressure.

When knocking signals are measured by a sensor of combustion pressure of prior art, the resonance point of the pressure-conveying member of the sensor overlaps the frequency of knocking signals and knocking signals of small magnitude of pressure are drowned out by the noise due to the resonance of the pressure-conveying member. Thus, the measurement of knocking signals is relatively difficult.

Referring toFIG. 5, the above problem will be described more specifically.FIG. 5shows an ordinary waveform of combustion pressure Wo. A waveform of knocking Wnis put on the waveform of combustion pressure Wo. “f0” and “fninFIG. 5stand for the frequency of combustion pressure and the frequency of knocking, respectively.

Because the frequency of knocking fnis considerably higher than the frequency of combustion pressure f0, combustion-pressure signals of which frequency f0is lower than the frequency of knocking fnare eliminated by a low-pass filter (LPF). Thus, the pressure signals of knocking can be extracted.

However, pressure signals of knocking are very small as compared with pressure signals of combustion pressure. Besides, various noises such as resonance of the pressure-conveying member with knocking and mechanical noises of the engine, or pseudo-knocking, may overlap the pressure signals of knocking.

In this case, the pressure signals of knocking are buried under the noises and it is difficult to distinguish the former from the latter. Therefore, the resonance frequency of the pressure-conveying member has to be kept away from the frequency band of knocking by adjusting the size of the pressure-conveying member and so on.

If the resonance frequency of the pressure-conveying member is kept away from the frequency band of knocking so that the noise due to the resonance of the pressure-conveying member does not overlap the pressure signals of knocking, the pressure signals of knocking remain small and errors may occur in distinguishing the pressure signals of knocking from the noise.

In the above conventional pressure-detecting device, the present inventor tried placing a pressure-sensing element closer to the side of the pressure-detecting environment in order to improve the sensor characteristics. Namely, the inventor tried decreasing the distance between a pressure-receiving diaphragm and the pressure-sensing element by making the pressure-conveying member shorter.

The reason for shortening the pressure-conveying member is as follows: When the pressure-detecting device is applied to a sensor of combustion pressure of an engine, for example, if the pressure-conveying member is long, the resonance frequency of the pressure-conveying member overlaps the frequency of knocking signals, which causes the resonance of the pressure-conveying member.

Then, knocking signals of small magnitude of pressure are drowned out by the noise due to the resonance of the pressure-conveying member.

Further, if the pressure-conveying member is long, the pressure-conveying member itself is liable to deform. Thus, the condition of contact between the pressure-conveying member and the pressure-receiving diaphragm and the condition of contact between the pressure-conveying member and the pressure-sensing element may change. When such a change in the condition of contact arises, the deterioration in the pressure-conveying precision is caused, affecting the sensor characteristics.

In this regard, if the pressure-sensing element is placed closer to the side of the pressure-detecting environment, the pressure-sensing element becomes closer to the pressure-receiving diaphragm to reduce the length of the pressure-conveying member. Thus, the problems of resonance and deformation liable to occur to the long pressure-conveying member can be minimized.

Conventionally, in the pressure-detecting device of this kind, a housing constituting the body of the device is provided with signal processing units such as a connector unit to take out signals from the pressure-sensing element. In the conventional device, the pressure-sensing element and the signal processing units are electrically connected by wire bonding or the like.

However, when the pressure-sensing element is placed closer to the side of the pressure-detecting environment, the distance between the pressure-sensing element and the signal processing units increases as much as the length of the pressure-conveying member shortened. Accordingly, when electrically connecting the pressure-sensing element with the signal processing units, the conventional wire bonding cannot serve the purpose.

In the case when connecting the pressure-sensing element and the signal processing units placed so apart that wire bonding cannot serve the purpose, considering the ease of handling, size reduction, ease of connection and so on, the inventor determined to use a flexible printed circuit board as a means of connection.

For decreasing the distance between the pressure-sensing element and the pressure-receiving diaphragm by shortening the length of the pressure-conveying member, and for using a flexible printed board to connect the pressure-sensing element with the signal processing units including the connector unit, the inventor built a prototype of the pressure-detecting device shown inFIG. 10.

FIG. 10is a schematic sectional view showing a whole configuration of a pressure-detecting device built as a prototype by the inventor.

The pressure-detecting device can be applied to a sensor of combustion pressure of an engine. In this case, a pipe section312of the housing310is threadedly engaged with a threaded hole of an engine block of the engine and the pressure-detecting device detects the pressure inside the combustion chamber (cylinder pressure) of the engine.

The metal housing310comprises a cylindrical body311and the cylindrical pipe section312extending from the body311. A male thread section313is formed on the periphery of the pipe section312of the housing310for the thread engagement with the engine block.

In the pressure-detecting device, the tip of the pipe section312of the housing310is fitted with a pressure-sensing element330which outputs signals in accordance with pressure. The pressure works on the pressure-sensing element330as shown by the arrow “Y” inFIG. 10.

The pressure-sensing element330is fitted, with molten glass, onto the surface of a diaphragm322of a hollow cylindrical metal stem320. The metal stem320serves as a support and has an opening321at one end and the diaphragm322at the other end.

A pressure-conveying member316is provided in the hollow of the metal stem320, and a pressure-receiving diaphragm315is welded onto the top of the metal stem320so as to cover the opening321.

The pressure is, as shown by the arrow “Y” inFIG. 10, conveyed from the pressure-receiving diaphragm315to the rear surface of the diaphragm322of the metal stem320through the pressure-conveying member316.

When the diaphragm322of the metal stem320deforms under pressure, an electric signal corresponding to the deformation is outputted from the pressure-sensing element330.

As shown inFIG. 10, a circuit board340is provided in the body311of the housing310. Also, an IC chip342is fixed onto the upper surface of the circuit board340by gluing, and formed on the IC chip342is a circuit to process signals outputted from the pressure-sensing element330.

The IC chip342and the circuit board340are electrically connected by bonding wires344. Besides, the circuit board340and the pressure-sensing element330are electrically connected by a flexible printed circuit board350. The flexible printed circuit board350is provided such that it extends in the pipe section312of the housing310along the length of the pipe section312.

One end351of the flexible printed circuit board350is electrically joined to the pressure-sensing element330, and a portion on the side of the other end352extends through the pipe section312toward the circuit board340. The other end352of the flexible printed circuit board350is electrically connected to the circuit board340by soldering or the like.

Further, a connector case360with a terminal361is disposed below the circuit board340in the housing310. The connector case360is provided as a connector unit to take out signals from the pressure-sensing element330.

The terminal361of the connector case360and the circuit board340are electrically connected by a spring362. Thus, the pressure-sensing element330is electrically connected to the connector case360, or the connector unit360, through the flexible printed circuit board350and the circuit board340.

According to the pressure-detecting device shown inFIG. 10, the pressure-sensing element330is disposed at the tip of the pipe section312to minimize the length of the pressure-conveying member as compared with the prior art pressure-conveying member in which the pressure-conveying member extends along the whole length of the pipe section. In this example, the pressure-conveying member316is housed in the metal stem320, the former being as substantially short as the latter.

As in the conventional method for manufacturing the pressure-detecting device of this kind, in the present prototype, after the pressure-sensing element330is fixed to the metal stem320serving as a support with molten glass or the like, the pressure-receiving diaphragm315is welded to the metal stem320while the pressure-conveying member316is put between the pressure-sensing element330and the pressure-receiving diaphragm315.

Then, in the assembly of the metal stem320, the pressure-sensing element330, and pressure-receiving diaphragm315, the flexible printed circuit board350is connected to the pressure-sensing element330and, while inserting the flexible printed circuit board350into the pipe section312of the housing310, the metal stem320is mounted on the tip of the pipe section312.

The pressure-receiving diaphragm315is welded while a load is applied to the surface of the pressure-receiving diaphragm315so that the rear side of the pressure-receiving diaphragm315presses the diaphragm322of the metal stem320through the pressure-conveying member316.

In such a manufacturing method, when welding the pressure-receiving diaphragm315, the pressure-conveying member316applies a load to the diaphragm322of the metal stem320serving as a support and the pressure-receiving diaphragm315. There are following two reasons why the welding is conducted while applying a load.

The first reason is as follows: If both the ends of the pressure-conveying member316are in contact with the diaphragm322of the metal stem320and the pressure-receiving diaphragm315, almost no loads being applied to them, the pressure-conveying member316loses contact with at least one of the diaphragms322and315when the pressure-conveying member316contracts due to its linear expansion coefficient.

The second reason is as follows: When the pressure-detecting device is applied to a sensor of combustion pressure of an engine, the pressure inside the combustion chamber may become negative. Accordingly, the pressure-receiving diaphragm315may bulge out and lose contact with the pressure-conveying member316.

Thus, to secure the contact between the pressure-conveying member316and the metal stem320as well as the pressure-receiving diaphragm315, by welding the pressure-receiving diaphragm315while applying a load to it, both the ends of the pressure-conveying member316are put in contact with the diaphragm322of the metal stem320and the pressure-receiving diaphragm315after the assembly so as to apply certain loads to the diaphragms322and315.

However, the method of welding the pressure-receiving diaphragm315to the metal stem320after fixing the pressure-sensing element330to the metal stem320serving as a support with molten glass or the like has following problems.

The first problem is as follows: The influence of the heat caused by welding is not negligible since the pressure-conveying member316is shortened and the distance between the welded portion of the pressure-receiving diaphragm315and the pressure-sensing element330is decreased. Therefore, the heat caused by the welding of the pressure-receiving diaphragm315gives thermal damage to the pressure-sensing element330on the metal stem320.

The second problem is as follows: After gluing the pressure-sensing element330onto the metal stem320, the pressure-receiving diaphragm315is welded while a load is applied from the pressure-conveying member316to the pressure-sensing element330through the diaphragm322of the metal stem320. Therefore, the load applied remains in the pressure-sensing element330after the welding of the pressure-receiving diaphragm315, and this leftover load causes an offset of the output.

Thus, the pressure-detecting device, wherein the pressure-sensing element outputting signals in accordance with pressure is fixed to the support, the pressure-conveying member is put between the pressure-sensing element and the pressure-receiving diaphragm, and the pressure-receiving diaphragm is welded to the support has problems such as thermal damage to the pressure-sensing element during the welding of the pressure-receiving diaphragm and the offset of the output caused by the load during the welding.

SUMMARY OF THE INVENTION

Under the circumstances, the object of the present invention is to provide a pressure-detecting device wherein the combustion pressure of an engine is conveyed through a rod-like pressure-conveying member to a sensing unit for outputting a signal in accordance with the conveyed pressure and which is capable of detecting knocking more reliably.

Also, in view of the above, another object of the present invention is to provide a pressure-detecting device wherein when the pressure-receiving diaphragm is welded to the support, the pressure-sensing element is prevented from being thermally damaged and being exposed to a load which may cause the offset.

The pressure-detecting device of the present invention comprises a sensing unit, which outputs signals in accordance with pressure, and a rod-like pressure-conveying member, of which one end is disposed on the side of the sensing unit and of which the other end extends out of the sensing unit, into a hole made in an engine. The combustion pressure working on the end of the pressure-conveying member, which is inserted in the hole, is conveyed through the pressure-conveying member to the sensing unit for the detection of the combustion pressure. The pressure-conveying member is designed to resonate at the frequency of knocking of the engine, and the frequency of knocking is detected based on the resonance of the pressure-conveying member.

Because the knocking frequency is extracted based on the resonance of the pressure-conveying member according to the present invention, it will do for the detection of knocking if it is checked whether a signal of knocking frequency has been outputted or not, no matter how large or small the amplitude of the signal of knocking frequency is.

Accordingly, the pressure-detecting device of the present invention is capable of detecting knocking more reliably than the pressure-detecting devices of prior art.

To achieve the above objects, according to the present invention, the following aspect characterizes the method for manufacturing a pressure-detecting device wherein the pressure-sensing element outputting signals in accordance with pressure is fixed to the support, and the pressure-receiving diaphragm is welded to the support while the pressure-conveying member is put between the pressure-sensing element and the pressure-receiving diaphragm, and the pressure working on the pressure-receiving diaphragm is conveyed through the pressure-conveying member to the pressure-sensing element for the detection of the pressure.

Namely, according to the present manufacturing method, the pressure-conveying member is put between the support and the pressure-receiving diaphragm, the pressure-receiving diaphragm is welded to the support while the pressure-receiving diaphragm applies a load to the support through the pressure-conveying member, and then the pressure-sensing element is fixed to the support.

According to such a method, the pressure-receiving diaphragm is welded to the support while the pressure-receiving diaphragm applies a load to the support through the pressure-conveying member before the pressure-sensing element being fixed to the support.

Thus, the contact between the pressure-conveying member and the support and the contact between the pressure-conveying member and the pressure-receiving diaphragm are secured.

Besides, when the pressure-receiving diaphragm is welded to the support while a load is applied, the support is not fitted with the pressure-sensing element; therefore, the pressure-sensing element is not thermally damaged by the welding or exposed to a load.

Thus, according to the present invention, when the pressure-receiving diaphragm is welded to the support, the pressure-sensing element is prevented from being thermally damaged and being exposed to a load which may cause an offset.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first embodiment of the present invention will be described below by referring to drawings.FIG. 1is a schematic sectional view of the pressure-detecting device100according to the first embodiment. The pressure-detecting device100is fitted to an engine200.

The pressure-detecting device100has a housing10with a pipe section10c, and the pipe section10cengages threadedly in a hole201of the engine200. The pressure-detecting device100can be applied to a so-called combustion-pressure sensor to detect the pressure in the combustion chamber202(cylinder pressure) of the engine200.

The housing10has a first hollow cylindrical division10aand a second division10b, and the second division10bhas the pipe section10cwhich is thinner than the first division10a. The first and second divisions10aand10bare made of a metal by cutting or cold forging.

The housing10is made of stainless steel or the like, and the one end of the first division10ais inserted into an expanded section of the second division10bby pressure fitting. Thus, the first division10ais fixed to the second division10b.

The first division10amay be joined to the second division10bby welding or gluing, or the former may threadedly engage in the latter.

The pipe section10chas a male-thread section11on the periphery thereof and female threads are formed inside the hole201of the engine200so that the pipe section10ccan threadedly engage in the hole201.

The pipe section10cis inserted in the hole201and fixed in the hole201through the medium of the male-thread section11so that the tip10dof the pipe section10cfaces the combustion chamber202. Thus, the pressure-detecting device100is fitted to the engine200.

A sensing unit20is fitted, threadedly or by press fitting, into the first division10aof the housing10. Thus, the sensing unit20is housed in the housing10.

The sensing unit20comprises a hollow cylindrical metal stem21and a strain gauge30. The metal stem21has an opening22at its top and a diaphragm23at its bottom as seen inFIG. 1. The strain gauge30is fixed on the diaphragm23with molten glass.

The metal stem21is a hollow cylindrical metal part, which is press-fitted into and fixed in the hollow of the first division10aof the housing10. The metal stem21may threadedly engage in the housing10.

As shown inFIG. 1, the opening22of the metal stem21is disposed near the top of the first division10aand the diaphragm23of the metal stem21is disposed near the bottom of the first division10a.

The strain gauge30may be a silicon-semiconductor chip with a bridge circuit of diffused resistive elements, etc.

When the diaphragm23of the metal stem21deforms under pressure, the strain gauge30deforms and its resistance value changes accordingly. The change of the resistance value is converted into an electric signal and outputted. The diaphragm23of the metal stem21and the strain gauge30determine the basic performance of the pressure-detecting device100.

The material of the metal stem21must be strong and of a low coefficient of thermal expansion because the metal stem21is exposed to high pressure and the strain gauge30comprising a silicon semiconductor, etc. has to be fixed to the metal stem21with glass of low melting point.

To be specific, the metal stem21may be made of an alloy of Fe, Ni, and Co or an alloy of Fe and Ni containing precipitation-enhancing materials of Ti, Nb, and Al or Ti and Nb. The metal stem21may be made by pressing, cutting, or cold forging.

A first circuit board40comprising a ceramic board, etc. is provided around the diaphragm23of the metal stem21. The first circuit board40is fixed, by gluing, to the first division10aof the housing10.

The strain gauge30and the first circuit board40are electrically connected by bonding wires42of aluminum (Al), gold, or the like.

A second circuit board50comprising a ceramic board, etc. is disposed under the first circuit board40as seen inFIG. 1. An IC chip44is glued onto the top surface of the second circuit board50as seen inFIG. 1.

The IC chip44is provided with a circuit to amplify and modulate the signals outputted from the strain gauge30. The second circuit board50and the IC chip44are electrically connected by bonding wires42of aluminum (Al), gold, or the like.

A spring45is provided between the first and second circuit boards40and50so as to connect them electrically.

The spring45is a conductive elastic body. For example, it is connected to one of the first and second circuit boards40and50by brazing or soldering and is kept in contact with the other board by its elasticity. Thus, the first and second circuit boards40and50are electrically connected.

A connector case70with a terminal61for external connection is provided on the bottom surface of the second circuit board50.

The connector case70is made of PPS (polyphenylene sulfide) resin or the like. The terminal61is inserted in the connector case70, both the parts formed as a unit.

A conductive connector63is disposed between the second circuit board50and the terminal61to connect them electrically. The conductive connector63may be a rubber block wherein a plurality of metallic pins are anisotropically arranged, or a spring, or a conductive adhesive.

The strain gauge30, IC chip44, first and second circuit boards40and50, and terminal61may be connected by other means of connection than the above bonding wires42, spring45, and conductive connector63.

As shown inFIG. 1, the bottom end12of the housing10is bent onto part of the connector case70so as to join the housing10and the connector case70as a unit. The terminal61can electrically be connected to the ECU of the vehicle through the medium of a cable or the like.

On the other hand, as shown inFIG. 1, the pipe section10cof the second division10bprotrudes from the part of the housing10wherein the sensing unit20is housed.

A pressure-receiving diaphragm13is welded to the tip10dof the pipe section10c. The opening of the tip10dis stopped up by the pressure-receiving diaphragm13.

The pressure-receiving diaphragm13faces the inside of the combustion chamber202and is exposed to the pressure inside the combustion chamber202as shown by the arrow Y inFIG. 1. The pressure-receiving diaphragm13is made of stainless steel or the like.

A pressure-conveying member80is inserted in the pipe section10c. The bottom end, as seen inFIG. 1, of the pressure-conveying member80is disposed on the side of the sensing unit20and the top end, as seen inFIG. 1, of the pressure-conveying member80is disposed on the side of the pressure-receiving diaphragm13. The pressure-conveying member80is made of stainless steel or the like.

The bottom end of the pressure-conveying member80is inserted in the metal stem21so as to come in contact with and apply a load to the diaphragm23. The top end of the pressure-conveying member80is inserted in the pipe section10cso as to come in contact with and apply a load to the pressure-receiving diaphragm13.

Thus, both the ends of the pressure-conveying member80are in contact with the diaphragms13and23, applying certain loads to them, for the reason described below.

If both the ends of the pressure-conveying member80are in contact with the diaphragms13and23, applying almost no loads to them, the pressure-conveying member80loses contact with at least one of the diaphragms13and23when the pressure-conveying member80contracts due to its linear expansion coefficient.

Besides, when the pressure inside the combustion chamber202becomes negative, the pressure-receiving diaphragm13bulges out into the combustion chamber202and may lose contact with the pressure-conveying member80.

Thus, to secure the contact between the pressure-conveying member80and the diaphragms13and23, both the ends of the former are put in contact with the latter so as to apply certain loads to the latter.

As described above, the rod-like pressure-conveying member80is so disposed that its one end is on the side of the sensing unit20and its other end extends out of the sensing unit20, into and through the pipe section10cin the hole201of the engine200.

The pressure inside the combustion chamber202is conveyed to the sensing unit20through the pressure-receiving diaphragm13and the pressure-conveying member80as shown by the arrow inFIG. 1.

Then, the diaphragm23of the metal stem21deforms under the conveyed pressure and the strain gauge30deforms accordingly. An electric signal in accordance with the deformation of the strain gauge30is outputted from the strain gauge30, and the combustion pressure is detected based on the signal.

The pressure-conveying member80is designed to resonate at the frequency fnof knocking of the engine200(seeFIG. 5), and the frequency fnof knocking is detected based on the resonance of the pressure-conveying member80.

To make the pressure-conveying member80resonate at the knocking frequency fn, the resonance frequency of the pressure-conveying member80is so adjusted that it is equal or near to the knocking frequency fn.

The knocking frequency fnis a value peculiar to each engine and determined by the bore of the cylinder; therefore, the resonance frequency of the pressure-conveying member80is adjusted in accordance with the knocking frequency fnof the engine200.

The resonance frequency of the pressure-conveying member80is adjusted and set at a frequency of the engine200at which there is no mechanical vibration of pistons, cylinders, etc. Namely, it is desirable to separate the resonance frequency of the pressure-conveying member80from the pseudo-knocking frequency of the engine200and set the resonance frequency nearer to the knocking frequency fn.

The resonance frequency of the pressure-conveyi55ng member80may be adjusted as follows. The resonance frequency f of the pressure-conveying member80is represented by the expression below.
f=(½π)·(k/m)1/2(1)
where m and k are the mass and the spring constant, respectively, of the pressure-conveying member80. Accordingly, the resonance frequency f of the pressure-conveying member80can be adjusted by simply changing its length, diameter, material density, etc. to adjust its mass and spring constant.

Besides, the resonance frequency f of the pressure-conveying member80can be adjusted by making part or the whole of the pressure-conveying member80hollow. For example, a pipe-like pressure-conveying member80can easily be formed by extruding.

The pressure-conveying members of pressure-detecting devices of prior art are made of solid rods.

On the other hand, by making at least part of the pressure-conveying member80hollow, its weight can be reduced and its resonance frequency f can be changed without changing its length, diameter, or materials.

If the pressure-conveying member80is partly hollow, it may be made by joining a hollow section and a solid section, or by stuffing the hollow with the material constituting the pressure-conveying member80or another material having a resonance frequency similar to that of the material constituting the pressure-conveying member80.

FIGS. 2A to 2Eshows pressure-conveying members80of partly hollow construction. Each of the pressure-conveying members80ofFIGS. 2A to 2Eis made by inserting a solid rod80binto a pipe or pipes80aand welding or brazing them together.

Besides, as shown inFIG. 3, depressions81may be made in the surface of the pressure-conveying member80to reduce its weight. The depressions81may be made by cutting or pressing. In this case too, the weight of the pressure-conveying member80can be reduced and the resonance frequency f of the pressure-conveying member80can be changed without changing its length, diameter, or materials.

Accordingly, if knocking occurs, the pressure-conveying member80resonates at the knocking frequency fn.

Then, the resonance is conveyed to the strain gauge30through the diaphragm23of the metal stem21, the strain gauge30is deformed, and an electric signal in accordance with the deformation is outputted from the strain gauge30.

An example of the method of assembling the pressure-detecting device100will now be described. First, the metal stem21, to which the strain gauge30is fixed with molten glass, is press-fitted into the first division10aof the housing10.

Next, the first circuit board40is fixed to the first division10aby gluing or otherwise to dispose the first circuit board40around the metal stem21. Then, the first circuit board40and the strain gauge30are connected by the bonding wires42.

The first circuit board40and the second circuit board50, on which the IC chip44is mounted by wire bonding, are connected by the spring45.

Then, the connector case70is fitted into the first division10aof the housing10and the bottom end12of the first division10ais bent onto part of the connector case70so as to fix the connector case70to the first division10atogether. Next, the second circuit board50and the terminal61are connected by the conductive connector63.

Thus, the first division10a, into which the sensing unit20and the circuit boards40and50are built, and the connector case70are assembled into a unit. On the other hand, the pressure-receiving diaphragm13is welded to the tip10dof the pipe section10cof the second division10bby welding.

Next, one end of the pressure-conveying member80is inserted into the opening22of the metal stem21and the other end of the pressure-conveying member80is inserted in the pipe section10c. Then, the first division10ais press-fitted into the second division10b. Thus, the pressure-detecting device100is completed.

The pressure-detecting device100is mounted on the engine200by threadedly engaging the male-thread section11of the housing10into the hole201of the engine200.

When the pressure-receiving diaphragm13is exposed to the pressure inside the combustion chamber202as shown by the arrow Y inFIG. 1, the pressure is conveyed to the sensing unit20through the pressure-conveying member80.

The pressure deforms the diaphragm23of the metal stem21and the strain gauge30converts the deformation into an electric signal and outputs the electric signal for the detection of the pressure. The electric signal is processed by the IC chip44and outputted to the outside through the terminal61.

By referring toFIG. 4, the workings of the pressure-detecting device100will now be described more specifically.FIG. 4shows the construction of the detector  circuit90of the pressure-detecting device100.

The detector circuit90is to detect the electric signals outputted from the strain gauge30and comprises the above IC chip44and first and second circuit boards40and50.

As shown inFIG. 4, the detector circuit90includes a band-pass filter (BPF)91and a band-elimination filter (BEF)92. The filter92may be a low-pass filter (LPF).

The band-pass filter91lets only the signals of knocking frequency fn(seeFIG. 5) pass. The band-elimination or low-pass filter92eliminates the signals of knocking frequency fn, etc. and lets the frequency signals of combustion pressure pass.

The electric signals outputted from the strain gauge30are sent to both the band-pass filter91and the band-elimination or low-pass filter92.

Accordingly, if knocking occurs, the strain gauge30outputs an electric signal of frequency fndue to the resonance of the pressure-conveying member80and the electric signal passes the band-pass filter91and is detected as a knocking signal. Thus, the knocking is detected.

When knocking does not occur, the strain gauge30does not output an electric signal of frequency fndue to the resonance of the pressure-conveying member80and no signal passes the band-pass filter91. Thus, no knocking is detected.

On the other hand, because signals of combustion pressure pass the band-elimination or low-pass filter92, they can be detected as signals from which signals of knocking frequency fnare eliminated if knocking occurs.

As described above, according to the first embodiment of the present invention, there is provided the pressure-detecting device100comprising (i) the sensing unit20which outputs signals in accordance with pressure and (ii) the rod-like pressure-conveying member80of which one end is disposed in the sensing unit20and of which the other end extends out of the sensing unit20, into and through the hole201of the engine200. The combustion pressure, to which the top end (as seen inFIG. 1) of the pressure-conveying member80is exposed, is conveyed through the pressure-conveying member80to the sensing unit20for the detection of the combustion pressure. The feature of the pressure-detecting device100is that the pressure-conveying member80resonates at the knocking frequency fnof the engine200and the knocking frequency fnof the engine200is detected based on the resonance of the pressure-conveying member80.

Because the knocking frequency fncan be extracted based on the resonance of the pressure-conveying member80, it will do for the detection of knocking if it is checked whether a signal of knocking frequency fnhas been outputted or not, no matter how large or small the amplitude of the signal of knocking frequency fnis.

Accordingly, the pressure-detecting device100is capable of detecting knocking more reliably than the pressure-detecting devices of the prior art.

Namely, as described earlier, the resonance frequency of the pressure-conveying member of the pressure-detecting device of the prior art is kept away from the frequency band of knocking so that the noise due to the resonance of the pressure-conveying member does not overlap the pressure signals of knocking. According to the first embodiment of the present invention, however, the pressure-conveying member80is allowed to resonate with knocking and a signal of the resonance frequency is detected.

If the pressure-conveying member80with depressions81for the adjustment of weight ofFIG. 3is used, the depressions81may be made before the pressure-detecting device100is mounted on the engine200as shown inFIG. 1or they may be made after the pressure-detecting device100is mounted on the engine200as shown inFIG. 1.

In the latter case, the housing10has suitably disposed holes. After the pressure-detecting device100is mounted on the engine200, a cutting tool is inserted into each hole to make a depression81.

Although the housing10of the above pressure-detecting device100is divided into the first division10aholding the sensing unit20and the second division10bfitted with the pressure-receiving diaphragm13, the housing10may be formed in one piece.

Although the above sensing unit20comprises a metal stem21and a strain gauge30, the sensing unit20may be constructed otherwise so long as it outputs signals in accordance with the pressure conveyed through the pressure-conveying member80.

Although there are provided the circuit boards40and50, IC chip44, bonding wires, etc. between the sensing unit20and the connector case70inFIG. 1, the construction can be changed.

The construction of the detector circuit90is not limited to the construction shown inFIG. 4so long as the detector circuit90is capable of detecting knocking frequency fnbased on the resonance of the pressure-conveying member80.

The main part of the present invention is that the combustion pressure of the engine200is conveyed to the sensing unit20through the rod-like pressure-conveying member80and that the pressure-conveying member80is allowed to resonate at the frequency fnof the knocking of the engine200and the frequency fnof the knocking is detected based on the resonance of the pressure-conveying member80. The other part of the present invention can be changed appropriately.

FIG. 6is a schematic sectional view showing a whole configuration of the second embodiment of pressure-detecting device300of the present invention.FIG. 7is an enlarged schematic sectional view of the tip portion of the pipe section312shown inFIG. 6.

The pressure-detecting device300can be used as a combustion-pressure sensor. In this case, the pipe section312is threadedly engaged with a threaded hole of the engine block of an engine and the pressure-detecting device300detects the pressure inside the combustion chamber of the engine.

The pressure-detecting device300has a housing310which comprises a hollow cylindrical body311and the above pipe section312which is in the shape of a thin, long hollow cylinder and thinner than the body311. The body311and the pipe section312are made of stainless steel or the like by cutting or cold forging. The pipe section312may be in the shape of a rectangular pipe.

The housing310may be made in one piece, or it may be made by making the body311and the pipe section312separately and then joining them together by welding, gluing or press-fitting, or threadedly, or otherwise.

Besides, a male-thread section313is formed on the periphery of the pipe section312for the thread engagement with the engine block. Thus, the housing310is constructed to have the thin, long pipe section312which protrudes from its one end.

The pressure-detecting device300is mounted on the engine block by threadedly engaging the male-thread section313in the above threaded hole of the engine block.

The pressure inside the combustion chamber works on the tip of the pipe section312as shown by the arrows inFIGS. 6 and 7.

The tip of the pipe section312is fitted with a pressure-sensing element330which outputs signals in accordance with pressure. The pressure-sensing element330may have a strain gauge's function of deforming under pressure and outputting a signal proportional to the magnitude of the pressure based on the magnitude of the deformation.

To be specific, as shown inFIG. 7, the pressure-sensing element330is fitted, with molten glass or otherwise, onto the bottom surface of a diaphragm322of a hollow cylindrical metal stem320. The metal stem320is the above support and has an opening321at its top and the diaphragm322at its bottom as seen inFIG. 7.

A flange323protruding outward is formed around the opening321of the metal stem320. The metal stem320may be in the shape of a rectangular pipe.

The part of the metal stem320under the flange323is inserted in the tip of the pipe section312. The flange323of the metal stem320and the tip of the pipe section312are joined together by gluing, welding, pressure welding, or the like.

As shown inFIG. 7, a diaphragm315is provided on the top of the metal stem320so as to cover the opening321. The diaphragm315is hereinafter referred to as “pressure-receiving diaphragm315” to distinguish it from the diaphragm322of the metal stem320.

The pressure-receiving diaphragm315is made of a metal such as stainless steel and in the shape of a disk, and its periphery is welded to the flange323of the metal stem320.

Thus, the pressure-receiving diaphragm315and the metal stem320are joined together. The pressure-receiving diaphragm315faces the combustion chamber and the combustion pressure works on it as shown by the arrows inFIGS. 6 and 7.

As shown inFIG. 7, a pressure-conveying member316is provided in the hollow of the metal stem320. Namely, the pressure-conveying member316lies between the pressure-receiving diaphragm315and the pressure-sensing element330. The pressure-conveying member316is made of ceramic or metal.

The bottom, as seen inFIG. 7, of the pressure-conveying member316is in contact with and applies a load to the diaphragm322, and the top, as seen inFIG. 7, of the pressure conveyor316is in contact with and applies a load to the pressure-receiving diaphragm315.

Accordingly, if the pressure-conveying member316contracts due to its linear expansion coefficient or if the pressure inside the combustion chamber becomes negative, the contact between the pressure-conveying member316and the diaphragms315and322is suitably maintained.

Although the pressure-conveying member316ofFIG. 7is in the shape of a sphere, it may be in any shape. The pressure inside the combustion chamber is conveyed from the pressure-receiving diaphragm315to the pressure-sensing element330through the pressure-conveying member316and the diaphragm322.

The pressure-sensing element330with the function of a strain gauge may be a silicon-semiconductor chip with a bridge circuit of diffused resistive elements, etc.

When the diaphragm322deforms under pressure, the pressure-sensing element330with the function of a strain gauge deforms accordingly, converts the change of its resistance due to its deformation into an electric signal, and outputs the electric signal.

The diaphragm322of the metal stem320and the pressure-sensing element330constitute a deforming unit which deforms under pressure. This deforming unit determines the basic performance of the pressure-detecting device300.

The material of the metal stem320must be strong and of a low coefficient of thermal expansion because the metal stem320is exposed to high pressure and the pressure-sensing element330comprising a silicon semiconductor, etc. has to be fixed to the diaphragm322with glass of low melting point.

To be concrete, the metal stem320may be made of an alloy of Fe, Ni, and Co or an alloy of Fe and Ni containing precipitation-enhancing materials of Ti, Nb, and Al or Ti and Nb, such as precipitation-hardening stainless steel. The metal stem320may be made by pressing, cutting, or cold forging.

As shown inFIG. 6, a circuit board340consisting of a ceramic board, etc. is provided in the body311of the housing310. The circuit board340is disposed to cover the lower opening of the pipe section312. The periphery of the circuit board340is fixed to the housing310by gluing or otherwise.

An IC chip342is fixed onto the upper surface of the circuit board340by gluing or the like, and formed on the IC chip342is a circuit to amplify and modulate signals outputted from the pressure-sensing element330.

The circuit board340and the IC chip342are electrically connected by bonding wires344of aluminum, gold, or the like. Besides, the pressure-sensing element330and the circuit board340are electrically connected by a wiring member350.

The wiring member350ofFIGS. 6 and 7is a flexible printed circuit board (FPC)350, but may be a lead wire or the like.

The flexible printed circuit board350may be a baseboard of polyimide resin on which a circuit of a conductor such as copper is laid. As shown inFIG. 6, the flexible printed circuit board350is so disposed that it extends in the longitudinal direction of the pipe section312.

The upper and lower ends351and352, as seen inFIG. 6, of the flexible printed circuit board350are bent. The upper bent end351is electrically and mechanically joined to the pressure-sensing element330by soldering or the like. To be specific, although not shown, conducting part of the upper bent end351is joined to a pad formed on the bottom surface of the pressure-sensing element330.

The part of the flexible printed circuit board350under the upper bent end351extends through the pipe section312toward the circuit board340.

The flexible printed circuit board350extends downward through a hole346made in the circuit board340so that the lower bent end352is disposed on the bottom surface of the circuit board340.

The lower bent end352of the flexible printed circuit board350is electrically connected to the bottom surface of the circuit board340by soldering or the like.

As shown inFIG. 6, a connector case360with a terminal361is disposed below the circuit board340.

The connector case360is made of PPS (polyphenylene sulfide) resin or the like. The terminal361is inserted in the connector case360, both the parts formed as a unit. The connector case360is provided as a connector unit to take out signals from the pressure-sensing element330.

The circuit board340and the terminal361are electrically connected by a spring362. Thus, the pressure-sensing element330is electrically connected to the terminal361through the flexible printed circuit board350and the circuit board340.

As shown inFIG. 6, the bottom end314of the body311of the housing310is bent onto part of the connector case360so that they are fixed together as a unit.

The terminal361can electrically be connected to the ECU or the like of the vehicle through a cable or the like (not shown). Thus, the pressure-detecting device300can send signals to and receive signals from the outside.

Now, by referring toFIGS. 8A to 8D, the process of making the pressure-detecting device300will be described.FIG. 8is a process drawing to illustrate the process of making the pressure-detecting device300and relates mainly to the welding of the pressure-sensing element330.

First, as shown inFIG. 8A, the pressure-conveying member316is put between the metal stem320and the pressure-receiving diaphragm315. To be specific, the pressure-conveying member316is put in the metal stem320and the pressure-receiving diaphragm315is put on the metal stem320so that the pressure-receiving diaphragm315covers the opening321of the metal stem320.

Next, while a load is applied from the pressure-receiving diaphragm315to the diaphragm322through the pressure-conveying member316, the pressure-receiving diaphragm315is welded to the flange323of the metal stem320. Thus, the pressure-receiving diaphragm315and the metal stem320are joined together.

Namely, while a downward load and a upward load are applied from the pressure-receiving diaphragm315and the diaphragm322, respectively, to the pressure-conveying member316, the entire periphery of the pressure-receiving diaphragm315is welded to the flange323by laser welding or the like.

Thus, the pressure-receiving diaphragm315, pressure-conveying member316, and metal stem320are assembled into a single unit. Then, as shown inFIG. 8B, the metal stem320is fitted with the pressure-sensing element330.

To be specific, glass of low melting point is put on the bottom surface of the diaphragm322and the pressure-sensing element330is put on the glass. Then, the glass is burned to fix the pressure-sensing element330to the bottom surface of the diaphragm322with the molten glass.

Next, as shown inFIG. 8C, the upper bent end351of the flexible printed circuit board350is connected to the pressure-sensing element330by soldering or the like.

Then, as shown inFIG. 8D, the flexible printed circuit board350is inserted in the pipe section312and the lower end352of the flexible printed circuit board350is inserted into the hole346of the circuit board340.

Then, the lower end352of the flexible printed circuit board350is bent and the lower bent end352is joined to the circuit board340by soldering.

Next, the circuit board340is joined and fixed to the body311of the housing310. Thereafter, the connector case360is fitted to the body311of the housing310and the bottom end314of the body311is bent onto part of the connector case360. Thus, the housing310and the connector case360are joined together.

When the connector case360and the housing310are joined together, the circuit board340and the terminal361are electrically connected by the spring362. Thus, the pressure-detecting device300is completed as shown inFIG. 6.

The pressure-detecting device300is mounted on the engine block by threadedly engaging the male-thread section313in the above threaded hole of the engine block.

The pressure inside the combustion chamber works on the pressure-receiving diaphragm315as shown by the arrows inFIGS. 6 and 7. The pressure is conveyed through the pressure-conveying member316to the diaphragm322of the metal stem320. The diaphragm322deforms under the pressure, and pressure-sensing element330converts the deformation into an electric signal for the detection of the pressure.

The electric signal is sent to circuit board340through the flexible printed circuit board350and processed by the IC chip342, and the processed signal is outputted to the outside through the terminal361.

The above process of making the pressure-detecting device300is characterized by the following feature.

The feature is that (i) the pressure-conveying member316is put between the pressure-receiving diaphragm315and the metal stem320, (ii) the pressure-receiving diaphragm315is welded to the metal stem320while the pressure-receiving diaphragm315applies a load to the metal stem320through the pressure-conveying member316, and (iii) the pressure-sensing element330is fixed to the metal stem320.

Thus, the pressure-receiving diaphragm315is welded to the metal stem320while the pressure-receiving diaphragm315applies a load to the metal stem320through the pressure-conveying member316, before the pressure-sensing element330is fixed to the metal stem320.

Therefore, the contact between the pressure-conveying member316and the pressure-receiving diaphragm315and the contact between the pressure-conveying member316and metal stem320are properly secured. If the pressure-conveying member316contracts due to its linear expansion coefficient or if the pressure inside the combustion chamber becomes negative, the contact between the pressure-conveying member316and the pressure-receiving diaphragm315and the contact between the pressure-conveying member316and metal stem320are maintained.

Besides, when the pressure-receiving diaphragm315is welded to the metal stem320while the pressure-receiving diaphragm315applies a load to the metal stem320through the pressure-conveying member316, the pressure-sensing element330is not yet fixed to the metal stem320. Therefore, the pressure-sensing element330is free from thermal damage and any load.

Thus, when the pressure-receiving diaphragm315is welded to the metal stem320, the pressure-sensing element330is prevented from being thermally damaged and being exposed to a load which may cause an offset.

The pressure-conveying member316may be in the shape of a rod as in the cases of prior art.

In the case of spherical pressure-conveying member316ofFIGS. 6 and 7, the pressure-conveying member316stays in stable point-contact with the pressure-receiving diaphragm315and the diaphragm322of the metal stem320and the number of contact spots between the pressure-conveying member316and the diaphragms315and322can be reduced.

Therefore, the change of condition of contact between the pressure-conveying member316and the diaphragms315and322can be kept under control, stable condition of contact can be secured at each contact point, and deterioration in the pressure-conveying precision can be prevented.

The spherical pressure-conveying member316does not deform easily as compared with rod-shaped ones of prior art. This fact too contributes to stable condition of contact between the spherical pressure-conveying member316and the diaphragms315and322.

The pressure-conveying member316shown inFIGS. 6 and 7is in the shape of a perfect sphere, but it may be in the shape of a sphere deformed to some extent; i.e., an oval sphere or a rugby ball.

The pressure-conveying member316may be a pillar with round ends for point contact with the pressure-receiving diaphragm315and the diaphragm322of the metal stem320.

The pressure-sensing element330is disposed near to the pressure-receiving diaphragm315to reduce the length of the pressure-conveying member316as compared with the pressure-conveying members of prior art. In this example, the pressure-conveying member316is housed in the metal stem320, the former being as substantially short as the latter.

Thus, the problems of resonance and deformation liable to occur to long pressure-conveying members can be prevented, which contributes to the improvement of the characteristics of the pressure-sensing element330and the sensitivity of the pressure-detecting device300.

FIG. 9is a schematic sectional view of a modified version of the second embodiment of the present invention.

In the second embodiment shown inFIGS. 6 and 7, the metal stem320are formed as a support and the pressure-receiving diaphragm315is welded to the flange323of the metal stem320.

In the modified version of the second embodiment ofFIG. 9, the metal stem320is fixed, by gluing or welding, to the inside of a pipe390of a diameter similar to that of the pipe section312and the pressure-receiving diaphragm315is welded to the pipe390. The pressure-conveying member316is cylindrical.

The pipe390and the metal stem320constitute a support. After the metal stem320is fixed to the inside of a pipe390, the pressure-conveying member316is put between the metal stem320and the pressure-receiving diaphragm315and the pressure-receiving diaphragm315is welded to the pipe390while a load is applied to the pressure-conveying member316.

Then, the pressure-sensing element330is fixed to the metal stem320with molten glass, the flexible printed circuit board350is joined to the pressure-sensing element330, the flexible printed circuit board350is inserted in the pipe section312, and the pipe390and the pipe section312are joined together.

The support is not limited to the above metal stem320or the above assembly of the metal stem320and the pipe390.

Any support will do if (i) it is capable of supporting the pressure-sensing element330and conveying the pressure from the pressure-conveying member316to the pressure-sensing element330and (ii) the pressure-receiving diaphragm315can be welded to it.

The pressure-sensing element330does not need to have the function of a strain gauge. Any pressure-sensing element will do if it can be fixed to the metal stem320and output a signal in accordance with the pressure from the pressure-conveying member316.

Further, according to the above embodiment, the housing310has the long pipe section312. Since the tip of the pipe section312is provided with the pressure-sensing element330, metal stem320, and pressure-receiving diaphragm315, the pressure-sensing element330and the circuit board340are connected through the flexible printed circuit board350.

However, the distance between the pressure-sensing element330and the circuit board340on the side of the connector unit may be decreased by modifying the shape of the housing to minimize the length of the pipe section or eliminating the pipe section so that the pressure-sensing element330and the circuit board340can be connected by bonding wires or the like.

According to the embodiment shown inFIG. 6, there are provided the IC chip342, circuit boards340, and various electric connectors between the pressure-sensing element330and the connector unit360in the housing310. However, the construction of this section is not limited to the above and it can be changed appropriately.

The main part of the present invention is that, in a method for manufacturing a pressure-detecting device wherein the pressure-sensing element330is fixed to the support320and the pressure-receiving diaphragm315is welded to the support320while the pressure-conveying member316being between the pressure-sensing element330and the pressure-receiving diaphragm315, the pressure-sensing element330is fixed to the support320after the pressure-receiving diaphragm315is welded while the pressure-conveying member316is put between the support320and the pressure-receiving diaphragm315and while the pressure-receiving diaphragm315applies a load to the support320through the pressure-conveying member316. The other part of the present invention can be changed appropriately.

Further, it is needless to say that the pressure-detecting device of the present invention is not limited to the sensor of combustion pressure (cylinder pressure) described above.