阅读说明：本技术 具有菊花链天线的rfid系统 (RFID system with daisy chain antenna ) 是由 格雷厄姆·默多克 庞大伟 加内什·纳根德拉 孙雨欣 于 2020-06-18 设计创作，主要内容包括：RFID系统(200)包括RFID读写器(202)、天线阵列(204)和长度补偿单元(215)。RFID读写器(202)被配置成询问RFID天线。天线阵列(204)包括可经由一系列线缆链路(208)连接至RFID读写器(202)的两个或更多个RFID天线(206)。每个RFID天线(206)与相应的线缆链路(208)相关联,并且每个线缆链路(208)具有线缆长度。长度补偿单元(215)与每个RFID天线(206)相关联,并且被配置成将RFID读写器(202)与相应的RFID天线(206)之间的总线缆长度调节为有效线缆长度。(The RFID system (200) includes an RFID reader (202), an antenna array (204), and a length compensation unit (215). The RFID reader (202) is configured to interrogate an RFID antenna. The antenna array (204) includes two or more RFID antennas (206) that may be connected to the RFID reader (202) via a series of cable links (208). Each RFID antenna (206) is associated with a respective cable link (208), and each cable link (208) has a cable length. A length compensation unit (215) is associated with each RFID antenna (206) and is configured to adjust a total cable length between the RFID reader (202) and the respective RFID antenna (206) to an effective cable length.)

1. An RFID system, comprising:

an RFID reader configured to interrogate an RFID antenna;

an antenna array comprising two or more RFID antennas connectable to the RFID reader via a series of cable links, each RFID antenna associated with a respective cable link, each cable link having a cable length;

a length compensation unit associated with each RFID antenna, the length compensation unit configured to adjust a total cable length between the RFID reader and the respective RFID antenna to an effective cable length.

2. The RFID system of claim 1, wherein the RFID tag is a RFID tag,

wherein each RFID antenna has an antenna impedance and the respective cable link of each antenna has a cable impedance, and the antenna impedance is different from the cable impedance such that each RFID antenna is impedance mismatched from its respective cable link, an

Wherein the length compensation unit associated with each RFID antenna is configured to adjust a bus cable length between the RFID reader and the respective RFID antenna such that reflections caused by the impedance mismatch have a predefined phase.

3. The RFID system of claim 2, wherein the antenna impedance of the respective RFID antenna is transformed along the effective cable length to a final impedance that is resistive and substantially nonreactive.

4. The RFID system of claim 2 or 3, wherein the antenna impedance is equal to an antenna resistance without reactance and the antenna impedance is transformed along the effective cable length to have an intermediate impedance value including reactance, and wherein the final impedance is substantially equal to the antenna resistance without reactance.

5. The RFID system of any preceding claim, the system further comprising a controller configured to activate one RFID antenna at a time by transmitting a first control signal to the antenna array.

6. The RFID system of claim 5, further comprising a bypass switch associated with each RFID antenna, the bypass switch responsive to the first control signal to bypass or connect the respective RFID antenna to the RFID reader.

7. The RFID system of claim 5 or 6, wherein the total cable length is a variable length depending on which of the two or more RFID antennas is a functioning antenna, and the total cable length comprises a sum of cable lengths of each cable link connecting the RFID reader and the functioning antenna.

8. The RFID system of claim 7, wherein a length compensation unit associated with the active antenna adjusts a total cable length between the RFID reader and the active antenna to the effective cable length.

9. The RFID system of any preceding claim, wherein the length compensation unit comprises a configuration of reactive electronic components that simulates an extension or shortening of the total cable length.

10. RFID system according to one of the preceding claims,

wherein one of the two or more RFID antennas is a functioning antenna and the effective cable length is the sum of the total cable length and a compensation length provided by a length compensation unit of the functioning antenna, an

Wherein the effective cable length is substantially equal to a defined length.

11. The RFID system of claim 10, wherein each length compensation unit is configured to have a different compensation length, the compensation length for each unit being a function of a plurality of cable links between the compensation unit and the RFID reader.

12. The RFID system of claim 10, wherein the compensation length of each length compensation unit is adjustable.

13. The RFID system of any preceding claim, wherein the two or more RFID antennas of the antenna array are connected in a daisy chain configuration via the series of cable links.

14. The RFID system of claim 5, wherein the controller is further configured to transmit a second control signal to at least one length compensation unit for setting an adjustable compensated length of the at least one unit.

Technical Field

The present disclosure relates generally to radio frequency antennas and, more particularly, non-limiting embodiments relate to antenna configurations for radio frequency identification systems.

Background

Radio Frequency Identification (RFID) systems typically include one or more antennas that can communicate with an RFID transponder (or "tag"), and an RFID reader (or "interrogator") that communicates with the one or more antennas. The antenna sends a Radio Frequency (RF) signal to the RFID tag, and any response received by the antenna from the RFID tag is relayed to the reader for further processing.

In RFID systems using multiple antennas, for example for inventory tracking in large areas or volumes (e.g., in warehouse racks, etc.), the operation of the antennas is typically controlled by one or more readers in communication with the antennas. Fig. 1A of the accompanying drawings shows a prior art system in which a large shelf 100 has four overlapping antennas 104, each connected via a cable 103 to a port 102 of an RFID reader 101. The reader 101 communicates with each of the antennas 104 that transmit RF signals to identify RFID tags that may be present on the shelf 100.

A disadvantage of the prior art system shown in fig. 1A is that a system with several antennas will result in a large amount of wiring being used, since each individual antenna coil 104 requires its own length of cable 103 to connect to the reader/writer 101. This can be quite cumbersome and can take up a lot of space.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.

Disclosure of Invention

In one aspect, there is provided an RFID system, including: an RFID reader configured to interrogate an RFID antenna; an antenna array comprising two or more RFID antennas connectable to an RFID reader via a series of cable links, each RFID antenna being associated with a respective cable link, each cable link having a cable length; a length compensation unit associated with each RFID antenna, the length compensation unit configured to adjust a total cable length between the RFID reader and the respective RFID antenna to an effective cable length.

Each RFID antenna has an antenna impedance and the respective cable link of each antenna has a cable impedance, and the antenna impedance may be different from the cable impedance such that the RFID antenna is impedance mismatched from its respective cable link. The length compensation unit associated with the RFID antenna may be configured to adjust the bus cable length between the RFID reader and the respective RFID antenna such that reflections caused by said impedance mismatch have a predefined phase.

The antenna impedance of the RFID antenna may be transformed along the effective cable length to a final impedance that is resistive and substantially nonreactive.

The antenna impedance may be equal to the antenna resistance without reactance, and the antenna impedance may be transformed along the effective cable length to have an intermediate impedance value including reactance. The resulting impedance may be substantially equal to the antenna resistance without reactance.

The system may also include a controller configured to activate one RFID antenna at a time by transmitting a first control signal to the antenna array. The system may also include a bypass switch associated with each RFID antenna that is responsive to the first control signal to bypass or connect the respective RFID antenna to the RFID reader. The total cable length may be a variable length depending on which of the two or more RFID antennas is the active antenna, and includes the sum of the cable lengths of each cable link connecting the RFID reader and the active antenna. The length compensation unit associated with the active antenna may adjust the total cable length between the RFID reader and the active antenna to an effective cable length.

The length compensation unit may comprise a configuration of reactive electronic components simulating an extension or shortening of the total cable length. The effective cable length may be the sum of the total cable length and the compensation length provided by the length compensation unit of the active antenna. The effective cable length may be substantially equal to the defined length.

Each length compensation unit may be configured to have a different compensation length, the compensation length for each unit being a function of a plurality of cable links between the compensation unit and the RFID reader.

The compensation length of each length compensation unit may be adjustable.

Two or more RFID antennas of an antenna array may be connected in a daisy chain configuration via a series of cable links.

The controller may be further configured to transmit a second control signal to the at least one length compensation unit for setting the adjustable compensation length of the at least one unit.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Drawings

Embodiments of the present disclosure will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1A is a schematic diagram of a prior art RFID system;

FIG. 1B is a schematic diagram of another prior art RFID system;

FIG. 2A is a schematic diagram of an RFID system having serially connected antennas;

FIG. 2B is a schematic diagram of a bypass switch used in the RFID system of FIG. 2A;

FIG. 2C is another schematic diagram of a bypass switch used in the RFID system of FIG. 2A;

FIG. 3A is a schematic diagram of a circuit with a mismatched antenna and cable configuration;

FIG. 3B is a Smith chart showing the impedance of the circuit of FIG. 3A;

FIG. 4A is a schematic diagram of the impedance in the circuit of a cable having a shortened length;

FIG. 4B is a schematic diagram of the impedance in the circuit with an increased length of cable;

FIG. 5A is a schematic diagram of a circuit with length compensation for an increased length cable;

FIG. 5B is a Smith chart showing the impedance along the circuit of FIG. 5A;

FIG. 6A is a schematic diagram of a circuit with length compensation for a shortened length cable;

FIG. 6B is a Smith chart showing the impedance along the circuit of FIG. 6A;

FIG. 7 is a schematic diagram of an embodiment of a length compensation unit;

FIG. 8 is a schematic diagram of an embodiment of a length compensation unit;

FIG. 9 is a schematic view of another embodiment of a length compensation unit;

FIG. 10 is a schematic view of yet another embodiment of a length compensation unit;

FIG. 11 is a schematic diagram of an embodiment of a length compensation unit with a local unit controller;

FIG. 12 is a schematic diagram of another embodiment of a length compensation unit with a local unit controller;

FIG. 13 is a schematic diagram of an embodiment of an RFID subsystem;

FIG. 14 is another schematic diagram of the RFID subsystem of FIG. 13;

FIG. 15 is a schematic diagram of an embodiment of a calibration circuit; and

FIG. 16 is a schematic diagram of another embodiment of a calibration circuit.

In the drawings, like reference numerals designate like parts.

Detailed Description

Fig. 1B shows another prior art antenna configuration 120 that attempts to reduce the amount of wiring used by using a serial configuration. The example shown in fig. 1B has a shelf 122 with three antennas 124 connected to a reader/writer 126. A length of cable 128 and bypass switch 130 connect each successive antenna 124 and to the reader/writer 126. Typically, this type of configuration keeps all antenna elements powered by connecting power at each shelf via a respective bypass switch 130. Individual antennas 124 are activated via a control mechanism that addresses a particular antenna 124 via a unique address. This approach is not only inefficient in terms of power usage, but also inefficient due to the complexity of implementing the addressing system.

Generally, conventional RFID systems (e.g., those shown in fig. 1A and 1B) are configured to operate as impedance-matched systems. The input impedance of the antennas 104, 124 will match the cables 103, 128, and the cables 103, 128 will match the output impedance of the readers 101, 126. Matching is done to simplify the design and maximize power transfer in the RF system. However, in the RFID system, the impedance-matched system has a problem that an operation bandwidth is narrow and thus a data rate is low. For RFID systems using antenna coils, the interrogation signal is an oscillating magnetic field. The interrogation field is reactive and maximum power transfer is not a useful performance metric because the reactive interrogation field is lossless. Any circuit losses are due to resistive losses and do not constitute a useful part of the interrogation signal. Maximizing power transfer is equivalent to maximizing losses that are not useful to the reactive system. Thus, the solution proposed herein is an impedance mismatched RFID system.

Impedance mismatch operation

In the impedance-mismatched RFID system, the output impedance of the RFID reader/writer is not matched with the impedance of the connection cable, and the impedance of the connection cable is not matched with the impedance of the RFID antenna. Benefits of this type of mismatch include wide bandwidth and high data rate operation.

Typically, the connection cable will be a coaxial cable with an impedance of Zo-50 ohms, although other types of cables may also be used. The antenna is typically a series-tuned coil with a low impedance of a few ohms (e.g., 2 to 5 ohms). When connected to a cable, the antenna 206 and the cable are impedance mismatched. Likewise, the reader output impedance will be low, typically 10 ohms, which is also mismatched with the cable impedance. The antenna impedance is transformed along the cable, and if the cable is a certain fixed and correct length, the transformed impedance will be a predictable value. A disadvantage of typical mismatch systems is that the cable is required to have a certain fixed correct length. One option is that each cable link comprises an additional length of cable to ensure that a particular length is provided, but this will result in bulky cabling. It would therefore be useful to find a way to provide reliable mismatch operation in the event that the cable length is not the desired defined length.

Overview of the System

Fig. 2A of the accompanying drawings shows an RFID system 200 having multiple antennas, for example as implemented in a cabinet 210 having a number of shelves 212. The RFID system 200 has an RFID reader 202 and a controller 222. In some embodiments, the RFID reader 202 powers the controller 222, and in some embodiments, the controller 222 forms a portion of the reader 202.

In some embodiments, RFID system 200 has an RFID reader 202 configured to interrogate RFID antennas, and system 200 has an antenna array 204 with two or more RFID antennas 206 connectable to RFID reader 202 via a series of cable links 208, each RFID antenna 206 being associated with a respective cable link 208. Each cable link 208 has a cable length. System 200 also has a length compensation unit 215 associated with each RFID antenna 206, and length compensation unit 215 is configured to adjust the total cable length between RFID reader 202 and its corresponding RFID antenna 206 to an effective cable length. The system 200 has a bypass switch 214 associated with each RFID antenna 206, each bypass switch 214 operable to bypass or connect the respective RFID antenna to the RFID reader 202. In some embodiments, the bypass switch 214 and the length compensation unit 215 form a combined unit with shared functionality (e.g., with a shared local controller).

In this manner, the antennas 206 are connected in series in a daisy chain arrangement, with each RFID antenna 206 being connected to the RFID reader 202 via a respective cable link 208 and via a respective length compensation unit 215. In system 200, the cable length to each antenna 206 becomes longer and does not have the single fixed length required for typical impedance mismatch operation. The variable total cable length is adapted by including a length compensation unit 215. By effectively increasing or decreasing the cable length, the length compensation unit 215 is able to ensure that the effective cable length is substantially equal to the defined cable length required for the impedance mismatch operation.

Each RFID antenna 206 has an antenna impedance ZA and each antenna's respective cable link 208 has a cable impedance Zo. The antenna impedance ZA is different from the cable impedance Zo such that the RFID antenna and its corresponding cable link are impedance mismatched. The length compensation unit 215 associated with the RFID antenna 206 is configured to adjust the bus cable length between the RFID reader and the respective RFID antenna such that reflections caused by impedance mismatches between the RFID antenna and the cable link are controlled to have a predefined phase. The phase of the reflection affects the effect the reflection has on the operation of the reader and the antenna and the correct value of the phase when it reaches the reader end will ensure that the impedance transformation is correct.

In this way, the system of fig. 2A is configured for mismatch operation, and the length compensation unit 215 makes the necessary electrical length adjustments needed to provide reliable mismatch operation if the cable length does not have a fixed correct length.

Length compensation

In order for the reader 202 to see the appropriate defined cable length, the length compensation unit 215 is configured such that the necessary electrical length adjustments are made to provide a reliable mismatch operation in case the cable length does not have a fixed correct length. The length compensation device is selected to electrically represent an extra length of cable if the cable is too short, or a "negative" length of cable if the cable is too long.

Figure 3A of the accompanying drawings shows an embodiment in which the impedance Z1 of the antenna 501 is given by R1. The impedance of the antenna 501 does not match the impedance Zo of the cable 502. The impedance Z1 is transformed down the length of the cable to a value of Z2, which value of Z2 is given by R2+ jX at the distal end of the cable. The total impedance seen by the reader is adjusted by the compensation element 503 with an impedance-jX such that the total impedance Z3 seen by the reader is equal to R2.

Fig. 3B shows a smith chart 300, the smith chart 300 showing the change in impedance from the antenna (point a) through the cable (point B) and including the compensating element 503 (point C).

A：Z1＝R1

B：Z2＝R2+jX

C：Z3＝R2

The embodiments shown in fig. 3A and 3B depend on a defined cable length, which is typically a relatively short length of less than 1.5 meters of 1/8 of wavelength. In the case of a deviation in the cable length, the impedance seen by the reader/writer is not optimal for reader/writer performance. For example, as shown in FIG. 4A, the shorter cable 402 will produce a load impedance Z3 of R2-jX1, and as shown in FIG. 4B, the longer cable 404 will produce a load impedance Z3 of R2+ jX 2.

When a different antenna 206 in the antenna array 204 is selected, the characteristics of the connection wiring change because the antennas from the reader 202 to the antenna selected as the functional antenna include varying cable lengths. For example, the bus cable length will be different when the first antenna 206.1 is connected to the reader/writer 202 via the first bypass switch 214.1 when compared to the bus cable length when the third antenna 206.3 is connected to the reader/writer 202 via the third bypass switch 214.3.

The RFID reader 202 is configured to operate at a predefined load impedance ZL. Thus, when a different antenna 206 in the antenna array 204 is selected, the total impedance seen by the reader 202 will change due to the changing bus cable length. In order to adjust the total impedance seen by the reader/writer 202 to be substantially equal or close to the predefined load impedance ZL, a length compensation unit 215 comprised in the connection between the reader/writer 202 and the active antenna 220 provides a length compensation which makes the total impedance seen by the reader/writer 202 substantially equal to the predefined load impedance ZL. In this manner, the same effective cable length is seen from the reader 202 regardless of which antenna is activated.

Ideally, the RFID antenna has the following antenna impedances: the antenna impedance has a resistance but no reactance. For mismatch operation, it is also preferred that the antenna resistance is transformed along the connection cable to a transformed impedance having a resistance without reactance, that is, having a zero phase. However, as shown in fig. 4B, when the connection cable is too long or too short, the transformed impedance includes reactance.

In some embodiments, the compensation unit compensates for the increase in bus cable length using a passive element configuration 510 as shown in fig. 5A. The impedance of the circuit shown in fig. 5A is shown in smith chart 512 of fig. 5B, where the impedance change is:

at a, including "] Δ X compensation impedance: z1 ═ R1-jAX;

at B, which has compensated for the additional Δ l: z2 ═ R2;

at C after/defined cable length: z3 ═ R3+ jX; and

at D, which includes a default precompensation-jX: z4 ═ R3.

In some embodiments, the compensation unit compensates for the reduction in bus cable length using a passive component configuration 610 as shown in fig. 6A. The impedance of the circuit shown in fig. 6A is shown in smith chart 612 of fig. 6B, where the impedance change is:

at A: z1 ═ R1;

at B, including the j Δ X compensation impedance: z2 ═ R1+ jAX;

at C after the shortened cable length: z3 ═ R4+ jX; and

at D, which includes a default precompensation-jX: z4 ═ R4.

It should be understood that various different configurations of capacitive and/or inductive elements may be used for a set of compensation units 215 associated with the array 204 of antennas 206 as shown in fig. 2A. In one embodiment 700 shown in fig. 7 of the drawings, five antennas 206 are connected in series via a cable link 208. Total cable length from reader/writer to selected active antenna/cable length equal to initial cable length 11 and up to active antenna 220 η' × Δ I_ηThe sum of (a) and (b).

In this embodiment, the default compensation length is considered to be:

1＝11+Δ11+Δ12，

wherein the shorter connection cables of the first two antennas need to have additional compensation lengths Δ 11+ Δ 12 and Δ 12, respectively, and the longer connection cables of the last two antennas need to have a reduction in length with compensation lengths- Δ 13 and- Δ 13- Δ 14, respectively.

For this purpose, the following configuration of the compensation unit is used: this configuration provides a default compensation 702 of-jX associated with the intermediate antenna 704 and a capacitive compensation unit 706 and an inductive compensation unit 708, which capacitive and inductive compensation units 706 and 708 are located farther and closer to the reader than the intermediate antenna 704, respectively, where the reactances are as follows:

in this way, the length compensation unit comprises a configuration of reactive electronic components which simulate an extension or a shortening of the total cable length as required.

Bypass switching

The controller 222 activates one RFID antenna 206 at a time by transmitting a bypass control signal to the antenna array 204. The controller 222 controls switching between the antennas 206 by controlling the bypass switch 214. The bypass switch is responsive to a bypass control signal to bypass or connect the respective RFID antenna to the RFID reader.

In some embodiments, a 3-bit (3-bit) control line may be provided, for example, to enable switching between antennas at each daisy-chain position. In other embodiments, the combined RF, DC, and control signals are sent from the controller 222 along the daisy-chain cable to the antenna array 204, and the bypass control signal causes the selected bypass switch 214 to switch in the selected length compensation unit 215 and the selected antenna 220. In some embodiments, the control signal is carried on the RF signal and/or the DC power signal according to the methods described in international patent application published as WO2009/149506 a1, the contents of which are incorporated herein by reference. In some embodiments, the control signal is added to the DC power signal, which also provides power to the local unit controller 1102 that controls the bypass switch 214. This is described in more detail elsewhere herein with reference to fig. 12. In some embodiments, control signals from controller 222 may direct the operation of local antenna controller 1310 as described elsewhere herein with reference to fig. 13. It should be understood that communication between RFID reader 202 and antenna 206 is bi-directional, such as via one or more of controllers 222, 1102, 1310.

The connection from the RFID reader 202 to the antenna 206 is made via the bypass switch 214, or alternatively through the antenna 206 and to the subsequent cable 208. By way of example, fig. 2B and 2C of the drawings illustrate the first bypass switch 214.1 of the system 200 shown in fig. 2A. The switch 214.1 may connect the cable 208.1 to the first antenna 206.1 via the length compensation unit 215.1 or bypass the antenna 206.1 and connect the cable 208.1 to the cable 208.2 leading to the second antenna 206.2. In the example shown, the second antenna 206.2 has been selected as the active antenna 220. Each bypass switch 214 operates to connect the selected active antenna 220 to the cable 208 from the reader/writer 202 while disconnecting the next segment of cable 208 after the switch 214. So in this example, switch 214.2 disconnects cable 208.3. Alternatively, when the bypass switch 214 is set to bypass the antenna 206, then the switch 214 disconnects its respective antenna and connects the preceding and subsequent cable links 208 so that the reader 202 can connect to the next antenna along the daisy chain. In this example, switch 214.1 would switch around antenna 206.1 and connect cable 208.1 and cable 208.2.

In some embodiments, the bypass switch may be implemented using a pin diode. In other embodiments, the bypass switch may be implemented using a relay.

Adjustable compensation

In some embodiments, the controller 222 controls the compensation units 215, wherein the compensation length of each length compensation unit is adjustable.

Fig. 8 to 12 show embodiments of the adjustable length compensation unit.

The length compensation can be made adjustable by making the impedance value selectable using a switch, as shown in fig. 8, fig. 8 showing a first embodiment of an adjustable length compensation arrangement 800. The illustrated length compensation has binary weighted inductive impedances that can each be selected using shunt switches 802. The inductive impedance may be adjusted from zero to + j7X in steps of jX.

Fig. 9 shows a second embodiment of an adjustable length compensation arrangement 900, in which adjustment can be made in positive and negative directions using an inductance 902 and a capacitance 904, respectively. The binary weighted inductor impedances may each be selected using shunt switches 802, and the total series impedance may be adjusted in steps of jX from-j 3X to + j 4X.

Fig. 10 shows a third embodiment of an adjustable length compensation arrangement 1000 in which adjustment can be made in both positive and negative directions using an alternative combination of inductance and capacitance. The binary weighted capacitance impedances may each be selected using shunt switches 802, and the total series impedance may be adjusted in steps of jX from-j 4X to + j 3X.

In some embodiments, the shunt switch 802 may be implemented using a pin diode. In other embodiments, the shunt switch 802 may be implemented using a manually set mechanical switch. In other embodiments, the shunt switch 802 may be implemented using a relay. In applications where switching is infrequent, a mechanical latching relay may be used so that the relay may be latched and held set. This is the case in applications where adjustments are made at power up and thereafter only infrequently when a particular antenna in the array is selected, and preferably configuration is saved at power down.

Fig. 11 shows another embodiment of a length compensation unit 1100, where the combined RF, DC and control signals are sent from the reader 202 along a daisy chain cable to the antenna array 204, and the control signals cause the selected bypass switch 214 to switch in the selected length compensation unit 215. The control signal is added to the DC power signal. The DC power signal provides power to a local unit controller 1102 that operates the bypass switch 214. The control signal 1104 also directs the operation of the unit controller 1102 to control the shunt switch 802 of the length compensation unit 1100. In this manner, the unit controller 1102 may adjust the length compensation (under the direction of the reader's controller 222) to ensure that the correct length compensation is selected for the active antenna.

Fig. 12 shows a further embodiment of a length compensation unit 1200 using the same combined RF, DC and control signal 1104 as described with reference to fig. 11. As shown here, when the reader 202 deselects the antenna 206, the bypass switch 214 turns off the length compensation unit 1200, however the unit controller 1102 remains active and is able to keep the switch 214 in a "bypass" state so that the reader 202 can communicate with the unit controller of the next antenna. This process is repeated for the following antennas and the like. In this manner, the reader 202 can communicate with each antenna 206 in the daisy chain in a sequential and repeatable manner. If the power is disconnected, all bypass switches 214 revert to the disconnected state, which automatically connects the reader 202 with the first antenna 206.1 in the daisy chain, due to the default setting of the first switch 214.1. The process of sequential control and operation may then be repeated.

Although fig. 8-12 show 3 shunt switches each configured to require 3 bits of control, a greater or lesser number of shunt switches may be used depending on the desired length compensation resolution.

Local controller

In fig. 13 and 14 of the drawings, an embodiment of an antenna subsystem 1300 is shown. The subsystem may be associated with, for example, one of the shelves 212 in the cabinet 210 shown in fig. 2A.

The subsystem 1300 has a plurality of antennas 1302, the plurality of antennas 1302 in communication with a tuner 1304 via a first multiplexer 1306 and with the length compensation unit 215 and the bypass switch 214 via a second multiplexer 1308. The subsystem 1300 has a local antenna controller 1310 that controls the operation of the tuner 1304, multiplexers 1306, 1308, the antenna 1302 via the multiplexers 1306, 1308, the length compensation unit 215, and the bypass switch 214 (e.g., via the length compensation unit 215).

As in the embodiments shown in fig. 11 and 12, the subsystem may receive the combined RF, DC, and control signal 1104, and when the reader 202 deselects the antenna 206, the bypass switch 214 turns off the length compensation unit 215, while the antenna controller 1310 remains active and is able to keep the switch 214 in a "bypass" state so that the reader 202 can communicate with the controller of the next antenna.

The tuner 1304 adjusts the resonant frequency of the antenna coil by adjusting the tuning capacitance so that the antenna coil is tuned to resonance. When tuned, the antenna input impedance has a low practical value, since the preferred antenna is a series resonant coil with low resistance and no reactance.

In some embodiments, the subsystem 1300 may be operably connected to one or more additional devices, displays, sensors, indicators, etc., to provide a user interface, for example, for the shelves 212 and/or the cabinet 210. An indicator light may show where the label is located, a display may show pickup information related to the shelf 212, etc.

Calibration

Prior to operation, such as at installation of RFID system 200 or when the system is powered up, the initial effective cable length must be measured to determine any adjustments to the desired cable length compensation.

Fig. 15 shows a first embodiment of a calibration circuit 1500, and fig. 16 shows a second embodiment of a calibration circuit 1600. Placing the shorting calibration switch 1502 at the antenna end 1504 of the length compensator 215 enables the reader 202 to set or calibrate the compensation length. Calibration switch 1502 is closed via a local controller (e.g., antenna controller 1310) under the direction of the reader's controller 222. Once closed, the reader 202 can monitor the phase and amplitude of the RF signal it delivers to the active antenna. The switch 1502 appears as a low impedance load and the phase (of phase and amplitude) should coincide with this value. That is, the RF current should be relatively large, the voltage relatively low, and the phase between the current and voltage should be zero degrees (in phase). If the compensation length is too short or too long, the current will be low, the voltage will be high, and the phase between the current and the voltage will be positive above zero or negative below zero. The reader's controller 222 adjusts the shunt switch 802 of the length compensation unit 215 to obtain a phase angle as close to zero as possible, at which point the length compensation will be at the best achievable setting.

Once set, the compensation settings will typically not change, as the length compensation will remain fixed unless the antenna or cable is physically moved or changed. Mechanical latching relays may be used in this application so that the relay can be latched and held in place. In this case, the adjustment can be made at power-up and then only infrequently when needed. This configuration is advantageously preserved by latching the relay when de-energized.

The length compensated unbalanced to balanced operation may be achieved by placing a balun (balun) between the compensation unit 215 and the antenna 206. The cable 208, the bypass switch 214, and the compensation unit 215 operate asymmetrically, and the antenna 206 may operate in a balanced state. Balanced operation has been found to be advantageous in reducing interference at antenna 206 and spurious coupling from antenna 206. The circuit of fig. 15 provides unbalanced operation, while the circuit of fig. 16 provides balanced operation.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments without departing from the broad general scope of the disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.