Combustion pressure measuring plug for a combustion engine

Disclosed is a pressure-measuring plug (100) for a combustion engine comprising a hollow plug body (102), a housing (110) attached to a distal end (102D) of the plug body (102), electronic components (116) arranged in the housing (110) and a sensing module (104) arranged in the hollow plug body and attached to a proximal end (102P) of the hollow plug body (102) The measuring plug further comprises an interconnection module (108) configured to electrically connect the electronic components (116) to the sensing module (104) through the hollow plug body. The interconnection module (108) comprises an elongated support structure (108A) with a first end (108P) provided with first terminals (108C) and a second end (108D) provided with second terminals (108B1), wherein the first terminals (108C) provide an electrical coupling which is flexible in axial direction. The interconnection module provides a reliable connection and could be used for mass production.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to European Patent Application No. 13169547.0, filed May 28, 2013.

TECHNICAL FIELD

The invention relates to a measuring plug and a method for assembling a measuring plug. More particularly, the invention relates to a piezo-resistive pressure-measuring plug for a combustion engine.

BACKGROUND ART

Advanced combustion strategies for gasoline engines in the future depend on the existence of accurate pressure feedback from each combustion cylinder during the entire cycle (compression—combustion—exhaust cycle). These strategies may or may not include Homogeneous Charge Compression Ignition (HCCI) combustion, and can result in high pressure release rates which require fast and accurate pressure response.

A piezo-resistive pressure-measuring plug is known from EP2138819A1. The pressure-measuring plug comprises a very small sensing element that can be placed near the combustion side and therefore a very high bandwidth sensor is feasible. By means of bonding wires, the sensing elements are electrically coupled with sensor electronics on a printed wiring board (PWB) which is located in the plug body. The sensor electronics is arranged to measure a resistance change of the piezo-resistive elements mounted on a sensing module and to generate a conditioned measurement signal therefrom. It has been found that the temperatures inside the body do not allow using a conventional PWB with standard components. Consequently, to obtain a reliable measuring plug, more expensive parts have to be used.

SUMMARY OF INVENTION

It is an object of the present invention to provide an improved measuring plug with a structure which protects the conditioning electronics from thermal overload. Another object of the invention to provide a pressure measuring plug which is at least one of: reliable, cheaper to manufacture, producible in high volume by means of semi- or full automatic production processes, long lasting and/or robust to harsh pressure media, withstanding the high temperature and vibration typical of an internal combustion engine.

According to a first aspect of the invention, this object is achieved by a measuring plug having the features of claim1. Advantageous embodiments and further ways of carrying out the invention may be attained by the measures mentioned in the dependent claims.

A measuring plug according to the invention comprises a hollow plug body, a housing, a sensing module and electronic components, for example connectors. The hollow plug body comprises a proximal end and a distal end. The hollow plug body further comprises a plug body axis. The housing is attached to the distal end of the plug body. The electronic components are arranged in the housing. The sensing module is arranged in the hollow plug body and comprises at least one electrical sensing element and a PWB. The PWB is electrically coupled to the at least one electrical sensing element. Furthermore, the sensing module is attached to the proximal end of the hollow plug body. The measuring plug further comprises an interconnection module configured to electrically connect the electronic components to the sensing module through the hollow plug body. The interconnection module comprises an elongated support structure with a first end and a second end. The first end and the second end are provided with first terminals and second terminals, respectively. The first terminals provide an electrical coupling which is flexible in axial direction of the hollow plug body. In an embodiment the first terminals are spring type terminals. In an advantageous embodiment, the sensing module is configured to measure pressure in a combustion chamber.

These features provide an electrical interconnection between a sensing module in the hollow plug body and electronics in the housing which can connect over a total distance of at least 100 mm. The interconnection can be made after the sensing module is attached to the hollow plug body. The spring type terminals provide a reliable electrical connection in the plug body which can withstand automotive engine temperature and vibration requirements. The interconnection module is inserted in the hollow plug body after the sensing module is attached at one end of the hollow plug body. While the interconnection module is inserted in the hollow plug body, the terminals of the interconnection module will make electrical contact with contact areas of the sensing module. This modular construction allows high volume assembly. The flexibility of the connection in axial direction allows using a material for the elongated support structure with a coefficient of thermal expansion which differs from the coefficient of thermal expansion of the hollow plug body. The difference in expansion is compensated by the flexible coupling. Furthermore, the flexible coupling in axial direction allows compensating variations in the length of the hollow plug body, interconnection module and sensing module

In an embodiment, each spring type terminal includes a helical compression spring. In an advantageous embodiment, the helical compression spring comprises a spring axis which is parallel to the plug body axis. Automotive applications have a large operating temperature range. As components are used with different coefficient of thermal expansion, stress in the electrical connection between sensing module and interconnection module should be avoided. This feature provides a reliable electrical connection which compensates variations in distance between modules due to temperature variations and vibrations.

In an embodiment, the PWB of the sensing module comprises a number of contact areas which are in a plain perpendicular to the plug body axis. The first terminals are touching the number of contact areas. These features allow to slide the interconnection module and to make a reliable connection between sensing module and interconnection module. Furthermore, this feature reduces the complexity of the manufacturing process of the measuring plug.

In an embodiment, the second terminals are a press fit terminals. The combination of press fit terminal at one side of the interconnection module and spring type terminals at the opposite side of the interconnection module provide a cost effective solution to make a reliable electrical connection at both sides of the interconnection module.

In an embodiment, the sensing module further comprises an alignment element. The PWB of the sensing module is located in axial direction of the plug body between the at least one electrical sensing element and the alignment element. The gal dam is configured to hold together a protective gel which covers the electrical sensing elements to protect them against corrosion. The alignment element and an end of the elongated support structure of the interconnection module comprise cooperating alignment structures configured to align the interconnection module with respect to the PWB. These features allow blind insertion of the interconnection module in the hollow plug body. In an advantageous embodiment, the alignment structure of the alignment element comprises two or more arrow-shaped structures having a tip pointing in the direction of the housing.

In an embodiment, the interconnection module comprises conductive strip-like elements. A first end of the strip-like elements forms at least a part of a first terminal. A second end of the strip-like elements forms a second terminal. The elongated support structure comprises longitudinal recesses parallel to the plug body axis and configured to receive the strip-like elements. These features provide an interconnection module which is cost effective and easy to be manufactured in high volume.

In a further embodiment, a longitudinal recess terminates in a through hole at the distal end of the elongated support structure. Both the elongated recesses and stripe-like elements comprise a structure which narrows in the direction of the distal end. These features prevents that the stripe-like elements can move in axial direction in a longitudinal recess.

In an embodiment, the interconnection module has a rotational symmetric structure. This feature provides an interconnection module which has more than one possible placement in the hollow plug body to provide a reliable electrical connection through the hollow plug body.

In an embodiment, the interconnection module has been obtained by an overmolding process. An overmolding process allows reducing the manufacturing costs for high volumes.

In an embodiment, the hollow plug body internally comprises a protrusion which defines the position of the proximal end of the interconnection module in the hollow plug body in a direction along the plug body axis. This feature reduces the variation in distance between proximal end of the interconnection module and PWB of the sensing module due to temperature changes.

In a further embodiment, the interconnection module further comprises a resilient O-ring at an end of the interconnection module located at the distal end of the hollow plug body. The combination of protrusion in the hollow plug body and O-ring secures the interconnection module in axial direction in the hollow plug body. Furthermore, the O-ring reduces the amount of stress in the interconnection structure due to different coefficient of thermal expansion of the hollow plug body and the elongated support structure of the interconnection structure.

In an embodiment, the sensing module and the interconnection structure comprises a central through hole configured for passing through the hollow plug body an electrical connection of an electrical element attached in the central through hole of the sensing module to the housing. This feature allows combining the measuring plug with another function at the proximal end of the hollow plug body which requires also an electrical connection through the hollow plug body. In an embodiment, the electrical element is a glow rod.

In a second aspect there is provided a method of assembling a measuring plug. The method comprises:providing an assembly of sensing module and hollow plug body, a PWB of the sensing module is located at distance of an open end of the hollow plug body; and,sliding the interconnection module in the hollow plug body to make an electrical connection between interconnection module and PWB of the sensing module.

Other features and advantages will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, various features of embodiments.

DESCRIPTION OF EMBODIMENTS

FIG. 1shows schematically a sectional view of an embodiment of a Pressure-measuring plug100for a combustion engine. The pressure-measuring plug100comprises a hollow plug body102, a sensing module104, an interconnection module108, electronic circuitry116and a housing110. The housing110is provided with a hexagonal periphery (not shown) to enable the mounting of the pressure-measuring plug with a wrench.

The hollow plug body102comprises distal end102D and a proximal end102P. The hollow plug body is provided with an external thread102A for mounting the plug body102into a cylinder head of a combustion engine. The hollow plug body102has a plug body axis102C. The plug body axis coincides with the longitudinal axis of the pressure measuring plug100. A base part110A of the housing110is attached to the distal end102D of the hollow plug body102by welding. The housing further comprises a cap1108which is provided with connector elements (not shown). The housing110is configured to accommodate electronic components116such as conditioning electronics and connector elements. The electronic circuitry or electronic components arranged to perform at least one of the following actions: temperature compensation of a signal obtained from the pressure sensor, i.e. piezo-resistive element, calibration of the signal obtained from the at least one electrical sensing element, internal fault detection, converting the electrical signal(s) from the at least one electrical sensing element to a conditioned measurement signal, i.e. a signal indicative of the pressure in the combustion chamber.

The sensing module104is arranged in the hollow plug body102. The region in the hollow plug body occupied by the sensing module is indicated inFIG. 1by A. The sensing module104is attached to the proximal end102P of the hollow plug body. The sensing module is configured to measure pressure in a combustion chamber of an engine. The sensing module104comprises a first elongated circular body part104B, a ring-shaped sensing structure104A and a second elongated circular body part104C. The first elongated circular body part104B is attached at one end to the proximal end102P of the hollow plug body102by welding and extends in the hollow plug body to position the ring-shaped sensing structure104A at a distance from the proximal end of the hollow plug body and thus away from the combustion chamber. The ring-shaped sensing structure104A is attached to the other end of the first elongated circular body part104B.

At least one electrical sensing element (not shown) is arranged on a surface of the ring-shaped sensing structure facing away the combustion chamber. The at least one electrical sensing element could be in the form of strain gauges (e.g. silicon strain gauges such as micro-fused strain gauges (MSG)). The strain gauges are preferably piezo resistive sensing elements. The at least one electrical sensing element is configured to measure in use deformation of the ring-shaped sensing structure104A due to pressure variations in the combustion chamber. The working principle of the sensing module is explained in more detail in patent application EP2138819 of applicant. The second elongated circular body part104C is arranged in the first elongated circular body part104B and coupled at one end to the ring-shaped sensing structure104A. The second elongated circular body part104C forms a space104G for receiving a rod-like element. Examples of a rod-like element are but are not limited to: glow rod, temperature rod, and dummy rod.

A ring-shaped support104D is attached to the outer circumference of the ring-shaped sensing structure104A. A printed wiring board104E is positioned on the ring-shaped support104D. The ring-shaped support104D is configured to position the printed wiring board (PWB)104E at a minimal distance from the ring-shaped sensing structure104A, so that forces acting on the printed wiring board in direction of the plug body axis be led to the outer circumference of the ring-shaped sensing structure104A. In this way, distortion of the measured signal due to varying forces in the pressure-measuring plug is reduced significantly.

The sensing module104further comprises an alignment element104F. The alignment element104F forms the end of the sensing module104which is located in the hollow plug body102. The alignment element104F can be seen in more detail inFIG. 9which shows a top view of the sensing module104. The alignment element comprises an alignment structure104F1which will be described in more detail later. The alignment element104F comprises a disc-shaped part with two larger openings104F2. Each larger openings104F2extends above an electrical sensing element104A1which is attached to the ring-shaped sensing structure, the bonding wires902and parts of the PWB104E. The bonding wires902electrically couple the electrical sensing element104A1to the PWB.

As described above, the sensing structure comprises an end in the hollow plug body which is a stack of the following elements: ring shaped sensing structure104A, ring-shaped support104D, PWB104E and alignment element104F. The alignment element104F is mechanically coupled to the ring-shaped support104D by coupling structures104D1of the ring-shaped support104D. The coupling structures104D are crushed in respective openings of the alignment element104F. It might be clear that other coupling structures are possible to attach the alignment element to the ring-shaped support.

The alignment element104F further comprises a central opening106, and four smaller openings104F3. Each of the four openings104F3is configured to guide flexible contacts of an interconnection structure to contact areas104E1of PWB104E and to keep the flexible contacts in its position on the contact area.

The sensing module104is assembled as one part which is slit in the hollow plug body102. In this way, the electrical sensing elements and the PWB are positioned at distance from the combustion chamber resulting less harsh operational conditions for the electronics in the plug body. A method of assembling a measuring plug comprises:providing an assembly of sensing module and hollow plug body, a PWB of the sensing module is located at distance of an open end of the hollow plug body; and,sliding the interconnection module in the hollow plug body to make an electrical connection between interconnection module and PWB of the sensing module.

The interconnection module108is configured to electrically connect the electronic components116in the cavity of the housing110to the PWB of the sensing module104through the hollow plug body102. The region in the hollow plug body occupied by the interconnection module is indicated inFIG. 1by B. The interconnection module108comprises an elongated support structure108A with a proximal end108P provided with first terminals108C and a distal end108D provided with second terminals108B1. The first terminals108C are spring type terminals in the form of helical compression springs having a spring axis which is parallel to the plug body axis (102C). The second terminals108B1are in the present embodiment press fit terminals. The second terminals108B1are electrically coupled to a PWB arranged in the housing110of the pressure-measuring plug100.

The helical compression springs provide a flexible electrical connection in axial direction. Flexible electrical connection in the context of the present application means that the distance between two electrical components coupled by the connection may vary in time in a pressure measuring plug, or between several measuring plugs, wherein the electrical coupling between the two electrical components maintains reliable. A flexible electrical connection in axial direction allows using a material for the elongated support structure with a coefficient of thermal expansion which differs from the coefficient of thermal expansion of the hollow plug body. The difference in expansion is compensated by the flexible coupling. Furthermore, the flexible coupling in axial direction allows compensating variations in the length of the hollow plug body, interconnection module and sensing module.

The interconnection module108is slit in the hollow plug body through the opening at the distal end102D of the hollow plug body.FIG. 2shows a cross sectional view wherein the interconnection module is not in its final position in the hollow plug body. In the final position of the interconnection module, the first end of the elongated support structure108P is lying against a protrusion102E on the internal wall of the hollow plug body102. In the present embodiment, the protrusion is a circular edge along the inner side of the hollow plug body102. The protrusion102E defines the axial position of the proximal end108P of the interconnection module in the hollow plug body102and consequently the distance between proximal end108P of the interconnection module108and the PWB104E of the sensing module104. The PWB104E comprises a number of contact areas104E1which are in a plain perpendicular to the plug body axis. After inserting of the interconnection structure108, the helical springs of the first terminals108C are touching the number of contact areas. The use of the protrusion102limits the force of the helical spring terminals acting on the PWB of the sensing module104. The coefficient of thermal expansion of the material of the elongated support structure and hollow plug body102might be different. The use of the protrusion reduces the influence of this with respect to the forces acting on the PWB of the sensing module by fixating the axial position of the interconnection module108in the hollow plug body.

The interconnection structure108further comprises a resilient O-ring108E at a circular edge of the distal end108D of the interconnection module108. The distal end108D is located at the distal end102D of the hollow plug body102. After the base part110A of the housing110is attached to the distal end102D of the plug body102, the resilient O-ring108E in combination with the protrusion102E fixates the interconnection module in axial direction in the hollow plug body102. The resilient O-ring108E further enables to reduce the amount of stress in the elongated support structure108A of the interconnection module108due to different coefficients of thermal expansion of the material of the elongated support structure and hollow plug body.

The sensing module104and interconnection module108comprises both a central passage which forms one through hole106from the cavity formed by the housing to the tip of the measuring plug. The through hole106is configured to receive a rod-like element such as but not limited to a glow rod, temperature sensing rod and dummy rod. Both the glow rod and temperature sensing rod require an electrical connection which is provided through the through hole106.

The alignment element104F of the sensing module104and the elongated support structure108A of the connection module108comprise cooperating alignment structures104F1,108A2. The alignment structures are configured to align the interconnection module108with respect to the sensing structure104such that the terminals of the interconnection module108make electrical contact with the corresponding contact areas on the PWB of the sensing module. The alignment structure104F1of the alignment element104F comprises two arrow-shaped structures with a tip pointing in the direction of the housing. The alignment structure104F1of the alignment element104F is configured to protrude into a central hole at the proximal end108P of the elongated support structure108. The surface of the central hole has an arrow-shaped profile which forms the alignment structure108A2A of the interconnection module108. The profile of the alignment structure108A2A comprises a tip pointing in the direction of the sensing module.FIG. 1shows the aligned position of the sensing module and the interconnection module.FIG. 2shows the situation wherein the tip of the alignment structure108A2A of the interconnection module touches the tip of the alignment structure104F1of the sensing module. This situation could occur when the interconnection module is blind inserted in the hollow plug body. The alignment structure will force the interconnection structure while moving in the direction of the sensing structure to rotate around the plug body axis102C. When the sensing module and interconnection module have the correct orientation with respect to each other, the interconnection module could be shifted in axial direction to the sensing module without rotation around the plug body axis until the proximal end of the interconnection module hits the protrusion102E of the hollow plug body102. The helical springs of the terminals108C will be compressed in axial direction and touch the contact areas on the PWB of the sensing module104. Thus the alignment structure of the sensing module and interconnection module is configured to rotate first the interconnection module to align the orientation of the modules after which the terminals of the interconnection module are moved in axial direction of the measuring plug to make electrical contact with the sensing module. The openings104F3of the alignment element104F guide the terminals in the form of helical springs to the corresponding contact areas on the PWB. The openings104F3further ensure that the helical springs could not bend in axial direction after compression. Bending of the helical spring in axial direction could result in loss of contact with the contact area of the PWB.

FIGS. 3-8show in more detail the interconnection module108.FIG. 3shows a side view of the interconnection module. The interconnection module108comprises conductive strip-like elements108B. The strip-like elements108B extends from the distal end108D to the proximal end108P the elongated support structure108A. At the distal end108D, the strip-like element108B comprises a press fit terminal. At the proximal end108P, the strip-like element108B comprises a needle-like end108B2which forms a part of the terminal. The needle-like end108B2is pushed and fixated in the opening of a helical spring.FIG. 3shows a side view of the interconnection module108. The elongated support structure108A, which could be made from a plastic material, comprises a first part108A1and a second part108A2. The O-ring108E is attached to the first part108A1.FIG. 6shows a top view of the interconnection module andFIG. 7shows a bottom view of the interconnection module. It can be seen that interconnection module has a rotational symmetric structure. The terminals are rotational symmetric distributed around the longitudinal axis of the interconnection module. The distribution in the present embodiment is such that interconnection module has two possibilities to electronically connect the sensing module to the electronic components in the housing.FIGS. 6 and 7shows further the central opening106which forms the passage from the housing to the tip of the measuring plug.FIG. 7shows further the two tips of the alignment structure108A2A and two recesses108A2B for receiving the alignment structure of the sensing module.FIG. 7further shows that the tip of the needle-like end108B2is inserted in the opening of the helical spring element108C.

FIG. 8is a sectional view of the interconnection module along the lines VIII-VIII inFIGS. 6 and 7. It should be noted that only one conductive strip-like element108B is show. This figure shows that the elongated support structure108A comprises longitudinal recesses602configured to receive the conductive strip-like elements108B. The recesses602extend parallel to the plug body axis. A longitudinal recess602terminates at one end in a through hole604of the first part108A1at the distal end108D) of the elongated support structure108A. The longitudinal recess602terminates at the opposite end in a through hole of the second part108A2. To fixate the stripe-like elements108B in longitudinal direction in the elongated support structure both the elongated recesses602and stripe-like elements108B comprises a structure which narrows in the direction of the ends of the elongated support structure108.

The first part108A1and the second part108A2are coupled by means of a snap fitting. Reference numeral108A1A indicates the snap fitting structure of the first part108A1and reference numeral108A2A indicates the snap fitting structure of the second part108A2of the elongated support structure108A.

The interconnection module108is assembles in the following way. First the four strip-like elements108B are positioned in the recess of the first part. This is done by inserting the end of the stripe-like element with the press-fit terminal108B1through an opening in the section of the first part forming the distal end of the elongated support structure. Subsequently, the strip-like element is positioned in the recess602. After this, the second part108A2is shifted over the needle-like ends of the stripe-like elements and snap fitted to the first part108A1. Then, the helical spring elements108C are attached to the needle-like ends of the conductive stripe-like elements. Finally, the resilient O-ring108E is attached to the distal end of the elongated support structure108A.

In an alternative way to assemble the interconnection module108, the helical springs are positioned around the needle-like ends of the strip-like elements. The springs could be attached to the needle-like ends by clamping, welding or deformation of the springs. Then the four strip-like elements108B are positioned in the recess of the first part. This is done by inserting the end of the stripe-like element with the press-fit terminal108B1through an opening in the section of the first part forming the distal end of the elongated support structure. Subsequently, the strip-like element is positioned in the recess602. After this, the second part108A2is shifted or pushed over helical springs located at the needle-like ends of the stripe-like elements and snap fitted to the first part108A1.

Preferably, an opening in the second part108A2has a slightly smaller cross section then a cross section of a helical spring. In this way, the helical springs are locked up in the openings of the second part108A2after insertion.

It should be noted that the assembly of elongated support structure and stripe-like elements could be obtained by an overmolding process.

FIG. 10shows a perspective view of a second embodiment of an interconnection for use in a measuring plug. This embodiment differs from the previous described embodiment in that it does not comprise a central hole. This makes the embodiment suitable for a measuring plug which measures only the pressure in a space. Another difference is that the alignment structure108A2A′ of the interconnection module is located at the outer surface. The corresponding alignment element104F with alignment structure104F1′ is also shown inFIG. 10. The resilient O-ring is not shown in this figure.

FIG. 11shows a perspective view of a third embodiment of an interconnection module. In this embodiment, the interconnection module comprises electronic circuitry1104. The electronic circuitry could be arranged to condition the electronic signals generated by the electronic sensing elements of the sensing module. At the distal end108D of the interconnection module, contact areas1108are provided. Spiral contact springs attached to a PWB in the housing1106could be used to make the electrical connection between electrical components in the housing and the circuitry on the interconnection module. In this embodiment, the alignment structure108A2A′ of the interconnection module108at the proximal end108P is at the outer surface of the elongated support structure108A.

In the embodiments described above helical spring elements are used to make the electrical connection between interconnection module and sensing module. Other spring type terminals could be used to make the connection, for example connection with leaf spring elements. The flexible coupling in axial direction could also be provided by a flexible conductive O-ring or tip part which is attached to a terminal of the interconnection module. A flexible conductive material is for example a conductive elastomer. It might also be possible to use press fit terminals to make the electrical connection between interconnection module and sensing module. However, if press fit terminals are used; the connection at the other side of the interconnection module should have a flexible coupling in axial direction of the plug body.

Described is a design of an interconnection system to separate the electrical sensing element from the signal conditioning electronic circuitry over a relative long distance. In this way, the electronic circuitry is protected for thermal overload. The interconnection system withstands the automotive engine cylinder temperature and vibrations and is suitable for high volume assembly and common processes. The interconnection structure108meets the automotive temperature and vibration requirements. The design is applicable for different lengths a variety of engine head lay-outs and other measuring plug sensors wherein the sensor is located at one end of the plug body and the conditioning sensor electronics is located at the other end of the plug body.

The press fit terminals at the distal end of the interconnection module guarantee easy assembly for mass production. The proximal end of the interconnection module has helical spring elements which will touch contact areas of a PWB which is attached to a support ring. The interconnection module is designed in this way that blind insertion is possible for easy assembly during mass-production as it aligns with an alignment feature integrated in the end part of the sensing module. The interconnection module enables ceramic integration due to its high temperature resistance.

While the invention has been described in terms of several embodiments, it is contemplated that alternatives, modifications, permutations and equivalents thereof will become apparent to those skilled in the art upon reading the specification and upon study of the drawings. The invention is not limited to the illustrated embodiments. Changes can be made without departing from the idea of the invention.