Composition and design of thermal flow sensor signal acquisition system

　　Heat flow, also known as heat flux density, refers to the amount of heat (vector) transferred per unit area per unit time. Its characterization is the amount of heat transferred, the size and direction of the transfer, and a heat flow sensor (heat flow meter or heat flow meter) is a component that measures the size of the heat flow. There are many methods for testing heat flow, including the transient method and the water card method. The working principle of the transient method heat flow sensor is to generate a voltage that has a certain relationship with the heat flow value through the thermoelectric effect. The Gardon type heat flow sensor is a transient method measurement.

　　When studying the Gardon type heat flux sensor, it was found that the output of the heat flux sensor is related to the temperature of its heat sink. When the temperature of the heat sink rises to 250°C, the output of the sensor is nonlinear with the heat flux density. Studying the relationship between the temperature of the heat sink and the output of the sensor helps to expand the test time of the sensor. The main purpose of the system design is to collect the temperature signal of the heat sink while collecting the sensor signal.

　　2 System composition and design

　　The acquisition system has two groups of signals: one is the voltage value corresponding to the heat flux value, that is, the measurement signal; the other is the voltage value output by the thermocouple that monitors the temperature, that is, the monitoring signal. Different methods are used to collect and process the two groups of signals: the test signal uses a dedicated amplifier and filter; the monitoring signal uses a dedicated processing device for the thermocouple. In this way, the acquisition system is fully functional, small in size, and easy to use.

　　The acquisition system needs to communicate with the host computer, and the host computer needs to control the working status of the acquisition card (start working, stop working, reading, etc.). Therefore, the interface between the acquisition system and the host computer uses the RS-232 serial port. The system composition block diagram is shown in Figure 1.

　　The entire test system includes test signal conditioning, temperature signal processing, control, storage, communication, power supply and other modules. The following mainly introduces the test signal conditioning and temperature signal processing modules.

　　2.1 Measurement signal conditioning module

　　The output thermoelectric potential of the Gardon type heat flow sensor is proportional to the incident heat flow. Since the actual output thermoelectric potential is relatively small, it may not meet the sampling requirements of the single-chip microcomputer. Therefore, the signal must be amplified and the center level must be adjusted. The heat flow sensor analog signal can only enter the A/D converter for analog-to-digital conversion after amplification, filtering and other processing. The test signal conditioning circuit is shown in Figure 2.

　　After the analog signal generated by the heat flow sensor passes through the test signal conditioning circuit in Figure 2, the signal amplification and filtering performance is good, and the distortion rate is small; and the use of the MAX291 filter can increase the reliability and stability of the signal. Avoid the various errors and drift effects of discrete components. Figure 3 is a comparison of the effects before and after the conditioning circuit filtering.

　　2.2 Temperature signal processing module

　　The monitored temperature signal is the output signal of the K-type thermocouple. The thermocouple signal conditioning circuit includes cold end compensation, linearization, and amplification circuits. The conditioned thermocouple signal is then sent to the A/D converter for analog-to-digital conversion. According to the data, the design and debugging of these three parts of the circuit are too difficult and occupy a large volume, so they are not the first choice for design. Therefore, a dedicated device MAX6675 is used for the conditioning and acquisition of thermocouple signals. MAX6675 is a serial K-type thermocouple analog-to-digital converter with cold end compensation, linear correction, and thermocouple disconnection detection produced by Maxim. Its temperature resolution is 0.25℃, the cold end compensation range is -20℃~+80℃, and the operating voltage is 3.0~5.5V. This device not only greatly simplifies the circuit design and reduces the size of the temperature signal processing module, but also can directly convert the analog signal into a 12-bit digital quantity, and the interface is simple, using the SPI serial interface, which is convenient for communication with the microcontroller. The device uses SO-8 packaging, which is small in size and high in reliability. Figure 4 shows the output waveform and timing of the MAX6675 controlled by a single-chip microcomputer. [page]

　　3 Software Control

　　The C8051 microcontroller is used as the controller. It controls the acquisition of measurement signals and the interface with the MAX6675, and exchanges data with the host computer through the RS-232 serial port. The host computer sends commands to the acquisition system through the serial port to control its working status. At the same time, the acquisition system also has a 1 GB Flash to realize the data storage function. The program flow of the system is shown in Figure 5.

　　4 Experimental Results

　　After the system is powered on, the host computer performs relevant operations on the system, the system starts the control of MAX6675, and stores relevant data. Figure 6 shows the relevant data of a test experiment.

　　This set of data can be used with relevant drawing software to obtain a curve of the heat sink temperature during the experiment, as shown in Figure 7. The test time is 1200 s.

　　5 Conclusion

　　The heat flow sensor acquisition system can complete the proposed design task well. The system volume is reduced due to the integration concept. In terms of monitoring the temperature of the heat flow sensor, the system has high accuracy, reproduces the temperature changes during the operation of the sensor, and corrects the sensor output by monitoring the temperature, thereby extending the sensor test time.