Apparatus and methods for wireless/RFID sensors

A wireless sensor is provided for use in near-to-zero or zero-power consumption applications. The sensor includes a sensing circuit, a modulator connected to the sensing circuit and configured to modulate an input signal using variations in input impedance produced by the sensing circuit to produce a modulated output signal, and a transmitting element such as an antenna to transmit the modulated output signal to a receiver. In some implementations, the sensor includes a pilot sequence generator that may be powered by a received signal from a transmitting node. The input signal is thus modulated by both pilot data and the input impedance. The input signal may be received from the transmitting node. Alternatively, a power source may be provided in the wireless sensor for generating the input signal and/or pilot data.

TECHNICAL FIELD

The present disclosure relates to sensors, particularly for use in wireless or RFID applications.

TECHNICAL BACKGROUND

Smart sensors, which are designed to sense physical or chemical environmental conditions, are useful in wireless sensor networks (WSNs) for home and industrial applications to enhance system performance or system safety. Generally, a smart sensor node is configured to detect a condition, process data, and send information to a base station. Sensor nodes therefore typically consist of a small processing unit, one or more sensors, a radio or optical communication device, and a power source, usually in the form of a battery or energy harvesting module. Some unpowered sensor systems comprising transmitting, sensing, passive or active frequency conversion, and receiving elements have been proposed.

RFID tags are widely used in the identification and tracking of objects. Passive RFID tags are powered by the power transferred by an RFID reader. Active RFID tags have a local power source, such as a battery, to power the RFID chips. In conventional RFID systems, only stored information can be read from RFID tags since they can provide no sensing data about changes in the objects to which they are attached.

Resonant circuits can accurately sense material properties without destruction or contact. Such circuits are typically based on high quality factor (Q) resonators. Among them, microstrip resonators are of greatest interest because of their small size, low cost and ease of fabrication in comparison to dielectric and waveguide high Q resonators.

Vector modulators are commonly used in radio and wireless communications to generate complex modulation schemes. The use of a vector modulator eliminates the need for up-converting chains and leads to a simple and low-cost realization of different modulation schemes acting directly in microwave frequencies. Six-port structures use additive mixing instead of conventional multiplicative mixers to modulate or demodulate in-phase and quadrature components. A basic six-port receiver consists of three quadrature hybrids and one power divider to determine the phase of a microwave signal.

In industrial environments, many factors can influence the performance of sensors or data communications, such as temperature variations, blockages, time delays, and quantization method. Conventional low-power or unpowered sensors typically cannot deal with these problems without additional subsystems that increase complexity and power consumption.

DETAILED DESCRIPTION

The examples presented herein provide various embodiments of novel wireless/RFID sensors that integrate sensing and transmitting circuits. These sensors may be employed in near-to-zero (N-Zero) or zero power consumption applications, such as industrial processes, health care, water or food processing, and environmental/earth monitoring, temperature, sound, pressure, and so on.

In one aspect, an RFID/wireless sensor node includes a direct conversion sensor with multi-port structures, which simplifies the sensing system by integrating the sensor and modulator, thereby eliminating the need for a separate digitizer.

In another aspect, a low-powered wireless sensor comprises a signal generator, a modulator structure, two sensing circuits, and a transmitting element. For an operating frequency of f0, a signal source at frequency of f0and a transmitting element at f0connect to input and output ports, respectively. The other ports connect to similar sensors. One sensor detects the physical, chemical, environmental, etc. variations or conditions to be detected in the sensor target (i.e. sensor data), while the other sensor provides reference data. The detected variations are used to modulate the input signal, which is transmitted to a gateway node for further analysis. The reference data can be used in a calibration procedure at the receiver.

In another aspect, a smart low-powered wireless/RFID sensor comprises a signal generator, a modulator structure, a sensing circuit, a pilot sequence generator, and a transmitting element. For an operating frequency of f0, a signal source at the frequency of f0and a transmitting element at f0connect to input and output ports, respectively. The other ports operate as sensing and pilot ports, which connect to a sensor and a pilot sequence generator, respectively. The variations detected by the sensor provide the in-phase (or quadrature) data. The pilot sequence generator has two switches and four predefined loads. A control signal controls the switches to generate the pilot data, which provides the quadrature (or in-phase) data. The in-phase and quadrature data is used to modulate the input signal, which is transmitted to a gateway node. The pilot data can be used for calibration and channel estimation to reduce the overall error at the receiver. The pilot data can be generated based on a known RFID, ZigBee®, or Bluetooth® standard by a microcontroller or transponder integrated circuit (IC) and used for item identification.

In another aspect, an unpowered wireless sensor comprises a receiving element, a frequency multiplier, a modulator structure, two sensing circuits, and a transmitting element. The receiving element absorbs a pure signal transmitted by a gateway node at f0/M. The M-times frequency multiplier comprises nonlinear elements and matching circuits, changes the frequency to f0and provides the signal to the input port of the modulator. A transmitting element at f0connects to the output port. The other ports connect to two similar sensors. The first sensor provides sensor data, while the other sensor provides reference data. The detected variations modulate the input signal, which is transmitted to a gateway node for further analysis. The reference data can be used in a calibration procedure at the receiver.

In another aspect, a smart unpowered wireless/RFID sensor comprises a receiving element, a frequency multiplier, a modulator structure, a sensing circuit, a low power pilot sequence generator, and a transmitting element. The receiving element absorbs a pure signal transmitted by the gateway node at f0/M. The M-times frequency multiplier comprises nonlinear elements, input and output matching circuits, and a DC-RF separator. The DC part of the frequency multiplier's output provides the power for the switches and the pilot sequence generator. The RF part of the frequency multiplier's output provides the input signal of the modulator at f0. A transmitting element at f0connects to the output port. The other ports are sensing and pilot ports, which connect to a sensor and a pilot sequence generator, respectively. The detected variations from the sensor provide the in-phase (or quadrature) data. The pilot sequence generator has two switches and predefined loads. A microcontroller or a mid-frequency signal (at f1), as pilot data, controls both switches. This provides the quadrature (or in-phase) data. The in-phase and quadrature data directly modulate the input signal, which is transmitted to a gateway node. The pilot data can be used for calibration and channel estimation to reduce the overall error at the receiver. Moreover, the pilot data can be generated based on a known RFID, ZigBee, or Bluetooth standard by a microcontroller or transponder IC, and used in an identification process.

Still further, in another aspect a single frequency unpowered wireless sensor comprises a receiving element, a circulator, a modulator structure, two sensing circuits, and a transmitting element. The receiving element absorbs a pure signal transmitted by the gateway node at f0and the receive path of circulator provides the signal to the input port of the modulator. A transmitting element at f0connects to the output port. The other ports connect to two similar sensors. A first sensor provides sensor data, while the other sensor provides reference data. The detected variations (sensor data) modulate the input signal, which is transmitted to a gateway node for further analysis. The reference data can again be used in a calibration procedure at the receiver. At the gateway, a circulator separates the transmitted pure signal from the received modulated signal plus the self-coupled transmitted pure signal. As the self-coupled component is known to the gateway, it can be removed from the received signal, which can then be demodulated to extract the sensor data.

In yet another aspect, a single/multiple frequency smart unpowered wireless/RFID sensor comprises a receiving element, a circulator, a divider or diplexer, a modulator structure, a sensing circuit, a low power pilot sequence generator, and a transmitting element. The receiving element absorbs pure signals transmitted by the gateway node at f0or various frequencies of f0and f1, and the receive path of circulator provides the signals to the input port of the modulator. In a single frequency system, the divider separates a part of input signal at f0to power up the node and provides the other part to the input port of the modulator. In a double frequency system, a diplexer separates absorbed frequencies and provides the component with frequency f1to power up the node, and the component with frequency f0to the input port of the modulator. Using a RF to DC converter (rectifier), the f1signal provides power for the switches and pilot sequence generator. The f0signal is the input signal to the modulator. A transmitting element at f0connects to the output port. The other ports are sensing and pilot ports, which connect to a sensor and the pilot sequence generator, respectively. The detected variations by the sensor provide the in-phase (or quadrature) data. The pilot sequence generator has two switches and predefined loads. A microcontroller or a mid-frequency signal (at fm) as the pilot data controls both switches. The pilot data provides the quadrature (or in-phase) data. The in-phase and quadrature data directly modulate the input signal and transmits to a gateway node. The pilot data can be used in calibration and channel estimation to reduce the overall error at the receiver. Again, the pilot data can be generated based on a known RFID, ZigBee, or Bluetooth standard with a microcontroller or transponder IC, and can also be used in an identification (such as an RFID) process. At the gateway, a circulator separates the transmitted pure signal from the received modulated signal plus the self-coupled transmitted pure signal. As the self-coupled part is known to the gateway, it can be removed from the received signal, which is then demodulated to extract the sensed data.

FIGS. 1 through 8illustrate the structure of different embodiments of a low-powered or unpowered wireless/RFID sensor in accordance with the aspects mentioned above.

Low-Powered Wireless Sensor

Referring first toFIG. 1, a first embodiment of a low-powered wireless sensor100is shown, comprising a signal generator102, a modulator110, first and second sensing circuits120,130, and a transmitting element104. In this example, the signal generator102is a low-powered crystal oscillator and the transmitting element104is an antenna. A power source140provides power to the signal generator102via a connection142. Although shown as a battery in this case, the power source140may be any suitable type of power source, such as a solar cell, a piezoelectric material, a super-capacitor, or a power-harvesting system.

The modulator110may be any type of circuit adapted to modulate a signal onto another signal, such as a planar multi-port direct conversion structure. Multi-port structures having more than four ports can collect different phase-shifted versions of the input signal; in this example, the modulator110is a direct conversion six-port modulator consisting of three hybrid 90° couplers112,114, and116and one in-phase power combiner118. However, a four-, five-, or six-port modulator structure (as shown inFIG. 2) may be employed with two (P3and P4), three (P3, P4and P5), and four (P3, P4, P5, and P6) sensing ports, respectively. A 100-ohm isolation resistor119is provided for the power combiner and a 50-ohm matching resistor106is used to terminate the unneeded port. The signal generator102at operating frequency f0connects to the input port P1, and the transmitting element104, configured to operate at f0, connects to the output port P2. The first pair of sensor ports P5, P6connect to the first sensing circuit120and the second pair of sensor ports P3and P4connect to the second sensing circuit130.

The sensing circuits120,130in this example are both microwave resonator sensors. However, any type of sensitive circuit that can provide variable input impedance in response to a detected sample (“sensor data”) may be used instead of the illustrated resonator sensors in this implementation, and is contemplated herein. One of the first and second sensing circuits120,130detects the variations or conditions to be monitored by the low-power wireless sensor; the other sensing circuit130or120operates as a reference, generating reference data. The data from the first and second sensing circuits120,130are each used by the couplers116,114to modify the input signal f0received at the port P1. The modified signals are combined at the combiner118and output as a modulated output signal at port P2, as discussed in further detail below. The modulated output signal is then transmitted by the transmitting element104as transmitted signal164and received by a receiving element162(e.g., an antenna) as received signal166at a node160. The transmitting element104and the receiving element162may be any suitable transmitting and receiving element configured for communication using a wireless (i.e., not fixed) connection, such as by electromagnetic, inductive, capacitive, and optical coupling or transmission. The node160may be a gateway node connected to other elements in a network or system, and may be configured to extract the sensor data from the received signal166for analysis, and to process data. Thus, the node160may include a microcontroller, and may be any suitable computing device provided with or in communication with a suitable receiving element162. The reference data encoded in the received signal166can be used by the node160for calibrating the first sensing circuit120.

In one illustrative application shown inFIG. 3, the low-powered wireless sensor node100ofFIG. 1is used as a material detector. A reference sample (Ref)222and a sample under test (SUT)232are placed on resonator sensing circuits220and230, respectively. The sensing circuit230detects the condition of the test sample232and generates sensor data, which directly modulates the input signal of frequency f0to generate a modulated output signal. The transmitting element104transmits the modulated output signal as the transmitted signal164having frequency f0to the node160via the receiving element162for further analysis.

Referring again toFIG. 1, the signal from the low-powered oscillator102is divided into in-phase (I) and quadrature (Q) paths by the first coupler112. For a given reference input signal aLO(t) at input port Pi, the output signal aT(t) at output port Pj in a six-port circuit as inFIG. 1can be represented as:

aT=∑i=36⁢Si⁢⁢1⁢S2⁢⁢i⁢Γi⁢aLO⁢⁢{Si⁢⁢1=Si⁢⁢1⁢ej⁢⁢θi⁢⁢1S2⁢⁢i=S2⁢⁢i⁢ej⁢⁢θ2⁢⁢i(2)
where Sijare scattering parameters between ports Pi and Pj and Γiis the reflection coefficient. In order to have a good reflection coefficient at the input port, the following equation should be satisfied:

A simple way to implement the above condition is using the quadrature reflection phase shifting characteristics of the couplers114and116. In these structures, reflections from ports P3-P4and P5-P6cancel each other out when the reflectors at P3and P5are the same as P4and P6, respectively. In that case, the sensing circuits220and230inFIG. 1produce variable loads for the sample under test (SUT)232and reference sample (Ref)222, respectively, complex reflection coefficients of ΓSUT=ISUT+jQSUTand ΓRef=IRef+jQRef, from P3to P4and from P5to P6, respectively. Furthermore, in the case of an ideal combiner118and couplers112-114, the transmitted signal at P2is simplified as:

aT=(ΓRef+j⁢⁢ΓSUT)2⁢aLO(3)
and the overall transmission coefficient from P1to P2is:
ΓT=(ΓRef+jΓSUT)/2  (4)

Therefore, the output signal is composed of two vector components, i.e. ΓRefand jΓSUTwhich are in a semi-orthogonal vector space and easy to separate at the receiver.

On the receiving end at the node160, a quadrature demodulator may separate the real and imaginary parts of the received signal as:
ΓR=IR+jQR,  (5)

In the ideal communication channel, the normalized transmitted signal164is equal to the received one166and considering the (4), we have:
ΓR=ΓT=(ΓRef+jΓSUT)/2,  (6)

At the first step, when the system is started in the free state (R0) for both the reference sample (Ref)222and the sample under test (SUT)232, the ΓRefis equal to the jΓSUTand can be calculated simply from ΓR0by:
ΓRef=ΓR0−jΓR0,  (7)

Then, for each sample under test (SUT)232we have:
ΓSUT=j(ΓRef−2ΓR),  (8)
and from the calculated ΓSUTand from Γ versus the relative permittivity εrof the sensor, the value of the relative permittivity εrof the sample under test232can be estimated.
Smart Low-Powered Wireless/RFID Sensor

FIG. 4illustrates a further embodiment of a low-powered sensor, here a smart low-powered wireless/RFID sensor300. The embodiments described herein referred to as “smart” are embodiments incorporating pilot data and identification data, as described below. It will be appreciated by those skilled in the art that the example “smart” sensors described here can include further capabilities—for example, additional sensing structures—generating additional data that may be encoded in the signals transmitted from the sensor300. The sensor300comprises a signal generator102, a modulator110, a sensing circuit120, a pilot sequence generator330, and a transmitting element104. The signal generator102, modulator110, sensing circuit120, and transmitting element104may be the same types as described above.

The signal generator102, operating at frequency f0, connects to the input port P1of the modulator110and the transmitting element104, also operating at frequency f0, connects to the output port P2. The sensing circuit120is connected to the sensor ports P5and P6, while pilot sequence generator330is connected to the pilot ports P3and P4. The modulator110produces a directly modulated output signal at port P2using sensor data generated by the first sensing circuit120and pilot data generated by the pilot sequence generator330. The pilot data may be known to the node160. In the examples herein, the pilot data may comprise a fixed value such as an identifier (e.g., such as that used in for RFID identification), or optionally other arbitrary or non-arbitrary data sequences comprising with ZigBee, Bluetooth, or other standards. The pilot data, when obtained by the node160, may be used in processing the sensor data also received by the node160.

The pilot sequence generator330has two N-state (multi-state) switches332and334(thus, N=2 in this example), each connecting to one of a corresponding set of N predefined loads340,350respectively. In the example ofFIG. 4, there are thus four predefined loads342(short circuit),344(open circuit),352(short circuit), and354(open circuit). A control signal338from the controller336, preferably a mid-frequency signal, provides the in-phase pilot data I(t) to control the multi-state switches332and334at ports P3and P4in the upper branch of the modulator110. For simplicity, only two marginal reflection states provided by the predefined loads are available for selection: short (loads342,352) and open (loads344,354) circuits. This fixes the complex reflection coefficients of Γ0and −Γ0for I(t)=0 and 1, respectively. In other implementations, the loads342,344,352,354may be provided by variable load devices such as resistors or capacitors with voltage- or current-controlled impedances.

In the bottom branch of the modulator110, the ports P5and P5are connected to the sensing circuit120, which provides a variable load with a complex reflection coefficient of FSUT. Any change in the input impedance of sensing circuit120, due to the variations or conditions of a sample under test (SUT)232placed on the sensing circuit120, is directly modulated in the output signal at P6. The combiner118is used to combine the signals of the upper and lower branches of the modulator110. In the case of ideal combiners118and couplers112-116, the output signal at port P2can be simplified as:
aRF=2S31S23((−1)I(t)Γ0+jΓSUT)aLO,I=0,1.  (9)

The output signal is thus composed of two vector components, i.e. (−1)I(t)Γ0and jΓSUT, with the first one being known at the node160. The output signal is transmitted using transmitting element104and is sent as transmitted signal164with frequency f0. The signal is received as received signal166by receiving element162of the node160, where the received signal166can be decoded and analyzed. The pilot data extracted from the received signal166may be used for identification (such as an RFID), sensor calibration, and channel estimation purposes at the node160.

The controller336in these examples can be a microcontroller adapted to implement any suitable signal type that can be used to provide pilot data, including standard signals, such as ZigBee, Bluetooth, and RFID. Thus, the sensor300can function both as an RFID tag or similar wireless identification tag, and as a sensor device in a wireless sensor network. Alternatively, the controller336may operate as a mid-frequency signal generator operating at frequency f1to generate a simple, unique data sequence while consuming less power than a controller generating standard signals.

A power source320provides power to the signal generator102via a connection322, to the controller336via another connection324, and to the switches332and334via a further connection326. Although shown as a battery in this case, as explained above the power source320may be any suitable type of power source, such as a solar cell, a piezoelectric material, a super-capacitor, or a power-harvesting system.

Unpowered Wireless Sensor

In another embodiment, an unpowered wireless sensor400is provided, as shown inFIG. 5. The sensor400comprises a receiving element402, a frequency multiplier410, a modulator110, first and second sensing circuits120,130, and a transmitting element104. The modulator110, sensing circuits120and130, and the transmitting element104may be the same types as described above. The unpowered wireless sensor400operates in conjunction with a node460, which as before may be a gateway node. The node460is provided with a receiving element462as well as a transmitting element472. The receiving and transmitting elements462,472may be antennas as in the illustrated example.

The node460transmits, via its transmitting element472, a pure signal474of frequency of f0/M which is received by the sensor400by its receiving element402as received signal476. An M-times frequency multiplier410, in this example a passive diode doubler (i.e., M=2), comprises nonlinear elements412, such as diodes or transistors, and input and output matching circuits414and416. The due to its nonlinearity, the output signal of the diode412has a harmonic at M times the frequency input (in this example, at 2× the input). Consequently, the output matching circuit416only passes the signal with frequency f0. This modified signal is provided as input at port P1of the modulator110.

The ports P3and P4of the modulator110connect to the first sensing circuit120, while the ports P5and P6connect to the second sensing circuit130. As in the first embodiment described above, one of the first and second sensing circuits120,130detects the variations or conditions to be monitored by the low-power wireless sensor, producing sensor data; the other sensing circuit130or120operates as a reference, generating reference data. If the unpowered wireless sensor400is implemented as a material detector, a reference sample222and a test sample232are placed on the first and second sensing circuits120and130, as described above with reference toFIG. 3. The sensor data from the first and second sensing circuits120,130are used by the couplers to modulate the input signal at frequency f0received from the input port P1. The modulated signal is output at port P2to the transmitting element104, which operates at frequency f0to transmit the modified output signal as output signal464to the node460. The architecture of the unpowered sensor400is thus similar to the low-powered sensor, except that the input signal at port P1is extracted from the received signal474, and there is no power source in the sensor400.

The receiving element462of the node460receives the signal from the sensor400as received signal466. On receipt of the signal466, the node160may extract the sensor data for analysis, while the reference data may be extracted for use in calibration. The frequency f0of the received wave466is in a different region of the frequency domain from the originally transmitted signal474at frequency f0/M.

The corresponding pairs of transmitting elements and receiving elements104,462and462,472may be antennas or any other suitable transmitting and receiving element, as mentioned above.

FIG. 6illustrates a smart unpowered wireless/RFID sensor500embodiment. The architecture is similar to the smart low-powered sensor examples above, except that the input signal and bias signals powering the sensor500are obtained from a received signal. The sensor500comprises a receiving element402, a frequency multiplier510, a modulator110, a sensing circuit120, a pilot sequence generator330, and a transmitting element104. These components may be the same type as the components described above with reference to the other illustrated embodiments. The node460may be configured in a similar manner to the node460described above with reference toFIG. 5, and receiving and transmitting elements472,402and104,462may be any suitable elements as discussed above.

In this embodiment, the node460transmits via its transmitting element472a pure signal476with a frequency of f0/M. The pure signal is received as received signal476by the receiving element402of the sensor500and passed to an M-times frequency multiplier510. The M-times frequency multiplier510(M=2 in this example) comprises nonlinear elements512and input/output matching circuits514/516. Then, a low-pass filter inside the DC-RF separator518extracts the DC component of the signal. This portion of the signal is used as the bias for the controller336and switches332,334of the pilot sequence generator330via lines520,522and524. The RF component having a f0harmonic is extracted using a band pass filter of the separator518, and is input to the port P1of the modulator110. The input signal to the modulator110at port P1therefore has frequency f0.

Ports P3, P4are connected to a pilot sequence generator330and ports P5, P6are connected to a sensing circuit120. The pilot sequence generator330and sensing circuit120may be configured as described above with reference toFIG. 4. As in the example ofFIG. 4, the pilot data generated by the pilot sequence generator330and the sensor data from the sensing circuit120provide the quadrature and in-phase data (or vice versa) used by the modulator110to directly modulate the signal output at port P2, which has frequency f0. Also as described above with respect toFIG. 4, the signal464transmitted by the transmitting element104connected to port P2is received as received signal466by receiving element462of the node460, where the received signal466can be decoded and analyzed. The pilot data extracted from the received signal466may be used for identification (such as an RFID), sensor calibration, and channel estimation purposes at the node460.

FIG. 7illustrates a further embodiment, a single-frequency unpowered wireless sensor600. The sensor600comprises a receiving/transmitting element603, a receive/transmit separator602, a modulator110, and two sensing circuits120,130. The modulator110and sensing circuits120,130may be components as generally described above with reference to other embodiments. The separator602may be a circulator.

The node660, which as before may be a gateway node, is provided with a receiver662(which in this example includes a down-converter and processor) and a signal generator664in communication with a circulator666. The signal generator664generates a pure signal at frequency f0, which is transmitted via the circulator666to the receiving/transmitting element668and thence as signal674to the sensor600. The receiving/transmitting elements104,668may be an antenna or any other suitable component as discussed above.

The receiving element603receives the signal generated by the node660as received signal676. This signal is received by a first port of the separator602, which passes the received signal through a second port to the input port P1of the modulator110. The output port P2of the modulator110is connected to a third port of the separator602. The other ports P3, P4, P5, and P6of the modulator110are connected to two similar sensing circuits120,130, generally as described above with reference toFIGS. 1 and 5. Thus, one sensing circuit120,130detects variations or conditions in the target to produce sensor data, while the other sensing circuit130,120operates as a reference, producing reference data. The sensor600can be used as a material detector in the same general manner as described above with reference toFIG. 3.

The sensor data and reference data produced by the sensing circuits120,130directly modulate the input signal in the modulator110. The modulated signal is output to port P2, and transmitted by the receiving/transmitting element603as output signal604. This signal is received by the receiving/transmitting element668of the gateway660, then passed via the circulator666to the receiver662. In a self calibration process, the self-coupling of signal664into the receiver662is determined. Then, the node660can remove the self-coupled signal667from the received signal, and extract the sensor data for analysis. The reference data may also be extracted by the node660, and used for calibration.

In a further embodiment, a single/multiple frequency smart unpowered wireless/RFID sensor700is provided, as illustrated inFIG. 8. The sensor700in this embodiment comprises a receiving/transmitting element603, a receive/transmit separator602, a modulator110, a sensing circuit120, and a pilot sequence generator330. These components are generally described above with reference to other embodiments.

The node660is provided with a receiver662and a signal generator664in communication with a circulator666. The signal generator664generates a pure signal at frequency f0, or at various frequencies f0and f1. The signal is transmitted via the circulator666to the receiving/transmitting element668and thence as signal674to the sensor600. The receiving/transmitting elements104,668may be an antenna or any other suitable component as discussed above.

The receiving/transmitting element603of the sensor700receives the signal generated by the node660as received signal676. The received signal676is provided to a first port of the separator602, which passes the received signal through a second port to a divider or diplexer718. In a single frequency system where the input signal at the separator602is at a single frequency f0, the divider718directs part of the input signal to an RF to DC converter710, and the remainder of the signal to the input port P1of the modulator110. The RF to DC converter710comprises nonlinear elements (such as diodes or transistors)712and input and output matching circuits714,716to produce an output DC signal, which provides power for the pilot sequence generator330via lines522(to the controller336) and the switches332and334(via line524). In a mixed frequency system, a diplexer718diverts the portion of the signal with frequency f1to the RF to DC converter710to power the pilot sequence generator330, and the frequency f0portion to the input port P1of the modulator110.

The output port P2of the modulator110is connected to a third port of the separator602, while ports P5and P6are connected to the sensing circuit120, and ports P3and P4are connected to the pilot sequence generator330. As described above, the pilot sequence generator330operates to produce pilot data input to the modulator110, while the sensing circuit120produces sensor data input to the modulator110. These inputs to the modulator110provide the quadrature and in-phase parts (or vice versa) that are used to directly modulate the input signal received at port P1. The modulated output signal is output at P2to the connected receiving/transmitting element603, which transmits the modulated output signal as signal604to the node660.

The receiving/transmitting element668of the node660receives the signal as received signal606, and passes the received signal to the receiver662via the circulator666. As mentioned above, the self-coupled portion of the signal667can be removed from the received signal at the receiver662, and the sensor and pilot data can be extracted for analysis, calibration, and channel estimation.

Experimental Results

Each sensor subsystem described above was designed, fabricated, and tested at an operating frequency of 2.45 GHz. The test system employed a R&S® ZVA67 vector network analyzer (VNA) and a Tektronix™ DPO71604C digital oscilloscope for scattering parameter and time domain signal measurements, respectively. Simple patch antennas were used for wireless transmission to and from the sensor. Signals received from the sensors were collected by the oscilloscope and processed to extract the sensing information and pilot data.

The overall system was then tested with various standard samples. All circuits are fabricated with printed circuit technology using RO4003 laminate from Rogers Corporation, Arizona, USA, which has a relative permittivity of about 3.55, thickness of 0.508 mm, and a loss tangent of 0.0027.

FIG. 9(a)is a schematic of the fabricated low-powered sensor represented schematically inFIG. 1, with dimensions indicated inFIG. 9(b)as set out in Table 1:

The sensing circuit in this example consists of a quadrature coupler and a sample area which provides a SUT-related capacitive loading for the coupler. Coupons were extracted from various Rogers Corporation lamintes by removing the metal foil, thus yielding samples with εr=2.2, 3.55, 4.5, 6.0, 10.2, and 12.85. Each of these samples were placed on the sample area.

FIG. 10is a schematic of a fabricated pilot modulator circuit with the same dimensions indicated in Table 1. The pilot modulator circuit is composed of a quadrature coupler and two low insertion-loss Infineon Technologies BGS12SN6 RF MOS switches SW1and SW2. The bias voltage VBiaswas 3.4V and the control voltage VCtlrepresenting pilot data was modified by a periodic pulse between 0V and 3V. The time between control and RF is about 500 ns for this type of switch; accordingly, the frequency of the pilot modulator circuit900was selected as 100 KHz to ensure sufficient time for stable switching between two states in each period. However, in principle there is no frequency limitation if a correspondingly faster switch is used.

FIG. 11is a schematic of a fabricated smart low-powered direct-conversion sensor, using the same dimensions as inFIG. 9, and in which one sensing circuit has been replaced by the pilot modulator circuit ofFIG. 10.

A compact smart unpowered direct-conversion sensor was also fabricated, as shown inFIG. 12, to demonstrate the application of the inventive concepts herein to miniature sensor nodes. The six-port structure was used. It can be seen that that by using a low-profile sub-miniature power dividers PD (PD2328J5050S2HF from Anaren™) and hybrid 90° couplers H90 (C2327J5003AHF from Anaren™), the footprint of the sensor structure can be significantly reduced. The selected switches (Infineon™ BGS12SN6) and mid-frequency signal generator (Abracon™ ASTMTXK 32.768KHZ) are also very small. The oscillator used in this example (Abracon™ TCXO 32.768 KHz) is high performance with ultra-low current consumption.

FIGS. 13(a) and (b)shows the measured (a) magnitude and (b) phase of the transmission coefficient (ΓT) between ports P2and P1of the fabricated smart low-powered direct-conversion sensor, using the six samples mentioned above. As expected for this symmetric structure and its capacitive loads, the magnitude of ΓT inFIG. 13(a)is approximately constant and close to 1.

The variations of phase of ΓSUTwith respect to the free state of sensor ΓRef(i.e. εr=1) is approximately a linear function of εr, as can be seen inFIG. 14. This curve can be used at the receiver to find the εrvalue of SUT from demodulated transmission coefficient and as they are linearly dependent, the calibration procedure is simple.

FIG. 15shows the calculated εrSUTand its error with respect to the actual value for the fabricated unpowered direct-conversion sensor. In these calculations, the calculated ΓSUTwas normalized by ΓRefand then mapped into the curve shown inFIG. 14to find εr. As can be seen inFIG. 15, was less than 16%. It should be noted that this error is a function of the transmission channel and may be degraded in a noisy environment.

FIGS. 16(a) and (b)show the measured (a) amplitude and (b) phase of transmission coefficient, respectively, of the fabricated pilot modulator in the fabricated smart unpowered direct-conversion sensor. The amplitude of ΓT inFIG. 16(a)is approximately constant and close to 1, and the phase difference between two states inFIG. 16(b)is 180°, which reflected design expectations.

FIGS. 17(a) and (b)show the constellation (imaginary versus real component of a signal) of the demodulated (a) pilot and (b) sensor data, respectively, for the smart direct-conversion sensor. The constellation of the pilot data was approximately constant while the sensor data varied in proportion to εr.

FIGS. 18(a) and (b)show (a) the calculated εrfrom the received signals for different samples in a calibration process and (b) the error of calculated εrwith respect to the actual value for the smart direct-conversion sensor, respectively. In these calculations, the free state and one sample, as indicated in the legends, were used in a linear approximation. Based on the result for the given sample, the other five εrvalues were calculated based on the extracted QSUT/ISUTand the above linear approximation. As can be seen inFIG. 18(b), the error related to the εrof the calibration sample. However, based on the desired range of εr, a sample can be used as the standard sample to minimize error. In the presented range of εr, the error of estimation was kept below 15%. It should be noted that this error is a function of the transmission channel and may be degraded in an industrial or other noisy environment.

It will be appreciated by those skilled in the art that the foregoing embodiments demonstrate a wireless sensor using a modulator, such as a multi-port direct conversion structure, in combination with at least one sensing circuit and either reference or pilot data to modify an input signal to the modulator. The resultant modulated output signal can then be transmitted via an antenna or other suitable means to a receiving unit, such as the aforementioned nodes, which can extract the sensor data from the sensing circuit and the reference/pilot data for analysis and processing. Optionally, the wireless sensor can be powered with an on-board power source; but alternatively, the sensor may obtain or harvest power from another source, or be powered by the signal transmitted by the receiving unit. The receiving unit may be a card reader, smart phone, or other device adapted for communication with the wireless sensor. The wireless sensor, in some embodiments, can thus function as a combined sensor node and RFID tag, and may furthermore operate at a variety of frequencies encompassing radio, telecommunications, and ISM bands. The wireless sensor may furthermore communicate in a wideband or ultra-wideband mode, or in multiple bands, to reduce power consumption or environmental noise. Some or all of the components of the wireless sensor may be provided in compact form, or as integrated circuits.

The present invention has been described above and shown in the drawings by way of example embodiments and applications, having regard to the accompanying drawings. These are merely illustrative of the present invention; it is not necessary for a particular feature of a particular embodiment to be used exclusively with that particular embodiment. Instead, any of the features described above and/or depicted in the drawings can be combined with any of the example embodiments, in addition to or in substitution for any of the other features of those example embodiments. One embodiment's features are not mutually exclusive to another exemplary embodiment's features. Further, it is not necessary for all features of an example embodiment to be used. Instead, any of the features described above can be used, without any other particular feature or features also being used. Accordingly, various changes and modifications can be made to the example embodiments and uses without departing from the scope of the invention as described herein.