Design of dry-type transformer intelligent controller system using ATmega16 microcontroller

The analog conversion control circuit is used to convert the temperature analog quantity into an electrical signal that can be recognized by the single-chip microcomputer. The conversion principle is shown in Figure 2. When the temperature changes, the resistance value of PT100 will change linearly with the change of temperature. Its voltage division value is compared with a fixed circuit voltage division value, and the result is sent to the operational amplifier and converted into an analog quantity within the A/D conversion range. The
A/D conversion accuracy in ATmega16 is 10 bits. Since the reference voltage is 5V, the analog signal must be converted into a voltage of 0 to 5V. Therefore, when designing this circuit, the parameters of each component are designed according to this requirement. At the same time, its linearization must also be considered. In order to make the calculation in the software design linear, when designing the hardware, the temperature must be linearly changed with the digital quantity converted to the single-chip microcomputer. From the circuit, it can be seen that: From the formula
,
it can be seen that the obtained A/D conversion voltage is not proportional to Rw and does not meet the linear requirements. If it meets the requirements, the conversion voltage is approximately proportional to Rw and is also approximately proportional to the temperature. In this way, the temperature of any point can be calculated by linear calculation. However, using linearization to calculate this approximately linear graph will also bring about slight errors, which can be solved in software design.
1.2 Output circuit
The output circuit is the embodiment of the calculation and control results of the analog-to-digital conversion value by the single-chip microcomputer, as shown in Figure 3. The control quantity output by the single-chip microcomputer is input to the JK port. If this signal is low level, the optocoupler device is turned on, turning on the CMOS transistor, so that the relay is energized, the normally open contact is closed, and the 220V voltage is output; otherwise, the output voltage is 0V.

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The software adopts a modular structure, including 1 main module and 5 submodules (button processing submodule, digital quantum module for setting upper limit temperature and collecting boundary points, communication submodule, fault output processing submodule and display submodule). The main module completes the initialization of each submodule and calls the fault output processing submodule and display submodule. The button processing submodule, digital quantum module for setting upper limit temperature and collecting boundary points and communication module work in interrupt mode, and the main module communicates with them through a shared RAM area. Since the analog input signal of the single-chip microcomputer application system contains various noises and interferences, this program uses digital filtering technology to filter. In addition, for the linearization problem mentioned above, we divide 0~200℃ into four areas and perform linearization calculations in each area. This is much more accurate than directly calculating in the 0~200℃ area and can achieve an accuracy of 0.1℃.
The functions of each submodule are as follows:
(1) The button processing submodule applies for an interrupt to ATmega16 when a key is pressed, and modifies the pre-set flag in the interrupt subroutine. (2) Setting the upper limit temperature and collecting boundary points The digital quantum module can enter the interface for modifying the upper limit temperature by entering a password when the key is pressed for a long time. The digital quantities corresponding to 0℃, 50℃, 100℃, 150℃, and 200℃ are collected through the buttons, and the results are stored in the E2PROM. This data is used as the boundary point to calculate any temperature between 0 and 200℃. (3) The communication submodule can connect to the remote controlled object through the LBC184 (converts RS232 signals to RS485 signals) chip and the single-chip microcomputer for RS485 communication. (4) The fault output submodule can determine whether an abnormal situation occurs on site by comparing the actual temperature with the upper limit temperature. At the same time, the flag bit is set to determine whether to perform A/D conversion and whether to display. (5) The display submodule converts the results of the linear calculation from binary to BCD code and sends them to the 5-digit LED display for display. 