Abstract: Traditional calorimeters mainly rely on manual operation, which involves cumbersome and complex experimental procedures and low measurement efficiency. Therefore, the research on microcomputer-controlled automatic calorimeters has significant practical implications. This paper designs an automatic calorimeter with a high degree of automation, thereby greatly improving work efficiency. Keywords : Calorimeter, Automatic measurement, Field-Bus, Single - chip Accurate measurement of calorific value is crucial for energy extraction and efficient utilization. Based on calorific value, the amount of fuel used can be determined, achieving the goals of energy conservation and reduced production costs. With increasing awareness of energy measurement, the use of calorific value meters is receiving more and more attention. The application of calorific value measuring instruments has become widespread. To be suitable for measuring the calorific value of combustibles such as coal and petroleum in industries and sectors such as power, coal, papermaking, petrochemicals, cement, agriculture, animal husbandry, pharmaceutical research, and education, the system should be flexibly configured to adapt to different needs. 2. System Functions Based on actual usage needs, the system should meet the following technical specifications: 1. The measurement process should be automatically controlled by a microcomputer, and the system should have good scalability; 2. The system should be able to automatically test each component of the instrument, automatically identify the oxygen bomb, and automatically select the heat capacity; 3. The entire process of temperature measurement, ignition, stirring, data processing, process judgment, result printing, data storage, and top cover lifting should be automated during the experiment, and a self-diagnostic system should be provided to ensure normal instrument operation; 4. After the system is connected to an electronic balance with a standard communication port, manual weighing is not required, and the sample mass can be automatically input; 5. The calibration of the system's heat capacity, the calculation of the bomb's calorific value, and the conversion of the calorific value for different coal bases are also automatically completed by the computer based on the known sulfur or hydrogen content and moisture content input by the operator; 6. The system can be used for the measurement of the calorific value of coal and oil, and two measurement formulas are available: the Swiss formula and the Bent formula. 3. Hardware Circuit Design of Calorific Value Measurement System 3.1 Block Diagram of Calorific Value Measurement System To meet the requirements of automatic testing, the mechanical system needs to be modified. The schematic diagram of the new mechanical system is shown in Figure 1. The water circulation system is composed of 2, 3, 4, and 9. The pump and upper and lower valves are controlled by electronic circuits to complete the water inlet, filling, and draining. The measuring cup 4 is made into a special shape, and the volume method is used instead of the weighing method to complete the water filling process of the inner cylinder. The actual test shows that the water filling error of this method is less than 1g, which fully meets the national standard requirements, thus making automatic water filling possible. [align=center] Figure 1 Block Diagram of Calorific Value Measurement System 1. Outer cylinder 2. Pump 3. Upper valve 4. Measuring cup 5. Temperature sensor 6. Stirrer 7. Inner cylinder 8. Oxygen bomb 9. Lower valve[/align] 3.2 Hardware Block Diagram of Calorific Value Measurement System To realize the functions mentioned above, the automatic calorific value measurement system adopts a collaborative working mode of PC, main single-chip microcomputer 89052 and slave CPU 8902051. The division of functions is shown in Figure 2. Temperature measurement, ignition wire detection and control, fan stirring, and network interface are implemented by the 89052, while valve, inner cylinder control, and oxygen bomb identification are completed by the 8902051. The PC is responsible for sample weight sampling and data collection, processing, calculation, storage, report generation, and hard copy output of each 89052. The PC and the two microcontrollers coordinate the work process through command interaction via a CAN network. [align=center] Figure 2 Hardware principle block diagram of the calorific value measurement system[/align] 3.3 Temperature parameter measurement To achieve adjustment-free temperature measurement, the platinum resistance temperature measurement circuit adopts a dynamic calibration algorithm based on a reference resistor, so that the measurement accuracy is only related to the few resistors that make up the bridge. 3.3.1 Principle of temperature measurement circuit The platinum resistance temperature measurement circuit is shown in Figure 3. The bridge circuit from left to right consists of the measurement branch, calibration branch, and level offset branch (reference branch). Among them, R1 and Rt form the measurement branch, R0, RK, and RL form the calibration branch, and R3 and R4 form the reference branch. Rt is a platinum resistance thermometer, RL is the low-range calibration resistor, RK is the high-range calibration resistor, and R0, R1, RK, and RL are precision wire-wound resistors. The platinum resistance thermometer uses a 4-wire connection to reduce measurement errors caused by changes in ambient temperature. r0, r1, r2, and r3 are the input connection line resistances of the sensor. A is an amplifier, and ADC is an analog-to-digital converter. [align=center] Figure 3 Platinum Resistance Temperature Measurement Circuit[/align] 3.3.2 Measurement Branch The switching of the multiplexer S in the measurement circuit is achieved by a dual 4-to-1 analog switch 4052 (the two internal analog switches are electrically isolated and completely independent). The output of 4052 is directly connected to the input of the amplifier. Y0, Y1, and Y3 of the 4052 are all connected to the reference branch of the bridge. When the 4052 is connected to X0, the digital value corresponding to the output voltage is NL; when the 4052 is connected to X1, the digital value corresponding to the output voltage is NH; and when the 4052 is connected to X3, the digital value corresponding to the output voltage is Nt. Channel X2 is reserved for ignition wire detection. The measurement branch (i.e., channel X3 of the 4052) is connected to ground via a series resistor IR5 and a capacitor IC5 to eliminate noise introduced during measurement. 3.3.3 Measurement Amplifier The amplifier in the temperature measurement circuit uses the precision monolithic integrated measurement amplifier AD620 from Analog Devices, Inc. (ADI). The AD620 is a monolithic instrumentation amplifier improved from a typical three-op-amp structure. It is a complete differential or subtraction amplification system. Due to the precision laser trimming of the internal matching resistors, it has excellent linearity and common-mode rejection. It uses only one external resistor to set the gain, ranging from 2 to 1000 unity gain without requiring an external resistor. 3.3.4 The analog-to-digital converter (A/D converter) in the temperature measurement circuit of the A/D converter adopts the ICL7135 from Intersil Corporation, USA. The ICL7135 is a 4.5-bit dual-slope integrating A/D converter using all-MOS technology. With a unipolar reference voltage (VR = +1V), it can perform A/D conversion on bipolar input analog voltages and automatically output polarity discrimination signals and automatic range control signals. It employs self-zero calibration technology to ensure long-term stability of the zero point at room temperature, with a zero-point temperature coefficient <2μV/℃. The analog input can be a differential signal, with extremely high input impedance and zero-point leakage current <10pA. It uses a word-bit dynamic scanning BCD code output method. After one A/D conversion, its data output selector pulse output terminal (STB) outputs 5 negative pulses, selecting the BCD code data output from the high byte to the low byte. 3.4 System Detection and Control Module The essence of the ignition wire detection is to detect the resistance of the ignition wire connected to the circuit. When the ignition wire is short-circuited, the resistance is very small; when it is open-circuited, the resistance is relatively large; under normal conditions, the resistance is between the two values. Valve position detection uses a photoelectric detection circuit composed of LEDs and phototransistors. The opening and closing of valves and lids are controlled by a reversible geared motor; ignition, stirring, and fan operation are controlled by bidirectional thyristors. 3.5 Network Interface Module Due to the system's requirements for real-time performance and complex data processing capabilities, sending experimental data to a PC via a network is appropriate. The CAN bus is a fieldbus with multi-master communication capabilities, and its short-frame structure ensures short transmission time and low error rate, effectively meeting system requirements. Therefore, this system uses a CAN bus to build a distributed automatic testing network. Each calorimeter is connected to the bus via an SJA1000 and an 82C250, while the PC is connected to the bus via an RS232-CAN external converter card. 3.6 Data Processing Module The data processing module mainly consists of PC monitoring software. Data is transmitted from the calorimeter to the PC via the RS232-CAN external converter card through the CAN bus. The monitoring software uses a multi-threaded approach to constantly monitor the serial port. Once data is received, it utilizes the powerful data processing capabilities of the PC for calculation. Simultaneously, the software's interface dynamically displays experimental data as real-time curves and uses animations to indicate the current experimental stage. The main functions of the data processing module are: operating the serial port to send and receive commands and data, setting experimental parameters (including algorithm selection), calculating calorific value, plotting temperature curves, animating the experimental progress, managing experimental data, and printing curves and experimental results. 4. Calorific Value Measurement System Software Design: The A/D converter ICL7135 is connected to the microcontroller via interrupt, and the entire system operation is completed in the interrupt service routine of interrupt INT0. Before calculating the temperature value, the relevant parameters of the temperature measurement circuit must be measured. Therefore, before measuring the temperature, i.e., during the preparation period, the microcontroller connects to different positions in the circuit via an analog switch 4052, and the A/D conversion sequentially obtains NH and NL, which are then sent to the PC via the network. After Nt measured in the initial, main, and final stages is also sent to the PC, the PC software calculates the temperature value by solving the quadratic transmission equation of the platinum resistance sensor. Regardless of whether it's NH, NL, or Nt, the microcontroller treats them all as data in the same format and sends them to the PC. The PC software processes them differently depending on the stage of the experiment. The main program flowchart is shown in Figure 4: [align=center] Figure 4 Microcontroller Main Program Flowchart[/align] 5 Error Analysis The constant system error of the fixed output and gain of the microcomputer-based calorimeter does not cause measurement error in calorific value, but their stability affects the accuracy of calorific value measurement. The fixed output temperature drift coefficient and the unity gain temperature drift coefficient describe the magnitude of this influence. The nonlinearity of the instrument's transmission characteristics has the greatest impact on the accuracy of calorific value measurement. Therefore, a linearity verification experiment should be performed before using a new calorimeter, i.e., using standard substances of different masses for verification. If the range of the measurement results meets the technical requirements, it indicates that the instrument's linearity is good. Based on the analysis of the error, the following methods can be obtained to improve the accuracy of calorific value measurement: (1) Try to make the initial temperature of the inner cylinder when measuring calorific value and when calibrating heat capacity as close as possible, that is, both should be close to the ambient temperature; (2) Keep the internal temperature of the machine as constant as possible during the experiment; (3) According to the estimated value of the calorific value of the sample, appropriately control the amount so that the temperature rise of the experiment is close to the temperature rise when calibrating heat capacity. 6 Summary and Innovation of this paper Starting from the actual needs, this paper analyzes in detail the principle and method of calorific value measurement, and designs an automatic calorimeter with a high degree of automation, thereby greatly improving the work efficiency. In addition, according to the actual application requirements, multiple automatic calorimeters are assembled into a distributed test system using fieldbus (CAN bus) technology, which is uniformly controlled and managed by PC. Each calorimeter is independent of each other and does not interfere with each other. In order to realize the automation and intelligence of the entire measurement process, this paper designs powerful monitoring software for this measurement system. Finally, the error generated by the measurement using this system is analyzed, and the conclusion is that the nonlinearity of the transmission characteristics has the greatest impact on the accuracy of calorific value measurement. Several measures to improve the measurement accuracy are proposed based on the source of error. References: [1] Su Wen. A brief discussion on the development and current status of flow meters [J]. China Instrument and Meter, 2002.6 [2] 51 series single-chip microcomputer design examples [M]. 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