1 Introduction
Temperature is a common and important physical parameter in production processes and scientific experiments. In industrial production, in order to produce efficiently, the main parameters in the production process, such as temperature, pressure, flow, speed, etc., must be effectively controlled. Temperature control accounts for a considerable proportion of the production process. Accurate measurement and effective temperature control are the main conditions for quality, high yield, low consumption and safe production.
2 System Overview
The whole temperature control system is mainly composed of computer control system (host computer), single-chip measurement and control system (lower position machine), temperature sensor group, power heating system and other parts. The system adopts the modular design idea, the setting method is flexible, and the measurement point can be increased by using the combination of multiple MCU measurement and control systems, which has good scalability. The system structure block diagram is shown in Figure 1.
The temperature measurement uses the high-precision temperature sensor PT100 to obtain the current temperature of the object. After low-power consumption, low input offset voltage, and linear OP07A, the signal is amplified and sent to the 8051F350 internal high-speed 24-bit A/D converter, according to system settings. The target temperature (sent by the host computer) and the control range, through the 6-way PWM control heater's working condition, the object reaches the target temperature and maintains a constant temperature state. At the same time, the internal temperature of the microcontroller can be used to store the temperature and system parameters set by each channel. When the system is powered off or reset, it can continue to operate, enhancing the anti-jamming performance of the system.
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3.1 main control circuit
The main control circuit of the temperature acquisition monitoring system uses a high-performance and powerful 8051F350. The 8051F350 is a fully integrated mixed-signal system-on-chip (SoC) from Cygnal with a CIP-51 microcontroller core that is fully compatible with the MCS51 instruction set; the machine cycle is reduced from a standard 12 system clock to a system clock. Cycles, processing power is greatly improved, peak speeds up to 25 MI / s; internal integration of almost all analog and digital peripherals and other functional components (including PGA, ADC, DAC, voltage comparison) required to form a microcontroller data acquisition or control system , voltage reference, temperature sensor, SMBus/I2C, UART, SPI, timer, programmable counter/timer array, internal oscillator, watchdog timer, power monitor, etc.).
3.2 Temperature acquisition measurement circuit
The temperature acquisition measurement part adopts PT100 with high precision, repeatability and wide application as sampling resistance; the signal amplification part adopts low power consumption, low input offset voltage and linearity OP07A; A/D module adopts high speed 24-bit inside 8051F350 A/D converter. The temperature acquisition measurement circuit is shown in Figure 2. In the figure, PT100 is a high-precision temperature sensor, and Z1 is a 3.6 V voltage regulator for protection.
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The serial communication adopts the CAN bus with differential pressure transmission, which has the advantages of long transmission distance and strong common mode interference, and the communication rate can reach 1 Mb/s. The CAN bus communication interface integrates the physical layer and data link layer functions of the CAN protocol, and can complete the framing processing of communication data, including bit filling, data block coding, cyclic redundancy check, and priority discrimination. The data segment is up to 8 bytes long and meets the general requirements for control commands, operating conditions and test data in the general industrial field. At the same time, 8 bytes do not take up too long bus time, thus ensuring the real-time communication. The CAN protocol uses CRC checking and provides corresponding error handling functions to ensure the reliability of data communication. In the whole temperature measurement and control system, the CAN bus is used as the data communication line, and the temperature measurement module is installed in the range close to the measurement point, so that the wiring from the sensor is relatively short, thereby reducing the interference.
3.4 Power Control Module
P10～P15 output 6 PWM waves, control the input control voltage of the thyristor module through photoelectric isolation and RC filter circuit, change the conduction angle of the thyristor module, and thus change the output power. The power control module circuit is shown in Figure 3. P10 and P10 are one PMW respectively. The control voltages K0 and K1 are changed by changing their ratio of high and low levels to control the power of the heater.
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The software design of the whole system consists of two parts: computer software (host computer) and microcontroller software. Among them, the computer software mainly completes the set temperature, the current temperature of the monitoring system and the calibration system. The MCU software completes A/D acquisition, serial communication and power module control.
The PC software is written in LabWindows/CVI. It combines the powerful and flexible C language platform with the measurement and control professional tools for data acquisition analysis and display, and utilizes its integrated development environment and interactive programming methods. The function panel and rich library functions greatly enhance the functions of the C language, providing an ideal software development environment for building test software, automatic test environment, data acquisition system, process monitoring system and other application software. It can be run away from the Labwindows/CVI development environment, and the user finally sees an operation panel similar to the actual instrument panel. The board and the PC are connected via an RS-232 serial cable.
The block diagram of the lower computer is shown in Figure 4. After the system is powered on, the lower computer program can make the single chip continuously collect the temperature. When the upper computer sends a command to set the acquisition flag, the temperature collected by the lower computer is sent to the panel of the upper computer and displayed, and then the collected. The temperature data is compared with the preset temperature value. When the collected temperature is lower than the set temperature, according to the collected temperature value and the set temperature difference, the P10 to P15 pins of the single-chip microcomputer output 6-channel PWM. The input control voltage of the thyristor module is controlled by the photoelectric isolation TLP521-2 and the RC filter circuit, and the conduction angle of the thyristor module is changed, thereby changing the output power. As the collected temperature value is getting closer to the set temperature value, the PWM value of the single-chip output is getting larger and larger, the conduction angle is getting smaller and smaller, and the output power is correspondingly smaller, until the collected temperature is equal to the set temperature. When the conduction angle is completely turned off. At the same time, the lower computer can also respond to the serial port interrupt at any time, which is convenient for the user to set or modify the set temperature and control range through the upper computer.
5 Conclusion
The system design uses a high-precision temperature sensor and a low input offset voltage, a linear good operational amplifier to form a signal conditioning circuit, while using a 24-bit A / D acquisition module, the system measurement accuracy of 0.02 ° C, control accuracy of 0.5 ° C, Meet the user's requirements for temperature control. The powerful 8051F350 microcontroller is used as the control core, which reduces the system's requirements for peripheral devices, simplifies circuit design, improves reliability, and reduces costs.
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