TECHNICAL FIELD

[0001] The present disclosure relates to sensors, particularly for use in wireless or RFID applications. TECHNICAL BACKGROUND

[0002] Wireless sensors are used in a variety of home, medical, and industrial applications for a variety of purposes ranging from health and safety, to stochastic data acquisition, to monitoring and controlling operating environments. Sensor systems are generally configured to detect a physical characteristic or state of a sensed material, and transmit information about the sensed material to a networked or remote device for further processing. With the proliferation of“smart” devices and the Internet of Things, the need for low-cost wireless sensors has increased. However, many current wireless sensor systems require relatively complicated measurement and post-processing of the frequency spectrum to extract the relevant sensor data.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Example embodiments of the present invention are described with reference to the following drawings. In the drawings, like elements are assigned like reference numerals. The drawings are not necessarily to scale unless indicated:

[0004] FIG. l is a circuit diagram of an equivalent circuit of two RLC tanks; [0005] FIG. 2 is a block diagram of select components of a wireless sensor system with a demodulator-based detector;

[0006] FIG. 3 is a block diagram of select components of a wireless sensor system with a modulator-based detector;

[0007] FIG. 4 is a block diagram of select components of an implementation of a wireless sensor system with a demodulator-based detector; [0008] FIG. 5 is a block diagram of select components of a first implementation of a transmitter node including a detector;

[0009] FIG. 6 is a block diagram of select components of a second implementation of a transmitter node including a detector;

[0010] FIG. 7 is a block diagram of select components of a third implementation of a transmitter node including a detector;

[0011] FIGS. 8A and 8B are three-dimensional and plan view schematics, respectively, of an example antenna employed in a transmitter node in a demodulator-based detector system;

[0012] FIG. 9 is a circuit diagram representative of a transmitter node for use with a demodulator-based detector system;

[0013] FIG. 10 is a graph of measured input reflection coefficients of the antenna of FIGS 8 A and 8B for different materials under test;

[0014] FIG. 11 is a graph of the measured amplitude of a received signal in the demodulator- based detector system;

[0015] FIG. 12 is a graph of the demodulated and extracted bits from a received signal in the demodulator-based detector system;

[0016] FIG. 13 is a graph of carrier signal frequency extracted using a zero-crossing technique for the demodulator-based detector system;

[0017] FIG. 14 is a plan view schematic of an antenna, balun, and coupler in a modulator- based detector system; and

[0018] FIG. 15 is a graph of the normalized frequency shift and normalized deviation voltage measured in the modulator-based detector system. DETAILED DESCRIPTION

[0019] The present disclosure provides a simple, low-power, and low-cost wireless sensor system for use in wireless sensor networks, having a sensing circuit comprising a signal source (e.g., a local oscillator) and a passive radiator (e.g., an antenna), in which the input impedance of the radiator is altered by proximity to the sensed material, thereby producing an altered output signal representative of a physical characteristic of the sensed material. The change to the output signal is then measured and may be output as a numerical value that can then be correlated to a property of the sensed material, without the need for additional power consuming components at the transmitter, or alternatively without the need for complex signal processing at the receiver. The sensor function of the system is realized using the radiator structure itself, which operates as both sensor and antenna at the same time, and can be combined with functions of smart radiofrequency identification (RFID) tags or other smart sensor devices.

[0020] The physical characteristic, in the non-limiting examples described herein, can be the relative permittivity (dielectric constant _r ) of the sensed material. As those skilled in the art will appreciate, relative permittivity as a sensed physical characteristic can be useful in a variety of applications including soil water content measurement, flow of the material measurement, water impurity, gas sensing and material characterization. The physical characteristic may also be associated with other properties of the sensed material, such as a presence or proximity property (whether the object is present or absent from a region monitored by the sensor).

[0021] The change in input impedance induced by the sensed material alters the load on the sensing circuit, with the result that the operating frequency of the sensing circuit, and therefore the carrier frequency of the signal transmitted by the sensing circuit, is altered from its original, “free” frequency (the“free” frequency is the operating frequency of the sensing circuit when it is not loaded by the sensed material). The signal with the shifted carrier frequency, optionally carrying additional data, may be transmitted by a transmitting node comprising the sensing circuit for receipt by a receiving node. The receiving node can then extract the carrier frequency and use the carrier frequency information to determine the physical characteristic or property of the sensed material, or transmit the information to another node in the network which then uses the information to determine the physical characteristic or property (e.g., a data processing system in communication with the receiving node). In this embodiment, the receiving node carries out the task of“detecting” the signal corresponding to the physical characteristic, because the shift in carrier frequency is extracted at the receiving node.

[0022] Alternatively, the shift in frequency may be detected at the transmitting node by employing a comparator at the transmitting node to compare the carrier frequency to a reference,“free” frequency, and this information may be encoded into the signal transmitted by the transmitting node to the receiving node. In this embodiment, the transmitting node carries out the task of“detecting”. Either way, the wireless sensor system and transmitter of the embodiments described herein provide a less complex means for monitoring a sensed material, compared to conventional spectrum monitoring techniques used in other radiofrequency (RF) wireless sensor systems.

[0023] Thus, the embodiments described below include wireless sensor systems in which detection (i.e., the required signal processing to detect a frequency shift in the sensing circuit) occurs either at the receiver or demodulator side of the system, or at the transmitter or modulator side of the system. In these systems, a sensing circuit comprising a radiator and a signal source is included in the transmitter of the system. The signal source in the illustrated embodiments is a signal generator, here a voltage-controlled oscillator operably connected to the radiator and receiving input from an optional binary sequence generator or other data source. The radiator in these examples is a dipole antenna, although other antenna configurations may be employed. Those skilled in the art will appreciate that the equivalent circuit of an antenna may be represented by an RLC (resistor-inductor-capacitor) tank circuit, in which the resonant frequency of the circuit is dependent on the inductance and capacitance of the tank. Further, an oscillator may also be modeled as a similar RLC tank circuit. FIG. 1 illustrates how these equivalent circuits may be combined. The resonant frequency of the combined circuits may therefore be expressed as [0024] Any physical characteristic of a sensed material being monitored by the transmitter of the wireless sensor system that modifies either the inductance or the capacitance of the circuit will therefore alter the resonant frequency.

[0025] The change in resonant frequency may be detected by a receiver node that receives a signal from the transmitter. A receiver-based or demodulator-based detection system is illustrated generally in FIG. 2. A transmitting node 100 includes a transmitter module 110, which includes the aforementioned signal source, connected to a radiator 120 such as a dipole or patch antenna. A power source is also provided for powering the transmitter module 110. The power source may be a battery, solar cell, piezoelectric material, super-capacitor, power- harvesting system, or other power-supplying mechanism as known in the art. The transmitting node 100 may further include a microcontroller or other components to carry out other functions, depending on the intended application of the wireless sensor system. Such commonly-known components that may be included in the implementation of the wireless sensor systems described herein have been omitted from the following description and accompanying drawings for ease of exposition, but those skilled in the art will appreciate that these components may be included in any of the transmitter or receiver embodiments described herein, as appropriate.

[0026] A sensing region close to the radiator 120 receives a sensed material 50 (here designated as material under test (MUT) 50) shown in FIG. 2. The sensed material may be placed on the transmitting node 100, or alternatively the transmitting node 100 may be affixed to the sensed material. For example, if the transmitting node 100 takes the form of a mountable or adhesive tag, the tag may be mounted onto the sensed material, such as a container, vehicle, or even human skin. Where the sensed material does not have a discrete surface to which the node 100 may be affixed— for example, in the case where a fluid is being monitored by the sensor— the sensed material may simply be present in proximity to the radiator 120 or in contact with a sensing area proximate to the radiator 120.

[0027] Optionally, additional data, such as pilot data, from a data source may be provided for encoding in the signal output by the transmitting node 100, particularly in the case where the transmitting node 100 is comprised in a smart sensor that is capable of at least some limited amount of data processing. The data source may be integrated within the transmitting node 100, or may be external to the transmitting node 100. The transmitter module 110 in this embodiment may therefore further include an encoder, such as a binary sequence generator (not shown in FIG. 2) connected to the signal source. The encoder may be employed for a variety of modulation techniques such as quadrature amplitude modulation, quadrature phase shift keying, frequency-shift keying, and so on. The binary encoder may be included in any of the example transmitting nodes described herein.

[0028] Proximity of the sensed material 50 to the radiator 120 results in a shift of the resonant frequency of the circuit comprising the transmitter module 110 and the radiator 120 from its original resonant frequency / ₀ or ff _ree (i.e., the free state frequency of the sensing circuit when it is loaded only by impedance due to the ambient atmosphere, which is expected to have the lowest dielectric constant with _r equal to approximately 1) to new operating frequency / _l and the resultant signal radiated by the radiator 120 accordingly has a carrier frequency / _x . The resultant signal can then be received by a receiving antenna 210 of a receiving node 200. Data encoded in the signal can be recovered using any suitable demodulation technique (here indicated by data recovery block 220), while the carrier frequency of the received signal is recovered separately (indicated by carrier frequency block 230, producing a digital number indicating the carrier frequency). At the receiving node 200, the recovered carrier frequency may be compared to a reference frequency value. The reference frequency value may be a stored value (e.g., stored in memory at the receiving node 200) since the free state frequency of the transmitter is already known, or else obtained from a reference signal from a local oscillator to determine a difference between the carrier frequency and the reference frequency. The recovered carrier frequency of the difference may then be correlated to a physical characteristic and/or property of the sensed material, for example based on stored information at the receiving node 200 or at a computing system (not shown) in communication with the receiving node 200. Again, it will be understood that additional components, such as microcontrollers, communications subsystems, etc. which may be present at the receiving node 200 to provide this functionality will be known to those skilled in the art, but are not included in the drawings for ease of exposition. [0029] An alternative transmitter-based or modulator-based detection system is generally illustrated in FIG. 3. In this alternative system, the shift in carrier frequency is detected at the transmitting node 100, rather than at the receiving node 200. The sensed material 50 loads the sensing circuit comprising the antenna 120 and the transmitter module 110, producing a signal at the shifted operating/carrier frequency. The signal is sampled at the transmitter node 100 and compared to a reference frequency by a comparator 130. The reference frequency may be generated by a reference oscillator included in the transmitting node 100, as described below. The output of the comparator 130 (e.g., a digital number) can then be provided as input to the transmitter 110 and encoded in the signal output by the transmitting node 100 using any appropriate means. The signal transmitted by the transmitting node 100 can then be received and decoded by any suitable demodulator 240 in a suitable receiving node 200 to obtain the carrier frequency shift and any additional encoded data.

Demodulator-Based Detection

[0030] A specific architecture for a wireless sensor system with receiver-based or demodulator-based detection is illustrated in FIG. 4. At the transmitter node 300, a signal source, here a voltage-controlled oscillator (VCO) 330, generates a signal at an initial operating frequency while unloaded (“free”). The VCO 330 is operatively coupled to an antenna 320, and therefore, when a sensed material 50 is proximate to the antenna 320, the loading of the sensing circuit formed by the VCO 330 and antenna 320 results in a change to the operating frequency of the sensing circuit, and the carrier frequency of the signal transmitted by the antenna 320. A binary sequence generator 310 may be included, which provides data (e.g., an identifier for use in RFID) that is encoded in the signal from the VCO 330 using any appropriate modulation technique to provide an output signal that is transmitted by the antenna 320 on the resultant carrier frequency. [0031] At the receiver node 400, the transmitted signal is received by a receiving antenna 410.

The received signal can be demodulated using any appropriate demodulation technique to recover the binary data. In this example, a non-coherent envelope demodulator 420 is employed. The received signal is also passed to a zero-crossing detector 430 and pulse counter 440 to recover the frequency of the carrier signal. Each time the zero-crossing detector 430 detects that the signal changes sign, a pulse is sent to the pulse counter 440. The number of pulses N may be counted for a time i, and accordingly the carrier frequency will be N/21. A longer sampling time t will produce a more accurate result. The resultant carrier frequency value may then be compared to a reference frequency value to determine the physical characteristic or property of the sensed material, for example by a microcontroller at the receiving node 400, or else transmitted by the receiving node to another system for processing by a communications subsystem of the receiving node 400. Again, it will be appreciated that these other components of the receiving node 400 are not shown in the accompanying figures for ease of exposition, but their selection and implementation will be known to those skilled in the art.

[0032] The recovery of the carrier signal frequency is therefore relatively uncomplicated, and does not require transmission of any additional data from the transmitter side of the wireless sensor system, or the addition of further components to the transmitter. Modulator-Based Detection

[0033] FIGS. 5, 6, and 7 illustrate three example architectures for a transmitting node 500 for use in a wireless sensor system. In the example of FIG. 5, a VCO 520, binary sequence generator 510, and antenna 530 are provided in the transmitting node 500 as in the example of FIG. 4; however, a power coupler 540 is added between the antenna 530 and VCO to obtain a sample of the sensing circuit signal, which is passed to a comparator module 560. The comparator module 560, which may be a phase frequency comparator, also receives as input a reference signal at a reference frequency. The reference frequency may match the free state frequency of the sensing circuit, but may instead be a different value. The reference signal may be provided by a separate reference oscillator 550. The output of the comparator 560 can therefore be a low-frequency or DC signal proportional to the difference between the sampled and reference frequencies. The oscillators employed in the transmitting node may be VCOs, as mentioned above, or other suitable signal generators such as crystal oscillators or other resonant structures. [0034] The comparator 560 output may then be incorporated into the output signal from the transmitting node 500. In the example of FIG. 5, the comparator output is used in analog modulation of the signal generated by the VCO 520 (e.g., amplitude, phase or frequency modulation), while other data from the binary sequence generator 510 may be encoded on the signal using digital techniques. The signal transmitted by the antenna 530 is therefore transmitted at a carrier frequency corresponding to the shifted operating frequency of the sensing circuit, and carries information about the frequency shift, as well as any other encoded data. Corresponding demodulation techniques are used at the receiving node, not shown, to extract the data encoded in the received signal and to recover the information about the frequency shift.

[0035] The transmitting node 500 of FIG. 6 includes similar components as the embodiment of FIG. 5, but the output of the comparator 560 is digitized by an analog to digital converter (ADC) and provided to the binary sequence generator 510, and directly provided from the generator 510 to the VCO 520 for up-conversion into the signal transmitted from the antenna 530. In this example, then, the information about the frequency shift is encoded digitally in the signal. Finally, the transmitting node of FIG. 7 is similar to the embodiment of FIG. 6, but adds a mixer 580 for up-converting the signal.

[0036] In some implementations, each sensor must be restricted to transmit within a specified frequency band to avoid collisions with other sensors in the system. Therefore, once the comparator output is used to determine the frequency shift, output from the comparator may then be provided as input to the VCO 520 to re-tune the output of the VCO 520 so that the final signal emitted by the antenna 530 is at the original free state frequency, to ensure that transmissions are confined to a predetermined band. Re-tuning the output may also be desirable to ensure that the antenna 530 emits a signal in its optimum frequency range, or in the optimum frequency range for the receiver (not shown). This enables the modulator-based sensor to effectively detect the measured parameter of the sensed material 50 without adversely impacting its transmission efficiency. [0037] Thus, in these embodiments, the transmitting node 500 carries out the detecting function by determining the shift in operating frequency caused by the proximity of the sensed material to the antenna 530. This information is then encoded into the same signal transmitted by the transmitting node 500 to the receiver. Depending on the encoding, the receiving node may not require any additional components, as the same components used to recover the pilot or other data encoded in the signal may also be used to extract the frequency shift information. The receiving node may then process the extracted frequency shift information, or pass the information to another component for further processing.

Experimental Results Demodulator-Based Detection

[0038] A demodulator-based detection system was built and tested at an operating frequency of about 2.45 GHz. Circuits for the system were fabricated with printed circuit technology using R04003 laminate from Rogers Corporation, Arizona, USA having a relative permittivity of 3.55, a thickness of 0.508 mm, and a loss tangent of 0.0027. For the sake of simplicity, an On-Off-Keying (OOK) modulation technique, which can be employed with both RFID and near-field communication (NFC), was employed.

[0039] FIGS. 8A and 8B illustrate the dipole antenna fabricated for this test system. The three- dimensional diagram of FIG. 8A illustrates the relative arrangement of the antenna ground plane 600 and dipoles 605, sensing area 630, and a balun 610 provided to balance the currents in the circuit. The sensing area 630 in this example is a region spaced from, but generally coincident with, the area of the dipoles 605. Dimensions of the antenna, indicated in the plan view of FIG. 8B, are provided in Table 1 below:

Table 1. Dimensions of fabricated antenna in FIG. 8B.

[0040] FIG. 9 illustrates an equivalent circuit for the tested demodulator-based detection system, including a data source and circuits representing the VCOand antenna. For the evaluation purposes of this system, the data source 710 was a simple 0101 data stream provided by a pulse generator operating at 30 KHz with a 3 3 V peak-to-peak voltage. The receiver node was a 2.45 GHz patch antenna connected to a Tektronix® DPO71604C digital oscilloscope and a direct-conversion receiver operating at a 100 GS/s sample rate. The digitized signals were passed to a simulation system in Simulink and MATLAB (The MathWorks, Inc., Massachusetts, USA) performing the non-coherent envelope demodulation and zero-crossing counter functions described with reference to FIG. 4 above.

[0041] The system was run in the free state (no sensed material on the sensing area 630, so _r = 1), and with a set of sample materials under test, placed on the sensing area 630 in turn. The samples were laminates from Rogers Corporation, having dielectric constants of _r = 4.5, 6.0, 10.2, and 12.85 and cut to 20 mm square shapes. Each sample had a thickness of 0.51 mm, with the exception of the sample with _r = 12.85, which was 1.5 mm thick. Thus, the physical characteristic under test and only characteristic resulting in a substantial change to the input impedance of the antenna was the dielectric constant.

[0042] FIG. 10 shows the measurement results of the input reflection coefficient of the proposed sensor antenna for the free state and the four different samples, obtained using a Rohde & Schwarz® ZVL13 vector network analyzer after a full one-port calibration. FIG. 11 is a plot of the received signal in the time domain. FIG. 12 is a plot of the demodulator output and extracted bits (i.e., the 0101 data stream), also in the time domain. FIG. 13 is a plot illustrating a relationship between the extracted carrier frequency from the received signal (a digital value produced by the zero-crossing counter function), versus the dielectric constants of the free state ( _r = 1) and the samples.

Modulator-Based Detection

[0043] A modulator-based detection system was built and tested at an operating frequency of about 915 MHz, i.e., the North American UHF RFID band. Circuits for the system were again fabricated with printed circuit technology using R04003 laminate with the same relative permittivity of 3.55, thickness of 0.508 mm, and loss tangent of 0.0027. The transmitting antenna was a parasitically-loaded dipole antenna, illustrated in plan view in FIG. 14. Samples of the material under test were substantially similar in dimension to the samples in the modulator-based detection system test, but with dielectric constants 2.2, 6, and 10.2 The samples were placed in a sensing area defined by an open copper loop 830 adjacent to the dipoles 805 of the antenna. The use of a parasitic element such as the sensing area in this example capacitively loads the dipoles without substantially altering the radiation characteristics of the antenna, as might result if the samples are placed immediately proximate to the antenna. The antenna was connected via a balun 810 (a 3 dB power divider with a 180° phase shift in one branch) to a first port (C) of a coupler 850, to balance the current received from the voltage-controlled oscillator (VCO) connected to a second port (A) of the coupler 850. The ground plane 800 of the antenna is shown in phantom in FIG. 14. A third port (D) of the coupler 850 was connected to a phase frequency comparator (PFC). The VCO and PFC employed were an Analog Devices® ADF4153 fractional -N frequency synthesizer evaluation board and HMC439 digital phase frequency detector, respectively. A remaining port (B) of the coupler 850 was left isolated, terminating in a matched load. The coupler 850 was used to extract a sample of the altered carrier frequency of the sensing circuit to provide as input to the PFC. A coupling factor of -14 dB was selected in order to obtain sufficient power for detection by the PFC without unduly depleting the radiated signal transmitted from the antenna. A separate reference signal was generated by a reference local oscillator, set to the free running frequency of the VCO, and input to the PFC for comparison to the sensing circuit input.

[0044] The output of the PFC was connected to a low-pass filter and an analog to digital converter (ADC). Since the input signals from the sensing circuit and reference oscillator may be out of phase, the low-pass filter was employed on the PFC output to accumulate the output signal over time, which correlated to the error between the two input signal frequencies. The ADC was then used to convert this detected error to a digital value. As noted above, output from the PFC may also be provided as input to the VCO to re-tune the operating frequency of the circuit to compensate for the shift induced by the sensed material, so that the operating frequency of the circuit returns to its original, free state value.

[0045] During testing, the initial free running frequency of the VCO, ff _ree , and the PFC free output voltage, f _ree , was determined (i.e., with no sample in the sample area, such that _r = 1). Each sample was then placed on the sample area in turn, and the ADC output was measured using a voltage meter over a 100 second period, taking the initial and final values of the measured voltage V _initia Vf _{t nai .} The 100 second time period was selected as a reasonable time period for manual measurement and recording of the output voltages in this test setup; however, it will be understood that measurements may be taken much more quickly in an automated system.

[0046] For each sample, the initial and final voltage measurements were used to compute the change in both voltage and frequency with reference to the free voltage and frequency of the system. These differences were expressed as normalized values across all samples as follows:

D — V i _nai V initial

[0047] FIG. 15 illustrates that the greater the dielectric constant _r , the greater the difference in carrier frequency in the loaded and free states. Thus, a change in carrier frequency due to loading of the signal generator at the transmitter may be used to detect a state of a material under test in a sensing area adjacent to the transmitter antenna. [0048] It will be appreciated by those skilled in the art that the foregoing embodiments demonstrate a wireless sensor system incorporating a sensing circuit at the modulator (transmitter) employing a signal generator and an antenna, and a detection function at either the modulator (transmitter) or demodulator (receiver). 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 any accompanying reference/pilot data for analysis and processing. Optionally, the transmitting node in the wireless sensor system can be powered with an on-board power source; but alternatively, the device may obtain or harvest power from another source, or be powered by the signal transmitted by the receiving unit. The receiving node may be a card reader, smart phone, or other device adapted for communication with the transmitter. The transmitter, in some embodiments, can function as a combined sensor node and RFID tag, and may furthermore operate at a variety of frequencies encompassing radio, telecommunications, and ISM bands, and may transmit a signal optically, magnetically, or electrically. 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 transmitter and receiver may be provided in compact form, or as integrated circuits.

[0049] 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.