WO1982000950A1 - Pulse detector - Google Patents

Abstract

Portable pulse detector (10). The pulse detector (10) comprises a band (14) in the form of a ring and is designed to fit around the wrist of the user. A ring stress detection circuit (16) detects the tension of the band and produces a pulse signal each time the band is stretched by the impulse of a person carrying the band. A pulse evaluation circuit (18) responsive to the pulse signals produces an output signal indicating the value of the pulse rate of the person wearing the band.

Description

PULSE DETECTOR

BACKGROUND OF THE INVENTION

The present invention is directed towards a pulse detector and, more particularly, to a portable pulse detector which monitors the heart rate by the detection of pulses and provides a visual indication of the heart rate of the user.

The need for an accurate pulse detector of this type is generally recognized by the medical profession for two primary reasons: (1) it is probably one of the most important guides in cardiac rehabilitation programs (e.g., after surgery or heart attacks), and (2) many people cannot monitor their pulse with sufficient accuracy to manually determine the heart rate.

Pulse detectors of the foregoing type are well known in the prior art. Such detectors generally include a pulse-detecting transducer for generating output signals representative of the patient's pulse, an electronic circuit for evaluating the patient's pulse rate and a display for providing a visual indication of the patient's pulse rate. The prior art patents fall into two broad classes: those which detect the pulse of the patient using pressure transducers and those which detect the pulse of the patient using light transducers. The foregoing class of patents is exemplified by U. S. Patent No. 3,426,747 which uses a pressure transducer which is biased against the radial artery of the patient and detects changes in the size of the radial artery resulting from the rush of blood therethrough. The latter class of patents is exemplified by U. S. Patent No. 3,359,975 which uses a light transducer attached to the earlobe of the patient and detects the rush of blood through the earlobe.

The prior art light transducer pulse detectors have not met with great commercial success since they generally must be attached to the earlobe of the patient and are therefore unattractive or must be placed against the finger of the patient and have not been particularly accurate. Prior art pressure responsive pulse detectors are less obtrusive but have been unable to consistently provide an accurate pulse reading. The inability of prior art pressure responsive pulse detectors to provide consistently accurate readings stems from the need to locate pressure transducers at a location corresponding to the radial artery, and the generation of noise signals resulting from movement of the wrist of the individual. It is a primary object of the present invention to overcome these drawbacks by providing a pressure responsive portable pulse sensor which need not be accurately placed over the radial artery and which is less sensitive to the generation of noise signals.

BRIEF DESCRIPTION OF THE INVENTION

In order achieve the foregoing and other objects, the present invention is directed towards a pulse detector comprising: a band having a generally hoop-like shape; hoop stress sensing means for detecting the tensing of said band and for generating a pulse signal each time said band is tensed by the pulse of a person wearing said band; and circuit means responsive to said pulse signals generated by said hoop stress sensing means for generating an output signal indicative of the value of the pulse of said person wearing said band.

In the preferred embodiment, the pulse detector takes the general form of a wristwatch having a casing housing both the hoop stress sensing means and the circuit means and an elastic band coupled to the casing. The elastic band is adapted to fit tensively around the wrist of the wearer and is mechanically coupled to the hoop stress sensing means which is preferably a piezoelectric transducer. The stored signals are read out of the memory at a high frequency which has the effect of compressing time and dilating frequency. As a result, the random noise components become widely separated in frequency leaving a predominant high frequency signal whose primary frequency (determined by the frequency of the pulse signals alone) is N times the frequency of the pulse signals. A phase locked loop is utilized to determine the dominant frequency of the signal read out of the memory and for generating an output signal indicative thereof. In the preferred embodiment, this signal is applied to an LCD (LIQUID CRYSTAL DISPLAY) display which provides a numerical indication of the pulse of the patient wearing the pulse detector.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in the drawings an embodiment which is presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

Figure 1 is a perspective of a pulse detector in accordance with the principles of the present invention.

Figure 2 is a side view, partially schematic and partially in section, of the pulse detector of Figure 1 with a modified wristband.

Figure 2A is a plan view taken along lines 2A-2A of Figure 2.

Figure 3 is a sectional view of a clasp forming part of the wristband illustrated in Figure 2. Figure 4 is a block diagram of the pulse analysis circuit of Figure 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein like numerals indicate like elements, there is shown in Figure 1 a pulse detector constructed in accordance with the principles of the present invention and designated generally as 10. In the embodiment illustrated, pulse detector 10 is similar in shape to a wristwatch and includes a casing 12 and an elastic band 14. As best illustrated in Figure 2, the casing 12 houses a piezoelectric transducer 16, a pulse analysis circuit 18 and an LCD display 20. Display 20 is visible through an opening 22 formed in the casing 12 and provides a visual indication of the patient's pulse. While this embodiment represents the preferred structure of the invention, it should be recognized that other embodiments may be employed without departing from the spirit or scope of the invention. For example, the band 14 could be adapted to fit around the neck of the patient and/or the pulse analysis and display circuits can be located at a remote location and could contain time keeping and out of limits visual and/or audio alarm functions as well. Referring again to Figure 2, the band 14 is connected to a thin disc 24 so that tensive forces produce a bending moment on disc 24. The piezoelectric transducer 16 mounted on the disc 24 is responsive to the bending moment. The disc sits on a pedestal 25 located on the bottom of case 12. The band 14 extends through openings 26 and 28 in case 12 and is attached to the disc 24 off the plane of the disc 24 so that tensive forces bend the disc about the pedestal that is acting as a fulcrum. The pedestal 25 may be formed of rubber which is bonded to both disc 24 and the case 12.

When band 14 is placed around the wrist of the patient, the band 14 assumes a generally oval shape. When the patient's heart pulses, blood flows through his wrist arteries effectively enlarging the circumference of his wrist. The enlarged circumference of the patient's wrist tends to stretch wristband 14 circum ferentially and thereby stress the same. This circumferential stress is referred to herein a "hoop stress". This stress is transmitted to disc 24 and is detected by piezoelectric transducer 16 which generates output signals indicative thereof.

The output signals generated by transducer 16 are of two primary types: relatively constant frequency pulses generated in response to each heartbeat of the patient and relatively random pulses (noise) generated as a result of movement of the patient's wrist. These signals take the general form illustrated at 30 in Figure 4. These pulses are applied to pulse analysis cir cuit 18 which evaluates the same and generates an outputsignal indicative of the pulse rate of the individual wearing pulse detector 10. A block diagram of pulse analysis circuit 18 is set forth in Figure 4 and will be described in further detail below. As should be apparent from the foregoing, the ability of the pulse analysis circuit 18 to accurately determine the pulse rate of the patient is determined in part by the sensitivity of the transducer 16 to variations in the circumference of the patient's wrist. The sensitivity of transducer 16 is, in turn, a function of the degree of mechanical coupling between the band 14 and the patient's wrist. If the band 14 is too loosely coupled to the wrist of patient, small changes in the diameter of the patient's wrist will cause little or no increase in the tensive forces in wristband 14. If the band 14 is too tightly connected to the patient's wrist, it could stop the flow of blood through the radial artery and thereby effectively remove the pulse from the wrist. Accordingly, the tightness of the band 14 should be adjusted to a level between these two extremes. Yet another factor effecting the quality of the output signal generated by transducer 16 is the tensile sensitivity of disc 24. Thus, the disc 24 may have a certain range of tensile forces in which its dimensions are most highly effected by changes in tension. The median tension placed on disc 24 by wristband 14 should be within a desired sensitivity range of disc 24.

In order to insure that the median tension in wristband 14 is at a desired level, it is possible to use an elastic wristband 14 which has been pre-sized to the individual. patient. Such an embodiment is illustrated in Figure 1. In lieu thereof, the wristband 14 may be adjustable as illustrated in Figure 2 using a releasable clamp 29. The structure of clasp 29 is shown in Figure

3. As shown therein, wristband 14 is divided into two halves 14a and 14b. Band half 14a is soldered to casing 30 at 31 while band half 14b is releasably coupled to casing 12 by releasable clamp 32. Clamp 32 includes a cylindrical stop 34 which is biased into contact with band end 14b by compression spring 36. A pair of cam followers 38 (Fig. 2) extend from opposite axial sides of stop 34 and extend through respective cam slots 40 located on either side of casing 30. When band half 14b is moved to the right as illustrated in Figures 2 and 3, stop 34 moves up and to the right against the force of spring 36 permitting the free movement of band half 14b into housing 30. When band half 14b is moved to the left as viewed in Figures 2 and 3, stop 34 is biased towards the base 42 of housing 30 thereby pinching band half 14b and preventing further movement to the left. As a result of the foregoing, band half 14b is freely movable into casing 30 but cannot be freely moved out of casing 30. If the patient wishes to remove band half 14b from casing 30, he will grasp the handles 44 connected to the end of either cam follower 38 and pull hold stop 34 up and to the right as viewed in Figure 3. As a result, band half 14b will no longer be pinched between stop 34 and base 42 and may be freely withdrawn from casing 30.

A block diagram of the pulse analysis circuit 18 is illustrated in Figure 4. As shown therein, pulse analysis circuit 18 receives an input signal 30 from input transducer 16. Input signal 30 includes both the pulse signals generated in response to the flow of blood through the patient's radial artery and noise signals responsive to various factors such as the movement of the patient's wrist. As noted above, the pulse signals have a relatively uniform frequency f while the noise signals are quite random. As a result, the fundamental frequency of signal 30 is the pulse frequency f.

The output of transducer 16 is applied to a band pass filter 46 which preferably passes a range of between .5 and 4 Hertz, although other ranges may be used. Filter circuit 46 serves to remove a significant portion of the noise from the input signal 30. A substantial amount of noise does, however, remain in this signal.

The output of filter 46 is coupled to an input amplifier 48 which amplifies the signal and applies it to analog digital converter (hereinafter A/D converter) 50. A/D converter 50 converts the analog signal at its input to a digital signal (preferably a four bit BCD signal) which is applied to input lines 58 of recirculating memory 52.

Memory 52 is an N stage shift register (N being an integer greater than 1) which stores N four bit digital signals representative of the shape of the input signal 30 over the last several seconds. In the preferred embodiment, memory 52 is a sixty-four stage memory which may be formed of four parallel sixty-four bit shift registers. Appropriate shift registers are manufactured by RCA under the product denation CD4031A.

Memory 52 is operable in two modes: an input mode and a recirculate mode. During the input mode, memory 52 applies the four bit signal appearing on lines 58 to the first stage of memory 52 each time a clock signal CL1 is applied to the clock input CL of memory 52. During the recirculate mode, the four bit digital signal appearing on output lines 60 of memory 52 is applied to the first stage of memory 52 each time the clock signal CL1 is applied to clock input CL of memory 52. In either mode of operation, each of the four bit words stored in memory 52 are shifted one stage towards the output whenever a clock signal CL1 is applied to the clock input CL of memory 52.

The mode of operation of memory 52 is controlled by memory control counter 56 whose operation is, in turn, controlled by system clock 54. System clock 54 generates clock signals CL1, CL2 and CL3 which may, for example, have frequency of 1092, 1.066 and 68 Hz, respectively. System clock 54 may include a crystal oscillator and an appropriate set of digital counters for generating the desired signals.

Memory control counter 56 is an N+1 counter whose count increases by one each time a new clock pulse CL1 is applied to its count input CL. The output of counter 56 is at a binary "1" level whenever its count is full (i.e., it has received N+1 clock pulses) and is at the binary "0" level at all other times. Whenever counter 56 is full, the count in counter 56 is reset to 1 responsive to the next clock pulse applied thereto. As a result, the output of counter 56 is at the binary "0" level for N consecutive clock pulses and at the binary "1" level for one clock pulse. Memory 52 operates in the input mode whenever a binary "1" is applied to its MODE input and operates in a recirculate mode whenever a binary "0" is applied thereto. As a result, all N binary signals stored in memory 52 are shifted through each of the stages of memory 52 before a new signal appearing on lines 58 is applied thereto. The effect of the foregoing is to generate a digital output signal (on lines 60) whose shape is identical to that of signal 30 but whose frequency is N times greater than the basic frequency f of the input signal 30. For simplicity of explanation, it will be presumed that meory 52 has 64 four bit stages (N = 64) and that the frequency of the signal appearing on lines 60 is 64f.

The signals appearing on lines 60 are applied to the input of digital to analog converter (hereinafter D/A converter) 62. D/A converter 62 converts the digital signals appearing on output lines 60 of memory 52 to an analog signal having a shape substantially identical to the shape of input signal 30 but having a frequency 64 times as great. This signal is applied to band pass filter 64 which serves to remove extraneous frequency components from the signal appearing at the output of D/A converter 62. The signal appearing at the output of filter

64 will have a fundamental frequency 64f which is 64 times greater than the fundamental frequency f (the pulse frequency of the patient) of the input signal 30 This signal is applied to a phase locked loop 66 which generates an output signal whose frequency is equal to the fundamental frequency (64f) of the output of filter 64. While any suitable phase locked loop may be used, one such circuit is manufactured by RCA under the product designation CD4046. By adding appropriate external components (resistors and capacitors) to phase locked loop 66, it is possible to limit the range of output signals generated by phase locked loop 66 to a particular frequency band. It is preferred that this frequency band correspond to the frequency band of band pass filter 64. The output of phase locked loop 66 has a frequency 64f which is substantially independent of frequency components of the input circuit 30 which are attributable to noise. This output is applied to a counter 68 which is preferably, but not necessarily, a three digit BCD counter. The count in counter 68 is increased at a rate determined by the frequency of the output of phase locked loop 66 whenever a binary "1" is applied to its enable input ENB. The enable input ENB of counter 68 receives the clock signal CL2 generated by system clock 54. As noted above, clock signal CL2 preferably has a frequency of 1.066Hz which will enable counter 68 for 15/16 ths of a second. The clock signal CL2 is also applied to one shot 70 which generates a single pulse at its output responsive to each clock pulse CL2. The pulse appearing at the output of one shot 70 is applied to one shot 72 which also generates a single pulse which clears the count in counter 68. As a result of the foregoing, the count in counter 68 is equal to the pulse rate in pulses per minute of the patient wearing pulse detector 10 at the instant one shot 70 produces an output pulse. In addition to being applied to one shot 72, the pulse generated by one shot 70 is applied to the latch input LT of latch and decoder LCD display driver circuit 74. Driver circuit 74 latches the signal appearing at the output of counter 68 responsive to the pulse generated by one shot 70. Accordingly, driver circuit generates a three digit BCD signal corresponding to the pulse rate of the patient. This signal enables an LCD display 20 in a known manner at a rate determined by the clock signal CL3. In the preferred embodiment, driver circuit 74 is comprised of three parallel latch and decoder LCD display driver circuits manufactured by RCA under the product designation CD4543. Display 20 is preferably a three digit LCD display manufactured by Beckman under the product designation 708-6-2. Driver circuit 74 and display 20 are enabled by the clock pulses CL3 generated by system clock 30 which preferably has a frequency of 68Hz. If desired, an alarm circuitry can be added to pulse analysis circuit 18 to alert the patient that his pulse rate is too high or too low. By way of example, the output of counter 68 can be coupled to a pair of comparators one of which compares the output of counter 68 to a predetermined minimum pulse rate (e.g., 35 pulses per minutes) and the other of which compares the output of counter 68 to a predetermined maximum pulse rate (e.g., 150 pulses per minute). The output of both counters can be coupled to an alarm circuit which goes off whenever the first counter determines that the actual pulse rate is below the predetermined minimum count and whenever the second count er determines that the pulse rate is above the predetermined maximum pulse rate . If desired, means (for example , a pair of potentiometers) can be provided for adjusting the level of the minimum and maximum pulse rates .

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims , rather than to the forego ing specification as indicating the scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A pulse detector, comprising: a band having a generally hoop-like shape; hoop stress sensing means for detecting the tensing of said band and for generating a pulse signal each time said band is tensed by the pulse of a person wearing said band; and circuit means responsive to said pulse signals generated by said hoop stress sensing means for generating an output signal indicative of the value of the pulse rate of said person wearing said band.

2. The pulse detector of claim 1, wherein said band is an elastic band.

3. The pulse detector of claim 1, wherein said band is adjustable.

4. The pulse detector of claim 1, wherein said hoop stress sensing means comprises: a piece of metal coupled to said band in such a manner that tensive forces in said band generate corresponding tensive forces in said piece of metal; transducer means for generating electrical signals representative of said tensive forces in said piece of metal, said electrical signals including said pulse signals.

5. The pulse detector of claim 4, wherein said transducer means comprises a piezoelectric transducer mechanically coupled to said piece of metal.

6. The pulse detector of claim 4 or 5, wherein said piece of metal is a thin metal disc.

7. The pulse detector of claim 6, wherein said disc is mounted in said casing in such a manner that said disc tends to bend in response to tensive forces in said wrist band.

8. The pulse detector of claim 4, wherein said band is elastic.

9. The pulse detector of claim 4, wherein said band is adjustable and permits the mean tension of said band to be adjusted to a desired level while the person is wearing said band.

10. The pulse detector of claim 1, further including display means responsive to said output signal for providing a visual indication of the. pulse rate of the person wearing said band.

11. The pulse detector of claim 10, wherein siad display means is a LCD display.

12. The pulse detector of claim 1, wherein said hoop stress sensing means and said circuit means are housed in the same casing.

13. The pulse detector of claim 10, wherein said hoop stress sensing means, said circuit means and said display means are housed in the same casing.