Patent Publication Number: US-2013249301-A1

Title: System And Method For Powering An RFID Module Using An Energy Harvesting Element

Description:
BACKGROUND  
     Radio frequency identification (RFID) technology is used in many different areas, including inventory control, point-of-sale transaction processing, determining the location of an individual, etc. An RFID element typically includes an integrated circuit and an associated antenna, the combination of which is sometimes referred to as an “RFID tag.” Some uses of RFID technology include determining the location of an individual in a particular geographical area and providing point-of-sale transactional processing for that individual. A high-frequency (HF) RFID tag is typically used at relatively short ranges, on the order of direct contact to about one foot, to support transactional interactions, such as point-of sale transactions, where an individual is charged for a product or service. Due to the nature of these transactions, they demand an affirmative action by the individual, such as swiping the RFID tag against a reader to initiate the transaction. 
     For RFID applications that do not demand an affirmative action by the individual, an ultra-high frequency (UHF) RFID tag can be used at relatively long ranges, on the order of 10-20 feet, to passively detect the proximity of an individual as they enter an area monitored for the presence of the UHF RFID tag. These UHF RFID tags are sometimes referred to as “far field” RFID tags. Such RFID applications can be useful for situations in which it is desirable to passively monitor for the presence of the wearer or allow the wearer to engage in an interactive experience without requiring any deliberate action by the wearer. 
     In some applications, one or more RFID tags can be located in a wearable item, such as a wristband, or other item, that can be worn by an individual. An example is an RFID wristband worn by a patient in a hospital or an attendee at an entertainment venue. These RFID tags typically employ HF technology and require the wearer to tap or swipe a reader to obtain the desired product or service. This “near field” tap or swipe results in a transactional type experience for the wearer, as described above. 
     One challenge with “far field” UHF RFID applications is that in order to avoid the need for a replaceable power source, such as a battery, the RFID circuitry generally requires a relatively large antenna to be able to provide the RFID tag with a signal having adequate signal strength. Such a large antenna does not readily lend itself to incorporation in a small, wearable item. 
     Therefore, it would be desirable to have a UHF RFID tag incorporated into a wearable item that does not require a replaceable power source or an inordinately large antenna. 
     SUMMARY 
     Embodiments of a system for powering a radio frequency identification (RFID) module include an energy harvesting system configured to passively generate a voltage, a voltage regulator configured to regulate the passively generated voltage and a controllable port through which the passively generated voltage is provided to the RFID module. 
     Other embodiments are also provided. Other systems, methods, features, and advantages of the invention will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention can be better understood with reference to the following figures. The components within the figures are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a schematic diagram illustrating a high-frequency (HF)/ ultra high-frequency (UHF) RFID assembly. 
         FIG. 2  is a plan view illustrating the RFID module of  FIG. 1 . 
         FIG. 3A  is a plan view illustrating a wristband assembly having the RFID assembly of  FIG. 1 . 
         FIG. 3B  is a cross-sectional view illustrating the wristband assembly of  FIG. 3A . 
         FIG. 4  is a cross-sectional view of a portion of the RFID module located within the wristband of  FIG. 3A  and  FIG. 3B . 
         FIG. 5  is a block diagram illustrating an embodiment of the energy harvesting system of  FIG. 4 , and additional related circuitry. 
         FIG. 6A  is a schematic diagram illustrating a first embodiment of the energy harvesting system of  FIG. 5 . 
         FIG. 6B  is a schematic diagram illustrating an alternative embodiment of the energy harvesting system of  FIG. 5 . 
         FIG. 6C  is a schematic diagram illustrating an alternative embodiment of the energy harvesting system of  FIG. 5 . 
         FIG. 7  is a schematic diagram illustrating another alternate alternative embodiment of the energy harvesting system of  FIG. 5 . 
         FIG. 8  is a schematic diagram illustrating another alternative embodiment of the energy harvesting system of  FIG. 5 . 
         FIG. 9  is a schematic diagram illustrating an example of using low frequency RF energy to create magnetic inductive coupling to supply the energy harvesting element. 
         FIG. 10  is a view illustrating an application of the RFID assembly of  FIG. 1 . 
         FIG. 11  is a flowchart describing an exemplary method for powering an RFID module using an energy harvesting element. 
     
    
    
     DETAILED DESCRIPTION 
     The system and method for powering an RFID module using an energy harvesting element can be used to increase the sensitivity of ultra-high frequency (UHF) circuitry in an RFID module having high-frequency (HF) and UHF circuitry. Shorter range HF RFID circuitry can be used to process a transaction that requires an affirmative action by a user, such as a swipe or other direct contact between the RFID tag and a reader. Such transactions typically involve a service or product for which there is a charge or fee and for which the individual must agree to pay. When used in an entertainment venue such as an amusement facility, longer range UHF RFID circuitry can be used to passively determine an individual&#39;s proximity in a geographical area. Applications of this nature could include interactive and personalized entertainment experiences, as well as capturing various operational metrics such as person counts and flow estimation, and possibly security or other location/behavior tracking applications. However, without a power source to provide external power to the UHF circuitry, the sensitivity, and therefore, the useful range, of the UHF RFID circuitry is limited. 
     Several challenges arise when combining and powering both UHF and HF circuitry in a single RFID module. There is limited space in a wristband or other wearable format to combine the two RF technologies. The performance and read range of a UHF antenna is reduced when located in close proximity to human skin, often necessitating providing increased power to the UHF circuitry to improve the sensitivity of the UHF antenna and related circuitry. As used herein, the term “RFID” encompasses all known RFID technologies including, for example, high-frequency (HF), ultra-high frequency (UHF), low-frequency (LF), active, passive and semi-passive, operating in frequencies ranging from approximately 800 MHz to approximately 5.8 GHz. 
       FIG. 1  is a schematic diagram illustrating a high-frequency (HF)/ultra high-frequency (UHF) RFID assembly  100 . The HF/UHF RFID assembly  100  is also referred to as the “RFID assembly.” The RFID assembly  100  comprises a HF/UHF RFID module  102 , also referred to as the “RFID module,” located on a backplane  104 . In an embodiment, the RFID module  102  comprises a high-frequency antenna  108  integrated onto a high-frequency subassembly  160 . The RFID module  102  also includes a planar UHF antenna  114  integrated onto a UHF subassembly  165  ( FIG. 2 ). The planar UHF antenna  114  comprises a substrate that can be formed using copper, aluminum, or another conductive material, onto which one or more UHF circuit elements can be formed. 
     The HF subassembly  160  also includes a ferrite isolator  112  separating and electrically isolating the high-frequency antenna  108  from the planar UHF antenna  114 . The RFID module  102  also includes a spacer  106  around which the planar UHF antenna  114  is assembled. The spacer  106  can be any high dielectric material, and, in an embodiment, can be made from polycarbonate, or another suitable material. The spacer  106  can be formed to have a curved structure designed to fit comfortably against the wrist of a wearer when the RFID assembly  100  is molded or otherwise contained within a wearable element, such as a wristband. 
       FIG. 2  is a plan view illustrating the RFID module  102  of  FIG. 1 . The HF subassembly  160  is mounted approximately as shown on the top surface of the RFID module  102 , adjacent to the UHF IC  124 . The HF subassembly  160  comprises the ferrite isolator  112 , which isolates the high-frequency antenna  108  from the surface of the planar UHF antenna  114 . The RFID module  102  also includes the planar UHF antenna  114  and UHF IC  124 , comprising the UHF subassembly  165  formed thereon. 
       FIG. 3A  is a plan view illustrating a wristband assembly  170  having the RFID assembly  100  of  FIG. 1 . The wristband assembly  170  includes a wristband portion  172  in which the RFID assembly  100  is contained. The RFID assembly  100  can be secured inside the wristband portion  172  by, for example, injection molding, or another fabrication technique. The backplane  104  can comprise a conductive foil, such as aluminum, copper, or another conductive material, and serves as an energy collection element to electrically excite the UHF subassembly  165 , and also serves to isolate the UHF subassembly  165  from the human skin, which absorbs the RFID energy. 
       FIG. 3B  is a cross-sectional view illustrating the wristband assembly  170 . The wristband assembly  170  includes the RFID module  102  applied over the backplane  104 , forming the RFID assembly  100 . The RFID assembly  100  is then molded within the wristband  172  to form the wristband assembly  170 . 
       FIG. 4  is a cross-sectional view of a portion of the RFID module  102  located within the wristband  172  of  FIG. 3A  and  FIG. 3B . In the view shown in  FIG. 4 , the RFID module  102  comprises the UHF subassembly  165  located adjacent to the energy harvesting system  180 . The energy harvesting system  180  can comprise and can interface to one or more energy harvesting device technologies that can provide power to the UHF IC  124  and the UHF antenna  114  to improve the sensitivity of the UHF circuitry. In an embodiment in which the energy harvesting system  180  is adapted to receive radio frequency (RF) energy, or energy coupled to the energy harvesting system  180  using magnetic coupling from which a voltage can be generated to power the UHF IC  124 , a loop antenna  184  having one or more concentric revolutions can be electrically coupled to the energy harvesting system  180  and located within the wristband  172 . The loop antenna  184  can comprise multiple revolutions, or windings, of wire adapted to receive RF or magnetic energy and, in an embodiment, can be joined with a clasp  186 , or other joining means to mechanically and electrically connect the multiple windings within the loop antenna  184  to form a continuous loop antenna that can be incorporated into the removable wristband  172 . 
       FIG. 5  is a block diagram illustrating a generalized embodiment of the energy harvesting system  180  of  FIG. 4 , and related circuitry. The energy harvesting system  180  comprises an energy harvesting element  502  adapted to provide a direct current (DC) or alternating current (AC) voltage on connection  504 . The power provided by the energy harvesting element  502  can either be provided as a DC voltage, or can be converted to a DC voltage. If the energy harvesting element  502  provides an AC voltage, an optional rectifier, or rectifier and boost element  510  is provided to rectify and convert the AC voltage to a DC voltage before it can ultimately be used to power the UHF IC  124 . The rectify and boost element  510  can optionally use diode voltage multiplier techniques to convert the AC voltage to a DC voltage, which can also be used to multiply the AC voltage level by fixed or adjustable increments. A filter/energy storage element  512  receives the signal on connection  508 , stores and conditions the energy provided by the energy harvesting element  502  and provides a DC output on connection  514 . A non-limiting example of the filter/energy storage element  512  is a capacitor. The signal on connection  514  is referred to as an input voltage, Vin, because it is provided as input to the UHF IC  124 . 
     The voltage on connection  514  is provided to a voltage regulator  520 . The voltage regulator  520  stabilizes the input voltage and provides a regulated voltage, Vout, on connection  522 . The output voltage on connection  522  is provided to a digitally controlled input/output (I/O) pin  530  on the UHF IC  124 . Although not required for operation of the energy harvesting system  180 , a digital I/O function, illustrated herein using a digital port control element  535 , which switches power input between the energy harvesting system  180  and another supply (not shown) in the UHF IC  124 , provides a simple and controllable way of switching to harvested energy for UHF IC  124  and ultimately the UHF antenna  114 . In an embodiment, the UHF IC  124  is adapted to operate in a frequency range of approximately 800 MHz to approximately 5.8 GHz. The energy harvesting element  502  can be implemented to passively obtain energy from a variety of sources including, but not limited to, radio frequency (RF), such as AM and FM radio, magnetic coupling of very low frequency (10&#39;s of kilohertz (KHz) energy, infrared (IR), visible, solar, ultraviolet, thermal, kinetic, or other sources. 
       FIG. 6A  is a schematic diagram illustrating a first embodiment of the energy harvesting system  180  of  FIG. 5 . In the embodiment shown in  FIG. 6A , the energy harvesting system  180  is adapted to passively harvest energy from radio frequency (RF) energy. One of the challenges when implementing RF energy harvesting technology using circuitry that is in contact with, or in close proximity to human tissue is that human tissue is permeable to RF energy at or below certain frequencies. As known, relatively low-frequency AM and FM radio transmissions easily permeate human tissue. When implemented within a wearable element or object, such as the wristband  172  illustrated in  FIGS. 3A and 3B , it would be desirable to have the ability to harvest energy using circuitry located within the wristband  172 , at frequencies at which RF energy easily permeates human tissue. Amplitude modulated radio transmissions at a frequency of approximately 1 MHz, and frequency modulated radio transmissions at a frequency of approximately 100 MHz easily permeate human tissue, so radio transmissions at these approximate frequencies are useful for passive energy harvesting when the energy harvesting source is located on or in a wearable object. 
     The energy harvesting system  180  comprises a loop antenna  184 , which can be implemented in a wearable object as described above in  FIG. 4 . The loop antenna  184  can optionally be resonated by use of a capacitor  620  to increase the available peak AC voltage that can be coupled via connection  606  to a rectifier  608 , which is illustrated in this embodiment using a diode, but which can be a synchronous rectifier or any other AC to DC converter. In response to the received RF energy, the energy harvesting system  180  produces the DC output, Vin, at connection  514 . 
       FIG. 6B  is a schematic diagram illustrating an alternative embodiment of the energy harvesting system  180  of  FIG. 5 . Optionally, the available DC voltage Vin on connection  514  can be increased, or boosted, using a supplemental voltage element, also referred to as a boosting element  630 . The boosting element  630  is illustrated in  FIG. 6B  as photocell  630 , but can be any other source of DC voltage. The boosting element  630  can provide a small supplemental voltage to forward bias the rectifier  608 , thus eliminating a zero voltage output condition that may occur when the output of the loop antenna  184  is insufficient to overcome the approximate 0.3V to 0.6V forward voltage drop of a typical rectifier diode. Because the AC output of the loop antenna  184  is a very low voltage signal, the optional boost element  630  allows Vin to be maintained at an acceptable level even with very low radio frequency input. Other ways to implement the optional boost element  630  include, but are not limited to, a piezoelectric voltage source, a diode with a transparent enclosure that uses ambient light to generate a small voltage, vibration of an electret element, and any another element that can generate a small voltage to forward bias the rectifier  608 . A capacitor  612  is used to temporarily condition voltage fluctuations in Vin, and in some cases to bridge gaps in harvested power availability. 
       FIG. 6C  is a schematic diagram illustrating an alternative embodiment of the energy harvesting system  180  of  FIG. 5 . In cases where a higher DC voltage, Vin, is desired, the AC output  606  of the loop antenna  184  can simultaneously be rectified and multiplied. As is known in the art, a voltage multiplier (in this example a voltage doubler) comprising the diode  608  and the capacitors  620  and  612  of  FIG. 6B  that form a first AC rectifier and filter, can be combined with an additional diode  609  and an additional capacitor  613  to form a rectifier for the other half cycle of the AC output of the loop antenna  184 . In this manner a DC voltage of two times Vin can be derived as opposed to Vin, as shown in  FIG. 6B . Optionally, voltage boost elements such as photocell  630  of  FIG. 6B  can be added in series with each additional diode to offset each additional diodes forward conduction voltage, thus allowing the doubling (or higher level multiplying) circuit to work with AC signals of very low amplitude. The voltage Vin at connection  514  can be provided to the voltage regulator  520 , as described above, and ultimately to the UHF IC  124 . 
       FIG. 7  is a block diagram illustrating another alternate alternative embodiment of the energy harvesting system  180  of  FIG. 5 . The energy harvesting system  700  can be implemented using an infrared energy source  702 . The infrared energy source  702  can be a light emitting diode (LED), or an array of such diodes, configured to emit light at infrared wavelengths, or can be an incandescent light source that is filtered to provide an infrared output, or any other infrared source. The output of the infrared energy source  702  is provided over connection  704  to an infrared photo detector  706 . The connection  704  can be air, or another medium through which infrared energy is conducted from the infrared energy source  702  to the infrared photo detector  706 . The infrared photo detector  706  provides a DC voltage output on connection  708  that is supplied to an energy storage element  712 . The energy storage element  712  may be a capacitor similar to the capacitors  512  and  612  described above. 
       FIG. 8  is a schematic diagram illustrating another alternative embodiment of the energy harvesting system  180  of  FIG. 5 . The energy harvesting system  800  is implemented using ultraviolet light. An ultraviolet light source  802  provides ultraviolet energy over medium  804  to an ultraviolet-to-visible, or ultraviolet-to-infrared light converter  806 . The medium  804  can be air, or any other medium that can conduct ultraviolet light. The light converter  806  (which in one embodiment can be phosphorescent material that glows in the infrared or visible light spectrum when illuminated by ultraviolet light) converts the received ultraviolet energy to visible or infrared light on medium  808 . This visible, or infrared light may be more efficiently converted to electrical energy by low cost silicon semiconductors than can the initial ultraviolet light. The medium  808  can be air, or any other medium that can conduct visible or infrared light to a visible or infrared light photo detector  810 . The visible or infrared light photo detector  810  receives the visible or infrared light from the ultraviolet-to-visible or ultraviolet-to-infrared light converter  806  and converts the light to a DC voltage signal on connection  811 . A DC voltage on connection  811  is provided to an energy storage element  812 , which is similar in function operation and structure to the capacitors  512  and  612  described above. 
       FIG. 9  is a schematic diagram illustrating an example of using low frequency RF energy to create magnetic inductive coupling to supply the energy harvesting element  502 . The loop antenna is schematically illustrated using reference numeral  184  as being located in the vicinity of a secondary loop antenna  904 . Other elements of the wristband assembly  170  are not shown in  FIG. 9  for ease of illustration, but are understood to be included with the loop antenna  184  as described in  FIG. 4 . 
     The secondary loop antenna  904  can be a long wound coil of conductive material, such as metallic windings, located within an attraction or area  902 . As an example, the attraction or area  902  can be an attraction at an amusement park through which, within which, or in the vicinity of which a wearer of the wristband assembly  170  may pass or enter. A low frequency oscillator  906  can be used to provide a very low frequency signal, such as on the order of tens of KHz, to excite the secondary loop antenna  904  to establish a magnetic field  908  in the vicinity of the attraction or area  902 . The magnetic field  908  couples to the loop antenna  184  via inductive coupling so as to provide low frequency magnetic energy to the loop antenna  184  and the energy harvesting system  180 . When implemented as shown in  FIGS. 6A through 6C , energy can be provided in the form of magnetic inductive coupling, thus generating a DC voltage and current as described above. 
       FIG. 10  is a view illustrating an application of the RFID assembly of  FIG. 1 . As shown in  FIG. 10 , a magnetic field created by an array of permanent magnets  1050  can also be used to generate an AC voltage by using magnetic field-cutting techniques to couple energy to the loop antenna  184  in the RFID assembly  100 . In the example shown in  FIG. 10 , an array of permanent magnets  1050  mounted along the top rail  1052  of a fence or banister  1054 , or any place where it might be expected that a person wearing an RFID assembly  100  might have their hand (and therefore wristband) in close proximity to the magnets, but moving past them, can induce a voltage in loop antenna  184 , and be used as harvested power, as described above. 
       FIG. 11  is a flowchart describing an exemplary method for powering an RFID element using an energy harvesting element. 
     In block  1102  a voltage is generated by the energy harvesting system  180  ( FIG. 5 ). In block  1104  the harvested energy is converted to a DC voltage as described above. 
     In block  1106 , the DC voltage is regulated to a predetermined level that is usable by the UHF IC  124  ( FIG. 5 ). In block  1108 , the regulated DC voltage is applied to the UHF IC  124 . 
     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of the invention.