Wireless and Battery-less Sensor Using RF Energy Harvesting

Transcription

1 DOI.56/etc4/. Wirele and Battery-le Senor Uing RF Energy Harveting Chritian Merz, Gerald Kupri, Maximilian Niedernhuber 3 Deggendorf Intitute of Technology, Edlmairtr , Deggendorf, Germany [email protected] [email protected] 3 [email protected] Abtract The contribution introduce a RF energy harveting circuit which can be ued to power a wirele and battery-le enor ytem. The enor can be powered wirelely over a ditance of about two meter. The baic principle of RF energy harveting are explained, it i hown how the ytem can be deigned and pecial conideration are dicued. The propoed RF energy harveting ytem operate at a frequency of MHz, o that far field propagation i preent. The ytem conit of an antenna, a matching circuit, a RF-to-DC converion circuit and a power management module, including a torage capacitor, a comparator and a DC-to-DC converter. The harveter produce a puled output voltage of.8 V at an input power of at leat -6 dbm. Key word: RF Energy Harveting, Wirele Power Tranfer, Battery-le Senor, RF-to-DC Converion, Electromagnetic Far Field Propagation Introduction Energy harveting i the proce of capturing, converion and toring of energy from the environment to upply low power device or aving the energy for later ue. There are many different type of energy harveting ource, e.g. olar, vibration, temperature or electromagnetic wave. Of thee ource, electromagnetic wave provide by far the leat power denity, which i indicated by table. [] Tab. : Different energy harveting ource. Source Technology Power Denity Solar Vibration Temperature Electromagnetic Wave Photovoltaic Cell Piezoelectric Element Thermal Generator mw/cm 8 µw/cm 6 µw/cm Antenna < µw/cm Becaue of the fact that the power denity of electromagnetic wave i very low, the energy feed-in and converion ha to be done very efficiently. The feed-in i performed by an antenna, which capture the incident electromagnetic wave and tranfer the reulting AC voltage to the harveter circuit. Between the antenna and the energy converion circuit there i the matching network. On the one hand, it ha the tak to enure the maximum power delivery from the antenna to the remaining circuit and on the other hand it minimize the ignal reflection at the interface of the antenna and the matching circuit. The energy converion circuit convert the RF power, which i delivered by the antenna, into DC voltage by uing a cacaded Greinacher circuit. After the rectification, the reulting DC voltage i tored at a capacitor. The energy at thi torage capacitor get accumulated until an upper threhold voltage i reached. After exceeding thi voltage, the torage capacitor dicharge until a lower threhold voltage i reached. Thi charging and dicharging of the capacitor i triggered by a comparator, which compare the voltage at the torage capacitor with an internal reference voltage and output a digital voltage indicating which one of the compared voltage i higher. The output of the comparator witche a DC-to- DC converter on and off, which increae the DC voltage level at the torage capacitor to another adjutable level, e.g..8 V. Fig. how the component of the propoed RF energy harveting ytem. etc4-34. European Telemetry and Tet Conference 8

2 DOI.56/etc4/. Fig.. Diagram of the propoed RF energy harveting ytem. Preconideration The election of the frequency band and the determination of the expected input power are very important apect that mut be dealt with before tarting the deign of the RF energy harveting ytem. For energy harveting purpoe, four frequencie are mainly ued in today indutry for different application. The frequencie have a major influence on the deign and on the tranmiion behavior of the harveting ytem. The frequencie are 5 khz, 3.56 MHz, 868 MHz and.4 GHz. Thee frequencie are allowed by the ETSI to be ued without permiion. The different frequencie lead to variou coupling mechanim at ditance of everal meter. At the frequencie of 5 khz and 3.56 MHz, motly inductive coupling i ued for power tranmiion. The frequencie of 868 MHz and.4 GHz typically are coupled electromagnetically. Thee coupling mechanim depend on the field region at which the power tranfer occur. The two field region are called near-field and far-field. The inductive coupling only occur at the near-field region and the electromagnetic coupling only take place at the far-field region. If the ditance d of the ender and the receiver of the tranmiion i below λ/(π), the near-field i preent. Above thi ditance, the region i conidered a the far-field. Within the near-field region, the magnetic field trength i dominant and decreae according to /d 3. At the farfield, the magnetic and electric field are in phae and create an electromagnetic wave. The field trength are decreaing within thi region according to /d. Becaue of thi, the ue of far-field propagation i preferred to nearfield propagation for long range application. The frequency band are divided into ubband, where different maximal field trength or tranmiion power are allowed. The 868 MHz band ha for example the ubband of MHz MHz, where a tranmiion power of Watt (ERP) i allowed without retriction. Becaue of thi, we ue the frequency of MHz for the power tranfer, which i the middle of the ubband. At the different frequencie, different antenna type are typically ued to capture the energy. At 5 khz and 3.56 MHz motly antenna coil are ued. At 868 MHz and.4 GHz mainly PCB-, chip-, patch- or monopole antenna are utilized depending on the application. To calculate the incident power at the antenna at a certain ditance R to the ender, the Frii tranmiion equation ha to be ued. It relate the received ignal power ( P r ) and the tranmitted ignal power ( P t ) a P r PG G ( ) 4R () rtt G t and G r are the antenna gain of the tranmitting and the receiving antenna and i the wavelength of the tranmitted ignal. The Frii formula i only valid for the following four condition. Firt, the tranmiion ha to take place in the far-field region at free pace condition. Second, the tranmitting and receiving antenna mut be correctly aligned and polarized. Third, the bandwidth of the tranmiion ha to be o narrow that one value for the wavelength can be aumed. Four, the two antenna and their tranmiion line are conjugate matched, o that no loe occur due to mimatching. Antenna The antenna ha the tak to harvet the incident electromagnetic wave that are propagated by a RF tranmitter. On the one hand, the gain of the antenna hould be a high a poible to increae the received power. etc4-34. European Telemetry and Tet Conference 9

3 DOI.56/etc4/. On the other hand, the antenna hould be a iotropical a poible, o that the direction at which the electromagnetic wave are captured doe not influence the harveting very much. Becaue the two feature cannot be achieved at the ame time, a compromie ha to be found. A quarter-wave monopole antenna i the bet compromie, becaue it ha a high gain of.5 dbi and an approximately iotropic antenna diagram, except at the antenna axi. The length of the quarter wave monopole at MHz i cm. The impedance of the antenna i 5 Ω, which i the ame value typical RF meaurement device have. Matching Circuit The matching circuit i needed to enure the maximum power delivery from the antenna to the remaining circuit and to minimize ignal reflection. A T-match circuit i ued to accomplih thi tak. It conit of two erie inductor and one parallel variable capacitor. The firt tep of the matching procedure i to determine the impedance of the harveting circuit without the antenna and matching circuit. Thi can be performed by imulation or experimentally by uing a network analyzer. In thi work, the impedance ha been determined experimentally. The impedance of the circuit doe not only depend on the frequency, but alo on the input power becaue of the diode, which are nonlinear device. The matching can be performed at one particular frequency and input power. The impedance of the circuit ha been meaured at a frequency at MHz and an input power of -6 dbm, which i the lowet power at which the harveter begin to work. The meaured impedance ha the value of 97 Ω + j 96 Ω. Thi impedance ha to be matched to the 5 Ω of the antenna. The econd tep of the matching procedure i the calculation of the reactive element of the T-match circuit. For thi calculation, the quality factor Q i a very important parameter. He decribe the broadband behavior of the matching and i defined a the relation of the center frequency f (in our cae: MHz) and the frequency bandwidth f = f - f, where f and f are the frequencie where the pectrum of the circuit i reduced to 3 db compared to f. For our calculation, we ue a Q factor of 3. A T-match network can be interpreted a two back-to-back L-type network with a common virtual reitance R, which i hown in fig.. Fig.. T-match circuit hown a two back-to-back L- network with common virtual reitor R. [] The virtual reitance R can be calculated uing eq. (), where R = 5 Ω i the impedance of the antenna. R R ( Q ) 5 3( ) 5 () The firt erial reactance can be computed with the following formula. QR 35 5 (3) S The firt parallel reactance p can be calculated a follow. R Q 3 p (4) For the L-network at the load end, the quality factor Q i defined by the virtual reitor R and the load reitor RL a: R Q (5) R L With thi value, the econd parallel reactance can be determined a follow. p R Q.38 p (6) The combined equivalent parallel reactance can be calculated with eq. (7). p 99.3 p p p (7) etc4-34. European Telemetry and Tet Conference

4 DOI.56/etc4/. The econd erial reactance can be calculated uing the following equation. Q R L L (8) Finally, the value of the reactive element L, L and C var can be calculated with the following three formula. L f nh 7 (9) C var i a variable capacitor with the range of.4 pf to 3 pf o that device tolerance, meaurement uncertaintie and inductive and capacitive influence caued by the microtrip line can be compenated. The matching with thee component lead to a good match at -6 dbm. Fig. 3 depict a mith chart that how the meaured impedance of the matched harveter circuit at the antenna input port from -8 dbm to 8 dbm L f nh 8 () C var f p pf5. () For L and L the commercial available value 7 nh and 8 nh have been ued. Fig. 3. Impedance of the matched harveter in dependence on the input power. Due to the matching, the voltage reflection coefficient decreae to approximately % at an input power of -6 dbm. Thi mean, that 98 % of the power which i captured by the antenna get delivered to the ret of the harveter circuit. Fig. 4 how the voltage coefficient in dependence to the input power from -8 dbm to 8 dbm. Fig. 4. Voltage reflection coefficient over input power. etc4-34. European Telemetry and Tet Conference

5 DOI.56/etc4/. Rectifier Circuit To convert the electromagnetic wave into DC voltage, a rectifier circuit i needed. Since the level of the collected power i very low, the rectifier circuit i baed on a cacaded voltage multiplier circuit. To accomplih an effective rectification with an acceptable output DC voltage, a 7-tage cacaded Greinacher circuit i ued. The circuit not only rectifie the incoming ignal but alo multiplie the peak amplitude. With increaing tage, the output DC voltage get higher, but the loe alo increae with each tage. Fig. 5 how a ingle tage Greinacher circuit. Fig. 5. Single tage Greinacher circuit. Fig. 5 compoe the elementary tage of the rectifier circuit where C (. nf) and D form a negative clamp and C (. nf) and D achieve peak rectification. C moothe the output voltage and act a a tage torage capacitor. The choice of the diode i a very important apect of the rectifier deign and i critical for the overall performance of the harveter circuit. Becaue the harveter circuit operate at a frequency of MHz and an input power of -6 dbm, the diode hould have a very fat witching time and a very low turn on voltage. Thee requirement can be achieved by uing Schottky diode, which ue a metalemiconductor junction intead of a emiconductor-emiconductor junction. Thi allow the junction to operate much fater and perform a very low forward voltage drop. The Schottky diode HSMS-85P from Avago Technologie have been elected. They have a maximum forward voltage of 5 mv. Energy Storage Capacitor To cumulate the DC voltage, which i delivered by the rectifier, a torage capacitor i ued. The value of the capacitor determine the amount of energy which can be tored. The leakage current of the capacitor hould be a mall a poible. Smaller capacitor charge more quickly, but lead to horter operation cycle. Larger capacitor charge more lowly, but provide higher operation cycle. Becaue of thi, the value of the torage capacitor depend on the application. The following equation can be utilized to etimate the neceary capacitor value. 5 V () C V I t out out on V out and I out are the voltage and average current at the output of the DC-to-DC converter and t on i the on-time of V out. For the propoed harveter, C ha the value of 4 µf, which lead to an on-time of approximately 4 m at an output voltage of.8 V and an average output current of 3.7 ma. Thee parameter are uitable to power a wirele low power enor ytem coniting of a MCU and a enor. Comparator A comparator i an electrical circuit that compare two different analog voltage and output a digital voltage that indicate which one of the compared voltage ha the larger value. The comparator i needed to make ure that the energy which i tored at the torage capacitor cumulate until a certain voltage ( V ) a i reached. x, After exceeding thi voltage, the comparator output witche to high which reult in dicharging the torage capacitor until a defined voltage ( V ) i i n, reached. Thi behavior i viualized by fig. 6. Fig. 6. Output voltage of the comparator depending on the torage capacitor voltage. For the propoed RF energy harveting circuit, we ue the Maxim MA964 low power comparator. He ha a minimum upply voltage of V and conume maximal only 7 na upply current. Thi low-voltage capability make the comparator very attractive to paive wirele device. The comparator ha an internal. V reference voltage. In the propoed RF energy harveting ytem, the upply voltage i provided by the voltage which i applied at the torage capacitor. The two witching voltage at which the comparator witche on and off can be dimenioned via external reitor. The reitor dimenioning i explained in detail at [3]. The external reitor dimenioning enure that the two witching etc4-34. European Telemetry and Tet Conference

6 DOI.56/etc4/. voltage occur at V = a.7 x, V and V =.94 V. The difference of thi voltage i n, i 33 mv and i called hyterei bandwidth. In addition, the comparator ha the purpoe to prevent the circuit from overvoltage, becaue the maximum voltage V at a the x, torage capacitor, which upplie the comparator and the DC-to-DC converter, i never exceeded, independent from the input power. If the input power increae, the operation cycle are jut carried out more quickly due to the fater charging of the capacitor, but the maximum voltage at the torage capacitor doe not change. [4] DC-to-DC Converter A DC-to-DC converter i an electronic circuit that i able to convert DC voltage from one level to another. For our harveter, we ue the LTC356L DC-to-DC converter from Linear Technology. He ha the tak to increae the voltage which i applied at the torage capacitor (94 mv.7 V) to a DC voltage of.8 V during one operation cycle. The output voltage can be adjuted between.5 V and 5.5 V by an external reitor divider tap. The device ha an input voltage range between.5 V to 5 V and i upplied by the torage capacitor. [5] Complete RF Energy Harveting Sytem The RF energy harveting circuit propoed in thi work ha been deigned with CadSoft EAGLE and fabricated uing PCB technology. The maximum harveting range of the ytem i approximately m and the minimum neceary input power i -6 dbm. Fig. 7 how the voltage at the output of the harveter (lower graph) and the voltage at the torage capacitor (upper graph) during one operation cycle. Thee voltage have been meaured with an ocillocope. It can be een at the figure that the voltage at the torage capacitor decreae from.7 V to 94 mv which are the two threhold voltage determined by the comparator reitor dimenioning. During the dicharge of the torage capacitor the DC-to- DC converter output a DC voltage of.8 V for a time of 4 m. Thi i indicated by the two vertical line at fig. 7. Within the on-time of the DC-to-DC output voltage, the load, which i typically a wirele enor ytem, i powered and able to execute one operation cycle. Such cycle conit for example of the determination and tranmiion of temperature and humidity value. After operation, the enor ytem wait on tandby until the next operation cycle begin. Fig. 7. Vcap and Vout over time during one cyle Concluion and Future Work In thi paper, a RF energy harveting ytem for the 868 MHz band i preented. It can be ued to energize low power device, e.g. wirele enor ytem. The complete RF energy harveting ytem can operate at input power of at leat -6 dbm and i optimized for a frequency of MHz and ha a maximum harveting range of m. It output an energy of up to 6 μj during one operation cycle. One future tak of thi work i the invetigation of the behavior of the preented RF harveter circuit if it i integrated into everal building and inulation material, e.g. reinforced concrete or teel wool. Integrated into thee material, the RF energy harveting circuit hould be able to power a wirele enor ytem, which meaure and tranmit the temperature and humidity value of thee material. Reference [] K. Dembowki, Energy Harveting fuer die Mikroelektronik, pp. 4 ff, t edition, VDE Verlag, Berlin,. [] C. Bowick, J. Blyler, C. Ajluni, RF Circuit Deign, page 7, Newne, nd edition, 7 [3] Maxim Integrated. (5, Sep.) Application Note 366: Adding Extra Hyterei to Comparator. [Online]. Available: [4] Maxim Integrated. (4, Mar.) Ultra-Small, Low- Power Single Comparator in 4-Bump UCSP and 5-SOT3. [Online]. Available: d/583 [5] Linear Technology Corporation. (4, Mar.) LTC356L/LTC356LB. [Online]. Available: etc4-34. European Telemetry and Tet Conference 3