Cardiac monitoring systems and methods

Abstract

A cardiac monitoring system (100) includes an array of force-sensitive resistors (10) spanning a sensor surface (50). Each resistor (10) is configured to change a corresponding resistance value (R) according to an amount of static pressure (P) applied by an object (200) on the sensor surface (50) at a corresponding position of the force-sensitive resistor (10). An array of piezoelectric transducers (20) is dispersed in the array of force-sensitive resistors (10). Each transducer (20) is configured to generate a corresponding time-dependent electrical signal (S) according to a corresponding vibration (F) applied by the object (200) on the sensor surface (50) at a corresponding position of the transducer (20). A controller (30) is configured to determine a heart rate (H1) of the object (200) based on a combination of a measured resistance value (R) of the force-sensitive resistor (10) and a time-dependent electrical signal (S) of the piezoelectric transducer (20).

Description

Technical Field

[0001] The present disclosure relates to systems and methods for measuring heart rate. Background Art

[0002] For example, piezoelectric sensors can be used to perform ballistocardiography (BCG) in a semi-contact manner through clothing or bedding / mattresses (such as chairs / beds) to measure heart rate, respiratory rate and even pulse wave velocity. Preferably, the sensor needs to be close to the relevant part of the body to transmit vibrations. Unfortunately, it is difficult to extract this contact information from the sensor signal. Therefore, artifacts are common and are more severe when the position of the object is uncertain.

[0003] US2018 / 0337325A1 describes a multi-element piezoelectric sensor for physiological measurements in bed. A monitoring system may perform a first scan of all piezoelectric sensors. The first scan may be, for example, a high-level scan to roughly determine the position of the user along the mat. The monitoring system may perform a second scan of one or more piezoelectric sensors, such as piezoelectric sensors that may be located in the immediate vicinity of the user's body. Piezoelectric sensors that may be located in the immediate vicinity of the user's body may be, for example, those that measure force while also being adjacent to piezoelectric sensors that do not measure force.

[0004] However, piezoelectric sensors typically generate time-dependent signals, which may require separate and continuous monitoring. It may also be difficult to balance accuracy for different time scales and pressure ranges. Further improvements in the accuracy of heart rate monitoring are still needed. Summary of the invention

[0005] Aspects of the present disclosure relate to a cardiac monitoring system and method. As described in the present disclosure, the cardiac monitoring system preferably uses a sensor surface having a combination of a (membrane-based) force-sensitive resistor and a piezoelectric transducer. The force-sensitive resistor can be configured to measure the amount of corresponding pressure applied by the object on the sensor surface. For example, the force-sensitive resistor can change the corresponding resistance value according to the pressure. The piezoelectric transducer can be dispersed among the force-sensitive resistor, such as between, and the piezoelectric transducer is configured to measure the corresponding vibration applied by the object on the sensor surface at the corresponding position of the transducer. For example, the piezoelectric transducer can generate a time-dependent electrical signal based on the vibration. The heart rate of the object can be determined based on a combination of corresponding signals from different types of sensors (e.g., the measured resistance value of the force-sensitive resistor and the time-dependent electrical signal of the piezoelectric transducer).

[0006] Compared with using piezoelectric sensors, using force-sensitive resistors can more accurately determine the (static) pressure distribution of an object. For example, force-sensitive resistors can be arranged in a (relatively) high-density grid, for example with shared scan lines in a passive matrix configuration. As will be understood, force-sensitive resistors can be better suited to measuring the range of different pressure signals and can be used to accurately determine the pressure distribution in a high-density grid. Using the accurate pressure distribution information of the force-sensitive resistor, the piezoelectric transducer that can generate the best signal can be selected. For example, the pressure distribution can be used to determine the area where the heart rate vibration on the body is most prominent. For example, piezoelectric transducers can be arranged in a (relatively) low-density grid, for example, so that dedicated circuit lines can be implemented between relatively few transducers and controllers. In this way, rapidly changing signals, such as heartbeats, can be clearly and accurately measured from a specific location. This can improve reliability and data integrity. Data from pressure sensors can also be used for many other applications, such as breathing, posture detection, etc. This can further enhance the application of sensors. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The above and other features, aspects, and advantages of the apparatus, system, and method of the present disclosure will be better understood from the following description, appended claims, and accompanying drawings, in which:

[0008] Figure 1A An exemplary embodiment of a cardiac monitoring system is shown;

[0009] Figure 1B shows a cross-sectional view of a cardiac monitoring system having a force sensitive resistor and a piezoelectric transducer;

[0010] Figure 2 shows a schematic layout of sensors on a sensor surface in a cardiac monitoring system and connections of the sensors to a controller;

[0011] Figure 3A and Figure 3B An example of machine learning using a neural network to improve the determination of heart rate is shown. DETAILED DESCRIPTION

[0012] The terminology used to describe specific embodiments is not intended to limit the invention. As used in this disclosure, the singular forms "a", "an", and "the" are also intended to include the plural forms, unless the context clearly indicates otherwise. The term "and / or" includes any and all combinations of one or more of the listed related items. It will be understood that the terms "comprises" and / or "comprising" specify the presence of the described features, but do not exclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as a subsequent step of another step, unless otherwise stated, the particular step may be directly followed by the other step, or one or more intermediate steps may be performed before performing the particular step. Similarly, it should be understood that when describing a connection between structures or components, unless otherwise stated, the connection may be established directly or may be established through an intermediate structure or component.

[0013] Aspects of the present disclosure relate to performing data fusion to improve the reliability of piezoelectric sensors for detecting heart beats or other physiological parameters such as respiratory rate. Preferably, a combination of piezoelectric sensors and membrane-based pressure sensors is utilized. For example, based on the distribution of pressure sensors, the piezoelectric sensor that is most likely to have good signal integrity can be selected.

[0014] In some embodiments, the quasi-static pressure distribution on an object (e.g., a bed, a chair, a crib, clothing) is first detected using pressure sensor technology, such as based on thermoplastic polyurethane (TPU) technology. From the pressure distribution, the heart rate and / or respiratory rate measurements can be extracted using a piezoelectric sensor that is most likely to provide accurate data. Preferably, the piezoelectric sensor is printed.

[0015] In some embodiments, the pressure sensor is used to reconstruct how someone is lying on the sensor. This can facilitate the reading of the piezoelectric sensor, for example, because the pressure sensor can provide a low-power solution to detect the presence of a person or object on the sensor system. Therefore, the pressure distribution obtained can be used to reconstruct a model of a person located on the pressure sensor. In one embodiment, the piezoelectric sensor is only read when a human body is detected. This can avoid computationally intensive analysis in other situations where computationally intensive analysis is not required. In another or further embodiment, based on a combination of physical positions relative to the body and body parts (such as arms, legs, neck), sensors located on the best body parts for detecting BCG are extracted, and among this group of piezoelectric sensors, piezoelectric sensors that are within the optimal pressure range are selected.

[0016] In some embodiments, piezoelectric sensors that have a higher probability of not being in contact with the body are used for noise cancellation, for example, for removing irrelevant (e.g., parasitic) vibrations. In other or further embodiments, sensors located on parts of the body where a BCG signal should not be present are used to remove other vibrations, such as vibrations related to breathing rather than heart rate, or vice versa (in the case of measuring breathing rate). In some embodiments, the breathing rate of the subject is extracted simultaneously from the pressure sensor signal and the piezoelectric sensor to provide redundancy for the obtained signal. Advantageously, since the motion of the subject is detected using a pressure sensor, motion artifacts can be actively suppressed. Alternatively, or in addition, motion artifacts can be removed spatially and / or temporally.

[0017] Utilizing an array of piezoresistive sensors allows data directly related to improved signal quality to be provided by defining the most accurate measurement locations and by measuring real-time spatial and temporal low frequency motion, e.g., below 1 Hz. It will be appreciated that utilizing the absolute pressure distribution provides more relevant data for improved measurements than, for example, using only piezoelectric sensors. Advantageously, static pressure sensor information can be used to better filter the piezoelectric data and to supplement the piezoelectric sensor data (in the case of respiration rate).

[0018] In some embodiments, the sensor is manufactured using stretchable ink and TPU technology. Accordingly, the sensor can be located closer to the human body, which improves the reliability of the signal. In other or further embodiments, the measurement can be supplemented by incorporating a (printed) temperature sensor in the sensor surface, for example to measure the temperature in a bed or chair.

[0019] The present invention is described more fully below with reference to the accompanying drawings, in which embodiments of the invention are shown. In the accompanying drawings, the absolute and relative sizes of systems, components, layers and regions may be exaggerated for clarity. The embodiments may be described with reference to schematic and / or cross-sectional views of possible idealized embodiments and intermediate structures of the invention. In the specification and drawings, the same numbers always refer to the same elements. Relative terms and their derivatives should be interpreted as referring to the orientation as described at the time or as shown in the drawings discussed. These relative terms are for convenience of description and, unless otherwise stated, these relative terms do not require that the system be constructed or operated in a specific orientation.

[0020] Figure 1A One exemplary embodiment of a cardiac monitoring system 100 is shown.

[0021] In some embodiments, for example, as shown, the cardiac monitoring system 100 includes an array of force-sensitive resistors 10 that span a sensor surface 50, such as a substrate. In one embodiment, each resistor 10 is configured to change a corresponding resistance value "R" based on the amount of (quasi) static pressure "P" applied by the object 200 on the sensor surface 50 at a corresponding position of the force-sensitive resistor 10. In other or further embodiments, the system includes an array of piezoelectric transducers 20 that are interspersed between the arrays of force-sensitive resistors 10, for example, on the same or another (overlapping) substrate. In one embodiment, each transducer 20 is configured to generate a corresponding time-related electrical signal "S" based on a corresponding vibration "F" applied by the object 200 on the sensor surface 50 at a corresponding position of the transducer 20. In a preferred embodiment, for example, as shown, controller 30 is configured to determine a heart rate “ H1 ” of subject 200 based on a combination of a measured resistance value R of force-sensitive resistor 10 and a time-dependent electrical signal “S” of piezoelectric transducer 20 .

[0022] The cardiac monitoring system 100 as described in the present disclosure can be applied to various environments and situations. In one embodiment, for example, as shown in the figure, various aspects or applications can be embodied as a bed 300 including a cardiac monitoring system 100 as described in the present disclosure. In some embodiments, the mattress of the bed 300 includes a cardiac monitoring system 100 for the subject 200 to lie on. In some embodiments, for example, as shown in the figure, the sensor surface 50 is embedded in the mattress. Alternatively, the sensor surface 50 can be set on the top of the mattress. In other or further embodiments, the sensor surface 50 can be set between one or more sheets or other bedding. For example, the subject 200 can be a patient lying in a hospital bed who needs to be continuously monitored. The cardiac monitoring system can also be incorporated into other types of furniture, such as a chair (not shown). For example, the system can be incorporated into the bottom and / or back portion of a chair (e.g., an ordinary chair or a car seat). For example, the sensor surface 50 including the force-sensitive resistor 10 and the piezoelectric transducer 20 can be clamped to the seat back and arranged on the top of the seat bottom. Alternatively, or in addition, the sensor surface 50 may be embedded in an interior layer of the seat, for example, embedded inside the seat cushion.

[0023] In a preferred embodiment, for example, as shown in the figure, the controller 30 receives as input the measured resistance value "R" of the force-sensitive resistor 10 and the time-related electrical signal "S" of the piezoelectric transducer 20, and generates as an output the heart rate "H1". Therefore, even when the object 200 is not in direct contact with the sensor surface 50, the cardiac monitoring system 100 can provide continuous, real-time measurement and analysis of the heart rate of the object 200. In some embodiments, for example, as shown in the figure, the controller 30 can be provided as an external device of the bed 300. For example, the controller 30 is connected to the bed 300 via an electrical connection. Alternatively, the controller 30 can be provided as a part of the bed 300. In one embodiment, for example, as shown in the figure, a display device is connected to the controller 30. The heart rate can also be output in other ways, for example, as an electrical data signal for further processing, or as an audio or tactile signal.

[0024] In some embodiments, Figure 1A As shown, a secondary cardiac monitoring device 40 may be provided temporarily or permanently and is coupled to the controller 30. In some embodiments, the secondary cardiac monitoring device 40 is attached to the subject 200 (e.g., wrist or chest) for a period of time to generate a secondary heart rate measurement H2 that may serve as a reference value. For example, the secondary heart controller 30 may compare the secondary heart rate measurement H2 with the primary heart rate measurement "H1." In the embodiment shown, the secondary heart rate measurement H2 is measured by a separate instrument, such as a dedicated secondary cardiac monitoring device 40, based on an electrical signal (electrocardiogram) from the heart area. In some embodiments, the secondary cardiac monitoring device 40 may be provided temporarily to check and / or calibrate the primary cardiac monitoring system 100. Alternatively, as will be described later in Figure 3A and Figure 3B As explained in , once the network has been trained, the secondary heart monitoring devices (e.g., electrodes) on the chest can be removed.

[0025] In some embodiments, for example, as shown, the weight, position or movement of the subject can cause a static or quasi-static pressure "P" on the sensor surface 50. For example, the pressure "P" may be relatively high in a particular area, for example, the pressure "P" is higher in the chest area of ​​the subject than in the legs of the subject. In a preferred embodiment, the pressure distribution is used to determine the location of the heart rate measurement. For example, based on the time scale, static pressure can be distinguished from dynamic vibrations "F" such as heart rate. As will be understood, force-sensitive resistors are generally able to measure static pressure, that is, even if the pressure is constant in time. Of course, the pressure may change, for example due to movement of the object, and the change in pressure will result in a new (static) pressure being measured. This may be in contrast to measurements using piezoelectric sensors, which are generally more sensitive to changes in vibration or pressure.

[0026] Figure 1B A cross-sectional view of a cardiac monitoring system 100 with a force sensitive resistor 10 and a piezoelectric transducer 20 is shown.

[0027] In a preferred embodiment, the force sensitive resistors 10 are arranged on or across the sensor surface 50. Most preferably, the force sensitive resistors 10 are distributed on the sensor surface 50 at equal intervals. This allows for easy correspondence between the signals and positions of the respective sensors. In another or further embodiment, the force sensitive resistors 10 may vary in distribution, orientation, number and / or shape. This may allow, for example, a higher concentration of sensors at relevant locations.

[0028] Typically, each force-sensitive resistor 10 includes a force-sensitive material 14, such as a conductive composite material or other material that changes its resistance when a force is applied in a membrane structure. In one embodiment, for example, as shown in the figure, the force-sensitive material 14 is disposed on a first flexible substrate 17, such as a thermoplastic polyurethane (TPU) substrate, wherein the force-sensitive material 14 faces a set of electrodes 12, 13 disposed on a second substrate 11 (e.g., a TPU substrate) relative to each other. In another or further embodiment, for example, as shown in the figure, the force-sensitive material 14 is kept separate from the electrodes 12, 13 by a spacer material 15, which is disposed between the substrates 17, 11 and surrounds the force-sensitive material 14. In some embodiments, for example, as shown in the figure, the first substrate 17 and / or the second substrate 11 is configured to bend toward the relative substrate under the action of a static pressure "P" applied to the force-sensitive resistor 10. Therefore, the force-sensitive material 14 contacts the electrodes 12, 13 and changes the resistance value "R". In another or further embodiment, when no pressure is applied, the sensor acts like an infinite resistor, i.e., an open circuit. Preferably, the greater the pressure applied to the sensor surface, the more the electrodes 12, 13 are in contact with the force-sensitive material 14, and the lower the resistance becomes. Preferably, the diameter D10 of the force-sensitive resistor is on the order of a few millimeters. For example, between one millimeter and fifty millimeters, preferably between five millimeters and twenty millimeters, such as ten millimeters.

[0029] In a preferred embodiment, piezoelectric transducers 20 are dispersed among the force-sensitive resistor 10. Typically, each piezoelectric transducer 20 includes a layer of piezoelectric material 24. Preferably, the piezoelectric material 24 is made of a polymer having a piezoelectric effect (e.g., PVDF-TrFE). The piezoelectric effect can be understood as a phenomenon in which electric charge and a corresponding field can accumulate in a particular material in response to an applied mechanical stress. For example, the piezoelectric transducer 20 can detect small changes in pressure, acceleration, temperature, strain, or force, and convert these small changes into electrical signals.

[0030] In one embodiment, for example, as shown, the piezoelectric material 24 is sandwiched between the bottom electrode 22 and the top electrode 23. In some embodiments, the bottom electrode 22 is disposed on a substrate 21, such as a TPU substrate. Preferably, the piezoelectric transducer 20 is configured to generate a corresponding time-dependent electrical signal S in response to a time-dependent mechanical stress applied to the piezoelectric material 24. Preferably, the diameter D20 of the piezoelectric transducer is on the order of a few millimeters. For example, between one millimeter and fifty millimeters, preferably between five millimeters and twenty millimeters, such as ten millimeters.

[0031] Figure 2 A schematic layout of sensors 10 , 20 on a sensor surface 50 in a heart monitoring system 100 and their corresponding connections to a controller 30 are shown.

[0032] As described in this example, the sensor surface 50 can be formed by one or more substrates that house the sensors. For example, the force-sensitive resistors 10 and / or the piezoelectric transducers 20 can be distributed (scattered) on a single substrate, or each type of sensor can be disposed on a separate substrate. Although the present figure shows that the force-sensitive resistors 10 and the piezoelectric transducers 20 are disposed on top of the substrate 50, in other embodiments, the substrates 11, 21 of the sensors can be part of the sensor surface.

[0033] In one embodiment, for example, as shown, each force-sensitive resistor 10 is coupled to a pair of shared circuit lines 30h, 30v. For example, each shared circuit line connects multiple force-sensitive resistors 10 to the controller 30. Therefore, the resistance value "R" of the corresponding force-sensitive resistor 10 can be determined, for example, by scanning the corresponding pair of shared circuit lines 30h, 30v connected to the force-sensitive resistor 10. For example, Figure 2 As shown, the first row of force-sensitive resistors 10 are horizontally coupled to a shared circuit line 30h, and the first column of force-sensitive resistors 10 are also coupled to a shared circuit line 30v. In another or further embodiment, for example, multiple shared circuit lines 30v, 30h are coupled to the controller 30 from one side and to ground on the other side. Of course, other configurations are possible.

[0034] In another or further embodiment, for example, as shown, each piezoelectric transducer 20 is connected to the controller 30 via a dedicated circuit line 30p. Piezoelectric transducers 20 can generally use dedicated circuit lines 30p, for example, because they generate a single time-related electrical signal, which is preferably detected separately. For example, the first piezoelectric transducer 20 on the first column of the array is individually connected to the controller 30 from one side via a dedicated circuit line 30p, and is connected to ground from the other side. In other embodiments, any other suitable means for connecting the force-sensitive resistor 10 and the piezoelectric transducer 20 to the controller 30 can be envisioned.

[0035] In a preferred embodiment, the sensor surface 50 includes more force-sensitive resistors 10 than piezoelectric transducers 20. For example, the force-sensitive resistors are arranged in a relatively high-density grid, while the piezoelectric transducers are arranged in a relatively low-density grid. For example, the number of force-sensitive resistors 10 can be at least two, three, four or five times higher than the number of piezoelectric transducers 20. It will be understood that, for example due to differences in signals and / or connections, it may be easier to connect multiple force-sensitive resistors 10 than to connect piezoelectric transducers 20. This can facilitate better measurement of pressure distribution using force-sensitive resistors 10, while using a small number of dispersed piezoelectric transducers 20, for example based on the distribution, to measure heart rate at a specific location.

[0036] In one embodiment, a plurality of shared circuit lines 30V, 30h and dedicated circuit lines 30p form a matrix readout system. In another or further embodiment, the controller 30 is configured to determine the pressure distribution based on the measured resistance value (R) of the force-sensitive resistor 10. In some embodiments, the pressure distribution is used to determine a region of interest (RoI) based on the position of the corresponding force-sensitive resistor 10 on the surface 50. For example, the region of interest is the area on the sensor surface 50 that the object 200 is in contact with. In a preferred embodiment, the controller 30 is configured to analyze the shape and / or magnitude of the pressure distribution to cover the shape of the object's body. In another or further preferred embodiment, the region of interest is determined at one or more specific locations in the pressure distribution corresponding to corresponding locations on the object's body.

[0037] In one embodiment, the controller 30 is configured to exclusively or primarily select the piezoelectric transducers 20 located in the region of interest for measuring the corresponding time-related electrical signal "S". Thus, the heart rate "H1" is determined by measuring the vibration "F" using a subset of the piezoelectric transducers 20 on the surface 50 located in the region of interest. For example, the region of interest is an area on the sensor surface 50 where the BCG signal of the object 200 is optimal. In some embodiments, only the piezoelectric transducers 20 located in the region of interest are activated. It will be understood that in this way, the controller does not have to continuously monitor all piezoelectric transducers 20 on the sensor surface 50.

[0038] In some embodiments, at least one of the piezoelectric transducers 20 outside the region of interest based on the force-sensitive resistor 10 is used for noise cancellation. For example, after determining the region of interest, one or more piezoelectric transducers 20i outside the region of interest can be used to cancel noise by canceling any vibration "F" detected by the non-activated piezoelectric transducer 20i. It will be understood that in this way, the vibration "F" not caused by the contact of the object 200 with the sensor surface 50 and any background noise are eliminated, and the above vibrations and noise are not taken into account when measuring the heart rate "H1".

[0039] Figure 3A and Figure 3B An example of machine learning utilizing a neural network 30n to improve the determination of heart rate "H1" is shown.

[0040] In one embodiment, for example, Figure 3A As shown, the controller 30 includes a neural network 30n, which is configured to receive at least a resistance value "R" to determine a region of interest. For example, the resistance value "R" is fed into the neural network 30n as an input, and the neural network is trained to output the region of interest. In a preferred embodiment, for example, as shown in the figure, according to the region of interest determined by the neural network, one or more time-related electrical signals "S(t)" are selected to determine the main measurement value of the heart rate "H1". In some embodiments, the network is trained for each individual object to provide the best accuracy taking into account the specific characteristics of the object. In other or further embodiments, the network is usually trained for other objects as well. Various combinations are also possible, for example, the network starts with basic training that can be improved for a specific individual. For example, the trained network may include a set of weights (w), offsets or other parameters, wherein the above-mentioned weights, offsets or other parameters determine the network that outputs the region of interest and / or heart rate according to the corresponding input signal.

[0041] In another or further embodiment, for example, Figure 3BAs shown, the time-dependent electrical signal "S(t)" can be input into the same or another neural network for use in determining a corresponding heart rate measurement. For example, the resistance value "R" and the time-dependent electrical signal "S" are fed as inputs into the neural network 30n. In a preferred embodiment, the neural network 30n is configured to output a value indicative of a primary measurement of heart rate "H1".

[0042] In some embodiments, for example, Figure 3A and Figure 3B As shown, the primary measurement of heart rate "H1" is compared with the secondary measurement of heart rate "H2" obtained by the independent heart monitoring device 40 to train the neural network 30n. For example, the neural network 30n can be trained by comparing the direct or indirect output of the neural network 30n with the secondary heart rate of the secondary heart monitoring device 40. For example, the difference between the primary heart rate measurement and the secondary heart rate measurement is fed back into the network as an error "Err", which may cause the weights "w" or other parameters of the neuron connections to change.

[0043] In some embodiments, a recurrent neural network (RNN) is used. The recurrent neural network is an artificial neural network in which the connections between nodes form a directed graph along a time series. This enables the network to exhibit dynamic behavior in time, for example, a time-dependent heart rate signal. Other or similar artificial networks may also be used. Other or further values ​​H' may also be output from the neural network 30n, or generally from the controller 30. For example, the neural network 30n may output physiological parameters such as respiratory rate, blood pressure, weight, posture detection, body position, body temperature, etc.

[0044] For the sake of clarity and concise description, the various features in the present disclosure are described as part of the same or separate embodiments, however, it will be understood that the scope of the present invention may include embodiments having a combination of all or some of the features described. Some aspects of the present disclosure relate to a non-transitory computer-readable medium having instructions stored thereon. In some embodiments, the non-transitory computer-readable medium, when executed by a controller in a cardiac monitoring system, causes the system to determine the heart rate of an object in direct or indirect contact with a sensor surface. Other aspects of the present disclosure relate to a method for cardiac monitoring. In some embodiments, the method includes: providing an array of force-sensitive resistors, the array of force-sensitive resistors spanning the sensor surface. In one embodiment, each resistor is configured to change a corresponding resistance value according to the amount of quasi-static pressure applied by the object on the sensor surface at a corresponding position of the force-sensitive resistor. In other or further embodiments, the method includes: providing an array of piezoelectric transducers, the array of piezoelectric transducers being interspersed in the array of force-sensitive resistors. In one embodiment, each transducer is configured to generate a corresponding time-dependent electrical signal according to a corresponding vibration applied by the object on the sensor surface at a corresponding position of the transducer. In a preferred embodiment, the method includes determining the heart rate of the subject from a combination of a measured resistance value of the force sensitive resistor and a time-dependent electrical signal of the piezoelectric transducer.

[0045] In interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or actions than those listed in a given claim; the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; any reference numerals in the claims do not limit the scope of the claims; multiple "ways" may be represented by the same or different items or structures or functions implemented; unless otherwise specifically stated, any disclosed device or part of the device may be combined together or separated into further parts. When one claim refers to another claim, this may indicate the synergistic advantages achieved by combining the respective features of these claims. However, the recitation of certain measures in mutually different claims does not mean that the combination of these measures cannot be used to obtain advantages. Therefore, the embodiments may include all valid combinations of claims, wherein each claim may in principle refer to any prior claim unless explicitly excluded by the context.

Claims

1. A cardiac monitoring system (100), comprising: an array of force-sensitive resistors (10) spanning a sensor surface (50), wherein each resistor (10) is configured to change a respective resistance value depending on an amount of static pressure applied by an object (200) on the sensor surface (50) at a respective location of the force-sensitive resistor (10); an array of piezoelectric transducers (20) interspersed between the force-sensitive resistors (10), wherein each transducer (20) is configured to generate a respective time-dependent electrical signal in response to a respective vibration applied by the object (200) to the sensor surface (50) at a respective location of the transducer (20); wherein the sensor surface (50) has more force-sensitive resistors (10) than piezoelectric transducers (20); A controller (30) configured to determine the heart rate of the object (200) based on a combination of the measured resistance value of the force-sensitive resistor (10) and the time-related electrical signal of the piezoelectric transducer (20); wherein the controller (30) is configured to measure pressure distribution using the force-sensitive resistor (10), and to measure the heart rate at one or more specific locations of the sensor surface (50) using a subset of the piezoelectric transducers (20) between the force-sensitive resistors (10) based on the measured pressure distribution.

2. The system according to claim 1, wherein: The controller (30) is configured to determine the pressure distribution based on the measured resistance value of the force-sensitive resistor (10), wherein the pressure distribution is used to determine a region of interest (RoI) based on the position of the corresponding force-sensitive resistor (10) on the surface (50), wherein the heart rate is determined by measuring vibration using a subset of the piezoelectric transducers (20), and the subset of piezoelectric transducers is located on the surface (50) in the region of interest (RoI).

3. The system according to claim 2, wherein: The controller (30) is configured to analyze the shape and / or size of the pressure distribution to cover the shape of the subject's body, wherein the region of interest (RoI) is determined at one or more specific locations in the pressure distribution corresponding to corresponding locations on the subject's body.

4. The system according to claim 2 or 3, wherein: At least one of the piezoelectric transducers (20) outside a region of interest (RoI) based on the force sensitive resistor (10) is used for noise cancellation.

5. The system according to claim 1, wherein: The resistance value is fed as an input into a neural network (30n), wherein the neural network is trained to output a region of interest (RoI), wherein one or more of the time-correlated electrical signals are selected based on the region of interest (RoI) to determine a measure of the heart rate.

6. The system according to claim 1, wherein: The resistance value and the time-dependent electrical signal are fed as inputs into a neural network (30n), wherein the neural network (30n) is configured to output a value indicative of a measurement of the heart rate.

7. The system according to claim 5, wherein: The measurement of the heart rate is compared with a secondary measurement of heart rate obtained by an independent heart monitoring device (40) for use in training the neural network (30n).

8. The system according to claim 1, wherein: Each of the force-sensitive resistors (10) includes a force-sensitive material (14) arranged on a first substrate (17), the force-sensitive material facing a group of electrodes (12, 13) arranged on an opposite second substrate (11), wherein the force-sensitive material (14) is kept separated from the group of electrodes (12, 13) by a spacing material (15), the spacing material being arranged between the first substrate (17) and the second substrate (11) and surrounding the force-sensitive material (14), wherein the first substrate and / or the second substrate (17) is configured to bend toward the opposite substrate under the action of static pressure applied to the force-sensitive resistor (10), so that the force-sensitive material (14) contacts the group of electrodes (12, 13) and changes the resistance value.

9. The system according to claim 1, wherein: Each of the piezoelectric transducers (20) includes a layer of piezoelectric material (24) sandwiched between a bottom electrode (22) and a top electrode (23), wherein the piezoelectric transducer (20) is configured to generate a corresponding time-dependent electrical signal in response to a time-dependent mechanical stress applied to the piezoelectric material (24).

10. The system according to claim 1, wherein: Each of the force-sensitive resistors (10) is connected to a pair of shared circuit lines (30h, 30v), wherein each shared circuit line connects multiple force-sensitive resistors (10) to the controller (30), wherein the resistance value of the corresponding force-sensitive resistor (10) is determined by scanning the corresponding pair of shared circuit lines (30h, 30v) connected to the force-sensitive resistor (10).

11. The system according to claim 1, wherein: Each of the piezoelectric transducers (20) is connected to the controller (30) via a dedicated circuit line (30p).

12. The system of claim 1, wherein: The sensor surface (50) comprises at least twice as many force-sensitive resistors (10) as piezoelectric transducers (20).

13. A bed or a chair comprising the cardiac monitoring system (100) according to claim 1.

14. A non-transitory computer-readable medium storing instructions which, when executed by a controller (30) in a cardiac monitoring system (100) according to claim 1, cause the system to determine a heart rate of an object (200) in direct or indirect contact with the sensor surface (50).

15. A method for cardiac monitoring (100), the method comprising: Providing an array of force-sensitive resistors (10) spanning a sensor surface (50), wherein each resistor (10) is configured to change a respective resistance value depending on an amount of static pressure applied by an object (200) on the sensor surface (50) at a respective location of the force-sensitive resistor (10); An array of piezoelectric transducers (20) is provided, the array of piezoelectric transducers being interspersed between the force-sensitive resistors (10), wherein each transducer (20) is configured to generate a respective time-dependent electrical signal in response to a respective vibration applied by the object (200) to the sensor surface (50) at a respective location of the transducer (20); wherein the sensor surface (50) has more force-sensitive resistors (10) than piezoelectric transducers (20), determining a heart rate of the subject (200) based on a combination of the measured resistance value of the force-sensitive resistor (10) and the time-dependent electrical signal of the piezoelectric transducer (20); The force-sensitive resistor (10) is used to measure pressure distribution, and a subset of the piezoelectric transducers (20) between the force-sensitive resistors (10) is used to measure the heart rate at one or more specific locations on the sensor surface (50) based on the measured pressure distribution.

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