GB2052752A - Improvements Relating to Beat Rate Sensors - Google Patents

Abstract

A portable electric beat monitoring device is provided which may take many forms, but advantageously may resemble a wristwatch case (40) having a strap (45) by which it may be attached to a wrist so that a light emitting diode (42), for example in a protuberance (41) on the base of the case (40) may transmit light which, when reflected, will be received by a light dependent resistor (43). The light intensity received will vary at each pulse beat and a convertor circuit (Figs. 2, 3 or 4), within the case will determine the time between pulse beats and convert this to a beat rate per minute, to be displayed as a digital display through a window (44). The device could alternatively monitor electrical, pressure, displacement, blood flow, density or acoustic changes associated with a pulse beat, such as by a diaphragm, a piezoelectric crystal, a microphone, a capacitor/resistor and/or indicator or an optical fringe detector. <IMAGE>

Description

SPECIFICATION Improvements Relating to Beat Rate Sensors This invention is concerned primarily with beat rate sensors in the form of physiological pulse sensing, detecting, and/or analysis. The invention is particularly though not exclusively applicable to sensing human heart beats in vivo, and under active conditions, for example whilst taking exercise.

According to the invention there is provided a portable electronic beat monitoring device having means for attachment to a body, a beat sensor arranged to be held in close contact with the body, a voltage source, an analogue or digital display and a convertor circuit having an input from the sensor and an output to the display for producing a display related to the beat frequency.

There are various physiological changes associated with the beat frequency of a heartpulse. Any one of these changes may be used to sense the heart-pulse, examples of which are as follows:~ (i) electrical changes (ii) pressure changes (iii) displacement changes (iv) blood flow changes (v) density changes (vi) sound changes These changes may be detected in many ways by means of suitable sensors, such as by: (i) piezo-electric cystal (ii) microphones (iii) capacitors/resistors and/or inductors (iv) optical fringes, for example Moiré fringes (v) scatter light intensity/frequency and/or phase sensors Preferably the sensor, convertor and display form a self contained microminiturized unit, without wires, which can be held in close contact with the wrist.Thus the unit would comprise a wristwatch-like device which can be strapped to the wrist and carries the display on the face. In this unit the heart-pulse may be detected by monitoring variations in reflected and/or transmitted light through tissues by means of a voltage generating light detector and light emitting diode next to the skin on the wrist. The convertor circuit samples the data and analyses it for display. The sampling part of the circuit preferably includes an automatic voltage sustaining device, which will hold the analogue voltage steady until the next sample. Preferably there is a timing circuit which automatically controls the sampling rate, and the same timing circuit (adjusted if required) may be used to calibrate a pulse frequency analysing portion of the convertor circuit.

Micro processor design concepts and circuitry, for example program controlled large scale integrated circuits (L.S.I.), may be used to sense/analyse and/or display data. However due to the large development costs associated with assembly language program developments (assembler, editor, loader etc), it is preferable to use standard off the shelf custom designed large scale integrated circuit design and technology which is tried and well established. However if facilities do exist for development the micro processor design concepts offer greater flexibility.

The optimum design concept to maintain low costs and at the same time maintain maximum performance would probably be a combination of both software and hardware techniques.

The invention may be performed in variolas ways and preferred embodiments thereof will now be described with reference to the accompanying drawings, in which~ Figure 1 is a schematic diagram of the circuit of a monitoring device of this invention; Figure 2 illustrates a modified circuit providing a digital display; Figure 3 is a block circuit diagram of a further embodiment of the invention; Figure 4 is a block circuit diagram of a still further form of monitoring device of the invention Figure 5 illustrates a monitoring device constructed with the appearance of a wristwatch; and Figure 6 illustrates a beat sensing device which may be used in association with the circuits illustrated in Figures 1 to 4.

In Figure 1 the converter circuit components and analogue display components were built using C. Mos technology. The whole device can be operated from a low voltage low power button cell battery 1. Moreover the whole device is small, compact and light enabling it to be supported on the wrist.

The circuit incorporates a sensor consisting of a light dependent resistor (L.D.R.) 4 a light emitting diode (L.E.D.) 2 and a standard resistor 3 arranged in a Wheatstone Bridge circuit. One advantage of using a bridge circuit is that common mode noise interference may be removed using a differential amplifier connected between points A and B. The common mode noise is particularly troublesome for detectors that have large impedances, as for example photodiodes. The L.D.R. has a low impedance when illuminated and that is one of the main reasons for using them as opposed to the photodiodes. However good common mode rejection may be obtained using a photodiode 21 in the modified circuit configuration for section S shown in Figure 1 A employing amplifiers 22.A further advantage is that the bridge circuit "amplifies" the change in the signal voltage produced by interrupted light on the L.D.R. 4 i.e. as the voltage at B falls and rises the voltage A does the opposite. Consequently, since it is the difference voltage that is amplified, this effect increases the effectiveness of the detector. (It would be desirable to use constant current source X).

Since individuals have different kinds of skin, for example the skin thickness and colour will be different for any two individuals, the transmiJited light intensity will vary from individual to individual. Hence the direct current voltage (D.C.) at A and B will also vary when different people use the device. The D.C. problems, associated with individual's skin differences, are solved using blocking capacitors 5. The heart-pulse causes the sensor to generate a voltage pulse or (a.c. signal) which passes through the capacitor unimpeded to a differential amplifier 6. The capacitance value of each capacitor 5 is important from the point of view of having sufficient energy to drive the next stage.

In addition to the wanted a.c. signals from the heart there are unwanted a.c. signals referred to as normal mode interference signals and transient signals which also pass through the capacitor to the amplifier. These latter unwanted signals, however, may be removed using selective band pass filters to allow the heart-pulse only to pass.

Sincs the unwanted signals are mainly due to unnecessary movement it is possible to build the device without filters provided the wrist is held still during the reading of the display.

The output from the differential amplifier 6 connected with a resistive feedback loop 7 and a gain of fifty approximately is used to trigger a monostable circuit consisting of two transistors Ti and T2 connected back to back. Its found that the heart-pulse, when applied to the base of T1 (say) causes the monostable to trip over and produce a square-wave output at the collector of T2. The square wave output has a fixed magnitude or height and duration or width. The duration of the pulse is determined by the time constant of the circuit components 8 and 9.

The monostable circuit is also arranged to be triggered separately from a standard clock 10 and/or a counter 11. The standard clock has a dual function, the first being to calibrate the display. This is done by standardising the pulses from the monostable to feed a linearized rate meter. The rate meter consists of a feed capacitor 12, a transistorized pump 13 and a store capacitor 14 across which there is connected a bleed resistor 15. The analogue voltage vfrom the rate meter will fluctuate about a mean position related to the frequency of the input pulses. If these are at a rate of 2 pulses/second then a sampled output of the rate meter can be used to calibrate the display. The second function of the Clock 10 is to sample and hold the voltage v by means of a pulse applied to the gate of a field effect transistor T3.The sample and hold frequency may also be done manually, rather than automatically, by the individual enabling the gate line E.

The importance of the sample and hold mode of operation of the converter cannot be over emphasised, for without this facility the device would not function adequately, especially when using a digital display as shown in Figures 2 and 3, because the display would be constantly changing and flickering.

The sample and hold circuit shown in Figure 1 samples the voltage v when T3 is closed by means of component 16 in the feedback loop of an OP amp 23. It then holds the voltage on the capacitance component 17 when the switch T3 is opened. The voltage sampled remains constant until the switch is closed once again.

The analogue display in Figure 1 makes use of a bank of operational amplifiers 19 connected as comparators i.e. in open loop high gain configuration, to compare the sampled voltage v with potential differences across a chain of series resistors 18 connected between the supply. The display consists of a chain of L.E.D.'s 20 which come on in series depending on the value of v. If the diodes are calibrated against a background scale the device will behave as a monitor of the individual heart-rate. Alternatively one diode at a time may be turned on using decoders. This second method has the advantage of reducing power consumption.

Figure 2 is a schematic diagram of a heart-rate monitor having a digital readout. Although the device is essentially similar in operation to that of Figure 1 in the sensor and convertor sections there are minor modifications such as a filter coil 24 to remove fast rising interference noises, a smoothing capacitor 25 to remove low frequency noises and the sensor is not connected in a bridge circuit configuration. The sensor/convertor circuits in Figure 1 and Figure 2 are however, interchangeable.

In Figure 2, however, the display is a three digit seven segment multiplexed display 26. A special off the shelf L.S.I. circuit 27 is used to convert the analogue voltage vfrom the sample and hold circuit to a binary coded decimal (B.C.D.) digital readout. The analogue to voltage conversion is done by a quantized feedback technique where

wherein: vin= sample and hold voltage vrer reference voltage.

The counts N are provided by an internal oscillator or clock on the chip 27. The digital display is calibrated using the reference clock 10 as in Figure 1. Moreover, since the digits are seven segment light emitting diodes, a decoder/driver 28 is necessary to match the binary coded decimal output from the chip 27.

The multiplexed display is also achieved by clock pulses D1, D2 and D3 generated on the chip 27 which make for a compact and efficient readout.

Figure 3 is a schematic diagram of an alternative digital heart-rate monitor. In this diagram high frequency clock pulses from a reference clock 30 are simultaneously fed into two counters, namely a binary counter in an analogue to digital or digital to analogue device 31 and a binary coded decimal counter in a second chip 32. The latter has facilities to latch up the input for display in either B.C.D. or alternatively seven segment digits 33. In operation the clock pulses generate a digital ramp voltage v' which is compared continuously by means of a comparator 34 with the sample and hold voltage v as shown. Once v' > v the comparator 34 flips over and inhibits the clock pulses passing into the B.C.D. counter and display 33.The counters may be reset either using the reference clock 30 or alternatively clock pulses from the monostable 35 i.e. the display may be updated each heart beat using the heart-pulse to change both sample and hold 36 and reset on the counters 31 and 32.

Figure 4 is a schematic diagram of a heart-rate monitoring device which does not work on the principles outlined in Figures 1,2 or 3, i.e. using rate meters and sample and hold convertor circuits. In Figure 4 the heart rate is measured essentially beat to beat. Essentially the period between beats as applied to a flip-flop 37 is used to gate clock pulses from a clock 38 into a difference counter or subtractor 39. The clock pulses are subtracted from a reference and the resultant number of times the subtraction is made is displayed i.e. the clock pulses are divided into a reference number, and the resultant number or divisions is displayed on the display 40. The beat to beat system is ideal for microprocessor applications. It is necessary to control the sampling frequency and display otherwise the display devices would be monitoring and changing too quickly for comprehension.

Figure 5 illustrates a device designed to resemble a wristwatch and having a digital and/or analogue heart-rate monitor housed in a wristwatch case 40 with a double protrusion 41 in the base of the case to house a light emitting diode (L.E.D.) 42 and a light dependent resistor (L.D.R.) 43. The protrusion 41 is to give depth into the underside of the wrist and in close proximity to the normal pulse taking position. The form of the protrusion can be altered to cover the whole of the base plate in contact with wrist. The angle between the source and detector is less than 1 800. The case has a display window 44 for viewing the display device and a strap 45 for holding the case on the wrist.

Alternatively other types of transducer may be attached to the wrist to detect heart pulses, for example~ (i) a pressure transducer either of the diaphragm or piezo-electric type may be placed in close proximity with the radial artery (ii) a tuned electrical oscillator circuit containing a resistance, capacitance and/or inductance as a possible variable element in contact with the radial artery (iii) a hall effect detector in contact with the blood as an electrical conductor moving in relation to fixed magnets on the base plate of the case 40 (iv) an ultrasonic transmitter/detector system used to measure pulses through the radial artery (v) a temperature detector used to detect temperature variations due to changes in blood flow.

Figure 6 illustrates an alternative method of attachment to body by placing a finger or thumb between a sensor 50 and a detector 51 in a clothes peg like structure 52 which is hinged at 53 and biased into a closed position by springs 54. Wires can lead from the detector 51 to a separate unit housing and the convertor circuit and display.

A number of monitoring devices could be incorporated into a microprocessor controlled system in which the heart rates of several people are being monitored and displayed under the control of a microprocessor. The monitoring devices of the system could for example be attached to people doing exercise on an indoor circuit training equipment or to a number of patients in a hospital. In this type of system application each person would necessarily be connected to microcomputer by means of an attachment lead. The input from the athlete or patient would depend on several factors according to the needs of the system. Three general methods which might be used to obtain information are (i) direct memory address (D.M.A.) (ii) interrupts (iii) program control The potential of using microprocessors lies in flexibility of design for data acquisition and analysis. For example whilst normally heart rates would be displayed, it is a relatively simple procedure to indicate fitness levels and performance. To do this it would be necessary to monitor recovery periods and data log the athlete over periods of weeks or months.

Another possibility is to use the heart rate monitor as a means of monitoring emotional stress as the heart rate usually increases as stress increases. This system could incorporate a unit to determine if the heart rate rises above a predetermined level associated with an alarm device which will operate if that level is exceeded.

It will be appreciated that the circuits of Figures 1 and 2 for example may readily be modified to enable the sensity level of monitoring to be adjusted.

Claims (15)

Claims

1. A portable electronic beat monitoring device having means for attachment to a body, a beat sensor arranged to be held in close contact with the body, a voltage source, an analogue or digital display and a convertor circuit having an input from the sensor and an output to the display for producing a display related to the beat frequency.

2. A device according to claim 1, wherein the beat sensor is constructed to monitor a physiological change associated with the beat frequency of a heart-pulse.

3. A device according to claim 2, wherein the beat sensor is constructed to monitor electrical, pressure, displacement, blood flow, density, or sound changes associated with a heart-pulse.

4. A device according to claim 2 or claim 3, wherein the beat sensor incorporates a diaphragm, a piezo-electric crystal, a microphone, a capacitor/resistor and/or inductor, an optical fringe detector, or a scatter light intensity/frequency and/or phase sensor.

5. A device according to any one claims 1 to 4, therein the sensor, convertor and display form a self contained microminiturized unit, without wires, which can be held in close contact with the wrist.

6. A device according to claim 5, wherein the unit comprises a wristwatch-like device which can be strapped to the wrist and carries the display on the face.

7. A device according to any one of claims 1 to 4, wherein the beat sensor is carried by an adjustable member which may be clipped about a part of the body where a beat is to be sensed.

8. A device according to any one of claims 5 to 7, wherein the heart-pulse is detected by monitoring variations in reflected and/or transmitted light or sound through tissues by means of a voltage generating light or sound detector and light emitting diode or sound generator which will be positioned next to the skin on the wrist.

9. A device according to any one of claims 1 to 8, wherein the convertor has a sampling section which includes an automatic voltage sustaining device, which will hold the analogue voltage steady until the next sample.

10. A device according to claim 9, including a timing circuit which automatically controls the sampling rate.

11. A device according to claim 10, wherein the timing circuit is used to calibrate a pulse frequency analysing portion of the convertor circuit.

12. A device according to any one of claims 1 to 11, including a memory for storing data of previous beat measurements and a comparator for comparing instantaneous measurements with the stored data.

13. A device according to any one of claims 1 to 11, including a unit to determine a rise in beat frequency above a predetermined level associated with an alarm device which will operate if that level is exceeded.

14. A portable electronic beat monitoring device substantially as herein described with reference to the accompanying drawings.

15. A monitoring system comprising a number of electronic beat monitoring devices as claimed in any one of claims 1 to 14, linked to a microprocessor for monitoring and displaying information related to each device.