This invention relates to improvements and new features in comparison to previous devices for infant apnea monitors using movement detection.

Apnea monitors are used to detect cessation of breathing and alert the caregiver so they may take appropriate action to correct the apnea event. There are many forms of apnea monitoring, using many direct contact methods to detect breathing such as impedance chest bands, pressure sensitive devices to detect movement or airflow and carbon probes to detect electrical signals from chest muscle movement. There are also indirect contact methods (ie. sensor is not directly attached to the infant) such as a piezo-based mat, which is fitted under a mattress to detect movement transmitted from the infant through the mattress.

This invention falls into the category of detecting apnea using indirect contact by way of a piezo sensor based mat. This type of movement sensor mat is normally placed underneath the mattress of an infant's crib, cot or bassinet and detects the chest movement associated with breathing as transmitted through the mattress. Research conducted on the various types of direct and indirect contact monitors above revealed that piezo mat based sensors returned the highest rate of false alarms by a significant margin. The current invention undertakes, by inventive steps, to improve the shortcomings of piezo based movement sensor mats and their apnea monitor units such as those found in Eh0514744, W08600996 and GB2192460. The present invention also includes innovative new features within the apnea monitor unit, which are lacking in previous inventions of this type.

As mentioned above a major shortcoming in previous piezo sensor mat-based apnea monitor inventions is a high false alarm rate. This often originates from the piezo based movement sensor mat and can largely be attributed to the following causes: Signal strength from the piezo based sensor mats can be lost through inefficient sensor mat design such as touching collector plate and base plate edges, foam separators between the collector plate and base plate and over- simple collector design. All of the above contribute to diverting movement signals directly to the base plate away from the piezo sensor element which is usually located at the center of the collector plate.

* Signal strength is lost from the piezo based sensor mat when the infant moves to the boundaries of the crib, cot or bassinet (hereafter referred to as a crib).

The sensor mat designs of previous inventions are not able to efficiently collect movement signals transmitted through the mattress at such obtuse vectors to the sensor mat edges, resulting in false alarms.

Attempts to provide wider sensing area in the crib by provision of an extra movement sensor mat or mats further degrades signal strength as the sensor mats have to be connected in parallel and fed into the same input amplifier stage. This causes destructive interference at the amplifier's input as depicted in Figure 2. The signals collected at each piezo {Signals (37) and (38)} are coupled together to give Signal (39). This resultant signal can often be weaker than the signal coming from the better-placed sensor mat as shown in Signal (37). Connecting piezo sensors in parallel also alters the stated input filter response of the apnea monitor as the capacitance value of the sensor mats combined is increased. This is undesirable as the input filter is then made more'low pass'in its response, which in turn makes the apnea monitor less sensitive, thus further increasing the incidence of false alarms.

The above shortcomings are eliminated in the present invention as described later.

The general arrangement of the present invention is described in Figure 1 with a piezo based movement sensor mat (1) connecting to a low pass filter (2) in order to reject non- breathing frequencies such as heart beat and other unwanted vibration. The low pass filter (2) outputs to an amplifier stage (3) whose output is fed into a voltage controlled oscillator (VCO), the higher the amplitude from the amplifier (3) results in a higher frequency being produced out of the VCO (4). Any number of independent stages of low pass filter (2), amplifier (3) and VCO (4) can be added according to the number of sensor mats required in the design. The output of the VCO (4) is fed into a microprocessor (6), which determines movement of the infant by way of detecting frequency change from the VCO (4). If no movement has been detected for a pre-selected time (usually 20 seconds) an alarm (5) will sound. The user has the choice of re-setting the alarm via the Smart Switch (7) or initiating the Cardio Pulmonary Resuscitation (CPR) aid via the CPR switch (35). The Smart Switch has other functions as described later.

According to the present invention, innovative new features are described below to assist caregivers in the use of apnea monitors of this type. A brief description of need and realisation of the need is given below: Cardio Pulmonary Resuscitation (CPR) aid-parents are often inexperienced in CPR techniques and soon forget the correct heart to breathing timing and ratio required in the event that CPR is required to correct a life threatening event evolving from an apnea alarm. The present invention provides an audible and visual heart to breathing timing and ratio to assist the user with CPR. This is achieved by software and hardware processing as described later.

Manual monitor suspend and automatic monitor resume mode-often, caregivers will forget to turn on the apnea monitor, putting the infant at risk without such monitoring. The present invention provides a means of detecting when an infant has been placed in a crib; the result being automatic resumption of the monitor to its primary mode of operation (apnea detection), requiring no further caregiver intervention. The operation of the monitor is suspended by manual intervention once the baby has been removed from the crib. This is achieved by software and hardware processing as described later.

Independent sensor mat amplification, processing and management-in order to overcome the shortcomings of movement signal degradation when an infant moves to the extremities of the crib or onto another movement sensor mat, independent sensor mat amplification, processing and management is provided. This ensures that the sensor mat with the best movement signal return has the focus for movement monitoring. This process is assisted by a 'sensor mat attached'sensing method, which automatically detects when an extra mat has been connected to the monitor. This feature is required to correctly manage additional sensor mats added to the monitoring system. The above is achieved in hardware and also by software frequency comparison processing algorithms as described later.

Sensor mat cable break or cable disconnected alarm-provides the caregiver with an audible and visual alarm that a sensor mat cable has not been connected correctly or has been removed or broken. This is achieved in hardware and by software processing as described later.

Smart Switch-in order to simplify the monitor to user interface a single Smart Switch has been invented to manage all but one of the apnea monitor's user activated features (this being the start CPR function). The following actions can be achieved by the Smart Switch: Monitor suspend (baby is out of crib) Manual monitor resume to primary mode (apnea detection) Apnea alarm reset CPR timing reset (CPR is activated by a separate switch) In the event of a sensor mat cable break alarm, mute alarm for twenty seconds then resume alarm if fault has not been corrected The above is achieved by software and hardware processing as described later.

Movement sensor mat features-the present invention eliminates the shortcomings of movement signal loss described previously by way of the following design: Instead of supporting the base and collector plates at the edges by, for example, rubber stops at each comer or across the whole area as in a foam or sponge separator method, the present invention uses the lever principle to attain greater mechanical advantage at the edges of the mat. This greatly increases the efficiency of the mat in transmitting movement signals across the collector plate (10) and towards the piezo sensor (12) when the baby is not directly over the sensor mat but at some obtuse angle from it The greater the leverage distance Figure 3 (32) the better. However, a practical limit exists whereby the elasticity of the collector plate will determine when the edge of the collector plate will bend and touch the base plate, thus detrimentally transmitting movement signals into the base plate and away from the piezo sensor at the center of the collector plate. Dimensions are not given, as different collector plate (10) materials will have different elasticity and hence a different bending moment. Suffice to say the leverage distance (32) should be optimised to the material used on the collector plate (10) for an envisaged maxim infant weight.

It can be seen from the above that movement sensing efficiency will be improved at the extremities of the sensor mat compared to other inventions in this category.

The piezo sensor mat design in Figure 3 {shown in side elevation in its entirety in Figure 4 (31)} shows a rubber fulcrum strip (8) on a rectangular collector plate (10). The rubber fulcrum strip (8) acting as the fulcrum point to the lever action. A variation of a circular collector plate with completely uniform lever characteristic is given at Figure 5.

The efficiency of the signal reaching the piezo sensor is further improved by the introduction of slots at Figure 3 (9) cut into the collector plate. The slots (9) effectively de-couple adjacent areas of the collector plate from each other in order to allow more of the movement signal disposed in that direction to reach the piezo sensor (12) at the center of the collector plate.

The piezo sensor is prevented from being excessively deformed or crushed between the collector plate Figure 4 (10) and base plate (33) by the addition of a rubber buffer strip (11). In order to give the piezo sensor further sensitivity, it is de-coupled from the rigid collector plate by a second flat semi-rigid rubber de-coupling washer (14), the de- coupling washer's outer diameter not exceeding that of the piezo sensor.

Consider the example in Figure 6 where the collector plate (34) is solid across its surface and without slots added such as those in the present invention. A movement signal from the infant (15) introduced at the outer edge of the collector plate is propagated uniformly across the collector. This is represented by movement signal vectors (16) (ie. signals with magnitude and direction) which, for the purposes of this description, are assumed to propagate uniformly across the collector plate (34) and (10). In the case of Figure 6 this results in potential movement signal strength at the piezo sensor being lost by dissipation of the signal within the collector plate. This is caused, for example, by friction within the crystal lattice of metal, polymer structure of plastic or the fiber structure of wood and cardboard. Further losses are caused by transmittal of the movement signal vectors (36) in unwanted directions away from the piezo sensor (12) causing the movement signal of that particular vector to dissipate to zero prior to reaching the piezo sensor.

The present invention in Figure 7 de-couples various sections of the collector plate thus diverting the direction of movement signal vectors (36) and concentrating them towards the piezo sensor (12). This results in more of the movement signal reaching the piezo sensor as shown in Figure 7 (36) and hence lessens the incidence of false alarms when an infant has moved to the extremities of the sensor mat. It has been found that eight slots equal-angled (45 degrees) around the 360 degree mat area provides good de-coupling but this would not preclude less (one to seven) or more than eight slots being added. Tt should be stipulated, however, that increasing the number of grooves eventually diminishes the collector area efficiency and potential movement signal will be lost in this way.

Explicit dimensions of base and collector plate, space between the collector plate and base plate and those of the piezo sensor, rubber buffer strip, de-coupling washer and rubber fulcrum strip are not stated as they will depend on the compression properties of the rubber. Also the bending limits of the collector plate and piezo element will vary between different manufacturing materials. Ideally dimensions will be minimised to retain good operation whilst preventing excessive deformation of the mattress (which would be uncomfortable to the infant) under which the sensing mat is located.

Ne feature£ and improvements within the apnea monitor main unit CPR feature-should the caregiver be required to initiate Cardio Pulmonary Resuscitation, pressing the CPR switch will be sensed as shown in Figure 8. The microprocessor (6) will sense a change in Logic State by the switching action of the CPR switch (17). An audible and visual timing ratio between breathing and heart compressions is then generated by software via the microprocessor (6) and output via the alarm sounding device (5) (this device is a piezo type sounder which can output any number of distinguishing audible tones when driven by a microprocessor). The timing is presented to the caregiver audibly and visually in order to assist with the CPR action.

The current heart compression to breaths ratio recommended for infants is 5 heart compressions to 1 breath with 100 chest compressions per minute against 20 breaths per minute. After 1 minute the CPR initiator will be alerted by another tone (5 seconds duration) which is a reminder signal to check the infant for life signs. It can be seen from this that there is a considerable amount of timing for the CPR initiator to maintain and hence the CPR feature is an extremely useful innovation as it removes the mental burden of timing in favour of greater concentration on resuscitating the infant. The CPR feature may be activated at any stage of the monitors operation (i. e. the CPR function takes priority over any of the monitor's other user activated features). The function may be cancelled by pressing the CPR switch again or selecting the Smart Switch as described later.

Manual monitor suspend and automatic monitor resume mode-a common problem with apnea monitoring is relying on the caregiver to remember to turn the monitor on once the infant has been placed in the crib. This is not an instinctive action and puts the infant's life at risk with the monitor being inadvertently left switched off. This could also promote tension between caregivers for failing to carry out the action of turning the monitor on.

According to this invention the monitor is always powered on (battery performance is not an issue as current technology implemented in this invention would yield over 12 months continuous operation with 4 x AA type batteries). The caregiver only then has to remember to suspend the monitor's operation when removing the infant from the crib. If they forget, they would soon be reminded, as the apnea alarm would activate due to no movement being detected.

During the suspend mode, the monitor is constantly looking for the return of the infant via saturated signals at the amplifier output. This is achieved as the infant is placed in the crib. The piezo element (12) will be significantly deflected, thus causing a large signal to be amplified. Experimentation with this feature shows that an amplifier stage will typically saturate at both the positive and negative rails of its output. This is useful as vibration generated from, say, walking past the crib or accidentally knocking the crib does not have this effect and can be discriminated by hardware and software processing.

Figure 9 (20) shows a signal at the amplifier (19) due to unwanted vibration. Figure 10 (21) shows the output of the amplifier when an infant is placed into the crib, bearing down into the movement sensor mat's area of operation. To process the signal in order to determine if it is unwanted vibration or an infant being placed in the crib the analogue signal at the amplifier output is fed to a voltage-controlled oscillator (VCO) (4). It can be seen that the upper and lower saturation limits of the amplifier will yield different frequency counts at the VCO output. These frequency counts are processed by the microprocessor (6) to determine if the monitor needs to be resumed from its suspend mode to the primary mode of operation (apnea detection). An upper and lower limit for re-activating the apnea monitor can therefore be set. It is useful to have more than one limit because, as shown in Figure (20), an excessive unwanted momentary vibration could activate one of the limits but is not usually sustainable enough (due to feedback characteristics of the amplifier) to ensure both limits are reached. If this proves not to be the case, software processing can further increase the number of times the limits have to be reached before the monitor is re-activated.

Independent sensor mat amplification, processing and management-this is an extremely useful feature as the monitor unit will decide, when two or more sensor mats are connected, which is the best sensor mat to use due to better signals coming from that sensor when compared to other sensor mats. Previous inventions connect additional mats in parallel which can degrade the monitor's performance by destructive interference as shown in Figure 2 and will also effect the input filter characteristics due to capacitance changes when paralleling piezo sensors, thus further reducing sensitivity.

The present invention employs an independent amplifier (3) and a voltage-controlled oscillator (VCO) (4) per movement sensor mat and then a software frequency process using a microprocessor (6) to determine which of the sensor mats is receiving the strongest movement signal. To assist the software process, a sensor mat attached detection method is used to inform the microprocessor as to which VCO frequency outputs need to be compared.

The frequency comparison procedure determines which sensor mat has the strongest movement signal by the following method. For this example two sensor mat channels are used but any number of additional mats can be added to the procedure.

Firstly, the number of sensor mats connected is determined by the sense method in Figure 11. The'sensor mat 1,2 etc'pin to the microprocessor is driven low when a sensor is connected due to the short (22) placed at the connecting terminal by the sensor mat wiring configuration. In the sensor mat wiring loom there will be six wires. Two take the piezo sensor signal to the low pass filter, two are shorted together as indicated (22) to provide a'mat connected'signal once the mat is connected to the apnea monitor at the connector (23) and two provide the cable break signal as described later. The connected sensors are then included in the frequency comparison process below.

Consider Figure 12, which shows the amplitude output from each amplifier stage. Each signal is fed through a separate VCO (25), (40), (41) and the corresponding frequency is read alternately by the microprocessor as shown (fl, f2, f3, f4, f5, f6, f7, etc). The third sensor mat channel (27) has no mat connected and is excluded from the procedure below.

Experimentation has shown that nine frequency samples per sensor mat is enough to determine which mat has the strongest signal. In the current invention this test would last for two seconds but would vary from microprocessor to microprocessor; the determining factor being the processing speed of the microprocessor for any given nine frequency samples.

The frequency spread sampling process is managed by the microprocessor (6), which also stores the frequency values obtained from each VCO. The microprocessor (6) then determines which sensor mat channel has the greatest frequency spread over the sample period. In this example it can be seen that Figure 12 (24) is the strongest signal and its corresponding VCO (25) output produces the greatest frequency spread (28). Once the best channel has been determined sense signals are then obtained exclusively from that sensor mat channel only until the next test is conducted. This frequency spread comparison test is conducted typically every five minutes or three seconds prior to an apnea alarm condition. This is because the infant may have simply moved out of the sensing area of the sensor mat providing the exclusive movement signals and rolled onto another sensor mat, which will be providing good movement signals.

Setisor mat cable break or cable disconnected alarm-previous apnea monitor inventions with piezo based sensor mats do not include a cable break (or removed) sensing alarm, instead relying on the removal of a sensor mat to force an alarm condition due to no signal received. This presents a risk situation as it has been shown that, with a piezo based sensor mat disconnected, the apnea monitor can be falsely triggered by static electricity at the open terminal contacts. A realistic example is if a curious child approaches the crib in which a monitored infant is placed and disconnects (or breaks) the sensor mat cable. If they then proceed to place their fingers across the monitor's sensor mat connector socket (or the broken cable's exposed wires), or play with the monitor unit, minute electrostatic discharge from their fingers, hair or body can be detected and amplified. This would be incorrectly registered within the apnea monitor as movement by the monitored infant.

The sensor mat cable break or cable disconnected alarm determines if a movement sensor mat has been disconnected or broken by using the same method for detecting sensor mat connection as in Figure 11 but using a different pair of wires shorted together. Software also determines on monitor start up, which sensor mats are originally connected. At least one mat must be connected at start up; otherwise an alarm condition would arise. An alarm condition exists if a mat is removed or sensor mat cable is broken prior to the monitor being shut down. An additional feature allows further sensor mats to be added after power up for inclusion in the sensor mat cable break detection method and also the independent sensor mat management feature described previously.

In any event, the sensor mat cable break alarm will sound continually until the sensor mat cable has been reconnected or the apnea monitor has been re=started to accept the missing movement sensor mat. There must always be at least one sensor mat connected or an alarm condition wdll exist even after re-starting the apnea monitor.

The sensor mat cable break alarm may be muted for twenty seconds by pressing the Smart Switch but it will resume if the alarm condition has not been corrected. The twenty-second muting period will be cancelled if the cable fault has been corrected.

Smart Switch-in order to simplify the monitor to user interface a single Smart Switch has been invented to manage all but one of the apnea monitor's user activated features.

This single switch can achieve the following actions: Monitor suspend (baby is out of crib) Monitor resume (Automatic Monitor Start manual override) Apnea Alarm reset CPR timing reset (CPR is activated by a separate switch) In the event of a sensor mat cable break alarm, mute alarm for twenty seconds then resume alarm if fault has not been corrected After detection of the Smart Switch (18) being depressed as shown in Figure 8, the above operating actions are then realised by software within the microprocessor. The caregiver will press the Smart Switch, which produces a response dependent upon which mode the main software program is currently operating in. A table of Smart Switch actions is givenbelow: Initial Apnea Monitor Apnea Monitor Mode After Pressing Smart Switch Mode suspendmodeapneadetection suspend mode'resume to primary mode of operation (apnea detection) apnea alarm reset apnea alarm, move to apnea detection CPR function | reset CPR function, move to apnea detection Sensor mat secondalarmmute,thenbackto'cablebreak'alarmif20 disconnected or broken conditionalarm not corrected.