Methods and apparatus for reducing power consumption of an active transponder

[0001] This application claims the benefit of U.S. Provisional Application No. 60/673,715, entitled “Method And Device For Reducing Power Consumption Of Active RFID Tags,” which was filed on Apr. 21, 2005, the disclosure of which is incorporated herein by reference.

[0002] The present invention relates to transponders, such as RFID tags, and in particular to methods and apparatus for reducing the power consumed by active transponders.

[0003] The use of radio frequency identification (RFID) systems is expanding rapidly in a wide range of application areas. RFID systems consist of a number of radio frequency tags or transponders (RFID tags) and one or more radio frequency readers or interrogators (RFID readers). The RFID tags include one or more integrated circuit (IC) chips, such as a complementary metal oxide semiconductor (CMOS) chip, and an antenna connected thereto for allowing the RFID tag to communicate with an RFID reader over an air interface by way of RF signals. In a typical RFID system, one or more RFID readers query the RFID tags for information stored on them, which can be, for example, identification numbers, user written data, or sensed data. RFID systems have thus been applied in many application areas to track, monitor, and manage items as they move between physical locations.

[0004] RFID tags can generally be categorized as either passive tags or active tags. Passive RFID tags do not have an internal power supply. Instead, the relatively small electrical current induced in the antenna of a passive RFID tag by the incoming RF signal from the RFID reader provides enough power for the IC chip or chips in the tag to power up and transmit a response. Most passive RFID tags generate signals by backscattering the carrier signal sent from the RFID reader. Thus, the antenna of a passive RFID tag has to be designed to both collect power from the incoming RF signal and transmit (or reflect, e.g., backscatter) the outbound backscatter signal. Due to power limitations, the ability to provide devices such as sensors or microprocessors on passive RFID tags is limited. Passive RFID tags do, however, have the advantage of a near unlimited lifetime as they obtain their power from the RF signal sent from the RFID reader.

[0005] Active RFID tags, on the other hand, have their own internal power source, such as, without limitation, a battery, a fuel cell or what is commonly known as a super capacitor. The internal power source is used to power the IC chip or chips and discrete circuit elements, which typically include an RF receiver, an RF transmitter, and some type of controller, such as microcontroller or other processor, and any other electronics provided on the active RFID tag. As a result, active RFID tags can include relatively high power devices such as sensors, microprocessors, receivers and transmitters. Also, because of the on-board power, active RFID tags typically have longer ranges and larger memories than passive RFID tags. The internal power source, however, also means that active RFID tags typically have a lifetime that is limited by the lifetime of the power source. Thus, periodic maintenance is required.

[0006] As noted above, multiple active RFID tags may be used to track, monitor, and manage multiple items/assets as they move between physical locations. In such an application, each active RFID tag is affixed to an item/asset that is located in a particular location or environment, such as in a building. In current RFID systems, the active RFID tags, when deployed in such a manner, are done so in a state where (i) an RF receiver of the tag is in an active state for receiving RF signals, and (ii) the controller is in a low power inactive (sleep) state to preserve power. When one or more of the active RFID tags are to be queried, the RFID reader sends out a wake-up signal that is received by the RF receiver of each tag. Upon receipt of the signal, the RF receiver in each tag will then send a signal to the controller of the tag that causes it to move from the inactive state to an active (wake-up) state. For example, in RFID systems implemented according to the ISO 18000 Part 7 standard, when one or more tags are to be queried, the reader will send out a 30 KHz tone lasting for a period of approximately 2.5 seconds. Upon receipt of the tone, the RF receiver in each tag will wake-up the controller in the tag. The RFID reader then sends out signals intended for particular ones of the tags. Those particular tags for which the signals are intended will then perform the requested action, and the remaining tags (i.e., those tags not currently of interest to the reader) will move back to a sleep state. Alternatively, in some implementations both the RF receiver and the controller of each tag are in a constant active state when deployed, and therefore a wake-up signal is not required.

[0007] The multiple active RFID tag arrangements just described present at least two power management problems. First, each active RFID tag that is deployed is required to have at least one component in an active, relatively high power consuming state at all times. In the first described arrangement, an RF receiver of each tag is always “on” so that it can listen for the wake-up signal. In the second described arrangement, both the RF receiver and controller of each tag are always “on.” Second, in the first described arrangement, when the RFID reader needs to query one or more particular tags, all of the tags that are deployed are woken up (for example, according to the ISO 18000, Part 7 standard), i.e., their controllers are caused to move to an active, relatively high power consuming state. Only when a particular tag determines that the query in question is not intended for it will it then move back to the sleep state. As will be appreciated, these problems result in unnecessary use of power from the power source (e.g., battery) of each tag, and therefore decreases the continuous uninterrupted operational lifetime of each tag by requiring periodic maintenance.

[0008] The invention relates to a transponder apparatus having an identifier associated therewith that includes a receiver for receiving an RF signal transmitted by an interrogator, a power source, such as a battery or supercapacitor, and a processing unit, such as a microprocessor, a microcontroller or a custom electronic device, that is operatively coupled to the power source. The processing unit is capable of being in an inactive, sleep state (low current draw) and an active state. The transponder apparatus also includes a buffer device that is in electronic communication with the receiver and with the processing unit. The buffer device is structured to: (i) receive an information signal based on the RF signal from the receiver, (ii) determine whether the information signal includes the identifier, and (iii) cause the processing unit to move from the inactive state to the active state and transmit at least a portion of the information signal to the processing unit only if it is determined that the information signal includes the identifier.

[0009] In one embodiment, the buffer device is operatively coupled to and powered by the power source. The buffer device in this embodiment may be in an active state at all times, or may be caused to move to an active state when the receiver receives the RF signal. Similarly, the receiver may be operatively coupled to and powered by the power source, and may be in an active state at all times. Alternatively, the buffer device and/or the receiver may be operatively coupled to and powered by an energy harvesting circuit that receives energy transmitted in space from a far-field source, such as the interrogator or a radio station, converts the received energy into a DC power signal, and provides the DC power signal to the buffer device and/or receiver.

[0010] In one embodiment, the transponder apparatus is one of a plurality of system transponder apparatuses in a transponder system, and the identifier is unique to the transponder apparatus in the system. In an alternative embodiment, the transponder apparatus is one of a plurality of system transponder apparatuses in a transponder system, and the identifier is common to a plurality of the system transponder apparatuses.

[0011] The invention also provides a method of moving a processing unit included in a transponder apparatus from an inactive, sleep state to an active state, wherein the transponder apparatus has an identifier associated therewith. The method includes steps of receiving an RF signal from an interrogator, converting the RF signal into an information signal, determining whether the information signal includes the identifier, and causing the processing unit to move from the inactive state to the active state and transmitting at least a portion information signal to the processing unit only if it is determined that the information signal includes the identifier.

[0012] It is an object of the invention to provide a transponder apparatus and associated method that reduces the power consumed by the transponder apparatus from, for example, a battery or other power source associated therewith.

[0013] It is a further object of the invention to provide a transponder apparatus and associated method that employs a processing unit that may be moved from an inactive (low current drawing) state to an active state.

[0014] It is still a further object of the invention to provide a transponder apparatus and associated method that employs a buffer device to determine whether to wake-up the associated processing unit based upon the receipt of an identifier associated with the transponder apparatus.

[0015] It is still a further object of the invention to provide a transponder apparatus and associated method that employs a buffer device to determine whether to wake-up the associated processing unit that is powered by the battery of the transponder apparatus.

[0016] It is still a further object of the invention to provide a transponder apparatus and associated method that employs a buffer device to determine whether to wake-up the associated processing unit that is powered by the battery of the transponder apparatus and that may be moved from an inactive state to an active state when it is necessary to determine whether the processing unit should be woken up.

[0017] It is still a further object of the invention to provide a transponder apparatus and associated method that employs a buffer device to determine whether to wake-up the associated processing unit that is powered by harvesting energy transmitted in space.

[0018] It is still a further object of the invention to provide a transponder apparatus and associated method that employs a receiver that is powered by harvesting energy transmitted in a space.

[0025] FIG. 1 is a block diagram of an RF transponder 5 according to one embodiment of the present invention. The RF transponder 5 includes a receiver 10 operatively coupled to an antenna 15. The receiver 10 may be any suitable RF receiver that is capable of demodulating an incoming RF signal such as, for example, the rfRXD0420 UHF ASK/FSK/FM receiver sold by Microchip Technology Inc. of Chandler Ariz. The receiver 10 is in electronic communication with a buffer device 20, the functionality of which is described in greater detail below. The buffer device 20 is in electronic communication with a processing unit 25, which may be, without limitation, a microprocessor, a microcontroller, or some other type of processor device. The processing unit 25 may further be another type of electronic device, such as a CMOS device or any other electronic circuit element provided on, for example, a semiconductor substrate or printed circuit board (PCB), which performs a particular function or functions. The processing unit 25 is capable of being placed into an inactive, sleep state where the current drawn by it is at a minimum. In addition, the processing unit 25 may be woken up, i.e., moved from the inactive, sleep state to an active state, upon receipt of an external input signal. An RF transmitter 30, such as the model rfPIC12F675F sold by Microchip Technology Inc. of Chandler Ariz., is in electronic communication with the processing unit 25. The RF transmitter 30 is, in response to commands received from the processing unit 25, able to transmit RF signals through an antenna 35 operatively coupled thereto. Like the processing unit 25, the RF transmitter 30 is capable of being placed into an inactive, sleep state where the current drawn by it is at a minimum, and can be woken up by receipt of an external input signal provided by the processing unit 25. The RF transponder 5 also includes a battery 40 which provides the power required for the operation of the receiver 10, the buffer device 20, the processing unit 25 and the transmitter 30. The battery 40 may alternatively be replaced by another power source, such as, without limitation, a fuel cell or a so called supercapacitor.

[0026] The RF transponder 5 is particularly adapted to be utilized in a transponder system 50, as shown in FIG. 2, wherein a plurality of similarly structured RF transponders 5 are deployed in a particular location, such as within a building. The transponder system 50 further includes an interrogator unit 55 which is in electronic communication with a host (central) computer system 60. Under the control of the host computer system 60, the interrogator unit 55 generates the RF signals that are required to selectively awaken the RF transponders 5 in the manner below. In addition, for reasons to be described below, each of the RF transponder 5 is assigned at least one identifier, such as a unique identification number, that allows the RF transponder 5 to be selectively identified. That identifier is stored by the buffer device 20 in each transponder device 5. Each RF transponder 5 may, in one embodiment, have a unique identifier that uniquely identifies the RF transponder 5 in the system 50. Alternatively or additionally, one or more RF transponders 5 may share a common identifier so that they may be grouped and identified together.

[0027] In operation, each of the RF transponders 5, in one embodiment, is deployed in a state wherein the receiver 10 and the buffer device 20 thereof are active, meaning they are drawing current from the associated battery 40 at a level that allows them to be fully functional. In addition, the processing unit 25 and the transmitter 30 of each RF transponder 5 are, in this embodiment, in the inactive, sleep state. As such, the extent to which they draw power from the associated battery 40 will be at a minimum. When it is desired to “wake-up” a particular RF transponder 5, such as when it is desired to obtain information from the RF transponder 5, an RF information signal of an appropriate frequency that includes, without limitation, the particular identifier assigned to the RF transponder 5 of interest and any appropriate commands is generated by the interrogator unit 55 and transmitted to each RF transponder 5 in the system 50. The information just described may be included in the RF information signal by any known technique, such as a known modulation technique like frequency shift keying (FSK) or amplitude shift keying (ASK). The RF information signal is received by the receiver 10 of each RF transponder 5, where it is demodulated to obtain a corresponding information (digital) signal. The information signal, which includes the identifier and other information such as one or more commands, is then provided to the buffer device 20 of each RF transponder 5.

[0028] The buffer device 20 of each RF transponder 5 receives the information signal and compares the identifier identifiers included within the information signal to the identifier that is stored in the buffer device. In each case, if the buffer device 20 determines that the received identifier does not match a stored identifier, then no further action is taken. In the case or cases where the buffer device 20 of a transponder 5 determines that the two identifiers do match, the buffer device 20 generates a DC wake-up signal and provides the DC wake-up signal to the sleep input (pin) of the associated processing unit 25. The DC wake-up signal causes the processing unit 25 to move from the inactive, sleep state to its active state. Once the processing unit 25 has moved to the active state, the buffer device 20 will then provide the other information, such as one or more commands, that was included in the information signal received from the associated receiver 10 to the processing unit 25. In the active state, the processing unit 25 is able to perform any action that is required by the received commands, such as waking up the associated RF transmitter 30 and causing it to transmit an information signal to the interrogator unit 55. When finished (or after some predetermined period of time), the processing unit 25 can return to an inactive, sleep state until subsequently woken up as described herein. The buffer device 20 may be any type of device that includes electronic circuitry for performing the functionality just described. For example, the buffer device 20 may be a PICFXX8 flash microcontroller sold by Microchip Technology Inc. of Chandler Ariz. or a custom designed electronic circuit, such as a custom CMOS circuit.

[0029] Thus, as will be appreciated, the system 5 provides improved performance by maximizing the life of the batteries 40 included in the RF transponders 5 by awakening the processing units 25 of only the one or more RF transponders 5 for which an information signal generated by the interrogator unit 55 is intended.

[0030] In an alternative embodiment of the RF transponder 5, the buffer device 20 is capable of being placed into an inactive, sleep state where the current drawn by it is at a minimum, and may be woken up, i.e., moved from the inactive, sleep state to an active state, upon receipt of an external input signal. The receiver 10 in this alternative embodiment of the RF transponder 5 is adapted to generate a wake-up signal for the buffer device 20 when it receives a information signal as described above. Thus, in operation, the buffer device 20 in this embodiment will remain in a low power sleep state until the information signal is received by the receiver 10, at which time it will be awakened to perform the comparison described above. As a result, less power will be continuously consumed by the RF transponder 5 in this embodiment.

[0031] FIG. 3 is a block diagram of an RF transponder 5′ according to an alternative embodiment of the present invention. The RF transponder 5′ is similar in operation to the RF transponder 5 shown in FIG. 1 and may be substituted for the RF transponder 5 in the system 50 shown in FIG. 2. Like the RF transponder 5, the RF transponder 5′ includes a receiver 10, an antenna 15, a buffer device 20, a processing unit 25, a transmitter 30, an antenna 35 and a battery 40 which function as described in connection with FIG. 1. The RF transponder 5′ differs from the RF transponder 5, however, in that instead of having a buffer device 20 that is operatively coupled to and continuously powered by the battery 40, the buffer device 20 receives operational power from an energy harvesting circuit 65 that harvests energy that is transmitted in space. As employed herein, the term “in space” means that energy or signals are being transmitted through the air or similar medium regardless of whether the transmission is within or partially within an enclosure, as contrasted with transmission of electrical energy by a hard wired or printed circuit boards. A number of methods and apparatus for harvesting energy from space and using the harvested energy to power an electronic device are described in U.S. Pat. No. 6,289,237, entitled “Apparatus for Energizing a Remote Station and Related Method,” U.S. Pat. No. 6,615,074, entitled “Apparatus for Energizing a Remote Station and Related Method,” U.S. Pat. No. 6,856,291, entitled “Energy Harvesting Circuits and Associated Methods,” and United States Patent Application Publication No. 2005/0030181, entitled “Antenna on a Wireless Untethered Device such as a Chip or Printed Circuit Board for Harvesting Energy from Space,” each assigned to the assignee hereof, the disclosures of which are incorporated herein by reference.

[0032] The preferred energy harvesting circuit 65 is shown in FIG. 4 and includes an antenna 70, which may be, without limitation, a square spiral antenna. The antenna 70 is electrically connected to a matching network 75, which in turn is electrically connected to a voltage boosting and rectifying circuit in the form of a charge pump 80. Charge pumps are well known in the art. Basically, one stage of a charge pump essentially doubles the effective amplitude of an AC input voltage and stores the resulting increased DC voltage on an output capacitor. The voltage could also be stored using a rechargeable battery. Successive stages of a charge pump, if present, will essentially increase the voltage from the previous stage resulting in an increased output voltage. In operation, the antenna 70 receives energy, such as RF energy, that is transmitted in space by a far-field source, such as an RF source. The RF source may be, for example, the interrogator unit 55, in which case the RF energy may be the information signal transmitted by the interrogator unit 55, or a local radio station, in which case the RF energy comprises ambient RF energy in the vicinity of the RF transponder 5′. The RF energy received by the antenna 70 is provided, in the form of an AC signal, to the charge pump 80 through the matching network 75. The charge pump 80 amplifies and rectifies the received AC signal to produce a DC signal. The matching network 75 preferably matches the impedance of the charge pump 80 to the impedance of the antenna 70 in a manner that optimizes the amount of energy that is harvested (i.e., maximum DC output). In one particular embodiment, the matching network 75 is an LC tank circuit formed by the inherent distributed inductance and inherent distributed capacitance of the conducing elements of the antenna 70. Such an LC tank circuit has a non-zero resistance R which results in the retransmission of some of the incident RF energy. This retransmission of energy may cause the effective area of the antenna 70 to be greater than the physical area of the antenna 70. The DC signal generated by the charge pump 80 is provided to the buffer device 20 provided in the RF transponder 5′ for powering the buffer device 20. Thus, the buffer device 20 in the RF transponder 5′ is able to be powered remotely without the need, as in the case of the RF transponder 5, to continuously consume power from the battery 40. As will be appreciated, this is advantageous in that it further extends the life of the RF transponder 5′ by further minimizing consumption of power from the battery 40.

[0033] FIG. 5 is a block diagram of an RF transponder 5″ according to a further alternative embodiment of the present invention. The RF transponder 5″ is similar in operation to the RF transponder 5 shown in FIG. 1 and the RF transponder 5′ shown in FIG. 3 and may be substituted for the RF transponder 5 or the RF transponder 5′ in the system 50 shown in FIG. 2. Like the RF transponder 5, the RF transponder 5″ includes an antenna 15, a processing unit 25, a transmitter 30, an antenna 35 and a battery 40 which function as described in connection with FIG. 1. The RF transponder 5″ differs from the RF transponder 5 in that instead of having a receiver 10 and a buffer device 20 that are operatively coupled to and continuously powered by the battery 40, it includes a combination receiver/buffer device 20′ that combines the functionality of the receiver 10 and the buffer device 20 described elsewhere herein in a single device. The receiver/buffer device 20′ receives operational power from an energy harvesting circuit 65 that harvests energy that is transmitted in space. As will be appreciated, this configuration is advantageous in that it still further extends the life of the RF transponder 5″ by even further minimizing the consumption of power from the battery 40.

[0034] While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.