WO2018096221A1

WO2018096221A1 - Sensor

Info

Publication number: WO2018096221A1
Application number: PCT/FI2017/050822
Authority: WO
Inventors: Panu KOPPINEN; Teuvo SILLANPÄÄ
Original assignee: Teknologian Tutkimuskeskus Vtt Oy
Priority date: 2016-11-24
Filing date: 2017-11-24
Publication date: 2018-05-31
Also published as: FI20165896L; FI20165896A

Abstract

A sensor (1) comprises an ultrasonic transducer (2) comprising a pressure sensitive element and a resonance cavity (6) arranged in connection with a transducer (2). The sensor (1) further comprises a measurement arrangement for measuring a change in an impedance of the transducer (2) in response to an effect of an external pressure affecting on the pressure sensitive element, and a compensation arrangement for compensating the effect of the external pressure affecting on the pressure sensitive element on the basis of the measured change in the impedance of the transducer (2). Also a method for operating the sensor (1).

Description

SENSOR

FIELD OF THE INVENTION

The present invention relates to a sensor to be used in microelectro- mechanical systems or devices. BACKGROUND OF THE INVENTION

A sensor to be used in microelectromechanical systems or devices comprises an ultrasonic transducer to put out or emit as well as to receive ultrasound or ultrasonic waves. The ultrasound or the ultrasonic waves may be applied in an identification of a substance or an agent, such as to identify a gas, or in a measurement of a property of the substance or the agent, such as to measure a composition or a concentration. A sensor comprising an ultrasonic transducer may also be used to measure pressure of a gas.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a novel sensor applica- ble to be used in microelectromechanical systems or devices and a method for operating the sensor.

The invention is characterized by the features of the independent claims.

A sensor comprises an ultrasonic transducer comprising a pressure sensitive element, a resonance cavity arranged in connection with a transducer, a measurement arrangement for measuring a change in an impedance of the transducer in response to an effect of an external pressure affecting on the pressure sensitive element, and a compensation arrangement for compensating the effect of the external pressure affecting on the pressure sensitive element on the basis of the measured change in the impedance of the transducer.

An advantage of the invention is to provide a combined sensor for identifying a liquid or a gas, or alternatively to determine or measure a composition or a concentration of the liquid or the gas, and additionally simultaneously the pressure of the liquid or the gas. The identification of the liquid or the gas, or alternatively the determination or the measurement of the composition or a concentration of the liquid or the gas is based on the ultrasound or the ultrasonic waves put out or emitted by the ultrasonic transducer. The measurement of the pressure of the liquid or the gas is based on the measurement of the change of the impedance of the transducer in response to an effect of the pressure of the liquid or the gas and compensating the effect of the pressure of the liquid or the gas to the pressure sensitive element, a measure of the compensation providing a magnitude of the pressure.

Some embodiments of the invention are disclosed in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which

Figure 1 shows schematically an axonometric view of a sensor;

Figure 2 shows schematically an axonometric view of a part of the sensor of Figure 1;

Figure 3 shows schematically an axonometric view of a part of the sensor of Figure 1;

Figure 4 shows schematically an axonometric view of a transducer of the sensor of Figure 1;

Figure 5 shows schematically a cross-sectional side view of a sensor according to Figure 1; and

Figure 6 shows schematically an example of a DC bias tuning of a reso- nance frequency of a capacitive micromachined ultrasonic transducer.

For the sake of clarity, the figures show some embodiments of the invention in a simplified manner. Like reference numerals identify like elements in the figures.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows schematically an axonometric view of a sensor 1. Figure 2 shows schematically an axonometric view of a part of the sensor 1 of Figure 1 and Figure 3 also shows schematically an axonometric view of a part of the sensor 1 of Figure 1. The sensor 1 comprises an ultrasonic transducer 2 being connected to or being in connection with an acoustic resonance cavity 6. Some exam- pies of sensors like that are disclosed in the following description.

The sensor 1 is a combo sensor that is intended to a simultaneous measurement of a composition or a concentration of a fluid, either for identifying the fluid or to measure a concentration of a specific constituent of the fluid, as well as to measure a pressure of the fluid. The fluid thus forms a substance or an agent properties of which is to be measured. The fluid may be liquid or gas. If the fluid is liquid, the fluid may be composed of only one liquid or it may be a mixture of two or more different liquids. Alternatively, if the fluid is gas, the fluid may be composed of only one gas or it may be a mixture of two or more different gases. Alternatively the fluid may be a mixture of at least one liquid and at least one gas.

The ultrasonic transducer 2 is configured to put out or emit ultrasound or ultrasonic waves or absorb ultrasound or ultrasonic waves or both to put out and receive ultrasound or ultrasonic waves. The ultrasound or ultrasonic waves are applied in the identification of the fluid or in the measurement of the composition or the concentration of the fluid. Figure 4 shows schematically an axono- metric view of a transducer 2 which may be applied in the sensor 1.

The sensor 1 comprises a base plate 3. The base plate 3 provides a body of the sensor 1. The base plate 3 comprises a space 4 or a room 4 for accommodating the transducer 2 in the sensor 1. The base plate 3 thus provides or forms a frame or a holder for the transducer 2. According to an embodiment the base plate 3 is formed of a silicon wafer but the base plate 3 may be made of any other material applicable to be used for providing the base or the frame for the transducer 2.

In the embodiment of the base plate 3 disclosed in Figures 1 to 3 the space 4 is a hole arranged through the base plate 3 in the thickness direction thereof. The space 4 is thus arranged to extend from a top surface 3' or a front surface 3' of the base plate 3 up to a bottom surface 3" or a backside surface 3" of the base plate 3.

The sensor 1 further comprises a silicon-on-insulator plate 5, i.e. a SOI plate 5, made of a silicon wafer and arranged on top of the base plate 3. The sili- con-on-insulator plate 5 at least partly defines the resonance cavity 6 that is a free space extending horizontally and vertically in the sensor structure level provided by the silicon-on-insulator plate 5. The cavity 6 is located on top of the space 4 where the transducer 2 is arranged to remain, the cavity 6 being arranged to be open to the space 4 so that the transducer 2 is arranged to be connected to or to be in open connection with the cavity 6 when the transducer 2 is assembled in the sensor 1.

The ultrasound or the ultrasonic waves are generated into the cavity 6 by the transducer 2. The cavity 6 is also arranged to receive the fluid to be identified or the property of which is to be measured. The cavity 6 may be formed of the silicon-on-insulator plate 5 by removing material away from the silicon-on- insulator plate 5 for example by etching either before it is stacked on top of the base plate 3 or after it has been stacked on top of the base plate 3.

In the embodiment of the sensor 1 disclosed above the cavity 6 was at least partly defined by the silicon-on-insulator plate 5. The cavity 6 in the sensor 1 may, however, have a numerous number of different implementations so that a combination of an ultrasonic transducer 2 connected to or being in connection with an acoustic resonance cavity 6 is provided in the sensor 1.

The sensor 1 of Figures 1 to 3 further comprises a flow channel 8 which is arranged in connection with the cavity 6 and which is at least partly defined by the silicon-on-insulator layer 5. The flow channel 8 is arranged to extend substantially horizontally through the silicon-on-insulator plate 5 via the cavity 6, and the transducer 2 forms a bottom of the flow channel 8 at the cavity 6. The flow channel 8 is intended for a fluid exchange in the cavity 6 of the sensor 1 when the fluid flowing through the cavity 6 is the substance or the agent which is to be identified or the property of which is to be measured with the sensor 1. The flow channel 8 thus brings the fluid flow into the cavity 6 in contact with the transducer 2. In the embodiment of the sensor 1 disclosed in Figures 1 to 3 both ends 8' 8" of the flow channel 8 are open out of the sensor 1 so that the fluid may flow into the flow channel 8 from the first end 8' of the flow channel 8 and out of the flow channel 8 from the second end 8" of the flow channel 8.

The flow channel 8 is formed of the silicon-on-insulator plate 5 by removing material from the silicon-on-insulator plate 5 either after it has been stacked on top of the base plate 3 or before it is stacked on top of the base plate 3. The material removal may be implemented for example by etching. The bottom 8"' of the flow channel 8 is thereby formed for example by an insulation layer of the silicon-on-insulator plate 5 at other portions of the flow channel 8 but not at the cavity 6 at which the material of the silicon-on-insulator plate 5 is totally removed so that at the cavity 6 the bottom 8"' of the flow channel 8 is formed by the top surface of the transducer 2. Alternatively the bottom 8"' of the flow channel 8 at other portions of the flow channel 8 but not at the cavity 6 is provided by the top surface 3' of the base plate 3, which may be implemented by etching the silicon-on-insulator plate 5 up to the top surface 3' of the base plate 3 or by forming the silicon-on-insulator plate 5 of two separate pieces that together form the silicon-on-insulator plate 5.

The sensor 1 disclosed above is arranged to comprise the flow channel 8. Depending on the intended application of the sensor 1, the sensor 1 may, however, be implemented without any flow channel 8. The sensor 1 further comprises a top element 7 on top of the silicon- on-insulator plate 5 for terminating the cavity 6. According to an embodiment of the sensor 1 the top element 7 is formed of a silicon wafer. The distance between the transducer 2 and the top element 7, or in other words a thickness of the sili- con-on-insulator plate 5 determines a cavity length of the cavity 6, i.e. a vertical dimension of the cavity 6. For a proper operation of the sensor 1 a resonance condition between the cavity 6 and the transducer 2 should be met. Generally in resonance condition the cavity length is a half or a quarter of the wavelength of the ultrasound or the ultrasonic waves or any integer multiple of the half or the quarter of the wavelength of the ultrasound or the ultrasonic waves put out by the transducer 2.

When the sensor 1 is assembled, the silicon-on-insulator plate 5 is stacked onto the base plate 3 and the cavity 6 and the flow channel 8 are formed as disclosed above unless the cavity 6 has been manufactured earlier in the sili- con-on-insulator plate 5. After that the transducer 2 may be inserted into the space 4 in the base plate 3 through the hole in the bottom surface 3" of the base plate 3. A horizontal dimensioning of the cavity 6 is arranged to be smaller than a horizontal dimensioning of the space 4, whereby, when the transducer 2 is moved towards the front surface 3' of the base plate 3, the transducer 2 will stop at its final location at the bottom of the cavity 6 when the transducer 2 meets the silicon-on-insulator plate 5 that at least partly defines the cavity 6.

After that the top element 7 is stacked onto the silicon-on-insulator plate 5 for providing a sensor 1 having a three-layer structure. The different layers of the sensor 1, i.e. the base plate 3, the silicon-on-insulator plate 5 and the top element 7 as well as the transducer 2 are glued together with adhesive that does not deform when drying.

According to an embodiment of the sensor 1, the top element 7 is an Application Specific Integrated Circuit, an ASIC. When the top element 7 of the sensor 1 is the ASIC, the sensor 1 may form an independently operable unit, i.e. all the electronics needed for the operation of the sensor 1 may be contained by the sensor 1 itself, or in other words, all the necessary electronics needed for the operation of the sensor 1 may be embedded into the ASIC. The sensor 1 may comprise electrical feed-through connections 16 arranged through the base plate 3, whereby the sensor 1 may be assembled in connection to a circuit board of the actual device where the sensor 1 is utilized or powered via the electrical feed- through connections 16 extending through the base plate 3. A cross-sectional side view of a sensor 1 of this type is shown schematically in Figure 5.

According to an embodiment of the sensor 1, the top element 7 is a micro hotplate. When the top element 7 is the micro hotplate, the cavity 6 and/or the flow channel 8 of the sensor 1 as well as the fluid flowing in the flow channel 8 may be heated to a temperature suitable for the intended measurement operation of the sensor 1.

According to an embodiment of the sensor 1, the transducer 2 is a ca- pacitive micromachined ultrasonic transducer, i.e. a CMUT. In CMUTs, an energy transduction is due to a change in capacitance in the transducer 2. The transducer 2 has a silicon substrate 9 formed of a silicon wafer and provides a base 9 of the transducer 2. The transducer 2 comprises a vacuum space 10 that is schematically shown later in Figure 5. The vacuum space 10 of the transducer 2 is formed in the silicon substrate 9. On top of the vacuum space 10 of the transducer 2 there is a thin vibrating membrane 11, i.e. a sound transducing membrane 11, such as a thin membrane, which provides a pressure sensitive element of the transducer 2, as explained in more detail later. The vibrating membrane 11 comprises a metallized layer that acts as an upper electrode of the first electrode of the transducer 2, together with the silicon substrate 9 which serves as a bottom electrode or the second electrode of the transducer 2.

In the embodiment of Figure 4 the transducer 2 comprises a number of transducer elements 23 that are separate from each other, each element 23 having the vibrating member 11 of its own, meaning that the transducer 2 is formed as a composition of several transducer elements 23 wherein each element 23 provides an operable transducer unit. Some of the transducer elements 23 may put out the ultrasound and the rest of the transducer elements 23 may receive the ultrasound. In the embodiment of Figure 5 there is a single transducer 2 that is arranged to both put out and receive the ultrasound. Electrical contacts of the transducer 2 are shown only very schematically with boxes denoted with reference signs 13, 14 and 15.

When an AC signal is applied across the contact elements 13, 14 with an AC voltage source 17 or oscillator 17, as shown schematically in Figure 5, the vibrating membrane 11 will produce the ultrasound or the ultrasonic waves in the fluid flowing in the flow channel 8 in connection with the cavity 6 of the sensor 1 and the transducer 2 at the bottom of the cavity 6. In that case the transduc- er 2 works as a transmitter. On the other hand, when the ultrasound or the ultrasonic waves are received onto the membrane 11 of the CMUT, it will generate al- ternating signal as the capacitance of the CMUT is varied, whereby the transducer 2 works as a receiver.

According to an embodiment the transducer 2 may be a piezoelectric micromachined ultrasonic transducer, i.e. a PMUT. PMUTs are based on the flex- ural motion of a thin membrane which is coupled with a thin piezoelectric film. The transducer 2 implemented as a PMUT can also function as a transmitter and a receiver depending on the intended use of the sensor 1.

General structures and operation principles of capacitive micromachined ultrasonic transducers and piezoelectric micromachined ultrasonic transducers are known for a person skilled in the art and therefore they are not considered here in more detail.

The identification of the fluid, or the measurement of the composition or the concentration of the fluid, is provided by transmitting the ultrasound or the ultrasonic waves by the transducer 2 to the fluid remaining in the cavity 6. A speed and a damping of the ultrasound or the ultrasonic waves is dependent on the fluid remaining in the cavity 6. By measuring at least one of the speed and the damping of the ultrasound or the ultrasonic waves the fluid remaining in the cavity 6 may be identified, or the composition or the concentration of the fluid remaining in the cavity 6 may be determined. For a proper operation of the sensor 1 the resonance condition between the cavity 6 and the transducer 2 should be met. Generally in resonance condition of the sensor the cavity length is a half or a quarter of the wavelength of the ultrasound or the ultrasonic waves or any integer multiple of the half or the quarter of the wavelength of the ultrasound or the ultrasonic waves put out by the transducer 2, or in other words, the ultrasound or the ultrasonic waves put out by the transducer 2 is arranged to have a wavelength fulfilling the resonance condition of the sensor 1.

When the sensor 1 is used as a pressure sensor, the pressure of the fluid being in contact with the cavity 6 and the transducer 2 through the flow channel 8 bends the sound transducer membrane 11 of the transducer 2 downward, because there is a vacuum space 10 on the other side of the membrane 11. In other words, the pressure of the fluid being in contact with the cavity 6 and the transducer 2, provides a static displacement of the membrane 11, and a transducer's 3 internal capacitance, i.e. a capacitance of the transducer 2, is changed in response to the bending of the membrane 11. A capacitor 18 shown schematically in Figure 5 is intended to present the capacitance C of the transducer 2. The bending or the static displacement of the membrane 11 causes also a change in the frequency of the ultrasound or the ultrasonic waves put out by the transducer 2 so that the transducer 2 is not any more in an optimal resonance condition relative to the cavity 6.

For measuring of the pressure of the fluid, the sensor 1 comprises a measurement arrangement for measuring a change in the capacitance of the transducer 2 in response to the bending of the sound transducing vibrating membrane 11, i.e. the pressure sensitive element of the transducer 2, by the external pressure affecting on the pressure sensitive element of the transducer 2. The measurement arrangement comprises contact lines 19, 20 connected at one end thereof in connection with the contact elements 13, 14 for the electrodes of the transducer 2, the other ends of the contact lines being connected to a measurement and control unit 21 which remains between the electrodes of the transducer 2 and contains suitable electronics for determining the change in the capacitance C of the transducer 2 and forms a part of the measurement arrangement too.

For compensating of the bending of the sound transducing membrane

11, or in other words, for compensating the change in the capacitance of the transducer 2 due to the pressure of the fluid being in contact with the cavity 6 and the transducer 2, the sensor 1 comprises a compensation arrangement for compensating the effect of the pressure affecting on the membrane 11. The compensa- tion arrangement comprises an adjustable or a regulable DC voltage source 22 which is coupled in parallel with the AC voltage source 17, as shown schematically in Figure 5. Alternatively the DC voltage source 22 could be coupled in series with the AC voltage source. The adjustable DC voltage source 22 provides means for adjusting a DC bias of the transducer 2, or in other words a bias tension of the vibrating membrane 11 for compensating the bending of the membrane 11 so that the sensor 1 will achieve the desired resonance frequency for an intended use of the sensor 1. The operation of the adjustable DC voltage source 22 is controlled by the measurement and the control unit 21 on the basis of the measured change in the capacitance C of the transducer 2, the measurement and the control unit 21 thus forming a part of the compensation arrangement too. The control connection from the measurement and the control unit 21 to the DC voltage source 22 is shown schematically with a line CL22 in Figure 5. The adjustable DC voltage source 22 is controlled so that the static displacement of the membrane 11 due to the external pressure affecting to it is compensated, whereby the sensor 1 will meet its resonance frequency. A magnitude of the control signal from the measurement and the control unit 21 to the adjustable DC voltage source 22 through the control connection CL22 is proportional to the static displacement of the membrane 11, i.e. to the pressure of the fluid flowing in the flow channel 8, and the magnitude of the pressure of the fluid can thus be read from the control signal from the measurement and the control unit 21 to the DC voltage source 22. The identification of the fluid, or alternatively the measurement of the composition or the concentration of the fluid is provided for example by an impedance measurement using the measurement and the control unit 21, which contains necessary equipment or electronics for obtaining the actual information or variable intended to be measured. In Figure 5 the arrow denoted with reference sign P indicates the measured pressure of the fluid and the arrow denoted with reference sign M indicates the indication information of the identified liquid or gas or the measured composition or the concentration of the liquid or the gas.

If the top element 7 is implemented with the ASIC, the measurement and the control unit 21, or especially the functionalities provided by it may be implemented with the ASIC.

In the embodiment disclosed above the change in the capacitance C of the transducer 2 was the quantity or variable to be determined for measuring the pressure of the fluid. Generally, depending on the actual implementation of the transducer 2 the quantity or variable to be determined for measuring the pres- sure of the fluid may be some other quantity or variable that describes a change in an impedance of the transducer 2, the capacitance C of the transducer 2 above thus describing only a specific quantity or variable in the impedance determination of the transducer 2.

In Figure 6 it is shown an example of the DC bias compensation of a transducer 2 for adjusting a resonance condition of the sensor 1, i.e. a resonance frequency of the transducer 2 relative to the cavity 6 when an external pressure to be compensated is affecting on the sound transducing membrane 11. In Figure 6 a transfer function of the transducer 2 in decibels is shown on y-axis and a frequency is shown on x-axis in megaherzes. In the example of Figure 6 three differ- ent DC biasing voltages is used, i.e.

20 V, Vwas = 15 V and 10 V. From Figure 6 it can be seen that with the DC biasing voltage

20 V a resonance condition of the transducer 2 relative to the cavity 6 is achieved at the frequency of about 2.69 MHz. With the DC bias compensation the effect of the fluid pressure affecting on the sound transducing membrane 11 may be compensated so that the resonance condition of the transducer 2 can be achieved despite of the external pressure affecting on the membrane 11, and the pressure of the fluid can be de- termined simultaneously from the magnitude of the control signal providing the DC bias compensation of the transducer 2.

The sensor 1 as presented may be used for various applications.

According to an embodiment, the sensor 1 may be used as a gas sen- sor. The sensor can for example be used to measure both a damping and either a speed or a velocity of the ultrasound in the gas, whereby the gas can be determined or identified based on these measurements by the impedance measurement using the measurement and control unit 21. Because the damping and the speed and velocity of the ultrasound depend on temperature and humidity of the gas, an accurate measurement may also require the measurement of the temperature and humidity. The humidity of the gas may also be determined only from the damping of the ultrasound if measured in a broad frequency range.

If the top element is implemented as a micro hotplate or if the top element comprises a micro hotplate, the temperature and/or humidity of the gas to be measured may be arranged to be a specific predetermined constant. In that case temperature and/or humidity measurement are not needed. This may be achieved for example by arranging the cavity to have a temperature that is substantially high relative to the temperature of ambient of the sensor.

The sensor 1 may be used correspondingly to identify different liquids or the composition or the concentration thereof.

According to an embodiment, the sensor 1 may be used as a pressure sensor. The pressure of the fluid can be measured by determining a deflection of the vibrating membrane of the transducer 2 because of the fluid affecting through the flow channel 8 and the cavity 6 to the vibrating membrane 11 of the transduc- er 2. This causes a change in the impedance of the transducer 2 which indicates the pressure of the fluid.

The measurement principle using the ultrasound or the ultrasonic waves for identifying the fluid or for measuring the composition or the concentration thereof, is generally known for a person skilled in the art and therefore it is not described herein in more detail. For example WO-publication 2009/071746 Al discloses some possible applications listed above in more detail.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1. A sensor (1) comprising

an ultrasonic transducer (2) comprising a pressure sensitive element, a resonance cavity (6) arranged in connection with the transducer (2), a measurement arrangement for measuring a change in an impedance of the transducer (2) in response to an effect of an external pressure affecting on the pressure sensitive element, and

a compensation arrangement for compensating the effect of the external pressure affecting on the pressure sensitive element on the basis of the meas- ured change in the impedance of the transducer (2).

2. A sensor as claimed in claim 1, c h a r a c t e r i z e d in that the pressure sensitive element is a sound transducing membrane (11) of the transducer (2), a bending of the membrane (11) being arranged to change in response to the external pressure affecting on it and in response to the compensation arrangement compensating the effect of the external pressure affecting on the membrane (11).

3. A sensor as claimed in claim 1 or 2, c h a r a c t e r i z e d in that the sensor comprises a flow channel (8) arranged in connection with the cavity (6) for a fluid exchange or a gas exchange in the sensor (1), the flow channel (8) extending substantially horizontally through the cavity (6) and the transducer (2) forming a bottom of the flow channel (8) at the cavity (6).

4. A sensor as claimed in any one of the preceding claims, c h a r a c t e r i z e d in that

the measurement arrangement is arranged to measure a change in a capacitance (C) of the transducer (2) in response to an effect of an external pressure affecting on the pressure sensitive element and the compensation arrange- ment is arranged to compensate the effect of the external pressure affecting on the pressure sensitive element on the basis of the measured change in the capacitance (C) of the transducer (2).

5. A sensor as claimed in any one of the preceding claims, c h a r a c - t e r i z e d in that the compensation arrangement comprises a DC voltage tuning arrangement comprising an adjustable direct current voltage source (22).

6. A sensor as claimed in any one of the preceding claims, charac- terized in that

the transducer (2) is one of a capacitive micromachined ultrasonic transducer and a DC biased tuneable piezoelectric transducer.

7. A sensor as claimed in any one of the preceding claims, charac- terized in that the sensor (1) comprises

a base plate (3) comprising a space (4) for accommodating the transducer (2), the base plate (3) forming a frame for the transducer (2),

a silicon-on-insulator plate (5) on top of the base plate (3), the silicon- on-insulator plate (5) at least partly defining the cavity (6) arranged in connec- tion with the transducer (2), and

a top element (7) on top of the silicon-on-insulator plate (5) for terminating the cavity (6).

8. A sensor as claimed in claim 7, characterized in that the top element (7) is an Application Specific Integrated Circuit (ASIC).

9. A sensor as claimed in claim 7 or 8, characterized in that electrical feed-through connections (16) are arranged through the base plate (3) for electrical signals.

10. A sensor as claimed in any one of the preceding claims, characterized in that

the sensor (1) is a combined gas and pressure sensor.

11. A method for operating a sensor (1) comprising an ultrasonic transducer (2) comprising a pressure sensitive element, and a resonance cavity (6) arranged in connection with the transducer (2), the method comprising

measuring a change in an impedance of the transducer (2) in response to an effect of an external pressure affecting on the pressure sensitive element, and

compensating the effect of the external pressure affecting on the pres- sure sensitive element on the basis of the measured change in the impedance of the transducer (2).

12. A method as claimed in claim 11, characterized by compensating the effect of the external pressure affecting on the pressure sensitive element on the basis of the measured change in the impedance of the transducer (2) so that the transducer (2) is in resonance with the cavity (6).

13. A method as claimed in claim 12, characterized in that the change in the impedance of the transducer (2) to be measured is a change in a capacitance (C) of the transducer (2) and that the effect of the external pressure affecting on the pressure sensitive element is compensated on the basis of the measured change in the impedance of the transducer (2).

14. A method as claimed in any one of claims 11 to 13, characterized by

compensating the effect of the external pressure affecting on the pressure sensitive element by adjusting a DC biasing of the transducer (2) with an adjustable direct current voltage source (22) forming at least part of a DC voltage compensation arrangement intended for compensating the effect of the external pressure affecting on the pressure sensitive element.

15. A method as claimed in any one of claims 11 to 14, characterized by

the pressure sensitive element being a sound transducing membrane

(11) of the transducer (2), a bending of the membrane (11) being arranged to change in response to the external pressure affecting on it and in response to the compensation of the effect of the external pressure affecting on the membrane (11).