Pulse measurement is a measurement that detects the presence or absence of a pulse. When there is a pulse, it blocks light, and when there is no pulse, it transmits light. The sensors used are infrared receiving diodes and infrared emitting diodes. The pulse measurement used for sports measurement generally has two modes: finger vein and ear vein. These two measurement methods have their own advantages and disadvantages. The pulse measurement is convenient and simple. However, because there are more sweat glands on the fingers, the finger clamps are used all the year round. The pollution may reduce the measurement sensitivity. The ear vein measurement is relatively clean, and the sensor uses environmental pollution. Less, easy to maintain. However, because the ear veins are weak, especially when the season changes, the measured signal is significantly affected by the ambient temperature, resulting in inaccurate measurement results.
Pulse signal picking
The pulse signal pickup circuit is shown in Figure 1. IC1A is connected as a unity gain buffer to generate a reference voltage of 2.5V.
The infrared receiving diode can generate electric energy under the illumination of infrared light, and a single diode can generate O.4 V voltage and 0.5 mA current. The BPW83 infrared receiving diode and the IR333 infrared emitting diode have a working wavelength of 940 nm. In the finger clip, the infrared receiving diode and the infrared emitting diode are placed opposite each other to obtain the best directivity. The larger the current in the infrared emitting diode, the smaller the emission angle, and the greater the intensity of the emission. In Figure 1, the RO selection of 100 Ω is based on the sensitivity of the infrared receiving diode to sense infrared light. When R0 is too large, the current through the infrared emitting diode is too small, and the BPW83 type infrared receiving diode cannot distinguish between the pulse and the pulseless signal. 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 with and without pulse. When the infrared light emitted by the infrared emitting diode directly illuminates 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 “Oâ€. When the finger is in the measurement position, two situations occur: one is the pulse-free period. Although the finger blocks the infrared light emitted by the infrared emitting diode, due to the dark current in the infrared receiving diode, a dark current of 1 μA causes 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 makes the finger translucency worse, the dark current in the infrared receiving diode decreases, and the Vi potential rises.
From this point of view, the so-called pulse signal pickup is actually obtained by the infrared receiving diode, the weak change of the dark current when there is a pulse and no pulse, and then amplified by IClB. The picked up signal is a voltage signal of about 2 μV.
Signal amplification
The low-pass amplifier is designed according to the calculation of the maximum pulse rate of the human body pulse after exercise for 240 times/minute, which is composed of IC2A and C04, as shown in Fig. 2. The corner frequency is determined by R07, C04, R08 and C05, and the magnification is determined by the ratio of R08 and R06.
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