Pulse measurement method based on photoelectric technology

1 Introduction

Pulse measurement is a measurement to detect whether there is a pulse. When there is a pulse, the light is blocked, and when there is no pulse, the light is highly transparent. The sensors used are infrared receiving diodes and infrared emitting diodes. There are two ways to measure pulse for sports measurement: 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 more sweat glands on the fingers, the finger clips are used all year round, and pollution may reduce the measurement sensitivity; ear pulse measurement is relatively clean, the sensor is used in a less polluted environment, and it is easy to maintain. However, because the ear pulse is weak, especially when the seasons change, the measured signal is significantly affected by the ambient temperature, resulting in inaccurate measurement results.

2. Pulse signal pickup

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 electrical energy under the irradiation of infrared light. A single diode can generate 0.4 V voltage and 0.5 mA current. The operating wavelength of the BPW83 infrared receiving diode and the IR333 infrared emitting diode are both 940 nm. In the finger clip, the infrared receiving diode and the infrared emitting diode are placed relative to each other to obtain the best directional characteristics. The larger the current in the infrared emitting diode, the smaller the emission angle, and the greater the emission intensity. In Figure 1, RO is selected as 100 Ω based on the infrared receiving diode's sensitivity to infrared light. If R0 is too large, the current passing through the infrared emitting diode is too small, and the BPW83 infrared receiving diode cannot distinguish between the signals 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 between the signals with and without pulse. When the infrared light emitted by the infrared emitting diode directly irradiates 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 measuring position, two situations will occur: one 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 lμA, which will cause the Vi potential to be slightly lower than 2.5 V. Second, there is a pulse period. When there is a beating pulse, the blood vessels make the light transmittance of the finger worse, the dark current in the infrared receiving diode decreases, and the Vi potential rises.

From this, it can be seen that the so-called pulse signal is actually picked up by the infrared receiving diode, and the slight change of dark current when there is a pulse and no pulse is obtained by amplification by IClB. The picked-up signal is a voltage signal of about 2μV.

3 Signal amplification

The low-pass amplifier is designed based on the calculation that the highest pulse rate of human body after exercise is 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 amplification factor is determined by the ratio of R08 and R06.

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

Considering that the maximum 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 taken into account, then:

It can be seen from this that C03 does not affect the analysis of frequency characteristics, its function is only to isolate DC.

The secondary amplifier and comparator is shown in Figure 3. Rpll is used to adjust the amplification factor of the system, and C06 is used to prevent the amplifier from self-excitation. With secondary amplification, the zero drift is not very obvious, about 0.1 V. Therefore, the threshold voltage of the comparator is designed to be 0.25 V to ensure that the interference signal is filtered out. The advantage of using a comparator is that it can effectively overcome the influence caused by zero drift and improve the accuracy of measurement.

4 Waveform Shaping

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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, D1 and T3 form an NOR gate. Only when both C and E are low, the output of the signal amplifier is high. The purpose of designing this circuit is to output a narrow pulse at the output end, 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, which is generally not expected to be too wide. The waveform shaping timing is shown in Figure 5.

5 Conclusion

When the amplifier is used in a cluster pulse meter, it is important to pay attention to the mutual influence between different signal channels. It is recommended to separate the power supplies of each amplifier. In addition, the measurement channel requires a switch circuit. When the finger clip is suspended, this switch circuit turns off the monostable circuit, cuts off the signal path, and prevents miscounting.

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

It is better to use two-stage amplification than three-stage amplification. The zero drift of some three-stage amplification circuit boards is large enough to reach full amplitude, making the measurement inaccurate. The amplification factor 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, and R07, R08 and R06 are usually metal film five-color ring resistors.

IClA, R02 and R03 form a voltage follower with a design value of 2.5 V. The accuracy is determined by R02 and R03. It is best to use metal film five-color ring resistors.

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

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