Passive wireless sensor

Abstract

The RFID sensor tag may include normal RFID circuits and functions (such as rectifier, modulator, logic and memory) as well as a resonator-based clock generator or oscillator. The oscillator is a stable resonator-based oscillator having a high Q value. The resonator-based oscillator is loaded with a sensor element which tunes the oscillation frequency, i.e. the oscillation frequency is sensitive to the measured quantity. Thereby, a passive RFID sensor with a resonator-based oscillator and a sensor element is provided, wherein the oscillation frequency can be made dependent on the sensor element if sensing is required. The concept is compatible for existing RFID tags and can enable the possibility to measure external quantities with-out reducing the read-out distance.

Claims

1. A passive wireless transponder, comprising: an antenna, a rectifier to convert a RF power of a received radio frequency (RF) interrogating signal into a direct current (DC), a tag oscillator configured to generate a modulation frequency and an internal clock, a demodulator adapted to demodulate a received RFID interrogating command from the received RF interrogating signal, a digital control part that uses the internal clock and is adapted to process the received RFID interrogating command and to output a digital response, a modulator adapted to generate a modulated backscattered signal carrying the digital response for communication with a backscattering principle to provide radio frequency identification (RFID) features, and a sensing element for sensing a predetermined variable, wherein the modulated backscattered signal carrying a digital response comprises sidebands that are offset from an interrogation frequency by a modulation frequency outputted from the tag oscillator, and wherein the tag oscillator is based on a resonator, and the sensing element is connected to the tag oscillator to load the tag oscillator, and the modulation frequency outputted from the tag oscillator and thereby the offset of the sidebands of the modulated backscattered signal are arranged to be dependent on and carry a sensed value of the predetermined variable.

2. A passive wireless transponder according to claim 1, wherein the sensing element further comprises a control logic and/or a memory for radio frequency identification (RFID) features.

3. A passive wireless transponder according to claim 2, wherein the radio frequency identification (RFID) features comprise identification and anti-collision.

4. A passive wireless transponder according to claim 1, wherein major part of the transponder is implemented with integrated circuit technique, and wherein the sensing element is an external component.

5. A passive wireless transponder according to claim 1, wherein the transponder comprises a plurality of sensing elements selectively, one at a time, enabled to load the tag oscillator such that the modulation frequency output from the tag oscillator is dependent on a sensed value of the predetermined variable of the enabled sensing element.

6. A passive wireless transponder according to claim 5, wherein the transponder comprises a selection means arranged to selectively enable one and disable the remaining ones of the plurality of sensing elements according to a predetermined sequence or according to a command received from a reader device.

7. A passive wireless transponder according to claim 1, wherein major part of the transponder is implemented with an integrated circuit technique, and wherein at least part of the tag oscillator comprises an external component.

8. A passive wireless transponder according to claim 7, wherein the external component is an external resonator.

9. A passive wireless transponder according to claim 1, wherein major part of the transponder is implemented with an integrated circuit technique, and wherein the sensing element and a resonator part of the tag oscillator are implemented with a resonant sensor or a MEMS resonant sensor.

10. A passive wireless transponder according to claim 1, wherein the tag oscillator based on a resonator comprises one of a LC oscillator, a RLC oscillator, a crystal oscillator, and an oscillator based on a MEMS (microelectromechanical systems) resonator, a SAW (surface acoustic wave) resonator, or a BAW (bulk acoustic wave) resonator.

11. A passive wireless transponder according to claim 1, wherein the tag oscillator is one of Collpitts oscillator, Meissner oscillator, Armstrong oscillator, and Hartley oscillator.

12. A passive wireless transponder according to claim 1, wherein the tag oscillator is a Collpitts LC or RLC oscillator which comprises an active amplifying device, a capacitive divider, and a feedback inductance.

13. A passive wireless transponder according to claim 12, wherein a voltage level of the modulation frequency output from the LC or RLC oscillator is dependent of a capacitive divider ratio of the capacitive divider.

14. A passive wireless transponder according to claim 1, comprising one or more of a high pass filter between the antenna and the rectifier, a low pass filter between a DC output of the rectifier and an input of the oscillator, and a band pass filter between an output of the oscillator and an input of the modulator.

15. A passive wireless transponder according to claim 1, wherein the rectifier comprises a rectifier diode D1, and wherein the rectifier diode also provides a mixer of the modulator.

16. A system comprising an RFID reader, and at least one passive wireless transponder that further comprises: an antenna, a rectifier to convert a RF power of a received radio frequency (RF) interrogating signal into a direct current (DC), a tag oscillator configured to generate a modulation frequency and an internal clock, a demodulator adapted to demodulate a received RFID interrogating command from the received RF interrogating signal, a digital control part that uses the internal clock and is adapted to process the received RFID interrogating command and to output a digital response, a modulator adapted to generate a modulated backscattered signal carrying the digital response for communication with a backscattering principle to provide radio frequency identification (RFID) features, and a sensing element for sensing a predetermined variable, wherein the modulated backscattered signal carrying a digital response comprises sidebands that are offset from an interrogation frequency by a modulation frequency outputted from the tag oscillator, and wherein the tag oscillator is based on a resonator, and the sensing element is connected to the tag oscillator to load the tag oscillator, and the modulation frequency outputted from the tag oscillator and thereby the offset of the sidebands of the modulated backscattered signal are arranged to be dependent on and carry a sensed value of the predetermined variable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 illustrates backscattering communication principle in an RFID system;

(3) FIG. 2 is a functional block diagram illustrating an example of a an RFID tag architecture;

(4) FIG. 3 is a circuit diagram illustrating an RC oscillator;

(5) FIG. 4A illustrates an example of a LC oscillator;

(6) FIG. 4B illustrates an example of a RFID sensor tag implemented with an integrated circuit chip and external components;

(7) FIG. 4C illustrates an example of a RFID sensor tag provided with two or more sensing elements;

(8) FIG. 5A is a system diagram that illustrates the communication principle for a RFID sensor according to exemplary embodiments of the invention;

(9) FIG. 5B is a spectral graph that illustrates the communication principle for a RFID sensor;

(10) FIG. 6 is a block and circuit diagram illustrating an RFID sensor device according to according to exemplary embodiments of the invention; and

(11) FIG. 7 is a graph illustrating the oscillation output voltage as a function of input voltage supply.

DETAILED DESCRIPTION OF THE INVENTION

(12) FIG. 2 shows a functional block diagram illustrating an example of radio frequency identity (RFID) transponder (tag) architecture. In the illustrated example the RFID tag 10 may comprise an antenna 21 directly matched to the tag's front end impedance (matching circuit is not shown) to communicate with a RFID reader 11; an analogue RF front end that may typically contain rectifier circuitry 22 to convert RF power into a direct current (DC), a clock generator or oscillator 23, a modulator 24 and a demodulator 25. There may also be a logic part or a digital control module 26 that may be configured to provide desired functions, such as to handle interrogating commands, execute the anti-collision protocol, perform the data integrity check, run memory read-write operations, and perform output control and data flow. The logic implementation usually follows a defined standard and a certain associated protocol. Further, memory storage 27 may be provided. Depending on a user's requirement, non-volatile memory storage may be needed if both read/write capability is implemented.

(13) As discussed above, the passive RFID tags utilize the modulated backscattering principle for communication. When a tag communicates with a reader, it modulates the received signal and reflects a portion of it back to the reader. In prior art RFID systems, the modulation frequency is the same as the clock frequency of the tag. In prior art RFID systems, the clock frequency generation 23 is realized with an inefficient RC-oscillator whose frequency is relatively unstable. Therefore, a relatively large band (40 or 160 kHz, ?15%) is allocated for the modulated response of the tags. The modulation frequency itself does not carry any information in current systems, and the spectral efficiency is poor (information band can be as low as one tenth of the overall band). The RC-oscillator also consumes relatively large power. An example of an RC oscillator containing a transistor M1, a resistor R1 and a capacitor C1 is illustrated in FIG. 3.

(14) Current passive wireless sensors cannot simultaneously provide sensing and the sophisticated features of the RFID technology. This can be enabled by equipping an RFID tag with a resonator-based oscillator (LC-oscillator, RLC-oscillator, crystal oscillator or similar) and sensor element in order to enable the sophisticated features of RFID and the possibility to measure external quantities without reducing the read-out distance.

(15) According to one embodiment of the invention the unstable RC-oscillator in an RFID tag device is replaced with a stable resonator-based oscillator having a high Q value, and the RFID is provided with a sensor element in order to enable the sophisticated features of RFID and the possibility to measure external quantities without reducing the read-out distance. Examples of resonator-based oscillators include Hartley, Collpits, Armstrong, and Meissner LC-oscillators or RLC-oscillators, different crystal oscillators, and oscillators based on MEMS (microelectro-mechanical systems), SAW (surface acoustic wave), and BAW (bulk acoustic wave) resonators. Exemplary embodiments described herein for a LC-oscillator can be similarly applied to a RLC-oscillator. It should be appreciated that a LC-oscillator will become as a RLC-oscillator, if a minimal amount resistance R is introduced into the LC-oscillator. In practice, all LC-oscillators can be considered as RLC-oscillators, because there is always resistance R present in the inductor L. On the other hand, it is preferable to minimize the resistive component R, because the resistive component R increases the power consumption and reduces the read-out distance. The invention can also be used with an RC-oscillator with reduced measurement resolution. The advantage of an RC-oscillator is that it can be integrated.

(16) The resonator-based oscillator, first of all, enables a huge improvement to the spectral efficiency of the RFID enabling simultaneous reading of numerous tags at different modulation frequencies (state-of-the-art readers already enable carrier frequency multi-access scheme, which should not be confused with the modulation frequency multi-access scheme). Furthermore, the resonator-based oscillator can be loaded with a sensing element which tunes the oscillation frequency, i.e. the oscillation frequency becomes sensitive to the measured quantity. In other words, a passive RFID sensor with a resonator-based oscillator and a sensor element is provided, wherein the oscillation frequency can be made dependent on the sensor element if sensing is required. Therefore, this concept is compatible for existing RFID tags and can enable the possibility to measure external quantities without reducing the readout distance.

(17) It is also possible to combine the resonator and sensor for example by designing a MEMS resonator in such a way, that its resonance is sensitive to a measured quantity. For example, the resonance frequency of a MEMS resonator can be made sensitive to temperature and strain and thus these parameters can be obtained from the resonance frequency.

(18) Embodiments of the invention offer many benefits. First, an RFID sensor tag can be interrogated with existing RFID readers, because they already measure the clock frequency (that is, the modulation frequency) of the tag. No hardware modification is required to existing RFID readers to communicate with a sensor RFID according to embodiments of the invention. RFID sensor tags can also be equipped with external sensor elements without any reduction in the read-out distance. In fact, the power consumption of an RFID tag has slightly decreased when an RC-oscillator is replaced with an oscillator based on a resonator (e.g. LC, RLC, quartz crystal, SAW or BAW resonator or MEMS resonator).

(19) An example of a resonator-based LC oscillator containing a transistor M1 and an LC resonator 41 with a parallel connection of inductor L1 and a capacitor C2 is illustrated in FIG. 4A.

(20) FIG. 4B illustrates an example of a RFID sensor tag wherein the normal RFID circuits and functions (such as rectifier 22, modulator 24, logic 26 and memory 27) as well as part of the clock generator 23 (such transistor M1 and capacitor C1) are implemented on an integrated circuit chip 43, whereas part of the oscillator 23 (such an inductor L1 or a resonator) and a sensing element 42 (such capacitive sensor Cs) may be implemented as external components connected to the integrated circuit chip 43. Alternatively, the entire resonator (such as the resonator 41) may be implemented with one or more external components, or some other kind of configuration may be employed. The concept allows utilizing potentially low-cost inductors, resonators and sensors or resonant sensors. For example, MEMS resonators and sensor or MEMS resonant sensors may be applicable. MEMS resonant sensor is a resonating device, whose resonance is made sensitive to a measured quantity. The measured quantity is obtained by measuring the resonance of the device. The cost of MEMS resonators and sensors is in the range of 0.5-2 , and that of mass produced RFID chips around 0.1. As a comparison, mass produced high frequency (HF) and near-field-communication (NFC) RFID tags with a sensor interface costs $2.59-3.46.

(21) FIG. 4C illustrates an example of a transponder or a RFID sensor tag provided with two or more sensing elements 42-1, 42-2, . . . , 42-N. The RFID sensor tag may include normal the normal RFID circuits and functions (such as rectifier 22, modulator 24, logic 26 and memory 27) as well as a resonator-based clock generator or oscillator 23, such as those described in other exemplary embodiments. Sensing elements 42-1, 42-2, . . . , 42-N can be selectively, one at time, enabled to load the oscillator 23 such that the modulation frequency output from the oscillator 23 is dependent on a sensed value of the predetermined variable of the enabled sensing element 42. In other words, one sensing element can be selected to load the oscillator at a time and thus affect the oscillation frequency. This way the transponder can be equipped with multiple sensors which can be read individually. The transponder may have a logical circuit that can switch on certain sensing element and switch off all the rest. For example, the transponder may comprise a selector 44, such as a switch circuit or an analog multiplexer arranged to selectively connect one and disconnect the remaining ones of the plurality of sensing elements 42-1, 42-2, . . . , 42-N to and from the oscillator 23. The selective enabling, switching or connecting may be carried out according to a predetermined sequence. Alternatively, a reader device can send a command to the transponder on which sensing element 42-1, 42-2, 42-N is switched on. For example, the logic 26 may receive a selection command from the reader and control the selector 44 accordingly.

(22) FIGS. 5A and 5B illustrate the communication principle for a RFID sensor according to exemplary embodiments of the invention. In the FIGS. 5A and 5B, f.sub.CW and f.sub.OSC represent the carrier frequency and oscillation frequency, respectively. The RFID sensor is actuated using an RF CW signal transmitted from the reader 11. First, the RF signal is converted to DC voltage by a rectifier 22 (such as rectifier 22 in FIG. 4B). The rectified voltage powers up an oscillator 23, which produces a low frequency sinusoid f.sub.OSC at its output. Finally, the oscillation signal f.sub.OSC is fed to the modulator 24 to realize the backscattering principle. The modulator 24 modulates the signals, and those going back to the antenna 21 depend on the matching between the antenna and the rectifier 21/modulator 24. As a consequence, there are sidebands f.sub.CW?f.sub.OSC and f.sub.CW+f.sub.OSC in the signal reflected from the sensor. The sidebands are offset from the carrier f.sub.CW by the oscillation frequency f.sub.OSC.

(23) For illustrative purposes, exemplary embodiments of the invention are described with only main parts: an antenna 21, a rectifier 22, an LC-oscillator 23, a modulator 24 and a sensing element 42, as illustrated in FIG. 6. The antenna 21 matched to the rectifier 22 with a suitable matching circuit as well known to a person skilled in the art. Moreover, there may be a high pass filter 51 between the antenna 21 and the rectifier 22, a low pass filter 52 between the DC output of the rectifier 21 and the input of the low-frequency oscillator 23, and a band pass filter 53 between the output of the oscillator 23 and the input of the modulator 24.

(24) A passive RFID sensor 50 typically harvest all the energy needed for its operation from the interrogation signal of the reader device 11. However, it is also possible to attach other energy harvesters to the presented sensor device to achieve an increased read-out range. Possible energy harvesters include photovoltaic and solar cells, thermocouples, vibration energy harvesters, wind turbines (also in microscopic level) and acoustic harvesters. The reader device 11 illuminates the tags with a continuous wave (CW), and the RFID sensors utilize a voltage rectifier 22 to produce the supply voltage V.sub.DC needed for the operation. Under small-signal conditions, in which the RFID sensors often operate at large distances, the efficiency of the rectifier is proportional to the peak AC voltage. Rectifier 22 may be based on a non-linear element, such as a diode D1, and it is used to convert AC voltage into DC. A most simple rectifier may contain a zero bias Schottky diode D1. The antenna 21 may be isolated from the rectifier diode D1 with high pass filter 51, such as a DC block capacitance, and the oscillator 23 may be isolated from the antenna 21 and the rectifier 22 at RF with the low pass filter 52, such as an RF choke, in order to prevent RF energy from dissipating in the DC load, i.e. in the oscillator 23.

(25) As was discussed earlier, the sensor utilizes the modulated backscattering principle for communication. In exemplary embodiments, the modulated backscattering may be realized by applying the oscillator output f.sub.OSC to the rectifier 22. In other words, rectifier diode D1 is also used as a part of the modulator 24. In the modulator 24, the rectifier diode D1 is used as a mixer. The band-pass filter 63, such as an RF choke, allows the oscillator frequency to f.sub.OSC pass through and stop other frequencies. The oscillation signal f.sub.OSC modulates the RF impedance of the rectifier diode D1. The voltage at the oscillation frequency f.sub.OSC mixes with the original input frequency f.sub.CW, generating signals at the sum (f.sub.CW+f.sub.OSC) and difference (f.sub.CW?f.sub.OSC) frequencies. As a consequence, there are sidebands in the signal reflected from the sensor 50. The reflected power by the RFID sensor depends on the diode parameters, RFID sensor input power (V.sub.DC), oscillator output voltage V.sub.OSC and the internal resistance of the antenna 21, for example.

(26) When used in a passive wireless sensor, an oscillator 23 must produce a large output voltage and its power consumption must be small. The power consumption of an RFID sensor 50 mainly takes place in the oscillator circuit 23 to generate the oscillator output voltage, on which the sensor read-out distance depends. Therefore, power consumption of an RFID sensor 50 can be made arbitrary small by designing an oscillator which can operate with ultra-low supply voltage. Moreover, larger read-out distance of the RFID sensor 50 can be achieved by producing a large oscillator output voltage. The previous analysis was made under an assumption that the sensor is not equipped with digital IC electronics. When digital electronics are included, the read-out distance may be limited due to the power consumption of the digital circuitry.

(27) Oscillator circuit 23 can be implemented with various oscillator topologies. Examples of classic oscillator topologies include Collpitts oscillator, Meissner oscillator, Armstrong oscillator, and Hartley oscillator. The Colpitts oscillator is perhaps the most widely used oscillator in the history, widely used in commercial signal generators up to 100 MHz and also because it is easy to implement. In exemplary embodiments a simple Colpitts oscillator topology has been chosen for description without intending to restrict the invention to this oscillator topology.

(28) Referring to FIG. 6, the exemplary oscillator 23 may be implemented by a low voltage Colpitts oscillator. The exemplary oscillator circuit contains common-gate amplifier, including a transistor M1, a capacitive divider composed of capacitors C1 and C2, and a feedback inductor L1.

(29) The power consumption of an oscillator depends heavily on the semiconductor fabrication process and transistor technology type. Common processes are silicon (Si), gallium arsenide (GaAs) and silicon germanium (SiGe). Typical technologies used are bipolar junction transistor (BJT), field-effect transistor (FET) and heterojunction bipolar transistor (HBT). The power consumption of an oscillator also depends on the bias circuit and threshold voltage of the transistor. The metal-oxide-semiconductor field-effect transistor (MOSFET) is a type of field-effect transistor (FET). As compared to BJTs, a MOSFET can be made quite small and its operation requires relatively low power. Therefore, in the exemplary embodiments, a MOSFET transistor M1 with a very low threshold voltage, such as ALD 800, may be employed.

(30) For ultra-low-voltage operation, the control terminal of MOSFET transistor M1 are connected to the supply voltage V.sub.DC directly, the drain terminal is connected to the supply voltage V.sub.DC via the feedback inductor L1, and the source terminal is connected to the ground via a series connection of the inductor L2 and the biasing resistance R.sub.bias. The sensing element 42 may provide a load resistance R.sub.L at the output of the oscillator. The required feedback may be achieved with a capacitive divider formed by a series connection of the capacitors C1 and C2 connected between the drain terminal and the ground. The middle node between C1 and C2 is connected to the source terminal and provides the output of the oscillator. The capacitive divider C1-C2 determines ratio between the oscillator voltages V.sub.DC and V.sub.OSC.

(31) The equivalent capacitance C.sub.eq and capacitive division ratio n.sub.c can written as

(32) $C_{\mathrm{eq}}=\frac{C_1{C}_2}{C_1+C_2}.n_c=\frac{{\underset{\_}{V}}_{\mathrm{osc}}}{V_{DC}}=1+\frac{C_1}{C_2}.$

(33) FIG. 7 illustrates the oscillation output voltage as a function of input voltage supply with parameter values listed in Table 1 below and with the aim of generating a signal at 100 kHz. The oscillator required a minimum supply voltage of 4 mV to start the oscillation which makes it feasible for use in a passive wireless sensor.

(34) TABLE-US-00001 TABLE 1 Parameters used in the oscillator simulation Feedback inductor L.sub.1 = 1 mH Bias inductor L.sub.2 = 10 mH Capacitor C.sub.1 = 3.3 nF Capacitor C.sub.2 = 16 nF Load resistance R.sub.L = 0.5 M? Feedback resistance R.sub.bias = 50 ?

(35) During the design of the oscillator, some key features have been realized which may be considered when designing an oscillator. The power consumption of an oscillator can be made arbitrarily low by decreasing the capacitive divider ratio and increasing the quality factor (Q) of the feedback inductor. It is found out that the rectified DC voltage depends on the diode parameters, input power and load impedance. Oscillator output voltage depends on the rectified DC voltage, oscillator circuit topology, and quality factor and characteristic impedance of the resonator. Moreover, power consumption by an oscillator can be made arbitrarily small by decreasing the capacitive divider ratio. Furthermore, modulated reflected power of the sensor depends on mixer diode parameters, input power of the sensor and output voltage of the oscillator.

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