Background Art Monitors for neonates and children have been known for many years and are used to monitor the breathing and/or heart rate and/or temperature of babies and'children.

Usually a system includes a monitor carried or worn by a patient which in use is in communication with a remote listening arrangement or alarm. More sophisticated monitoring systems are available in hospitals for monitoring younger babies and neonates.

One type of baby monitor is a pulse oximeter. The pulse oximeter includes a sensor which is worn close to the wearer's skin. It detects oxygenation levels in the blood. When the level of oxygenation drops below a certain value an alarm is triggered.

Another type of baby monitor is a pulse or breathing monitor, which detects pressure variations, created by a pulse or breathing and triggers an alarm if no pulse or breathing is detected. This type of monitor may include a microphone arranged to detect breathing and/or crying.

Therefore two broad categories of monitors exist. These may be referred to as:"baby listeners"and"baby monitors". Baby listeners can usually include a radio unit (transceiver) either battery or mains powered which is kept near the baby. Parents or a baby-minder are able to listen to voice transmissions at a distance, through a radio unit (transceiver) which may be mains or battery powered. Two-way communication may be possible giving instant opportunity to respond appropriately.

Baby monitors monitor heart beat or breathing of a baby in a cot using a sensor which is usually"wired"to a cot side transmitter. The transmitter can act as an alarm unit if heart rate or breathing is not sensed for more than a preset period. The alarm can be transmitted to a remote receiver unit.

An example of another type of baby monitor is a mat on which the baby is intended to lie.

Pressure variations arising as a result of the baby's breathing are detected and transmitted via a hardwire link to a radio frequency (rf) transmitter, which in turn sends a signal to the parent at a remote listening station. Although the aforementioned type of system has been extremely popular, a disadvantage has been that, especially with more mobile babies and infants, they have tended to move away from the mat, either by rolling off it or crawling.

The present invention arose partly in order to overcome this problem, and also to provide a cheaper more reliable monitoring system.

Disclosure of Invention According to a first aspect of the invention there is provided a physiological monitor, which in use is adapted to be worn on a user's body, and is capable of detecting movement within the body, the monitor including: a transducer for providing an electrical signal indicative of said movement; threshold detection means adapted to discriminate between signals arising from internal body movements and unwanted signals so as to provide a status signal indicative of the user's status; and a transmitter which transmits the status signal.

The monitor may also be used to monitor movement of the user's body.

Preferably the transducer includes an electret microphone which detects acoustic signals generated by movements of, for example, internal fluids such as blood, within the body.

The electret microphone may also be used to detect sounds generated by muscle movements, or even to detect vocal sounds from the user.

Because a threshold detection means is included, it is possible to distinguish between unwanted signals, such as external noise and vibrations; and body movements such as

vibration caused by muscle movement, blood flow through arteries, breathing and other physiological movements occurring as a result of normal musculo-skeletal interaction.

Alternatively a dual operational amplifier may be used to discard unwanted signals.

Advantageously an energy source is provided. This facilitates a portable, self-contained monitor. This may be achieved by one or more batteries. These may be mercury batteries.

The monitor and its power consumption is crucial because monitors should preferably not require their batteries to be changed or recharged more than two or three times per year.

On this basis batteries should have a life of at least 10,000 hours. As a status signal is provided, typically every 15-20 seconds, and preferably a radio frequency (rf) transmitter transmits the status signal to a remote receiving station, considerable demand is placed upon design of sampling, monitoring and control circuitry. Also rf transmission components need to be optimised so they consume as minimum current as possible.

These problems have been solved by optimisation of certain key stages and components and one way in which this is achieved is explained in detail below.

Advantageously the transducer includes a pressure sensor, for example a piezoelectric sensor, which is arranged to detect very small movements and vibrations. Alternatively motion sensors may be used. However, as the piezoelectric sensor is sufficiently sensitive to detect movements as small as 0.05 mm, this sensor is preferred.

Detected signals are filtered and amplified, typically by an amplification factor between 10-200. The detection threshold is advantageously set to exclude frequencies higher than substantially lOHz, as the range below IOHz was found to be optimum for detecting vibrations and sounds of interest. A variable detection time may be set, during which, if no body movements are detected an alarm is triggered. The time interval is preferably 10 seconds.

A retriggerable switch may be arranged to detect every second; although this may be varied to more or less depending upon constriants placed upon power consumption.

Provided signals are detected within the predetermined interval no alarm is triggered.

However, if no signal is detected during the predetermined period an alarm is triggered.

An example of a retriggerable switch includes a retriggerable monostable and astable oscillator.

A failsafe may also be included which is arranged to activate if/when an energy supply drops below a given limit.

An advantage of intermittent status transmission is that the transmitter does not have to operate continuously and this reduces current consumption.

Brief Description of Drawings The invention will now be described, by way of example only, and with reference to the Figures in which: Figures la and lb show an overall diagrammatical view of a physiological monitoring system, including a monitor ; and Figure 2 is a circuit diagram of an embodiment of the monitor.

Detailed Description of Preferred Embodiments Referring to Figures la and lb, a baby shown diagrammactically at (10), is positioned in a cot or incubator (12). A monitor (14) is placed around the baby's ankle or wrist. The monitor (14) includes a transducer (16) and sensor (18) arranged to detect movements of the order of 0.05 mm. Threshold discriminator (19) is arranged to discard or ignore signals greater that about IOHz. A radio frequency (rf) transmitter (20) is also housed in monitor (14). Transmitter (20) transmits to a remote receiving station (22). Receiving station (22) is connected to audible and visible alarms (24) and may be adapted to receive signals from a plurality of other monitors (not shown).

Each of the aforementioned elements are now described in greater detail and with specific reference to Figure 2.

Sensor To detect sound signals generated by a living body, a relatively sensitive, small and low cost acoustic transducer (16) is used. Most microphones tested tended to be large and have high power consumption. An electret microphone was found to be the only transducer which met requirements of size and power consumption.

After selecting a suitable electret microphone it was discovered that acoustic coupling from a wearer's body (10) was enhanced by removing any felt pad attached to the front face of the sensor housing (26). It is believed that by doing this, vibration energy is coupled more effeciently to the electret sensor by creating a cavity over the area of body (10) which contacts sensor (18). This cavity enables'pressure waves'to be transmitted to sensor (18) when the surface of the skin rises and falls due to the physiological factors.

Amplification The amplification stage is shown diagrammatically as (30). Frequency content of sounds detected are predominantly in the range lHz to IOHz. Frequencies below lOHz exist primarily due to body movement. A dc blocking capacitor (32) (lOOpF) cuts off signals below lOHz. Selecting a value of capacitor to give an even lower cut-off frequency is possible. However, practically capacitor (32) becomes physically large and unsuitable for small, portable application. A compromise is therefore achieved between size and characteristics by using this value capacitor. Blocking capacitor (32) is necessary to block the dc voltage applied to signal line (34) in order to power the microphone's integral amplifier.

Signal amplification is needed in addition to any internal amplifier of the transducer (16), due to the fact that only very small amounts of energy are detected. This stage of amplification is provided by a single stage operational amplifier (35) which incorporates a 100nF feedback capacitor (36) to limit any high frequency response of the amplifier.

Shunt capacitor (36) produces a 3dB roll off around IOHz. This is because amplifying above this frequency only increases electrical background noise and consequently

degrades performance. Choosing this cut-off frequency also means that domestic mains pickup and most other types of interference present in a typical domestic or industrial environment is reduced.

Initial signal amplification is performed within the sensor (18). This is because ultra low noise operational amplifier (35) for second stage of amplification is not critical. The main design criteria is the operational amplifier (35) should be ultra low power to maximise battery life, operate from low voltage supplies, again for battery use, and be available in a small package to minimise the size and weight of the monitor. An Analog Devices OP113 was used in a prototype.

Initially gain of the operational amplifier (35) is adjustable (typically between x14- x150). However, once a typical level of amplification is established then gain is advantageously fixed by replacing the potentiometer with a single resister (not shown).

This reduces component count and eliminates the need for factory adjustment. Replacing with a resistor also further reduces physical size and cost.

Threshold Detector Threshold detector (40) includes a comparator (41) (LM311) which performs threshold detection. A dc threshold level is derived by resistively dividing supply line (42). Where the supply voltage may vary (e. g. as in battery operated equipment) this threshold voltage may need to be generated by tapping voltage across a reference diode (not shown) as opposed to using a resistor chain. If a diode is used then the voltage drop across it can also be used directly to provide a reference voltage to set the dc operating point in the amplifier section. This further reduces the number of components required.

A potentiometer may be used to set the actual threshold level, but this is preferably replaced with fixed components once a typical threshold level has been established. This again helps to reduce the component count and overall costs.

By fixing the gain and threshold levels a certain level of performance or sensitivity is established for the monitor (14). This is optimum when maximum sensitivity is achieved

without the monitor (14) responding to internally generated electrical noise or external background sounds and vibrations.

An alternative to using a comparator is to use a dual operational amplifier (not shown).

This may be in the form of an Integrated Circuit (IC). One half of the IC may then be used for the amplifier and filter circuit, whilst the other half can be configured as a comparator (with appropriate modification for hysteresis). Performing threshold detection in this way further reduces the circuit complexity and the number of components used.

This not only lowers manufacturing costs but also reduces current consumption and space required to house the components.

A retriggerable pulse generator (50) comprises a retriggerable monostable and an astable oscillator. Output (45) from the threshold detector (40) is used to trigger the monostable oscillator each time comparator section'detects'body sounds. Providing that detection occurs more frequently than, say, approximately once a second, then monostable oscillator is repeatedly retriggered and its output remains active indefinitely. In the event that detection does not occur within a predefined time interval, say ten seconds, then the output from the monostable oscillator changes state and indicates an alarm condition.

Output from the monostable oscillator may be used directly to provide supply current to a low power CMOS astable oscillator (52). Oscillator (52) is configured to generate narrow pulses, which in turn are used to power transmitter (60), for as long as the astable power supply remains present.

In an alternative embodiment, detection of output from comparator is used to control directly transmitter (60). This has the advantage of reducing the overall number of components, but doing this does present two potential problems, both of which may result in significantly increased power consumption in the transmitter section. These problems arise from the fact that the rate of detection of body sound, and the duration that each sound peak exceeds the threshold (and hence the width of the detected output pulse), are not very well defined.

Fortunately, by the inclusion of a monostable between the output of comparator (41) and transmitter (60) it is possible to obtain accurately controlled transmit burst widths, which overcomes the latter of these problems. Unfortunately, this does not necessarily improve the transmitter efficiency as the rate of detection remains uncontrolled.

Transmitter In order to maximise battery life, current consumption was minimised wherever possible.

Consequently, it was decided to pulse the transmitter (60) rather than run it continuously.

Pulse generator (50) performs this function. By transmitting very short bursts of low RF power in this way average battery current drain is kept well below 1mA for the transmitter section. The pulses are as short as possible in duration but sufficiently long to allow receiver (22) to detect the presence of a transmitted carrier. In the event of an 'alarm condition', the pulse generator (50) is turned off. This in turn switches off the transmitter (60). This is then detected by receiver (50).

Transmitter (60) is a simple, single transistor, crystal controlled oscillator as this provides a frequency stable source using a relatively small number of components. Also, to reduce complexity, the oscillator output is neither modulated nor encoded in any way.

For multi-channel applications, where a number of monitors may be used together, some form of encoding of each transmitter (60) is required. Alternatively a range of different transmit frequencies may be used. This ensures that monitors do not interfere with each other.

One solution, for multi-channel use, is to use frequency diversity as this essentially requires different crystals to be installed in each oscillator and therefore does not add significantly to the cost or complexity of a sensor. Other options may be to use a proprietary encoder/decoder chipset but these may require additional circuitry in the transmitter.

If encoding were used, then one form is to use pulse position modulation within transmit bursts from a transmitter, similar to that used in commercial infra-red transmitter systems.

If necessary, further complexity may be added with the use of a small microprocessor (not shown) to control the transmit bursts with the aim of sending data over to the receiver.

Although data rate is low, information such as amplitude of detected body sounds and the rate at which detect threshold is exceeded may be sent to the receiver. Doing this potentially enables the system to detect the presence of body movement in addition to the normal background body signals, and thus the monitor may also act as a tag or locator.

Burst transmissions are detected at the receiver, and then envelope or amplitude detected to reconstruct an astable signal at the transmitter. The repeated detection of these transmitted bursts provide a'no alarm'condition. For an alarm to be raised, the receiver simply has to detect the absence of one or more of these burst signals. This then (normally) indicates either the transmitter/sensor battery has failed, the sensor has been removed from the body, or the body noise has reduced below the pre-defined threshold.

Battery The main constraint placed on the battery power source is that it should be an integral part of a'standalone'monitor intended to be worn on the body. By implication this means that a battery power source of some description is needed. To this end a range of batteries of differing were investigated, including rechargeable cells. Unfortunately, the power density of all rechargeable cells tested appeared to be far less than that of comparable sized primary cells. This made their use less attractive as the cells would then require regular charging to ensure that they did not fail during use.

A more significant factor against the use of a rechargeable cells is due to the fact that they need to be recharged, necessitating the inclusion of a charging circuit in the design.

Although this is possible in an alternative embodiment, in the present embodiment the additional components impact on the component count, cost and size and also require the provision of a mechanism for coupling power to the battery. This could either be achieved by the use of metal contact pads, or by some form of inductive coupling. Either way, both these approaches undoubtedly further add to size and/or complexity of the wearable unit, especially if the housing is required to be washable/water resistant. Also, if contact pads were to be used, then provision needs to be made to ensure that there is adequate

protection for the battery, in the event that these contact pads are accidentally shorted together.

The invention has been described by way of examples only, and variations may be made to the aforementioned embodiments without departing from the scope of invention.