GB2087691A - Electrical signal transmission - Google Patents

Abstract

A system for transmitting and receiving electrical signals employs a receiver (5 to 8) having a loop aerial (4) extending around a space in which signals are to be received. A transmitter produces inductive signals which can be picked up by the receiver within the space bounded by the aerial. This system can be used to monitor the activity of a person by connecting the transmitter to a sensor, such as an arrangement of mercury switches, so that recurrent signals are transmitted during normal activity of the person. Departures from such normal activity give rise to changes in the pattern of transmitted signals which can be detected by the receiver. <IMAGE>

Description

SPECIFICATION Electrical signal transmission This invention relates to a system for the transmission and reception of electrical signals.

In the case where electrical signals are to be transmitted over relatively short distances within the confines of a building or other predetermined space as for example where data is to be transmitted to a central computer from peripheral equipment or where alarm signals are to be transmitted from remote sensors to a central control device, it is known to use systems in which signal transmission is effected along cable links and also systems in which signal transmission is effected via v.h.f. radio waves. These known systems, however, may not be wholly satisfactory in that cable links may involve installation problems and may be inconvenient or unsuitable for use with protable transmitters, and with v.h.f. radio links it may be difficult or impossible to restrict transmission and/or reception to the confines of the predetermined space.

One object of the present invention is to provide an electrical signal transmission/reception system which does not rely on the use of cable links yet with which signal transmission and reception can be readily confined to a predetermined space.

According to one aspect of the invention therefore there is provided a system for the transmission and reception of electrical signals comprising a receiver having a loop aerial extending around a space in which signals are to be received and a transmitter utilisable within said space, said transmitter and receiver being adapted respectively to transmit and receive inductive signals.

With this arrangement it will be appreciated that signals can be transmitted without requiring any cable link yet the range of transmission and reception can be readily restricted substantially to the said space. Moreover, due to the use of inductive transmission/reception techniques, effective and reliable operation can be achieved at low levels of power consumption and with little tendency to cause interference with nearby equipment.

With regard to the transmitter, this may take any suitable form but preferably comprises appropriate digital and/or linear circuitry capable of producing oscillations in a frequency range of 1 KHz to 150 KHz.

The transmitter may have an aerial in the form of a short ferrite rod.

The transmitter may be adapted to transmit simple impulse signals or sustained tones or modulated carrier waves. Modulation where employed may be FM or AM and may be used to transmit speech or for any other purpose.

The receiver has a loop aerial which may be in the form of a wire running around the said predetermined space, for example around a room, around the periphery of a building orthe like. The loop may be fixed or movable as desired and may enclose an area of say a few hundred square centimetres to several hundred square metres. Also it is possible to use multiple loops, for example three mutually orthogonal loops which can be used for direction finding purposes. Such multiple loops may be movable about their axes, or the loops may be fixed and the relative electrical properties thereof may be adjustable by adjustment of associated circuitry thereby to give an effect equivalent to movement.

The or each loop may be connected to a receiving circuit of any suitable form capable of responding to inductively received signals. Such receiving circuit may comprise a tuned circuit and appropriate filters, amplifiers, detection and conditioning circuits and the like.

The system of the invention may be used for any suitable purpose and in any suitable environment.

Thus for example, the system may be used in commercial premises or industrial premises or sites to transmit data to a central computer for example from remote data input keyboards or from sensors or limit switches or the like. The system can also be used for security purposes to transmit alarm signals to a central control location from remote manually operable switches or automatic sensors or portable personal alarm devices or the like. It is also possible to use the system for communication purposes if desired using speech transmission. Further, the system may be used for remote actuation or identification purposes, whereby for example, equipment is actuated to unlock a door, to start machinery, to switch off an alarm, or take other action when a person carrying the transmitter enters the confines of the loop.

If desired a plurality of separate receivers with separate loops may be used at adjacent regions this being possible without appreciable cross-talk due to the limited range of the inductive transmission/reception techniques. It is also possible to use a multi-channel system with multiple transmitters operating within a common loop. Further, the system of the invention can be adapted for two-way communication whereby as and when desired the loop aerial is used for inductive transmission purposes and the transmitter of the invention is used as or in conjunction with inductive receiving apparatus.

It is to be understood that the or each transmitter may be capable of producing multiple different transmissions separately or simultaneously, appropriate automatic or manual switching devices being provided to change or select such transmissions and the system having a single multi-channel discriminating receiver or alternatively multiple different receivers using the same loop aerial or separate loop aerials in the same or different localities.

It is desirable to be able to monitor the wellbeing of persons exposed to danger, as for example in the case of personnel working in hazardous conditions or elderly people living alone, and it is known to use electrical and electronic alarm systems for this purpose. However, with such systems it can be difficult to achieve sensitive monitoring whilst at the same time avoiding spurious actuation thereof in circums tances not corresponding to alarm conditions.

A further object of the present invention is to provide a monitoring system which can be used to monitor the well-being of persons and which is capable of sensitive yet reliable operation.

According to a further aspect of the invention therefore there is provided a monitoring system comprising a signal generating device movable within a zone, said device comprising at least one sensor for detecting recurrent stimulae and a transmitter operable to transmit signals corresponding to detection of said stimulae; a receiving device operable to receive said signals within said zone; and a control system connected to said receiving device and operable to produce an alarm output in response to a predetermined change in said received signals corresponding to a departure of said stimulae from a predetermined norm thereof.

With this arrangement in so far as the signal received are assessed in relation to a predetermined stimulae norm it will be appreciated that it is possibleto effect monitoring with both sensitivity and reliability. In particular it is possible to make allow ante for normal non-dangerous irregularities in said recurrent stimulae and thereby avoid spurious production of an alarm output in response to the occurrence of such irregularities.

Most preferably the system is applied to the monitoring of the well-being of a person as mentioned above in which case the said signal generating device will be adapted to be carried or worn by such person. It is however to be understood that the invention is not restricted to such field of application and, for example, it is also possible to use the system for monitoring animals or machines.

In a particularly preferred embodiment the said sensor is a movement sensor, for example a mechanical position switch such as a mercury switch which responds to a change in attitude and/or to abrupt movement thereof. Such sensor may be adapted to be worn by a person, particularly on such person's arm or leg and thus for example may be incorporated in a garment such as a jacket. In one embodiment a plurality of sensors is provided around one (or each) cuff of a protective jacket or coat. Alternatively or additionally it is possible to use sensors which respond to stimulae other than movement, for example body functions such as metabolic rate, heart rate or the like.

The transmitter may be of any suitable form and may be associated with the sensor or sensors in any suitable manner. Thus, for example, the sensor or sensors and transmitter may be combined in a single housing or unit or alternatively may be provided in separate interconnected units. The transmitter may operate on any suitable form of radiated energy although preferably inductive radiation is utilised as described in the first aspect of the present invention.

The transmitter may have an aerial which may be directional in relation to the receiving device such that reception of signals is prevented or at least severely limited when the transmitting aerial has a predetermined disposition.

The receiving device may take any suitable form depending on the nature of the transmitter, and thus a suitable inductive receiver will be used in the case where inductive radiation is utilised.

The said control system may function in any suitable manner. Thus such system may be adapted simply to respond to the absence of a received signal after a predetermined period of time. Alternatively, the system may be adapted to respond to a significant departure from a predetermined signal pattern or trend over a predetermined period of time. In the latter case, where a person is to be monitored a normal signal pattern for such person may be derived by monitoring (and possibly continually or continuously updating) transmitted signals generated by the person and such normal pattern may be utilised as a reference for comparison with subse quent received signals averaged or filtered over predetermined periods of time.

Following production of an alarm output an alerting signal may be produced at or in the locality of the signal generating device and an opportunity may then be given for the control system to be reset and the alarm output cancelled as for example by appropriate feed of stimulae to the sensor or sensors of the signal generating device or by operation of a control on the device which initiates transmission of an appropriate reset signal.

The system of the invention may be used to monitor the well-being of persons working in adverse conditions or elderly, infirm or ill persons for example living alone. The system may also be used to give warning of undesired intrusion into the said zone of operation of the receiver. In the latter respect the system may also incorporate a presence detector which operates using infra-red, active or passive ultrasonic techniques or the like, such detector being operable to actuate an alarm unless overridden by detection of an acceptable signal produced by said signal generating device. With this arrangementthe alarm can be actuated for example in the event that a burglar or other unauthorised person not carrying said signal generating device enters the said zone, or in the event that an authorised person enters the said zone without his signal generating device.In the latter case, where the signal generating device is incorporated in a protective garment it will be appreciated that the system can operate to encourage personnel to wear required protective garments before entering the said zone (which may be a hazardous zone).

The invention will now be described further by way of example only and with reference to the accompanying drawings in which: Fig. lisa circuit diagram of a transmitter of one form of a system according to the invention; Fig. 2 is a circuit diagram of an alternative form of transmitter; Fig. 3 is a circuit diagram of a receiver for use with the transmitter of Fig. 1 or 2; Fig. 4 is a diagram of a signal generating device of one form of a monitoring system according to the invention; and Fig. 5 is a diagram of the receiver and control device of the monitoring system.

Fig. 1 shows a transmitter circuit which is powered from a 3v d.c. supply (not shown) and comprises a transistor 1 having coils 2, 3 in its base and collector circuits which are wound on a common ferrite rod thereby to define a feedback loop. The transistor 1 may be of a general purpose npn kind (such as a BC107). The ferrite rod may be say 5cm long and 6 to 7mm diameter. The coil 3 may comprise 160 turns of 0.16mm wire and the coil 2 comprise 16 turns of the same wire. The circuit also comprises resistors and capacitors in the base, emitter and collector circuits of the transistor as shown.

The parameters of the circuit are selected such that it oscillates at a frequency in the range 1KHz to 150KHzwith a high Q (sayofthe orderof30)the voltage appearing across the coil 3 being say 6 to 8v.

The circuit is however arranged (particularly by selection of the ferrite rod aerial, the turns ratio of the coils 2, 3 and the circuit gain) such that there is a low level of power consumption (say 0.25 ma at 3v) and a low level of transmitted power from the ferrite rod aerial (say of the order of microwatts), such transmitted power being principally in the form of inductive radiation and comprising little RF radiation. Further the circuit is arranged such that modification of the properties thereof by external influences (for example by introduction of ferromagnetic material close to the ferrite rod aerial) will tend to kill the oscillations rather than to modify the nature thereof.

The circuit of Fig. 1 is adapted to produce only a continuous single frequency oscillation but it is possible to modulate this, using AM techniques, to produce a single steady tone, a switched single tone, selectable multiple tones, speech transmissions or the like.

The transmitter of Fig. 1 is intended for use in a predetermined space, for example within a building, within a room, within an outdoor industrial site, within a vehicle, or other space. Substantially around the horizontal periphery of such space there is provided a loop aerial 4 (i.e. a looped length of wire or other conductor) and this is connected to a receiving circuit as shown in Fig. 3.

The aerial loop 4 is linked inductivelyto a resonant circuit 5 tuned to the pre-set inductive carrier frequency of the transmitter, and the output of this circuit is fed via a band pass filter 6, a phase-locked loop 7, and amplifying stages 9, to usual conditioning circuits 8 which act to detect, demoduiate, amplify and utilise (as by operating a loudpseaker, or switching circuits or the like) the received signals. As appropriate, the receiver may be adapted to produce a digital output signal which after amplification and processing with a Schmitttriggercan be used directly by digital circuits.

Having regard to the inductive nature of the principal mode of operation of the transmitter and receiver, it will be appreciated that transmission and reception can be severely limited to the confines of the predetermined space around which the loop 4 is disposed. There will thus be little tendency either for signals to be transmitted outside such space or for unwanted signals to be picked up by the receiver.

Moreover, efficient, reliable operation can be achieved utilising simple equipment and with low power levels.

The transmitter/receiver system may be used for any suitable purpose and the equipment may correspondingly take any suitable form.

Thus, the system may be used to transmit information to a central computer in which case one or a plurality of transmitters may be incorporated in or connected to office equipment, remote automatic sensors or the like at static or mobile locations such as to transmit data signals, corresponding to operation of such equipment or sensors, to said receiver which is connected to the computer via appropriate interface devices to feed received data signals thereto for processing by the computer.

The system may also be used as an automatic safety or security system in which case one or a plurality of transmitters may be connected to alarm sensors, such as burglar alarm sensors, fire sensors, smoke detectors or the like, such that alarm signals are transmitted in correspondence with operation of the sensor, to said receiver which acts to operate an alarm device such as an audible warning device, or visual warning device or a circuit which takes remedial action such as locking doors, activating sprinklers or the like.

The system may further be used as a personnel identification system in which case one or a plurality of transmitters may be carried by one or more persons, such transmitter or transmitters being arranged to transmit a signal either automatically or when specifically actuated by the respective person (e.g. by operation of a press-button or other control on a housing of the transmitter), said receiver being arranged to receive such transmitted signal and effect corresponding actuation or deactuation of equipment. In the case of actuating signal transmission, the receiver may act to switch on a machine in a factory, to switch on a vehicle ignition circuit or to take other similar action which is required to be taken in correspondence with activation of the transmitter by and/or entry into the zone encompassed by the aerial of the respective person.For example, the aerial may extend around the driver's seat of a car or around the cab of a commercial vehicle such that the ignition circuit is switched on when a driver carrying the transmitter (for example in his pocket) sits on the seat or enters the cab. This arrangement may be in place of or in addition to the usual ignition key switch and the transmitter may be continuously operable or alternatively may operate only when activated by the driver. Such transmitter may produce a signal which is coded so that the ignition can be switched on only by an authorised driver having the appropriate transmitter. The ignition switch may automatically switch off when the transmitter is switched off and/or removed from the influence of the aerial loop. Alternatively the circuit may latch in the switched-on position until switched off (by appropriate action such as operation of an ignition key switch) ready for subsequent reactuation by the transmitter (and possibly also the ignition switch). In the case of deactuating signal transmission, the receiver may act to switch off or inhibit a machine, a door-locking device, an alarm system (such as a burglar alarm, a theft alarm, an ultrasonic intrusion detector) or to take other similar action when the transmitter is activated and/or is introduced into the zone encompassed by the aerial.For example, a room or vehicle or other zone may have the aerial around same and connected to said receiver linked to an alarm which is arranged normally to be triggered on actuation of alarm sensors connected thereto, such receiver acting to trigger a relay or switching transistor or the like such as to switch off the alarm when an authorised person carrying the transmitter enters the protected area. The transmitter may transmit a general or coded signal continuously or only when manually activated. The alarm may automatically reset when the transmitter is switched off or removed from the area or alternatively by a switch may be provided on the alarm for use in resetting same.

Yet further, the system may be used as a remote signalling system for purposes of remote control of equipment or machinery or for purposes of remote alarm actuation. Thus, for example, the transmitter may be carried by a person to enable such person to effect remote control of the operation of a machine or device or to actuate an alarm in the event that such person is attached or otherwise in danger.

Where a portable transmitter is used this may comprise a small battery-powered housed unit and preferably with such unit the ferrite rod aerial is positioned centrally of the housing to limit interference by external ferromagnetic material. Due to the low power consumption, a portable battery transmitter can conveniently have a long operational life before battery changing is required even where continuous use is required.

Fig. 2 shows an alternative form of transmitter which utilises digital techniques. This transmitter has a digital voltage-controlled oscillator 10 which feeds a coil 11 on a ferrite rod aerial via an amplifying transistor 12. The produced oscillation may be amplitude-modulated with a signal fed via a low pass filter 13, the resulting digitally constructed sine wave (at 14) being passed via a reconstruction filter 15 to the transistor 12. Alternatively, the oscillation may be frequency-modulated by a signal fed via a band width limiting and anti-aliasing low pass filter 16 to the voltage controlled oscillator 10.

The monitoring system of Figs. 4 and 5 is for use in monitoring the well-being of persons working in a hazardous environment, for example, technicians, scientists, biologists, process workers or other operatives working with dangerous chemicals or processes.

The system utilises a respective signal generating device for each person to be monitored and a common receiver operative to receive signals generated by said devices within a predetermined zone.

Each signal generating device comprises two arrangements 21, 22 each of five sensors 23 responsive to movement and a transmitter. The sensors 23 comprise mercury switches mounted on the sides of two pentagonal supporting structures 24 adapted to fit respectively around the wrists of the person being monitored. Each supporting structure 24 may be in the form of a bracelet or the like although preferably such structure is incorporated in a cuff of a protective jacket or coat that is a laboratory coat, a reinforced jacket or the like. The mercury switches 23 of both arrangements 21, 22 are connected together in any suitable series and/or parallel combination, and such combination is connected to the input of the respective associated transmitter 25.In use, whenever the person being monitored moves either or both arms, the switches 23 can change the conductive states thereof (for example by passing from normally-closed to open states) due to adjustment of the attitudes of the switches and/or by application of shock impulses to such switches. Itwill be understood that only a small movement or shock will be required to change the switches from the shortcircuit to open-circuit states as for example when the person being monitored is writing or otherwise engaged in light manual tasks. The switches 23 may all change state simultaneously or alternatively (as for example when the person rotates his wrist or arm) successively.

As the switches 23 change conductive state impulses are fed to the input of the transmitter 25.

The transmitter 25 comprises a housed batterypowered unit capable of transmitting low power limited range radiated signals via an aerial which may be incorporated in the housing or which may be a separate aerial connected to the housing. The transmitter 25 may comprise any suitable radio frequency or other radiation transmitting device although preferably an inductive transmitter as described with reference to Figs. 1 to 3 is used. The transmitter including the aerial may be mounted in a pocket or at any suitable position on the aforesaid protective garment so as to be conveniently portable.

The transmitter 25 has input circuitry incorporating high bandwidth monostablecircuitssothatthe irregulartrain of sporadic pulses produced by the sensors initiates monostable operation which in turn triggers operation of the transmitter to effect transmission of corresponding inductive signals of relatively low power.

The receiver 26, as shown in Fig. 2, is connected to an aerial 27 in the form of a wire loop extending around the periphery of the zone in which monitoring isto be effected. Signals transmitted by the transmitter 25 of the or each person working within such zone are picked up by the receiver 26 and processed with control circuitry including a signal decoding unit 28 of analogue or digital form; an alarm processing circuit 29 using microprocessor technology; a stop-go circuit 30; and an alarm and annunciation circuit 31. Essentially the control circuitry actsto assess the received signals and to acti- vate an alarm (which may be an audible and/or visual device or any other suitable device) in the event that such signals correspond to an alarm condition.

Thus, the unit 28 may distinguish the signals from the different respective transmitters (where a plurality of people are being monitored) and the signals from each transmitter may be averaged or filtered over successive predetermined periods of time by the circuit 29 to produce outputs proportional to the activity of the respective person during such periods.

Each such output can then be compared by the circuit 29 with a predetermined reference level which may or may not be time-dependent, an output being fed to the alarm circuit 31 whenever an alarm condition is detected. An adaptive process may also be used involving variable filter coefficients rather than a variable reference; or the arrangement may be such that the circuit 29 is reset by each received signal and the output is fed to alarm circuit 31 if a reset signal is not recieved after a predetermined period of time.

With this arrangement in the event that a person collapses or otherwise becomes incapacitated the pattern of transmitted signals will change as for example by termination of same and the alarm can be actuated to summon assistance. Where the arrangement is such that signals must terminate to effect alarm actuation, the transmitter aerial may be polarised in relation to the receiver aerial such as to prevent or limit the reception of transmitted signals (having regard to the low power operation of the transmitter) when the monitored person (and hence the transmitter aerial) is in a generally horizontal (or other predetermined) position thereby avoiding alarm actuation due to signals produced by involuntary muscle spasms.

Where the received signals over a period of time are compared with a reference value, such reference value may be appropriate to the normal activity of the monitored person at the particular day of the week and time of the day when such comparison is made. In this case the processing circuitry 29 will contain appropriate stored data derived by prolonged monitoring of signals produced by the person in question. Moreover, such data may be continually updated. Provision may be made for changing or modifying the basis of comparison to allow for changed working conditions or other factors.

More specifically, the arrangement may be such that an activity profile is derived for each person being monitored in relation to the total time during which such monitoring is to be effected on a daily or weekly basis; such profile representing periods during which the person is inactive (e.g. sleep or rest periods), periods of low activity (e.g. where the person is engaged in reading or other sedentary occupation), and periods of high activity (e.g. manual labour).When assessing the current signal output, the system may therefore compare such output during successive discrete periods of time with the reference level already predetermined for such periods; or alternatively the system may provide a continuous comparison representative of the cumulative level of activity over a pereceding period of time (say the preceding hour or day) compared with the expected level for that period based on a continuously updated weighted average of the historical activity level, an alarm output being produced if there is a sustained significant difference either above or below the reference level. With this arrangement the system can develop its own activity reference profile to accommodate a wide range of system requirements and applications.

When an alarm condition is detected the alarm and annunciation circuit 31 is actuated and a local alarm of an audible and/or visual nature in the proximity of the transmitter giving rise to the alarm condition is operated. The person carrying the transmitter now has an opportunity to cancel the alarm condition either by moving his arms to initiate signal transmission or alternatively by operating a control on the transmitter which transmits on a second channel a reset signal which actuates the stopgo circuit 30 to reset the circuit 29. If such remedial action is not taken by the person within a predetermined period of time (say 30 seconds), the alarm circuit 31 operates a main alarm which may be remote from the monitored zone, for example at a main control location.

With the arrangement so far described it is possibleto achieve sensitive monitoring with great reliability. In particular it is possible to make allowance for normal irregularities in activity of a person being monitored and also for differences as between different persons whereby it is possible to respond rapidly to a genuine alarm condition whilst at the same time avoiding frequent spurious actuations.

The monitoring system may be set to operate continuously or for predetermined periods related for example to a predetermined work cycle. Alternatively the system may be switched on by an operator when required for use.

It is also possible to arrange for the system to commence operation automatically when a person enters the said monitoring zone. This may be effected using a presence detector (of infra-red or ultrasonic operation) which actuates the system when a person enters the zone; or alternatively using an actuation circuit which actuates the system when signals are recieved from the signal generating device of a person entering the zone. In each case provision may be made for the system to be deactuated, by a key, code or reset button, when the zone is vacated. Where a presence detector is used it will be understood that the system can act as an intruder alarm; that is the alarm will be triggered if a person enters the zone without at the same time producing acceptable signals.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiments which are described by way of example only.

Claims (23)

1. Asystem for the transmission and reception of electrical signals comprising a receiver having a loop aerial extending around a space in which signals are to be received and a transmitter utilisable within said space, said transmitter and receiver being adapted respectively to transmit and receive inductive signals.

2. Asystem according to claim 1, wherein the transmitter operates in the range 1 to 150 KHz.

3. A system according to claim 1 or 2, wherein the transmitter has an aerial on a ferrite rod.

4. A system according to any one of claims 1 to 3, wherein the transmitter is adapted to transmit FM or AM modulated inductive signals.

5. A system according to any one of claims 1 to 4, wherein the receiver has a single fixed loop aerial.

6. A system according to any one of claims 1 to 4, wherein the receiver has multiple aerial loops arranged in mutually orthogonal positions.

7. A system according to any one of claims 1 to 6, wherein said receiver is adapted for connection to data processing equipment to feed thereto data from said transmitter.

8. A system according to any one of claims 1 to 6, wherein said transmitter is adapted to be connected to an automatic alarm sensor so as to transmit signals to the receiver corresponding to operation of said sensor.

9. A system according to any one of claims 1 to 6, wherein said transmitter is adapted to transmit a signal to be used by said receiver for the switching on of equipment.

10. A system according to claim 9, wherein said receiver is arranged to switch on a vehicle ignition circuit.

11. A system according to any one of claims 1 to 6, wherein said transmitter is adapted to transmit a signal to be used by said receiver for the switching off or inhibition of equipment.

12. A system according to claim 11, wherein said receiver is arranged for the switching off or inhibition of an alarm system.

13. A system according to claim 1, substantially as hereinbefore described with reference to and as illustrated in Figs. 1 to 3 of the accompanying drawings.

14. A monitoring system comprising a signal generating device movable within a zone, said device comprising at least one sensor for detecting recurrent stimulae and a transmitter operable to transmit signals corresponding to detection of said stimulae; a receiving device operable to receive said signals within said zone; and a control system connected to said receiving device and operable to produce an alarm output in response to a predetermined change in said received signals corresponding to a departure of said stimulae from a predetermined norm thereof.

15. A system according to claim 14, applied to the monitoring of the well-being of a person and wherein said signal generating device is adapted to be carried or worn by such person.

16. A system according to claim 14 or 15, wherein said sensor is a movement sensor.

17. A system according to claim 16, wherein said sensor comprises a mechanical position switch such as a mercury switch.

18. A system according to claim 16 or 17, with claim 15, wherein said sensor is adapted to be worn on a person's arm or leg.

19. Asystem according to claim 18, wherein there are a plurality of said sensors adapted to fit around a cuff.

20. Asystem according to any one of claims 14to 19, wherein said control system is adapted to respond to the absence of a received signal after a predetermined period of time.

21. A system according to any one of claims 14 to 19, wherein said control system is adapted to respond to a significant departure from a predetermined signal pattern ortrend over a predetermined period of time.

22. A system according to any one of claims 14 to 21,when using an inductive transmitting/receiving system according to any one of claims 1 to 13.

23. A system according to claim 14 substantially as hereinbefore described with reference to and as illustrated in Figs. 4 and 5 of the accompanying drawings.