Sensor and a sensor system

1. A wireless sensor comprising;

an electrostatically actuated ultrasound transmitter loaded with a fluid-filled cavity and comprising a light-construction diaphragm oscillator,

a passive sensor element located at the opposite end of the cavity to the ultrasound transmitter,

an antenna matched to the ultrasound transmitter, the antenna being configured to receive radio frequency signals (f 1 , f 2 ), and

a conductor arrangement connecting the antenna to the ultrasound transmitter for using the radio frequency signals for providing energy for driving the ultrasound transmitter,

wherein the antenna for receiving radio frequency signals (f 1 , f 2 ) fed by a reader device is configured for sending radio frequency signals (f 3 ) based on the interaction between the ultrasound transmitter and the cavity and the ultrasound transmitter is configured to actuate at the frequency difference of an incoming signal.

2. A sensor in accordance with claim 1 , wherein the antenna is a loop antenna.

3. A sensor in accordance with claim 1 , wherein the antenna is a dipole antenna.

4. A sensor in accordance with claim 1 , wherein the light-construction diaphragm oscillator forms one electrode of the transmitter and a fixed electrode forms another electrode for the ultrasound transmitter and wherein the antenna is electrically connected to said electrodes.

5. A sensor in accordance with claim 4 , wherein the ultrasound transmitter of the antenna is electrically connected to said electrodes through a matching circuit.

6. A sensor in accordance with claim 1 , wherein the distance of the passive sensor element from the ultrasound transmitter is such that a resonance condition is met.

7. A system for measuring pressure, sound pressure variation, magnetic field, acceleration, shaking, or gas composition, comprising;

at least one ultrasonic sensor having a cavity, an ultrasound transmitter and a passive sensor element sensitive to a parameter to be measured,

a reading device capable of reading a variation in the sensor caused by a change in the parameter to be measured,

wherein the sensor includes at least one antenna for wireless communication with the reading device, and

wherein the reading device is configured to

generate two frequencies and sending them to the at least one sensor, and

detect a third frequency backscattered or reflected from the sensor and wherein the ultrasound transmitter is configured to actuate at the frequency difference of an incoming signal.

8. A system in accordance with claim 7 , wherein at least one of the antennas is a loop antenna.

9. A system in accordance with claim 7 , wherein at least one of the antennas is a dipole antenna.

10. A system in accordance with claim 7 , wherein in the sensor has a light-construction diaphragm oscillator forming one electrode of the transmitter, and a fixed electrode forms another electrode for the ultrasound transmitter whereby the antenna is electrically connected to said electrodes.

11. A system in accordance with claim 10 , wherein in the sensor the ultrasound transmitter of the antenna is electrically connected to said electrodes through an matching circuit.

12. A system in accordance with claim 7 , wherein the ultrasound transmitter of the sensor is formed by a piezo element.

13. A system in accordance with claim 7 , wherein at least one sensor includes several cavities with different basic frequencies connected to the same antenna in order to give an identity for the sensor unit.

14. A method in connection with an ultrasound sensor, the method comprising:

forming ultrasound in an electrostatically actuated ultrasound transmitter loaded with a fluid-filled cavity,

matching an antenna or a near-field coupling element to the ultrasound transmitter,

adapting a reader device to feed two frequencies (f 1 and f 2 ) to the ultrasound sensor, whereby the ultrasound transmitter actuates at the frequency difference of an incoming signal, and wherein the ultrasound sensor backscatters signals at an intermodulation frequency (f 3 ) of two actuation frequencies (f 1 and f 2 ), and

using the intermodulation response (f 3 ) of the ultrasound sensor to measure the dimensions of the fluid-filled cavity that is used to load the ultrasound transmitter or the propagation properties of sound in the fluid, whereby external measured quantity affects either the dimensions of the cavity, the propagation properties of sound in the cavity or both.

15. The method of claim 14 , wherein the reader device is inductively coupled to the ultrasound transmitter via a first coil connected to the reader device and a second coil connected to the ultrasound transmitter.