RFID systems typically include RFID memory devices and RFID readers (the latter are also known as RFID reader/writers or RFID interrogators). RFID systems can be used in many ways for locating and identifying objects to which the memory devices are attached. RFID systems are particularly useful for tracking large numbers of objects being processed, inventoried, or handled. In such cases, an RFID memory device is usually attached to an individual item, or to its package.
In principle, an RFID interrogator transmits a Radio Frequency (RF) wave to one or more RFID memory devices. A memory device that senses the interrogating RF wave responds by transmitting back another RF wave. The memory device generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways. The reflected-back RF wave may further encode data stored internally in the memory device, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID memory device typically includes an antenna system, a power management section, a radio section, and frequently a logical section, a memory, or both. In earlier RFID memory devices, the power management section included an energy storage device, such as a battery. RFID memory devices with an energy storage device are known as active tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID memory device can be powered solely by the RF signal it receives. Such RFID memory devices do not include an energy storage device, and are called passive tags.
International Patent Applications WO 2004/084131 and WO 2004/083798 both describe passive RFID memory devices that utilize arrays of vibrating members to encode data. These members may take the form of cantilever and bridge structures, and may have different resonant frequencies from one another so that the presence or absence of a vibrating member of a particular frequency may be equated to a logical “1” or “0”, and may represent binary code, a status flag or the like. A determination of the presence or absence of a member may be made by applying an excitation signal to the array and by analysing the response to determine if it is indicative of a particular member's resonant frequency. The arrays may be fabricated using MEMS technology (microelectromechanical systems technology), which is also known as MST (Micro System Technology) and micromachining. MEMS technology includes fabrication technologies for integrated circuits, and technologies specifically developed for micromachining. It generally relates to the fabrication of components with dimensions in the range of micrometers to millimeters.
Each such RFID memory device includes electrically or electronically active elements to electrodynamically couple the motion of the vibrating member and an external interrogation circuit. One electrodynamic interaction which may be employed in a RFID tag is Lorentz force/Faraday induction. A conductor runs along or through the vibrating members and extends beyond the vibrating members on the RFID memory device to electrical terminals. A coil antenna interconnects the terminals. The vibrating member is located in a region of non-zero magnetic field. An alternating electrical current is induced in the coil antenna (and hence the conductor) by a corresponding coil in the interrogation circuit. The lines of this magnetic field are so oriented that a Lorentz force associated with the alternating electrical current flowing through the conductor tends to displace the vibrating members from an equilibrium position.
The energy associated with movement and displacement of the vibrating member, as well as other energy storage mechanisms such as electrostatic energy, will be manifest between the electrical terminals in the RFID memory device as relationships between terminal voltages and currents. These relationships can be expressed as an equivalent electrical circuit. Such equivalent circuits will include inductive or capacitive elements. In addition, mechanical resonances of a vibrating member are typically damped by a number of mechanisms including intrinsic causes such as visco-elastic and thermo-elastic loss. Such losses also manifest in the equivalent electrical circuit as resistive elements.
Each vibrating member is fabricated to have a distinct resonant frequency. When the frequency of the applied alternating electrical current corresponds to the resonant frequency of a particular vibrating member, that member is caused to mechanically resonate. The oscillating motion of the beam in a magnetic field causes by Faraday Induction an electromotive force in the circuit consisting of the conductive path through or comprising the resonators and the components attached to these, including an electromagnetic coupling. An oscillating electromagnetic signal is therefore transmitted back the external interrogation circuit. This additional contribution of electromotive force can be interpreted as a increment in impedance, as the ratio of the electromotive force to the current passing through the beam.
An RFID memory device may have the property that the magnitude of the impedance change caused by the mechanical resonance of a particular vibrating member is much smaller that the “background” impedance directly associated with the electrical elements of the equivalent electrical circuit within the RFID memory device. In the case of Lorentz force/Faraday induction based coupling, for typical configurations, the resonant impedance change is several orders of magnitude smaller than the background impedance, namely the ohmic resistance of the beam conductors and electrical interconnections. A technique for resolving the impedance change caused by the mechanical resonance of a particular vibrating member is described in copending U.S. provisional patent application entitled “Ringup/Ringdown Interrogation of RFID tags” filed on 3 Oct. 2008 to the present Applicant.
It would be desirable to provide an alternate memory device and interrogation method that has advantages over existing RFID memory devices and interrogation methods.
The above discussion of background art is included to explain the context of the present invention. It is not to be taken as an admission that any of the documents or other material referred to was published, known or part of the common general knowledge at the priority date of any one of the claims of this specification.