WO2012140310A1

WO2012140310A1 - Method and apparatus for monitoring an environmental variable

Info

Publication number: WO2012140310A1
Application number: PCT/FI2011/050339
Authority: WO
Inventors: Juhani Virtanen; Juha Vattulainen
Original assignee: Upm Rfid Oy
Priority date: 2011-04-15
Filing date: 2011-04-15
Publication date: 2012-10-18

Abstract

A sensor (100) comprises: - an RFID transponder (120) having an RFID chip (110), and - a capacitor (10) located outside the chip (110), 5 wherein the capacitance (Cx) of the capacitor (10) depends on an environmental variable (M_W, Δy, F_Z), the sensor device (100) is arranged to provide measured data (XDATA) based on the capacitance (Cx) of the capacitor (10), the transponder (120) is arranged to send a response (RES) to an interrogation signal (ROG) such that the response (RES) contains the 10 measured data (XDATA), and the sensor device (100) is arranged to monitor the capacitance (Cx) of the capacitor (10) by using operating energy extracted from a radio-frequency field (ROG).

Description

METHOD AND APPARATUS FOR MONITORING AN ENVI RONMENTAL VARIABLE

FI ELD OF THE INVENTION

The present invention relates to monitoring an environmental variable. In particular, the present invention relates to detecting moisture.

BACKGROUND

Buildings contain materials which may be damaged due to moisture. For example, moisture may cause rotting of wood in structural elements of a building. The presence of moisture may also cause growth of mould fungus, which is harmful to the health of humans and animals. Sometimes the damages may be limited or avoided if the moisture can be detected at an early stage.

SUMMARY

An object of the invention is to provide a method for monitoring an environmental variable. An object of the invention is to provide an apparatus for monitoring an environmental variable.

According to a first aspect of the invention, there is provided a sensor according to claim 1 . According to a second aspect of the invention, there is provided a method for monitoring an environmental variable according to claim 1 2.

According to a third aspect of the invention, there is provided an apparatus according to claim 27.

Further aspects of the invention are defined in the other claims. A sensor (1 00) may comprise:

- a transponder (1 20), and

- a capacitor (1 0),

wherein the capacitance (Cx) of the capacitor (1 0) depends on an environmental variable (M_w, Ay, F_z), the sensor device (1 00) is arranged to provide measured data (XDATA) based on the capacitance (Cx) of the capacitor (1 0), the transponder (1 20) is arranged to send a response (RES) to an interrogation signal (ROG) such that the response (RES) contains the measured data (XDATA), and the sensor device (1 00) is arranged to monitor the capacitance (Cx) of the capacitor (1 0) by using operating energy extracted from a radio-frequency field (ROG).

RFI D is an abbreviation for radio frequency identification. In particular, the environmental variable may be moisture content. The capacitance of the capacitor may depend on the moisture content.

In an embodiment, the environmental variable may be the level of a liquid. The capacitance of the capacitor may depend on the level of a liquid.

In an embodiment, the environmental variable may be the concentration of a first substance mixed or dissolved in a second substance. The capacitance of the capacitor may depend e.g. on the concentration of a first substance mixed or dissolved in a second substance.

In an embodiment, the environmental variable may be (mechanical) displacement. The capacitance of the capacitor may depend on the displacement. In an embodiment, the environmental variable may be a force acting on a stressed structural element. The capacitance of the capacitor may depend on said force.

Thanks to the invention, the environmental variable may be monitored in locations, which are difficult or impossible to access. The environmental variable may be monitored through a barrier without using galvanic feedthroughs. In particular, there is no need for an unobstructed line-of sight between the sensor and a reader.

When identification data is obtained from the sensor, measured data obtained from said sensor may be unambiguously associated with an item or location of the sensor.

The sensor may be arranged to extract operating energy from a radio frequency field. An RFID reader may be arranged to energize the sensor by providing a radio frequency field, and the RFID reader may be arranged to receive measured data from the sensor. Thus, the life of a battery does not limit the lifetime of the sensor. In particular, there is no need to replace the battery of the sensor during the lifetime of the sensor. Thus, the operating lifetime of the sensor may be very long.

Thanks to the invention, there is no need to use a large battery, and the total thickness of the sensor may be very small. For example, the total thickness of the transponder may substantially equal to the thickness of the RFID chip. Thanks to the invention, the battery may even be completely omitted. Thus, the use of toxic chemicals may be reduced or avoided.

Thanks to the invention, the manufacturing costs for the sensor may be substantially reduced. Due to the potentially low price a high number of sensors may be attached to various different locations. Due to the potentially low price, sensors may be attached to a high number of items.

Because there is no need to change batteries, the electrically conductive parts of the sensor may be hermetically encapsulated to withstand various environmental conditions. In particular, the sensor may be encapsulated to withstand corrosion. The encapsulation does not need to have a sealed lid for changing a battery.

An item equipped with the sensor may be moved through a stationary reader station in order to identify the item and to obtain measured data from the sensor. However, the potentially long lifetime also makes it possible to utilize the sensor in applications where the sensor remains in place for an extended period of time. Measured data may be obtained from a stationary sensor by using a movable RFID reader, in particular by using a portable RFID reader.

The sensor may be a moisture sensor. Moisture measurements have various applications. Capability to measure moisture via wireless techniques may broaden the application areas further.

For example, buildings may contain materials which may be damaged due to moisture. For example, moisture may cause rotting of wood in structural elements of a building. The presence of moisture may also cause growth of mould fungus, which is harmful to the health of humans and animals. Sometimes the damages may be limited or avoided if the moisture can be detected at an early stage. Usability of moisture detection increases naturally if the price of the sensors can be reduced. Wireless measurement methods, without need to line-of- sight access to the sensor device, further increase the applications to new areas. In an embodiment, the lifetime of the sensor may be very long, e.g. longer than e.g. 50 years without a need to replace a battery of the sensor. A sensor may be permanently embedded e.g. inside a wall of a building and the sensor may used for detecting moisture throughout the lifetime of the building.

In an embodiment, one or more moisture sensors embedded in a building element allow reliable long term monitoring of the condition of the building element. Building elements should normally be substantially dry. If moisture is detected in a building element, this may be an indication that a corrective action is required. The presence of moisture may damage building elements. The presence of moisture may cause growth of mould fungus, which is harmful to the health of humans and animals.

If the moisture content increases at a high rate, this may be an indication that immediate action is required e.g. to stop water flow from a leaking device. A control system may be arranged to automatically close a valve when rapid increase of moisture content is detected.

If the moisture content increases at a lower rate, this may be an indication that heating and/or ventilation of the building should be adjusted. A control system may be arranged to control heating, cooling and/or ventilation of a building based on moisture data obtained from a moisture sensor.

In an embodiment, moisture associated with a very large number of items or distribution of moisture in very large areas may be monitored at relatively low costs. This may be utilized e.g. when determining need for irrigation for agricultural land.

In an embodiment, sensors comprising standard RFI D transponders may be used as cheap disposable detectors.

The embodiments of the invention and their benefits will become more apparent to a person skilled in the art through the description and examples given herein below, and also through the appended claims.

BRI EF DESCRI PTION OF THE DRAWINGS

In the following examples, the embodiments of the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows a sensor comprising a capacitor for monitoring an environmental variable, Fig. 2a shows, in a three-dimensional view, a sensor comprising a dipole antenna,

Fig. 2b shows, in a three-dimensional view, a sensor comprising a coil antenna, shows, in a three-dimensional view, monitoring a plurality of items, shows, in a top view, a sensor having capacitor plates in the same plane, shows, in a cross-sectional view, the sensor of Fig. 4a, shows, in a three-dimensional view, the sensor of Fig. 4a, shows, in a cross-sectional view, the sensor of Fig. 4a, when exposed to moisture, shows, in a cross-sectional view, the sensor of Fig. 4a, with an optional moisture-absorbing layer, shows, in a three-dimensional exploded view, a moisture- detecting capacitor having capacitor plates in two different planes, shows, in a cross-sectional view, the capacitor structure of Fig. 5a, shows a circuit diagram for monitoring the capacitance, shows, by way of example, voltage and signal waveforms associated with the set-up of Fig. 6, shows, by way of example, alternative voltage and signal waveforms associated with the set-up of Fig. 6, shows a circuit diagram for monitoring the capacitance, wherein the capacitor is used as a part of a high-pass filter, shows a circuit diagram for monitoring the capacitance, wherein the capacitor is used as a part of a low-pass filter, shows, by way of example, voltage and signal waveforms associated with the set-up of Fig. 9a, shows a circuit diagram for monitoring the capacitance, shows an alternative circuit diagram of a digital output stage, shows an alternative circuit diagram of a digital output stage, shows a circuit diagram for monitoring the capacitance, wherein the output node of a driving unit has been connected to an input node of a monitoring unit, shows, by way of example, voltage and signal waveforms associated with the set-up of Fig. 13, shows an apparatus comprising a sensor and a reader, shows an apparatus comprising a sensor and a reader, shows an item comprising a sensor, shows an item comprising a several sensors, shows manufacturing an item, which comprises one or more sensors, shows an item comprising one or more sensors, shows a building comprising one or more sensors, shows a sensor having an additional capability of monitoring temperature, shows, in a top view, a sensor comprising a movable portion for monitoring displacement, shows, in a top view, the sensor of Fig. 22a in a state where the movable portion has been displaced, shows, in a three-dimensional view, a sensor comprising a movable portion for monitoring displacement, and shows, in a side view, a sensor arranged to detect displacement of a second item with respect to a first item.

DETAILED DESCRI PTION

Referring to Fig. 1 , a sensor device 1 00 may comprise an RFI D transponder 120, a capacitor 1 0, a driver unit 91 for varying the charging state of the capacitor 1 0, and a monitoring unit 92 for monitoring the charging state of the capacitor 1 0.

The capacitance Cx of the capacitor 10 depends on an environmental variable. The environmental variable may be e.g. humidity, level of a liquid pressure, force, or mechanical displacement. In particular, the capacitor 1 0 may be arranged such that presence of moisture LQ in the vicinity of the plates 1 1 , 1 2 of the capacitor 1 0 has an effect on the capacitance value Cx of the capacitor 1 0, when compared with a situation where there is no moisture in the vicinity of the plates 1 1 , 1 2. The driver unit 91 may charge the capacitor 1 0, and the monitoring unit 92 may be arranged to monitor corresponding variations in the voltage V_Cx of the capacitor 1 0.

The driving unit 91 may be arranged to charge the capacitor 1 0 by using energy extracted from the radio-frequency field ROG, and the monitoring unit 92 may be arranged to provide a digital signal S₂, which depends on variations in the voltage V_Cx of the capacitor 1 0.

The capacitor 1 0 may be connected e.g. between a terminal N1 of the driver unit 91 and a terminal N2 of the monitoring unit 92. Figs 6, 9a, 9b, 1 1 and 13 show various alternative arrangements for monitoring the capacitance Cx of the capacitor 1 0 by using the terminal N1 of the driver unit 91 and by using the terminal N2 of the monitoring unit 92. The capacitor 1 0 does not form a part of the oscillating radio frequency circuitry comprising radio frequency unit RXTX1 and antenna 140. Thus, the sensor 100 may operate such that the capacitance Cx does not have an effect on the radio frequency tuning of the RFI D transponder 1 20. Figs. 4a-5b show capacitor structures, which can be used to implement a moisture-detecting capacitor 1 0.

The transponder 120 may be attached to a substrate 1 30 so as to form an RFI D tag. The substrate 1 30 may be e.g. a plastic film, paper, or cardboard. The substrate 1 30 may be adhesive-lined so as to form an adhesive label. The transponder 1 20 may comprise protective layers to form a sealed structure. The transponder 1 20 may encapsulated so as to withstand various environmental conditions, e.g. moisture and/or other corrosive substances. The transponder 120 may be arranged to send a response RES to an interrogation signal ROG. The interrogation signal ROG may be sent from a mobile reader 200 or a stationary reader 200. In particular, the mobile reader may be a portable reader. The reader 200 and the transponder 1 20 may be arranged to communicate e.g. according to the EPC Gen2 protocol. EPC Gen2 is a abbreviation for "EPCglobal UHF Class 1 Generation 2". The protocol has been incorporated e.g. in the standard ISO 18000-6C (frequency band 860-960MHz). (Reference is made to the latest versions of the protocol and standard as in force on 1 5 April 201 1 ). The transponder 1 20 may comprise an RFID chip 1 1 0 connected to one or more antenna elements 140 via terminals N3, N4. The chip 1 1 0 may be a semiconductor element, which comprises e.g. resistors and transistors. The chip 1 1 0 is a radio frequency identification chip, which means that be chip may be arranged to receive a radio frequency interrogation signal ROG via the antenna 140 and to send a response RSP via the antenna 140 such that the response RSP contains identification data I D1 . The antenna 140 converts the radio signal ROG to an electrical signal coupled to the chip 1 1 0, and the antenna 140 also converts an electrical signal generated by the chip 1 1 0 into the radio signal RSP.

The RFI D chip 1 1 0 may comprise a plurality of miniature capacitors e.g. to implement logical gates and oscillators. However, the sensing capacitor 1 0 is located outside the RFI D chip 1 1 0. In other words, the capacitor 1 0 is an external capacitor. Thus, the chip 1 1 0 may be effectively protected e.g. from corrosive substances and/or stressing forces, and/or the size of the chip 1 1 0 may be minimized.

Electromagnetic interrogation signal ROG transmitted in a wireless manner is converted into an electrical signal by the antenna elements 140. The chip 1 1 0 may comprise a radio frequency unit RXTX1 , a control unit CNT1 , and a memory MEM1 . The radio frequency unit RXTX1 may comprise a signal receiver RX1 , and a signal transmitter TX1 . The receiver RX1 may also be called as a signal demodulator. The transmitter TX1 may also be called as a signal modulator. The radio frequency unit RXTX1 may also be called as an analog radio frequency interface. The radio frequency unit RXTX1 may comprise connection terminals N3, N4, which may be connected to at least one antenna element 140. The antenna elements may from e.g. a dipole antenna (Fig. 2a) or an inductive antenna (coil antenna, Fig. 2b). The radio frequency unit RXTX1 , the control unit CNT1 , and the memory MEM1 may be implemented on the same semiconductor chip 1 1 0. Also the driving unit 91 and the monitoring unit 92 may be implemented on the RFI D chip 1 1 0.

The receiver RX1 may provide an input signal SIN based on the received interrogation signal ROG. The control unit CNT1 may be arranged to enable transmission of first information I D1 e.g. when the input signal SIN contains a first (correct) password code (which matches with a reference code previously stored in the chip 1 1 0). The first information I D1 may comprise e.g. identification data of the transponder 120. The identification data I D1 may comprise e.g. an electronic item code (EPC). A unique electronic item code assigned to an item may be stored in a transponder 1 20 as a binary number.

The control unit CNT1 may be arranged to enable transmission of second information INF2 e.g. when the input signal SIN contains a second (correct) password code (which matches with a reference code previously stored in the chip 1 1 0). The second information INF2 may comprise e.g. measured data XDATA, previously stored measured data (reference data REFDATA), location data (LOCDATA) and/or calibration data (CALDATA). The second information INF2 may comprise a capability parameter, which specifies e.g. - whether the transponder is capable of detecting moisture,

- whether the transponder is capable of detecting displacement,

- whether the transponder is capable of monitoring temperatures,

- whether calibration data for the transponder exists,

- calibration data, and/or

- identification code for relevant calibration data.

The second information INF2 may be stored e.g. in the memory MEM1 of the transponder 1 20. The response RES transmitted by the transponder 1 20 may comprise the first information I D1 and/or the second information INF2. The information I D1 and/or INF2 may be retrieved from the memory MEM1 by the control unit CNT1 . The control unit CNT1 may send an output signal SOUT to the radio frequency unit RXTX1 . The output signal SOUT may comprise the information INF2. The transmitter TX1 may generate the radio-frequency response RES based on the output signal SOUT- The input signal S|_N and the output signal SOUT may be digital signals.

A dipole antenna may transmit information from the transponder 1 20 to a reader 200 by back scattering. Alternatively, an inductive antenna may be used. A coil antenna of the transponder 120 may cause modulation of the load for the reader 200. This modulation can be used for transmitting data from the transponder 120 to the reader 200.

The transponder 1 00 may substantially energetically passive, i.e. the radio frequency unit RXTX1 may be powered by energy extracted from an incoming radio frequency signal. The radio frequency unit RXTX1 may operate without a battery. The transponder 1 20 may be powered e.g. by electro-magnetic energy transmitted from the reader 200. The combination of an antenna structure 140 and a radio frequency unit RXTX1 of a transponder 100 may be arranged to provide operating power for the transponder 1 20 by extracting energy of an in-coming electromagnetic signal ROG. The radio frequency unit RXTX1 may comprise a voltage supply VREG1 , which is arranged to extract operating power from an incoming radio frequency signal. In particular, the voltage supply VREG1 may be arranged to extract operating power from the interrogation signal ROG. The operating power may be distributed to from the voltage supply VREG1 to the radio frequency unit RXTX1 . Operating power may be distributed to from the voltage supply VREG1 also to the control unit CNT1 , the memory MEM1 and to the units 92, 92.

Consequently, operating lifetime of the sensor 100 may be very long. Operating lifetime refers to a time when the transponder is capable of responding to an interrogation signal. In fact, the operating lifetime may be substantially infinite. There is no need to change a battery during the operating lifetime of the transponder. The transponder may be very small, as there it is not necessary to reserve a considerable space for the battery.

The transponder may be substantially energetically passive, i.e. energy for operating the radio frequency unit RXTX1 , the temperature monitoring unit 55, the control unit CNT1 , the memory MEM1 , the driving unit 91 and the monitoring unit 92 may be extracted from a radio frequency field. Energy for operating the radio frequency unit RXTX1 , the temperature monitoring unit 55, the control unit CNT1 , the memory MEM1 , the driving unit 91 and the monitoring unit 92 may be extracted from an interrogation signal ROG sent from a reader 200. An energetically passive transponder 1 00 may comprise an energy-storing capacitor or a rechargeable battery for storing operating energy extracted from an interrogation signal ROG. The information I D1 and/or INF2 may be transmitted e.g. by modulating a carrier frequency of the response RES at a modulation frequency f_LF- The modulation frequency f_LF may also be called as a "link frequency".

The monitoring unit 92 may be arranged to generate digital data XDATA (in particular digital moisture data) by using operating energy extracted from a radio frequency field, in particular by using operating energy extracted from an interrogation signal ROG. The monitoring unit 92 may be arranged to generate the data XDATA such that the data XDATA depends on the capacitance Cx.

The frequency of the energizing electromagnetic radio frequency field may be e.g. greater than or equal to 1 00 kHz, in particular greater than or equal to 1 MHz. The frequency of the electromagnetic radio frequency field may be substantially equal to the carrier frequency of the interrogation signal ROG.

The transponder 1 20 may be arranged to transmit measured data XDATA to a reader 200 e.g. when requested by an interrogation signal ROG. The measured data XDATA may also be called as measurement data. The response RES to the interrogation signal may contain the measured data XDATA. The response RES may be sent such that the carrier frequency f₀ of the response RES does not depend on the measured data XDATA. The response RES may be sent such that the carrier frequency f₀ of the response RES does not depend on the capacitance Cx of the capacitor 1 0.

Fig. 2a shows a sensor 1 00 comprising a dipole antenna. The chip 1 1 0 and the antenna elements 140 may be attached to a substrate 1 30 in order to form a sensor tag. The sensor 1 00 may be attached to an item 300a (Fig. 3) e.g. by using an adhesive. The sensor 1 00 may also be embedded inside an item. In particular, the presence of moisture LQ may have an effect on the capacitance Cx of the capacitor 1 0.

SX, SY and SZ denote orthogonal directions.

Fig. 2b shows a sensor 1 00 comprising an inductive antenna 140.

The total thickness of a sensor 1 00 (in the direction SZ) may be smaller than or equal to 1 mm. The sensor 1 00 may be flexible. The sensor 1 00 may further comprise an adhesive layer (not shown). A sensor 1 00 may further comprise a release layer, which protects the adhesive layer. The release layer can be removed before the sensor 1 00 is attached to an item by the adhesive layer. Referring to Fig. 3, a monitoring apparatus 700 may comprise one or more sensors 100, a reader 200, and an optional auxiliary unit 400.

The auxiliary unit 400 may also be called a data storage unit or a data processing unit.

The sensors 1 00a, 1 00b, 100c may be attached to items 300a, 300b, 300c. The environmental variable may be monitored by using response signals RES sent by the sensors 1 00a, 100b, 100c. Alternatively, the markings 300a, 300b, 300c may also refer to different portions 300a, 300b, 300c of a single item 300 (See Fig. 1 8).

A sensor 1 00a attached on an outer surface of the item 300a may be used for monitoring the environmental variable in the vicinity of the item 300a or for monitoring the environmental variable on the surface of the item 300a.

In particular, the environmental variable may be the (local) moisture content M_w. The unit of moisture content M_w may be e.g. kg/m³. A sensor attached on an outer surface of an item 300a may be used for providing an early warning of moisture. For example, the item 300 may be a package containing a moisture-sensitive material. The sensitive material may be e.g. paper or medicine. When moisture is detected on the surface of the item 300a, the item may be e.g. rapidly moved to a dry atmosphere in order to prevent damaging the moisture-sensitive material inside the item. A sensor 1 00a inside an item 300a may be used for monitoring of moisture inside the item 300a. For example, the item 300a may be a package, and the sensor 300a may be used for monitoring moisture inside the package. The package may contain e.g. foodstuff or a moisture-sensitive chemical substance (e.g. cement).

A plurality of tagged items 300a, 300b, 300c may stored in a storage, and a user may rapidly monitor an environmental variable associated with the items 300a, 300b, 300c by using the reader 200. A user may also make an inventory of the items 300a, 300b, 300c stored in said storage by receiving identification information carried by the responses RES.

The signals RES may be received by a reader 200. Measured data XDATA1 , XDATA2, XDATA3 obtained from the transponders 1 00a, 100b, 1 00c may be stored in a auxiliary unit 400, which may be separate from the reader 200. The auxiliary unit 400 may comprise e.g. a memory for storing measured data XDATA and/or calibration data CALDATA (See Fig. 16).

The reader 200 may be movable or moved with respect to a location reference LOCREF. An item 300a may be movable or moved with respect to a location reference LOCREF. The location reference LOCREF may be e.g. a building. A reader 200 may be movable or moved with respect to a stationary item 300a. An item 300a may be movable or moved with respect to a stationary reader 200. A mobile (portable) reader 200 may be moved in the vicinity of tagged items 300a, 300b, 300c. The reader 200 may be moved e.g. between receiving a first response and a second response. In case of a stationary reader 200, the items 300a, 300b, 300c may also be moved between receiving a first response and a second response.

COM1 denotes communication between the reader 200 and the auxiliary unit 400. The reader 200 may comprise a body 202 and a user interface 500. The user interface 500 may comprise e.g. a display for visually displaying measured information carried by responses RES1 , RES2, RES3 The user interface 500 may comprise e.g. a keypad or a touch-sensitive screen for receiving data and/or commands.

Each sensor 1 00a, 1 00b, 1 00c, may be arranged to transmit a response RES to an interrogation signal ROG. A response RES1 transmitted by the sensor 1 00a may comprise measured data XDATA1 and/or identification data I D1 associated with the first sensor 100a. The measured data XDATA1 may be used for monitoring an environmental variable (e.g. moisture content M_w) associated with the item 300a or portion 300a. The identification data I D1 may be used for identifying the item 300a or portion 300a.

A response RES2 transmitted by the sensor 1 00b may comprise measured data XDATA2 and/or identification data ID2 associated with the second sensor 100b. The measured data XDATA2 may be used for monitoring an environmental variable associated with the item 300a or portion 300b. The identification data I D2 may be used for identifying the item 300a or portion 300b.

A response RES3 transmitted by the sensor 1 00c may comprise measured data XDATA3 and/or identification data ID3 associated with the second sensor 1 00c. The measured data XDATA3 may be used for monitoring an environmental variable associated with the item 300a or portion 300c. The identification data I D3 may be used for identifying the item 300a or portion 300c.

Based on the identification data I D1 , measured data XDATA1 associated with the sensor 1 00a may be associated with the identity of the sensor 1 00a. Based on the identification data I D1 , measured data XDATA1 associated with the sensor 1 00a may be associated with the identity of the item 300 or portion 300a. Based on the identification data I D2, measured data XDATA2 associated with the sensor 1 00b may be associated with the identity of the sensor 1 00b. Based on the identification data I D2, measured data XDATA2 associated with the sensor 1 00b may be associated with the identity of the item 300b or portion 300b.

Based on the identification data I D3, measured data XDATA3 associated with the sensor 100c may be associated with the identity of the sensor 1 00c. Based on the identification data I D3, measured data XDATA3 associated with the sensor 100c may be associated with the identity of the item 300c or portion 300c.

By using the identification data I D1 , I D2, I D3, the number of the items 300a, 300b, 300c can be counted, and the type of the items can be identified.

When using several sensors 1 00a, 1 00b, 1 00c, measured data obtained from the sensors may be processed effectively and reliably when measured data obtained from a particular sensor is associated with identification data obtained from the same sensor.

A reader device 200 may be arranged to send an interrogation signal ROG for energizing a sensor 1 00a, and arranged to receive a response RES1 from the sensor 1 00a such that the response RES1 contains measured data XDATA1 and identification data I D1 .

The reader 200 may comprise a display 500 for displaying measured information such that measured information is associated with an identifier, wherein the measured information is determined from the measured data XDATA1 and the identifier is determined based on the identification data I D1 . The measured information may be the same as the measured data XDATA1 . The identifier may be the same as the identification data I D1 . However, the measured information may also be different from the measured data XDATA1 . The identifier may be different from the identification data I D1 . For example, the measured data may be e.g. a numerical value "90", and the identification data may be an identification number e.g. "5432". The corresponding displayed measured information may be e.g. the word "wet" and the corresponding identifier may be e.g. "bathroom floor" or "bottom of washing machine"

The reader 200 may be arranged e.g. to compare the measured data XDATA1 with reference data REFDATA, and to provide an alarm based on said comparison.

Figs. 4a- 5b show structures for implementing a moisture-detecting capacitor 10.

Fig. 4a shows a moisture sensor 1 00 where the capacitor plates 1 1 , 1 2 of the capacitor 1 0 are implemented in the same plane. This may facilitate manufacturing of the sensor 100 and/or to reduce manufacturing costs. Also the capacitor plates 1 1 , 1 2 and the antenna elements 140 may be in the same plane. The capacitor plates 1 1 , 1 2 and the antenna elements 140 may be implemented on a substrate 1 30.

The capacitor plates and/or the antenna elements may be e.g. conductive foils. The material of the foils may comprise e.g. aluminum, copper, silver, brass or steel. The form of the plates and/or the antenna elements may be implemented e.g. by etching, laser cutting, water jet cutting and/or deposition. The plates and/or the antenna elements may also be printed by using conductive ink or conductive paste. If one or more external resistors are needed in addition to the external capacitor 1 0 e.g. to implement one of the circuits shown in Figs. 9a, 9b, 1 1 and 1 3, the resistors (R3, R4 or R7) may be produced on the substrate 1 30 e.g. by applying resistive paste or resistive ink on the substrate 1 30. A resistor may be printed on the substrate by using resistive ink.

The plates 1 1 , 1 2 may comprise several plate portions 1 1 a, 1 1 b, 1 1 c, 1 2a, 12b, 1 2c in order to increase the capacitance Cx. The plate portions of the first plate 1 1 and the plate portions of the second plate 12 may be interlaced so as to form a finger capacitor. At least one plate portion 1 1 b of the first plate 1 1 may be located between plate portions 1 2a, 1 2b of the second plate 12. Referring to Fig. 4b, the capacitor plates 11, 12 and the terminals N1, N2 may be covered with a protective layer 131. The capacitor plates 11, 12 and the terminals N1, N2 may be encapsulated between the substrate 130 and the protective layer 131. The substrate 130 and the protective layer 131 should be electrically insulating so as to allow operation of the plates 11, 12 as a capacitor. At least one of the capacitor plates 11, 12 may be hermetically encapsulated in order to avoid electrochemical corrosion of the capacitor plates due to moisture. This may provide a long operating time even in the presence of moisture. Advantageously, both capacitor plates 11, 12 may be hermetically encapsulated. The substrate 130 and the protective layer 131 may be impermeable to moisture in order to hermetically encapsulate the plates 11, 12. The capacitor plates 11, 12 may be encapsulated e.g. within a polymer or glass.

Fig.4c shows, in a three-dimensional view, the sensor of Fig.4a.

Fig.4d shows, in a cross-sectional view, the sensor of Fig.4a when moisture LQ is present in the vicinity of the capacitor 10. The moisture may be adsorbed e.g. on the substrate 130 and/or on the protective layer 131. The capacitance Cx of the capacitor 10 depends on the dielectric constants of materials in the vicinity of the capacitor plates 11, 12. Moisture, i.e. water has a high dielectric constant when compared with air. The capacitor 10 may have a first capacitance value when a higher amount of water is present in the vicinity of the capacitor plates 11, 12. The capacitor 10 may have a second lower capacitance value when a lower amount of water is present in the vicinity of the capacitor plates 11, 12. Thus, the presence of water in the vicinity of the capacitor plates 11, 12 may be detected by monitoring the capacitance Cx of the capacitor 10. A change in the amount of water LQ in the vicinity of the capacitor plates 11, 12 may be detected by monitoring a change in the capacitance Cx of the capacitor 10. The moisture LQ may be e.g. water which has condensed onto the sensor 100 or which has been transferred to the vicinity of the sensor 1 00.

Referring to Fig. 4e, the sensor 1 00 may further comprise moisture-absorbing material 132. The moisture-absorbing material 1 32 may be used to increase the amount of water LQ in the vicinity of the capacitor 10. The moisture- absorbing material 1 32 may be used to absorb water vapor from the gas phase into the material 1 32. The moisture-absorbing material 132 may also be used to increase the time constant of the sensor 1 00. The moisture absorbing material 1 32 may be e.g. paper or cardboard.

Referring to Fig. 5a, the plates 1 1 , 1 2 of the capacitor 1 0 may also be in different planes. The plates 1 1 , 1 2 may be separated by a material layer 1 32. The first capacitor plate 1 1 may be encapsulated within a layer of protective material 1 31 a in order to protect the plate from electrochemical corrosion. The second capacitor plate 1 2 may be encapsulated within a layer of protective material 1 31 b in order to protect the plate from electrochemical corrosion. Fig. 5a is an exploded view. During normal operation, the protective layer 131 a (or plate 1 1 ) may be in contact with the material layer 1 32, and also the protective layer 1 31 b (or plate 1 2) may be in contact with the material layer 132. The material layer 1 32 may be moisture-absorbing and/or porous such that the amount of water between the plates 1 1 , 12 may vary according to the moisture content M_w of the environment ENVI. The plate 1 1 and the protective layer 1 31 a may have holes or gaps 1 5 in order to accelerate transfer of water (or water vapor) from the environment ENVI to the material layer 1 32 and/or to accelerate transfer of water from the material layer 1 32 back to the environment ENVI. Liquid water and/or water vapor may be transferred through the holes/gaps 1 5 from the environment ENVI to the material layer 1 32 and/or from the material layer 1 32 to the environment ENVI. Fig. 5b shows, in a cross-sectional view, the stacked capacitor structure of Fig. 5a.

In an embodiment, a first capacitor plate 1 1 of the capacitor 10 may be surrounded by a second capacitor plate 1 2 in order to minimize coupling of electric interference to the capacitor 1 0. In other words, the second capacitor plate 1 2 may be arranged to operate as a Faraday cage around the first capacitor plate 1 1 . The second capacitor plate 1 2 may have e.g. holes 1 5, gaps, or a mesh structure in order to facilitate transfer of moisture to the space between the capacitor plates 1 1 , 1 2.

In an embodiment, the first capacitor plate 1 1 and the second capacitor plate 12 of the capacitor 1 0 may be surrounded by a third conductive element, which is arranged to operate as a Faraday cage. The third conductive element may be connected e.g. to the ground node N5 or N6 shown in Fig. 6.

Variation in the capacitance Cx of the capacitor 1 0 may be monitored e.g. by using one of the arrangements shown in Figs. 6-14. Referring back to Fig. 1 , an RFI D chip 1 10 may comprise a driver unit 91 and a monitoring unit 92. The driver unit 91 may have a node (terminal) N1 , which may be arranged to operate as a digital output. The monitoring unit 92 may have a node (terminal) N2, which may be arranged to operate as a digital input. In particular, a response RES sent by the sensor 1 00 may comprise measured data XDATA, which depends on the state of the digital input N2. In an embodiment, a signal provided by the output N1 may be controlled by sending an interrogation signal ROG to the sensor 1 00.

Referring to Fig. 6, the driver unit 91 may comprise a driver stage comprising a switch SW1 and a resistor R1 . The state of the switch SW1 may be controlled according to a digital signal Si provided by the control unit CNT1 (Fig. 1 ). The signal Si may control the switch SW1 e.g. via a buffer BUF. One node N5 of the switch SW1 may be connected to a first ground GND1 . One node N7 of the resistor R1 may have a DC voltage V₀ with respect to the ground GND1 . The voltage V₀ may be higher than a minimum voltage V_H corresponding to the high logical state. DC denotes direct current. The switch SW1 and the resistor R1 may be connected to the output node N1 . The switch SW1 may be e.g. a transistor.

The monitoring unit 92 may provide a digital signal S₂ based on the voltage of the input node N2. The unit 92 may have hysteresis, but it does not need to have. The unit 92 may comprise e.g. a Schmitt trigger. The unit 92 may be connected to a reference ground potential GND2 at a node N6. The input node N2 may be optionally connected to a node N8 by a resistor R2. The node N8 may have e.g. a voltage V₀. The resistor R2 may reduce a risk of overvoltage-induced damage to the unit 92. Furthermore, the resistor R2 may be arranged to pull the node N2 to a high logical state in a situation where the node N2 floating freely.

The second ground potential GND2 may be connected to the first ground GND1 e.g. via an internal connection 93 inside the chip 1 1 0.

The chip 1 1 0 may optionally have a connection terminal for making an external connection to the node N5. The chip 1 1 0 may optionally have a connection terminal for making an external connection to the node N6.

The capacitor 1 0 may be connected between the node N1 of the driver unit 91 and the node N2 of the monitoring unit 92.

V-i denotes the voltage of the node N1 , and V₂ denotes the voltage of the node N2.

Fig. 7 shows, by way of example, temporal behavior of voltages and logical states in case of the arrangement of Fig. 6. "0" refers to the low logical state and "1 " refers to the high logical state. V_H denotes a voltage level where a rising voltage is considered to be at the high logical state. V_L denotes a voltage level where a decreasing voltage is considered to be at the low logical state.

The uppermost curve of Fig. 7 shows the digital signal S-i , which is used to control charging (and/or discharging) the capacitor 1 0, by using the buffer BUF and the switch SW1 . Referring to the second curve from the top, the voltage V of the node N1 may be switched to zero at the time t| . Referring to the third curve from the top, the voltage V₂-V of the capacitor 10 may start to (exponentially) increase at the time t-i . The capacitor 1 0 may now be charged via the resistor R2 and the switch SW1 .

The third curve from the top shows temporal evolution of the voltage difference V₂-V-| .

Referring to the fourth curve from the top, the voltage V₂ at the input node N2 may reach the voltage level V_H at the time t₂. Referring to the lowermost curve of Fig. 7, the digital output of the monitoring unit 92 may also change state at the time t₂, respectively.

The length of the time period At between the time ti and the time t₂ depends on the capacity of the capacitor 1 0. Thus, the capacitance Cx or a change in the capacitance Cx may be monitored by monitoring the time t₂ and/or the length of the period At. The control unit CNT1 may be arranged to provide the measured data XDATA by e.g. monitoring the length of the time period At.

For example, the control unit CNT1 may be arranged to count how many clock pulses of the local oscillator 52 (Fig. 21 ) correspond to the length of the time period At.

At the time t₃, the voltage of the node N1 may be switched to V₀, and the capacitor 1 0 may be discharged via the resistors R1 and R2. Consequently, the above-mentioned charging and discharging cycle may be repeated in order to make a new measurement, if desired.

Referring to Fig. 8, the capacitance Cx or a change in the capacitance Cx may be monitored by varying the duration of a charging period and/or by varying the duration of a discharging period. In particular, the capacitance Cx or a change in the capacitance Cx may be monitored by varying the pulsation frequency of the signal S-i . The uppermost curve of Fig. 8 shows temporal evolution of the signal S-i .

The second curve from the top shows the voltage of the node N1 . The third curve from the top shows the voltage difference V₂-V-| . The charge stored in the capacitor 10 is proportional to the voltage difference V₂-V-| . The fourth curve from the top shows the voltage of the node N2, and the lowermost curve of Fig. 8 shows the digital output S₂ of the monitoring unit 92.

When the period Δΐ_Ρι of the driving pulse is short, the fluctuations of the voltage V₂-V of the capacitor 10 may be so small that the voltage V₂ at the node N2 does not reach the low logical level V_L.

When the capacitor 10 is charged during a longer time period, the voltage V₂- V-i of the capacitor 10 may have larger fluctuations. With a longer period Δΐ_Ρ2, the voltage V₂-V of the capacitor 10 may fluctuate so that the voltage V₂ at the node N2 reaches the low logical level V_L at a time t-i .

At the time t₂, the voltage V₂ at the node N2 may reach the high logical level I_H. The length of the time period At between the time ti and the time t₂ depends on the capacitance Cx of the capacitor 10. Thus, the capacitance Cx or a change in the capacitance Cx may be monitored by monitoring the time t₂ and/or the length of the period At.

The capacitance Cx or a change in the capacitance Cx may also be monitored by determining the shortest period At_P2 of the driving pulse Si which is still capable of changing the state of the logical output signal S₂.

Referring to Fig. 9a, a combination of the capacitor 10 and a resistor R3 may be connected as a high-pass filter between the nodes N1 , N2. The capacitance Cx of the capacitor 10 has an effect on the cut-off frequency of the high pass filter. Thus, the capacitance Cx or a change in the capacitance Cx may be monitored by varying the frequency and/or timing of the driving signal Si and by monitoring the cut-off frequency of the high-pass filter. Referring to Fig. 9b, a combination of the capacitor and a resistor R4 may be connected as a low-pass filter between the nodes N1 , N2. The capacitance Cx of the capacitor 1 0 has an effect on the cut-off frequency of the low- pass filter. Thus, the capacitance Cx or a change in the capacitance Cx may be detected by varying the frequency and/or timing of the driving signal Si and by monitoring the cut-off frequency of the low pass filter.

Fig. 1 0 shows waveforms associated with Fig. 9a. When the duration Δΐ_Ρ3 of the charging is short, the charge stored in the capacitor 10 is low, and the voltage V₂ of the node N2 reaches the low logical level V_L when the switch SW 1 is closed i.e. at the time t-i .

When the duration Δΐ_Ρ4 of the charging is longer, the charge stored in the capacitor 1 0 is higher, and the change of the output signal S₂ from high logical level to low logical level (at time t₃) and a change of the voltage V1 to zero (at time t₄) do not take place simultaneously.

Thus, when the duration Δΐ_Ρ3 of the charging is short, the waveform of the signal S₂ may be identical to the waveform of the signal S-i . On the other hand, when the duration Δΐ_Ρ4 of the charging is longer, the timing of the signal S₂ may deviate from the timing of the signal S-i .

Thus, the capacitance Cx or a change in the capacitance Cx may be monitored by varying the frequency and/or timing of the driving signal Si and by monitoring the time difference t₃-t₂ and/or by monitoring the time difference t₄-t₃.

Referring to Fig. 1 1 , the capacitor 1 0 and a resistor R4 may be connected in series between the nodes N1 , N2. The operation of this set-up may be substantially similar to the operation of the set-up of Fig. 6, the resistor R4 may just reduce the rate of charging and discharging.

Referring to Fig. 1 2a, the driver unit 91 may comprise two switches SW1 , SW2, which are arranged to operate in a push-pull configuration instead of the A-class driving stage of Figs. 6, 9a, 9b, 1 1 and 1 3. Referring to Fig. 1 2b, the driver unit 91 may comprise a switch SW2 and a resistor R6 to implement an alternative A-class driving stage instead of the arrangement of Figs. 6, 9a, 9b, 1 1 and 1 3. Referring to Fig. 1 3, the node N1 may also be directly connected to the node N2. The capacitor 1 0 may be connected e.g. between the ground and the node N2 so that the the capacitor can be charged and discharged by the node N1 of the driving unit 92. A resistor R7 may be connected in series with the capacitor 1 0 in order to reduce the current coupled through the switch SW1 to a level, which does not damage the switch SW1 .

Fig. 14 shows, by way of example, voltage and signal waveforms associated with Fig. 1 3.

The uppermost curve of Fig. 14 shows the digital signal S-i , which controls the switch SW1 . At the time t_{1 ;} the switch SW1 is closed (i.e. SW1 is switched to the conducting state), and the capacitor 1 0 is discharged via the resistor R7 and the switch SW1 . The voltage V₃ at the node N9 decreases, as shown in the third curve from the top.

At the time t₃, the switch SW1 is opened, and the capacitor 1 0 is charged via the resistors R1 , R2 and R7. The capacitance Cx of the capacitor 1 0 has an effect on the time period At between the time t₄ and t₃. Thus, the capacitance Cx or a change in the capacitance Cx may be monitored by charging and discharging the capacitor 1 0 and by monitoring the length of the time period At (=t₄-t₃). Referring to Fig. 1 5, a temperature monitoring apparatus 700 may comprise one or more sensors 1 00, and a reader 200.

The reader 200 may stationary or movable with respect to the location reference LOCREF. In particular, the reader 200 may be portable. The reader 200 may comprise a control unit 21 0 (CNT2) for controlling operation of the reader 200, and a radio frequency unit RXTX2 for transmitting interrogation signals ROG, and for receiving response signals RES. The radio frequency unit RXTX2 may be arranged to operate such that it provides a radio frequency field which in turn energizes the transponder 120. The system 700 may be arranged to operate such that the transponder 120 extracts operating energy from the interrogation signal ROG provided by the reader 200. The radio frequency unit RXTX2 may comprise a transmitter TX2 and a receiver RX2.

The reader 200 comprises at least one antenna 205 for transmitting an interrogation signal ROG. Also the response RES may be received by the (same) antenna 205. The antenna 205 may be e.g. a dipole antenna or an inductive antenna (i.e. a coil).

The antenna 205 may also be a leaky waveguide antenna (not shown) arranged to distribute the electromagnetic radio frequency interrogation signal ROG to a large area. A leaky waveguide antenna may also be arranged to obtain electromagnetic radio frequency response signals RES from said large area. In particular, the leaky waveguide antenna 205 may comprise a microstrip waveguide. The reader 200 may comprise a memory MEM2 for storing identification data I D1 (identifier F1 ) and measured data XDATA1 (measured information) associated with the identification data I D1 (identifier F1 ).

The memory MEM2 (or a memory MEM9, see Fig. 1 6) may further comprise computer program code, which when executed by the control unit CNT2 is for carrying out the method according to the present invention.

The reader 200 may comprise a user interface 500. The user interface 500 may comprise e.g. a display 501 (See e.g. Fig. 3) for displaying measured information and/or identification data. The user interface 500 of the system 700 may comprise a display arranged to visually display measured information and/or identification data.

Referring to Fig. 1 6, the apparatus 700 may comprise one or more additional units and/or functionalities when compared with the set-up of Fig. 1 5. The apparatus 700 (i.e. a system 700) may comprise an auxiliary unit 400.

The auxiliary unit 400 may comprise a control unit CNT4 (41 0) for processing data. In this case, the auxiliary unit 400 may also be called as a data processing unit. The control unit 41 0 (CNT4) may be arranged to control operation of the auxiliary unit 400 and/or for controlling operation of the reader 200.

The control unit CNT4 may be arranged to provide a control signal S_CNT based on the measured data XDATA. In particular, the measured data XDATA may be moisture data. The control signal S_CNT may be used for controlling e.g. a heater element or a ventilation fan (See Fig. 20). The control signal S_CNT may be used for controlling an actuator, heater, cooling unit, ventilation unit, pump etc. based on measured data XDATA obtained from the transponder(s).

The auxiliary unit 400 may optionally comprise one or more of the following memory units or memory areas MEM3, MEM4, MEM5, MEM6, MEM7, MEM8, MEM9.

The auxiliary unit 400 (or the reader 200) may comprise a memory MEM3 for storing measured data XDATA (measured information). The control unit CNT4 may be arranged to store the measured data XDATA (measured information) such that measured data XDATA obtained from a sensor 1 00 is associated with identification data obtained from said sensor 1 00.

First measured data XDATA1 obtained from a first sensor 1 00a may be associated with identification data I D1 obtained from the first sensor 1 00a. Second measured data XDATA2 obtained from a second sensor 1 00b may be associated with identification data I D2 obtained from the second sensor 100b. The sensor 1 00 may optionally have a capability of monitoring temperature (See Fig. 21 ). The auxiliary unit 400 (or the reader 200) may comprise a memory MEM4 for storing temperature data TDATA obtained from a sensor 100. The temperature data TDATA may be stored such that it is associated with corresponding identification data.

The auxiliary unit 400 (or the reader 200) may comprise a memory MEM5 for storing time data TIMEDATA. The control unit CNT4 may be arranged to retrieve a time associated with a specific measured data value stored in the memory MEM3 by using the time data TIMEDATA stored in the memory MEM6.

The apparatus 700 may be arranged to store history data associated with a sensor 1 00. The history data may comprise measured data values obtained at different times from the same sensor 1 00. The history data may be stored e.g. in the memory MEM2, MEM3 and/or MEM5.

The auxiliary unit 400 (or the reader 200) may comprise a memory MEM6 for storing location data LOCDATA. The control unit CNT4 may be arranged to retrieve a location associated with a specific measured data value stored in the memory MEM3 by using the location data LOCDATA stored in the memory MEM6.

Location data LOCDATA stored in the memory MEM6 may be used to determine a location of a sensor 1 00 based on the identification data I D1 .

A memory MEM7 may comprise reference data REFDATA. The control unit CNT4 may be arranged to perform an action based on a comparison between measured data XDATA and reference data REFDATA.

The memory MEM8 may store calibration data CALDATA associated with the identifier I D1 of a sensor 1 00. Absolute values of an environmental variable may be calculated from the measured data XDATA by using the calibration data CALDATA. In particular, absolute moisture or humidity values may be calculated from measured moisture data by using the calibration data CALDATA. The auxiliary unit 400 may comprise a user interface 500 (not shown). The reader does not need to comprise a user interface 500.

A memory MEM9 may store computer program code PROG, which when executed by a data processor is for operating the apparatus 700 according to the invention. The apparatus 700 may comprise a computer-readable medium MEM9 storing computer program code PROG, which when executed by data processor CNT2, CNT4 is for executing a measurement, displaying and/or control.

The control unit CNT4 may be arranged to control a system 700 based on the comparison between measured data XDATA and reference data REFDATA. The control unit CNT4 may be arranged to provide a control signal S_CNT based on the comparison between measured data XDATA and reference data REFDATA.

The control unit CNT4 may be arranged to initiate an alarm procedure based on the comparison between measured data XDATA and reference data REFDATA.

Measured data XDATA1 obtained from a first sensor 1 00a may be e.g. compared with reference data REFDATA, which has been determined based on measured data, which has been obtained earlier from the same sensor 100a.

Measured data XDATA1 obtained from a first sensor 1 00a may be e.g. compared with reference data REFDATA, which has been determined based on measured data XDATA2 obtained from a different sensor 1 00b.

COM1 denotes communication between the reader 200 and the auxiliary unit 400. The communication COM1 may take place e.g. via a mobile telephone network, internet, Wireless Local Area Network (WLAN), Bluetooth, electrical cable, and/or optical cable. The reader 200 may optionally comprise a navigation unit NAV1 for determining the position of the reader 200. The navigation unit NAV1 may comprise e.g. a satellite navigation device, a laser distance meter and/or an ultrasonic distance meter. In particular, the navigation unit NAV1 may be a GPS device (GPS is an acronym for Global Positioning System). The navigation unit NAV1 may be arranged to determine the position of the reader 200 with respect to one or more reference devices. The reference devices may be e.g. optical prisms, crosshair patterns, laser units, or radio beacons.

The reader 200 and/or the auxiliary unit 400 may comprise a user interface 500. The user interface 500 may comprise e.g. a display 501 (not shown) for displaying measured information and/or identifiers. The user interface 500 may comprise e.g. a keypad (not shown) or a touch screen for receiving commands from a user.

Several sensors 1 00a, 100b, 1 00c may be simultaneously within the interrogation range of the reader 200. Sometimes a user can not be sure whether he is gathering temperature data from the right sensor. For that purpose, the maximum interrogation range of the reader 200 may be adjustable. The interrogation range may be adjusted to be so short that only the sensor closest to the reader responds to an interrogation signal. In this case, the position of the (responding) sensor 1 00 may be approximated by the position of the transponder 1 20 (during the transmission of the response). The interrogation range may be adjusted e.g. by using the user interface 500. The interrogation range may also be called as the maximum reading distance. The temperature monitoring system 700 may be arranged to provide measured information such that the measured information is associated with an identifier. Consequently, reliable measurements may also be made when several sensors 1 00a, 1 00b, 1 00c are simultaneously within the interrogation range of the reader 200. The identity of a sensor 1 00a may also be associated with the location of said sensor 1 00a. For that purpose, the location of the sensor 1 00a should be determined. In particular, the the location of the transponder 1 00a may be measured.

Sometimes a sensor may be embedded in a material such that is is not directly visible. The location of a transponder 1 00 may be measured with respect to a location reference LOCREF by using a reader 200. Determining a location (x,y) of a sensor 100a with respect to the location reference LOCREF may comprise:

- determining the location of a reader 200 with respect to the location reference LOCREF, and

- determining the position of the sensor 1 00a with respect to the reader 200.

The location of the reader 200 may refer to the location of an antenna 205 of the reader 200. The location of the sensor 1 00a may refer to the location of an antenna 140 of the transponder 1 20. In an embodiment, the interrogation range of the reader 200 may be set to be so short that the position of a responding sensor 1 00a may be approximated by the location of the reader 200. In other words, the position of the sensor 100a may be approximated by the position of the reader 200a if the distance between the reader 200 and the sensor 1 00a may be assumed to be smaller than a predetermined value.

The interrogation range may be limited e.g. by adjusting the amplitude of the interrogation signal ROG and/or by setting a minimum level for an acceptable response RES. If the amplitude of the response RES is not greater than or equal to the minimum level, the response RES received by the reader 200 may be rejected.

The position of a sensor 1 00 with respect to a reader 200 may also be determined e.g. by triangulation.

Fig. 1 7 shows an item 300 comprising a sensor 1 00. Fig. 1 8 shows an item 300 comprising several sensors 100a, 100b, 1 00c. A first portion 300a of the item may comprise a first sensor 1 00a, a second portion 300b of the item may comprise a second sensor 1 00b, and a third portion 300c of the item may comprise a third sensor 1 00c.

Measured data may be obtained from the sensors 1 00a, 1 00b, 1 00c by a reader 200. In this case, the measured data obtained from the sensor 1 00c may reveal that the environment in the vicinity of the sensor 1 00c has become wet.

Referring to Fig. 1 9a and 19b, one or more sensors 1 00a, 1 00b, 1 00c may be embedded or attached in an item already when the item 300 is manufactured. The item may be manufactured e.g. from materials 31 1 , 31 2. The sensors 100a, 1 00b, 1 00c may be capable of sending corresponding identification data I D1 , I D2, I D3. Thus, the same sensors may be used for monitoring the environmental variable and for identifying the item 300 and/or location in question. In particular, the sensors 1 00a, 1 00b, 1 00c may be moisture sensors.

Referring to Fig. 20, a system 700 may be controlled based on moisture data obtained from one or more moisture sensors 1 00a, 1 00b, 100c.

The system 700 may be e.g. a building. The building 700 may comprise a building element 300, which in turn may comprise one or more moisture sensors 100a, 100b, 100c. The item 300 may be e.g. a floor element, a wall element, a ceiling element or a roof element. The item 300 may be e.g. a block of thermal insulation or a wooden beam. Each sensor 1 00a, 100b, 1 00c may be used to detect moisture in the vicinity of said sensor.

Heating, cooling, ventilation and/or a liquid valve 760 may be controlled based on moisture data obtained from one or more moisture sensors 1 00a, 100b, 1 00c. An auxiliary unit 400 may be arranged to control operation of a heating device 710 based on moisture data obtained from one or more moisture sensors 1 00a, 1 00b, 1 00c. An auxiliary unit 400 may be arranged to control operation of a cooling device 71 0 based on moisture data obtained from one or more moisture sensors 1 00a, 1 00b, 1 00c. An auxiliary unit 400 may be arranged to control operation of a ventilation unit 720 based on moisture data obtained from one or more moisture sensors 1 00a, 1 00b, 100c. An auxiliary unit 400 may be arranged to control a liquid valve 760 based on moisture data obtained from one or more moisture sensors 1 00a, 100b, 100c. For example, if moisture LQ is detected under a device 750, a liquid valve 760 may be automatically closed in order to shut off water flow to the device 750 and in order to minimize damage to the building structures. The device 750 may be e.g. a washing machine. A moisture sensor 1 00 having an additional temperature-monitoring capability may be used e.g. to determine a dew point. When the temperature of a gas (air) is decreased, the state of the sensor 1 00 may be changed from a dry state to a wet state at a certain temperature called as the dew point. When the temperature of a gas is decreased, the dew point is the highest temperature where atmospheric water vapor condenses from the gas on a smooth planar surface. In particular, when the temperature of air decreases in the evening or during nighttime, a material or a surface in the vicinity of the capacitor 1 0 may become wet at the dew point. Thus, the temperature where the capacitance Cx of the capacitor 1 0 rapidly increases may be considered to be the dew point.

The moisture sensor 1 00 may be arranged to detect moisture LQ. The moisture LQ may be e.g. (liquid) water, which has been condensed onto the sensor 1 00 from vapor phase. Water may also be transferred onto the sensor 100 due e.g. to a leak in a water pipe. The moisture LQ may be e.g. free- flowing water, or water which is absorbed in a substance. In particular, the moisture LQ may be water which is absorbed in a porous or fibrous substance.

The sensor 1 00 may be arranged to detect a change in the dielectric constant of a medium, which is located in the vicinity of the sensor 1 00. The dielectric constant of water is approximately equal to 80. The dielectric constant of ice is approximately equal to 3. The dielectric constant of ethanol is approximately equal to 26. The dielectric constant of ethylene glycol is approximately equal to 37. The dielectric constant of gasoline is approximately equal to 2. The dielectric constant of kerosene is approximately equal to 2. The dielectric constant of dry wood may be e.g. in the range of 2.5 to 7. The dielectric constant of air is approximately equal to 1 . The dielectric constant may also be called as the relative permittivity. The sensor 100 may be arranged to monitor the presence of a first substance in a second substance, wherein the dielectric constant of the first substance is substantially different from the dielectric constant of the second substance.

For example, the sensor 1 00 may be used to monitor e.g. the content of water in lubrication oil. The sensor may be used to monitor e.g. the content of water in a fuel, wherein the fuel may be e.g. gasoline, kerosene, ethanol, wood or peat. The sensor may be used to monitor e.g. the content of water in a liquid coolant comprising an anti-freezing substance, e.g. glycol. The sensor may be used to monitor e.g. the content of ethanol in gasoline. The sensor may be used to monitor the content of water in ethanol or the content of ethanol in water.

A predetermined minimum level of humidity may be required when storing and/or transporting certain foodstuffs, medicines and/or chemicals. A predetermined maximum level of humidity may be required when storing and/or transporting certain foodstuffs, medicines and/or chemicals. A package for food, medicine or chemical may contain a moisture sensor 1 00 in order to monitor humidity in the package. A control unit CNT4 may be arranged to adjust humidity in the package or in a storage room such that humidity is maintained in an optimum range.

The dielectric constant (=3) of ice is substantially lower than the dielectric constant of water (=80). Thus, the moisture sensor 1 00 may be used to detect freezing of water and/or melting of ice. The sensor 1 00 may be used to detect the level of a liquid. For example, the sensor 1 00 may be used to detect the level of water in a vessel or to detect the level of water in a well. The vessel or the well may contain substantially clean water or waste water.

The sensor 100 may be used to detect moisture in agricultural land. Irrigation (watering) may be controlled based on moisture data obtained from one or more moisture sensors embedded in soil. The reader 200 may be attached to a vehicle to obtain measurement data XDATA from sensors positioned to monitor humidity of a large agricultural area.

The dielectric constant of water depends on temperature. Absolute moisture data may be calculated from the moisture data XDATA obtained from a moisture sensor 1 00 by using a temperature-dependent function. In particular, absolute moisture values M_w may be determined by multiplying a moisture data value XDATA with a temperature-dependent coefficient. The moisture data M_w may be e.g. as a numerical value which is in the range of 0 to 1 000. An absolute moisture data value M_w may be expressed e.g. as a percentage where 0% indicates that no water is present in the vicinity of the moisture sensor 1 00, and the value 1 00% indicates that the moisture sensor 100 is completely immersed in water.

Referring to Fig. 21 , a sensor 1 00 may also be capable of monitoring temperature of the RFID chip 1 1 0 or capable of monitoring temperature in the vicinity of the RFI D chip 1 10. Temperature data TDATA obtained from the moisture sensor 1 00 may supplement the other measured data XDATA obtained from the same sensor 1 00.

For example, the link frequency of the transponder 1 20 may depend on a frequency f_Cu< of a local oscillator 52 of the RFI D chip such that the temperature of the chip 1 1 0 or a change in the temperature of the chip 1 1 0 may be determined from variations in the link frequency. A reader 200 may be arranged to monitor variations in the link frequency and to determine the temperature of the chip 1 10 based on variations in the link frequency. The sensor 1 00 may comprise a temperature sensor 57. The temperature sensor 57 may be e.g. a temperature-dependent resistor, a P-N junction, or a thermocouple. The sensor 1 00 may comprise a temperature monitoring unit 55, which is arranged to provide digital temperature data TDATA based on an analog temperature signal S_AN obtained from the temperature sensor 57. A response sent from the transponder 1 20 of the sensor 1 00 may comprise the temperature data TDATA.

Fig. 22a shows a sensor device 1 00, which may be used e.g. as a position detector. The sensor 1 00 may comprise a first portion POR1 and a second portion POR2 such that the second portion POR2 is movable with respect to the first portion POR1 . The capacitance Cx 1 0 of the capacitor 1 0 may depend on the position of the second portion POR2 with respect to the first portion POR1 .

The portion POR1 may be in contact with a first item 300a, and the movable portion POR2 may be in contact with a second item 300b (See Fig. 23b). Thus, the sensor 1 00 may be used for monitoring position of the second item 300b with respect to the first item 300a. The sensor 1 00 may be used for monitoring proximity of the second item 300b with respect to the first item 300a.

The portions POR1 , POR2 may be attached to the items 300a, 300b e.g. by an adhesive. The portions POR1 , POR2 may also be supplied separately. The portions POR1 , POR2 may be supplied e.g. as adhesive labels such that a first roll of labels comprises the first portions POR1 , and a second roll of labels comprises the second portions POR2.

The sensor 100 may comprise an auxiliary capacitor plate 1 3, which is arranged to move (or to be movable) in the vicinity of the first capacitor plate

1 1 and the second capacitor plate 1 2. The first plate 1 1 and the second plate

12 may be fixed to the first portion POR1 of the sensor. The auxiliary plate 1 3 may be fixed to the second movable portion POR2. In this case, the capacitor 10 may actually comprise two capacitors connected in series. The first capacitor comprises the first plate 1 1 and the auxiliary plate 1 3, and the second capacitor comprises the auxiliary plate 1 3 and the second plate 1 2. Movement of the portion POR2 changes the distance between the plates 1 1 , 13 and/or changes the distance between the plates 1 3,1 2. Consequently, movement of the portion POR2 may change the capacitance Cx. By using the auxiliary plate 1 3, it is not necessary to use a flexible conductor for connecting at least one of the plates 1 1 , 1 2 to the terminals N1 , N2 (see the conductor 1 8 in Fig. 23a). Consequently, the geometry of the sensor 1 00 may be simplified, operating life of the sensor may be increased and/or the manufacturing of the sensor may be simplified.

Fig. 22b shows a situation where the first portion POR1 has been displaced in the direction SY with respect to the second portion POR2. Ay denotes the magnitude of the displacement (i.e. shift). The capacitance Cx corresponding to the situation of Fig. 22b is lower than the capacitance Cx corresponding to the situation of Fig. 22a.

The displacement Ay may also associated with a force, which depends on the (magnitude of) the displacement Ay. Consequently, the sensor 1 00 may also be used to monitor the force. For example, a first portion 300a of a structure may be mechanically connected to a second portion 300b of the structure by elastic material. The first portion 300a may be displaced with respect to the second portion 300b when the structure is stressed. For example, a wooden beam of a building may comprise the portions 300a, 300b. The portions POR1 , POR2 of a sensor 1 00 may be attached to the structure in order to monitor whether the stress and/or displacement is smaller than a predetermined limit.

A layer of insulating material between the first capacitor plate 1 1 and the auxiliary capacitor plate 1 3 may prevent galvanic contact between the first plate 1 1 and the auxiliary plate 1 3. A layer of insulating material between the second capacitor plate 1 2 and the auxiliary capacitor plate 1 3 may prevent galvanic contact between the second plate 1 2 and the auxiliary plate 1 3.

Referring to Fig. 23a, the second plate 12 of the capacitor 1 0 may be movable with respect to the first plate 1 1 . The distance d1 between the first plate 1 1 and the second plate 12 may be variable. The plates 1 1 , 12 may be connected to the terminals N1 , N2 of the RFID chip 1 10 by conductors 17, 18. At least one of the conductors 17, 18 may be flexible so as to allow variation of the distance d1 between the plates 1 1 , 12. The first plate 1 1 may be fixed to a first portion POR1 of the sensor 100, and the second plate 12 may be fixed to a second movable portion POR2 of the sensor 100. The second portion POR2 may be displaced with respect to the first portion POR1 . Δζ denotes the displacement of the portion POR2 in the direction SZ, with respect to the portion POR1 .

Referring to Fig. 23b, the portion POR1 of the sensor 100 may be in contact with a first item 300a, and the portion POR2 of the sensor 100 may be in contact with a second item 300b. Thus, a displacement of the second item 300b with respect to the first item 300a may cause change in the capacitance Cx. Consequently, the sensor 100 may be used for monitoring the position and/or displacement Δζ of the item 300a (or the item 300b), based on variations in the capacitance Cx. LINO denotes the position of the upper part of the item 300b in a reference situation. A layer of compressible dielectric material 19 may be disposed between the plates 1 1 , 12. The material 19 may be reversibly compressible. The compressible dielectric material 19 may be e.g. foam plastic, foam rubber or plastic foil comprising air-bubbles (similar material is typically used as a cushioning packing material). The compressible dielectric material 19 may be attached to the plates 1 1 , 12 e.g. by an adhesive.

A force F_z acting on the second portion POR2 may cause elastic compression of the material 19. The distance d1 may depend on the force F_z. Thus, the sensor 100 may also be used for monitoring a force F_z.

The sensor 100 may be used as weighing device. The sensor 100 may be used to e.g. monitor whether the weight of the item 300a is smaller or greater than a predetermined value. The item 300a may be e.g. a part of a seat. The sensor 100 may be used to e.g. monitor whether a person is sitting on a seat. The width of the person may compress the material 1 9 such that the corresponding variation in the capacitance Cx may be detected. The sensor 1 00 may be used to detect presence of a person, animal or an item. The material 1 9 may have closed cells or open cells. When the material 1 9 has closed cells (closed gas cavities), the sensor 100 may also be used as a pressure sensor to monitor (absolute) ambient pressure.

When detecting proximity or position, the portion POR2 of the sensor 1 00 of Fig. 22a may also be omitted if the item 300b has an electrically conductive surface. However, an electrically conductive item 300b may partially prevent propagation of the radio frequency signals ROG, RES.

The RFI D sensor 1 00 may also be arranged to operate as a capacitive chemical sensor for detecting the presence of a chemical substance (said substance may be different from water). The layer 1 32 shown in Figs. 4e and 5b or the material 1 9 of Fig. 23a may have a suitable chemical composition and a suitable physical structure so as to allow operation of the sensor 1 00 as a chemical sensor for monitoring concentration of a chemical substance (analyte).

For example, a (selective) chemical reaction between the material 1 9 (or layer 1 32) and a chemical compound may cause swelling or shrinking of the material 1 9. The swelling or shrinking may change the distance d1 between the capacitor plates 1 1 , 1 2, which in turn may cause a detectable change in the capacitance Cx.

For example, the material 1 9 (or layer 1 32) may comprise an ion-exchange substance e.g. to selectively detect analyte ions in a liquid medium. An ion- exchange mechanism may replace a portion of counter-ions of the ion- exchange substance with the analyte ions. The removal of the counter-ions from the ion-exchange substance may cause a change in the permittivity, which in turn may cause a detectable change in the capacitance Cx. For example, a chemical reaction between the material 1 9 (or layer 1 32) and a chemical compound may have e.g. an effect on the hydrophilic or hydrophobic properties of the material 1 9. This may change the capability of the material 1 9 to absorb water when the sensor is exposed to a gas containing water vapor. This may change the capability of the material 1 9 to absorb water when the capacitor 1 0 is immersed in an aqueous solution. Thus, the presence of the chemical compound may be monitored by detecting a change in the capacitance Cx.

For example, a chemical reaction between the material 1 9 (or layer 1 32) and a chemical compound may promote condensation of water vapor from the gas phase to the material 1 9.

The material 19 (or layer 1 32) may comprise e.g. sintered particles, packed particles, fibrous material or porous material in order to reduce the time constant of a chemical reaction between the chemical substance (analyte) and the material 1 9.

The sensor 1 00 may be arranged to provide measured data XDATA based on a variation (variations) in the capacitance Cx. The sensor 100 may send a response RES to an interrogation signal ROG such that the response RES contains the measured data XDATA. A reader 200 may be arranged to send the interrogation signal ROG and/or to receive the response RES.

Information about an environmental variable may be determined from the measured data XDATA. The environmental variable may be e.g. position of a first element 300a with respect to a second element 300b, weight of an element 300a, force acting on the sensor, pressure, presence of a second element 300b in the vicinity of a first element 300a, stress of a structural element 300, or concentration of a chemical substance. In particular, the measured data XDATA may be moisture data XDATA. The moisture data XDATA may be used for monitoring moisture content, level of a liquid, or the concentration of a first substance mixed or dissolved in a second substance. The environmental variable may be e.g. moisture content, level of a liquid, or the concentration of the first substance. The environmental variable may be different from the temperature of the chip 1 10. The environmental variable may be different from the temperature of the sensor 100.

In general, the reader 200 and the sensor 1 00 may be arranged to communicate e.g. according to the EPC Gen2 protocol. EPC Gen2 is a abbreviation for "EPCglobal UHF Class 1 Generation 2". The protocol has been incorporated e.g. in the standard ISO 1 8000-6C (frequency band 860- 960MHz). (Reference is made to the latest versions of the protocol and standard as in force on 1 2 January 201 1 ).

The reader 200 and the sensor 1 00 may be arranged to communicate e.g. according to one or more of the following standards:

ISO/I EC 1 8000-2A (frequency band 125/1 34.2 kHz, interrogation range e.g. up to 2 m)

ISO/I EC 1 8000-2B (frequency band 1 25/1 34.2 kHz)

ISO 18000-3 (frequency band 1 3.56 MHz, interrogation range e.g. up to 3 m) ISO 18000-7 (frequency band 433 MHz)

ISO 1 8000-6A (frequency band 860-960MHz, interrogation range e.g. up to 3 m)

ISO 18000-6B (frequency band 860-960MHz)

ISO 18000-6C (frequency band 860-960MHz)

EPCglobal Class 0 (frequency band 860-960MHz)

EPCglobal Class 1 (frequency band 860-960MHz)

EPCglobal Class 1 Gen 2 (frequency band 860-960MHz)

ISO 18000-4 (frequency band 2.45 GHz, reading range e.g. up to 1 2 meters) Proximity cards: ISO/I EC 14443 (frequency band 1 3.56 MHz, interrogation range e.g. up to 1 2.5 cm)

Vicinity cards: ISO/I EC 1 5693 (frequency band 1 3.56 MHz, interrogation range e.g. up to 1 .5 m)

(Reference is made to the latest versions of the protocols and standards as in force on 1 5 April 201 1 ). An RFI D chip manufactured under a trade name "UCODE G2il_+" by a company "NXP Semiconductors" may be used as the chip 1 10 of a the sensor 1 00. The chip G2il_+ comprises connection terminals (pads) marked as "VDD", OUT", "RFP", and "RFN" in the datasheet. The terminals "RFP" and "RFN" may be used for connecting antenna elements to the chip. According to the manufacturer (NXP Semiconductors), the terminals "OUT" and "VDD" may be used for connecting a tamper alarm loop to the chip. If the tamper alarm loop is broken, i.e. when the chip detects that the terminal "OUT" is not connected to the terminal "VDD", a response RES sent by the chip may comprise data which specifies that the tamper loop is broken. However, when the chip G2il_+ is used as a part of the sensor 1 00, the terminals "VDD" and "OUT" may be used for a different purpose. The terminal "OUT" may be used as the node N1 of the driving unit 91 , and the terminal "VDD" may be used as the node N2 of the monitoring unit 92. The terminal RFN may be used as the (grounded) node N5 or N6 (See e.g. Fig. 6).

Sensors 1 00a, 1 00b, 1 00c may be energetically passive so that they can be permanently embedded within the building structures or to other locations which are difficult to access.

The sensor 100a may be arranged to store measured data XDATA1 in a memory (register) MEM1 located in the transponder 1 00a. The (digital) measured data XDATA1 may be stored in the register located in the transponder 1 00a such that the value of the measured data XDATA1 may be accessed and read by sending a predetermined interrogation signal ROG to the transponder 1 00a.

The sensor 1 00a may be arranged to send a response RES such that the measurement XDATA is included in the response e.g. by using pulse code modulation (PCM), by using pulse interval encoding PI E, and/or by using Manchester encoding. The sensor 100a may be arranged to send a response RES, which contains the measured data XDATA in pulse code modulated format PCM and/or in a pulse interval encoded format PI E and/or in a Manchester encoded format. Charging of the capacitor 1 0 may be carried out by using energy extracted from a radio frequency field coupled to the antenna 140 of the sensor 1 00. Monitoring of the state of the capacitor Cx may be carried out by using energy extracted from a radio frequency field coupled to the antenna 140 of the transponder 1 00. In particular, the analog-to-digital conversion (quantization) by the monitoring unit 92 (from the analog voltage V₂ to the digital signal S₂) may be carried out by using energy extracted from an interrogation signal ROG coupled to the antenna 140 of the sensor 100. According to preferred embodiments of the invention, the sensor 1 00 may be substantially energetically passive. This may provide a small size and a substantially infinite operating life.

However, in principle, the sensor 1 00 could also be an active device or a battery assisted device.

In case of a battery-assisted device, the response RES may be transmitted by using reflected power of the interrogation signal ROG (by using passive reflected power), but power provided by a battery may be used for processing information and/or storing information. This set-up may provide a long lifetime without a need to change the battery. However, if it is not possible to change the battery, the operating life of the battery still sets an upper limit for the operating life and size of the transponder. In practice the maximum operating life of a battery may be e.g. 1 year, 5 years, or 1 0 years.

In case of an active device, the battery provides operating power for the radio frequency unit RXTX1 . In this case, the operating life of the battery sets an upper limit for the operating life and size of the transponder. For the purposes of measuring a temperature with an RFI D transponder, a reference is made to a patent application PCT/FI201 1 /050020.

For the purposes of measuring location of an RFI D transponder, a reference is made to the patent application PCT/FI201 1 /050020. For the person skilled in the art, it will be clear that modifications and variations of the devices and the methods according to the present invention are perceivable. The drawings are schematic. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.

Claims

1 . A sensor device (100), comprising:

- an RFID transponder (120) having an RFID chip (1 10), and

- a capacitor (10) located outside the chip (1 10),

wherein the capacitance (Cx) of the capacitor (10) depends on an environmental variable (M_w, Ay, F_z), the sensor device (100) is arranged to provide measured data (XDATA) based on the capacitance (Cx) of the capacitor (10), the transponder (120) is arranged to send a response (RES) to an interrogation signal (ROG) such that the response (RES) contains the measured data (XDATA), and the sensor device (100) is arranged to monitor the capacitance (Cx) of the capacitor (10) by using operating energy extracted from a radio-frequency field (ROG).

2. The sensor device (100) of claim 1 wherein the transponder (120) is arranged to send a response (RES), which contains identification data (ID1 ).

3. The sensor device (100) of claim 1 or 2 wherein the transponder (120) is arranged to send a response (RES), which contains the measured data (XDATA) in a pulse code modulated format (PCM), in a pulse interval encoded format (PIE) and/or in a Manchester encoded format.

4. The sensor device (100) according to any of the claims 1 to 3, comprising:

- a driving unit (91 ) arranged to charge the capacitor (10) by using energy extracted from the radio-frequency field (ROG), and

- a monitoring unit (92) arranged to provide a digital signal (S₂), which depends on variations in the voltage (V_Cx) of the capacitor (10).

5. The sensor device (100) according to any of the claims 1 to 4 wherein capacitor plates (1 1 , 12) of the capacitor (10) are in the same plane.

6. The sensor device (100) of claim 5 wherein a first capacitor plate (1 1 ) of the capacitor (10) comprises a plurality of portions (1 1 a, 1 1 b, 1 1 c) and a second capacitor plate (12) of the capacitor (10) comprises a plurality of portions (12a, 12b, 12c) such that at least one portion (1 1 a, 1 1 b, 1 1 c) of the first plate (1 1 ) is positioned between portions (12a, 12b, 12c) of the second plate (12).

7. The sensor device (100) according to any of the claims 1 to 6 wherein at least a first plate (1 1 ) of the capacitor (10) is encapsulated in order to protect the first plate (1 1 ) from electrochemical corrosion.

8. The sensor device (100) according to any of the claims 1 to 7 wherein the capacitance (Cx) of the capacitor (10) depends on the amount of water in the vicinity of the capacitor (10).

9. The sensor device (100) of claim 8 comprising a layer of moisture- absorbing material (132), which is positioned such that the capacitance (Cx) of the capacitor (10) depends on the dielectric constant of the moisture- absorbing material (132).

10. The sensor device (100) according to any of the claims 1 to 9, wherein the transponder (120) is arranged to send a response (RES) to an interrogation signal (ROG) such that the response (RES) contains information (TDATA) about a temperature of the sensor device (100).

1 1 . The sensor device (100) according to any of the claims 1 to 7 wherein the capacitance (Cx) of the capacitor (10) depends on the displacement (Δζ) of a first portion (POR1 ) of the sensor device (100) with respect to a second portion (POR2) of the sensor device (100).

12. A method for monitoring an environmental variable (M_w, Ay, F_z) by using a sensor device (100), the sensor device (100) comprising an RFID transponder (120), and a capacitor (10) such that the capacitance (Cx) of the capacitor (10) depends on the environmental variable (M_w, Δζ, F_z), the method comprising:

- receiving an interrogation signal (ROG) to the transponder (120),

- providing measured data (XDATA) based on the capacitance (Cx) of the capacitor (10), and

- sending a response (RES) to the interrogation signal (ROG) such that the response (RES) contains the measured data (XDATA), wherein the transponder (1 20) comprises a RFI D chip (1 1 0), the capacitor (1 0 ) is located outside the chip (1 1 0), and the sensor device (1 00) is arranged to monitor the capacitance (Cx) of the capacitor (1 0) by using operating energy extracted from a radio-frequency field (ROG).

13. The method of claim 1 2 comprising sending a response (RES), which contains identification data (I D1 ).

14. The method of claim 1 1 comprising sending a second response (RES2) from a second sensor device (1 00b) such that the second response (RES2) contains second identification data (I D2).

15. The method according to any of the claims 1 2 to 14 comprising associating the measured data (XDATA) with a location (x,y).

16. The method according to any of the claims 1 2 to 1 5 comprising charging the capacitor (1 0), and providing a digital signal (S₂), which depends on variations in the voltage (V_Cx) of the capacitor (1 0).

17. The method according to any of the claims 12 to 1 6 comprising detecting the presence of moisture (LQ) by monitoring the capacitance (Cx).

18. The method according to any of the claims 1 2 to 1 7 comprising detecting a dew point based on measured data (XDATA) obtained from the sensor device (1 00).

19. The method according to any of the claims 1 2 to 1 8 comprising sending a response (RES), which contains information (TDATA) about a temperature of said sensor device (1 00).

20. The method according to any of the claims 12 to 1 9 comprising detecting a level of a liquid (LQ) by monitoring the capacitance (Cx).

21 . The method according to any of the claims 1 2 to 20 wherein the sensor device (1 00) is attached to an element (300) of a building.

22. The method according to any of the claims 12 to 21 comprising controlling a heating unit (71 0), controlling a cooling unit (71 0), controlling a ventilation unit (720) and/or controlling a valve (760) based on measured data (XDATA) obtained from the sensor device (1 00).

23. The method according to any of the claims 1 2 to 1 6 comprising monitoring an amount of a first substance in a second substance.

24. The method according to any of the claims 12 to 1 6 comprising detecting a displacement (Δζ) of a first portion (POR1 ) of the sensor device (1 00) with respect to a second portion (POR2) of the sensor device (1 00) by monitoring the capacitance (Cx).

25. A computer program (PROG) comprising computer program code, which when executed by data processor (CNT2, CNT4) is for executing the method according to any of the claims 1 2 to 24.

26. A computer-readable medium (MEM9) storing computer program code (PROG), which when executed by data processor (CNT2, CNT4) is for executing the method according to any of the claims 1 2 to 24.

27. An apparatus (700) comprising a sensor device (100) according to any of the claims 1 to 1 1 .

28. The apparatus (700) of claim 27 further comprising a control unit (CNT4) arranged to control a heating unit (71 0), to control a cooling unit (71 0), to control a ventilation unit (720) and/or to control a valve (760) based on measured data (XDATA) obtained from the sensor device (1 00).