Interpretation of the working principle of pulse measurement

1 Introduction

    Pulse measurement is a measurement that detects whether there is a pulse. When there is a pulse, the light is blocked, and when there is no pulse, the light transmission is strong. The sensors used are infrared receiving diodes and infrared emitting diodes. The pulse measurement used for sports measurement roughly has two methods: finger pulse and ear pulse. These two measurement methods have their own advantages and disadvantages. Finger pulse measurement is more convenient and simple, but because there are many sweat glands on the finger, the finger clip is used all year round, and pollution may reduce the measurement sensitivity; the ear pulse measurement is relatively clean, and the sensor is used to pollute the environment. Less and easy to maintain. However, because the ear pulse is weak, especially when the season changes, the measured signal is significantly affected by the ambient temperature, resulting in inaccurate measurement results.

    2 Pickup of pulse signal

    The pulse signal pickup circuit is shown in Figure 1. IClA is connected as a unity gain buffer to generate a 2.5V reference voltage.

    The infrared receiving diode can generate electric energy under the irradiation of infrared light, and a single diode can generate 0.4 V voltage and 0.5 mA current. The working wavelength of BPW83 infrared receiving diode and IR333 infrared emitting diode are both 940 nm. In the finger clamp, the infrared receiving diode and infrared emitting diode are placed relative to each other to obtain the directivity characteristics. The greater the current in the infrared emitting diode and the smaller the emission angle, the greater the emission intensity produced. In Figure 1, the RO selection of 100 Ω is based on the consideration of the infrared light sensitivity of the infrared receiving diode. R0 is too large, the current passing through the infrared emitting diode is too small, and the BPW83 infrared receiving diode cannot distinguish the signal with and without pulse. On the contrary, if R0 is too small, the current passing through is too large, and the infrared receiving diode cannot accurately distinguish the signal when there is pulse and when there is no pulse. When the infrared light emitted by the infrared emitting diode is directly irradiated on the infrared receiving diode, the potential of the inverting input terminal of IC1B is greater than the potential of the non-inverting input terminal, and Vi is "O". When the finger is in the measurement position, two situations will occur: one It is the pulseless period. Although the finger blocks the infrared light emitted by the infrared emitting diode, due to the dark current in the infrared receiving diode, there is still a dark current of 1μA which will cause the Vi potential to be slightly lower than 2.5 V. The second is the pulse period. When there is a beating pulse, the blood pulse deteriorates the light transmittance of the finger, the dark current in the infrared receiving diode decreases, and the Vi potential rises.

    From this point of view, the pickup of the so-called pulse signal is actually obtained by the infrared receiving diode, the weak change of the dark current when there is pulse and no pulse, and then the amplification of IClB. The picked-up signal is a voltage signal of about 2μV.

    3 Amplification of the signal

    The low-pass amplifier is designed according to the number of pulses of the human body after exercise up to 240 times/min. It is composed of IC2A and C04, as shown in Figure 2. The turning frequency is determined by R07, C04, R08 and C05, and the magnification is determined by the ratio of R08 and R06.

    According to the transfer function of the second-order low-pass filter, we can get



    Considering that the human pulse is 4 Hz, the low frequency characteristics are satisfactory.

    It should be noted that the above analysis is made under the condition of ignoring C03. If C03 is considered, then:

    It can be seen that C03 does not affect the analysis of frequency characteristics, and its function is only to block the direct current.

    The two-stage amplifier and comparator are shown as in Fig. 3. Rpll is used to adjust the magnification of the system, and C06 is used to prevent self-excitation of the amplifier. With two-stage amplification, the zero drift is not very obvious, around 0.1 V. Therefore, the threshold voltage of the comparator is designed to be O.25 V to ensure that interference signals are filtered out. The advantage of using the comparator is that it can effectively overcome the influence caused by the zero drift and improve the accuracy of the measurement.



    4 Wave shaping

    The waveform shaping circuit is shown in Figure 4. IC3A is a CD4528 monostable multivibrator with an effective pulse width of 0.05 s. Its width is determined by R22 and C20. IC3B also forms a monostable multivibrator with a pulse width of 240ms. D2, Dl and T3 form a NOR gate. Only when C and E are both low, the output of the signal amplifier is high. level. The purpose of designing this circuit is to output a narrow pulse at the output, and no signal will interfere with the output within the time determined by R13 and C07. The length of the charging time of R23 and C21 determines the width of the counting pulse. Generally, it is not desired to be too wide. The waveform shaping sequence is shown in Figure 5.



    5 concluding remarks

    When this amplifier is used in a cluster pulse measuring instrument, we must pay attention to the mutual influence between different signal channels. It is recommended to separate the power supply of each amplifier. In addition, the measurement channel needs a switch circuit. When the finger clip is suspended, the switch circuit closes the monostable circuit, cutting off the signal path, and preventing miscalculation.

    Several years of production practice has proved that the amplification processing circuit is stable and reliable. The following are some of the experience I gained in the design.



    Using two-stage amplification is better than three-stage amplification, and the zero drift of individual three-stage amplification circuit boards is large enough to reach full amplitude, making the measurement inaccurate. The magnification of each single-stage amplifier should not be greater than 30 to avoid self-oscillation.

    The high-frequency corner frequency of this signal amplifier is determined by C05, C04, R07, R08 and R06. C05 and C04 are usually polypropylene capacitors or polycarbonate capacitors. R07, R08 and R06 are usually metal film five-color ring resistors.

    IClA, R02 and R03 form a voltage follower, the design value is 2.5 V, the degree is determined by R02 and R03, the metal film five-color ring resistor is used.

    The leakage of DC blocking capacitor C03 should be small, and tantalum electrolytic capacitor is better.

    IClA and IC1B should select operational amplifiers with small bias current and small input offset voltage. Considering the cost performance, I used TLC2264 and TLC2262.