GB2547631A

GB2547631A - Heart rate and activity monitor system filters

Info

Publication number: GB2547631A
Application number: GB1600136.4A
Authority: GB
Inventors: Moss Alon
Original assignee: Cloudtag Inc
Current assignee: Cloudtag Inc
Priority date: 2016-01-05
Filing date: 2016-01-05
Publication date: 2017-08-30
Also published as: GB201613194D0; GB201600136D0

Abstract

A heart rate and activity monitor comprises a sensor package 1 comprising a pair of capacitance sensors capable of sensing changes in bio electric field indicative of heart activity (ECG). The package comprises a signal generator for generating a high frequency signal three or four orders of magnitude greater than the heart rate (for example 2 kHz). Filters are tuned to this frequency and allow a processor to validate the sensed signal and determine that the bio electric field is being measured. A communications package 9 may be separably connected to the sensor package to form a clamp for securing to clothing, and may incorporate wireless communications and a power cell. The communications package may further incorporate a motion sensor for identification of user activity. When disconnected the communications package 9 may be mounted on a wrist band. Shielding and guard amplifiers may be provided to the sensor plates to reduce external interference.

Description

HEART RATE AND ACTIVITY MONITOR SYSTEM FILTERS

Technical field

The present invention concerns a system of components and processes whereby a human or animal heart rate (pulse) and other factors indicative of metabolism can be accurately and conveniently sensed and recorded during normal and vigorous exercise activity.

Prior Art

Conventional heart rate monitors are used to detect a human or animal pulse by sensing the electric field generated by the heart as it progresses through a cycle of contraction and relaxation. Conventionally the electrical potential at the skin surface is sensed via one or more “wet” conductive biosensor electrodes in intimate contact with the subject’s skin. Electrical contact is generally achieved via a conductive gel. However, such systems suffer from a range of well-known problems including: a requirement for the skin to be prepared clean and free of hair, and variations in sensitivity caused by metabolic changes such as perspiration and, partial, or total displacement during activity.

Dry contact sensors have also been used but are even more vulnerable to imprecise sensing due to the skin condition of the subject even when the skin is well prepared.

Non-contact sensors have been researched as exemplified by the disclosure in WO2015061282A.

The electric field does propagate beyond the skin. As remarked in WO2015061282A development in “dry” biosensor electrodes has focused on non-contact capacitive sensors in which two capacitive sensors comprising capacitor plates generate signals on two separate channels. Different input load capacitances are provided at each channel. When disposed close to the skin of a subject the input coupling capacitance of each capacitor plate can in theory be determined from the different input load capacitances.

An object of a non-contact sensor is to facilitate mounting the monitor for use close to the chest and heart without resorting to straps and adhesives. For example to clamp or pin the monitor to the an item of clothing. However, mounting in this way inevitably means that the sensor will move relative to the subject users skin, both towards and away from the skin and parallel to the skin. Such movements will induce fluctuations in the output signal which obscure the fluctuations induced by the beating heart (the heart field).

In practice the electric field generated by the heart action is very weak and is liable to fluctuate in frequency and amplitude during exercise. The heart field is also weak by comparison with unwanted electric fields generated by other user muscular activity, in particular involuntary muscular activity caused by respiration and voluntary muscular activity caused by exercise. The sensors are also liable to pick up electrical noise from nearby extraneous machinery. The unwanted electric fields have hitherto proven difficult to eliminate reliably with a monitor system which is convenient and practical to use.

In a medical environment it is desirable to collect a great deal of information about the changes to the electric field caused over time by the heart function, because these can indicate underlying disease conditions. In a non-medical environment the main interest in ECG sensing is to determine pulse and hence metabolic rate and performance during exercise. A relatively accurate estimate of energy expended (calories) during exercise can then be calculated from the record of pulse (heart rate) over time and other characteristics of the individual such as weight and age. Non-medical or sports heart rate monitors commonly use a dry contact electrode sensor package containing the bio-electrode strapped to the chest of a user via a band. Such electrodes are susceptible to poor connectivity depending on the condition and preparation of the subjects skin. Signals from the sensor package are fed to a processing and recording device, often a wrist borne device or smart phone. Communication is via a wire or wirelessly via a protocol such as ZIGBEE®, BLUETOOTH ® or WiFi. Where wireless communication is employed each of the sensor package and PRD must have separate power supplies, usually in the form of compact cells. The cells require frequent inconvenient and expensive replacement. Rechargeable cells require ports for charging plugs which may add to the weight and size of the package and may be vulnerable to fouling or damage.

There is also an interest in monitoring general levels of activity and fitness, that is to say, non-exercise oriented activity throughout a user’s day.

The present invention aims to address at least one of the technical problems described above in a heart rate and activity monitoring system.

Statements of Invention

According to the present invention there is provided a heart rate and activity monitor system comprising: a heart rate monitor package; and a communications package; wherein the heart rate monitor package contains; a pair of capacitance sensors capable of sensing changes in a bio-electric field indicative of heart activity, means to connect the heart rate monitor package to the communications package; wherein the signals from each capacitance sensor are filtered through a motion differential amplifier disposed in one of the heart rate monitor package or the communication package and arranged to subtract signal changes caused by relative movement of the heart rate monitor sensor package and the user.

The sensors are separated and will sense a difference in strength of the heart electric field. The difference will be amplified by the motion differential amplifier. Changes in field strength caused by movements common to each sensor will be suppressed.

According to a second aspect of the present invention there is provided a heart rate and activity monitor system comprising: a heart rate monitor sensor package; a communications package; wherein the heart rate monitor package has a subject skin facing side and an opposite clip facing side and contains a pair of capacitance sensors capable of sensing changes in a bio electric field indicative of heart activity, and means to separably connect the heart rate monitor package to the communications package; a guard amplifier arranged to receive signals from the capacitance sensor and disposed in one of the heart rate monitor package or the communication package; wherein a conductive layer is disposed to partially envelop each capacitive sensor on the clip facing side and is coupled to a guard amplifier stage to shield the capacitance sensor from extraneous signals.

The conductive layer leaves the subject facing side of each capacitive sensor exposed. Preferably the conductive layer extends into one or more side walls forming a cup around the capacitive sensor. Preferably the conductive layer is electrically connected to the guard differential amplifier stage to provide an active guard ring which attenuates the capacitive sensor signals induced by random motion of the monitor.

According to a third aspect of the present invention there is provided a heart rate and activity monitor system comprising: a heart rate monitor sensor package; and a communications package; and a pair of capacitance sensors capable of sensing changes in a bio electric field indicative of heart activity, a fast signal generator to generate a signal with a predetermined frequency “fast” in comparison with the frequency of the heart and to project an oscillating electric field at a frequency corresponding to the fast frequency; said capacitance sensors arranged to communicate with a fast filter module tuned to the “fast” frequency and a processor module responsive to a signal passed by the fast frequency filter to confirm that the capacitance sensors are sensing a signal from the subject’s bio-electric field.

The fast signal generator will preferably generate a fast signal of around 2kHz. The projected electric field will induce a fast response signal in the subject’s bio electric field sympathetic to the fast signal which is stronger and otherwise easier to detect than the heart signal because the frequency is predetermined and three or four orders of magnitude higher. The fast frequency filter can therefore be narrowly tuned to filter out all signals other than the fast response signal. The heart signal has a very slow frequency in the range 0.7-6Hz. The received fast response signal may be passed to a quality assessment module adapted to assess characteristics indicative of the quality of the fast response signal. Reception of the fast response signal will indicate the quality of connection to confirm the functionality of the monitor system.

The system will preferably include an adaptive filter responsive to the quality of the fast response signal to alter the adaptive filter pass band and thereby eliminate unwanted signals and help isolate the heart signal. The adaptive filter may be an amplitude filter and the pass band may be altered according to the quality of the fast response signal.

The monitor may include any possible combination of the first second and third aspects of the invention and will preferably include each of the first second and third aspects and the preferred features mentioned above.

The means to connect the heart rate monitor package to the communications package will preferably be means to separably connect including a plug and socket to facilitate deliberate connection and disconnection during normal use.

The communications package will preferably include a rechargeable power supply, and a charge transfer module to receive charge from the heart rate monitor package.

The communications package will preferably include a wireless communications module to transmit data received from the sensors to a recording and readout device such as a smart phone, tablet.

The heart rate monitor package will include a power supply which may include at least one storage cell, to power the systems in the heart rate monitor package.

The heart rate monitor package will preferably include a charge transfer module having means to receive a charge from an external power supply and transfer the charge onto the heart rate monitor package power supply. An external power supply may be any source of electric charge.

The means to separably connect the heart rate monitor package to the communications package power supply will advantageously comprise a heart rate monitor package magnet arranged to attract a communications package magnet disposed within the communications package. One of the means to separably connect may further comprise a flexible strap or “U” shaped spring supporting conductors and connectors to electrically connect the heart rate monitor package to the communications package. Where a flexible strap is used the magnetic attraction between the heart rate monitor package and the communications package will be sufficient to enable a span of clothing to be firmly clamped between the heart rate monitor package and the communications package thus securing the combined heart rate monitor package and communications package to the user. Where a spring is used the magnet will assist the spring in a clamping action. The connected heart rate monitor package and communications package can conveniently be clamped around the edge of conventional clothing such as a sports bra or the collar of a shirt close to the chest and underlying heart.

Charging of the communications package is achieved from the heart rate monitor package while connected.

While connected the signals acquired by the heart rate monitor package are transmitted to the communications package. The communications package then retransmits the signals to a processing and readout device such as an application equipped smartphone or any other computer to be recorded, processed and stored.

When the heart rate monitor package is not in use it may be stored connected to a charger, preferably an inductive charger.

The communications package may contain additional sensors such as a temperature sensor or acceleration sensor. The communications package may be adapted to be coupled to a body fastening such as a wrist band, finger ring, brooch, thigh band, ankle band, belt, or headband whereby it can be secured to a subjects wrist, finger, leg, lower torso, neck or forehead to capture data from the movement and general activity of the user.

According to a fourth aspect of the present invention there is provided a heart rate and activity monitor system comprising: a heart rate monitor sensor package having non-contact sensors capable of sensing changes in a bio electric field indicative of heart activity, a communications package including a motion sensor; and means to connect the communications package to the heart rate monitor package; wherein the system has at least one active motion sensor filter responsive to movements sensed by the motion sensor to alter the pass band of said motion sensor filter to filter out signals received from the non-contact sensors corresponding to the motion sensed by the motion sensors.

During activity such as running the movements induced by muscular activity generate electric field fluctuations with an approximately regular frequency cycle. The frequency may be quite close to the pulse frequency. By sensing the actual gross physical movements a movement sensor filter can be tuned to remove electric field fluctuations corresponding to such movements and facilitate revealing the actual pulse signal.

The movements of large muscle groups involved in physical exercise result in electric field fluctuations with an amplitude large by comparison with the heart signal. The motion sensor may be used to identify which received signals correspond to such motions and set an active amplitude filter, responsive to the motion sensor signals, to eliminate such signals. Because the motions sensed may vary suddenly as the activity changes these filters will update frequently.

Brief Description of the Figures

An embodiment of a heart rate and activity monitor system constructed in accordance with the present invention will now be described, by way of example only, with reference to the accompanying illustrative figures, in which:

Figure 1 is an isometric view of an heart rate monitor package and communications package separated, to be brought into contact;

Figure 2 is an isometric view of the heart rate monitor package and communications package coupled together for use during exercise;

Figure 3A is an isometric view of the heart rate monitor package with the cover removed to show some internal details;

Figure 3B is a side elevation of the communications package;

Figure 3C is a bottom view of the communications package:

Figure 4 is a sketch of the connected heart rate monitor package and communications package clamped to a sports bra during use;

Figure 5A is an isometric view of a wrist band and mount for the communications package;

Figure 5B is an isometric view of a wrist band with the communications package mounted; and

Figure 6 is a block diagram of the system components.

Figure 7 A is a sectional view through a circuit board incorporating a pair of capacitance sensors;

Figure 7B is a isometric view of the skin facing side of the circuit board;

Figure 7C is a plan view of the circuit board;

Figure 8A is a circuit diagram of the guard amplifier stage and frequency band pass filter:

Figure 8B is a circuit diagram of an op amp stage,

Figure 9A is circuit diagram of a fast frequency generator;

Figure 9B is a circuit diagram of a fast frequency band pass filter, and

Figure 10 is a flow chart illustrating the control process for an adaptive frequency band filter responsive to movement sensed by a motion sensor.

Detailed Description

The system of the embodiment shown comprises a heart rate monitor package 1 sometimes known as a Smartag ™ a communications package 2 sometimes known as the Pill ™ and a wrist band 3. The heart rate monitor package 1 comprises a rear or skin side housing part 4 intended to sit next to the skin of a user. A front housing part 5 is secured by screws to the rear housing part 4 to form a sealed hollow enclosure. A “U” shaped spring 6 extends from a base of the heart rate monitor package 1 to support a clasp 7 having two resiliently deformable arcuate horn shaped elements 8 adapted to engage in a groove 9 extending around the periphery of the communications package 2. Conductors (not shown) run through the spring 6 from the electronics in the heart rate monitor package 1 to connectors 10 which couple with corresponding connectors 11 formed in the underside of the communications package 2. The connectors 10, 11 and spring 6 are adapted to facilitate deliberate connection and separation of the heart rate monitor package and the communications package 2.

Figure 6 illustrates the electronics in the heart rate monitor package 1 and communications package 2. The electronics in the heart rate monitor package 1 is powered by a heart rate monitor package power supply 29 comprising a rechargeable chemical cell and an inductive charging module In the heart rate monitor package 1 the electric field generated by a heart pulse is sensed by a pair of non-contact capacitance sensors 12. The sensors are shielded from external electric fields by an active shield 13. Signals received from the capacitance sensors 12 are filtered by a low and high pass differential amplifier 14 to isolate signals with the frequency appropriate to a heart pulse, ie approximately 0.6Hz to 3.3 Hz. A high frequency fast test signal is generated by the module 15 and emitted from the sensors 12. The response is sensed by the sensors 12 to confirm reliable sensor coupling with the body’s electric field and to control a further active filter 16 responsive to the coupling from the fast test filter. Finally an amplitude band filter 17 removes signals with an amplitude exceeding or below the amplitude expected from a heart.

The filtered signals are fed to a communications module and transmitted via conductors in the spring 6 to the communications package 2.

Communications package 2 includes an input module 18 to communicate signals received from the heart rate monitor package 1 to wireless communications module 19 which transmits the data signals to a recording and processing device such as a smart phone (not shown). The communications package 2 includes a communications package power supply 20 including a power storage cell charged by charge received from the heart rate monitor package power supply 21 during connection over two or more of the conductors in the spring 6.

In use the communications package 2 is mounted into the clasp 7 which is then clamped around the edge of an item of clothing such as the bottom of the sports bra 22 as shown in figure 4. A magnet 23 is disposed in the heart rate monitor package 1 and works in concert with a magnet or ferromagnetic material (not shown) provided in the communications package 2 to close the gap between the front of the heart rate monitor package 1 and the back of the communications package 2. To further enhance the grip of the system on the fabric ribs 24 are formed on the front of the heart rate monitor package 1.

When the communications package 2 is charged and recordal of an accurate heart rate is not desired the communications package 2 can be separated from the clasp 7 and secured in a frame 25 attached to a wrist strap 26 as shown in figure 5A and 5B.

In this condition sensors such as an accelerometer 27 and thermometer 28 are able to sense motion and temperature of the user and produce an indication of activity throughout the day.

When not in use to determine heart rate the heart rate monitor package can be charged using a substantially conventional inductive charger.

The communications package 2 is retained in the frame 25 by means of a spring loaded latch 26. Depressing the latch releases the communications package 2.

Figures 7 A to 7C illustrate the structure of a PCB 32 in which each of a pair of capacitance sensors 12 are embedded. Each sensor consists of a main plate 12a embedded in the skin facing surface 33 of the PCB 32. A shield plate 12b is annular and overlies the opposite clip facing side of the main plate 12a. Shield plate 12b is formed with a rim wall 12c which encircles the main plate. A hole 12d is formed through the shield plate 12b and provides an insulated passage for a conductor 12e communicating the main plate to a guard differential amplifier. A conductor 12f communicates electrically with the shield plate 12b and perforates the clip side of the PCB to communicate with and be grounded by the guard amplifier stage. Thus the capacitance sensor 12 is effectively shielded from extraneous electric field signals by a faraday cage formed by the shield plate. The PCB 32 is embedded in the heart rate monitor package as shown in figure 3.

As shown in figure 8 each capacitance sensor is connected via the conductors to the inputs of a guard differential amplifier stage 34 as shown in figure 8A. The output from each guard differential amplifier stage is delivered to a frequency band pass filter 35 tuned to eliminate frequencies below 0.7 Hz and above 3.5 Hz before non-unity gain signal amplification at gain amplification stage 36.

Fig 9A illustrates the high frequency fast test signal generator designed to generate a 2kHz sine wave in fluctuation in an electric field projected from a 2kHz pad (not shown) located on the skin side of the heart rate monitor package 1. The signal generator circuit may be built onto the PCB 32.

Fig 9B illustrates the two fast frequency test channels connected to each of the two capacitance sensors 12’ and 12”. To be concise only one of the two identical channels will be described. Signals received from the first capacitance sensor electrode 12a are communicated to a high pass filter stage 37 tuned to pass frequencies received above 1590Hz. The remaining signal is then passed to a low pass filter stage 38 which eliminates signal with frequencies above 2340H The human body does not naturally generate electric fields with frequency fluctuations of this size so any signal emitted from the low pass filter stage 38 will have been induced by the fast test signal emitted by the generator of figure 9A.

Apart from confirming that there is effective signal pickup from the capacitance sensors 12, the quality of the fast test signal received through the low pass filter 38 can be used to adjust the settings of other analogue or digital filters of the system and thereby improve reception of the heart signal.

Figure 10 illustrates the process of operation of a digital filter wherein the signals from the accelerometer 27 are read for a predetermined period and digitised at step 101.

At step102 the accelerometer readout is analysed for regular cycles of movement and the movement frequencies are identified. The movement frequencies are then applied to a frequency bandwidth filter at step 103 to remove signals of corresponding frequency from the detected electric field signal to facilitate revealing the heart signal.

Claims

Claims A heart rate and activity monitor system comprising: a heart rate monitor sensor package; and a communications package; and a pair of capacitance sensors capable of sensing changes in a bio electric field indicative of heart activity, a fast signal generator to generate a signal with a predetermined frequency “fast” in comparison with the frequency of the heart and to project an oscillating electric field at a frequency corresponding to the fast frequency, said fast frequency being substantially three or four orders of magnitude higher than the heart rate; said capacitance sensors arranged to communicate with a fast filter module tuned to the “fast” frequency and a processor module responsive to a signal passed by the fast frequency filter to confirm that the capacitance sensors are sensing a signal from the subject’s bio electric field. A system according to claim 1 wherein the fast frequency is 2kHz. A system according to claim 2 wherein the received fast response signal is passed to a quality assessment module adapted to assess characteristics indicative of the quality of the fast response signal. A system according to claim 3 wherein an adaptive filter is responsive to the quality of the fast response signal to alter the adaptive filter pass band and thereby eliminate unwanted signals and help isolate the heart signal. A system according to any one of the preceding claims wherein the heart rate monitor package contains; means to connect the heart rate monitor package to the communications package; wherein the signals from each capacitance sensor are filtered through a motion differential amplifier disposed in one of the heart rate monitor package or the communication package and arranged to subtract signal changes caused by relative movement of the heart rate monitor sensor package and the user. A system according to any one of the preceding claim wherein a guard amplifier is arranged to receive signals from the capacitance sensor and disposed in one of the heart rate monitor package or the communication package; wherein a conductive layer is disposed to partially envelop each capacitive sensor on the clip facing side, leaving the subject facing side exposed and is coupled to a guard amplifier stage to shield the capacitance sensor from extraneous signals. A system according to claim 6 wherein the conductive layer extends into one or more side walls forming a cup around the capacitive sensor. A system according to claim 6 or claim 7 wherein the conductive layer is electrically connected to the guard differential amplifier stage to provide an active guard ring which attenuates the capacitive sensor signals induced by random motion of the monitor A system according to claim 5 wherein the means to connect the heart rate monitor package to the communications package is means to separably connect. A system according to claim 9 wherein the heart rate monitor package includes at least one storage cell, to power the systems in the heart rate monitor package and a charge transfer module to receive a charge from an external power supply and transfer the charge onto the heart rate monitor package power supply A system according to claim 10 wherein the communications package will preferably include a rechargeable power supply, and a charge transfer module to receive charge from the heart rate monitor package while connected. A system according to any one of the preceding claims wherein the communications package will preferably include a wireless communications module to transmit data received from the sensors to a recording and readout device. A system according to any one of the preceding claims wherein the connected heart rate monitor package and communications package cooperate to form a clamp. A system according to any one of the preceding claims wherein the communications package can be separated from the heart rate monitor package and coupled to a body fastening. A system according to any preceding claim wherein the communications package includes a motion sensor. A system according to claim 15 wherein an active motion sensor filter is responsive to movements sensed by the motion sensor to alter the pass band of said motion sensor filter to filter out signals received from the non-contact sensors corresponding to the motion sensed by the motion sensors.