Design of human body temperature measuring instrument based on pyroelectric sensor p7187

The pyroelectric infrared detector is a photoelectric detection device composed of a pyroelectric infrared sensor, a Fresnel lens and an electronic circuit. It can detect the infrared rays radiated by human body movement without contact and convert them into electrical signals for output. This article introduces the principle and basic design circuit of a p7187 pyroelectric sensor for rapid measurement of human body temperature.

1 Pyroelectric effect

The surface of some strong dielectric materials (PZT, LiTaO3, etc.) receives infrared radiation energy, causing temperature changes on the surface. As the temperature rises or falls, the charge on the surface of these materials changes. This phenomenon is called the pyroelectric effect. Figure 1 shows the movement of charge on the crystal surface as the temperature changes [1].

It can be seen that when infrared rays irradiate the pyroelectric element, its internal polarization changes greatly, and the changed part is released as charge. The charge taken out from the outside becomes the output voltage of the sensor. It can be seen that the pyroelectric sensor only has an output voltage when the temperature changes.

2 P7187 pyroelectric sensor equivalent circuit

Common pyroelectric sensors include P2613, P3782, P7187, etc. According to Faraday's law, the body temperature of the human body is about 37℃, and the wavelength of the most infrared radiation is about 10μm. P7187 is more sensitive to wavelengths in the range of 7-20μm. It uses two pyroelectric element PZT plates. The surface of the PZT plate absorbs infrared rays, and a pair of electrodes for taking out charges are installed inside and outside the light-receiving surface. It can sensitively capture the object or light source being measured and has high sensitivity. These two light-receiving electrodes are connected in reverse series, which can effectively prevent the influence of background fluctuations and false operations when interfering light is irradiated (one is the false operation caused by environmental changes, and the other is the false operation when using the light modulator) on the sensor. When the two light-receiving electrodes are irradiated with infrared rays at the same time, the output voltages cancel each other out and there is no output. Only when the human body moves, there is a voltage output. The output voltage reflects the movement of the human body more accurately.

3 Basic circuit and parameter selection of infrared thermometer measurement system

3.1 Circuit Configuration

After amplification, frequency selection and filtering, the sensor output signal is added and corrected with the room temperature measurement element output, and finally an output voltage proportional to the temperature of the object being measured is obtained. [page]

3.2 Parameter Selection

The signal output by the sensor is coupled to the in-phase amplifier A1 through a 47μF capacitor, and the closed-loop gain of A1 is between 23 and 24. At the same time, A1 also acts as a high-pass filter with a cutoff frequency of fL = 0.3 Hz.

A2 is a low-pass filter with a closed-loop gain of approximately 1 and a cut-off frequency of fH = 7 Hz.

A1 and A2 filter out the signals below 0.3 Hz and above 7 Hz respectively, so that the output signal is only the 1 Hz infrared radiation signal modulated by the modulator.

The temperature compensation part is composed of a temperature-sensitive diode and an operational amplifier A4. It detects the temperature Ta of the modulator and uses the nonlinearity of the temperature-sensitive diode for temperature compensation.

According to the Stefan-Boltzmann law, when the temperature of the modulator device is T0 and the temperature of the measured object is T0, the output voltage of the infrared sensor is:

To obtain a voltage V proportional to the absolute temperature of the object to be measured, the signal should be added to the above formula for compensation. V(Ta) is provided by the temperature compensation circuit. The temperature compensation curve can be approximately regarded as a fourth-order curve. This process will be completed in adder A3. The function of A3 is to add the signal voltage to the output of the temperature correction part.

4 Conclusion

The infrared thermometer has a sensitivity of 20 mV/℃, a resolution better than 0.2℃, and an error within 0.3℃. It has the advantage of non-contact rapid temperature measurement, which provides an important means to reduce mutual infection. However, it is only suitable for non-contact temperature measurement occasions where the distance between the measured object and the sensor unit is about 10 cm, and the surface temperature of the human body is not only related to the human body temperature, but also affected by many factors such as blood circulation under the body surface, thermal conductivity and surface heat exchange conditions. The thermometer measures the temperature of the human body surface, and the measured person should be required to stay in the measurement environment for a long enough time so that the surface heat exchange conditions of the measured person are the same or similar.