DISTANCE MEASURING DEVICE FOR DETECTING SLEEP DISORDERS

Technical Domain

The present invention relates to a distance measurement device which is capable of very accurately measuring distances, in particular when mounted on a living being through a support, without having to use voltages or magnetic field values which would harm the human body.

Technological Background of the invention

The patent applications US 2007/027SS66 A1 and DE4114398 disclose sleep disorder detectors comprising a distance measuring devices. The distance measuring device comprising an emitter and a receiver, said emitter containing a resonance circuit having a resonance frequency, said emitter further being arranged to produce a magnetic field emission intermittently, said emission having a predetermined energy, said receiver being connected to a conditioning and measuring unit comprising a detector, said receiver further being arranged to pick up at said resonance frequency the magnetic field emitted by said emitter, said receiver being further arranged to provide a first signal representing the voltage level associated to said picked up magnetic field emission to the input of said detector, said detector being arranged to determine a residual energy value by correlation of said first signal with a second predetermined signal having a waveform representative of a signal to be picked up by the receiver, said detector being further arranged to produce a distance measurement signal representing the distance between emitter and receiver, based on said residual energy value. According to the known device, the strength of the measured magnetic field determines the distance between the emitter and the receiver and can be used to accurately measure the distance between 2 points. Unfortunately, those devices present the drawback to require a relatively high voltage level to charge the resonance circuit. Said higher voltage results on one hand in a higher power consumption of the device, certainly when taking in account that this distance measurement is used to monitor the movement of the mouth of a living being, and so performed almost continuously with small time intervals in between. As the invention is mainly used while mounted on the face of a living being, high energy values should be avoided due to safety risks. This high energy values will also imply the use of strong shielded wiring between said emitter, said receiver and said detector in order to avoid interference with the measured signal.

Brief summary of the invention

The object of the invention is to produce a distance measuring device that is capable of measuring distances very accurately, using a low magnetic field value, produced by a resonant circuit excited by a low level voltage source, without having a detrimental effect on the health of the human body.

This object is solved by a distance measurement device according to the independent claim 1.

The object is further solved by a sleep disorder detector with such a distance measurement device, by a movement analyze comprising such a distance measurement device and by a method for measuring a distance for identifying a sleep disorder according to the independent claim.

It has been indeed realized according to the present invention that it is possible through an optimized, more efficient charging circuit to solve at least a part of the previously stated drawbacks by providing a device in which the voltage used to charge the capacitor can be many times lower than the desired voltage level reached across said capacitor. The emitter comprises a resonance circuit and the conditioning and measuring unit comprises an energizing circuit. The specific arrangement of said resonance circuit and said energizing circuit allows said energizing circuit and said resonance circuit to switch between different charging states to create and amplify a magnetic field, generated by said resonance circuit. Therefore, even if the voltage source generates a low-level voltage, this creates a magnetic field value which will be further amplified until the energy value of said magnetic field reaches the predefined maximum energy value. The amplification of the magnetic field by switching the 4 switches alternatively a predefined number of times in order to alternate between the different possible configuration of the resonance circuit makes it possible to use a low voltage, although providing enough resolution to the distance measuring device according to the present invention, in order to measure distances very accurately.

The distance measuring device is indeed intended to measure the movement of the mouth of a human being while sleeping. While some movements are with a quite big amplitude and are easy to detect, others are less easy to measure and require a high resolution of the distance measurement device. As mentioned in the prior art, to measure the breath effort upon sleeping, the distance measuring device should be able to measure very light mouth movement (less than 0,1 mm for mouth openings up to 5 cm compared with the mouth closed position) and this, regularly.

Other embodiments of the distance measuring device according to the present invention are mentioned in the appended claims.

In one embodiment, said emitter and said conditioning and measuring unit and/or said receiver and said conditioning and measuring unit being connected to each other by using a cable selected in the group consisting of a shielded single core cable, a coaxial cable, a shielded twisted pair cable, and a twisted pair cablelt has been now identified surprisingly, while the prior art was teaching to have a high voltage circuit to be able to reach enough accuracy for the measure, that it is also possible to reach a very high accuracy of the measure by using a low voltage input, which creates the same magnetic field in the connections between emitter and respectively receiver and the detector if carefully selecting the right cable, to connect the elements without having detrimental effect of cross influence.

Advantageously, in the distance measuring device according to the present invention, said detector is arranged to implement said correlation with said second signal, which second signal is formed by said waveform representing the signal to be picked up as obtained in the absence of perturbation. In a further preferred embodiment, said second signal can alternatively be formed by said waveform being represented by a sinusoidal waveform in synchronization with the first signal itself multiplied by a time window with reduced taper factor or by a square waveform in synchronization with the first signal.

Preferably, the emitter is housed in a protective case and arranged to produce said magnetic field outside said case with a strength less than 2 mTesla, preferably less than lmTesla, such as less than 800 pTesla, less than 600 pTesla, less than 500 pTesla, less than 100 pTesla, more preferably less than 1 pTesla.

Advantageously, said emitter further comprising an inductance and a capacitor connected in series with one another and connected by means of electrical conductors to said conditioning and measuring unit comprising said energizing circuit, said energizing circuit comprising a voltage source.

The present invention also relates to a sleep disorder detector comprising a distance measuring device according to the present invention.

Preferably, the sleep disorder detector according to the present invention comprises a casing for being placed directly on the head of a living being as to measure the movement of the mouth.

Advantageously, said detector comprises an analyzer having an input connected to the distance measuring device and is arranged to receive (directly or later) said distance measurement, said analyzer can also be an external analyzer (i.e. a computer provided with the required software), said analyzer being arranged to compare and match the measured signal with a collection of characteristic value sets describing movement patterns of the mouth of a living being, and to identify a characteristic set of movement pattern within said collection characteristic value sets describing movement patterns, producing a detection signal upon.

Favorably, said characteristic value sets further comprises a first characteristic set of characteristic values describing a sudden closing of the mouth, a second characteristic set of characteristic values indicating a slow opening of the mouth, a third characteristic set of characteristic values indicating a slow closing of the mouth, and a fourth characteristic set of characteristic values indicating an increase in the amplitude of the signal at the breathing frequency followed by a decrease in the signal at the breathing frequency.

Preferably, said analyzer is arranged to add the distance measure signal to the characteristic set which matched to distance measure signal, to be used and to improve the matching to this pattern in the future. Said analyzer is further arranged to produce a specific sleep disorder signal upon recognition of a specific sequence of any of the first, second, third and fourth characteristic set of characteristic values.

Advantageously, said detector comprising a decision element using said detection signal to provide an indication of insufficient, correct or excessive treatment for the targeted sleep disorders.

In one embodiment, the method for measuring a distance for identifying a sleep disorder comprising the steps of :

i) Production of a magnetic field intermittently having a maximum predefined energy value by an emitter having a resonance circuit having a resonance frequency,

ii) picking up by said receiver at said resonance frequency the magnetic field produced by the emitter, and providing the associated voltage level to the input of said conditioning and measuring unit comprising said detector, to whom said receiver is connected,

iii) Determining by said detector based on the ratio between the maximum energy value and the residual energy value, a distance measurement signal representing the distance between said emitter and said receiver, said determining step of said distance measurement signal being done by correlation of said first signal with a second predetermined signal having a waveform representative of a signal to be picked up by the receiver, said second signal comprising a time window having a predetermined duration and comprising at least an initial sub-period and a final sub-period, said second signal being an alternating signal synchronized with the first signal and whereof the amplitude is attenuated during the initial and the final periods,

wherein said method further comprises a step of alternating opposite charging states of the resonance circuit, thereby creating and amplify a magnetic field, generated by said resonance circuit, until the energy value of said magnetic field reaches the predefined maximum energy value, said emitter and receiver being connected to said conditioning and measuring unit comprising said detector by using a cable selected in the group consisting of shielded single core, coaxial, shielded twisted pair, twisted pair, ...

The method further comprising a multiplication and integration with said second signal formed by said waveform representing the signal to be picked up as obtained in the absence of perturbation by said detector.

The invention will now be described in more detail with the help of the drawings that depict a preferential embodiment of a device according to the invention and a sleep disorder detector comprising a distance measuring device according to the invention. In the drawings:

FIG 1: Illustrates the basic structure of the measuring device.

FIG 2: Illustrates the energizing of the emitter and the signal emitted by the emitter and the signal received by the receiver.

FIG 3: Illustrates the energizing topology for the resonant circuit, forming the emitter.

FIG 4: Shows the different circuit states and the corresponding states of the switches.

FIG 5: Shows a schematic representation of the energizing circuit of the emitter.

FIG 6: Illustrates how the invention can be mounted on the face of a human being.

DETAILLED DESCRIPTION OF THE INVENTION

The distance measuring device uses a robust distance sensor for physiological use. Using a magnetic field to measure the distance offers many advantages, as it passes through clothing, pillows, sheets, .... The magnetic field being used for the measurement is extremely small, and periodic, at the measuring rate. The maximum amplitude reaches a value less than lmTesla, considered to be non-harmful to health in the event of continuous exposure, preferably a value less than 1 micro Tesla, preferably less than 0,4 micro Tesla. The distance measurement device comprises preferably an emitter 1, a receiver 2 and a measuring and conditioning unit 3. The measuring and conditioning unit 3 comprises preferably an energizing circuit, a detector and a power source.

The distance measurement device is preferably designed to operate with loose magnetic coupling, which means that the emitter 1 affects the receiver 2, but the attenuation between the emitter 1 and the receiver 2 does not allow the receiver 2 to affect the emitter 1 in return. The detection method is preferably implemented in digital processing which allows very accurate measuring while still making use of small components, assuring an easy to implement ergonomic system.

FIG. 1 illustrates an example of a basic structure of a device according to the invention. In the shown example, the energizing circuit, the detector and the power source of the measuring and conditioning unit 3 are all in the same device, the measuring and conditioning device 3. However, it would also be possible to arrange the energizing circuit, the detector and/or the power source in at least two distinct devices. In the shown example, the emitter 1, the receiver 2 and the measuring and conditioning unit 3 are arranged in three distinct devices, the emitter device 1 (including the emitter 1), the receiver device 2 (including the receiver 2) and the measuring and conditioning device 3 (including the measuring and conditioning unit 3). The different devices are connected by at least one cable 4. In another example, it is also possible to arrange measuring and conditioning unit 3 in the emitter device 1 or the receiver device 2. In another example, the energizing circuit, the detector and the power source can be arbitrarily distributed among the emitter device 1, the receiver device 2 and/or the measuring and conditioning device 3.

The emitter 1 is configured to emit or generate or produce a magnetic field. The emission/production/generation of a magnetic field is preferably a local magnetic field which does not emit an electromagnetic wave using the principle of magnetic coupling. However, it would also be possible to use the emission of a (electro)magnetic wave to the receiver 2. The emitter 1 comprises an inductor for emitting/producing/generating the magnetic field of the emitter 1. The emitter 1 comprises preferably a resonance circuit with the inductor and a capacitor. Preferably, the inductor and the capacitor are connected in series. However, it would also be possible that the inductor and the capacitor of the resonance circuit of the emitter 1 are connected in parallel. However, the series connection of the capacitor and the inductor gives the best result. The inductor comprises a first terminal and a second terminal for connecting the inductor. The capacitor comprises a first terminal and a second terminal for connecting the capacitor. The resonance circuit comprises a first terminal and a second terminal for connecting the resonance circuit. The terminals can be actual terminals for connecting cables or PCB conductor tracks or could be virtual entrances and/or exits in the components. If the inductor and the capacitor are connected in series, the second terminals of the inductor and the capacitor are (directly) connected and the first terminal of the capacitor is connected to the first terminal of the resonance circuit and the first terminal of the inductor is connected to the second terminal of the resonance circuit of the emitter. If the inductor and the capacitor of the resonance circuit of the emitter 1 are connected in parallel, the first terminal of the inductor is connected with the second terminal of the capacitor and with the second terminal of the resonance circuit, and the first terminal of the capacitor is connected with the second terminal of inductor and with the first terminal of the resonance circuit.

The power source is preferably a direct current (DC) power source. The term power source in here is used equivalently with the terms electric source, voltage source or current source. The power source comprises preferably a first pole and a second pole. In the shown example, the first pole is positive and the second pole is negative. In another example, the first pole is negative and the second pole is positive. The power source is preferably a battery or an accumulator. In Fig. 3, the first pole or the positive pole or the voltage difference between the two poles corresponds to 3,3 Volt. Obviously other values are possible. Preferably, the potential difference between the two poles has a value smaller than 20 Volt, preferably than 10 Volt, preferably than 8 Volt, preferably than 6 Volt, preferably than 5 Volt, preferably than 4 Volt. In one embodiment, the second pole corresponds to ground.

The conditioning and measuring unit 3 and/or the energizing circuit is configured to control or drive the emitter 1 and/or the resonance circuit to generate the magnetic field. The energizing circuit is connected with the power source and with the resonance circuit of the emitter 1. The energizing circuit is configured to drive the resonance circuit of the emitter 1 based on the power of the power source to generate the magnetic field. The energizing circuit is configured to produce intermittently a magnetic field at the emitter 1 or its resonance circuit. Preferably, a measurement magnetic field pulse (short measurement pulse) is intermittently generated at the emitter 1, preferably periodically with a measurement or sampling frequency. Each measurement pulse detected at the receiver 2 is used to measure one distance. By measuring a sequence of measurement pulses, a sequence of distances can be measured. The measurement pulse comprises preferably an alternating magnetic field with a maximum magnetic field amplitude increasing in a first portion of the measurement pulse and decreasing in a second subsequent portion of the measurement pulse. In the following, the generation of the magnetic field in the resonance circuit of the emitter 1 (for one measurement pulse) is described.

The energizing circuit is configured to charge the resonance circuit of the emitter 1 in a charging period (of the measurement pulse). The energizing circuit comprises at least one charging state in which the resonance circuit of the emitter 1 is connected to the power source to introduce energy in the resonance circuit (and thus amplifying the magnetic field). The energizing circuit comprises preferably at least one switch configured to change the connection between the power source and the resonance circuit of the emitter 1 during the charging period to generate the magnetic field, in particular to maximise the energy entering in the resonance circuit. Preferably, the at least one switch is configured to change between at least two charging states defining to connection states between the power source and the resonance circuit of the emitter 1 during the charging period. Preferably, the energizing circuit comprises at least two, preferably four switches for switching the energizing circuit between at least two charging states.

Fig. 3 shows an example of the energizing circuit. Fig. 4 and 5 show different states of the energizing circuit. Preferably, the energizing circuit comprises a first switch Ml, a second switch M2, a third switch M3 and a fourth switch M4. Preferably, the at least one switch or the first switch Ml, the second switch M2, the third switch M3 and the fourth switch M4 are transistors, preferably metal-oxide-semiconductor field-effect (MOSFET) transistors. Preferably, the first switch Ml and the second switch M2 are n-type MOSFET transistors and the third switch M3 and the fourth switch M4 are p-type MOSFET transistors. Preferably, (the first terminal of) said capacitance (or the first terminal) of the resonance circuit is connected to (the first terminal of) the first switch Ml and (the first terminal of) the fourth switch M4 of the energizing circuit. Preferably, (the first terminal of) said inductance (or the second terminal) of the resonance circuit is connected to (the first terminal of) the second switch M2 and (the first terminal of) the third switch M3. Preferably, said energizing circuit is further connected to a voltage source between the fourth and third switches M4, M3, i.e. the second terminal of the third switch M3 and the second terminal of the fourth switch M4 are connected to the voltage source, more precisely to the first pole of the voltage source. The second terminal of the first switch Ml and the second terminal of the second switch M2 are connected to the second pole of the voltage source which is sometimes also referred as ground GND (see Fig. 3).

Preferably, in a first charging state a first voltage (unequal zero or charging the resonance circuit) is applied to the resonance circuit and in a second charging state a second voltage (unequal zero or charging the resonance circuit) is applied to the resonance circuit. Preferably, the first and second voltage have opposite polarity and the same absolute amplitude.

Preferably, in the first charging state, the first terminal of the resonance circuit or the first terminal of the capacitor is connected to the first or positive pole of the power source, and the second terminal of the resonance circuit or the first terminal of the inductor is connected to the second or negative pole of the power source (see Fig. 5, charging state 1). Preferably, in the first charging state, the first switch Ml is open or not connected, the second switch M2 is closed or connected, the third switch MB is open or not connected, and the fourth switch M4 is closed or connected (see Fig. 4 and 5, charging state 1).

Preferably, in the second charging state, the first terminal of the resonance circuit or the first terminal of the capacitor is connected to the second or negative pole of the power source , and the second terminal of the resonance circuit or the first terminal of the inductor is connected to the first or positive pole of the power source (see Fig. 5, charging state 2). Preferably, in the second charging state, the first switch Ml is closed or connected, and the second switch M2 is open or not connected and/or the third switch M3 is closed or connected, and the fourth switch M4 is open or not connected, (see Fig. 4 and 5, charging state 2).

Preferably, the first and the second charging state are alternated a plurality of times during the charging period. Thus the energizing circuit creates a predefined series of voltage pulses alternating between positive and negative voltage, in order to energize the emitter 1 or its resonance circuit and thus amplify the magnetic field (i.e. increasing the (maximum) amplitude of the oscillating magnetic field). Preferably, the energizing circuit is configured to switch the at least two switches such that the first charging state follows directly after a second charging (without any other switching state of the energizing circuit in between) and the second charging state follows directly after a first charging state (without any other switching state of the energizing circuit in between). This excludes obviously the charging state first applied in the charging period. Preferably, each sequence of one first charging state and one subsequent second charging state or each sequence of one second charging state and one subsequent first charging state has the length corresponding to the length of one resonance cycle of the resonant frequency of the resonance circuit of the emitter 1. Fig. 2a shows an example of such a series of voltage pulses with positive and negative voltages during the charging period. The current flowing in the inductance of the emitter 1 is shown in Fig. 2b). The current in the inductance of the emitter 1 will result in a magnetic field. The magnetic field will have a direction and amplitude based on the current.

The charging period lasts preferably until the energy value of said magnetic field reaches a predefined maximum energy value. This can be reached by detecting the energy value of the magnetic field and stopping the charging period, when the magnetic field detected reaches the predefined maximum energy value. Alternatively, the charging period could have a (fixed or configurable) predetermined length corresponding to the maximum energy value. Many parameters of the resonant circuit can be used as energy value of the magnetic field. It could be the absolute amplitude or the square of the amplitude of the magnetic field or of the current in the inductance or the (last) maximum of the absolute amplitude or the square of the amplitude of the magnetic field or of the current in the inductance or an integration about the absolute amplitude or the square of the amplitude of the magnetic field or of the current in the inductance during the charging period or the voltage across the capacitor or many more.

The energizing circuit creates thus in the charging period a n AC current in the inductor of the emitter 1 whose (maximum) amplitude increases due to the power or energy received during the charging period from the power source. The energizing circuit creates thus an oscillating or alternating magnetic field with the (maximum) amplitude of the magnetic field increasing.

The energizing circuit comprises preferably further a resonance state. Preferably, the energizing circuit is switched in the resonance state during a resonance period following the charging period. The resonance state allows the resonance oscillation of the resonance circuit. In the resonance state, the first terminal of the resonance circuit or the first terminal of the capacitor is connected to ground, and the second terminal of the resonance circuit or the first terminal of the inductor is connected to ground (see Fig. 5, resonance state). Preferably, in the resonance state, the first switch Ml is closed or connected, and the second switch M2 is closed or connected and/or the third switch M3 is open or not connected, and the fourth switch M4 is open or not connected, (see Fig. 4 and 5, resonance state).

The energizing circuit is preferably configured during a measurement pulse to alternating between the two charging states during the charging period and subsequently to switch to a resonance state during the resonance state.

The energizing circuit comprises preferably an initial state. In the initial state, the resonance circuit is open, i.e. not forming a resonating closed loop. In the initial state, the all four switches Ml to M4 are open. The initial state is preferably applied between two measurement pulses, before a measurement pulse, after a measurement pulse, during a stand-by-state and/or during a switched off state of the distance measurement device.

The invention makes use of preferably 4 states:

i) Initial state: all switches are in an off state, the circuit is neither energizing nor resonating.

ii) Charging state 1: Switches M2 & M4 are closed while Ml & M3 are open. The voltage source will allow the inductance to let current increase and hence capacitor to charge towards the supply voltage, causing the resonance circuit to start oscillating and creating a magnetic field as shown in FIG 2.

iii) Charging state 2: Switches M2 & M4 open, whilst Ml & M3 are closed. This will cause an inverted voltage to be applied to the dipole formed by capacitor and inductance, reversing the phenomenon of charging state

1, boosting the inversion of current in the inductance and the discharge of the capacitor.

iv) Resonance state: Switches M3 & M4 opened, whilst Ml & M2 are closed, the resonance circuit is completely coupled off the energizing circuit and will continue to oscillate until all energy is converted into a magnetic field. The combination of energizing and resonance circuit will alter a predefined amount of times between charging state 1 & charging state 2, causing the amplitude of the magnetic field created by the inductance to reach the predefined value. This alternation between both charging states allows the circuit to reach a voltage level across the capacitor many times higher than the voltage level of the source. The energizing circuit comprised in the conditioning and measuring unit will alternate a predefined amount of times between charging state 1 and charging state 2, until the desired energy level has been reached (such as 15, 20, 25, ... times higherthan the voltage level of the source). This lower charging voltage lowers the effect of stray coupling, allowing to use a selective group of cables to be used to connect emitter, receiver and conditioning and measuring unit.

The receiver 2 is configured to pick up the magnetic field generated by the emitter 1. Picking up the magnetic field means preferably measuring, detecting or sensing the magnetic field (generated by the emitter 1). Picking up the magnetic field could also mean to receive the magnetic field or the electromagnetic wave from the emitter 1. The receiver 2 is preferably configured to transform the magnetic field picked-up into a first signal, e.g. a voltage signal. The first signal is sent to (an input of) the detector. The first signal is preferably an analogue signal. However, it is also possible that the first signal is a digital signal. The receiver 2 comprises preferably an inductor. Preferably, the receiver 2 comprises preferably a resonance circuit comprising the inductor and a capacitor. Preferably, the capacitor and the inductor are connected in parallel. However, it would also be possible to connect the capacitor and the inductor in series.

The detector is configured to measure or determine the distance between the emitter 1 and the receiver 2 based on the magnetic field picked- up and/or based on the first signal. The detector is preferably configured to determine a residual energy value by correlation of said first signal with a second predetermined signal having a waveform representative of a signal to be picked up by the receiver 1. Said second signal comprising a time window having a predetermined duration and comprising at least an initial sub-period and a final sub-period. Said second signal being an alternating signal synchronized with the first signal and whereof the amplitude is attenuated during the initial and the final periods. Said detector being arranged to determine based on the ratio between a maximum energy value of the emitted magnetic field and said residual energy value, a distance measurement signal representing distance between said emitter 1 and said receiver 2. In one embodiment, said detector is arranged to implement said correlation with said second signal, which second signal is formed by said waveform representing the signal to be picked up as obtained in the absence of perturbation. The method for detecting the distance based on the first signal is described in more detail in US2007/0273366 whose content shall be incorporated by reference for the sake of brevity.

The emitter 1 and the receiver 2 use the principle of resonance circuits that are able to influence each other if they have a mutual resonance frequency. The use of these circuits significantly increases the accuracy of the distance measuring device. Due to the low voltage values required to charge the emitter circuit, the influence of said voltage on the distance measure signal produced by the receiver 2 will be negligible. The voltage obtained at the side of the receiver 2, in the event of compatibility of the resonant circuits, is shown on FIG 2c).

The envelope of this signal depends on the quality factor of the resonant circuits being used. In the event of poor compatibility of the resonant circuits, this envelope can be significantly modified, with the possible appearance of a beat phenomenon. The maximum value of the voltage observed at the receiver varies as a function of the distance between emission inductance and receiving inductance according to the relationship:

Where a is the overall detection gain, including the effect of the amplifiers, b a possible offset due to the detection circuits and d the distance between the emitter and the receiver In the preferred embodiment of the invention, the resonant frequency is chosen preferably between 5 and 8 kHz, so as to maximize the quality factor of the resonant circuits whilst remaining below the radio frequencies. The use of miniature inductances can however lead to the adoption of frequencies going up to 50, perhaps even 100 kHz, according to the capabilities of the detection circuit. The value and the size of the resonance inductances are adapted so as to be able to base the detection on a signal having lOto 30 periods of resonance of significant amplitude. Shorter durations degrade the detection performance, whilst larger values require too great an accuracy, in practice, in the tuning of the resonant circuits.

The emitter 1 is arranged to produce a magnetic field by means of the resonant circuit formed by the inductance and the capacitor. The resonant circuit has an associated resonant frequency and the receiver 2 is arranged to pick up at said resonant frequency the magnetic field emitted/produced by the emitter 1 and to provide the associated voltage level to the input of the detector. As illustrated in FIG 2a, the emitter 1 produces a magnetic field from a series of charging pulses supplied by the energizing circuit. Thus this magnetic field is produced intermittently and each emission has a predetermined energy, in particular determined by the value of the inductance and the capacitor, the voltage level of the energizer and the number of charging pulses.

The conditioning and measurement unit 3 could comprise further an analyser for analysing a sequence of distances from the detector to determine an analysis result about the human being on which the distance measurement device is mounted. The analysis result could be a sleep disorder.

The distance measurement device comprises preferably at least one cable 4 connecting the conditioning and measurement unit 3 with the emitter 1 and the receiver 2. Preferably, the at least one cable 4 comprises a first cable connecting the energizing circuit with (the resonance circuit of) the emitter 1. The first cable comprises a first conductor connecting the first terminal of the resonance circuit with a first terminal of the energizing circuit. The first terminal of the energizing circuit is connected to the first terminals of the first switch Ml and the fourth switch. The first cable comprises a second conductor connecting the second terminal of the resonance circuit with a second terminal of the energizing circuit. The second terminal of the energizing circuit is connected to the first terminals of the second switch M2 and the third switch MB. Preferably, the at least one cable 4 comprises a second cable connecting the detector with (the resonance circuit of) the receiver 2. The second cable comprises a first conductor connecting a first terminal of the resonance circuit of the receiver 2 with a first terminal of the detector. The second cable comprises a second conductor connecting a second terminal of the resonance circuit of the receiver 2 with a second terminal of the detector. The first cable and/or the second cable is a cable selected in the group consisting of a shielded single core cable, a coaxial cable, a shielded twisted pair cable, and a twisted pair cable.

Fig 6. The device is then mounted on a support arranged to be applied onto the head of a living being so as to measure movements of the mouth. The operating parameters of the sensor are adjusted to produce a peak- to-peak measurement noise less than 0,1 mm for mouth openings up to 5 cm compared with the mouth closed position. The measurement reference can be placed anywhere along the median line of the face, above the upper lip, as shown in FIG. 6.

A common implementation of the present invention places this reference under the nose (A). Another common implementation places this reference on the forehead (B). Alternatively, the reference point can be placed inside the mouth, on the teeth, palate or gums. The measurement point of the sensor must be placed so as to monitor the movements of the mandible as well as possible. In the preferred implementation of the invention, it is placed in the hollow under the lower lip, where the relative movements between the bone structure and the skin are smallest. Alternatively, the measurement point can be situated at the point of the chin, under the chin, or inside the mouth, on the teeth or gums.

In the preferred embodiment of the invention, the two elements of the sensor are held in place by means of a comfortable harness, small in size and easily positioned. Supporting the reference point can be performed by any known means. Supporting the measuring point is more difficult. To minimize discomfort for the patient, structures situated close to the mouth and on the cheeks must be very light.

Moreover, effective support over the whole opening dynamics should be ensured without imposing any force on the mandible which would risk affecting the measurements.

Finally, independence as regards movements of rotation and inclination of the head forwards or backwards is essential. The preferred embodiment of the invention has a sensor structure of elongated shape provided with a link imposing a small traction from the hollow situated under the lower lip along a line passing underthe ear lobe. Alternatively, an additional support for the sensor can be provided by a chin strap.

It should be understood that the present invention is not limited to the described embodiments and that variations can be applied without going outside of the scope of the appended claims.