WO1988004152A1

WO1988004152A1 - Pulse rate detector

Info

Publication number: WO1988004152A1
Application number: PCT/GB1987/000862
Authority: WO
Inventors: Israel Kon
Original assignee: Israel Kon
Priority date: 1986-12-01
Filing date: 1987-12-01
Publication date: 1988-06-16
Also published as: GB8628698D0

Abstract

A pulse rate detector comprising a transmitter (1) and a receiver (3), in which the transmitter (1) operably transmits a signal which is received by the receiver (3) subsequent to modulation by part of the body of a user, wherein the power of the transmitted signal is increased from an initial value until a pre-determined signal power is received by the receiver (3).

Description

PULSE RATE DETECTOR

The present invention relates to a pulse rate detector as may be employed tor example in an exercise monitoring apparatus.

Known pulse rate detectors are based upon the detection of small variations in the power or intensity of a light beam after transmission through or reflection from a suitable part of the body of the user. The detected variations result from, and can therefore be used to monitor bloodflow pulsations, that is the pulse arising from the pumping action of the users heart. It is well established that pulse rate and characteristics of changes thereof are directly indicative of health, fitness, safe levels of exercise etc.

Conventionally, a pulse rate detector comprises a light source and a photoelectric receiver housed in respective sections of a clip. The clip is attached to a part of the user's body, such as an ear lobe, at which the pulse is most readily detected. However, skin and tissue characteristics can vary significantly between different people. It is a characteristic of conventional detectors that they employ a relatively high intensity light beam so as to ensure that sufficient light intensity impinges upon the receiver in all cases. The most significant result of this is that the conventional detectors are extremely sensitive to disturbances, as may be caused by motion of the light source and receiver relative to the user's body, interference by external light sources etc. Thus, conventional pulse rate detectors attempt to detect reliably and accurately a very small signal, pulse induced light intensity variations, modulating a very large signal., the intensity level of the transmitted or reflected light beam.

With a view to mitigating the above described disadvantages, the present invention provides a pulse rate detector comprising a transmitter and a receiver in which the transmitter operably transmits a signal which is received by the receiver subsequent to modulation by part of the body of a user, wherein the power of the transmitted signal is increased from an initial value until a pre-determined signal power is received by the receiver.

In one embodiment of the present invention the power of the transmitted signal is held constant at the value which first results in the pre-determined power of the received signal. Essentially, in this embodiment of the invention subsequent variation in the power of the received signal is assumed to result from bloodflow pulsations. The pre-determined power level for the received signal is preferably the minimum level enabling reliable detection.

In an alternative embodiment of the invention, the power of the transmitted signal is allowed to oscillate in accordance with variations in the power of the received signal once the pre-determined level has been obtained. In this embodiment the pulse rate can be measured from variations in the power of the transmitted signal.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which :

Figure 1 is a block circuit diagram of an arrangement for implementing the present invention.

Figure 2 illustrates eight examples for implementing one of the components of the circuit shown in figure 1,

Figure 3 shows at (a) a perspective view and at (b) an end view of the main unit of one embodiment of the present invention, and Figure 4 illustrates two examples of clips for holding the sensor to be connected to the main unit shown in Figure 3.

The block circuit diagram of figure 1 can be used to represent various embodiments of the present invention In figure 1, a light source is indicated by reference numeral 1 and the intensity of the transmitted light beam is regulated by the light source regulator unit 2. A photo-electric detector is indicated by reference numeral 3 and output therefrom is amplified by amplifiers 4 and 5. It is to be appreciated that the light source 1 and photo-electric detector 3 form a sensor which is remotely located in relation to the remainder of the circuitry shown in Figure 1. A threshold detector unit 7 is associated with amplifiers 4 and 5. Output from amplifiers 4 and 5 is input to integrating amplifier 6, the output of integrating amplifier 6 being input to the light source regulator unit.

The above described components are associated with establishing and maintaining the power or intensity of the transmitted signal. In addition the circuit of figure 1 comprises a voltage comparator 8, two threshold detectors 9 and 10 and an AND gate 11. Inputs for voltage comparator 8 are taken from the output of amplifier 5. The output from comparator 8 is supplied to threshold detector 9, whose output provides a DATA signal and also provides an input to AND gate 11. One of the inputs to voltage comparator 8 is supplied via an RC network to threshold detector 10. Output from threshold detector 10 provides a DATA VALID signal and a second input to AND gate 11. Output from AND gate 11 provides a VALID DATA signal. Collation of the two sections of the circuit of figure 1 will now be described.

The sensor including light source 1 and photoelectric receiver 3 is located on a suitable part of the body of a user. Light from light source 1 is received by the photoelectric receiver 3 subsequent to modulation by the said part of the body of the user. This may involve reflection from or transmission through the said part of the body. When the pulse rate detector is first switched on light source 1 is not actuated and no effective signal is output by the photoelectric receiver 3. Under these conditions the output from amplifier 5 is set to a high voltage level which is thus applied to the input of integrating amplifier 6. The output of amplifier 6 will rise in a time dependant manner as determined by the integrating network values associated with the integrating amplifier 6. Thus, a gradually increasing input voltage is applied to light source regulator unit 2 and this causes a current to flow through light source 1. Light source 1 emits a light signal which is received by the photoelectric receiver 3 after modulation by the body of the user. In one embodiment the threshold or bias unit 7 is arranged such that an increasing signal from photoelectric receiver 3 results in variation of the output from amplifier 5 until a threshold value is obtained by the signal from receiver 3. Thus, the circuit allows the current in light source 1 to increase until the pre-determined power of received signal is obtained. Subsequently, the circuit maintains this level of current flow through light source 1 regardless of changes in the optical characteristics of that part of the user's body which is modulating the transmitted light signal. This condition applies for changes in modulation of the transmitted signal on a time scale slower than the integrating time constant of amplifier 6. If this were not so, the system would also compensate for changes in modulation of the transmitted signal resulting from blood flow pulsations - with the result that pulse rate would not be detected.

The output of amplifier 5 contains a common mode voltage, superimposed on which is the signal resulting from blood flow pulsation. In order to extract the signal indicative of blood flow pulsation, the output from amplifier 5 is applied to both the inverting and non-inverting inputs of a high common mode rejection amplifier, namely amplifier 8. All signals which are common to both inputs of amplifier 8 are effectively cancelled since they do not produce any signal in the output from amplifier 8. A capacitor 12 is connected between one of the inputs of amplifier 8 and ground. Constant and very slowly fluctuating voltages are treated as common mode signals which are cancelled by amplifier 8. More rapdily fluctuating and pulsating voltages appear as differential signal at the inputs of amplifier 8 and result in considerable output fluctuations at the output of amplifier 8. Such output signals from amplifier 8 are detected by threshold detector 9 which shapes the signals in a manner suitable for subsequent processing by a microprocessor (not shown). This shaped signal, indicated as DATA in figure 1, corresponds directly to the bloodflow pulsations.

The signal applied to the input terminal of amplifier 8 connected to capacitor 12 is also applied to threshold detector 10 via an RC network comprising resistor 13 and capacitor 14. When the output from amplifier 5 is such as to result in a constant current flow through light source 1, the output of detector 10 changes. This change in output of detector 10 occurs after a delay determined by the values of components 13 and 14. The change in output indicates that the consant current state has been reached and consequently the DATA signal can subsequently be considered as corresponding correctly to the bloodflow pulsations. Thus, the output signal from detector 10 is a DATA VALID signal and the combination with the DATA signal, obtained using AND gate 11. provides the VALID DATA signal.

An alternative embodiment of the invention can be implemented by modification of the circuit of Figure 1. This modification includes the removal of the integrating circuitry associated with amplifier 6 and detection of fluctuations in the current flow to light source 1. That is, the circuit compensates for changes in received signal power by altering the current flow to light source I. Fluctuations in the current flow thus correspond to fluctuations in received signal power resulting from changes in optical characteristics resulting from bloodflow pulsations.

In both embodiments, the characteristics of the part of the user's body modulating the transmitted light regulates the amount of light transmitted. Thus, the power of the transmitted light is automatically optimised regardless of differences between users and different conditions of use. This provides the very significant benefit that the ratio of the pulse signal to the transmitted light signal is improved, often very considerably when compared with conventional arrangements. Typically the output offset voltage of a conventional arrangement might be 27 volts. With the embodiment illustrated in figure 1 it may be possible to reduce this to as low as 2 volts. Since the pulse rate signal will typically have a value of little more than 1/2 volt, the dramatic improvement will be immediately apparent.

The above described benefit of the present invention can be utilised to provide further practical advantages. In particular, the self-regulating and self biasing features of the invention enable reliable and accurate pulse detection at a greatly increased number of locations on the body of a user. For example, a light source and receiver, using the reflection technique, may be incorporated in a headband so as to monitor pulse from the user's forehead. This may be of special advantage to athletes.

Figure 2 provides eight examples of implementation of the threshold or bias unit 7 shown in figure 1. A resistive divider is illustrated in figure 2(a). Figures 2(b), 2(c) and 2(d) illustrate different forms of constant current source which may be user to implement unit 7. A beneficial variation incorporating ambient light compensation is shown in figure 2(e). Various diode arrangments are shown in figures 2(f) (Zenner diode), 2(g) (LED or reference diode), and 2(h) (single or multiple diode).

A physical embodiment of a pulse rate detector incorporating the present invention is illustrated in figures 3 and 4. Figure 3 illustrates the circuitry housing unit 15 and figure 4 illustrates two alternative sensor housings. Figure 4(a) illustrates an ear lobe clip and figure 4(b) illustrates a finger clip. Connections to the light source and receiver are provided by suitable leads within a common sheathing 16 which is joined to the main unit 15. As depicted in figure 3(b), the rear of unit 15 may be provided with a velcro (Registered Trade Mark) or similar attachment means 17. The circuitry is battery powered and unit 15 is readily carried by the user.

The front panel of unit 15 carries a keypad 18, a liquid crystal display 19 and three LEDs 20. Internally, unit 15 houses a microprocessor for processing the signals generated by the circuit arrangement of figure 1. The microprocessor also implements other functions, as will be described, in association with the keypad 18, display 19 and LEDs 10 so as to provide an extremely useful exercise monitoring apparatus. This appartus may conveniently be referred to as an Aerocal Aerobic Computer.

The Aerocal Aerobic Computer is used for monitoring the activities during aerobic exercise. Aerobic exercise is defined as the form of exercise in which a person reaches his/her maximum safe heart rate (exercise rate) and maintains this rate for a period of at least 12 minutes. The user enters his/her level, according to his/her own physical condition, and age to the computer and target heart rates will be calculated. During aerobic exercise, the computer measures the user's heart rate and gives indications on the status of the user. A timer is available for keeping the period of aerobic exercise.

The Aerocal Aerobic Computer has the following features:

1) Two digits age setting (maximum 99 years)

2) Three levels (1. 2. and 3 corresponding to low, average, and high) for target heart rate during aerobic exercise.

3) Continuously measures and displays the user's heart rate during exercise. 4) Three colour LED's. which blink with the heart rhythm, to indicate different heart rate conditions.

i) Orange LED blinks when user's heart rate is below the target heart rate.

ii) Green LED blinks when it reaches the target heart rate.

iii) Red LED blinks when the target heart rate is exceeded.

5) Count-down timer with minute and second display (maximum 99 minutes and minimum 12 minutes).

6) Buzzer for alarm ringing after preset time on the timer expired. Beeps will be heard during hazardous condition.

Operation of the Aerobic Computer consists of five stages; namely power up stage, stand-by mode, setting mode, review mode, and run mode.

Three target heart rates (the expected target rate, the upper and lower limit target heart rates) will be calculated from the equation: Target Heart Rate (THR) = (220 - Age) x R Where R = 86% to 75% (mean= 80.5%) for Level 3 75% to 68% (mean= 71.5%) for Level 2 68% to 68% (mean= 64.5%) for Level 1

Mean value of R is used in the calculation for the display of target heart rate to the user, and the upper and lower boundary values are used in calculating the upper and lower limit target heart rates.

1) Power Up

Upon power up. will be all

flashing and zero will be assigned to age, level, and the timer.

If either of the above remains at zero, run mode cannot be entered.

Press <ENTER> key to enter into setting mode.

2) Stand-by Mode

The Aerobic Computer enters this mode whenever it is in idle.

When no key has been pressed for more than one minute during setting mode or review mode, stand-by mode is automatically entered. - Display will be all blank.

- Press <ENTER> key to enter into review mode.

Press and hold <ENTER> key for more than two seconds to enter into setting mode. 3) Setting Mode

- Upon entry, will become flashing and the

preset timer value will be displayed. The timer will be set to zero after the power up stage (i.e. after replacing the batteries).

- Only the minute portion can be set. The second portion is set to zero.

- When a digit key is pressed, the digit will be shifted into the least significant digit of the minute portion and the previous most significant digit will be shifted out.

- Press <ENTER> key to set age.

- During age setting, will become flashing

and the preset age will be display. Age will be set to zero after the power up stage. Two digits are provided, and the setting procedure is the same as in timer setting.

- Press <ENTER> key to set level.

During level setting, will become

flashing and the preset level will be displayed Level will be set to zero after the power up stage.

One digit is provided for level setting.

Press <1>, <2>, or <3> accordingly.

Press <ENTER> key to display the expected target heart rate. "E" will be displayed if a timer value of less than 12 minutes, zero timer value, or zero level has been set.

Press <ENTER> key again to return to stand-by mode.

4) Review Mode - Upon entry.

will come up on the display along with the preset timer value.

- Press <ENTER> key to display age. will come up along with the preset age.

Press <ENTER> key again to display level. will come up along with the preset level,

- Press <ENTER> key to display the expected target heart rate.

- Press <ENTER> key and return to stand-by mode.

If instead, <START> key is pressed, run mode will be entered.

5) Run Mode - Run mode can only be entered from review mode, during target heart rate display to ensure that the user has reviewed all the input parameters before starting the exercise. Also three conditions have to be satisfied:

i) Timer value not less than 12 minutes ii) Age not equal to zero iii) Level not equal to zero

Upon entry, power will be supplied to the linear devices and the user's heart rate is measured. If no pulse is registered within one minute when in run mode, the computer will automatically return to stand-by mode.

- During run mode, timer and the user's heart rate will be displayed.

The user's heart rate is calculated from the average rate between four consecutive measurements.

One of the LED's will be blinking in synchronism with the incoming pulses.

If the user's heart rate is below the lower limit target heart rate, the orange LED will be blinking. The timer stops count down at this stage.

- If it is within the upper and lower limits, the green LED will become blinking and the timer will start to count down.

If it is over the upper limit, the red LED will be blinking and a beep sound in synchronism with the heart beat will be heard. The timer continues to count down. If no pulse is registered with the computer, which usually means that the user does not put on the sensor properly, "L" will be displayed and the computer will return to stand-by mode after one minute if this condition continues.

- If a pulse rate higher than 200 bpm is registered, "H" will be displayed.

- When the timer has expired, two rapid beeps will be heard and the timer stays at zero.

- Press <START> key anytime and return to stand-by mode. Power to the linear devices will then be cut off.

Claims

CLAIMS :

1. A pulse rate detector comprising a transmitter and a receiver, in which the transmitter operably transmits a signal which is received by the receiver subsequent to modulation by part of the body of a user, characterised in that the power of the transmitted signal is increased from an initial value until a pre-determined signal power is received by the receiver.

2. A pulse rate detector as claimed in claim 1, wherein the power of the transmitted signal is held constant at the value which first results in the pre-determined power of the received signal.

3. A pulse rate detector as claimed in claim, 1, wherein the power of the transmitted signal is varied in accordance with variations in the power of the received signal after the pre-determined level has been obtained.

4. A pulse rate detector as claimed in any preceding claim comprising a threshold circuit which establishes the said pre-determined signal power.

5. A pulse rate detector as claimed in any preceding claim, further comprising a common mode rejection amplifier connected to extract the signal indicative of bloodflow pulsation from the overall signal received from the receiver.

6. A pulse rate detector as claimed in claim 5, further comprising a signal shaping circuit which processes output from the common mode rejection amplifier.

7. A pulse rate detector as claimed in claim 2, comprising a signal integrating circuit connected to produce a linear increase in the power supply to the transmitter until said pre-determined power signal is obtained.

8. A pulse rate detector as claimed in claim 4, wherein the threshold circuit comprises a constant current source.

9. A pulse rate detector as claimed in claim 4, wherein the threshold circuit includes an ambient light compensation means.

10. A pulse rate detector as claimed in claim 4, wherein the threshold circuit includes a Zenner diode.

11. A pulse rate detector as claimed in claim 4. wherein the threshold circuit includes a light emitting diode.

12. An exercise monitoring apparatus including a pulse rate detector as claimed in any preceding claim together with a display device adapted to display pulse rate as measured by the pulse rate detector.