WO2003098534A1

WO2003098534A1 - Data tagging system using capacitive coupling

Info

Publication number: WO2003098534A1
Application number: PCT/GB2003/002191
Authority: WO
Inventors: Glen Pitt-Pladdy; Peter Symons
Original assignee: Innovision Research & Technology Plc
Priority date: 2002-05-21
Filing date: 2003-05-21
Publication date: 2003-11-27
Also published as: GB0211662D0; AU2003234012A1

Abstract

A data communication system has a base unit (122a) having a first capacitive coupler (103a), a signal supplier (30) for supplying a time-varying signal and an inductor (32) for inductively connecting the signal supplier to the first capacitive coupler (103a); and a subsidiary unit (129a) storing data, the subsidiary unit having a second capacitive coupler (125a) for coupling capacitively to the first capacitive coupler, a power supplier (34) for deriving a power supply from a signal coupled to the second capacitive coupler, and an output provider (35, 36, 37) for outputting stored data when the subsidiary unit is powered.

Description

DATA TAGGING SYSTEM

This invention relates to the field of data tagging systems .

Such a system comprises a tagged item provided with an electronic passive data storage device, more conveniently known as a 'tag', which stores data electronically in a machine-readable form and a reader. By passive, it is meant that the tags do not have a power source. Rather, the reader supplies a signal from which the tag derives electrical power when the tag is in range of the reader. Thus, the tag does not need to be provided with a power source such a battery. Upon powering up, the tag outputs the stored data to the reader.

Inductive and capacitive methods of interrogating the tag are known. With inductive methods, a high frequency reader signal is supplied by excitation of a coil which electromagnetically couples to a resonant tuned circuit of the tag when the tag is in range. The resonant tuned circuit consists of a separate coil and a capacitor. A disadvantage of this method is that the coils increase the cost of the system and that different reader signals may interfere.

ith capacitive methods, a time-varying reader signal is coupled to the tag when respective coupling electrodes of the tag and reader are close enough to provide sufficient capacitive coupling. A disadvantage of this method is that, in practice, the range is not as great as with the inductive method. However, there are applications for which the cost of an inductive system is too high and may be undesirable because the high frequency reader signal may interfere with other equipment, and where conventional capacitive systems cannot be used because the range is not sufficient.

According to a first aspect of the present invention, there is provided a data communication apparatus, comprising: a base unit having first capacitive coupling means, a signal supplying means for supplying a varying, typically oscillating, signal and inductor means for inductively connecting the signal supplying means to the first capacitive coupling means; and a subsidiary unit having second capacitive coupling means for coupling capacitively to the first capacitive coupling means, power supply means for deriving a power supply from a signal coupled to the second capacitive coupling means, and output means for outputting stored data when the subsidiary unit is powered.

Preferably, the subsidiary unit comprises a memory for storing the data.

In operation of the data communication apparatus, the inductor means at least partially resonates with the capacitance provided by the first and second capacitive coupling means, thus to some extent tuning out that capacitance so reducing the effective impedance thus increasing the signal strength received by the subsidiary unit at a given distance. This enables the maximum range (between the base unit and the subsidiary unit) over which (for a given reader signal output power) sufficient power can be coupled to the power supply means to be increased, so allowing the subsidiary unit to be positioned further away from the base unit.

The data communication system may be included in a drug delivery system comprising a medical drip bag and an infusion controller, with the infusion controller incorporating the base unit (reader) and the drip bag incorporating the subsidiary unit (tag). In this case, the tag may carry data specifying the minimum and maximum allowable rates at which the contents of the drip bag may be pumped into a patient. This provides the advantage that the infusion controller may be arranged to raise an alarm if an operator attempts to specify an inappropriate rate of infusion.

According to another aspect of the present invention, there is provided a tagged container for containing an electrically conducting fluid, for use with a data communication system having a reader (having first capacitive coupling means, a signal supplying means for supplying a varying, typically oscillating, signal and inductor means for inductively connecting the signal supplying means to the first capacitive coupling means), the tag of the container comprising: second capacitive coupling means for coupling capacitively to the first capacitive coupling means, power supply means for deriving a power supply from a signal coupled to the second capacitive coupling means, and output means for outputting stored data when the tag is powered, wherein the second capacitive coupling means comprises an electrode and wherein the electrode extends along the container so that the electrode is capacitively coupled to the electrically conducting fluid when the container contains such a fluid.

According to another aspect of the invention, there is provided a tagged container for containing an electrically conducting fluid, for use with a data communication system having a reader (having first capacitive coupling means, a signal supplying means for supplying a varying, typically oscillating, signal and inductor means for inductively connecting the signal supplying means to the first capacitive coupling means), the tag comprising: second capacitive coupling means for coupling capacitively to the first capacitive coupling means, power supply means for deriving a power supply from a signal coupled to the second capacitive coupling means and output means for outputting stored data when the tag is powered, wherein the second electrical coupling means comprises an electrode and wherein the electrode extends into the container so that the electrode is ohmically connected to the electrically conducting fluid when the container contains such a fluid.

The tagged container may be a medical drip bag containing a liquid such as, for example, saline solution, blood plasma or a medicament, or may be, for example, an printer cartridge or container containing printing liquid such as ink.

According to another aspect of the invention, there is provided a tag for use with a data communication system having a reader (having first capacitive coupling means, a signal supplying means for supplying a varying, typically oscillating, signal and inductor means for inductively connecting the signal supplying means to the first capacitive coupling means), the tag comprising: second capacitive coupling means for capacitively coupling to the first capacitive coupling means, power supply means for deriving a power supply from a signal coupled to the second capacitive coupling means and output means for outputting stored data when the tag is powered, wherein the second capacitive coupling means comprises an elongate antenna.

The present invention also provides an item carrying such a tag.

Embodiments of the invention will now be described by way of example only, with reference to the following drawings in which:

Fig 1 shows a diagrammatic view of a drug delivery system comprising two medical drip bags connected to an infusion controller;

Fig 2 shows a block schematic diagram to illustrate electrical components of the drug delivery system; Fig 3 shows a circuit diagram illustrating the coupling of the electrical components of the drug delivery system;

Fig 4 shows a block schematic diagram to illustrate electrical components of an alternative embodiment of the drug delivery system;

Fig 5 shows a circuit diagram illustrating the electrical components of the alternative embodiment;

Fig 6 shows a circuit diagram illustrating the electrical coupling between major parts of the alternative embodiment;

Fig 7 shows a diagrammatic perspective view of a drug delivery system comprising a medical drip bag connected to an infusion controller;

Fig 8 shows a block schematic diagram to illustrate electrical components of the drug delivery system;

Fig 9 shows a circuit diagram illustrating the coupling of the electrical components of the drug delivery system;

Fig 10 shows a circuit diagram similar to Figure 3 but with some modification of the electrical components;

Fig 11 is a diagrammatic part-cut-away view of part of a drip bag embodying the invention;

Fig 12 shows a functional block diagram of a tag to which data may be written; and Figure 13 shows a circuit diagram illustrating the coupling of the electrical components of another embodiment of the drug delivery system.

Figure 1 shows a frontal elevation view of drug delivery system 100 embodying the invention.

An infusion controller 101 is used to administer medicaments 106 contained within two drip bags 104a, 104b to patients. The infusion controller 101 has two pumps (not shown) to pump the medicaments 106 at respective predetermined rates. The medicaments pass from the drip bags 104 through respective drip chambers 107, then through respective drip tubes 108 into the infusion controller 101. The infusion controller 101 may be used to control the rate at which two different medicaments are administered to a single patient or may be used to control the rate at which identical medicaments are administered to different patients.

The infusion controller 101 is mounted to a stand 102, a lower part 102b of which holds the infusion controller 101 off the floor at a height convenient for an operator (not shown) such as a nurse. (The means of attachment between the infusion controller 101 and the stand 102 is not shown.) The top of the stand 102 has two hooks 103 from which suspended the drip bags 104 are suspended. The top of each drip bag 104 is provided with a mounting hole 105 which is shown engaged with the appropriate hook 103.

The operator programs the infusion controller 101 to administer the medicaments 106 at the appropriate rates. A keypad 5 allows the operator to enter commands and data into the infusion controller 101 whilst a display 6 allows the operator to check that the commands and data have been correctly entered, and to monitor the operation of the infusion controller 101.

The hooks 103a, 103b are attached to an upper portion 102a of the stand via respective readers 122a, 122b. The readers 122 allow the infusion controller 101 to read respective drip bag information associated with each of the drip bags 104. In this embodiment, the drip bag information identifies the medicament 106 within the drip bag 104.

The drip bag information is stored in tags 109. Each of the drip bags 104a, 104b is provided with a respective tag 109a, 109b. In this embodiment, each tag 109 is mounted on its respective drip bag 104 near the mounting hole 105.

To read the drip bag information from the tags 109, the infusion controller 101 activates the readers 122, causing them each to apply (with respect to the electrical potential of the stand 102) a time-varying electrical signal to their respective hook 103a, 103b. These electrical signals are capacitively coupled from the hooks 103 to the tags 109a, 109b. The electrical signals provide a power supply to the tags 109. In response, each of the tags 109 modulates the applied electrical signal in accordance with the drip bag information stored within it. The readers 122 demodulate the respective tag modulations and thus recover the stored drip bag information from the tags 109.

Thus the readers 122 allow the infusion controller 101 to verify that the "correct" combination of drip bags 104 has been placed onto the hooks 103 (e.g. one drip bag 104 containing blood plasma, the other containing morphine solution), and to verify that the drip bags have been placed on the hooks 103 with the "correct" permutation (e«g» blood plasma on the left hook 103a, morphine on the right hook 103b) . By correct, it is meant that the combination, and the permutation, is consistent with the program entered into the infusion controller 101 by the operator (via the keypad 5).

However, as well as verifying that the drip bags 104 have been correctly placed onto the hooks 103, the infusion controller 101 also needs to be able to verify the fluid path from the drip bags 104, through the infusion controller 101 to the patient(s). For example, the infusion controller 101 may be used to administer blood plasma to one patient and morphine solution to another patient. It is, of course, vital that the respective medicaments are delivered at the required rates. Moreover, if the drip bags are intended to supply medicaments to different patients, it is imperative that the correct medicament is supplied to the correct patient.

Therefore, each of the readers 122 incorporates an ultrasonic transducer (not shown in Figure 1) for vibrating a respective one of the hooks 103a, 103b. When the transducer of, say, the reader 122a is activated, vibrations from the left hook 103a pass along the drip bag 104a, through the drip chamber 107a, along the drip tube 108a and into the infusion controller 101. Inside the infusion controller 101, two acoustic-to-electrical signal converters in the form, in this example, of microphones (not shown in Figure 1) are used to verify the fluid path. This will now be explained in more detail with reference to Figure 2.

Figure 2 shows a part diagrammatic, part representational view of the drug delivery system 100 of Figure 1. For clarity, only the uppermost and lowermost portions of the drip bags 104 have been shown. For brevity, the explanation of Figure 2 will mainly be in terms of the parts on the left side (suffixed 'a') of the infusion controller 101; the operation of the parts on the right- hand side (suffixed 'b') is identical and so will not generally be discussed.

The infusion controller 101 has two pumps 121a, 121b, for pumping the medicaments from the drip bags 104a, 104b, respectively. (In this embodiment the pumps are peristaltic pumps.) Each pump 121 is provided, at an upstream end, with a respective microphone 201a, 201b.

To verify that the fluid path from the drip bag 104a passes through the pump 121a, the ultrasonic transducer (not shown) in the reader 122a is activated, causing ultrasonic oscillations to propagate along the drip tube 108a. If the drip tube 108a has been (correctly) routed then the microphone 201a will detect the ultrasonic oscillations. On the other hand, if ultrasonic oscillations are detected at the microphone 201b then the drip tube has been (incorrectly) routed to the pump 121b. Alternatively, if ultrasonic oscillations are not detected at microphone 201a or at microphone 201b then either (i) the drip tube 108a has not been routed to either of the pumps pump 121 or (ii) there is a break somewhere along the fluid path between the hook 3a and the microphone 201a.

Note that it is sufficient to check the routing of the drip tubes 108 (to avoid a transposition) only after an empty drip bag 104 has been replaced with a full drip bag 104. However, it is preferred that the contents of the drip bags 104 and the routing of the drip tubes 108 is periodically verified so that an alarm can be raised by the infusion controller 101 if a transposition should inadvertently arise during the time interval between replacement of the drip bags 104.

The infusion controller 101 comprises a central processing unit (CPU) 120 which executes a program stored in ROM (not shown) and which temporarily stores data in RAM (not shown). The CPU 120 reads operator inputs from the keypad 5 and causes the display 6 to display information for viewing by the operator. The CPU 120 is connected to the reader 122a and, upon activation by the CPU 120, the reader 122a energises the hook 103a with a time-varying electrical signal. This electrical signal is capacitively coupled to the tag 109a.

The tag 109a comprises an integrated circuit (IC) 129a to which are connected an electrode 125a and an antenna 126a. The electrode 125a passes approximately 225° around the perimeter of the hole 105a to ensure that, when the drip bag 104a is suspended from the hook 103a, the distance between the electrode 125a and the hook 103a is minimised (thus maximising the electrical coupling between the hook 103a and the electrode 125a). In this embodiment the separation between the hook 103a and the electrode 126a is about 0.5mm, giving a coupling capacitance between them of about IpF. The antenna 126a is capacitively coupled to the upper part 102a of the stand. In this embodiment the distance between the antenna 126a and the stand 102a is approximately 10cm but may range from about 5cm to 20cm in other embodiments. In this embodiment, the antenna 126a comprises a length in the range of 15mm to 25mm of 0.25mm diameter wire.

In operation, the reader 122a energises the hook 103a with respect to electrical potential of the stand 102. Thus the capacitive coupling of the antenna 126a to the stand 102 completes an electrical loop (not shown in Figure 2) from the reader 122a to the hook 103a, from the hook 103a to the electrode 125a, from the electrode 125a through the tag IC 129a, from the tag IC 129a to the antenna 126a, from the antenna 126a to the stand 102a and from the stand 102a back to the reader 122a.

By means of this electrical loop, the reader 122a couples electrical power to the tag IC 129a. In response, the tag IC 129a modulates the electrical signal applied by the reader 122a. This modulation is demodulated by the reader 122a. The CPU 120 reads the demodulated drip bag information from the reader 122a, thus enabling the CPU 120 to identify the medicament 106a within a drip bag hanging from the hook 103a.

Amplifiers 202a, 202b amplify the signals from their respective microphones 201 and pass the amplified signals to the CPU 120.

Figure 3 shows a circuit diagram illustrating the electrical components of the reader 122a and of the tag IC 129a and of their coupling.

The reader 122a comprises an oscillator 30 which produces a signal of 5V (volts) pk-pk (peak to peak) at a frequency of 13.56MHz. The oscillator is connected via an impedance 31, which has a value of lOkΩ (kilo-ohms), to a demodulator 33 and to one terminal of an inductor 32. The other terminal of the inductor 32 is connected to the hook 103a. The demodulator 33 is used to supply a demodulated signal to the CPU 120. The inductor 32 has a value of 280μH (micro-Henries). A piezo-electric transducer 135 is electrically connected to the CPU 120 and mechanically connects the hook 103a to the stand 102a. When activated by the CPU 120, the transducer 135 produces ultrasonic vibrations which are mechanically coupled to the drip bag 104a. (Of course, there is a corresponding transducer (not shown) in the other reader 122b. )

The tag IC 129a comprises a rectifier 34 for deriving, from the oscillator signal, a power supply for the tag IC 129a, a clock signal generator 35 and a data & logic unit 36 which stores, in this example, 104 bits of data representing the drip bag information. The data may relate to one or more of the following: the date of manufacture of the drip bag, the lot code, the date of expiry of the contents, minimum and maximum flow rates that are suitable for the product and may also provide a code or a string of characters identifying the contents of the drip bag 104a.

The data output of the data & logic unit 36 is connected to the gate of a MOSFET 37. The source of the MOSFET 37 is connected to the electrode 25, the drain of the MOSFET 37 is connected to one end of an impedance 38, the other end of which is connected to the electrode 26.

When the tag IC 129a is coupled to the reader 122a, the hook 103a and the electrode 125a form a capacitor Cl while the antenna 126a and stand 102a form a capacitor C2.

Thus a complete electrical loop is formed, consisting, in sequence, of the oscillator 30, the impedance 31, the inductor 32, the capacitance Cl, the tag IC 129a, and the capacitance C2 which is connected back to the oscillator 30.

The operation of the reader 122a will now be described with reference to Fig 3. When the reader 122a is activated by the CPU 120, the AC voltage from the oscillator 30 is supplied through the impedance 31 to the inductor 32 and the demodulator 33 and, assuming the drip bag 104a is supported by the hook 103a, the AC voltage from the inductor 32 is coupled to the tag IC 129a through capacitors Cl and C2 and then back to the oscillator 30.

The rectifier 34 rectifies the AC voltage coupled across the electrode 125a and antenna 126a and provides a DC voltage power supply (typically at about 1.2V to 1.5V and at 20μA to 50μA) to the clock generator 35 and to the data & logic unit 36. The clock generator 35 provides a clocking signal to the data & logic unit 36 which then outputs a pulse train representing the data bits stored within the data & logic unit 36. The output of the data & logic unit 36 drives the gate of the MOSFET 37, turning the MOSFET 37 on and off in response to the pulse train. As the MOSFET 37 is turned on and off, the impedance 38 is connected across the electrodes 25, 26 and then disconnected.

Whenever the impedance 38 is connected across the electrode 125a and antenna 126a, the impedance of the tag IC 129a is reduced and this reduction reduces the impedance between the hook 103a and the stand 102a, thereby decreasing the voltage signal to the demodulator 33. The demodulator 33 detects and amplifies the reductions and passes the reconstructed pulse train representing the data read out from the data & logic unit 36 to the CPU 120.

The demodulator 33 thus detects changes in the amplitude of the signal resulting from the tag IC 129a modulating its own impedance in accordance with the data stored therein. The impedance 31 provides a degree of isolation between the oscillator 30 and the demodulator 33. Without the impedance 31, changes in the amplitude resulting from the tag IC 129a would be reduced by the (low) source impedance of the oscillator 30.

As can be seen from Fig 3, the capacitors Cl and C2 are in series and can be regarded as a single capacitor of value C_eff. The value of inductor 32 is chosen so that at 13.56MHz it partially resonates with C_eff. The actual resonance frequency of C_eff and inductor 32 will depend on the value of C_eff (which of course, for given electrode and plate dimensions, depends on the separation between the constituent elements of Cland C2 ) , the value of the impedance 31 and on the effective impedance of the tag IC 129a (which is time-varying as a result of the data being modulated by the tag IC 129a) .

The effect of the partial resonance is that the voltage at the hook 103a is "stepped up" by the inductor 32 so that instead of the 5V pk-pk voltage of the oscillator appearing at the hook 103a, a voltage of as much as 100V pk-pk appears .

The stepped up voltage dramatically increases the AC current flowing between the reader 122a and the tag IC 129a, thus improving the ability of the reader 122a to couple sufficient power to the tag IC 129a though C_eff (which is of the order of 0.5pF). This is because, of course, I=V/Z where Z is the loop impedance at 13.56MHz. An alternative way of considering the effect of the partial resonance is that the impedance of the loop is reduced, thus improving the ability of the reader 122a to couple sufficient power to the tag IC 129a.

Of course, the actual voltage will depend on the degree of resonance: the nearer the resonant frequency of C_eff and inductor 32 to 13.56MHz, the greater the stepping up effect, and the greater the reduction in the effective impedance of C_eff.

The electrode 125a and the antenna 126a of the tag IC 129a are of elongate form and this causes them to act, to some extent, as antennae, thereby increasing the coupling between the tag IC 129a and the reader 122a. Although the antenna 126a is electrically 'short' (i.e. its length is only a small fraction of a wavelength at the frequency of operation), the inventors have, surprisingly, found that the use of an antenna 126a increases the range over which communication between the tag IC 129a and reader 122a can successfully be performed. In particular, the inventors have found that the elongate nature of the antenna 126a has a greater influence on the achievable communication range than the area (and hence capacitance) of an otherwise comparable capacitive plate. For example, in trials they found that the antenna 126a gave better performance than a capacitive plate several mm² in area, even though such a plate has more capacitance than the 15mm to 25mm long antenna 126a. Furthermore, the inventors have noticed that, at high frequencies, the impedance of the antenna 126a is lower than would be expected from consideration of its capacitance. For example, when the drip bag 104a is suspended from the hook 103a, the antenna 126a has a capacitance of about IpF. The impedance of the this capacitance would be expected to decrease with frequency, as X_c=l/2πfC. However, the impedance has been measured as substantially lower than that expected from that formula.

Even though in many applications the antenna 126a would not be regarded as an antenna due to its high impedance, in this embodiment the impedance between the reader 122a and tag IC 129a is also high (typically of the order of lOkΩ to lOOkΩ and predominantly capacitively reactive, due to the low coupling capacitances between them, with the effective capacitance C_eff typically about 0.5pF) and thus the antenna 126a contributes significantly to the transfer of electrical energy between the reader 122a and tag IC 129a. By contrast, conventional antennas are typically designed to be coupled to an electrical system having an impedance of 50Ω and so have dimensions corresponding to the wavelength at their intended frequency of operation.

Conventional capacitive systems generally use capacitor plates having an area of several square centimetres, separated by about 0.1mm (millimetre), giving a transfer capacitance between the transponder and the tag of the order of 20pF. At the frequencies commonly employed by such systems, for example 13.56MHz, such transfer capacitances give an impedance between the transponder and the tag of the order of 500Ω. With such relatively low impedances, it is relatively easy for such systems to couple sufficient electrical power to the tag and to reliably receive data from the tag. In the present embodiment, the transfer impedance is much greater and so the antenna 126a helps to electrically couple the reader 122a and tag IC 129a.

The actual AC current flowing through the electrode 125a and antenna 126a may be of the order of 1mA but, as shown by C_byPass in Figure 3, most of this current bypasses the tag IC 129a. C_bypass represents the 'bypass' capacitance of the tag IC 129a. The term bypass capacitance in this context is the inherent capacitance of the tag IC 129a between the electrode 125a and antenna 126a. Bypass capacitance allows the coupled electrical signal to bypass the tag IC 129a instead of being rectified by the rectifier 34 for use within the tag IC 129a. The bypass capacitance can be minimised by careful design and semiconductor processing of the tag IC 129a to minimise capacitances due to the substrate (not shown) of the tag IC 129a and due to static protection diodes (not shown). Another way of minimising the bypass capacitance is to select an electrode configuration for the tag IC 129 which minimises the bypass capacitance. For example, the tag IC 129 may be flip-chip mounted or may be bonded onto a PCB instead of being packaged in, say, a dual-in-line (DIL) package.

In the drug delivery system 100, the transducers 135 in the readers 122a, 122b were alternately stimulated and the microphones 201 were monitored, in order to verify the fluid path from the drip bags 104 to the pumps 121. This had the result that only one fluid path could be monitored at any given time. In an alternative embodiment, the transducers 135 of the readers 122a, 122b may be driven with respective ultrasonic frequencies. In this alternative embodiment, a fluid path is deemed correct if the appropriate microphone 201 detects a signal at the correct frequency. If the wrong frequency is detected at a microphone 201 then the wrong drip line 108 has been routed to the pump 121 associated with that microphone; if no signal is detected at that microphone then either no drip line 108 has been routed to the pump 121 associated with that microphone, or there is a break in the fluid path. The use of different frequencies allows the two fluid paths to be monitored simultaneously. Two or more fluid paths may also be monitored simultaneously (even if the same ultrasonic frequency is used for each fluid path) if the electrical signals to the transducers 135 are suitably modulated or encoded so that the microphones 201 can distinguish the fluid paths.

In the drug delivery system 100, the transducers 135 and microphones 201 were used to emit and detect, respectively, ultrasonic acoustic signals. Of course, acoustic signals at a lower, audible, frequency may be used (with appropriate modification of the transducers 135 and microphones 201, if required). Ultrasonic frequencies may be preferred in many applications as they will be inaudible to human patients. Infrasonic frequencies may also be used.

Other embodiments of the infusion controller 101 may cater for four or more drip bags 104; in such embodiments it becomes increasingly important to verify not only that the correct drip bags 104 have been loaded onto the hooks 103 but also that the drip bags 104 have been loaded onto the correct hooks 103.

Figure 4 shows a part diagrammatic, part representational view of a drug delivery system 200. The drug delivery system 200 has many features in common with the drug delivery system 100 of Figure 1. However, whereas the drug delivery system 100 used acoustic signals to verify the fluid path from the drip bags 104 to the pumps 121, the drug delivery system uses electrical signals to verify the fluid paths.

The drug delivery system 200 has an infusion controller 301 which is similar to the infusion controller 101 except that the microphones 201 have been replaced with electrical plates 251a, 251b which capacitively couple to the drip tubes 108a, 108b, respectively. Also, the readers 122 have been replaced with readers 222 which omit the transducers 135.

In operation, the readers 222 interrogate tags 209a, 209b mounted on the drip bags 104a, 104b, respectively. The tags 209 are similar to the tags 109 except that the antennas 126 have been replaced with strips of metal foil 226a, 226b, respectively. (As shown by Figure 5, the electrical interaction between the reader 222a and the tag IC 129a is unchanged.)

The strips 226 extend from the hooks 105 substantially along the entire length of their respective drip bags 104. Thus, whatever the level of medicament within the drip bags, there will always be some degree of capacitive coupling between the strips 226 and the medicaments 106. The medicaments (which may be for example saline solution, blood plasma or a drug solution) comprise an electrolyte (and therefore are electrically conducting) .

The readers 222 are able to read the drip bag information stored in the tag ICs 129 as was described above. However, as well as being capacitively coupled to the stand 102a, the strips 226 are capacitively coupled to the medicaments within their respective drip bags 104. The AC signal from the oscillators 30 passes down to the drip chambers 107. In this embodiment, the drip chambers 107 are provided with electrical bypasses 207a, 207b, respectively. The bypasses 207 capacitively couple to the medicament 106 both upstream and downstream of the drip chambers 107, thus ensuring continuity for the AC signal from the drip bags 104 down to the drip tubes 108, and thus into the infusion controller 301.

Inside the infusion controller 301, the AC signal capacitively couples to the plates 251. The signal from the plates 251a, 251b is amplified by amplifiers 252a, 252b, respectively, and passed to the CPU 120.

To verify the liquid paths of the medicaments from the drip bags 104 to the patient or patients, the readers 222 are alternately energised. For example, to verify the liquid path of the left hand drip tube 108a, only the left hand reader 222a is energised. If the drip bags 104 and drip tubes 108 have been correctly installed then, when the left hand reader 222a is energised, only the left hand plate 251a will detect an AC signal. If the right hand plate 251b detects an AC signal then the drip tube 108a has been incorrectly routed to the right hand pump 121b; if no signal is detected then either there is a break in the liquid path or the drip tube 108a has not been routed to the pump 121a. (If both plates 251 detect an AC signal then two drip bags 104 may have been suspended from the hook 103a, or there may be a malfunction inside the infusion controller 301.)

Of course, instead of using electrical bypasses 207 to provide capacitive continuity through the drip chambers 107, in an alternative embodiment the drip chambers 107 may be formed from an electrically conducive material that provides ohmic continuity. For example, electrically conductive plastics materials may be used.

Figure 6 shows a circuit diagram illustrating the electrical coupling between the reader 222a, the tag IC 129a and the plate 252a.

The reader 222a energises the hook 103a with a time- varying AC electrical signal with respect to the electrical potential of the stand 102a. In this embodiment, the stand 102a is ohmically connected to earth potential.

The AC signal from the hook 103a is capacitively coupled to the electrode 125a through a capacitance which has been designated Cl. The AC signal passes through the tag IC 129a (including via the bypass capacitance C_bypass which for clarity has not been shown in this Figure) . The AC signal then passes to the strip 226a where it couples to the ambient ground through a capacitance designated Ca_mbi_ent/ ^{to tne} stand 102a through a capacitance designated C_stand, and to the medicament 106 in the drip bag 104a through a capacitance which has been designated

The capacitances C_ambi_ent and C_stand provide the main return loop for the AC current back to the reader 222a. However, the AC signal also flows through a higher impedance loop along the drip tube 108 to the infusion controller 301. This higher impedance loop comprises C_bag/ capacitances C_dripl, C_drip2 and C_plate, and an impedance Z_tube. The capacitances C_dripl and C_drip2 are the bypass capacitances between the electrical bypass 207a and the upstream and downstream medicaments, respectively. From C_drip2, the AC signal passes along the drip tube 108a. At AC frequencies the drip tube 108a will have an impedance which has been designated Z_tube. Z_tube will depend on the length and diameter of the drip tube 108a and also on the electrolyte within the drip tube 108a. This is because the impedance of an electrolyte at a given frequency depends not only on the concentration of the electrolyte but also on the species of the ions that constitute the electrolyte.

Once coupled to the plate 251a, the AC signal passes to the amplifier 252a. The amplifier 252a amplifies the signal on the plate 252a with respect to the electrical potential of the stand 102a. Thus the AC signal passes through the amplifier 252a to the stand 102a and hence completes a return loop to the reader 222a.

Fig 7 shows a drug delivery system comprising an infusion controller 1 and a drip bag 2. The infusion controller 1 is secured by securing means (not shown) to the midpoint of a stand 3, 4. Thus the infusion controller 1 is held at a height off the floor that is convenient for an operator (not shown) such as a nurse. The lower part 4 of the stand has casters while the upper part 3 of the stand has a hook 3a from which the drip bag 2 is suspended.

A keypad 5 allows the nurse to specify the rate at which the infusion controller 1 administers the contents of the drip bag 2 to a patient (not shown), for example 100ml (milli-litres) per hour. A display 6 allows the nurse to check that the desired rate has been entered correctly.

A drip tube 7 exits the bottom of the drip bag 2 and conveys medicament from the drip bag 2 to the infusion controller 1. The drip tube 7 is routed into and out of the infusion controller 1 through a removable panel 8. Inside the infusion controller 1, a peristaltic pump (not shown) acts on the drip tube 7 to pump the medicament into the patient at the specified rate.

A tag 9 is embedded within an appendage 10 of the drip bag 2. The tag 9 comprises an integrated circuit (IC) having a data store that provides information about the drip bag 2 and its contents. The information may relate to one or more of the following: the date of manufacture of the drip bag, the lot code, the date of expiry of the contents, minimum and maximum flow rates that are suitable for the product and may also provide a code or a string of characters identifying the contents of the drip bag 2.

The drip bag 2 may be formed from two sheets of suitable plastics material that have been welded together around their periphery to form a seam 11 with the appendage 10 being provided by regions of the two sheets that extend outwards from the seam 11. The periphery of these regions are also seam welded (this seam is not shown) so that the tag 9 is held captive within the appendage 10.

Fig 8 shows a block schematic diagram to illustrate electrical components of the drug delivery system.

The infusion controller 1 has a central processing unit (CPU) 20 which executes a program stored in a ROM (not shown) and stores data in a RAM (not shown). The CPU 20 monitors the keypad 5 to determine commands and data inputted by the nurse, and then uses these inputs to determine an appropriate schedule of activation for a peristaltic pump 21. In this embodiment the schedule specifies both the timings of activation of the pump 21 and the rate at which the pump 21 pumps when activated. The display 6 is used to confirm commands and data with the nurse, and to indicate the status of the infusion controller 1. The status information may indicate whether the pump 21 is actually pumping and whether the infusion controller 21 is running off a mains electricity supply or if it is being powered by an emergency power source (e.g. internal rechargeable battery). It will of course be appreciated that the power supply connection shave been omitted from Fig 8 in the interests of simplicity.

As so far the infusion controller 1 is of known form. However, the infusion controller 1 also incorporates a reader 22 which, under the control of the CPU 20, supplies power to and communicates with the tag 9 when the drip bag 2 is supported on the stand 3,4 as shown in Fig 7 so as to enable coupling between the reader 22 and the tag 9.

Thus, the reader 22 has a capacitive plate 23 which couples capacitively to an elongate electrode 25 of the tag 9 through the ambient air and has a capacitive plate 24 (which may be provided by part of the housing of the infusion controller 1) which couples to the drip tube 7 through the plastics material of the drip bag 2 and drip tube 7. The tag has a further electrode 26 that couples with the medicament within the drip bag 2 and drip tube 7. In this example, the electrodes 25 and 26 are in the form of flexible wires. The medicament (which may be for example saline solution, blood plasma or a drug solution) comprises an electrolyte (and therefore is electrically conducting), thus when the drip bag 2 is supported on the hook 3a, a complete electrical circuit path is formed between the reader 22 and tag 9.

Figure 9 shows a circuit diagram illustrating the electrical components of the reader 22 and of the tag 9 and their coupling.

The reader 22 shares many parts in common with the readers 122 and so will not be discussed further. The tag 9 shares many parts in common with the tag 109 and so will not be discussed further.

When the tag 9 is coupled to the reader 22, the reader plate 23 and the tag electrode 25 form a capacitor Cl while the tag electrode 26 and the electrolyte within the drip bag 2 form a capacitor C2 and the electrolyte within the drip tube 7 and the reader plate 24 form a capacitor C3. The electrolyte within the drip bag 2 and drip tube 7 that connects C2 to C3 has an impedance that has been represented by Z_tube in Figure 3.

Thus a complete electrical loop is provided, consisting, in sequence, of the oscillator 30, the impedance 31, the inductor 32, the capacitance Cl, the tag 9, the capacitance C2 , the impedance Z_tube and the capacitance C3 which is connected back to the oscillator 30.

When the reader 22 is activated by the CPU 20, the AC voltage from the oscillator 30 is supplied through the impedance 31 to the inductor 32 and the demodulator 33 and, assuming the drip bag 2 is supported by the hook 3a, the AC voltage from the inductor 32 is coupled to the tag 9 through capacitors Cl, C2 and C3 and then back to the oscillator 30.

As can be seen from Fig 9, the capacitors Cl, C2 and C3 are in series and can be regarded as a single capacitor of value C_eff. The value of inductor 32 is chosen so that at 13.56MHz it partially resonates with C_eff. The actual resonance frequency of C_eff and inductor 32 will depend on the value of C_eff (which of course, for given electrode and plate dimensions, depends on the separation between the constituent elements of Cl, C2 and C3), the value of ^z _tube the value of the impedance 31 and on the effective impedance of the tag 9 (which is time-varying as a result of the data being modulated by the tag 9 ) .

Fig 10 shows a circuit diagram similar to Figure 3 but with alternative embodiments 9' and 22' of the tag IC 129 and reader 122.

In the reader 22', the oscillator is replaced by a variable frequency oscillator 40 and the inductor is replaced by a variable inductor 41 both controlled by a reader controller 43. The reader controller 43 causes the frequency of the oscillator 40 and the inductance of the variable inductor 41 to be varied in order to maximise the coupling of the reader 22' to the tag 9'.

As discussed above, C_ef£ depends upon the separation of the tag 9' from the reader 22' and so the reader controller 43 causes the inductance of the variable inductor 41 to be varied to ensure that it resonates with C_eff at the same frequency as the frequency being generated the oscillator 40. Thus the reader 22' can achieve full resonance of C_eff and the variable inductor 41, maximising the AC voltage at plate 23, over a range of values of C_eff instead of at a single value.

By contrast, the embodiment shown in Fig 3 can only achieve full resonance of C_eff and the inductor 32 for a predetermined value of C_eff. When the actual value of C_eff is not the same as the predetermined value, the resonance is only partial and so the AC voltage at the plate 23 is not maximised (although it is nonetheless improved compared to systems that do not have an inductor 32 ) .

The impedance of an electrolyte is generally a function of frequency, with minima and maxima at frequencies characteristic of the electrolyte. Therefore, the reader controller 43 is arranged to vary the frequency of the oscillator 40 in order to operate at a frequency which minimises the impedance, Z_tube, of the electrolyte within the drip tube 7, and thus maximises the coupling of the reader 22' to the tag 9'. Of course, adjusting the output frequency of the oscillator 40 will necessitate the reader controller 43 also re-adjusting the inductance of the variable inductor 41 to maintain it in resonance with C_eff.

Whereas the tag IC 129a shown in Fig 3 used an impedance 38 to modulate the coupled electrical signal with the data stored within data & logic unit 36, the tag 9' shown in Fig 4 uses a resistor 42 which is connected between the drain of the MOSFET 37 and the output of the rectifier 34.

When the MOSFET 37 is switched on, the resistor 42 acts as a shunt and increases the power consumption of the tag 9 ' . This increase in power consumption reduces the input signal to the demodulator 33. The demodulator detects the reduction and passes a signal to the CPU 20 indicative of the pulse train from the data & logic unit 36 and thus representing the data read out from the data & logic unit 36. Figure 11 shows an alternative embodiment 2 ' of a drip bag 104. The drip bag 2' may be formed from two sheets of suitable plastics material that have been welded together around their periphery to form a seam 11 with an appendage 10 being provided by regions of the two sheets that extend outwards from the seam 11. The periphery of these regions are also seam welded (this seam is not shown) so that the tag 109 is held captive within the appendage 10.

Here, instead of the tag IC 129 being connected to an electrode 125 and an antenna 126, it is connected to a small plate 50 and a contact 51. The plate 50 has an area of about 2mm² and is formed as a patch of metal deposited by evaporation or another suitable deposition technique on the surface of an appendage 10. The contact 51 passes through a seam 11 and is therefore ohmically connected to the medicament within the drip bag 2 ' .

The contact 51 should of course be formed of or coated with an electrically conductive material that does not detrimentally affect the medicament or patient, for example plated with an inert metal such as platinum.

When the drip bag 2' is used in conjunction with the infusion controller 301, the ohmic connection of the tag 109 to the medicament provides the advantage that the electrical coupling between the reader 222, tag 109 and plate 251 is improved.

The direct connection of contact 51 with the medicament (electrolyte) within the drip bag 2" and drip tube 108 allows the drip bag 2' and drip tube 108 to act as an elongate antenna, thereby increasing the coupling between the tag 109 and the reader 222. This effect can be used to enable the range (between the appendage 10 and the infusion controller 301) over which communication between the reader 222 and plate 251 can successfully be maintained to be increased.

Empty drip bags 2 ' may be supplied to medical product manufacturers for filling with a medicament and labelling. The data in the data & logic unit of a tag 109 is pre-stored. The IC of the tag 109 may be replaced by a writeable tag IC such as described in WO 02/052419 so that the medical product manufacturers may program the tags 109 with data giving information concerning the medicament.

Figure 12 shows a functional block diagram of an alternative IC for use within the tag 109. The circuitry of the IC has some similarities to the circuitry shown in Fig 3 for the tag IC 129a. However, instead of the RC clock 35 and the data & logic unit 36, the alternative tag IC has a CPU 90, a memory 92 and an analogue to digital convertor (ADC) 91, all of which are powered up by the rectifier 34 when the tag 109 is in range of the reader 122.

The memory 92 comprises ROM, RAM and non-volatile EEPROM. The CPU 90 executes a program stored in the ROM, thereby allowing data stored in the EEPROM to be overwritten with new data received from the reader 122. The ADC 91 monitors the DC voltage produced by the rectifier 34 and outputs a signal to the CPU 90 indicative of the DC voltage. The CPU 90 is connected to the gate of the MOSFET 37 so that the CPU 90 can change the effective impedance of the alternative tag IC by connecting an impedance 38 across the alternative tag IC.

The reader 122 is able to transmit data to the alternative tag IC by varying the amplitude of the AC voltage so as to represent data bits to be written. The varying AC voltage causes the DC voltage from the rectifier 34 to change accordingly. The ADC 91 digitises the resulting DC voltages and the CPU 90 decodes the changes to determine the data transmitted by the reader 122. Finally, the CPU 90 writes the received data into the EEPROM.

Of course, as an alternative to using the ADC 91, a comparator could be used to recover the data from the changes in the DC voltage output of the rectifier 34.

Figure 13 shows a circuit diagram illustrating the coupling of the electrical components of another embodiment of the drug delivery system. In this embodiment, the inductor 32 is provided as part of the tag instead of being provided in the reader.

Further embodiments

In an embodiment described above, an impedance 31 was used to increase the effective source impedance of an oscillator 30, and thus improve the ability of the demodulator 33 to detect changes in the signal level resulting from modulation by the tag 9. In other embodiments, the impedance may be inherent in the oscillator. For example, the oscillator may be designed to have an output resistance of, say, 10Ω or may have a complex output impedance that is both resistive and reactive.

In an embodiment described above, a reader 22 was described in which an oscillator 30 produced an AC signal at a frequency of 13.56MHz. In other embodiments, other frequencies may be used.

In an embodiment described above, a demodulator 33 detected changes in an amplitude resulting from modulation by a tag 9. In alternative embodiments, other modulation methods such as phase modulation may be used. For example, if phase modulation is used then the tag will be arranged to change its reactive impedance in response to the data stored therein, and the demodulator arranged to compare the phase of the signals on either side of the impedance 31.

In an embodiment described above, a demodulator 33 was connected to the 'low voltage' terminal of an inductor 32. In alternative embodiments, the demodulator 33 may instead be connected to the 'high voltage' terminal of the inductor 32. However, it is preferred that the demodulator 33 is connected to the low voltage terminal because high voltages (of the order of 100V) can appear at the high voltage terminal of the inductor 32. In general, it is difficult to design demodulators that can withstand such high voltages and, in any case, such demodulators would tend to be more expensive than those designed to accept lower voltage inputs. Furthermore, it is important to minimise the stray capacitance at the plate 23. Connecting the demodulator 33 to the low voltage terminal instead of the high voltage terminal of inductor 32 avoids loading the plate 23 with the input capacitance of the demodulator 33.

In an embodiment described above, a variable inductor 41 was used. The inductance may be varied by selecting an appropriate electrical tap of the coil windings using PIN diodes or by using a gyrator circuit to electrically transform a variable capacitor (e.g. a varactor variable capacitance diode) so that it behaves as a variable inductor. Alternatively, a variable capacitor could be connected between the "high" voltage terminal (i.e. the terminal coupled to the hook 103a) of the inductor 32 and ground and adjusted to keep the effective capacitance "seen" by hook 103a constant, thereby compensating for changes in the capacitance presented by the tag. Alternatively, a fixed inductor could be, in effect adjusted, by connecting a variable capacitor Alternatively, a fixed inductor could be, in effect adjusted, by connecting a variable capacitor in parallel with the inductor. Modifying the capacitance of the variable capacitance would modify the apparent inductance of the inductor/capacitor combination. Note that in some applications the inherent capacitance across the reader (i.e. analogous to C_bypass of the tag) may dominate the coupling capacitance to the tag. In an embodiment described above, a reader 22 was connected to two plates 23, 24. In alternative embodiments, a reader may be provided with three or more plates located at different positions on the reader for communication with the tag 9. The reader may evaluate different pairs of plates and use whichever pair gives the best coupling with the tag 9, or may have several sets of impedances 31, inductors 32 and demodulators 33 working simultaneously, in which case the demodulator giving the strongest data signal is selected for use.

In an embodiment described above, a drip bag 2 was suspended from the hook 3a of the stand 3, 4. In alternative embodiments, an electrically conductive upper part 3 of the stand may replace the plate 24.

In an embodiment described above, a platinum plated electrode 51 was placed in direct electrical connection with a medicament within a drip bag 2. In alternative embodiments, the electrode may instead be formed from a conductive plastics material.

In an embodiment described above, a drip bag 2' was described which was provided with an appendage 10 containing a tag 9. In alternative embodiments, a compartment within the drip bag 2 ' may be provided to house the tag 9.

In an embodiment described above, a reader was described which was able to both read data from and write data to a tag. In alternative embodiments, separate transponders may be used to write data to a tag and to read data from a tag .

In an embodiment described above, a tag 9 was described which was based on an IC. In alternative embodiments, discrete components may be connected together as a hybrid circuit on a substrate.

In an embodiment described above, a tag 9 was described which had a data & logic unit 36 storing 104 bits of data. In alternative embodiments, the clock generator and the data & logic unit 36 may be replaced by a multivibrator oscillator circuit with the output of the multivibrator driving the gate of the MOSFET 37. For example, the multivibrator may be arranged to oscillate at 5kHz. Whenever this alternative tag receives sufficient power from a reader, it will modulate the applied electrical field at a frequency of 5kHz. Other tags may be arranged to oscillate at other frequencies. Thus the reader can distinguish the various tags by measuring the frequency of modulation of the tag.

In an alternative embodiment, the tag may be provided with a battery so that the reader is only required to couple a data signal to or from the tag, instead of also powering the tag.

In an embodiment described above, the tag used a MOSFET to shunt either an impedance or a resistance across the tag. In an alternative embodiment, the resistance or impedance may be connected in series with the tag, and the MOSFET may be connected in parallel with the impedance or resistance. When the MOSFET is switched on, it shunts the impedance or resistance, thereby decreasing the effective resistance of the tag by shorting out the impedance or resistance. Modulation methods are also discussed in GB 2365267, the contents of which are herein incorporated by reference.

The present invention may be applied to devices having replaceable or attachable components or items such as a tool or a plug-in key or component that can be attached to or fitted into or onto the device, where a separation of the order of 1mm between the reader and the tag will be expected.

As examples, the present invention may be applied to watches (where, for example, the bezel may be replaceable), electronic games, puzzles, toys, vehicle body panels or parts such automotive, aircraft, shipping and other vehicle component parts, and so on provided that the communication between the passive data storage device and the reader unit can be shielded from interference by the vehicle engine, domestic equipment such as electric kitchen knives having attachable knives components, food processors having replaceable blades and mixing implements, smart packaging where the passive data storage device includes data regarding the product, for example cooking times in the case of a food product, for example microwave cooking times, smart cups or coffee maker receptacles that include data indicating how you like your coffee, water filters with attachable filters and/or jugs, beverage makers with attachable jugs or receptacles, vacuum cleaners having attachable tools and/or dust bags, filters and so on, polishers having different attachable components; personal care items such as electrical shavers or razors where the attachable component is a razor or shaver head, hair dryers having attachable brushes, diffusers and so on, hair tongs having different attachable tools and other personal care items, where the passive data storage device can be embedded in, for example, a plastics portion of the attachable component, home/office electrical devices such cameras where the attachable component is a lens carrying data identifying an exposure time, medical electrical devices having different attachable tools, blades and so on, tips for pipettes to calibrate the pipette, a fascia or other cover portion in the case of, for example electrical devices such as televisions, video recorders, DVD players, stereos, remote controls for any of these, answering machines, personal computers, laptops, games consoles, telephones, telephone accessories, vending machines, polishers, answering machines, watch bezels and the like where the attachable components may be different clip-on covers and the control data gives different technology features. The attachable component could also be a battery.

The present invention may be applied where the component is not necessarily attachable to the device but is an accessory that can be brought into proximity with the electrical device. For example the accessory may be a toy, a promotional item, promotional literature, an advertising hoarding or similar advertising material, a ticket, identity card, phone card, debit card or other mobile commerce product, packaging such as a product box or wrapping, vending machine or access device carrying a data storage device that contains data that affects an electrical device (be it a mobile communications device or other electrical or electronic device such as a toy, computer, games console, telephone and so on) carrying a reader unit. As examples, a washing powder (or dishwasher powder) box may carry a tag or data storage device that can be read by a reader incorporated in the washing machine (or dishwasher) to enable the washing cycle to be controlled in accordance with the type of powder being used, an identification device may carry a voice print ID to be detected by a reader unit, tyre valves may contain data that instructs a pump how much to inflate a tyre, a patient needs device may carry a data storage device that provides data to a drug dispenser, for example.

Where the device has communications facilities, then the data read by the reader unit to communicate with another device, for example to enable payment for a service or product to be effected.

Claims

1. A data communication system, comprising: a base unit having first capacitive coupling means, signal supplying means for supplying a time-varying signal and inductor means for inductively connecting the signal supplying means to the first capacitive coupling means; and a subsidiary unit storing data, the subsidiary unit having second capacitive coupling means for coupling capacitively to the first capacitive coupling means, power supply means for deriving a power supply from a signal coupled to the second capacitive coupling means, and output means for outputting stored data when the subsidiary unit is powered.

2. A data communication system according to claim 1, wherein the inductor means comprises at least one of an inductor.

3. A data communication system according to claim 1 or 2, wherein the inductor means is variable.

4. A data communication system according to claim 3, further comprising control means for adjusting the inductance of the inductor means so that in operation the inductor means at least partially resonates with a capacitance which comprises at least the capacitance of the first capacitive coupling means.

5. A data communication system according to claim 4 , wherein the control means is arranged to adjust the inductor means for full resonance with the said capacitance.

6. A data communication system according to any one of claims1 to 5, wherein the signal supplying means comprises an oscillator.

7. A data communication system according to claim 6, wherein the oscillator is a variable frequency oscillator.

8. A data communication system according to any of claims 1 to 4, wherein the signal supplying means comprises a variable frequency oscillator and the system further comprises oscillator control means for adjusting the frequency of the variable oscillator so that the inductor means at least partially resonates with a capacitance which comprises at least the capacitance of the first capacitive coupling means.

9. A data communication system according to claim 5 or 6, wherein the signal supplying means comprises a variable frequency oscillator and the control means is operable also to adjust the frequency of the variable oscillator so that the inductor means at least partially resonates with the said capacitance.

10. A data communication system according to any one of claims 7 to 9, further comprising impedance control means for adjusting the frequency of the variable frequency oscillator so as to minimise the impedance presented by the first capacitive coupling means to the inductor means .

11. A data communication system according to any of claims 1 to 10, wherein the subsidiary unit has a memory storing the data.

12. A data communication system according to any one of claims 1 to 11, wherein the subsidiary unit has a writeable memory and the base unit further comprises modulation means for modulating a signal supplied to the first capacitive coupling means in accordance with data to be written into the memory.

13. A data communication system according to claim 12, wherein the modulation means is operable to modulate the amplitude of the signal supplied to the first capacitive coupling means in accordance with data to be written into the memory.

14. A data communication system according to claim 13, wherein the modulation means is operable to cause the amplitude of the signal from the signal supplying means to vary in accordance with data to be written into the memory.

15. A data communication system according to any one of claims 12 to 14, wherein the modulation means is operable to modulate the phase of a signal supplied to the capacitive coupling means in accordance with data to be written into the memory.

16. A data communication system according to any one of claims 1 to 15, wherein the output means is arranged to output data by modulating a signal and the base unit further comprises demodulation means for recovering the modulation.

17. A data communication system according to 16, wherein the output means is arranged to output data by modulating the signal coupled to the second capacitive coupling means.

18. A data communication system according to claim 16 or 17, wherein the output means is arranged to output data by modulating the amplitude of the signal coupled to the second capacitive coupling means.

19. A data communication system according to claim 16, 17 or 18, wherein the output means is arranged to output data by modulating the phase of the signal coupled to the second capacitive coupling means .

20. A data communication system according to any of claims 16 to 19, wherein the demodulation means is coupled to the first coupling means by the inductor means .

21. A data communication system according to any of the preceding claims, wherein the first capacitive coupling means comprises an electrical plate.

22. A data communication system according to any of claims 1 to 21, wherein the first capacitive coupling means comprises an elongate electrode having antenna-like properties.

23. A data communication system according to any of the preceding claims, wherein the second capacitive coupling means comprises an elongate electrode having antenna-like properties.

24. A data communication system according to any of claims 1 to 23, wherein the second capacitive coupling means comprises an electrical plate.

25. A data communication system according to any of the preceding claims, wherein the power supply means comprises a rectifier.

26. A data communication system according to any of claims 1 to 25, wherein the output means comprises an impedance for changing the impedance of the subsidiary unit in accordance with the stored data.

27. A data communication system according to claim 26, wherein the impedance comprises a resistance.

28. A data communication system according to claim 26 or 27, wherein the output means comprises a switch for controlling coupling of the impedance to the second capacitor means in accordance with the stored data.

29. A base unit for the data communication system of claim 1, comprising: capacitive coupling means; signal supplying means for supplying a time-varying signal; and inductor means for inductively connecting said supplying means to the capacitive coupling means.

30. A base unit according to claim 29 having the base unit features set out in any of claims 2 to 28.

31. A subsidiary unit for use in a data communication system according to claim 1, the subsidiary unit comprising: capacitive coupling means for coupling capacitively to the capacitive coupling means of a base unit; power supply means for deriving a power supply from a signal coupled to the capacitive coupling means; and output means for outputting stored data when the subsidiary unit storing data is powered.

32. A subsidiary unit for use in a data communication system, the subsidiary unit comprising: capacitive coupling means for coupling capacitively to the capacitive coupling means of a base unit; power supply means for deriving a power supply from a signal coupled to the capacitive coupling means; and output means for outputting stored data when the subsidiary unit storing data is powered, wherein the capacitive coupling means comprises an elongate electrode having antenna-like properties.

33. A subsidiary unit for use in a data communication system, the subsidiary unit comprising: a data store; capacitive coupling means for coupling capacitively to the capacitive coupling means of a base unit; power supply means for deriving a power supply from a signal coupled to the capacitive coupling means; and output means for outputting data from the data store via the capacitive coupling means when the subsidiary unit storing data is powered, wherein the output from the data store is coupled to the capacitive coupling means via an inductor.

34. A subsidiary unit according to claim 33, wherein said capacitive coupling means comprises an elongate electrode having antenna-like properties.

35. A subsidiary unit according to claim 31 having the subsidiary unit features set out in any of claims 2 to 28.

36. A liquid supply system comprising: a container for containing liquid comprising an electrolyte; a device for receiving liquid from the container when the container is coupled to the device; a liquid supply path for supplying liquid from the container to the device when the container is coupled to the device; the device comprising a base unit having first capacitive coupling means, signal supplying means for supplying a time-varying signal and inductor means for inductively connecting the signal supplying means to the first capacitive coupling means; and the container carrying a subsidiary unit storing data, the subsidiary unit having second capacitive coupling means for coupling capacitively to the first capacitive coupling means when the container is coupled to the device, power supply means for deriving a power supply from a signal coupled to the second capacitive coupling means, and output means for communicating stored data to the base unit when the subsidiary unit is powered, with communication between the container and the device being at least partly via liquid in the liquid supply path.

37. A liquid delivery system comprising: a base unit having first capacitive coupling means, signal supplying means for supplying a time-varying signal and inductor means for inductively connecting the signal supplying means to the first capacitive coupling means; a liquid container arranged to be mounted adjacent the base unit; a main body for receiving liquid from the container; a liquid supply path for coupling the liquid container to the main body; the liquid container carrying a subsidiary unit storing data, the subsidiary unit having second capacitive coupling means for coupling capacitively to the first capacitive coupling means when the container is mounted adjacent the base unit, power supply means for deriving a power supply from a signal coupled to the second capacitive coupling means, and output means for communicating stored data to the base unit when the subsidiary unit is powered, the base unit and the main body having first and second communication means with the first communication means of the base unit being arranged to communicate with the second communication means of the main body in response to the base unit receiving data from the tag to enable the main body to determine whether or not the correct container has been coupled to the liquid supply path.

38. A drug delivery system comprising: a stand; a reader coupled to the stand, the reader having first capacitive coupling means, signal supplying means for supplying a time-varying signal and inductor means for inductively connecting the signal supplying means to the first capacitive coupling means; a drip bag supported by the stand for containing an electrolyte to be delivered to a patient, a drip tube providing a liquid supply path for supplying electrolyte to a patient from the drip bag, an infusion controller having an infusion pump for pumping electrolyte along the drip tube, the drip bag carrying a subsidiary unit storing data, the subsidiary unit having second capacitive coupling means for coupling capacitively to the first capacitive coupling means when the drip bag is supported by the stand, power supply means for deriving a power supply from a signal coupled to the second capacitive coupling means, and output means for communicating stored data to the base unit when the subsidiary unit is powered, the base unit and the infusion controller having first and second communication means with the first communication means of the base unit being arranged to communicate with the second communication means of the infusion controller in response to the base unit receiving data from the tag to enable the infusion controller to determine whether or not the correct drip bag has been supported on the stand.

39. A system according to claim 37 or 38, wherein the first and second communication means are arranged to communicate by at least one of an acoustic and an electrical path.

40. A system according to claim 37, 38 or 39, wherein the first and second communication means are arranged to communicate via the liquid supply path.

41. A drug delivery system comprising: a drip bag for containing an electrolyte to be delivered to a patient, a drip tube for supplying electrolyte to a patient from the drip bag, an infusion controller having an infusion pump for pumping electrolyte along the drip tube, the infusion controller comprising a base unit having first capacitive coupling means, signal supplying means for supplying a time-varying signal to the first capacitive coupling means; and the drip bag carrying a subsidiary unit storing data, the subsidiary unit having second capacitive coupling means for coupling capacitively to the first capacitive coupling means when the drip bag is coupled to the infusion controller, power supply means for deriving a power supply from a signal coupled to the second capacitive coupling means, and output means for communicating stored data to the base unit when the subsidiary unit is powered, wherein the electrolyte provides at least part of an electrical path from the subsidiary unit to the base unit.

42. A drug delivery system according to claim 38, wherein the electrolyte provides at least part of an electrical path from the subsidiary unit to the base unit.

43. A drug delivery system according to claim 41 or 42, wherein the second capacitive coupling means is resistively connected to the electrolyte

44. A data communication system comprising: a base unit having first capacitive coupling means, signal supplying means for supplying a time-varying signal to the first capacitive coupling means; and a fluid container carrying a subsidiary unit storing data, the subsidiary unit having second capacitive coupling means for coupling capacitively to the first capacitive coupling means, power supply means for deriving a power supply from a signal coupled to the second capacitive coupling means, and output means for communicating stored data to the base unit when the subsidiary unit is powered, wherein the fluid provides at least part of an electrical path from the subsidiary unit to the base unit.

45. A data communication system according to claim 44, wherein the second capacitive coupling means is resistively connected to the electrolyte.

46. A data communication system according to claim 44 or 45, wherein the base unit has inductor means for inductively connecting the signal supplying means to the first capacitive coupling means.

47. A system according to any of claims 1 to 28 and 33 to 46, wherein the second coupling means comprises an inert metal.

48. A system according to any of claims 1 to 28 and 33 to 47, wherein the second coupling means comprises an electrically conductive plastic.

49. A data communication system according to claim 44, 45 or 46 having the features set out in any of claims 2 to 28.

50. An infusion pump comprising a base unit having the base unit features set out in any of claims 1 to 28 and 37 to 48.

51. A drip bag comprising a subsidiary unit having the subsidiary unit features set out in any of claims 1 to 28 and 37 to 48.

52. An accessory comprising a subsidiary unit having the subsidiary unit features set out in any of claims 1 to

28 and 36 to 48.

53. A container for containing an electrically conducting fluid, comprising a subsidiary unit having the subsidiary unit features set out in any of claims 1 to 28 and 36 to 52.

54. A container for containing an electrically conducting fluid, comprising the features of the container set out in claim 36 or 37.