FI125492B

FI125492B - pressure sensor

Info

Publication number: FI125492B
Application number: FI20145723A
Authority: FI
Inventors: Teuvo Sillanpää; Hannu Sipola; Heikki Seppä; Niklas Doktar
Original assignee: Teknologian Tutkimuskeskus Vtt Oy; Wärtsilä Finland Oy
Priority date: 2014-08-15
Filing date: 2014-08-15
Publication date: 2015-10-30
Also published as: FI20145723A; WO2016024041A1

Description

PRESSURE SENSOR

TECHNICAL FIELD OF THE INVENTION:

The present invention relates to a pressure sensor. According to certain embodiments, the invention relates to a pressure-measuring plug for an engine, such as a marine diesel engine, with a plug body for insertion into a cylinder head of the engine. The invention further relates to a computer readable medium having stored thereon a set of computer implementable instructions.

BACKGROUND OF THE INVENTION:

Different pressure sensors are known, by means of which pressure can be measured in a measurement environment. The pressure sensors may be, for example, used for pressure measurement in a combustion chamber of a cylinder of an engine. Such engines may comprise, for example, low-speed or medium-speed diesel engines for ship propulsion. Cylinder pressure sensors are key components in closed loop engine control. The existing cylinder pressure sensors are typically of the piezoresistive or piezoelectrical type. Piezoresistive sensors include a strain gauge which is attached to the bending membrane. The bending membrane deforms the gauge and the electrical resistivity of the sensor gauge changes. The change of the electrical resistivity can be measured. Piezoelectrical sensors include a piezoelectrical ceramic which is compressed with pretension in between a moving membrane and a rigid support. Applying pressure to the membrane causes an electrical charge which can be measured.

Document US 4,500,864, for example, discloses a pressure sensor which converts fluid pressures into electrical signals by the deformation of a diaphragm. The pressure sensor is comprised of a strain gauge which includes a resistance body of amorphous metal material which may be formed directly on the diaphragm by means of a physical vapor deposition process. An electric circuit board is secured to the main body and is connected to the strain gauge by means of a lead wire and is connected to an external circuit by means of lead wires.

One of the main problems of existing piezoresistive or piezoelectrical cylinder pressure sensors is their poor reliability. The lifetime of the sensors can be ten times shorter than the required lifetime. Typically there is a metallic membrane which bends due to pressure. The measurement elements are in direct contact with the bending membrane. Thermal insulation of the measurement element is important and very challenging. Further, there must be an electrical contact to the measurement elements. The high mechanical and thermal stresses during operation of the engines may break the measuring elements and/or electrical contacts. Quite complicated mechanical solutions are necessary to overcome the aforementioned problems. New pressure sensors may, for example, improve reliability and lifetime.

Document US 5,714,680 teaches a method and apparatus for measuring pressure in a combustion chamber of an internal combustion engine with a non-intrusive, metal-embedded fiber optic pressure sensor. A Fabry-Perot Interferometer is arranged in a terminated, single mode fiber to function as a pressure gauge. The fiber Fabry-Perot Interferometer (FFPI) is embedded in a metal part which is disposed in the cylinder head of the engine. The metal part and FFPI experience a longitudinal compression in response to the pressure in the chamber. In another aspect of the invention, a non-intrusive fiber containing the FFPI is embedded in a hole drilled or otherwise provided in the metal housing of a spark plug. The spark plug is threaded into the cylinder head of an internal combustion engine and is directly exposed to the combustion chamber pressure. Consequently, the spark plug housing and FFPI experience a longitudinal strain in response to the pressure in the chamber .

Further, document WO 2009/071746 A1 discloses a sensor and a method for measuring pressure, variation in sound pressure, a magnetic field, acceleration, vibration, or the composition of a gas. The sensor comprises an ultrasound transmitter and a cavity arranged in connection with it. According to the invention, the sensor comprises a passive sensor element located at the opposite end of the cavity to the ultrasound transmitter, the distance of which from the ultrasound transmitter is selected in such a way that the resonance condition is met at the ultrasound frequency used. The ultrasound transmitter comprises a diaphragm oscillator, which is connected to the surrounding medium, and the sensor includes means for measuring the interaction between the ultrasound transmitter and the cavity. The sensor is neither notably heat-resistant nor pressure-resistant .

Ultrasonic sensors for detecting an ultrasonic wave are described in US 2008/0073998 A1. The ultrasonic wave is output from an ultrasonic generator and reflected by an object to be detected. The ultrasonic sensor includes: an acoustic matching layer having a first surface and a second surface, the first surface which receives the ultrasonic wave, and the second surface which is opposite to the first surface; and an ultrasonic detection element including a vibration member. The vibration member is coupled with the second surface of the acoustic matching layer. The acoustic matching layer is repeatedly deformable in accordance with a standing wave, which is generated in the acoustic matching layer by the received ultrasonic wave. The vibration member resonates with the repeat deformation of the acoustic matching layer.

Document JP 2002267558 A teaches a telemetric pressure sensor having a vibrating part deformed when external force is applied, and is capable of measuring the applied external force. Being excited by supersonic waves, the vibrating part is vibrated with resonance frequency. The vibrating part is formed by a semiconductor material formed on a board of a semiconductor material.

Document JP H08233670 A discloses a capacitive pressure sensor .

Document US 2004/231424 A1 describes a vibrating type pressure sensor allowing a corrosive gas pressure to be directly applied thereto.

Document US 2007/245806 A1 discloses a combustion pressure sensor for detecting a pressure in the combustion chamber of an internal combustion engine.

SUMMARY OF THE INVENTION:

An object of certain embodiments of the present invention is to provide a pressure sensor. In particular, an object of certain embodiments of the present invention is to provide a cylinder pressure sensor with improved reliability and lifetime, which sensor comprises a plug body for insertion into a cylinder head of an engine. An object of further embodiments of the invention is to provide a pressure sensor for high temperature applications with high pressure changes, wherein mechanical and/or thermal stresses resulting from a hot measurement environment are reduced in order to avoid breaking of the measurement element and/or the electrical contacts. Furthermore, an object of certain embodiments of the present invention is to provide a computer readable medium.

These and other objects are achieved by the embodiments of the present invention, as hereinafter described and claimed. According to an aspect of the invention, there is provided a pressure sensor comprising an ultrasound transmitter, and a cavity arranged in connection with this, which is in resonance mode at an ultrasound frequency used, and wherein the sensor comprises a bending membrane, located at the opposite end of the cavity to the ultrasound transmitter, and the ultrasound transmitter comprises a diaphragm oscillator configured to operate either at a resonance frequency of a diaphragm or below said frequency, which oscillator is connected to the surrounding medium, and the sensor contains means for measuring the interaction between the ultrasound transmitter and the cavity, and a thermal insulation circuit board is arranged between the ultrasound transmitter and a readout circuit board, wherein the thermal insulation circuit board causes a temperature gradient between the ultrasound transmitter and the readout circuit board.

According to an embodiment, the distance of the diaphragm from the bending membrane is selected in resonance operation such that a cavity resonance frequency equals a diaphragm frequency. The sensor is based on an ultrasound transmitter, which comprises an oscillation source in the form of a diaphragm. An ultrasound cavity is arranged in connection with the ultrasound transmitter and filled with the surrounding medium, i.e. a gas or gas mixture. The dimensions of the cavity are arranged to resonate at the operating frequency of the ultrasound transmitter. Typically one of the main dimensions of the cavity, i.e. the cavity length, is dimensioned to be a quarter, a half, or a multiple of these, of the wavelength of the ultrasound. The measurement can be realized either in one port configuration or in two port configuration. In one port configuration the ultrasound transmitter works as a transmitter and as a receiver. In other words, the ultrasound transmitter contains means for sending an ultrasonic wave and measuring the interaction between the transmitter and the cavity. In two port configuration at least two components are needed. One of them will work as a transmitter and a second one will work as a receiver. The operation principle is the same as in one port solution.

In one port configuration by measuring the impedance of the ultrasound through the input impedance or input power of the transmitter, the impedance will be observed to be strongly dependent on the deviation of the cavity relative to the aforementioned length. If operation takes place in the series resonance of the ultrasound, small changes in the impedance of the transmitter will be directly proportional to the cavity length and the width of resonance will be proportional to the damping in the cavity. The geometry of the sensor is arranged in such a way that the variable being measured affects the length of the cavity.

According to an embodiment, the pressure sensor comprises a plug body. The plug body includes a surrounding wall and the bending membrane. The ultrasound transmitter, the thermal insulation circuit board and the readout circuit board are arranged in the plug body.

According to another embodiment, the pressure sensor is configured to be inserted into a cylinder head of an engine. One side of the bending membrane forms a facing surface which is configured to directly face a combustion chamber of the engine.

In an embodiment, an air insulation gap is arranged between the surrounding wall of the plug body and at least one of a first sleeve, the thermal insulation circuit board, a second sleeve, and the readout circuit board.

In another certain embodiment, the sensor is configured to control a resonance frequency of the transmitter by means of bias adjustment such that the resonance frequency of the transmitter essentially equals a resonance frequency of the cavity. According to a certain embodiment, the sensor is configured to measure the impedance of the transmitter at fixed excitation frequency fe which equals the resonance frequency of the coupled resonator f0 when the bending membrane is in stationary rest position. In another certain embodiment, the sensor is configured to control an excitation frequency in a way that an acoustic cavity length is kept at a value which equals the acoustic cavity length when the bending membrane is in stationary rest position.

According to a certain embodiment, a thermal distribution disc is arranged in the cavity between the ultrasound transmitter and the bending membrane. The thermal distribution disc comprises at least one disc pin which is arranged on one side of the thermal distribution disc and faces the bending membrane. The at least one disc pin and a peripheral edge of the thermal distribution disc are directly in contact with the bending membrane.

In an embodiment, a first cavity length between the ultrasound transmitter and the bending membrane is in stationary rest position of the bending membrane one quarter, one half, or a multiple of these, of the wavelength of the ultrasound frequency used. According to another embodiment a second cavity length between the ultrasound transmitter and the thermal distribution disc is in stationary rest position of the bending membrane one quarter, one half, or a multiple of these, of the wavelength of the ultrasound frequency used.

According to a certain embodiment, the sensor includes a height adjuster which is arranged between the thermal insulation circuit board and the bending membrane. The height adjuster is a height adjustment ring or bimetal spring and is capable of adjusting the first cavity length between the ultrasound transmitter and the bending membrane depending on the cavity temperature in such a way that the resonance condition is met.

According to another certain embodiment, the sensor includes a height adjuster which is arranged between the thermal insulation circuit board and the thermal distribution disc. The height adjuster is a height adjustment ring or bimetal spring and is capable of adjusting the second cavity length between the ultrasound transmitter and the thermal distribution disc depending on the cavity temperature in such a way that the resonance condition is met.

According to another aspect of the invention, there is provided a computer readable medium having stored thereon a set of computer implementable instructions capable of causing a processor, in connection with the pressure sensor according to any one of claims 1 to 20, to calculate a pressure in a measurement environment and/or output pressure information depending on an effective cavity length.

Considerable advantages are obtained by means of the embodiments of the present invention. In an embodiment, there is provided a pressure sensor with improved reliability and lifetime. Mechanical stresses can be reduced due to the noncontact displacement measurement of the bending membrane. Thermal insulation of the measurement element, i.e. the ultrasound transmitter, is not required. The thermal insulation circuit board, which is arranged separately from the readout circuit board, further reduces thermal stresses to the readout circuit board during measurement in a measurement environment. According to certain embodiments, an air insulation gap will isolate the readout circuit board from the surrounding wall of the sensor housing. Additionally, a metal sleeve will conduct heat to the upper part of the surrounding wall and/or a cap of the housing. The invention is especially advantageous for high temperature applications with high pressure changes. Compared to piezoresistive or piezoelectrical sensors, a significant improvement in sensitivity is further achieved. The sensor can be used directly to replace piezo sensors and is less sensitive to, for example, problems arising from torsion.

In a certain embodiment, there is provided a cylinder pressure sensor including a thermal distribution disc for further reduction of thermal stresses in the measurement area during operation of an engine. Thermal stresses can be additionally reduced by means of an air insulation gap and a metal sleeve which conducts the heat to the upper part of the surrounding wall and the cap. The temperature in the area where the readout circuit board and readout LNA (Low Noise Amplifier) and impedance to pressure converter unit are located can be reduced, for example to temperatures in the range between 150 [°C] and 100 [°C] or to temperatures less than 100 [°C]. Mechanical and thermal stresses resulting from the combustion chamber of the cylinder during operation of the engine do not break the ultrasound transmitter or the electrical contacts. The lifetime of the cylinder pressure sensor can be significantly improved and meets the requirements according to certain embodiments. Quick and easy insertion and removal of the plug body is further possible.

Additionally, the mechanical structures of the embodiments of the invention are uncomplicated, thus reducing the price of the sensors and improving reliability and lifetime.

BRIEF DESCRIPTION OF THE DRAWINGS:

For a more complete understanding of particular embodiments of the present invention and their advantages, reference is now made to the following descriptions, taken in conjunction with the accompanying drawings. In the drawings :

Fig. 1 illustrates a schematic cross-sectional view of a pressure sensor according to a first embodiment of the invention,

Fig. 2 illustrates a schematic cross-sectional view of a pressure sensor according to a second embodiment of the invention,

Fig. 3 illustrates a schematic cross-sectional view of a pressure sensor according to a third embodiment of the invention,

Fig. 4 illustrates a schematic cross-sectional view of a pressure sensor according to a fourth embodiment of the invention,

Fig. 5 illustrates a schematic side view of a plug body of a pressure sensor according to a fifth embodiment of the invention, and

Fig. 6 illustrates a schematic graph of a pressure curve in a combustion chamber evolving throughout an engine operating cycle.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION:

In Fig. 1 a schematic cross-sectional view of a pressure sensor 1 according to a first embodiment of the invention is illustrated. The pressure sensor 1 comprises an ultrasound transmitter 2 which is arranged in a plug body 15 and configured to output an ultrasonic wave. The ultrasound transmitter 2 comprises an oscillation source in the form of a diaphragm. An acoustic cavity 3 is arranged in connection with the ultrasonic transmitter 2 and a passive bending membrane 5 is located at the opposite end of the acoustic cavity 3. The cavity 3 is filled with a gas mixture such as air but may be also filled with another gas or gas mixture. The bending membrane 5 is an integral part of the plug body 15 and typically made of metal, for example made of stainless steel. The thickness of the passive bending membrane 5 may be, for example, in the range between 0.8 [mm] and 1.5 [mm] and the diameter of the passive bending membrane 5 may be, for example, in the range between 8.0 [mm] and 15.0 [mm]. The distance between the ultrasound transmitter 2 and the passive bending membrane 5, i.e. the effective cavity length li, determines the cavity resonance frequency fc and may be, for example, li = η * λ/2 in stationary rest position, wherein n is an integer number and λ is the ultrasonic wavelength. The ultrasound transmitter 2 is mounted to a thermal insulation circuit board 4, for example a ceramic thermal insulation board. The cavity length lx between the bending membrane 5 and the ultrasound transmitter 2 is fixed by means of a first metal sleeve 6. In another embodiment, the cavity length li may be li = η * λ/4, li = (1 + 2 * η) * λ/4, li = 1 [mm], or li = 1.5 [mm], for instance.

The diaphragm of the ultrasound transmitter 2 is configured to create a resonating ultrasound in the cavity 3. The ultrasound transmitter 2 creates both electrical series and parallel resonance. The impedance of the series resonance is typically 1 - 10 kQ and the impedance of the parallel resonance is 10 - 100 times greater, always according to the quality factor of the resonator. If the sound cavity is, for example, li = λ/2 in length, it will amplify the resonance of the ultrasound. The reflected ultrasound wave arrives at the ultrasound diaphragm in phase, which increases the movement of the diaphragm. It can be assumed that practically all of the sound will be reflected back from the bending membrane 5, because there is a huge acoustic mismatch in between the bending membrane 5 and the gas or gas mixture in the cavity 3. If the bending membrane 5 radiates power out, the quality factor of the cavity diminishes and the resolution decreases. In practice, this means that the impedance of the series resonance diminishes .

The ultrasonic resonance cavity principle relies on the fact that the impedance of the resonance cavity depends on the distance between the surface of the bending membrane 5 and the detector's surface relative to the ultrasonic wavelength λ. The cavity impedance is a minimum when the distance between the surfaces is one half of the ultrasonic wavelength λ. Bending of the membrane 5 due to the pressure changes in an measurement environment will manifest a modulation of the effective cavity length li and cause a mismatch to the ultrasonic wavelength λ. Thus, the cavity impedance is detuned. Monitoring of the changes of cavity impedance provides the required information on the displacement. The readout circuit for detection of cavity impedance changes can be implemented electronically.

Bending of the bending membrane 5 during operation of the sensor 1 leads to a change of the effective cavity length li and therefore also to a change of the resonance frequency fc of the cavity 3. According to a certain embodiment, the sensor 1 is configured to control a resonance frequency ft of the ultrasound transmitter 2 by means of bias adjustment such that the resonance frequency of the transmitter ft essentially equals a resonance frequency of the cavity fc in order to achieve a resonating ultrasound. The term "essentially equals" means that the resonance frequency of the transmitter ft differs from the resonance frequency of the cavity tc by less than the factor of 1/Q. In an optimal case the resonance frequency of the transmitter ft exactly equals the resonance frequency of the cavity fc. A signal fo represents a resonance frequency of the cavity coupled resonator. In an optimal case the resonance frequency of the cavity coupled resonator f0 exactly equals the resonance frequency of the transmitter ft and the resonance frequency of the cavity fc. The operation frequency is selected such that a resonance bandwith Af = fo/Q is at least equal or more than the bandwith needed for the pressure measurement. In the equation fo is the resonance frequency of the cavity coupled resonator and Q is the quality factor of the resonance. The Q value of the resonance may be, for example, 2 00 and the bandwith may be, for example 10 kHz and thus the operation frequency is at least 2 MHz. The electronics can then calculate the effective pressure in the measurement environment by means of the resonance frequency of the cavity coupled resonator f0.

According to a certain embodiment, the system 1 is further configured to control the excitation frequency of the ultrasound transmitter fe in such a way that the excitation frequency of the ultrasound transmitter fe equals the resonance frequency fo of the coupled resonator when the bending membrane is in stationary rest position. In this case, the information about the effective cavity length li as well as the pressure in the measurement environment is contained in the transmitter impedance. Otherwise, the measurement may take place in such a way that the cavity impedance is measured and kept constant by means of adjustment of the operation frequency fe. In other words, the sensor 1 is configured to control the excitation frequency fe in a way that the acoustic length of the cavity is kept at a value which equals the acoustic cavity length when the bending membrane is in stationary rest position. In this case, the information about the pressure is contained in the excitation frequency fe, thus providing a large linear cavity length measurement range.

The ultrasound transmitter 2 is typically attached to the thermal insulation circuit board 4 by means of an adhesive. The adhesive used in the embodiment may be, for example, a high temperature glue. The adhesive used is selected in such a way that the adhesive used will have sufficient adhesion to the ultrasound transmitter and the thermal insulation circuit board 4 depending on the expected temperature range of the measurement environment. The thermal insulation circuit board 4 is arranged parallel to a readout circuit board 7. The thermal insulation circuit board 4 is configured to cause a negative temperature flow gradient in the direction from the ultrasound transmitter 2 to the readout circuit board 7 in order to reduce thermal stresses to the readout circuit board 7 during measurement in a measurement environment. The thermal insulation board 4 is further configured to send electric signals between the ultrasound transmitter 2 and the readout circuit board 7 bidirectionally. According to certain embodiments, the thermal insulation circuit board includes vias filled with conducting material in order to provide electrical contacts with excellent electrical properties on both sides of the thermal insulation circuit board 7, thus providing a high durability of the thermal insulation board 4. This is especially important when using the pressure sensor 1 as a cylinder pressure sensor in an engine. The readout circuit board 7 is arranged via pins 12 and a protrusion of a second metal sleeve 8 separately from the thermal insulation circuit board 4 in such a way that the influence of the thermal flow from the surrounding wall 9 of the plug body 15 is minimized. The pins 12 are typically made from conducting material and configured to send the electric signals between the thermal insulation circuit board 4 and the readout circuit board 7. The pins may be, for example, so called pogo pins and each located on an electrical contact of the thermal insulation circuit board 4. According to other embodiments, feed-throughs for electric cables are provided in the thermal insulation circuit board 4. The second metal sleeve 8 conducts the heat to the upper part of the surrounding wall 9 and the cap 10. The second metal sleeve 8 is thermally anchored to the surrounding wall 9 and/or the cap 10 at the end of the second metal sleeve 8 which is opposite to the thermal insulation circuit board. In other words, the second metal sleeve 8 represents a heat sink. The thickness of the surrounding wall 9 may be, for example, in the range between 1.5 [mm] and 3.5 [mm] and the thickness of the second metal sleeve 8 may be, for example, in the range between 1.0 [mm] and 3.0 [mm] . Additionally, an air insulation gap 11 is arranged between the surrounding wall 9 of the plug body 15 and the first sleeve 6, the thermal insulation circuit board 4, and the second sleeve 8. The thickness of the air insulation gap 11 may be, for example, in the range between 0.5 [mm] and 2.0 [mm].

According to certain embodiments, the pressure sensor 1 is configured to be inserted into a cylinder head of a reciprocating piston engine such as a diesel engine. The cylinder pressure sensor 1 may be, for example, inserted into cylinder heads of low-speed diesel engines or medium-speed engines. Low-speed engines are primarily used to power ships and typically operate in the range from 60 [rpm] to 200 [rpm]. Medium-speed engines are typically used in ship propulsion, electrical generators, and mechanical drive applications. Cylinder pressure sensors 1 according to certain embodiments of the invention are typically designed for operation up to 600 [rpm] . Pressure measurement is typically possible in the range between p = 0 [bar] and p = 400 [bar].

By means of the cylinder pressure sensor 1 according to the embodiments it is possible to realize noncontact displacement measurement of the bending membrane 5, thus reducing mechanical stresses. As the bending membrane 5 is directly contacted on one surface side with the combustion chamber of the cylinder, the aforementioned surface side of the bending membrane 5 becomes extremely hot, for example 300 [°C], 360 [°C] or 400 [°C]. The readout sensor mounting base, i.e. the thermal insulation circuit board 4, works as a circuit board for the ultrasonic transmitter and thermal barrier for the readout electronics which are located in a separate readout circuit board 7. The temperature in the area where the readout circuit board 7 is located may be, for example, 150 [°C], 120 [°C], 100 [°C] or less than 100 [°C], thus reducing the risk of thermally damaging the electronics. Due to the noncontact displacement measurement, further thermal insulation of the ultrasound transmitter 2 is not required. The temperature in the area where the ultrasound transmitter 2 is located is between the temperature of the bending membrane 5 and the temperature in the area where the readout circuit board 7 is located. The temperature in the area where the ultrasound transmitter is located may be, for example, 220 [°C], 200 [°C], or 180 [°C]. According to certain embodiments, the pressure sensor 1 includes a temperature sensor in order to measure the cavity temperature. The pressure sensor 1 is advantageous for high temperature applications with high pressure changes.

In Fig. 2 a schematic cross-sectional view of a pressure sensor 1 according to a second embodiment of the invention is illustrated. The pressure sensor 1 comprises an ultrasound transmitter 2 which is arranged in a plug body 15 and configured to output an ultrasonic wave. An acoustic cavity 3 is arranged in connection with the ultrasonic transmitter 2 and a passive bending membrane 5 is located at the opposite end of the acoustic cavity 3. According to other embodiments, also several oscillating ultrasound transmitters 2 may be used in parallel. The ultrasound transmitter 2 is mounted to a thermal insulation circuit board 4 and the distance li between the bending membrane 5 and the thermal insulation circuit board 4, i.e. the effective cavity length, is fixed by means of a first metal sleeve 6. The readout circuit board 7 is arranged on top of and perpendicular to the thermal insulation circuit board 4. Due to the orientation of the readout circuit board, specific components of the readout circuit board 7 are arranged more far away from the hot bending membrane 5 than according to the solution presented in Fig. 1. Further, on both sides of the readout circuit board 7 the gas or gas mixture volume is the same, i.e. the heat distribution is essentially the same on both sides of the readout circuit board. The second metal sleeve 8 conducts the heat to the upper part of the surrounding wall of the plug body 15 and the cap 10. Additionally, an air insulation gap 11 is arranged between the surrounding wall 9 of the plug body 15 and the first sleeve 6, the thermal insulation circuit board 4, and the second sleeve 8. The thermal insulation circuit board 4 is configured to cause a negative temperature flow gradient in the direction from the ultrasound transmitter 2 to the readout circuit board 7 in order to reduce thermal stresses to the readout circuit board 7 during measurement in a measurement environment.

In Fig. 3 a schematic cross-sectional view of a pressure sensor 1 according to a third embodiment of the invention is illustrated. The pressure sensor 1 comprises an ultrasound transmitter 2 which is arranged in a plug body 15 and configured to output an ultrasonic wave. An acoustic cavity 3 is arranged in connection with the ultrasonic transmitter 2 and a passive bending membrane 5 is located at the opposite end of the acoustic cavity 3. The ultrasound transmitter 2 is mounted to a ceramic thermal insulation circuit board 4. The ceramic thermal insulation circuit board 4 is configured to cause a negative temperature flow gradient in the direction from the ultrasound transmitter 2 to the readout circuit board 7 in order to reduce thermal stresses to the readout circuit board 7 during pressure measurement in a measurement environment. A thermal distribution disc 13 is further arranged in the cavity 3 between the ultrasound transmitter 2 and the bending membrane 5. The thermal distribution disc 13 comprises a disc pin 14 which is arranged on one side of the thermal distribution disc 13 and faces the center of the bending membrane 5. Only the disc pin 14 and the peripheral edge of the thermal distribution disc 13 are directly in contact with the bending membrane 5. By means of the thermal distribution disc 13 and the disc pin 14 heat can be conducted away from the center of the bending membrane 5, i.e. the measurement area, to the peripheral edge of the thermal distribution disc 13. The distance between the ultrasound transmitter 2 and the upper surface of the thermal distribution disc 13, i.e. the effective cavity length 12, may be, for example, 12 = η * λ/2, 12 = n * λ/4, or 12 = (1 + 2 * n) * λ/4, wherein n is an integer number and λ is the ultrasonic wavelength.

Bending of the membrane 5 due to the pressure changes, for example in a combustion chamber of a cylinder of a diesel engine, will manifest a modulation of the effective cavity length I2 and cause a mismatch to the ultrasonic wavelength λ. Thus, the cavity impedance is detuned. Monitoring of the changes of cavity impedance provides the required information on the displacement. The readout circuit for detection of cavity impedance changes can be implemented electronically.

In Fig. 4 a schematic cross-sectional view of a pressure sensor according to a fourth embodiment of the invention is illustrated. A height adjustment ring 16 is arranged between the thermal insulation circuit board 4 and the second metal sleeve 6. Other embodiments may include bimetal springs instead of the height adjustment ring 16. The speed of sound changes depending on the temperature of the gas or gas mixture in the cavity 3. The height adjustment ring 16 and the bimetal springs are capable of adjusting the cavity length li between the ultrasound transmitter 2 and the bending membrane 5 depending on the cavity temperature in such a way that the resonance condition is met. Springs 19 are arranged between the thermal insulation circuit board 4 and the second sleeve 8 in order to allow adjustment of the cavity length li depending on the cavity temperature. With increasing cavity temperature the height adjustment ring 16 or the bimetal springs will increase the cavity length 1χ by moving the thermal insulation circuit board in the direction of the cap 10 and vice versa. Such a pressure sensor 1 is advantageous in applications with high temperature changes.

In Fig. 5 a schematic side view of a plug body of a pressure sensor according to a fifth embodiment of the invention is illustrated. The plug body 15 of the sensor 1 is configured to be inserted into a cylinder head of a marine diesel engine. The plug body 15 includes a thread 17 on the outside surface of the surrounding wall 9 for uncomplicated and quick insertion and removal of the sensor 1. The ultrasound transmitter 2, the thermal insulation circuit board 4 and the readout circuit board 7, which are not shown in Fig. 5, are arranged in the plug body 15. During operation of the engine the bending membrane 5 is in direct contact with the combustion chamber of the cylinder. The sensor 1 is connected to an electronic system such as a computing device 18 which comprises a computer readable medium. A set of computer implementable instructions is stored thereon. The instructions are capable of causing a processor of a computing device 18, in connection with the pressure sensor 1, to calculate a pressure p in the combustion chamber and output pressure information depending on an effective cavity length. The pressure information may be, for example, p = 180 [bar].

In Fig. 6 a schematic graph of a pressure curve in a combustion chamber evolving throughout an engine operating cycle is illustrated. Fuel is injected into a combustion chamber of a cylinder of an engine at tinj. Due to the motion of the piston head, the cylinder pressure in the combustion chamber increases. Ignition of the fuel fed into the combustion chamber takes place at tign before reaching the maximum cylinder pressure. Then the cylinder pressure decreases until the end of the operating cycle. The continuously or step-wise measured pressure p can be displayed by means of a computing device 18. Displaying of the pressure information may take place in real time and the information may be additionally stored. The pressure sensor may further be connected to another system, for example to a fuel pump, in order to continuously or stepwise provide pressure input data to the other system.

Although the present invention has been described in detail for the purpose of illustration, various changes and modifications can be made within the scope of the claims. In addition, it is to be understood that the present disclosure contemplates that, to the extent possible, one or more features of any embodiment may be combined with one or more features of any other embodiment.

It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

The invention is in particular not limited to a pressuremeasuring plug for a diesel engine with a plug body for insertion into a cylinder head of the diesel engine. The invention can be also used for other applications.

List of reference numbers: 1 pressure sensor 2 ultrasound transmitter 3 cavity 4 thermal insulation circuit board 5 bending membrane 6 first sleeve or first spacer 7 readout circuit board 8 second sleeve 9 surrounding wall 10 cap 11 air insulation gap 12 pin 13 thermal distribution disc 14 disc pin 15 plug body 16 height adjuster 17 thread 18 computing device 19 spring fo resonance frequency of the cavity coupled resonator fc resonance frequency of the cavity fe ultra sound excitation frequency ft resonance frequency of the transmitter 11 first cavity lenqth 12 second cavity lenqth n integer number P pressure Q Q value of the cavity coupled resonator tign time of ignition tinj time of injection λ wavelength

Claims

A pressure sensor (1) comprising: - an ultrasonic transmitter (2), and - a cavity (3) arranged in connection therewith, which cavity is at the ultrasonic frequency used in the resonance state, and wherein - the sensor (1) comprises a flexible membrane (5) (3) opposite to the ultrasonic transmitter (2), - the ultrasonic transmitter (2) comprises a diaphragm oscillator configured to operate either at or below the resonant frequency of the diaphragm, wherein the oscillator is coupled to the surrounding medium; characterized by: - a thermal insulation circuit board (4) disposed between the ultrasonic transmitter (2) and the display circuit board (7), wherein the thermal insulation circuit board (4) provides a temperature gradient between the ultrasonic transmitter (2) and the display circuit board (7).

The pressure transducer (1) according to claim 1, wherein the distance of the diaphragm from the bending membrane (5) is selected in resonance operation such that the resonance frequency (fc) of the cavity is equal to the frequency of the diaphragm.

The pressure sensor (1) according to claim 1 or 2, wherein the pressure sensor (1) comprises a plug body (15).

A pressure transducer (1) according to claim 3, wherein the plug body (15) comprises a surrounding wall (9) and a bending membrane (5).

The pressure transducer (1) according to claim 3 or 4, wherein the ultrasonic transmitter (2), the thermal insulation circuit board (4) and the display circuit board (7) are arranged inside the plug body (15).

The pressure sensor (1) according to any one of claims 1 to 5, wherein the pressure sensor (1) is configured to be mounted on the cylinder end of the engine.

The pressure sensor (1) according to any one of claims 4 to 6, wherein the air insulating opening (11) is arranged on the wall (9) surrounding the plug body (15) and at least one of the first sleeve (6), thermal insulation board (4), second sleeve (8) and display board. (7).

The pressure sensor (1) according to claim 7, wherein the second sleeve (8) is configured to conduct heat from the direction of the thermal insulation board to the upper part and / or cover (10) of the surrounding wall (9).

The pressure transducer (1) according to any one of claims 1 to 8, wherein the transducer (1) is configured to control the resonance frequency (ft) of the transmitter (2) by bias control such that the resonance frequency (ft) of the transmitter (2) is substantially the same 3) Rasonance frequency (fc).

The pressure transducer (1) according to any one of claims 1 to 9, wherein the transducer (1) is configured to control the tuning frequency (fe) of the ultrasonic transmitter (2) such that the tuning frequency (fe) of the ultrasonic transducer (2) with the flexible membrane (5) in a stationary rest position.

The pressure transducer (1) according to any one of claims 1 to 10, wherein the transducer (1) is configured to control the tuning frequency (fe) so that the acoustic cavity length is maintained at a value equal to the acoustic cavity length when the flexible membrane is in a stationary rest position.

The pressure sensor (1) according to any one of claims 1 to 11, wherein the heat distribution plate (13) is arranged in the cavity (3) between the ultrasonic transmitter (2) and the flexible membrane (5).

The pressure sensor (1) according to claim 12, wherein the heat distribution plate (13) comprises at least one plate pin (14) disposed on one side of the heat distribution plate (13) and facing the bending membrane (5).

The pressure sensor (1) according to claim 13, wherein the at least one plate pin (14) and the outer edge of the heat distribution plate (13) are in direct contact with the bending membrane (5).

The pressure transducer (1) according to any one of claims 1 to 11, wherein the first cavity length (li) between the ultrasonic transducer (2) and the flexing membrane (5) is one quarter, half, or one of their multiple (n) the wavelength (λ) of the ultrasonic frequency used.

The pressure transducer (1) according to any one of claims 11 to 14, wherein the second cavity length (12) between the ultrasonic transmitter (2) and the heat distribution plate (13) is one-fourth, one-half or one of their multiple (n) the wavelength (λ) of the ultrasonic frequency.

The pressure transducer (1) according to claim 15, wherein the transducer (1) includes a height regulator (16) disposed between the thermal insulation circuit board (4) and the flexible membrane (5).

The pressure transducer (1) according to claim 17, wherein the height adjuster (16) is a height adjustment ring or a bimetallic spring and is capable of adjusting the first cavity length (li) between the ultrasonic transmitter (2) and the deflection membrane (5).

The pressure transducer (1) according to claim 16, wherein the transducer (1) includes a height regulator (16) disposed between the thermal insulation circuit board (4) and the heat distribution board (13).

The pressure transducer (1) according to claim 19, wherein the height adjuster (16) is a height adjusting ring or a bimetallic spring and is capable of adjusting a second cavity length (12) between the ultrasonic transmitter (2) and the heat distributor plate (13).