WO2004001445A1

WO2004001445A1 - An rfid transponder and a method of communicating identification code in electromagnetic noisy environments

Info

Publication number: WO2004001445A1
Application number: PCT/DK2002/000105
Authority: WO
Inventors: Per Jørgen BRAAGAARD
Original assignee: Tss Traffic Supervision Systems A/S Under Konkurs
Priority date: 2001-02-15
Filing date: 2002-02-15
Publication date: 2003-12-31
Also published as: AU2002231615A1

Abstract

The invention relates to an RFID transponder for communication of an identification code with an RFID-reader, where the transponder includes a parallel resonance circuit including a capacitor (C) and a first antenna (L1) for receiving of a carrier wave sent by the RFID-reader, wherein the transponder further includes a contact (SW1) for respectively breaking or closing of a resonance voltage in the parallel resonance circuit, wherein a peak voltage over the capacitor (C) is held by breaking of the contact (SW1), wherein the contact is controlled based on the first signal and is connected in series with the capacitor (C), a clock regenerator arranged for providing a clock signal based on the sent carried wave, phase-locked loop (PLL) for locking the phase of the clock signal to the phase of the sent carrier wave, and signal means arranged to provide the first signal based on the said clock signal. Hereby, a transponder is provided which makes a reliable reading possible, even in electromagnetically noisy environments and over longer distances. The invention also includes a method for the same purpose.

Description

An RFID transponder and a method of communicating identification code in electromagnetic noisy environments.

This invention is concerned with an RFID transponder for the communication of an identification code to an RFID reader, which transponder comprises

• a parallel resonant circuit comprising a capacitor (C) and a first antenna (Li) for receiving a carrier wave transmitted by the RFID reader. The invention further comprises a method for the communication of an identification code to an RFID reader, which method comprises • the reception in a parallel resonant circuit of a carrier wave transmitted by the RFID reader. In RFID (Radio Frequency Identification) it is typical to use a transponder/tag, which interacts with a reader, equipped with an antenna or similar device, in order to transmit information including e.g. a unique identification code and other relevant information. The antenna is typically used both to transmit and receive information between the transponder and the reader/antenna, with the result that the antenna, when has to receive a signal from the transponder, is particularly susceptible and sensitive to electromagnetic interference. Such interference can originate both from external sources and also from the antenna itself, since in the typical case said antenna continuously emits a carrier wave which is very large in relation to the very small signal from the transponder.

This can be a major problem, since the signal/noise ratio in conventional RFID communication is of vital importance in order to ensure that the transmission and/or reception of information is of sufficiently high quality to prevent the loss of information. Another problem with existing tags and antennas/RFID readers occurs when the antennas/RFID readers are used for both reception and transmission in communication with the tag/transponder, because the tag is generally powered by the carrier wave from the transmitting antenna. This requires the signal voltage from the transmitting antenna to be of significant magnitude (typically several hundred volts) in order to secure a certain range and to ensure that the tag receives enough energy to be able then to transmit and, if necessary, store and process information. The signal then reflected from the tag, i.e. the tag signal, destined to be received by the antenna, is typically very weak in relation to the transmitted carrier wave. In other words, the ratio of the signal from the tag to the transmitted carrier wave is very small. It is moreover extremely difficult with previously known technologies to obtain a suitably large and reliable scanning distance, particularly in electromagneticaliy noisy environments such as for example electric trains, industrial environments etc.

It is an object of the invention to make available a transponder which permits improved interaction with an antenna/RFID reader, whereby the effect of noise from external sources in the form of unwanted electromagnetic interference is eliminated or at least reduced.

Another object of the invention is to make available a transponder with a substantially improved tag signal such that the ratio of the tag signal to the noise received by the scanning unit is substantially increased. Another object of the invention is to make available a transponder which transmits a square-wave tag signal.

A further object of the invention is to make available a transponder which provides improved sensitivity or scanning distance with regard to interaction with an RFID reader/antenna. Yet a further object is to make available a transponder which permits reliable reading even in electromagneticaliy noisy environments.

This and other advantages are achieved by an RFID transponder of the type described initially, which transponder further comprises:

• a switch for switching alternately off and on a resonant voltage in the parallel resonant circuit, a peak voltage being maintained across the capacitor when the switch opens, said switch being controlled by a first signal and connected in series with the capacitor;

• a clock regenerator arranged to generate a clock signal based on the transmitted carrier wave; • phase locking means for locking the phase of the generated clock signal to the phase of the transmitted carrier wave; and

• signalling means arranged to generate the first signal based on said clock signal. In a preferred embodiment said switch opens when the resonant voltage is substantially zero. In a preferred embodiment said switch closes when the resonant voltage would be substantially at its maximum. The phrase "would be at its maximum" refers to the fact that the resonant voltage does not change when the switch is off, but the time at which the resonant voltage would be at a maximum is known from the phase locking means. In one embodiment the transponder additionally comprises frequency division means for dividing the frequency of said clock signal, resulting in a second clock signal.

In a preferred embodiment the transponder additionally comprises a memory device containing a unique and unambiguous ID code, a checksum part and a synchronization part. In a further preferred embodiment said first signal is generated in Manchester encoding on the basis of said clock signal or said second clock signal and at least part of the contents of said memory device.

In one embodiment said first signal is generated as the output of an XOR device from said second signal and the contents of said memory device. In a preferred embodiment the transponder further comprises:

• a second coil or a tap point on the first antenna, arranged so that a voltage is induced by the carrier wave transmitted from the RFID reader, and

• a power supply circuit for supplying power to said clock regenerator, signalling means, frequency division means, memory device and phase locking means, said second coil or tap point on the first antenna being electrically connected to the power supply circuit.

In one embodiment the transponder comprises a ferrite rod on which said first antenna and said second coil are wound. It is a further object of the invention to make available a method having the same objects mentioned in relation to the above RFID transponder.

This and other advantages are achieved by a method of the type initially described, said method comprising:

• the switching off and on, based on a first signal, of a resonant voltage in the parallel resonant circuit, a peak voltage being maintained on switching off; • the generation of a clock signal from the transmitted carrier wave;

• the phase locking of the generated clock signal to the phase of the transmitted carrier wave; and

• the generation of the first signal from said clock signal.

In one embodiment said switching offtakes place substantially at the zero of a resonant voltage in said resonant circuit.

In one embodiment said switching on takes place when the resonant voltage would be substantially at its maximum. In one embodiment, the method further comprises frequency division of said clock signal, resulting in a second clock signal.

In one embodiment the method further comprises the read-out of at least part of the contents of a memory device containing a unique and unambiguous ID code, a checksum part and a synchronization part.

In one embodiment said first signal is generated in Manchester encoding from said clock signal or said second clock signal and at least part of the contents of said memory device.

In one embodiment said first signal is generated as the output of an XOR device from said second signal and the contents of said memory device. In one embodiment the method further comprises:

• the induction of a voltage by the carrier wave transmitted from the RFID reader in a second coil (L₂) or a tap point on the first antenna (L , and

• the generation of a power supply from the second coil (L₂) or the tap point on the first antenna . The method and the embodiments thereof possess the same advantages for the same reasons as the RFID transponder and will therefore not be rehearsed again.

Figure 1 shows a schematic block diagram of an embodiment of an RFID transponder according to the invention; Figures 2A through 2E show a series of examples of the resonant voltage in the resonant circuit in different situations (the internal power supply voltage is also shown in Figures 2A and 2E); Figure 3 shows a flowchart of the method according to the invention. Figure 1 shows a schematic block diagram of an embodiment of an RFID transponder according to the invention. The figure shows a tuned resonant circuit consisting of a first antenna/coil ^~ _\ (103) and a capacitor C (104) connected in parallel. The resonant circuit further includes a switch SW_t (106), which either breaks or does not break the current flowing in the resonant circuit, depending on whether said switch (106) is off or on, open or closed, etc. The resonant circuit (103, 104) is acted on by a carrier wave transmitted with a certain signal strength by an RFID reader, when the latter comes within a certain range, through the transmitted signal/carrier wave being transferred by induction to the resonant circuit (103, 104).

The switch (106) is preferably a controlled switch which is controlled by a first signal (labelled "DATA" in the figure), as will be more explained in greater detail hereafter. When the switch (106) is closed, a resonant voltage/current exists/flows across/in the resonant circuit, which charges the capacitor C (104). When the switch (106) is open, the capacitor C (104) maintains the peak voltage. The switch (106) opens and closes based on a first signal (DATA), and preferably opens when the resonant voltage/current is substantially zero.

When the switch (106) closes again, the peak voltage is restored substantially instantaneously (corresponding to time A in Figure 2D), which results in a very high/maximum resonant voltage. The switch (106) preferably closes at the time when the resonant voltage would have been substantially at its maximum (that is, if the switch had not been open). This time is known via the phase locking device (see below).

This has the effect of achieving practically ideal square-wave modulation (see e.g. Figure 2 A) of the received RF-AM modulated tag signal, which gives considerably superior modulation (approx. 100%) and square-wave modulation, inasmuch as transients are minimized or avoided. With previously known solutions a square- wave modulation signal is not achieved, but rather, typically, a triangle signal of smaller amplitude.

It also achieves a modulation index very close to 100%, such that the signal in the resonant circuit is as strong as possible, with the result again that the tag signal/noise ratio in the antenna of the reading unit is greatly improved, hence increasing the reading distance and enabling the transponder to operate in environments that are electromagneticaliy very noisy. Electrically connected to the resonant circuit (103, 104, 105, 106) is a phase-locking device

("phase-locked loop") (107) and a clock regenerator ("clock extractor") (107). By this means a clock signal is produced which is locked in phase with the RFID reader carrier wave.

In this manner a clock signal is generated which has the form of a square- wave signal having the same phase as the transmitted carrier wave signal. The frequency of the clock signal is divided by frequency dividing means (108) by a certain divisor, preferably 32, in order to produce a suitable baud rate/bit rate for reading out the contents of the memory device (109). The contents of the memory device (109) are read out one bit at a time at the said baud rate/clock frequency.

The memory device (109) preferably contains a unique and unambiguous ID code of e.g. 18 data bits, a checksum part (CRC) of e.g. 4 bits, and a synchronization part consisting of a sync word of e.g. 10 bits.

The contents of the memory device (109), e.g. 32 bits, are read out, as previously mentioned, one bit at a time using the frequency-divided clock signal and are used together with the frequency-divided clock signal for Manchester encoding resulting in a first signal (DATA), which controls the switch SWi (106). The first signal thus contains information conforming to the Manchester standard. More specifically, an XOR device (110) may be used to produce the first signal (DATA) by applying an XOR function to the stream of read memory bits and the frequency-divided clock signal.

The figure further shows a power supply circuit (101) for supplying the components/blocks

(107, 108, 109, 110) with energy/voltage, so that the transponder does not require a battery or similar device. The power supply circuit (101) is preferably arranged to be supplied with energy during the reading of the transponder by an RFID reader via a second coil L₂ (102) or alternatively via a tap point on the antenna/coil L (103), implying that the transponder normally obtains its energy supply via the carrier wave from the transmitting antenna. The power supply circuit (101) may comprise, for example, a diode, a rectifier or similar device whereby a DC voltage is generated from the induced AC voltage in the coil L₂ (102) or alternatively at a tap point on the antenna coil Li (103). Preferably, the power supply circuit (101) comprises in addition a POR (POwer Reset) circuit, which resets the components/blocks (107, 108, 109, 110) to an appropriate initial state when the voltage induced by the transmitted carrier wave across the coil L₂ (102) or the tap point on L_\ (103) exceeds a certain threshold, whereupon the circuit powers up from a quiescent state. The first coil L (103) and the second antenna/coil L (102) are preferably wound on a material (105) having good magnetic properties such as for example a magnet, ferrite rod or similar. Figure 2a shows a graph of the resonant voltage (above, CH2 measured on L₂) and a graph of the voltage (CHI) delivered by the power supply circuit (101 in Figure 1). The upper graph (201) shows the resonant voltage which is formed from the carrier signal transmitted from an RFID reader. The graph corresponds to the situation where an RFID reader is approaching the transponder, and it will be seen that the amplitude of the resonant voltage increases (from left to right) with the approach of the RFID reader until it reaches a maximum amplitude. The lower graph (202) shows that the power supply circuit is not activated until the applied voltage (from L₂ or a tap point on Li in Figure 1) exceeds a certain threshold. The threshold is exceeded at time A. When the threshold is exceeded at time A, the active components of the transponder are reset and activated and the phase locking means locks the phase of the upper signal/resonant voltage (201) onto the phase of the transmitted carrier signal.

After an initializing process, modulation begins at time B, meaning that the switch opens and closes under the control of the first signal as previously explained. Preferably the switch opens substantially at the zeros of the resonant voltage. When the switch closes, the full/maximum peak voltage is instantaneously applied to the capacitor and the coil L_ls with the result that the amplitude of the resonant voltage instantaneously attains its maximum value without a build-up as in conventional RFID IC designs. After modulation begins (after time B), a Manchester encoding of the signal is generated. In the example shown, the bit sequence "0, 1, 1, 1, 1, 1, 1, 1, 0" is generated/labelled in Manchester encoding.

This bit sequence is an example of the 10 sync bits used in a laboratory set-up.

In Manchester encoding a zero is generated when the signal is falling at the start of a period and a one is generated when the signal is rising at the start of a period.

Alternatively, other well-known encoding schemes than Manchester encoding may be employed.

The length of a bit period (i.e. the portion of a signal representing one bit) is shown and indicated at (205) in the figure for a single bit (the example shows a bit period of 256 milliseconds, equal to RF/32).

The instantaneous onset of the peak voltage/resonant voltage obtained by the use of the switch results in a square-wave signal with a high AM modulation index. This results in a greatly improved signal/noise ratio, whereby interference from electromagnetic noise is reduced/eliminated, since the modulated signal has a maximal modulation index of approximately 100% of the maximum amplitude, as may be seen from the fact that the amplitude of the square- wave signal is at approximately the same level as the signal after build-up and prior to modulation beginning (prior to time B).

Other solutions produce a triangle signal, since the capacitor must first be charged up and does not receive the peak voltage instantaneously. The charging time is dependent on the Q factor of the resonant circuit. See for example the build-up process in Figure 2A (approximately the first 1.5 milliseconds).

Figure 2B illustrates an enlarged portion of a square wave (corresponding to e.g. (203) in Figure 2A). The length of the signal shown corresponds approximately to one bit period, i.e. the time it takes to read out one bit from the memory device. The length equivalent to one bit period is one grid square more than the length shown in the figure.

Figure 2C illustrates a portion of the resonant voltage in which the effect of the switch opening at time A can be seen. When the switch goes off, the voltage drops substantially instantaneously to zero (apart from noise).

Figure 2D illustrates a portion of the resonant voltage in which the effect of the switch closing at time A can be seen. As may be seen, the charge is restored substantially instantaneously, after which the sine wave continues.

Figure 2E shows in its upper portion a graph of the resonant voltage arising under the action of the transmitted carrier wave, when the carrier wave vanishes at time A. The carrier wave can be caused to vanish by the RFID reader moving away from the transponder. The graph illustrated is from a laboratory trial in which the reader is not merely moved away from the transponder but is switched off in order to reduce the action of the carrier wave as rapidly as possible. If the reader were moved away, a longer decay would have occurred. The lower graph shows the voltage supplied by the power supply circuit. It can be seen that the voltage is maintained for a certain time after the carrier wave from the RFID reader vanishes (approx. 30 milliseconds).

The reason for this is that the power consumption is very small and that the load capacitor (not shown) in the power supply circuit maintains the voltage for a certain time. This achieves the major advantage that the transponder continues sending a signal, which is particularly convenient if the RFID reader is mounted on a mobile object such as for example a railway carriage, a vehicle etc. The RFID reader can still receive the signal from the transponder after it has ceased to be directly supplied with energy, which significantly increases the reading distance. Figure 3 shows a flowchart of the method according to the invention.

At step (301) the method starts. At step (302) a check is made whether a voltage is being induced above a certain threshold, i.e. whether an RFID reader is transmitting a carrier wave signal within range and has been doing so for a certain length of time (corresponding to the behaviour at time A in Figure 2A). If this is not the case, nothing happens until the threshold is exceeded.

When the threshold is exceeded, step (303) is executed, wherein the phase of the induced signal (induced by the carrier wave transmitted from the RFID reader) is locked to the phase of the transmitted carrier wave. After this, step (304) is executed, wherein components etc. are reset, initialized and receive energy. This corresponds to the steep voltage rise shown at (202) in Figure 2A.

Steps (303) and (304) may alternatively be executed simultaneously or in the reverse order.

After this, in step (305), a signal is generated and transmitted. The signal is preferably a substantially square signal with a high modulation index. The signal is further preferably encoded in accordance with Manchester encoding. This can be achieved as explained in connection with Figure 1 and Figures 2A-E.

The generated signal is transmitted until there is no energy left, i.e. a certain time after the carrier wave from the RFID reader ceases to be present. By this means an improved interaction between a receiver and an RFID reader is achieved, wherein the effect of noise from external sources in the form of unwanted electromagnetic interference is eliminated or at least reduced, making reliable reading possible even in electromagneticaliy noisy environments. In addition the signal/noise ratio is substantially improved. The sensitivity of interaction between the receiver and the RFID reader is also improved.

Claims

What is claimed is:

1. An RFID transponder for the communication of an identification code to an RFID reader, which transponder comprises • a parallel resonant circuit comprising a capacitor (C) and a first antenna (Li) for receiving a carrier wave transmitted by the RFID reader, wherein the transponder additionally comprises

• a switch (SWi) for switching alternately off and on a resonant voltage in the parallel resonant circuit, a peak voltage being maintained across the capacitor (C) when the switch (SWi) goes off, said switch being controlled by a first signal and connected in series with the capacitor (C);

• a clock regenerator arranged to produce a clock signal from the transmitted carrier wave;

• phase locking means (PLL) for locking the phase of the generated clock signal to the phase of the transmitted carrier wave; and

• signalling means arranged to generate the first signal based on said clock signal.

2. An RFID transponder according to claim 1, wherein said switch opens when the resonant voltage is substantially zero.

3. An RFID transponder according to claims 1 and 2, wherein said switch closes when the resonant voltage would be substantially at its maximum.

4. An RFID transponder according to claims 1 to 3, wherein the transponder further comprises frequency division means for dividing the frequency of said clock signal, resulting in a second clock signal.

5. An RFID transponder according to claims 1 to 4, wherein the transponder further comprises a memory device containing a unique and unambiguous ID code, a checksum part and a synchronization part.

6. An RFID transponder according to claims 4 and 5, wherein said first signal is generated in Manchester encoding from said clock signal or said second clock signal and at least part of the contents of said memory device.

7. An RFID transponder according to claims 4 and 5, wherein said first signal is generated as the output of an XOR device (110) from said second signal and the contents of said memory device.

8. An RFID transponder according to claims 1 to 7, wherein the transponder additionally comprises

• a second coil (L₂) or a tap point on the first antenna (L_\) arranged so that a voltage is induced by the carrier wave transmitted from the RFID reader, and

• a power supply circuit for supplying power to said clock regenerator, signalling means, frequency division means, memory device and phase locking means, said second coil (L₂) or tap point on the first antenna (Li) being electrically connected to the power supply circuit.

9. An RFID transponder according to claims 1 to 8, wherein the transponder comprises a ferrite rod on which said first antenna (Li) and said second coil (L₂) are wound.

10. A method for the communication of an identification code to an RFID reader, said method comprising

• the reception in a parallel resonant circuit of a carrier wave transmitted by the RFID reader, wherein the method further comprises

• the switching alternately off and on, under the control of a first signal, of a resonant voltage in the parallel resonant circuit, a peak voltage being maintained on switching off;

• the generation of a clock signal derived from the transmitted carrier wave; • the phase locking of the generated clock signal to the phase of the transmitted carrier wave; and

• the generation of the first signal from said clock signal.

11. A method according to claim 10, wherein said switching off occurs when a resonant voltage in said resonant circuit is substantially zero.

12. A method according to claims 10 and 11, wherein said switching on occurs when the resonant voltage would be substantially at its maximum.

13. A method according to claims 10 to 12, wherein the method further comprises the frequency division of said clock signal resulting in a second clock signal.

14. A method according to claims 10 to 13, wherein the method further comprises the reading out of at least part of the contents of a memory device containing a unique and unambiguous ID code, a checksum part and a synchronization part.

15. A method according to claims 13 and 14, wherein said first signal is generated in accordance with Manchester encoding from said clock signal or said second clock signal and at least part of the contents of said memory device.

16. A method according to claims 13 and 14, wherein said first signal is generated as the output of an XOR device from said second signal and the contents of said memory device.

17. A method according to claims 10 to 16, wherein the method further comprises

• the induction of a voltage by the carrier wave transmitted from the RFID reader in a second coil (L₂) or a tap point on the first antenna (Li), and

• the generation of a power supply from the second coil (L₂) or the tap point on the first antenna (Li).