Abstract:
A heart rate monitor for use with road bikes and other exercise equipment to calculate the user&#39;s heart rate while exercising. The monitor includes only two, first and second, electrodes and a signal processor mounted on the handle bars in a position that contacts the user when the bike is in normal use. The first electrode is disposed at a first location on the handle bars for contacting the user and for sensing the user&#39;s heart rate. The first electrode produces a first sensed signal representative of the user&#39;s heart rate. The second electrode is disposed at a second location on the handle bars for contacting the user and for sensing the user&#39;s heart rate. The second electrode produces a second sensed signal representative of the user&#39;s heart rate. The signal processor produces a difference signal indicative of a difference between the first and second sensed signals. The signal processor further includes a power spectrum analyzer calculating the user&#39;s heart rate by measuring the power spectrum of the difference signal and producing a processed heart rate signal as a function of the measured power spectrum whereby the processed heart rate signal is indicative of the heart rate of the user as sensed by the first and second electrodes. A battery pack powers the signal processor, the battery pack adapted to be mounted on the road bike or other exercise equipment. A method of monitoring the heart rate of a person is also included.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. Ser. No. 09/260,160, filed Mar. 3, 1999 which will issue on Sep. 5, 2000 as U.S. Pat. No. 6,115,629. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to heart rate monitors for use in association with exercising equipment. In particular, the invention relates to a two electrode monitor employing digital signal processing of signals detected by the electrodes. 
     2. Description of the Prior Art 
     Heart rate monitors for use with road bikes and other exercise equipment are known in the art. In general, such monitors tend to require four electrodes, two for each hand, in-order to accurately detect a beat to beat heart rate. Some patented prior art monitors include the following. U.S. Pat. No. 3,702,113 illustrates a heart rate monitor with only one electrode per hand. U.S. Pat. No. 5,243,993 discloses a heart rate monitor which includes a digital signal processor 44 for implementing autocorrelator 26, signal processor 28 and arbitrator 30. Similarly, Antii Ruha, Sami Sallinen &amp; Seppo Nissilä, in A Real-Time Microprocessor ORS Detector System with a 1-ms Timing Accuracy for the Measurement of Ambulatory HRV, IEEE Transactions on Biomedical Engineering, March 1997, teach the use of digital signal processing in connection with monitoring a patient&#39;s heart rate. 
     U.S. Pat. No. 5,337,753 is directed to a four-electrode (two electrodes per hand) heart rate monitor. Specifically, it calls for a first live electrode and a first common electrode mounted on one half of an elongate member in spaced relationship with each other and a second live electrode and a second common electrode mounted on the other half of the elongate member in spaced relationship with each other. 
     Referring next to U.S. Pat. No. 5,337,753, it discloses an apparatus for measuring heart rate having at least the following features: (i) sensor means for generating an input signal including the biopotential signal produced by the user&#39;s heart, (ii) autocorrelating means responsive to the input signal for periodically generating an autocorrelation signal of the input signal over a predetermined time period and, (iii) signal indication means responsive to the autocorrelation signal for detecting the presence of at least one periodic signal in the autocorrelation signal and generating a heart rate signal corresponding to the detected signal (s). 
     U.S. Pat. No. 4,319,581 describes an apparatus having a first hand grip for attachment to one end of the handlebar of a bicycle or stationary exercise device, a second hand grip including a second means for electrically contacting the other end of said handlebar, third and fourth means disposed upon said second hand grip in spaced apart relationship and adapted to electrically engage said other hand of said user, and signal monitoring means for measuring electric pulses created in the body and for indicating the heart pulse rate of such body. 
     There is a need for a low cost, accurate monitor which digitally processes signals on a power spectrum basis rather than a peak to peak basis, which employs only two electrodes and which includes an isolation barrier. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a low cost heart rate monitor which employs only two electrodes. 
     It is a further object of this invention to provide an accurate heart rate monitor which may be used in combination with road bikes and other exercise equipment. 
     It is also an object of this invention to provide a heart rate monitor which through digital signal processing based on the power spectrum provides heart rate information which is reliable. 
     The invention meets these needs and objects in at least three ways. First, the invention provides only one electrode per hand which avoids the problem of sweat accumulation between electrodes. Second, the invention uses an isolation barrier which allows the monitor to meet Underwriter Laboratories&#39; and other safety specifications. Third, the invention uses digital signal processing to calculate the power spectrum, namely the application of discrete Fourier transforms with Goertzel&#39;s algorithm, to substantially reduces noise from motion artifacts, especially when the user is exercising rigorously, so that an accurate beat rate can be detected. 
     In one form, the invention-comprises a heart rate monitor for use with road bikes and other exercise equipment to calculate the user&#39;s heart rate while exercising. The monitor includes first and second electrodes and a signal processor. The first electrode is disposed at a first location on the road bikes and other exercise equipment for contacting the user and for sensing the user&#39;s heart rate. The first electrode produces a first sensed signal representative of the user&#39;s heart rate. The second electrode is disposed at a second location on the road bikes and other exercise equipment for contacting the user and for sensing the user&#39;s heart rate. The second electrode produces a second sensed signal representative of the user&#39;s heart rate. The signal processor produces a difference signal indicative of a difference between the first and second sensed signals. The signal processor further includes a power spectrum analyzer calculating the user&#39;s heart rate by measuring the power spectrum of the difference signal and producing a processed heart rate signal as a function of the measured power spectrum whereby the processed heart rate signal is indicative of the heart rate of the user as sensed by the first and second electrodes. 
     In another form, the invention comprises a heart rate monitor for use with road bikes and other exercise equipment to calculate the user&#39;s heart rate while exercising having only two electrodes and a signal processor. 
     In another form, the invention comprises an exercise apparatus comprising an exercise device for exercising the user and a heart rate monitor such as noted above for use with road bikes and other exercise equipment to calculate the user&#39;s heart rate while exercising. 
     The invention also includes a method of monitoring the heart rate of a person comprising the steps of: 
     contacting the person with a first electrode for sensing the person&#39;s heart rate, the first electrode producing a first sensed signal representative of the person&#39;s heart rate; 
     contacting the person with a second electrode for sensing the person&#39;s heart rate, the second electrode producing a second sensed signal representative of the person&#39;s heart rate; 
     producing a difference signal indicative of a difference between the first and second sensed signals; 
     measuring the power spectrum of the difference signal; and 
     producing a processed heart rate signal as a function of the measured power spectrum whereby the processed heart rate signal is indicative of the heart rate of the person as sensed by the first and second electrodes. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a block diagram and FIGS. 1B and 2B are schematic diagrams of one preferred embodiment of the hardware of the heart rate monitor according to the invention. 
     FIG. 2A illustrates in block diagram form the process employed by the digital signal processor according to the invention for generating adaptive heart beat peaks from the digital difference signal. 
     FIG. 2B illustrates the inputs and outputs of the digital signal processor-of the invention. 
     FIG. 2C illustrates the overlapping windows of successive samples which are used by the matched filter software of the digital signal processor of the invention to calculate the heart beat signal indicative of the heart rate. 
     FIG. 3 illustrates the process employed by the digital signal processor according to the invention to generate a heart rate calculation from the adaptive heart beat peaks. 
     FIG. 4 is a schematic diagram of the isolation barriers according to the invention. 
     FIG. 5 is a graph which illustrates the difference signal which is the raw data from the electrodes generated by a person exercising at 2 mph and which further illustrates the following signals corresponding thereto: the analog bandpass filtered (BPF) difference signal, the matched filter (MF) heart beat signal, the adaptive heart beat peak signal and the heart rate signal measured by an ECG monitor (Polar). 
     FIG. 6 is a graph comparing the power spectrum of the ECG signal and the adaptive heart beat peak signal of FIG.  5 . 
     FIG. 7 is a graph which illustrates the difference signal which is the raw data from the electrodes generated by a person exercising at 5 mph and which further illustrates the following signals corresponding thereto: the analog bandpass filtered (BPF) difference signal, the matched filter (MF) heart beat signal, the adaptive heart beat peak signal and the electrocardiogram (ECG) signal. 
     FIG. 8 is a graph comparing the power spectrum of the ECG signal and the adaptive heart beat peak signal of FIG.  7 . 
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A is a block diagram and FIGS. 1B and 2B are schematic diagrams of one preferred embodiment of the hardware of the heart rate monitor according to the invention. In general, the heart rate monitor is contemplated for use in combination with road bikes and other exercise equipment to calculate the user&#39;s heart rate while exercising. For example, the exercise device which may be used in combination with the monitor may be a device selected from the group consisting of a treadmill, a bicycle, a skiing machine, and a step machine. In each of these devices, a handle is provided. The handle is adapted to be grasped by the user&#39;s hands. A first electrode  102  and a second electrode  104  are provided on the handle for contacting the user&#39;s hands. In general, the first electrode  102  may be disposed at any location on the road bikes and other exercise equipment as long as it is capable of being in contact with the user. Similarly, the second electrode  104  is also disposed at any location on the road bikes and other exercise equipment so that it can contact the user. Each of the electrodes senses the user&#39;s heart rate. The first electrode  102  produces a first sensed signal representative of the user&#39;s heart rate. The second electrode  104  produces a second sensed signal representative of the user&#39;s heart rate. 
     The electrodes. are connected through an electrostatic discharge (ESD) protection circuit to a differential amplifier  108 . The equipment illustrated in block diagram form in FIG. 1A after the ESD protection circuit  105  comprises a signal processor  105  for converting the analog signals detected by the electrodes into a digital heart rate. Initially, differential amplifier  108  produces a difference signal via line  106 . The difference signal  106  is indicative of a difference between the first and second sensed signals provided by the electrodes  102  and  104 . 
     The difference signal  106  is then passed through a bandpass filter  110  such as an analog filter which only allows signals having a frequency of at least 0.72 Hz and a frequency not more than 100 Hz to pass to an amplifier  112  which amplifies the signal. Preferably, amplifier  112  would have a gain of 10. The amplified signal is then provided to a 12 bit analog to digital converter having a 1,000 Hz sampling frequency. For example, this converter  114  may be an AD7896 manufactured by Analog Devices. The converter provides a digital difference signal  116  to an isolating circuit  118  which provides electrical isolation between the user and the power source. The output of the converter  114  includes 1,000 samples per second which are provided through the isolation circuit  118  to a digital processor  120 . For example, processor  120  may be an ADSP-2186 as manufactured by Analog Devices. In general, the converter  114  may be a sampling circuit or sampling software for converting the analog difference signal  106  after it has been filtered by bandpass filter  110  and amplified by amplifier  112  into a corresponding digital difference signal  116 . 
     As shown in FIG. 1B, a hand grip connector HG provides the connection to the electrodes  102  and  104  which are provided to the differential amplifier  108  and converted to digital DATA signals. Accompanying the DATA signals are a clock signal CLK providing a timing reference for the digital data and a chip select CS providing a hand shake protocol. 
     FIG. 2A illustrates the process employed by the digital signal processor  120  according to the invention for generating adaptive heart beat peaks from the digital difference signal  116  provided as 1,000 samples per second to the digital signal processor  120  via the isolating circuit  118 . As will be described below in greater detail regarding FIG. 2B, the digital signal processor  120  comprises a microprocessor. In one aspect of the invention, the microprocessor employs software (stored in EPROM 222, FIG. 2B) which executes a digital signal processing algorithm including a digital band pass filtering algorithm  202  which passes signals in the range of 5-28 Hz to provide a filtered digital difference signal via line  204 . Such signals correspond to the bandwidth of the QRS complex. Next, as illustrated by block  206 , the processor  120  employs a matched filter algorithm to provide a heart beats signal via line  208  from the filtered digital difference signal provided via line  204 . In general, the matched filter is applied to the average QRS complex of successive, overlapping windows of the filtered digital difference signal in order to detect the heart beats, eliminate noise and generate a digital signal representative thereof. 
     In one aspect of the invention, the digital processor employs overlapping windows as illustrated in FIG. 2C to evaluate the waveform being provided by the series of samples. 
     In particular, referring to FIG. 2C, assume that the converter  114  begins sampling at time T 0  so that at time T 1 , 4,096 samples have been provided to the processor  120 . These samples are considered to be the first part of window  1 . The 4,096 samples between times T 1  and T 2  are the first part of window  2  and the second part of window  1 . Similarly, the 4,096 samples between times T 2  and T 3  are considered to be the first part of window  3 , the second part of window  2 , and third part of window  1 . Finally, the 4,096 samples between times T 3  and T 4  is considered to be the first part of window  4 , the second part of window  3 , the third part of window  2 , and the last part of window  1 . At time T 4 , window  1  is considered to be a completed waveform and at time T 5  window  2  is completed so that these windows can now be compared to each other according to matched filtering by processor  120 . As illustrated in FIG. 2C, window  2  overlaps window  1  by 75%. Essentially, window  2  drops the first 4,096 samples provided during time T 0  to T 0  and adds the 4,096 samples provided between time T 4  and T 5 . Subsequent windows  3  and  4  are similarly defined based on the previous windows. Therefore, by comparing successive windows to each other, noise is minimized or eliminated and a waveform representative of the heart beats results after the matched filter comparison. 
     Next, the digital signal processor  120  employs software which functions as an adaptive peak detector  210  so that the digital signal provided via line  212  is converted into heart beats peaks. 
     FIG. 2B illustrates the inputs and output of the digital signal processor  120 . Operation of the digital signal processor  120  provides a POWER CONTROL signal and a start clock (reset) signal SCLKO to control the dc/dc power supply (see FIG. 4) and a voltage source VCC which also illuminates a light emitting diode to visually indicate that power has been applied to the digital signal processor  120 . In addition, processor  120  loads its instructions from an EPROM 222 which is programmed with the various algorithms and functions as noted herein along with any other applications needed to operate the monitor. A microprocessor supervisor circuit  224  resets the processor whenever the voltage VCC drops below a minimum. An oscillator  226  provides a reference clock signal to the processor  120 . In addition, the processor  120  generates the clock signal CLK, a chip select signal CS and receives the DATA signal as processed from the electrodes. 
     Optionally, the processor may have serial and/or parallel ports for debugging and diagnostics. After processing, the processor  120  provides a POLAR OUT signal in the form of a square wave  228  having a period frequency corresponding the calculated heart rate. This POLAR OUT signal is a standard format which interfaces with most road bikes and other exercise equipment and is the same formatted signal transmitted by wireless heart rate detectors. Optionally, if the road bikes and other exercise equipment accommodates wireless detectors transmitting rf signals, the processor can be programmed so that such rf signals override the signals being detected by the electrodes. 
     Now that the heart beats peaks have been established by the processor  120 , the heart rate calculation can be executed as illustrated in FIG.  3 . Initially, as shown in FIG. 3, the heart beat peaks are processed through a triangular window  302  and then provided to software which evaluates the heart beat peaks to determine a power spectrum calculation  304 . The power spectrum is calculated using Goertel&#39;s algorithm, but there are other ways to calculate it. Windowing can be done many different ways. Although the preferred embodiment used triangular, many other windowing functions would work. This yields a processed heart rate signal which will have a power spectrum peak in the range of 1-2 Hz if the beats per second are in the range of 60-120 bpm. Alternatively, the processed heart rate signal may have a power spectrum peak in the range of 2-3 Hz when the heart rate is in the range of 120-180 bpm. 
     The digital signal processor  120  at step  310  compares peak  2  to peak  1  multiplied by a factor of 1.2. If the processed heart rate signal has a peak  2  which is greater than peak  1  times 1.2, the processor  120  proceeds to step  312  to define the heart rate as the frequency rate of peak  2  times 60. On the other hand, if peak  2  is equal to less than peak  1  times 1.2, the processor  120  proceeds to step  314  to define the heart rate as equal to the frequency  1  of peak  1  times 60. 
     One aspect of the invention is that the above configuration allows for electrical isolation between the electrodes and the road bikes and other exercise equipment as shown in FIG.  4 . In particular, a transformer XFMR provides isolation between the +/−5 volt power signals and the right side of FIG. 4 including the user engaging the hand grip HG, and the VCC source signal, the POWER CONTROL signal and the start clock (reset) SCLKO signal generated by the digital signal processor  120 . In addition, optisolators I/O provide isolation between the clock signal CLK, chip select signal CS and DATA signal on one side of the optoisolators and their complementary signals on the other side of the optoisolators, where the user is positioned and engaged. 
     FIG. 5 is a graph illustrating the various waveforms which are processed beginning with the analog signals provided by the electrodes  102  and  104  to the processed digital heart rate signals which define the heart rate. Waveform  502  illustrates the raw data in the form of the analog difference signal being provided via line  106 . Waveform  504  illustrates the analog difference signal after processing by bandpass filter  110 . Waveform  506  illustrates the analog heart beat signal after it has been processed by the matched filter software  206  and is represented by line  208 . Waveform  508  is a digital signal and represents the digital heart beat signal after it has been processed by adaptive peak detector  210 . This signal defines the heart beat peaks which are processed from the power spectrum as illustrated by FIG. 3 to make the heart rate calculation. By comparison, ECG waveform  510  is derived from an electrocardiogram (ECG) of the heart being monitored by the invention. A comparison of waveforms  508  and  510  indicates that the calculated heart rate of 94 bpm as illustrated by waveform  508  is the same as the actual heart rate of 94 bpm as detected and illustrated by the ECG waveform  510 . The waveforms of FIG. 5 are representative of an individual exercising at a rate of 2 mph on a treadmill and are based on actual monitored data. FIG. 6 further compares the detected ECG power spectrum  602  to the calculated power spectrum of the detected adaptive heart beat peaks signal  604 . FIG. 6 illustrates a close coordination between the detected and monitored power spectrums. 
     Similarly, FIGS. 7 and 8 are representative of an individual exercising at a rate of 5 mph on a treadmill and illustrate actual monitored data. FIG. 7 is a graph illustrating the various waveforms which are processed beginning with the analog signals provided by the electrodes  102  and  104  to the processed digital heart rate signals which define the heart rate. Waveform  702  illustrates the raw data in the form of the analog difference signal being provided via line  106 . Waveform  704  illustrates the analog difference signal after processing by bandpass filter  110 . Waveform  706  illustrates the analog heart beat signal after it has been processed by the matched filter software  206  and is represented by line  208 . Waveform  708  is a digital signal and represents the digital heart beat signal after it has been processed by adaptive peak detector  210 . This signal defines the heart beat peaks which are processed from the power spectrum as illustrated by FIG. 3 to make the heart rate calculation. By comparison, ECG waveform  710  is derived from an electrocardiogram (ECG) of the heart being monitored by the invention. A comparison of waveforms  708  and  710  indicates that the calculated heart rate of  106  bpm as illustrated by waveform  708  is the very similar to the actual heart rate of 110 bpm as detected and illustrated by the ECG waveform  710 . FIG. 8 further compares the detected ECG power spectrum  802  to the calculated power spectrum of the detected adaptive heart beat peaks signal  804 . FIG. 8 illustrates a close coordination between the detected and monitored power spectrums. 
     By way of illustration only, the peak  1  and peak  2  calculations of steps  306  and  308 , respectively, may be accomplished by executing the following algorithm:          Peak        (   j   )       =       ∑     PowerSpectrum                   (   j   )         ENBW                            
     For j=i−span/2, . . . i+span/2, 
     where i=peak index, 
     where PowerSpectrum(j)=power in bin j, and 
     where ENBW=equivalent noise bandwidth of the window (e.g., 1.5). 
     As an example of determining the peak calculation for 2 mph, the digital signal processor  120  would perform the following calculations:          Peak                   (   j   )       =       ∑     j   =     i   -   1         i   +   1              PowerSpectrum                   (   j   )       1.5                              
     PowerSpectrum[12]=1.21E-5 
     PowerSpectrum[13]=1.68E-5 
     PowerSpectrum[14]=5.24E-6 
     Peak [13]=(1.21E-5+1.68E-5+5.24E-6)/1.5 
     Peak [13]=2.28E-5 
     From the ECG waveform  510  in FIG. 5, the peak [13] reads 2.28E-5, which is the same as the peak as calculated above by the digital signal processor  120 . 
     Similarly, by way of illustration only, the calculation of frequency  1  and frequency  2  by steps  306  and  308 , respectively, may be accomplished by employing the following algorithm in the digital signal processor  120 :          Freq        (   j   )       =       ∑     PowerSpectrum                   (   j   )     *     (     j   *   dF     )           ∑     PowerSpectrum        (   j   )                                  
     For j=i−span/2, . . . i+span/2, 
     where i=peak index, 
     where PowerSpectrum(j)=power in bin j, and 
     where dF=frequency bin width. 
     As an example of determining the frequency calculation for 2 mph, the digital signal processor  120  would perform the following calculations:          Freq        (   j   )       =             ∑     j   =     i   -   1         i   +   1            PowerSpectrum                   (   j   )     *     (     j   *   dF     )             ∑     j   =     i   -   1         i   +   1            PowerSpectrum        (   j   )           *   60     +   BPMoffset             dF   =       1000   /   16384     =       6.104      E     -   2               BPMoffset   =     47.61                 bpm               Freq        (   13   )       =       60   *             [         (       1.21      E     -   5     )          (       12   *   6.104      E     -   2     )       +                     (       1.68      E     -   5     )          (       13   *   6.104      E     -   2     )       +                   (       5.24      E     -   6     )          (       14   *   6.104      E     -   2     )       ]               (       1.21      E     -   5     )     +     (       1.66      E     -   5     )     +     (     5.24   -   6     )           +   47.61               Freq        (   13   )       =   94.47                          
     From the ECG  510  in FIG. 5, the frequency[1] reads 94.47, which is the same as the frequency as calculated above by the is digital signal processor  120 . 
     It is particularly contemplated that the invention may be used with any display console that has a polar interface. The interface to the display console consists of three wires which mimics a polar receiver. In general, the system would operate in the following manner. The digital signal processor  120  detects when the hand grips are held and starts to search for a pulse. The signal input to the digital signal processor (i.e., the samples) are very noisy and their pulse amplitude is relatively small. Therefore, the processor uses the digital filtering algorithm  202  to define the samples. Preferably, the processor  120  records sixteen seconds of signal information that the digital band pass filtering algorithm  202  reduces to a filter digital difference signal. For this sixteen second period, the processor will not indicate a heart rate to its display console. After the sixteen second period, and after a heart rate is detected, updates are averaged every four seconds. 
     The sixteen seconds of initial data after the hand grips are first held are not displayed. Preferably, the processor  120  would be programmed to indicate through the display that the monitor is in the mode of searching for a pulse rate. 
     It is possible to display a heart rate at 8 to 12 seconds, but it is lower in resolution and less reliable. 
     The invention also includes a method of monitoring the heart rate of a person comprising the steps of: 
     contacting the person with a first electrode for sensing the person&#39;s heart rate, the first electrode producing a first sensed signal representative of the person&#39;s heart rate; 
     contacting the person with a second electrode for sensing the person&#39;s heart rate, the second electrode producing a second sensed signal representative of the person&#39;s heart rate; 
     producing a difference signal indicative of a difference between the first and second sensed signals; 
     measuring the power spectrum of the difference signal; and 
     producing a processed heart rate signal as a function of the measured power spectrum whereby the processed heart rate signal is indicative of the heart rate of the person as sensed by the first and second electrodes. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     As various changes could be made in the above products and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.