SIGNALS

TECHNICAL FIELD OF THE INVENTION

The invention relates to sensors for measuring a physiological parameter of a subject, for example photoplethysmographic (PPG) sensors, and in particular relates to an apparatus comprising such a sensor and a method of operating the same for improving the quality of signals obtained by the sensor.

BACKGROUND TO THE INVENTION

Photoplethysmography (PPG) sensors and pulse oximetry sensors are being used more and more in wearable devices and apparatus for monitoring the heart rate and/or arterial oxygen saturation (Sp02). An important problem with respect to sensors that are wearable or part of a wearable apparatus, and especially with respect to

photoplethysmography and pulse oximetry (or other sensors that rely on a measurement of light), is ensuring optimal contact pressure with which the light sensor and/or light source is applied to the part of the body of the subject from which the measurement is to be made. It has been shown that the contact pressure greatly influences the amplitudes of

photoplethysmography and pulse oximetry signals ("Reflectance forehead pulse oximetry: effects of contact pressure during walking" by Dresher RP, Mendelson Y, Conf Proc IEEE Eng Med Biol Soc. 2006; 1 :3529-32), and in particular that errors in the measurements increase when insufficient contact pressure is applied.

There is therefore a need for an apparatus having a sensor and a method of operating the same in which the sensor can be applied to a subject in order to obtain signals with a sufficient or required quality.

SUMMARY OF THE INVENTION

According to a first aspect, there is provided an apparatus, comprising a physiological parameter sensor comprising a light source and a light sensor for measuring a physiological parameter of a subject; an actuator for adjusting the pressure of the contact between the nhvsioloeical narameter sensor and the subject; an accelerometer for measuring the accelerations of the subject and/or the physiological parameter sensor; a control unit configured to (i) control the actuator to apply the physiological parameter sensor to the subject with a first contact pressure; (ii) obtain a signal using the light sensor and measure the accelerations of the subject and/or the physiological parameter sensor using the

accelerometer; (iii) process the measurements of the accelerations and the obtained signal to determine a second contact pressure at which a measurement of the physiological parameter can be obtained using a signal from the light sensor and at which relative motion of the subject and the physiological sensor due to accelerations is reduced or avoided; (iv) control the actuator to apply the physiological parameter sensor to the subject with the second contact pressure; and (v) measure the physiological parameter of the subject using the physiological parameter sensor.

According to a second aspect, there is provided a method of operating an apparatus that comprises a physiological parameter sensor and an actuator for adjusting the pressure of the contact between the physiological parameter sensor and a subject, wherein the physiological parameter sensor comprises a light source and a light sensor for measuring a physiological parameter of the subject, the method comprising (i) using the actuator to apply the physiological parameter sensor to the subject with a first contact pressure; (ii) obtaining a signal using the light sensor and measuring the accelerations of the subject and/or the physiological parameter sensor using an accelerometer; (iii) processing the measurements of the accelerations and the obtained signal to determine a second contact pressure at which a measurement of the physiological parameter can be obtained using a signal from the light sensor and at which relative motion of the subject and the physiological sensor due to accelerations is reduced or avoided; (iv) controlling the actuator to apply the physiological parameter sensor to the subject with the second contact pressure; (vi) using the physiological parameter sensor to measure the physiological parameter of the subject.

According to a third aspect, there is provided a computer program product having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable computer or processor, the computer or processor is caused to perform the method described above.

According to a fourth aspect , there is provided an apparatus, comprising a physiological parameter sensor comprising a light source and a light sensor for measuring a physiological parameter of a subject; an actuator comprising an electro-active material, EAM, for adjusting the pressure of the contact between the physiological parameter sensor and the subiect: a control unit configured to (i) control the actuator to apply the physiological parameter sensor to the subject with a first contact pressure; (ii) obtain a signal using the light sensor; (iii) determine if the signal meets a quality metric; (iv) if the signal does not meet the quality metric, control the actuator to adjust the contact pressure of the physiological parameter sensor to the subject; (v) repeat (ii)-(iv) until a signal that meets the quality metric is obtained; and (vi) measure the physiological parameter of the subject using the

physiological parameter sensor.

In some embodiments the physiological parameter sensor is a

photoplethysmography sensor, or an Sp02 sensor, or a sensor based on laser Doppler, laser speckle velocimetry, near-infrared spectroscopy or microcirculation microscopy.

In some embodiments the actuator is configured to adjust the pressure of the contact of the light source and the light sensor to the subject.

In alternative embodiments the actuator is configured to adjust the pressure of the contact of one of the light source and light sensor to the subject.

In alternative embodiments the actuator is configured to adjust the pressure of the contact of the light source to the subject and the apparatus further comprises a second actuator that comprises an EAM and that is configured to adjust the pressure of the contact of the light sensor to the subject, and wherein the control unit is configured to control one or both of the actuator and the second actuator to adjust the contact pressure in (iv).

In some embodiments the electro-active material is an electro-active polymer, EAP. The EAP may be a dielectric EAP, a ferroelectric polymer, a crystalline polymer, CP, a liquid crystal elastomer, LCE, an ionic polymer-metal composite, IPMC, or a carbon nanotube, CNT, polymer.

In some embodiments the control unit is configured to obtain a signal using the light sensor when the light source is emitting light.

In some embodiments the control unit is configured to determine if the signal meets the quality metric by determining a measure of the amplitude or intensity of the measured light and determining that the signal meets the quality metric if the amplitude or intensity is above a threshold value.

In alternative embodiments the control unit is configured to determine if the signal meets the quality metric by determining a measure of the amplitude or intensity of the measured light and determining that the signal meets the quality metric if the amplitude or intensity obtained at the current contact pressure is above the amplitude or intensity obtained at other contact pressures. In alternative embodiments the control unit is configured to determine if the signal meets the quality metric by determining a measure of the peak amplitude or peak intensity of the measured light and determining that the signal meets the quality metric if the peak amplitude or peak intensity is above a threshold value.

In alternative embodiments the control unit is configured to determine if the signal meets the quality metric by determining a signal to noise ratio, SNR, for the signal and determining that the signal meets the quality metric if the SNR is above a threshold value.

In alternative embodiments the control unit is configured to determine if the signal meets the quality metric by determining if the signal is sufficient for determining the physiological parameter of the subject.

In some embodiments the control unit is configured to obtain a signal using the light sensor when the light source is not emitting light. In some embodiments the control unit is configured to obtain a signal using the light sensor when the light source is not emitting light during intervals between measurements of the physiological parameter by the physiological parameter sensor.

In some embodiments the control unit is configured to determine if the signal meets the quality metric by determining a measure of the amplitude or intensity of the measured light when the light source is not emitting light and determining that the signal meets the quality metric if the amplitude or intensity is below a threshold value.

In some embodiments the control unit is configured to determine if the quality of the signal increased or decreased after controlling the actuator to adjust the contact pressure in a particular direction, and to determine to adjust the contact pressure in the opposite direction with the next adjustment if the quality of the signal decreased.

In some embodiments the control unit is configured to, following an increase of the contact pressure, determine if the quality of the signal has increased or decreased, and determine to (a) decrease the contact pressure with the next adjustment if the quality of the signal has decreased, or (b) increase the contact pressure with the next adjustment if the quality of the signal has increased. In some embodiments the control unit is configured to, following a decrease of the contact pressure, determine if the quality of the signal has increased or decreased, and determine to (a) increase the contact pressure with the next adjustment if the quality of the signal has decreased, or (b) decrease the contact pressure with the next adjustment if the quality of the signal has increased.

In some embodiments the control unit is configured to repeat (ii)-(iv) for a nluralitv of contact pressures; identify a contact pressure that provides a signal that meets the quality metric and to control the actuator to apply the physiological parameter sensor to the subject at the identified contact pressure.

In some embodiments the control unit is configured such that in the event that two or more contact pressures are identified that provide a signal that meets the quality metric, the control unit is configured to control the actuator to apply the physiological parameter sensor to the subject at the lowest one of the two or more identified contact pressures.

In some embodiments the apparatus further comprises a movement sensor for measuring the movements of the physiological parameter sensor and/or the subject, and wherein the control unit is further configured to process the measurements of the movements of the subject and/or the sensor and to control the actuator to adjust the contact of the physiological parameter sensor with the subject using the actuator according to the measured movements.

In some embodiments the control unit is configured to determine a control signal for the actuator from the measured movements.

In some embodiments the movement sensor measures movements in three dimensions, the apparatus further comprises a second actuator comprising an EAM and a third actuator comprising an EAM that are arranged such that the actuator, second actuator and third actuator are capable of applying pressure to the physiological parameter sensor in respectively different directions, and wherein the control unit is configured to control the actuator, second actuator and/or third actuator to apply pressure to the physiological parameter sensor based on the measured movements.

According to a fifth aspect, there is provided a method of operating an apparatus that comprises a physiological parameter sensor and an actuator comprising an electro-active material for adjusting the pressure of the contact between the physiological parameter sensor and a subject, wherein the physiological parameter sensor comprises a light source and a light sensor for measuring a physiological parameter of the subject, the method comprising (i) using the actuator to apply the physiological parameter sensor to the subject with a first contact pressure; (ii) obtaining a signal using the light sensor; (iii) determining if the signal meets a quality metric; (iv) if the signal does not meet the quality metric, adjusting the contact pressure of the physiological parameter sensor to the subject using the actuator; (v) repeating steps (ii)-(iv) until a signal that meets the quality metric is obtained; and (vi) using the physiological parameter sensor to measure the physiological parameter of the subiect. In some embodiments the physiological parameter sensor is a photoplethysmography sensor, or an Sp02 sensor, or a sensor based on laser Doppler, laser speckle velocimetry, near-infrared spectroscopy or microcirculation microscopy.

In some embodiments the steps of using the actuator and adjusting the contact pressure comprise using the actuator to adjust the pressure of the contact of the light source and the light sensor to the subject.

In alternative embodiments the steps of using the actuator and adjusting the contact pressure comprise using the actuator to adjust the pressure of the contact of one of the light source and light sensor to the subject.

In alternative embodiments the actuator is for adjusting the pressure of the contact of the light source to the subject and a second actuator that comprises an EAM is for adjusting the pressure of the contact of the light sensor to the subject, and wherein step (iv) comprises adjusting the contact pressure using one or both of the actuator and the second actuator.

In some embodiments the electro-active material is an electro-active polymer, EAP. The EAP may be a dielectric EAP, a ferroelectric polymer, a crystalline polymer, CP, a liquid crystal elastomer, LCE, an ionic polymer-metal composite, IPMC, or a carbon nanotube, CNT, polymer.

In some embodiments the step of obtaining a signal using the light sensor comprises obtaining a signal using the light sensor when the light source is emitting light.

In some embodiments the step of determining if the signal meets the quality metric comprises determining a measure of the amplitude or intensity of the measured light and determining that the signal meets the quality metric if the amplitude or intensity is above a threshold value.

In alternative embodiments the step of determining if the signal meets the quality metric comprises determining a measure of the amplitude or intensity of the measured light and determining that the signal meets the quality metric if the amplitude or intensity obtained at the current contact pressure is above the amplitude or intensity obtained at other contact pressures.

In alternative embodiments the step of determining if the signal meets the quality metric comprises determining a measure of the peak amplitude or peak intensity of the measured light and determining that the signal meets the quality metric if the peak amplitude or peak intensity is above a threshold value. In alternative embodiments the step of determining if the signal meets the quality metric comprises determining a signal to noise ratio, SNR, for the signal and determining that the signal meets the quality metric if the SNR is above a threshold value.

In alternative embodiments the step of determining if the signal meets the quality metric comprises determining if the signal is sufficient for determining the physiological parameter of the subject.

In some embodiments the step of obtaining a signal using the light sensor comprises obtaining a signal using the light sensor when the light source is not emitting light. In some embodiments the step of obtaining a signal using the light sensor comprises obtaining a signal using the light sensor when the light source is not emitting light during intervals between measurements of the physiological parameter by the physiological parameter sensor.

In some embodiments the step of determining if the signal meets the quality metric comprises determining a measure of the amplitude or intensity of the measured light when the light source is not emitting light and determining that the signal meets the quality metric if the amplitude or intensity is below a threshold value.

In some embodiments the method further comprises the step of determining if the quality of the signal increased or decreased after using the actuator to adjust the contact pressure of the physiological parameter sensor in a particular direction, and determining to adjust the contact pressure in the opposite direction with the next adjustment if the quality of the signal decreased.

In some embodiments the method further comprises the step of determining if the quality of the signal has increased or decreased following an increase of the contact pressure, and determining to decrease the contact pressure with the next adjustment if the quality of the signal has decreased and to increase the contact pressure with the next adjustment if the quality of the signal has increased. In some embodiments the method further comprises the step of determining if the quality of the signal has increased or decreased following a decrease of the contact pressure, and determining to increase the contact pressure with the next adjustment if the quality of the signal has decreased and to decrease the contact pressure with the next adjustment if the quality of the signal has increased.

In some embodiments the method further comprises repeating (ii)-(iv) for a plurality of contact pressures; identifying a contact pressure that provides a signal that meets the quality metric and step (iv) comprises using the actuator to apply the physiological parameter sensor to the subject at the identified contact pressure.

In some embodiments in the event that two or more contact pressures are identified that provide a signal that meets the quality metric, step (iv) comprises using the actuator to apply the physiological parameter sensor to the subject at the lowest one of the two or more identified contact pressures.

In some embodiments the method further comprises the steps of measuring the movements of the physiological parameter sensor and/or the subject, and adjusting the contact of the physiological parameter sensor with the subject using the actuator according to the measured movements.

In some embodiments the step of processing the measurements of the movements comprises determining a control signal for the actuator from the measured movements.

In some embodiments the step of measuring the movements comprises measuring the movements in three dimensions, and the step of adjusting the contact of the physiological parameter sensor using the actuator according to the measured movements comprises applying pressure to the physiological parameter sensor using the actuator, a second actuator comprising an EAM and/or a third actuator comprising an EAM that are arranged such that the actuator, second actuator and third actuator are capable of applying pressure to the physiological parameter sensor in respectively different directions according to the measured movements.

According to a sixth aspect, there is provided a computer program product having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable computer or processor, the computer or processor is caused to perform any of the method aspects or embodiments set out above.

According to another aspect, there is provided a method of operating an apparatus that comprises a sensor and an actuator for adjusting the contact of the sensor with a subject, wherein the sensor is for measuring a physiological parameter of the subject, the method comprising measuring the movements of the subject and/or the sensor and adjusting the contact of the sensor with the subject using the actuator according to the measured movements.

In some embodiments the method further comprises the step of processing the measurements of the movements to determine a control signal for the actuator from the measured movements. In some embodiments the step of measuring the movements comprises measuring the movements in three dimensions, and the step of adjusting the contact of the physiological parameter sensor using the actuator according to the measured movements comprises applying pressure to the physiological parameter sensor using the actuator, a second actuator and/or a third actuator that are arranged such that the actuator, second actuator and third actuator are capable of applying pressure to the physiological parameter sensor in respectively different directions according to the measured movements.

Various additional embodiments of the method corresponding to the above embodiments of the third aspect are also contemplated.

According to another aspect, there is provided an apparatus, comprising a sensor for measuring a physiological parameter of a subject, a movement sensor for measuring the movements of the subject and/or the sensor, an actuator for adjusting the contact of the sensor with the subject, and a control unit that is configured to process the measurements of the movements of the subject and/or the sensor and to control the actuator to adjust the contact of the sensor with the subject using the actuator according to the measured movements.

In some embodiments the control unit is configured to process the measurements of the movements to determine a control signal for the actuator from the measured movements.

In some embodiments the movement sensor measures movements in three dimensions, the apparatus further comprises a second actuator and a third actuator that are arranged such that the actuator, second actuator and third actuator are capable of applying pressure to the physiological parameter sensor in respectively different directions, and wherein the control unit is configured to control the actuator, second actuator and/or third actuator to apply pressure to the physiological parameter sensor based on the measured movements.

Various additional embodiments of the apparatus corresponding to the above embodiments of the fourth aspect are also contemplated.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

Figure 1 is a block diagram of an apparatus according to an embodiment; Figure 2 is an illustration of a sensor and an electro-active material according to an embodiment attached to a finger of a subject;

Figure 3a and 3b show a block diagram of an exemplary dielectric electro- active polymer;

Figure 4 is a flow chart illustrating a method of operating an apparatus according to an embodiment;

Figure 5 illustrates an exemplary PPG signal obtained using a PPG sensor; Figure 6 is a graph illustrating the relationship between contact pressure and PPG signal amplitude for an exemplary sensor;

Figure 7a and 7b illustrate an embodiment in which ambient light is measured by the sensor;

Figure 8a, 8b and 8c illustrate various arrangements of a sensor and actuator according to the invention;

Figure 9a and 9b illustrate alternative arrangements of a light source and light sensor on a finger of a subject;

Figure 10 is a flow chart illustrating a method of operating an apparatus according to an embodiment or further aspect;

Figure 11 illustrates exemplary measurements from an accelerometer;

Figure 12 illustrates an exemplary driving signal for an electro-active material; Figure 13 is a flow chart illustrating a method of operating an apparatus according to another embodiment;

Figure 14 illustrates an exemplary arrangement of a sensor and a plurality of actuators according to an embodiment; and

Figure 15 illustrates exemplary driving signals for the embodiment of Figure 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 is a block diagram illustrating an apparatus 2 according to an aspect of the invention. The apparatus 2 is to be worn or otherwise carried by a subject so that a measurement of a physiological parameter of the subject can be made. The apparatus 2 comprises a physiological parameter sensor 4 for measuring a physiological parameter of the subject through contact with a part of the body of a subject. The contact may be direct contact between the physiological parameter sensor 4 and the skin of the subject, or may be contact with the skin via some other medium. In preferred embodiments, the physiological parameter sensor 4 measures a physiological parameter of the subject using light that is reflected or transmitted by a part of the body of the subject. Thus, in these preferred embodiments, the physiological parameter sensor 4 comprises a light sensor or multiple light sensors 6 that measure light. The or each light sensor 6 may be sensitive to (i.e. measure) a specific wavelength or a range of wavelengths of light. In some embodiments the physiological parameter sensor 4 may comprise one or more light sources 8 that output light at one or more specific wavelengths (with the light sensor(s) 6 being sensitive at least to those wavelengths).

For example the physiological parameter sensor 4 can be for measuring the heart rate, or parameters related to the heart rate (such as heart rate variability, etc.), and/or blood oxygen saturation (Sp02). In the former case the physiological parameter sensor 4 can be a photoplethysmography (PPG) sensor, and in the former or latter case the physiological parameter sensor 4 can be an Sp02 sensor. As appreciated by those skilled in the art, a PPG sensor can comprise a light source 8 and a light sensor 6 (such as a photodetector) that is sensitive to at least the wavelength(s) of light emitted by the light source 8. In an exemplary, non-limiting, implementation, the light source 8 can be a green light emitting diode (LED) that emits light at wavelengths in the range of 500-600 nm (or a red LED that emits light at wavelengths in the range of 600-700 nm) and the photodetector or other type of light sensor 6 can be sensitive to light at wavelengths below lOOOnm. Also as appreciated by those skilled in the art, an Sp02 sensor comprises a plurality of light sources 8 and at least one light sensor 6 that is sensitive to at least the wavelengths of light emitted by the light sources 8. The plurality of light sources 8 can include, for example, a near-infrared LED that emits light at wavelengths in the range of 800-1000 nm and a red LED that emits light at wavelengths in the range of 600-700 nm. The light sensor 6 can be sensitive to light at wavelengths below lOOOnm (although alternatively separate light sensors 6 that are respectively sensitive to light in the ranges of 800-1000 nm and 600-700 nm can be provided).

In alternative embodiments the physiological parameter sensor 4 can be another type of light-based sensor that is sensitive to the quality of the contact between the subject and the sensor 4, such as, for example, laser Doppler, laser speckle velocimetry, near- infrared spectroscopy and microcirculation microscopy.

As shown in more detail below, in some embodiments the light sensor 6 and light source 8 can be located close to (e.g. next to) each other within the apparatus 2 (in which case the light sensor 6 can measure the light from the light source 8 that reflects from the nart of the bodv of the subject that the physiological parameter sensor 4 is in contact with) or they can be arranged on generally opposite sides of the part of the body of the subject (in which case the light sensor 6 measures light from the light source 8 that is transmitted by (i.e. that passes through) the part of the body of the subject).

The apparatus 2 also comprises a control unit 10 that is connected to the physiological parameter sensor 4 and that receives the signals or measurements from the physiological parameter sensor 4. The control unit 10 controls the operation of the apparatus 2 according to the invention. The control unit 10 can comprise one or more processors, processing units, multi-core processors or processing modules for performing various functions.

A memory module 12 is provided that is connected to the control unit 10 and that is for storing computer readable program code to be executed by the control unit 10 to perform the method according to the invention. The memory module 12 can also be used to store the measurements from the physiological parameter sensor 4 and other components of the apparatus 2 during operation of the apparatus 2.

As described in more detail below, the contact pressure of the physiological parameter sensor 4 to the subject is adjusted during use of the apparatus 2 in order to improve the quality of the contact between the physiological parameter sensor 4 and the subject, and thus improve the quality of the signal (and therefore the quality of the physiological parameter measurement) obtained by the physiological parameter sensor 4. Therefore, the apparatus 2 comprises an actuator 14 that is configured to adjust the contact pressure between the physiological parameter sensor 4 and the subject in response to a control signal from the control unit 10.

The actuator 14 is arranged in the apparatus 2 with respect to the physiological parameter sensor 4 in such a way that actuation of the actuator 14 by the control unit 10 results in the pressure with which the physiological parameter sensor 4 is pressed into the subject being changed (e.g. increased or decreased). For example, as shown in Figure 2, the actuator 14 and physiological parameter sensor 4 can be arranged so that the sensor 4 is between the actuator 14 and the subject (the subject's finger in this case), so that actuation of the actuator 14 leads to the sensor 4 being pushed onto the subject's finger with an increased/decreased pressure. Also shown in Figure 2 is a band 16 that is used in an exemplary embodiment to attach the apparatus 2 to the subject's finger. The specific arrangement of the apparatus 2, physiological parameter sensor 4 and actuator 14 depends on the nature of the actuator 14 and physiological parameter sensor 4. In some embodiments, in particular where the physiological parameter sensor 4 comprises a separate light sensor 6 and light source 8, the actuator 14 may be configured or arranged in the apparatus 2 such that it applies pressure to just one of the light sensor 6 and light source 8. In other embodiments, the actuator 14 can be configured or arranged to apply pressure to each of the light sensor 6 and light source 8. In yet further embodiments, each of the light sensor 6 and light source 8 can have a respective actuator 14 that apply pressure independently to the light sensor 6 and light source 8. These embodiments are described in more detail below with reference to Figures 8 and 9.

In preferred embodiments, the actuator 14 comprises an electro-active material (EAM) that changes its shape and/or configuration in response to an electrical signal or electrical field from the control unit 10. The amount of shape change produced by the electro- active material for a given current or voltage depends on the type of electro-active material used and its geometry. The use of electro-active materials is advantageous since the actuator 14 can be relatively small and doesn't require any additional hardware in order to operate, electro-active materials can easily be integrated into wearable apparatus 2 (such as clothing, a watch or a patch), electro-active materials require little power to operate which makes them particularly suited to portable applications, and their small size means that the overall blood flow in the limb or digit is not appreciably affected when pressure is applied to the sensor 4.

The electro-active material can be an electro-active polymer (EAP), such as a dielectric EAP, a ferroelectric polymer (for example polyvinylidene fluoride, PVDF), a crystalline polymer (CP), a liquid crystal elastomer (LCE), an ionic polymer-metal composite (IPMC) or a carbon nanotube (CNT) polymer.

The table below illustrates some properties of the exemplary electro-active polymers mentioned above.

T„1, 1 The Max. Strain represents the maximum deformation (as a percentage of its own size) upon application of the optimal potential (in Volts). The Max. Pressure is the maximum pressure generated upon application of the optimal potential (in Volts). The Max. Efficiency is the maximum efficiency of converting a potential to a deformation/pressure. The Driving Electric Field is the voltage required to drive the polymers. The Cycle Time indicates how fast the polymers respond to the application of a voltage.

In one preferred embodiment the electro-active material is a dielectric EAP. A dielectric EAP is formed of two electrodes that 'sandwich' a polymer. A dielectric EAP is illustrated in Figure 3. The dielectric EAP 14 comprises two electrodes 18 with a polymer 20 therebetween (as shown in Figure 3(a)). A voltage is applied to the electrodes 18 which causes electrostatic forces (OM) between the electrodes 18 and which pulls the electrodes 18 together and compresses the polymer 20 (as shown in Figure 3(b)). This causes the polymer to expand outwards in the plane of the electrodes (which is indicated by arrows 22).

Dielectric EAPs are capable of producing high strains. In an alternative preferred

embodiment, the electro-active material is a ferroelectric polymer (PVFD).

In alternative embodiments to the use of an electro-active polymer, the electro- active material can be any type of material that changes its shape and/or configuration in response to an electrical signal or field, such as a piezoelectric material.

In other embodiments, the actuator 14 can comprise an alternative means to an electro-active material for adjusting the contact pressure between the physiological parameter sensor 4 and the subject in response to a control signal from the control unit 10. For example, the actuator 14 can comprise a component that can be inflated (such as a balloon, bag or cuff) so that it expands or contracts in a defined direction and thus applies pressure to the sensor 4.

Returning to Figure 1, in further embodiments of the apparatus 2, the apparatus 2 can comprise a sensor 24 for measuring the movements or motion of the subject and/or the apparatus 2. In particular, the sensor 24 is for measuring at least the movements or motion of the subject and/or the apparatus 2 so that the control unit 10 can control the actuator 14 to adjust the contact pressure and compensate for or avoid a reduction in signal quality due to relative motion of the subject and the apparatus 2. In preferred embodiments the movement sensor 24 is an accelerometer and thus measures the accelerations of the apparatus 2 (and particularly of the physiological parameter sensor 4) and/or the subject. In some embodiments the accelerometer 24 is a one- or two-dimensional accelerometer that measures acceleration in one or two dimensions, whereas in other embodiments the accelerometer 24 is a three-dimensional accelerometer that measures the accelerations in three dimensions, and in other embodiments the accelerometer 24 comprises three one- dimensional accelerometers arranged orthogonally to each other. The accelerometer 24 measures the magnitude and direction of the acceleration acting on the apparatus 2 and outputs an acceleration signal indicating the acceleration in one, two or three dimensions to the control unit 10. The accelerometer 24 can operate according to any desired operating or sampling frequency to measure the acceleration, for example 50 Hz.

In this illustrated embodiment of the invention, the apparatus 2 comprises a single unit or device that is worn or carried by the subject and that measures the physiological parameter of the subject. In alternative embodiments, the control unit 10 (or the functions performed by the control unit 10 according to the invention) can be located remotely from the physiological parameter sensor 4 (for example in a unit that is worn on a different part of the body of the subject, in a base unit or computer that can be located in the subject's home, or a remote server located in the premises of a healthcare service provider), in which case the apparatus 2 will comprise a sensor unit to be worn by the subject (that is similar to that shown in Figure 1) and that comprises suitable transmitter, transceiver or communication circuitry for transmitting the measurements to a control unit in the remote unit. In either embodiment, the apparatus 2 can be part of a monitoring system which comprises a display or other visual indicator (that can themselves be part of or separate from the apparatus 2) that can be used to indicate the measured physiological parameter to the subject or a clinician.

In embodiments of the invention the apparatus 2 can be sized and/or shaped so that it can be worn or carried on a part of the body of the subject, for example on a finger, hand, arm, chest, leg, forehead, etc. The apparatus 2 can be provided with or be part of some means to enable the apparatus 2 to be held in contact with the subject so that the

physiological parameter sensor 4 can obtain the required measurements of the physiological parameter of the subject. For example, the apparatus 2 can be provided with or be part of a belt or strap, or the apparatus 2 can be part of an adhesive patch, a watch or an item of clothing such as a glove, jumper, or an arm, head or chest band.

In practical implementations, the apparatus 2 may comprise other or further components to those shown in Figure 1 and described above, such as a user interface that allows the subject to activate and/or operate the apparatus 2, and a power supply, such as a battery, for powering the apparatus 2. The user interface may comprise one or more components that allow a user (e.g. the subject) to interact and control the apparatus 2. As an exarrmle. the one or more user interface components could comprise a switch, a button or other control means for activating and deactivating the apparatus 2 and/or measurement process. The user interface components can also or alternatively comprise a display, or other visual indicator (such as a light) for providing information to the subject about the operation of the apparatus 2, including displaying the determined physiological parameter. Likewise, the user interface components can comprise an audio source for providing audible feedback to the subject about the operation of the apparatus 2, including an audible indication of the determined physiological parameter.

A method of operating the apparatus 2 according to an embodiment is shown in Figure 4.

In a first step, step 101, the physiological parameter sensor 4 is applied to the subject with a first contact pressure using the actuator 14. As noted above, one or more actuators 14 may be provided in the apparatus 2 (for each of a light sensor 6 and a light source 8), in which case step 101 can comprise applying the pressure using at least one of the actuators 14.

Step 101 may comprise providing a control signal from the control unit 10 so that the actuator 14 changes state, shape or configuration from a default state, shape or configuration to apply the first contact pressure. Alternatively, the first contact pressure may be the contact pressure applied when the actuator 14 is in the default state, shape or configuration (so no control signal may be supplied by the control unit 10.

Then, in step 103, a signal is obtained using the physiological parameter sensor 4. Where the physiological parameter sensor 4 comprises a light sensor 6, a signal representing the intensity of light is measured in step 103. In some embodiments, as described in more detail below, the signal obtained in step 103 can be obtained when a light source 8 in the physiological parameter sensor 4 is emitting light, and/or when the light source 8 is not emitting light (in which case the signal output by the light sensor 6 represents the intensity of the ambient light at the light sensor 6).

An exemplary signal obtained by a light sensor 6 is shown in Figure 5. The upper line represents the PPG signal obtained using red light and the lower line represents the PPG signal obtained using infrared light.

In step 105 the signal is analysed to determine if the signal meets a quality metric. This step can comprise determining the quality of the signal and comparing the quality to a quality metric, such as a threshold or other criteria (e.g. is the quality the maximum available value). The quality of the signal can be measured in different ways. In some embodiments, the Quality of the signal can be measured in terms of the amplitude or intensity of the measured light (e.g. the AC amplitude), and the signal can meet the quality metric if a peak amplitude or peak intensity is above a threshold value or a signal-to-noise ratio (SNR) is above a threshold value, or the signal can meet the quality metric if the maximum amplitude or intensity obtained at the current contact pressure is above the maximum amplitude or intensity obtained at other contact pressures (in other words the amplitude, intensity or SNR is an optimal/maximum value). In other embodiments, the signal can meet the quality metric if the signal is sufficient for determining the physiological parameter of the subject (which can for example be determined by comparing a signal-to- noise ratio or a ratio of the AC amplitude to the DC amplitude for the signal to a threshold value, or determining a value for the physiological parameter from the signal and estimating the reliability or quality of the physiological parameter, for example if the physiological parameter is within normally accepted values, etc.). It will be appreciated that in some embodiments, the threshold value can differ depending on the physiological parameter to be determined. For example, the signal quality required to determine heart rate is much lower than the signal quality required to determine the Sp02, and the thresholds can be set accordingly. In yet further embodiments, the quality of the signal can relate to the amplitude or intensity of light when a light source 8 in the physiological parameter sensor 4 is not emitting light (i.e. the quality of the signal relates to the amount of ambient light being measured by the light sensor 6 that can affect the quality of the physiological parameter measurement), with the quality, for example, corresponding to the amplitude or intensity of light being below a threshold value. It will be appreciated that in some embodiments, more than one measure of quality can be evaluated to determine if the signal meets the quality metric, and in some embodiments each measure of quality may need to meet the appropriate threshold or criteria for the quality metric to be met. For example, both the quality of a signal obtained when the light source is emitting light and the quality of a signal obtained when the light source is not emitting light can be evaluated to determine if the contact between the physiological parameter sensor 4 and the subject meets the quality metric.

If the signal does not meet the quality metric, then the method moves to step 107 in which the contact pressure of the physiological parameter sensor 4 is adjusted using the actuator 14. As noted above, this adjustment can comprise adjusting the pressure with which either or both of a light sensor 6 and light source 8 (if present) are applied to the subject. This step comprises the control unit 10 outputting a control signal to the actuator 14 that causes the actuator to change state, shape or configuration and apply the physiological narameter sensor 4 to the subject with a different pressure. It will be appreciated that step 107 can comprise controlling the actuator 14 to increase the pressure with which the

physiological parameter sensor 4 is applied to the subject, or controlling the actuator 14 to decrease the pressure with which the physiological parameter sensor 4 is applied to the subject. The method then returns to step 103 in which a signal is obtained using the physiological parameter sensor 4 and the signal analysed (step 105).

If the signal does meet the quality metric in step 105 then the method passes to step 109 in which the actuator 14 is controlled to maintain the current contact pressure between the physiological parameter sensor 4 and the subject, and the physiological parameter sensor 4 is used to measure the physiological parameter of the subject.

It will be appreciated that by analysing the signal quality and adjusting the contact pressure in response to that analysis, the method shown in Figure 4 provides closed- loop feedback control over the applied pressure until a signal that meets the quality metric is obtained. In particular embodiments, the amplitudes of light (PPG) signals can be measured frequently or continuously as the apparatus 2 is being worn and the sensor contact pressure adjusted in a closed- loop fashion to optimise the signal-to-noise of the signal (i.e. to optimise the AC amplitude of the PPG signal).

The graph in Figure 6 illustrates how a normalised PPG foot-to-peak amplitude changes with the applied contact pressure (measured in kPa). It will be noted that there is a pressure or range of pressures that provide the largest PPG signal peak amplitudes, and below or above these pressures the PPG amplitude is significantly lower (and may be too low to provide a reliable measure of the physiological parameter). At the lower contact pressures, the coupling between the sensor 4 and the subject may be too poor to enable the sensor 4 to detect the signal from the subject, or (in the case of a light sensor 6) may allow ambient light from the environment around the subject into the sensing region around the sensor 4 and thus reduce the quality of the measured signal. At the higher contact pressures, the pressure may be so high that the flow of blood through that part of the body of the subject is reduced, which, particularly in the case of a PPG sensor, reduces the ability of the sensor 4 to successfully obtain a PPG signal.

Thus, it can be seen from Figure 6 how adjusting the contact pressure of the sensor 4 can lead to a change in the amplitude of the obtained signal, and therefore the adjustment of the contact pressure (for example using a feedback loop as shown in the flow chart of Figure 4) can enable a contact pressure to be identified that provides a signal with a required or sufficient quality (i.e. that meets the quality metric). For example, if the required aualitv is the neak PPG amnlitude obtainable using the sensor 4, an initial contact pressure of 16 kPa gives a normalised PPG amplitude of 0.5. Decreasing the contact pressure using the control loop and analysing the quality of the signal will lead to an increase in the normalised PPG amplitude, with a pressure of around 10 kPa providing the highest possible normalised PPG amplitude.

It will be appreciated that during operation of the control loop the control unit

10 can determine the change in signal quality resulting from a change in contact pressure (i.e. determine whether the signal quality increased or decreased following the change in contact pressure), and make the next adjustment of the contact pressure in the appropriate direction (i.e. increase or decrease) accordingly. For example, based on Figure 6, if the first contact pressure is 16 kPa and the control unit 10 controls the actuator 14 to increase the contact pressure to 18 kPa in response to determining the signal amplitude at 16 kPa (0.5) does not meet the quality metric, then the control unit 10 will determine that the signal quality has now decreased (to around 0.35). In that case, the control unit 10 will determine that the next adjustment to the contact pressure should be a decrease in order to improve the signal quality.

It will be appreciated that although Figure 6 illustrates contact pressures along the x-axis, in practice the apparatus 2 will not be aware of or measure the actual contact pressure of the sensor 4 and instead adjusts the contact pressures on the basis of the control signal from the control unit 10. In the case where the voltage of the control signal controls the amount of actuation of the actuator 14, increasing or decreasing the voltage of the control signal will cause an increase or decrease in the amount of actuation of the actuator 14 and thus the contact pressure applied.

In some embodiments, the method in Figure 4 can be performed until a contact pressure is identified that provides a signal at the required quality (i.e. a contact pressure where the signal meets the quality metric). For example, where a signal quality above a threshold value is required, the method can be performed until a contact pressure is found that provides a signal at or above that quality. However, it will be noted from Figure 6 that in some cases there may be at least two contact pressures where the signal quality is first achieved (i.e. either side of the peak amplitude at around 10 kPa). To improve the comfort of the subject while wearing and using the apparatus 2, the contact pressure should be as low as possible. In this case, it would be preferable for the contact pressure to be the lower one of the two that provides the signal with the required quality. Therefore, in some embodiments, rather than perform the method in Figure 4 until a contact pressure is identified that provides a signal at a required quality, the control unit 10 can repeat steps 103 (obtaining a signal), 105 (determining if the signal meets the quality metric) and 107 (adjusting the contact pressure) so that the signal quality at a range of contact pressures is determined (in effect the control unit 10 determines information similar to that shown in Figure 6). The control unit 10 can then identify a suitable contact pressure (or rather the corresponding actuator control signal) that provides the signal at the required quality and at the lowest possible contact pressure, and control the actuator 14 to provide that contact pressure before measuring the physiological parameter using the sensor 4.

As noted above, the signal can be analysed in a number of different ways to determine if the signal meets the quality metric. In some of the embodiments, the signal is obtained when a light source 8 in the sensor 4 is active and emitting light (as is the case when the sensor 4 is being used to measure the physiological parameter).

As noted above, in some embodiments, the quality of the signal can relate to the amplitude or intensity of light when a light source 8 in the physiological parameter sensor 4 is not emitting light. In these embodiments, the quality of the signal indicates how much ambient light is leaking into the light sensing region (i.e. where the light sensor 6 is located on the subject) from the environment. Poor contact between the light sensor 6 and the subject will allow ambient light to leak in and may partially or completely mask the light from the light source 8 that has reflected from or passed through the subject.

Thus, in these embodiments, in step 103 the control unit 10 controls the light sensor 6 to measure light when the light source 8 is inactive and not emitting light. That way, the signal from the light sensor 6 will represent the amount of ambient light. Since the physiological parameter sensor 4 may be used to periodically measure the physiological parameter of the subject, the control unit 10 can control the light sensor 6 to measure the light level during intervals between physiological parameter measurements when the light source 8 is active. Figure 7(a) illustrates intervals during which background/ambient light

measurements can be made. This example relates to an Sp02 sensor in which measurements at both red and infrared wavelengths of light are required in order to determine the oxygen saturation. The red and infrared light sources are activated at separate times to enable the red and infrared light measurements to be obtained, and in accordance with this embodiment the light sensor 6 is activated to measure the background light at one or more intervals when the light sources 8 are switched off.

The signal from the light sensor 6 may be analysed in step 105 to determine the amplitude or intensity of the light during this or these intervals, and that amplitude or intensity can be compared to a threshold value determine if the signal meets the quality metric. Tf the arrmlitude or intensity is above the threshold value, then this can indicate that the amount of ambient light is too high and a signal from the light sensor 6 during a physiological parameter measurement will be of insufficient quality (i.e. does not meet the quality metric) and thus does not meet the quality metric. In that case the control unit 10 can control the actuator 14 to increase the contact pressure with the aim that the increased contact pressure will improve the contact and reduce the amount of ambient light measured by the light sensor 6.

In other cases, rather than specifically activate the light sensor 6 during intervals in which the light source 8 is inactive, the control unit 10 may control the light sensor 6 to obtain a signal continuously during operation of the apparatus 2 (so during periods when the light sources 8 are active and inactive). Two exemplary signals obtained in this way are shown in Figure 7(b). The first exemplary signal, signal 30, is obtained when there is good contact between the light sensor 6 and the subject. Signal 30 has a low (almost zero) amplitude between physiological parameter measurements (with the physiological parameter measurements corresponding to the peaks in the light amplitude) and clear peaks in the signal (signal 30 has the higher peaks in Figure 7(b)). Signal 30 thus has a high SNR and would provide good light measurements for obtaining the physiological parameter measurement. The second signal in Figure 7(b), signal 32, is obtained when the contact between the light sensor 6 and the subject is not as good. Signal 32 has a significant non-zero amplitude between physiological parameter measurements (with the physiological parameter measurements corresponding to the peaks in the light amplitude). As a result of the ambient light leaking into the light measurements, the peaks obtained using the light sensor 6 are lower than for signal 30 (this can be due to, for example, the effect of autogain). Signal 32 thus has a much lower SNR than signal 30 and may not provide a light measurement that meets the quality metric and allows the physiological parameter measurement to be obtained.

As noted above, in embodiments where the physiological parameter sensor 4 comprises a separate light sensor(s) 6 and light source(s) 8, the actuator 14 may be configured or arranged in the apparatus 2 such that it applies pressure to just one of the light sensor 6 and light source 8, or to both of the light sensor 6 and light source 8. Where there are multiple light sources 8 (for example in the case of an Sp02 sensor), each of the light sources 8 may have a respective actuator 14, or there may be a single actuator 14 for both light sources 8.

Figure 8(a) illustrates an embodiment in which the actuator 14 is configured to apply pressure just to the light sensor 6 and not the light source 8. With this arrangement, it is possible to improve the quality of the contact between the light sensor 6 and the subject and, for example, reduce the amount of ambient light affecting the light measurements.

Figure 8(b) illustrates an embodiment in which a first actuator 14 is provided to apply pressure to the light sensor 6 and a second actuator 14 is provided to apply pressure to the light source 8. With this arrangement, it is possible for the control unit 10 to independently control the pressure with which the light sensor 6 and light source 8 are applied to the subject. For example, in this embodiment the control unit 10 may identify from the measured light signal that the ambient light level measured by the light sensor 6 is low (which can indicate that the contact between the light sensor 6 and the subject is acceptable), but the maximum signal amplitude is also low, in which case the control unit 10 can control the second actuator 14 to apply the light source 8 to the subject with a different pressure to improve the coupling of the light from the light source 8 to the subject. Conversely, the control unit 10 may identify from the measured light signal that the maximum signal amplitude is adequate (which can indicate that there is a good coupling between the light source 8 and the subject) but that the ambient light level measured by the light sensor 6 is too high (e.g. above a threshold), in which case the control unit 10 can control the first actuator 14 to apply the light sensor 6 to the subject with a different pressure to improve the contact of the light sensor 6 to the subject and reduce ambient light artefacts.

Figure 8(c) illustrates an embodiment in which the actuator 14 is configured to apply pressure just to the light source 8 and not the light sensor 6. With this arrangement, it is possible to improve the quality of the contact between the light source 8 and the subject and, for example, improve the coupling of light from the source 8 into the subject.

Figure 9 illustrates two ways in which the embodiment in Figure 8 can be applied to a subject's finger. In Figure 9(a) the light sensor 6 and light source 8 are arranged on opposite sides of the finger, whereas in Figure 9(b) the light sensor 6 and light source 8 are arranged on the same side of the finger.

It will be appreciated that although Figures 2 and 9 show the sensor 4 (light sensor 6 and light source 8) and actuator 14 on particular sides of the subject's finger, these illustrated arrangements are not limiting and other orientations of or locations for the apparatus 2 on the subject are possible.

As noted with reference to Figure 1, in some embodiments of the apparatus 2, the apparatus 2 comprises a sensor 24 for measuring the movements or motion of the subject and/or the apparatus 2. It will be appreciated that accelerations, sudden jerks or impacts of the armaratus 2 or the nart of the body of the subject that the apparatus 2 is attached to can cause the quality of the contact between the physiological parameter sensor 4 and the subject to change. Therefore, in some embodiments, the control unit 10 can control the actuator 14 to adjust the contact pressure and compensate for or avoid a reduction in signal quality due to relative motion of the subject and the apparatus 2/sensor 4. This adjustment of the contact pressure can be performed by the control unit 10 in addition to the adjustments made as part of the method in Figure 4. In alternative implementations, these adjustments can be made independently of the method in Figure 4 (so without a check to determine if the signal meets the quality metric, and without an adjustment in contact pressure in response to the check on signal quality).

The flow chart in Figure 10 illustrates the operation of the control unit 10 according to this embodiment. In step 111, the movements of the subject and/or the sensor 4 are measured using the movement sensor 24 (e.g. accelerometer). Figure 11 illustrates an exemplary movement measurement in the form of an acceleration measurement that comprises acceleration signals along each of three orthogonally arranged axes.

Then, in step 113, the control unit 10 controls the actuator 14 to adjust the pressure of the contact between the sensor 4 and the subject to compensate for relative motion of the subject and sensor 4 and thus a potential reduction in signal quality. For example, where the apparatus 2 is arranged on the subject as shown in Figure 2, a rapid downward movement of the subject's hand may result in a reduction in the contact pressure between the apparatus 2 and the subject, in which case the control unit 10 can control the actuator 14 to increase the contact pressure between the sensor 4 and the subject.

In step 113 the control unit 10 derives a control signal for the actuator 14 from the measured movements. In some embodiments, the control unit 10 can determine the control signal from the movements (acceleration) in the direction along an axis that is orthogonal to the plane of the skin of the subject at the point of contact with the apparatus 2 (for example in the case of the example shown in Figure 2, the axis is the vertical axis). Where the movement sensor 24 measures movements/accelerations in three dimensions, the control unit 10 may need to process the three-dimensional motion signal to determine the required component of movement/acceleration. Such processing is known to those skilled in the art and will not be described herein.

In a simple embodiment, the control unit 10 can determine the control signal for the actuator 14 directly from the movement/acceleration signal. In particular, the control signal for the actuator 14 can be determined as the inverse of the movement/acceleration signal. Figure 12 illustrates an exemplary control signal (the bottom signal) that is derived by inverting the acceleration signal (the top signal) measured along the y-axis of the

accelerometer 24 (where the y-axis is the axis that is parallel to the direction of actuation of the actuator 14). Thus, the control signal determined in this embodiment causes the actuator 14 to directly counteract the detected acceleration. It will be appreciated that in practice the control unit 10 may need to apply some other processing and/or amplification to the inverted acceleration signal in order for the signal to be in the appropriate form to control the actuator 14, but such processing will be known to those skilled in the art.

In an alternative embodiment, the control unit 10 can determine the control signal for the actuator 14 as the ratio of the measured movement (e.g. acceleration) and movement artefacts that occur in the signal from the light sensor 6. Movement artefacts that occur in the signal can be identified through correlation with the acceleration signal. When the light sensor 6 moves away from the skin of the subject, the detected light intensity decreases. In this embodiment, rather than just correct the sensor contact for the movements observed in the acceleration signal, the extent to which the movements affect the quality of the signal from the light sensor 6 is estimated and the actuator 14 used to apply the necessary corrections. Another approach to determine the control signal based on movement artefacts is for the control unit 10 to analyse the movement signal to determine the amplitudes of acceleration and compare the amplitudes to the amplitude of the signal from the light sensor 6 to determine the effect that the acceleration has on the light signal, and determine the control signal for the actuator 14 on the basis of the comparison.

The flow chart in Figure 13 illustrates a further method of operating an apparatus 2 to compensate for relative motion of the physiological parameter sensor 4 and the subject due to accelerations (e.g. rapid movements, sudden jerks or impacts) of the apparatus 2 and/or the part of the body of the subject that the apparatus 2 is attached to. In the embodiment described above with reference to Figure 10, the control unit 10 controls the actuator 14 to adjust the contact pressure and compensate for or avoid a reduction in signal quality due to relative motion of the subject and the apparatus 2/sensor 4. In the method illustrated in Figure 13, the control unit 10 controls the actuator 14 to apply the sensor 4 to the subject with a contact pressure at which a measurement of the physiological parameter can be obtained using a signal from the sensor 4 (i.e. the contact pressure is such that the signal from the sensor 4 is of sufficient quality that a measurement of the physiological parameter can be obtained) and at which relative motion of the subject and the physiological sensor due to accelerations is reduced or avoided. The reduction in relative motion can be such that the relative motion of the subject and the physiological sensor is kept within tolerated limits.

Briefly, in this embodiment, the light source 8 is used to emit light, the light sensor 6 is used to measure light, an accelerometer 24 is used to measure the accelerations of the subject and/or the physiological parameter sensor 4, and the control unit 10 analyses the measured light signal and acceleration signal to determine a pressure that the sensor 24 should be applied to the subject with in order to be able to obtain a measurement of the physiological parameter in the presence of the measured accelerations/movements.

In the absence of motion (e.g. when the measured accelerations are below a threshold), the applied pressure is optimised by maximising the amplitude of the measured light signals (e.g. maximising the AC modulations in the measured light signal). It will be appreciated that maximising the amplitude often does not mean applying the highest possible contact pressure since that may lead to a reduction in the blood flow in the volume of tissue beneath the sensor 24 as the tissue is compressed, but some intermediate pressure value.

When motion is detected (e.g. when the measured accelerations are above a threshold), the measured accelerations (or the signal representing the measured accelerations) can be used to Overrule' the light measurements (e.g. ignore the contact pressure that would be determined from analysing the measured light signals) and the contact pressure can be set based on the need to avoid movement artefacts. Thus in this embodiment when the accelerations exceed the threshold, only the measurements of acceleration are used to determine the contact pressure.

Alternatively, both the measurements of acceleration and the signal from the light sensor 6 can be used to determine the contact pressure. In particular the control unit 10 can adjust the contact pressure to find an optimum between the amplitude of the light signals (e.g. the AC modulations in the light amplitude) and the extent of the motion artefacts. It will be appreciated that applying the sensor 4 to the subject with a high pressure will stop the sensor 4 from moving relative to the subject (and thus avoid motion artefacts), but this pressure may lead to no AC modulations/blood pulsations being measured or detectable in the signal from the light sensor 6, so this embodiment aims to find a contact pressure that balances these two requirements. In some embodiments the control unit 10 can manage this balance by aiming to obtain a light signal with a quality as high as possible (e.g. the amplitude of AC modulations as high as possible) while keeping movement artefacts below an acceptable level. One way in which the control unit 10 can adjust the contact pressure to find an optimum between the amplitude of light signals used for a PPG measurement and the extent of the motion artefacts according to the method in Figure 13 is for the control unit 10 to analyse (i) the relation between measured accelerations and changes in the light sensor signal, (ii) changes in accelerations and consequent PPG signal artefacts resulting from changes in contact pressure (this is performed in a closed-loop manner); (iii) the AC component due to the heartbeats; (iv) changes in the AC component due to heartbeats as a result of changes in contact pressures (this is also performed in a closed-loop manner). Based on these relations, and the desired physiological parameters to be derived, the control unit 10 can set the contact pressure such that motion artefacts are kept within application-specific limits (i.e. limits set based on the use for the PPG signal, e.g. measuring heart rate, Sp02, arrhythmia detection, etc. which have different minimum quality/maximum artefact requirements for the light signal) and the desired parameter can be measured with sufficient reliability.

Thus, in the first step of Figure 13, step 121 , the physiological parameter sensor 4 is applied to the subject with a first contact pressure using the actuator 14. As noted above, one or more actuators 14 may be provided in the apparatus 2 (for each of a light sensor 6 and a light source 8), in which case step 121 can comprise applying the pressure using at least one of the actuators 14.

Step 121 may comprise providing a control signal from the control unit 10 so that the actuator 14 changes state, shape or configuration from a default state, shape or configuration to apply the first contact pressure. Alternatively, the first contact pressure may be the contact pressure applied when the actuator 14 is in the default state, shape or configuration (so no control signal may be supplied by the control unit 10).

Then, in step 123, a signal is obtained using the physiological parameter sensor 4 and measurements of the acceleration of the subject and/or the physiological parameter sensor 4 are made using an accelerometer 24. Where the physiological parameter sensor 4 comprises a light sensor 6, a signal representing the intensity of light is measured in step 123. In some embodiments, as described above with reference to Figure 4, the signal can be obtained when a light source 8 in the physiological parameter sensor 4 is emitting light, and/or when the light source 8 is not emitting light (in which case the signal output by the light sensor 6 represents the intensity of the ambient light at the light sensor 6). It will be appreciated that the accelerometer 24 will measure the acceleration of the subject and/or the physiological parameter sensor 4 depending on where the accelerometer 24 is located. If the accelerometer 24 is located in the same housing as the physiological parameter sensor 4, the accelerometer 24 will measure the accelerations of the physiological parameter sensor 4. If the accelerometer 24 is separate from the physiological parameter sensor 4, the accelerometer 24 can measure the accelerations of the subject.

In step 125 the measurements of the accelerations and the obtained signal are processed (for example as described above) to determine a second contact pressure at which a measurement of the physiological parameter can be obtained using a signal from the light sensor (i.e. the light sensor signal is of sufficient quality to enable a measurement of the parameter to be obtained) and at which relative motion of the subject and the physiological sensor due to accelerations is reduced or avoided (i.e. as noted above, the relative motion is, at least, kept within tolerated limits). Thus, in this step a second contact pressure is determined that enables a measurement of the physiological parameter to be obtained while accelerations that cause relative motion are occurring.

Once the second contact pressure has been determined, the control unit 10 controls the actuator 14 to apply the physiological parameter sensor 4 to the subject with the second contact pressure.

As noted above, controlling the actuator 14 to apply the physiological parameter sensor 4 to the subject with the second contact pressure can comprise applying either or both of a light sensor 6 and light source 8 (if present) to the subject at the second contact pressure. Thus this step comprises the control unit 10 outputting a control signal to the actuator 14 that causes the actuator to change state, shape or configuration and apply the physiological parameter sensor 4 to the subject with a different pressure. It will be

appreciated that the second contact pressure can be higher than the first contact pressure and therefore step 127 can comprise controlling the actuator 14 to increase the pressure with which the physiological parameter sensor 4 is applied to the subject, or the second contact pressure can be lower than the first contact pressure and therefore step 127 can comprise controlling the actuator 14 to decrease the pressure with which the physiological parameter sensor 4 is applied to the subject.

After step 127, the control unit 10 measures the physiological parameter of the subject using the physiological parameter sensor 4 (e.g. the control unit 10 analyses the signal from the light sensor 6 to determine the physiological parameter).

It will be appreciated that in some embodiments step 125 can comprise determining the second contact pressure directly from the measured accelerations and the obtained signal (for example by using a look up table that relates contact pressure to acceleration and a aualitv of the obtained signal, or by calculating the required contact pressure from the measurements of the acceleration and a quality of the obtained signal). In alternative embodiments, step 125 can comprise implementing a control/feedback loop in which an adjustment to the contact pressure is determined based on the measurements of the acceleration and the obtained signal, the contact pressure is adjusted and new measurements of acceleration and light signal are obtained until a contact pressure is found that enables a measurement of the physiological parameter to be obtained using a signal from the light sensor with relative motion of the subject and the physiological sensor due to accelerations being reduced or avoided. Thus, steps 123-127 can be repeated until a signal that meets a quality metric is obtained.

In some embodiments, step 125 comprises determining whether the accelerations exceed a threshold (e.g. determining whether there is significant acceleration, and thus a significant risk of motion artefacts). If the accelerations do not exceed the threshold, then step 125 can comprise determining whether the obtained signal meets a quality metric, and if the obtained signal does not meet the quality metric, the control unit 10 can determine the second contact pressure as an adjustment to the first contact pressure that will improve the signal with respect to the quality metric. It will be appreciated from the above discussion that this adjustment can be an increase or a decrease in the first contact pressure.

In some embodiments, in step 125 the control unit 10 is configured to determine from the measurements of the accelerations whether the accelerations exceed a threshold. If the accelerations exceed the threshold (i.e. there is significant acceleration, and thus a significant risk of relative motion of the subject and the physiological parameter sensor), the control unit 10 determines an increase to the first contact pressure to compensate for or avoid a reduction in quality of the signal due to the accelerations.

In some embodiments, the control unit 10 continues to monitor the accelerations affecting the apparatus 2 and/or subject, and if the accelerations reduce below the threshold, the control unit 10 can reduce the contact pressure accordingly. In particular, after the control unit 10 has controlled the actuator to apply the physiological parameter sensor to the subject with the second contact pressure and the physiological parameter of the subject has been measured using the physiological parameter sensor, the control unit obtains a further signal using the light sensor 6 and measures the accelerations of the subject and/or the physiological parameter sensor, determines from the measurements of the accelerations whether the accelerations are below the threshold, determines a decrease in the contact pressure if the accelerations are below the threshold, and the actuator is controlled to decrease the contact pressure of the physiological parameter sensor on the subject.

In some embodiments, step 125 comprises determining from the measurements of the accelerations whether the accelerations exceed a threshold, comparing the signal to a quality metric if the accelerations exceed the threshold, and determining an adjustment to the contact pressure based on the comparison and the measurements of the accelerations.

In the above embodiments, where the obtained signal is compared to a quality metric, the signal can be analysed to determine a quality of the signal and this quality can be compared to a quality metric, such as a threshold or other criteria (e.g. is the quality the maximum available value). The quality of the signal can be measured in different ways. In some embodiments, the quality of the signal can be measured in terms of the amplitude or intensity of the measured light (e.g. the AC amplitude), and the signal can meet the quality metric if a peak amplitude or peak intensity is above a threshold value or a signal-to-noise ratio (SNR) is above a threshold value, or the signal can meet the quality metric if the maximum amplitude or intensity obtained at the current contact pressure is above the maximum amplitude or intensity obtained at other contact pressures (in other words the amplitude, intensity or SNR is an optimal/maximum value). In other embodiments, the signal can meet the quality metric if the signal is sufficient for determining the physiological parameter of the subject (which can for example be determined by comparing a signal-to- noise ratio or a ratio of the AC amplitude to the DC amplitude for the signal to a threshold value, or determining a value for the physiological parameter from the signal and estimating the reliability or quality of the physiological parameter, for example if the physiological parameter is within normally accepted values, etc.). It will be appreciated that in some embodiments, the threshold value can differ depending on the physiological parameter to be determined. For example, the signal quality required to determine heart rate is much lower than the signal quality required to determine the Sp02, and the thresholds can be set accordingly. In yet further embodiments, the quality of the signal can relate to the measured amplitude or intensity of light when a light source 8 in the physiological parameter sensor 4 is not emitting light (i.e. the signal relates to the amount of ambient light being measured by the light sensor 6 that can affect the quality of the physiological parameter measurement), with sufficient quality, for example, corresponding to an amplitude or intensity of light being below a certain threshold value. It will be appreciated that in some embodiments, more than one measure of aualitv. weighted individually, can be evaluated to determine if the signal meets the quality metric, and in some embodiments each measure of quality may need to meet the appropriate threshold or criteria for the quality metric to be met. For example, both the quality of a signal obtained when the light source is emitting light and the quality of a signal obtained when the light source is not emitting light can be evaluated to determine if the contact between the physiological parameter sensor 4 and the subject meets the quality metric. In further embodiments, the apparatus 2 may comprise multiple actuators 14 that are arranged in the apparatus 2 such that they can adjust the contact of the sensor 4 to the subject in different directions. An exemplary embodiment is illustrated in Figure 14. In this embodiment, in addition to an actuator 14 that is arranged to press the sensor 4 into the subject, two further actuators 40, 42 are provided that can press on and therefore move the sensor 4 over the skin of the subject (i.e. along a plane that is perpendicular to the direction in which the first actuator 14 acts). The three actuators 14, 40, 42 are thus arranged so that they apply pressure to the sensor 4 in orthogonal directions. The control unit 10 can control the actuators 14, 40, 42 individually in response to the measured (three-dimensional) movements to try and maintain the contact of the sensor 4 to the subject.

As in the embodiment above, the control signal for the actuators 14, 40, 42 can be determined as the inverse of the movement/acceleration signal. Figure 15 illustrates exemplary control signals (the right hand signals) that are derived by inverting the acceleration signals on each of the measurement axes (the left hand signal), assuming that the X-, y- and z-axes are parallel to the direction of actuation of respective actuators). Thus, the control signal determined in this embodiment causes the actuator or actuators (as appropriate) to directly counteract the detected acceleration in that or those directions.

In some embodiments, to prevent the contact pressure being continuously adjusted by the actuator(s) 14 in response to small fluctuations in the measured acceleration (which may be caused by noise in the movement measurements or by small movements of the subject due to, for example, blood flow, the control unit 10 may 'smooth' the measured acceleration using a filter, or may only provide a control signal to the actuator 14 when the measured acceleration exceeds a threshold value (in which case the actuator 14 will only change the applied pressure when there is a sufficiently large movement or acceleration).

In some embodiments the change in contact pressure in response to measured movements may only be temporary while the movement, acceleration, jerk or impact is occurring. Once that movement has passed, as indicated by the measured movements, the control unit 10 can return the actuator 14 to the state it was in prior to the measured movement (i.e. and so the actuator 14 applies the sensor 4 to the subject with the pressure it was applying prior to the movement).

It will be appreciated that the three-actuator arrangement shown in Figure 14 can also be used in the method of Figure 4, in which case the control unit 10 can operate any one or more of the actuators 14, 40, 42 to adjust the contact of the sensor 4 to the subject in response to determining that the signal measured by the sensor 4 does not meet the quality metric.

Thus, there is provided an apparatus having a sensor and a method of operating the same in which the sensor can be applied to a subject in order to obtain signals from which a measurement of a physiological parameter can be obtained.

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.

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 fulfil 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.