AUTOMATIC ASSOCIATION OF MOBILE ITEMS

BACKGROUND

Field

[0001] The present description relates to detecting when two items are near to each other and, in particular to identifying transactions based on proximity detection and generating records based of the transactions.

Related Art

[0002] Location systems are currently used in inventory tracking and management systems and in some service facilities. For example, location systems such as Pango, and Ekahau are used to attach tags to pieces of equipment, to patients, to doctors, and etc. These location system can determine the location of a person or piece of mobile equipment within a range of about 5m. This is sufficient to determine a wing or corridor and limit the location to within a few rooms of, for example, a hospital. Such location systems are sometimes referred to as building location systems. [0003] Such a system may be used to find a person or piece of equipment. Once the location is determined to within a range of 5m or so, another person can go to that approximate location and easily find the person or piece of equipment. Such a system may also be used to determine inventory. It can be used not only to determine whether a container is in a warehouse or store room, but also whether a doctor or patient is in a hospital and perhaps also which ward the doctor or patient is in. [0004] The location system may be based on a WiFi location system (e.g. Ekahau). WiFi systems are limited in the accuracy that they provide. In other systems, infra-red transmitters are attached to wearable tags. These tags are tracked by infra-red receivers mounted on walls etc. throughout the building. Infra-red systems are limited to line-of- sight communication. In another example, ultra- wide-band signals are used, with tags containing transmitters and receivers mounted on the building infrastructure. Ultra-wide band systems, however, require a large number of specialized receivers to be able to perform location tasks.

SUMMARY OF THE INVENTION

[0005] Mobile items may be automatically associated with each other and the association recorded. In one embodiment of the invention, a wireless transmitter transmits a near field signal. A wireless receiver receives the transmitted near field signal from the wireless transmitter and determines, based on the received near field signal, whether the transmitter is close to the receiver, and a communications interface, coupled to the wireless receiver and to a recordkeeping system, transmits a record to the recordkeeping system, the record indicating that the wireless transmitter has been close to the wireless receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The invention may be more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, in which like reference numerals refer to corresponding parts throughout the several views of the drawings, and in which:

[0007] Figure 1 is a block diagram of a transmitter according to an embodiment of the present invention;

[0008] Figure 2 is a block diagram of a receiver according to an embodiment of the present invention;

[0009] Figure 3 is a block diagram of a context for the use of the transmitter and receiver of Figures 1 and 2 as a Stage I system according to an embodiment of the invention;

[0010] Figure 4 is a block diagram of a context for the use of the transmitter and receiver of Figures 1 and 2 as a Stage II system according to an embodiment of the invention;

[0011] Figure 5 is a vector diagram of a magnetic field produced by a magnetic coil according to an embodiment of the invention;

[0012] Figure 6 is a vector diagram of magnetic fields for a transmitter and receiver according to an embodiment of the invention;

[0013] Figure 7 is a vector diagram of magnetic fields for a two coil transmitter according to an embodiment of the invention;

[0014] Figure 8 is a vector diagram of magnetic fields for a two-coil receiver according to an embodiment of the invention;

[0015] Figure 9 is a graph of the range error versus the relative angle between a magnetic transmitter and a receiver according to certain estimates;

[0016] Figure 10 is an example of a hardware implementation of a wearable three-axis tag according to an embodiment of the invention;

[0017] Figure 11 is a timing diagram of duty cycles for a transmitter and a receiver according to an embodiment of the invention; and

[0018] Figure 12 is a graph of power consumption versus On time for a transmitter, receiver combination according to certain estimates.

DETAILED DESCRIPTION

[0019] While existing system are sufficient for approximately locating people and things, there are other applications for which they are not reliable or not accurate enough. For example, if people and equipment could be located accurately enough to determine a particular room, then the location measurements could contribute to a billing system. So, for example, if it could be determined that a doctor is with a patient, then the doctor's time could automatically be charged to the patient. Any size increment of time up to three minutes or less could form the basis of a billable event. Similarly, if a patient is with an analysis or treatment device, then a measurement of the time that the patient is with the device could form the basis of a billable event for use of the equipment. [0020] One way to achieve such a transaction determination system is with a transmitter/receiver pair that uses magnetic (near field) signals to determine the distance between the transmitter and the receiver. This distance can then be communicated via WiFi or some other communications interface to a computer system that performs billing operations.

[0021] There are a variety of different system architectures that may be used to detect and report transactions. In this description a first type (Type I) and a second type (Type II) are specifically described, but other variations are also possible. [0022] In Type I, tags are attached to the items the association of which needs to be recorded, for example to doctors, patients and each item of equipment. In Type II, tags are attached to items as in the previous case, but tags are also attached to fixed items

(such as beds, doors, walls, etc.) and the association uses two steps. In the first step, the bed can determine if a patient is near the bed. In the second step, the bed can determine if a doctor is near the same bed. If both are detected, then the doctor and patient must be together. Transmitter

[0023] An example of hardware that may be attached to a doctor, a patient, a bed, a kidney dialysis machine, etc, is shown in Figure 1. Figure 1 shows a possible transmitter design. The magnetic transmitter 10 contains a data register 11 to hold the transmitter's unique identifier. This may be a simple identification number in binary, decimal, or hexadecimal form. The data register may also be used to hold additional information and program instructions. The data register may be a static or rewritable memory including a ROM (Read Only Memory), PROM (Programmable ROM), EPROM (Erasable PROM), RAM (Random Access Memory) NVRAM (Non- Volatile RAM) or any of variety of other types of registers. The unique identifier in the data register may be stored in the register at the time of its manufacture or it may be written into the register when the transmitter is commissioned. Alternatively, with additional equipment, the register value may be changed periodically through the magnetic coil link.

[0024] A Manchester encoder 12 is coupled to the data register to convert the data in the data register into a data word that can be decoded. An FM encoder 13 is coupled to the Manchester encoder to then encode the Manchester-encoded data into a signal suitable for transmission via a magnetic link. The FM encoded signal is then amplified using a power-amplifier 14 that is coupled to the FM encoder. The power amplifier drives the amplified signal into a transmit coil 15.

[0025] The transmitter 10 may have a single coil, two coils or more. In the example of Figure 1, there are three coils 15-1, 15-2, 15-3 mounted in orthogonal directions shown as x, y, and z directions of a Cartesian coordinate system. Other orientations may be used instead. The coils need not be orthogonal. These coils may be excited together or excited alternately as controlled by a switch 18.

[0026] In one example, the signal may be encoded using frequency modulation (FM) over a low-tone of 114KHz, and a high-tone of 126KHz. The transmit coil may be made from 40 turns of wire on a former of diameter of 5- 10cm, or it may be made from a set of

traces on a multi-layer PCB, or it may be made from coils commercially manufactured for magnetic links.

[0027J The magnetic transmitter 10 of Figure 1 also has a timer/trigger 16 to cause the device to send a magnetic message. This might cause the tag to transmit the contents of the register continuously or at regular intervals, for example every 10 seconds. [0028] The transmitter 10 also has a power source 18. The power source may be a battery, an RF (Radio Frequency) energy harvester, a photovoltaic cell, a mains power supply, or some other supply of power e.g. power-over-ethernet or power-over-USB (Universal Serial Bus).

Receiver

[0029] Figure 2 shows an example of a magnetic receiver 20. The magnetic receiver has a receive coil 21, or a set of receiver coils 21-1, 21-2, 21-3. These may be similar to the transmit coils described above and 1, 2, 3, or more may be used. The receive coils may be coupled to a switch 22 to select the active receiver coil from among the group of coils. The example of Figure 2 shows a switch that selects one of three coils, however, a switch may also be used to select coils in combinations. With a single coil or with some multiple coil configurations, a switch may be unnecessary.

[0030] The switch 22 is coupled to a low-noise amplifier (LNA) 23 to amplify the signal from the selected receive coil or coils. The signal from the LNA is coupled to an automatic gain controller (AGC) 24 that amplifies the signal to a level that can be decoded by an FM decoder 25. The FM decoder is coupled to the AGC at one end and to a Manchester decoder 26 at the other. The Manchester decoder extracts the data from the signal that was encoded by the Manchester encoder in the transmitter. The decoded signal is then passed to a data register 31. The data register is a temporary storage device coupled to the Manchester decoder. The data register is also coupled to a microprocessor 27.

[0031] The microprocessor can read from the data register to obtain the data that has been stored there. It may also have a clock so that it can time when signals are received and measure the duration from when a signal is first received from a transmitter to when the transmitter moves out of range. In addition to the data register 31, the microprocessor

is also coupled to the AGC 24. In one example, the AGC produces as an output a signal that represents the amplitude of the received signal. The connection between the microprocessor and the AGC may be used to allow the microprocessor to determine the signal strength of the magnetic signal from the transmitter or to receive an indicator from the AGC of the signal strength of the magnetic signal from the transmitter. The signal strength measurement may be used in an algorithm to determine the distance between the transmitter and receiver.

[0032] The microprocessor is also connected to a communications interface. In the example of Figure 2, this interface is a WiFi radio 28 and a connected antenna 29. The WiFi interface allows the receiver to operate as a client on a WiFi network. A variety of other communication interfaces may also be used, both wired and wireless. If the receiver is mounted to a fixed piece of equipment or equipment that is otherwise tethered to external connections, then a wired communications interface, such as Ethernet, may be preferred. As with the example of Figure 1, the receiver is also coupled to a power source 33. This power source may, for example, be any one or more of the types described above for Figure 1.

[0033] The hardware diagrams of Figures 1 and 2 are provided as examples of how a transmitter and receiver may be designed. A wide range of variations may be made depending on the particular application. The encoding, transmission, amplification and data may be modified as desired. In addition, the devices may be combined so that a single device may act as both a transmitter and a receiver. In both devices, a power control system may be provided to switch the device from a standby, inactive or sleep mode to an active mode based on timed events, or sensed events.

Type I System

[0034] Figure 3 shows an example of a Type I system 30 as mentioned above. The Type I and Type II systems are shown as examples of how the transmitter and receiver of Figures 1 and 2 may be used. In both Type systems there may be more or fewer transmitter and receivers attached to more or fewer people or things. While a hospital or doctor's office is shown, the same principles may be applied to many other contexts.

[0035] In the example of Figure 3, a patient 35 is fitted with a "tx tag" 36 or "exciter" such as that shown in the example of Figure 1. However, other types of transmitters may be used instead. The transmission from the patient's tag is shown by the solid arrow in Figure 3.

[0036] A doctor 37 is fitted with a "rx tag" 38. The patient's exciter occasionally transmits its identification number through its magnetic coils or from some other type of wireless transmitter. When the doctor comes close, then the signal from the exciter is received by the doctor's receive tag 38. This tag may measure the distance in any of a variety of different ways and may also associate times with the measurements. [0037] The receive tag is also equipped with a WiFi communications interface 39. This transmits the patient's tag number and the doctor's tag number to an antenna 40 of a WiFi AP (Access Point) 41. The wireless connection is indicated by the dotted arrow. The AP is coupled to a recordkeeping system 42 such as a transaction-based time billing system 42. The AP may also be coupled to an inventory tracking system, a personnel location system or a variety of other types of systems. The doctor's tag may send the time at which the doctor came close to the patient and the time that the doctor left the patient. It may send the duration of the contact between the doctor and the patient, or it may send no time or duration information. Instead, the times and duration may be determined by the billing system 42 based on when messages are received from the receiver. [0038] Similarly, a piece of equipment 43 may also be fitted with a receive tag 44. The equipment may be diagnostic equipment, test equipment, support equipment or any other kind of equipment for which interactions with patients should be recorded. The receive tag on the equipment also receives the periodic transmissions from the patient's tx tag 36. This is shown by the solid line arrow. In a manner similar to the doctor, when the patient comes close to the equipment of vice versa, the rx tag on the equipment is able to receive the patient's signal. This information may be put together into a record that is then sent over a WiFi interface 45 to the WiFi AP 41. This is indicated by the dashed arrow from the equipment to the WiFi AP. Type II System

[0039] Figure 4 shows an example of a Type II system 50. In this system, the doctor 37 still has a receiver tag 38 that can act as a WiFi client 39. The equipment 43 has a similar

type of tag with a magnetic receive portion 44 and a WiFi client portion 45. As in Figure 3, these two receivers communicate through their communication interfaces 39, 45 to the antenna 40 of the WiFi AP 41 that is coupled to the billing system 42. [0040] In contrast to Figure 3, in Figure 4, the patient 35 is also equipped with a receive tag 46 similar to the one used by the doctor and the equipment. This receive tag also can act as a WiFi client 47 and communicate with the WiFi AP. The WiFi connections from the doctor, patient, and equipment to the antenna 40 of the WiFi AP are all shown in Figure 4 as dotted line arrows to the WiFi AP.

[0041] Figure 4 also shows a bed 48 that is equipped with an exciter tag 49. This tag sends out a magnetic proximity signal in the same way that the patient tag of Figure 3 sends out a signal. As explained above in the context of Figure 1, this signal may contain an encoded, modulated version of the transmitter's identification number. This signal is shown as the three solid line arrows emanating from the bed 48. When the respective receiver comes close enough, it receives the signal transmitted from the bed. This allows, the receive tags on the doctor, the equipment, and the patient to all report when they are near the bed. The billing system may then infer that if the patient and the doctor are both near the bed, then they are near each other. Similarly if the patient and the equipment are both near the bed, then they are near each other. This proximity may be used to infer a transaction for billing purposes, or to locate the doctor, the equipment and the patient based on knowledge of the location of the bed.

[0042] Stated another way, in Figure 4, a "tx tag" 49 or "exciter" with a magnetic transmitter is attached to a bed 48. This tag may be mains powered. It transmits magnetic signals that can be received by "rx tags" 46, 44, 38 attached to patients 35, equipment 43, and doctors 37. These magnetic signals may be designed so that the range from transmitter to receiver can be determined. Using WiFi, the tags then transmit their range from the tx tag to an AP 41. This AP is part of a computer network, connected to the hospital billing system 42. The signal from the rx tags may simply contain the identification number of the transmitter 49 and the identification number of the receiver. The AP or the billing system may be used to make any and all timing or billing determinations. Alternatively, the processor of the rx tag may be used to add a wide range of additional information to the message.

[0043] Such exciters can also be placed in close proximity to operating tables, and in recovery rooms etc, so that time spent in the pre-operating room, the operating room, and in the recovery room can be automatically recorded. Tags may be placed in, on, or underneath tables, beds, or gurneys, or mounted on walls that the patient will be positioned near.

[0044] Another use of the tags may be made if the doctors, patients and equipment all have tags that are WiFi clients. The tags may all be tracked using a normal WiFi positioning system. This would allow the general location of the doctor and patient to be known (for example in a ward or in a surgery. This may be added to the information about the time when they come together. By combining this information additional inferences may be made. For example, if a doctor and a patient come near each other in a surgery room, then that time may be recorded as the time at which surgery starts. This recorded time may then be used for billing and other purposes.

[0045] It may be noted that Type 1 systems do not require any infrastructure other than the WiFi network, which is often already installed in work places. However, the magnetic transmitter has to be powered from a battery which is small enough to be worn on a body. In order to conserve battery power, the magnetic transmitter may be limited in range. A 2m range may be a viable limitation for type of magnetic transmitter and receiver described here.

[0046] The type II system, described herein may have a disadvantage of requiring additional infrastructure. However, there is only one magnetic transmitters and it is attached to a larger piece of equipment (a bed in the example). As a result, it may use large batteries (for example D-cells) or may be mains powered. A gurney may require batteries so that it may be moved from room to room. Many other types of beds are coupled to mains power to power adjustment motors and monitoring equipment. It may be simple to connect the magnetic transmitter into the bed's primary power supply. A larger power supply may be used to provide much more power for the magnetic transmitter. Ranges of up to 10m can be obtained using the type of equipment described above. Magnetic Link

[0047] A magnetic link may be modeled as a simple connection between two things, a magnetic transmitter and a magnetic receiver. Transmitters and receivers may be combined into transceivers that both transmit and receive and many different transmitter, receivers, transceivers and other components may be operated together to build a working system.

[0048] Figure 5 shows a magnetic field B produced by a coil A, when driven by a sinusoidal current nl. The coil is roughly circular and lying in the x-z plane. Accordingly, the field for a clockwise current is in the general vertical direction of the y axis. This magnetic field is given by:

and

_ psin(fl)

Be ^~ r 5 — \ ^z)

where

_{p =} »o ^Ln > ^A > (3)

4π

And: μ _Q is the permeability of free space;

/ is the peak current through the transmit coil; ω is the angular frequency of the current in the coil; n, is the number of turns in the transmit coil; A ₁ is the area of the transmit coil;

r is the distance from the transmitter to the receiver, assumed to be » A, although the diagram has shown these as comparable to aid clarity; θ is the angle from the y axis as shown in Figure 5.

[0049] These fields add to produce a single magnetic field vector having magnitude B at an angle θ +ψ from the y axis where

And

.tan θ. .„ ψ = arctan( ) (5)

Voltage in a receiver coil

[0050] The AC (alternating current) voltage v induced in a receiver coil by a field B is given by:

v — n _r A _r — cos.9 (6) dt

Where: n _r is the number of turns on the receiver coil; A _r is the area of the receiver coil;

6 is the angle of the receiver coil relative to the B field.

[0051] Substituting the previous expression for B, and considering only the voltage magnitude V yields

Putting

β = -&- _{Ln A n A ω (8)}

4.π Then _{v =} s^ r i ₍₉₎

[0052] Figure 6 shows the relative orientation of the transmitter, the receiver, and the B field. For the transmitter on the y-axis, the angle of the B field relative to the receiver axis is given by:

& = θ+ ψ - φ (10)

So _{v =} βcos(θ + ψ - φ)

[0053] Having established the equations for a single transmit coil, a system with two transmit coils and one or two receive coils can be modeled. A two coil transmitter is shown in Figure 7 and two coil receiver is shown in Figure 8. The one- and two- receiver-coil systems are described separately below. Multi-Axis Transmitter

[0054] Consider the example of a system that has two orthogonal transmit coils, two orthogonal receive coils and is free to move in two axes (x,y). This illustrates the essential principles of three-axis, six coil (three transmitter, three receiver) systems, but the algebra is simpler.

[0055] For a system with two transmit coils operated at different times, four components of the magnetic field that can be considered are:

K - ² 4-cosø; B _xr = 2-4sin0 (12,13)

B _yθ = -£-smθ ; B _xθ = -£-cosθ (14,15)

[0056] Where B _yr represents the r-axis field induced by the y-axis coil etc. Simple algebra yields:

2£ ₊ 2£ _{+ 2} & _{+ 2} & = 3£L ( ₁ 6)

[0057] This result is very useful because the RHS (Right Hand Side) of the equation indicates that there is an aspect of the magnetic field magnitude B that is independent of the orientation of the transmitter and receiver coils but is dependent on the distance between them.

Two-axis transmitter, two-axis receiver systems

[0058] Based on the equations above, the voltage induced in two receivers by two transmitters can be described. For this example, it is assumed that the transmitters are operated alternately.

[0059] Writing V as the voltage induced by the p axis transmitter in the q axis coil

(where the axes are relative to the local coordinate systems shown in the diagram), four of the measurements that can be made are:

V _χy = kB _y cosθ ; V _yt = kB _y sin3 (17,18)

V _xy = kB _x sin & ; V _x , = kB _x cos 3 (19,20)

where k = n _r A _t ω (21)

These are all positive because V _xy etc represent magnitudes. Combining these:

Vi +Vi +Vi +V^ kHBJ! + B}) (22)

Since the r and θ axes are orthogonal

Bl = B _x ² _θ + Bl and B) = B _y ² _θ + B _y ² _r . (23,24)

So

f ^(j ; íβ =^ 6 (25)

And

^r = ^5k2p / ^S j ^V l ⁺ K ⁺ K +^ ⁽²⁶⁾

[0060] Indicating that the distance between the transmitter and receiver can be measured without knowledge of their individual (φ ) or relative (θ ) orientation.

Single Coil Receiver

[0061] In an environment where the transmitters and receivers have different cost constraints, it can be useful to use a three-axis transmitter and a single axis receiver. An example of such a situation is if the transmitters are used as signposts and are permanently installed, but receivers are mobile devices designed for small size, low power, and low cost. One example application is to use a three axis transmitter on the hospital bed of Figure 4 and single axis transmitters for the patient, doctor and mobile equipment.

[0062] Again, to simplify the algebra, the example considers a two-axis transmitter and a receiver coil confined to the plane of the transmitters. A similar result is expected with a three-axis transmitter and an unconstrained receiver coil. For a single coil, the voltages can be measured: y * + y * ₌ £*!£!(( 3 _cos * ø + i). _C os ² 3 + (3. sin ² θ + 1 ). sin ² S) = ⁵ ^f * (\ + 3cos ² θ cos ² & + 3sin ² θ sin ² 8)

(27)

Since

1 < (1 + 3 cos ² θ cos ² & + 3 sin ² θ sin ² 3) < 4 (28)

[0063] This equation provides the value of r to a value within a multiple of \Jϊ , or

± 12% of the correct value. This is adequate for many purposes.

[0064] For a system with three transmitter axes and one receiver axis, although the signal strength varies with orientation this only affects the range by ± 12%. Therefore, if the distance between the transmitter and receiver is small enough, then communication is guaranteed.

Single Transmitter, Single Receiver Systems

[0065] Note that workable single-transmit coil, single receive-coil systems can be made workable despite their sensitivity to the relative orientation of the coils. The signal strength received by a magnetic coil includes a term that varies with the cosinusoid of the angle between them, so the measured range between two coils might be greater than it actually is. However, the actual range will never be smaller than the measured value.

People tend to move around, which may be used to change the orientation of the transmitter and the receiver. As a result, by making a set of range measurements as either the transmitter or receiver is moved and taking the smallest one of the measured (range) values, then a more accuracy range value can be obtained.

[0066] A final helpful property of magnetic links is that the signal strength drops with the cube of the distance, or conversely, the range estimate varies with the cube-root of signal strength. This tends to flatten the error function, as shown in the graph of Figure 9.

Figure 9 shows the error in the normalized estimated range on the vertical axis and the corresponding angle between the two coils for that error on the horizontal axis. If the angle between the transmitter and the receiver were to be chosen at random, then in 60% of the cases (36 degrees to 144 degrees) the range error would be less than 20%, and in

75% of the cases the error is less than 40%.

Two axis transmitter, two axis receiver systems

[0067] A disadvantage of single-axis transmitter, single-axis receiver systems is that the link has infinitely deep nulls, and will not work at all if the receiver coil is orthogonal to the B field. This is shown in Figure 9 at the areas near 0 degrees and 180 degrees. A system with a two-axis transmitter and a two-axis receiver does not have infinitely deep nulls and so, when the transmitter and receiver are close enough, they are almost certain to be able to communicate. Such coil arrangements are suitable for systems where

identical tags must identify each other and determine the range. The advantage of a two- axis tag over a three-axis tag is simply one of cost. Example Design

[0068] Figure 10 shows an example of a three-axis tag 100 in a low-profile design that may be worn by a person or placed unobtrusively on movable equipment or walls. Such a design may be used to build either the transmitter of Figure 1, the receiver of Figure 2 or a transceiver that performs both functions using a single group of three orthogonal coils.

[0069] In Figure 10, two magnetic wire coils 101, 102 are made using standard commercially available devices intended for 125KHz magnetic links. These are soldered to a PCB 104 in directions perpendicular to each other. A third coil 104 is made using wire traces, such as copper or aluminum that are written onto the PCB. The axes of the two coils that are soldered to the PCB are both parallel to the surface of the PCB, but the PCB trace coil's main axis is perpendicular to the circuit board. [0070] Figure 10 also shows a coin cell battery 105 and an electronics block 106 that would be powered by the coin cell. The electronics block may take a variety of different forms, including those shown in Figures 1 and 2. The entire tag may be mounted onto a credit card sized substrate and enclosed in a plastic housing (not shown). The tag may be sealed by the housing against moisture and other contaminants.

Example parameters for a working tag are as follows:

• PCB coil aperture up to 1600 cm ²

• Typical PCB-mount coil aperture = 200 cm ² -turns

• Transmitter Q = 10

• Receiver Q = 10

• Carrier frequency = 125KHz

• Current drive to transmitter = 8mA

• Receiver sensitivity = 50μV

• Receiver signal amplitude measurement resolution = IdB

• Protocol per ISO 24730-2 (draft)

Power Conservation

[0071] For many applications, power conservation may be important. One example is shown in Figure 3, a type I system in which two battery-powered tags, one being worn by a doctor, and one being worn by a patient communicate. However, power conservation

techniques may be applied to a wide range of different implementations. To connect, the receiver tag needs to be able to recognize the presence of the transmitter tag. Although it would be possible for the transmitter to transmit continuously, or for the receiver to listen for transmissions continuously, this would maximize the power consumption of the device. A typical transmitter might consume 3OmW, and a receiver might consume 300μW. If either one were operated continuously, then it may quickly discharge its battery. The battery consumption for the transmitter is particularly troublesome. Short battery lives for some tags may make an entire system impractical. [0072] In many applications, the system does not need to instantaneously recognize the presence of a nearby tag. There may be some response time R which is an acceptable delay between when the tags are brought into proximity and when the tags recognize their proximity. By enabling the transmitter and the receiver for short intervals, but with suitable timing, it is possible for the receiver to receive the message from the transmitter without either one having to run continuously.

[0073] Figure 11 shows an example of how this might be done. In Figure 1 1, the upper line shows a transmitter duty cycle and the lower line shows a receiver duty cycle. The horizontal axis is a time line and when the duty cycle line is low, the receiver or transmitter is not receiving or transmitting. When the duty cycle line is high, then the receiver or transmitter is receiving or transmitting. The duty cycle line may also be considered as an enable signal that is provided by a timer to the receiver or transmitter circuitry.

[0074] As shown by the enable signals, the transmitter is enabled for some short amount of time every time period R. R represents a response time for the system. For example, if the system must recognize proximity within a time R, then the transmitter is enabled every R seconds. In the hospital examples described above time is billed for in terms of several minutes. There may be a minimum five minute charge. When five minutes is the most accurate unit of time required, a response time of tens of seconds may be tolerated. In a manufacturing context, a response time of milliseconds may be required. [0075] The receiver on the other hand is enabled for a time duration of r with time period T to detect if a transmitter is present. The time r may be as short as possible, for example, just long enough for the receiver to recognize the presence of the transmitter.

[0076] So to ensure that the receiver receives a signal from the transmitter whenever it transmits, the transmitter is enabled for a period T+r. This is shown in Figure 11 as the duration or width of the enable signal for the transmitter. As T increases, the power consumption of the transmitter increases, and the power consumption of the receiver decreases. In the example of Figure 4 in which the bed is the transmitter and the other are all receivers, if the bed is coupled to the power mains for power, a large T may be used.

[0077] When both tags have a similar battery and a similar expectation of battery life as suggested in Figure 3, the battery life of the tags may be maximized when the power drawn by each tag is the same. Figure 12 shows a graph of transmitter operation or enabled time on the horizontal axis against power consumption on the vertical axis. The graph is based on a particular receiver and transmitter tag on a scale similar to that shown in Figure 10. For this example, the receiver responds in 15 seconds, the transmitter consumes 3OmW, the receiver consumes 350μW, and the receiver must be enabled for 3ms to recognize a transmitter. The power consumed by each tag will be approximately 5OuW, and this is reached when the transmitter wakes for 23ms each 15 seconds. The dotted line for average power consumption shows that there is a minimum for total average power consumption where the solid line for average transmitter power and the dashed and dotted line for average receiver power cross. Other Considerations

[0078] The approaches described herein may be used in any situation in which the association of two items leads to useful information. For example, in a motor vehicle repair facility, tags could be attached to cars to be repaired, and worn by mechanics. Since car repair bill often include a component for a mechanic's time and the rate for different mechanics may be different, the system can detect which mechanics were near the car and for how long. The repair bill may use these measurements to include a component for the mechanic's time, as indicated by the time that the mechanic and the car are in close proximity.

[0079] A lesser or more equipped transmitter or receiver than the examples described above may be preferred for certain implementations. Therefore, the configuration of the exemplary tags 10, 20 and systems 30, 50 will vary from implementation to

implementation depending upon numerous factors, such as price constraints, performance requirements, technological improvements, or other circumstances. The particular nature of any attached devices may be adapted to the intended use of the device. Any one or more of the devices, interfaces, or interconnects may be eliminated from this system and others may be added. For example, a variety of different connections to the access point may be provided based on different wired or wireless protocols. In addition, the particular configuration and frequencies of the magnetic coils may be adapted to suit different applications.

[0080] While embodiments of the invention have been described in the context of identifying doctor and equipment transactions in a hospital, the approaches and techniques described here may be applied to a wide variety of different contexts in which significant data may be obtained by determining when one thing comes close to another. In addition, embodiments of the invention may be applied to professional or consumer equipment and to inventory systems as well as service provider systems. [0081] In the description above, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some of these specific details. In other instances, well-known structures and devices are shown in block diagram form.

[0082] The present invention may include various steps. The steps of the present invention may be performed by hardware components, such as those shown in the Figures, or may be embodied in machine-executable instructions, which may be used to cause general-purpose or special-purpose processor or logic circuits programmed with the instructions to perform the steps. Alternatively, the steps may be performed by a combination of hardware and software.

[0083] The present invention may be provided as a computer program product which may include a machine-readable medium having stored thereon instructions which may be used to program an agent or a computer system to perform a process according to the present invention. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnet or optical cards, flash memory, or other type of machine-

readable media suitable for storing electronic instructions. Moreover, the present invention may also be downloaded as a computer program product, wherein the program may be transferred from a remote computer to a requesting computer by way of data signals embodied in a carrier wave or other propagation medium via a communication link (e.g., a modem or network connection).

[0084] Many of the methods and apparatus are described in their most basic form but steps may be added to or deleted from any of the methods and components may be added or subtracted from any of the described apparatus without departing from the basic scope of the present invention. It will be apparent to those skilled in the art that many further modifications and adaptations may be made. The particular embodiments are not provided to limit the invention but to illustrate it. The scope of the present invention is not to be determined by the specific examples provided above but only by the claims below.