WO2013072839A2

WO2013072839A2 - Dual-mode capacitive measurement

Info

Publication number: WO2013072839A2
Application number: PCT/IB2012/056374
Authority: WO
Inventors: Reinder Haakma; Mark Thomas Johnson
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-11-15
Filing date: 2012-11-13
Publication date: 2013-05-23
Also published as: WO2013072839A3

Abstract

The present invention relates to a sensor for providing capacitive measurement of electrophysiological and balisto-graphic signals, the sensor comprising a first capacitive plate arranged to be placed in the proximity of the skin of a human or animal body, wherein the first capacitive plate is arranged for providing a first measurement signal as a measure of an electrophysiological signal of the human or animal body, and a voltage offset signal arranged to be inputted to the first capacitive plate. The present invention further relates to a method of using such a sensor and a system for use with such a sensor.

Description

Dual-mode capacitive measurement

FIELD OF THE INVENTION

The present invention relates to dual-mode capacitive sensors, systems using dual-mode capacitive sensors and method for performing measurements using dual-mode capacitive sensors.

For sleep monitoring, it is desired to register electro -physio logical signals, such as EEG, EOG and chin-EMG, in combination with cardio-respiratory signals and bodily movement signals. EEG, EOG and chin-EMG are typically measured by sensors touching the head. Electro-physiological cardio-respiratory signals are not available at the head, however balisto-graphic signals (movement) are. The most unobtrusive way to measure electrophysiological signals is using capacitive sensors. Such capacitive sensors are disclosed in WO 2006/066566 A2 which describes a sensor system and method for the capacitive measurement of electromagnetic signals having a biological origin. The use of capacitive sensing compared to traditional glued electrodes is advantageous as it results in reduced skin irritation during prolonged use and thus increased patient comfort.

In order to get reliable electro-physiological signal from capacitive sensors, significant attention has been given to the reduction of motion artifacts.

For sleep monitoring it is advantageous to, in addition to measuring EEG, EOG and chin-EMG, also measure cardio-respiratory signals and bodily movements. One problem is that electro-physiological signals for cardio-respiratory are not available at the head. However, cardio-respiratory activity lead to small movement by the (skin of) the head. By measuring these movements, cardio-respiratory information such as heart rate and breathing rate can be assessed. It would be advantageous to achieve a device and system and method for performing such combined measurements. In general, the invention preferably seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination. In particular, it may be seen as an object of the present invention to provide a method that solves the above mentioned problems, or other problems, of the prior art.

SUMMARY OF THE INVENTION To better address one or more of these concerns, in a first aspect of the invention a sensor for providing capacitive measurement of electrophysiological and balisto- graphical signals is presented. The sensor comprising a first capacitive plate arranged to be placed in the proximity of the skin of a human or animal body, wherein the first capacitive plate is arranged for providing a first measurement signal as a measure of an

electrophysiological signal of the human or animal body. The sensor comprises a voltage offset signal arranged to be inputted to the first capacitive plate. The sensor may

advantageously be connected to an apparatus comprising a processor arranged for receiving said first measurement signals from the first capacitive plate and providing an output signal from the voltage offset signal and the first measurement signal.

The sensor allows the same capacitive sensor to be used to measure as well an electro-physiological signal as a balisto-graphical signal. This is advantageous in that it reduces the number of sensors that a patient is subjected to. This is especially advantageous when performing the measurements on children as they may not comprehend the need for studying their sleep pattern. The sensor thus includes a capacitive sensor providing measurement of mechanical movement in addition to the electro-physical signals.

Advantageously the sensor may comprise a second plate arranged to be placed in the proximity of the skin of a human or animal body, wherein the first capacitance between the first capacitive plate and the skin of the human or animal body is different from the second capacitance between the second capacitive plate and the skin of the human or animal body, and wherein the second capacitive plate is arranged for providing a second

measurement signal as a measure of an electrophysiological signal of the human or animal body. When such a sensor may then be connected to the before mentioned processor and the processor may then be arranged for receiving said first and second measurement signals from the first and second capacitive plates providing an output signal from the voltage offset signal and the first and/or second measurement signals. Using two capacitive plates in the same sensor provides the possibility to performed simultaneous measurements of both electro- physical signals and balisto-graphic signals.

Advantageously the sensor provides possibility for measuring an electrophysiological signal using a first compensation voltage signal to minimize the signal strength from the output voltage of the first capacitive plate. The first compensation voltage is preferably related to the voltage offset signal.

Advantageously the sensor provides possibility for measuring a balisto- graphic signal using a second compensation voltage signal based on the first compensation voltage signal to minimize the signal strength from the output voltage of the second capacitive plate. The second compensation voltage is preferably related to the voltage offset signal via the first compensation voltage.

Advantageously the sensor may comprise a switch for selectively operating the sensor for obtaining a balisto-graphical signal or an electro-physiological signal. The switch may respond to a control signal from an apparatus using the sensor.

A second aspect of the present invention relates to a method of using a capacitive sensor. The method may comprise the steps of obtaining in one time interval from the capacitive sensor a first signal being an electro-physiological signal, and obtaining in a second time interval from the capacitive sensor a second signal being a balisto-graphical signal. This may be achieved using a capacitive sensor and system as defined in relation to the first aspect of the present invention.

A third aspect of the present invention relates to a method of using a set of identical capacitive sensors. The method may comprise the steps of obtaining from a first group of capacitive sensors a first signal being an electro-physiological signal, and obtaining from a second group of capacitive sensors a second signal being a balisto-graphical signal. Using several, identical, capacitive sensors allow more precise measurements to be performed. As mentioned the sensor may be divided into groups. These groups may be chosen so that the two sets of measurements are performed distributed around a given area or volume, e.g. around a head of a person or animal. Alternatively the groups may be defined so that one type of measurements is performed in one region of e.g. a head and the other type of measurements is performed in another region.

Advantageously the first signal and the second signal are obtained simultaneously. This may be performed using the above-mentioned group definitions.

Advantageously the first signal and the second signal are obtained in sequence. This may e.g. be performed using a single capacitive sensor, or by switching one or more of the capacitive sensors from one type of measurements to the other type of measurements. Advantageously when doing this the first group and second group of capacitive sensors may be define dynamically. Advantageously the capacitive sensor or sensors may include a switch for selectively operating the capacitive sensor for obtaining the first signal or the second signal. As mentioned this will allow one sensor, or a set of sensors, to sequentially perform two types of measurements. Advantageously the switch may operate a compensation voltage between an activated state and a deactivated state. This may allow the sensor to perform the two types of measurements.

A fourth aspect of the present invention relates to an apparatus configured to obtain two signals relating to a body of a human, the sensor comprising a set of identical capacitive sensors, a processor configured for obtaining from a first group of the capacitive sensors a first signal being an electro-physiological signal, and the processor is configured for obtaining from a second group of the capacitive sensors a second signal being a balisto- graphical signal. The apparatus then is able to monitor and register information relating to sleep, as discussed above. The apparatus may advantageously be operated using the method according to the present invention. The combination of the capacitive sensors and the apparatus provides the advantage of co-registering the measurements. Advantageously each of the capacitive sensors may include a switch for selectively operating the capacitive sensor for obtaining the first signal or the second signal. By having a switch in the sensor it is possible for an apparatus using the sensor to selectively obtaining one of the two types of signals. Advantageously the switch may operate a compensation voltage between an activated state and a deactivated state.

Advantageously the set of identical capacitive sensors may be arranged in a matrix or in a sequential line. This may e.g. be included in a head band, a pillow or other suitable arrangement.

In general the various aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which FIG. 1 to 8 are schematic illustrates circuits representing sensors according to the present invention,

FIG. 9 is a schematic illustration of a pillow integrating a matrix of sensors according to the present invention,

FIG: 10 is a schematic illustration of a set of sensor arranged in series, and

FIG 11 is a schematic illustration of a set of sensor arranged in a matrix,

FIG: 12 is a schematic illustration of steps of a method, and

FIG. 13 is a schematic illustration of a system.

DESCRIPTION OF EMBODIMENTS

Fig. 1 schematically illustrates the basic capacitive sensor model. A signal is obtained from the human body, Vbio, via a skin-sensor transition providing a capacitive signal Ce measured via the circuit illustrated. Generally, contactless (capacitive) measurement of electrophysiological signals is able to overcome disadvantages of skin irritation during prolonged usage: namely restriction of the patient from free moving and providing less comfort for the patient, i.e. the patient is aware of being monitored.

In the technique according to one or more embodiments described in the following, a capacitor is effectively formed in which the human tissue acts as one of the capacitor plates and the plate electrode of the sensor acts as the other capacitor plate. In the capacitive sensing, no galvanic contact to the skin is needed (i.e. contactless sensing), thereby not needing skin preparation and a sticky patch with conductive gel for establishing a good electrical contact. It is apparently advantageous, in particular, when a lengthy measurement has to be conducted.

In Fig. 1 the electrode 10 and the skin of the human body 12 define a capacitor-like structure. The body of the human or animal subject produces the voltage Vbio which over the circuit 14 is supplied to the operational amplifier 16. The output from the operational amplifier 16 is the voltage Vout.

The output voltage may be determined using the following equation:

The first term within square brackets, on the right-hand side of the equation, represents the motion artifacts in Vout(t). The second term on the right hand side, within square brackets, represents the bioelectric signal that is filtered by the capacitive sensor. The formula reveals that the motion artifacts in the output of the capacitive sensor are

proportional to both the DC voltage and to the motion of the electrode with respect to the skin. Reducing the DC voltage difference between the skin and electrode reduces the motion artifacts in the output of the capacitive sensor. The electro-physiological signal is acquired by introducing a feedback system that reduces motion artifacts by eliminating the DC voltage using a compensation voltage. The compensation voltage is tuned in such a way that the average signal strength of the output voltage is minimized (more precise, the integral of Vout(t)2 over some time interval is minimized). This is possible because drift of the DC voltage over time is far slower than the variation of movement and electro-physiological signals.

Fig. 2 schematically illustrates an embodiment of a sensor. The sensor is arranged close to or in contact with the body or skin 3 of a patient, and comprises a parallel plate capacitor connected to the high impedance buffer amplifier 2. The buffer 2 connected to the measurement electrode, being a capacitive sensor electrode 1, is able to convert the impedance level from high-ohmic to low-ohmic, such that it can be transported across a cable to the measurement system without being affected by interference. In the sensor the voltage of the reference electrode 4 is controlled by a voltage source 5 connected to the reference electrode 4. In this embodiment, the voltage source 5 is connected between the reference electrode 4 and ground or reference potential 6. A resistor 7 is connected between the reference potential 6 and a buffer amplifier 2.

The sensor, system and method of the present invention provide the possibility of using the same capacitive sensor for measuring an electro-physiological signal as well as a balisto-graphical signal, either simultaneously or time-sequentially. The invention also allows for groups of such sensors to dynamically switch between modes for optimal signal acquisition.

Switching between electro-physiological and balisto-graphical sensing can be done in at least two ways: (1) the less sophisticated solution in which the sensor can either sense electro-physiological signals or mechanical/balisto-graphic signals, and (2) the sophisticated way in which the sensor simultaneously measure both. The latter has the best solution for drift in DC voltage and for compensation of this.

The first solution simply introduces a switch that allows the sensor to operate in two modes:

• With the compensation voltage being de-activated: for measuring mechanical activity (in Fig. 3: left part of the signal).

• With the compensation voltage being activated: for measuring electrophysiological signals (in Fig. 3: right part of the signal); As seen from Fig. 3 with the compensation voltage being de-activated, the movement component in the signal Vout(t) easily dominates the electro-physiological component.

Fig. 4 is a schematic circuit diagram representing such a sensor. The sensor is a sensor for providing capacitive measurement of electrophysiological and balisto-graphic signals. The sensor comprises a first capacitive plate arranged to be placed in the proximity of the skin of a human or animal body, wherein the first capacitive plate is arranged for providing a first measurement signal as a measure of an electrophysiological signal of the human or animal body by minimizing of Vout as described above, and a voltage offset signal - in this case 0V - arranged to be inputted to the first capacitive plate when it is disconnected from the adaptive controller.

One drawback of this embodiment is that there is no control exerted over the DC voltage. Drift in DC voltage will affect the transfer function (amplification factor) between mechanical movement and output voltage. The sensor will still operate as described.

In Fig. 5 an embodiment of a sensor is illustrated via a circuit diagram wherein the compensation voltage is set to a fixed voltage thus reducing the impact of variations in DC voltage as the transfer function will now vary around Voffset+VDc and not around VDC.

In Fig. 6 is schematically illustrated an embodiment where the part of the circuit used to minimize the power of the output voltage in' electro-physiological' mode, is used in 'balisto-graphic' mode to keep the average power of the output voltage at a particular level, preferably not equal to zero. This approach is suitable for repetitive motion as originating from cardiac and respiratory activity, in particular when the periodicity of the signals is of more importance that amplitude, e.g. for measuring heart rate and/or respiration rate.

A more sophisticated solution combines two sensors into one in such a way that the DC voltage of both sensors is about the same, and that they drift in the same way. A schematic circuit diagram is illustrated in Fig. 7. In Fig. 7 it is seen that the first sensor measures the electro-physiological signal using compensation voltage to minimize the signal strength of the output voltage Vout(t). The second sensor uses a compensation voltage derived from, but not equal to, the compensation voltage of the other sensor, e.g. by adding a constant voltage, thus assuring that mechanical movements are clearly detectable (Vmech) and the transfer of the mechanical movements to the output voltage remains the same over time by compensating drift in DC voltage.

The advantage of this embodiment is that drift is fully compensated for and that both electro-physiological and balisto-graphic information is measured at the same time.

In Fig. 7 the sensor is to be arranged in the vicinity of tissue of a human or animal body the tissue having a potential Vbio. The sensor comprises two capacitive sensor plates PI, P2, not explicitly illustrated here, arranged in two capacitive measurement channels. The probe for each capacitive channel only comprises one sensor plate PI or P2, which in use is placed in the proximity of the skin of a user so that the tissue of the user will act as the other capacitor plate. In Fig. 7 two plates PI and P2, will in use provide two capacitive couplings with the tissue of a user so as to provide capacitive measurement at almost the same position of the probe in relation to the user tissue. The first capacitance between the first capacitive plate PI and the skin of the human or animal body is made different from the second capacitance between the second capacitive plate P2 and the skin of the human or animal body. The first and second capacitive plates PI, P2 are arranged for providing a first and second measurement signal Vmech and Vout at outputs as a measure of two electrophysiological signals of the human or animal body. The two coupling capacitor plates PI and P2 of the probe are placed so close that they effectively measure the same signal of the user. The outputs separately provide signals from the 2 sensors, thus allowing parallel measurements to be performed simultaneously.

Further, Fig. 8 schematically illustrates a circuit representing an embodiment of a sensor where the electro-physiological component is removed in Vmech by subtraction. In another variant of this embodiment, it is also possible to use two capacitive sensor plates for signal input, in a manner similar to that shown in figure 7. In most applications, the above-discussed sensors will typically be used in groups, e.g. in a line or matrix. The invention also provides possibility that different sensors in such groups operate in different modes and that the mode these sensors operate in, can change dynamically, in particular for optimal and balanced signal acquisition under changing head position.

It is presently envisioned that the sensor are used in the context of unobtrusive sleep monitoring. Within sleep monitoring, the sensors may be integrated in various embodiments of products, e.g.:

headbands,

Night caps,

sleeping masks,

CPAP masks,

pillows and pillow covers,

mattresses and bed linen,

basically anything that touches the head in the bed and/or during sleep.

Fig. 9 schematically illustrates a pillow where the sensors are integrated in a matrix form. The integration of sensors in a pillow provides a system where the user is less aware that he or she is under observation and measurements are being performed. The pillow implementation may be preferred if the patient does not move too much, for such active patients a head band or the like is preferred.

Fig. 10 is a schematic illustration of a set of sensor, all denoted with the reference numeral 20, arranged in series. The sensors 20 may be addressed, controlled and read individually. The sensors 20 may be any of the types described above. In the embodiment illustrated here 4 sensors are used. In other embodiments other numbers of sensors may be used depending on the area or volume needed to be monitored.

Fig. 11 is a schematic illustration of a set of sensor all denoted with the reference numeral 30, arranged in a matrix. The sensors 30 may be addressed, controlled and read individually. The sensors 30 may be any of the types described above. The leads 40 is used as communication lines. In the embodiment illustrated here 12 sensors are used. In other embodiments other numbers of sensors may be used depending on the area or volume needed to be monitored. Fig. 12 is a schematic illustration of steps of a method 50 of using a set of identical capacitive sensors comprising the steps of obtaining 52 from a first group of capacitive sensors a first signal being an electro-physiological signal, and obtaining 54 from a second group of capacitive sensors a second signal being a balisto-graphical signal. Fig. 13 is a schematic illustration of an apparatus 60 configured to obtain two signals relating to a body of a human using a sensor 62. The sensor comprising a set of identical capacitive sensors, preferably as described in relation to any of the embodiments described above. A processor 64 configured for obtaining from a first group of the capacitive sensors a first signal being an electro-physiological signal, and the processor 64 configured for obtaining from a second group of the capacitive sensors a second signal being a balisto- graphical signal. Based on these signals the above mentioned method may be performed.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless

telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A sensor for providing capacitive measurement of electrophysiological and balisto-graphic signals, the sensor comprising:

a first capacitive plate arranged to be placed in the proximity of the skin of a human or animal body, wherein the first capacitive plate is arranged for providing a first measurement signal as a measure of an electrophysiological signal of the human or animal body,

a voltage offset signal arranged to be inputted to the first capacitive plate.

2. The sensor according to claim 1, comprising a second plate arranged to be placed in the proximity of the skin of a human or animal body, wherein the first capacitance between the first capacitive plate and the skin of the human or animal body is different from the second capacitance between the second capacitive plate and the skin of the human or animal body, and wherein the second capacitive plate is arranged for providing a second measurement signal as a measure of an electrophysiological signal of the human or animal body.

3. The sensor according to claim 1, comprising a first compensation voltage signal to minimize the signal strength from the output voltage of the first capacitive plate.

4. The sensor according to any one of the claims 1-3, wherein the sensor comprises a switch for selectively operating the sensor for obtaining a balisto-graphical signal or an electro-physiological signal.

5 A method of using a capacitive sensor comprising the steps of:

obtaining in one time interval from the capacitive sensor a first signal being an electro-physiological signal, and

obtaining in a second time interval from the capacitive sensor a second signal being a balisto-graphical signal.

6. A method of using a set of identical capacitive sensors comprising the steps of: obtaining from a first group of capacitive sensors a first signal being an electro-physiological signal, and

obtaining from a second group of capacitive sensors a second signal being a balisto-graphical signal.

7. The method according to claim 6, wherein the first signal and the second signal are obtained simultaneously.

8. The method according to claim 6, wherein the first signal and the second signal are obtained in sequence.

9. The method according to any of the claims 5-8, wherein the first group and second group of capacitive sensors are defined dynamically.

10. The method according to any of the claims 5-9, wherein the capacitive sensor or sensors include a switch for selectively operating the capacitive sensor for obtaining the first signal or the second signal.

11. The method according to claim 10, wherein the switch operates a

compensation voltage between an activated state and a deactivated state.

12. An apparatus configured to obtain two signals relating to a body of a human, the sensor comprising:

a set of identical capacitive sensors,

a processor configured for obtaining from a first group of the capacitive sensors a first signal being an electro-physiological signal, and

the processor configured for obtaining from a second group of the capacitive sensors a second signal being a balisto-graphical signal.

13. The apparatus according to claim 12, wherein each of the capacitive sensors include a switch for selectively operating the capacitive sensor for obtaining the first signal or the second signal.

14. The apparatus according to claim 12 or 13, wherein the switch operates a compensation voltage between an activated state and a deactivated state.

15. The apparatus according to any of the claims 12-14, wherein the set of identical capacitive sensors are arranged in a matrix or in a sequential line.