1 Introduction
Power supply technology, especially numerical control power supply technology, is a highly practical engineering technology that involves many disciplines such as electrical, electronics, system integration, control theory, and materials. The development of computer and communication technology has provided broad prospects for the development of power electronic technology, and at the same time put forward higher requirements for the power supply. Ordinary power supplies can no longer meet the needs of reality due to the shortcomings such as low accuracy. It was not until the emergence of single-chip technology and voltage conversion modules that the development of precision numerical control power supplies became possible. The numerical control current source designed in this article uses PIC16F877A single-chip microcomputer as the core component, and modules such as keyboard, display, D/A, and switching power supply are peripheral circuits.
2 Design requirements and overall design ideas
2.1 Design requirements
The design requirements: input 220V, output up to 12V; control the output current through the keyboard with a step length of 0.01A; use LED to display the output current with an accuracy of 0.02A; the current source steady current range is (0.2-1)A.
2.2 Overall design ideas
This design uses a switching power supply to meet the requirements of output range, accuracy and ripple. According to the system requirements, after adopting D/A conversion, the power amplifier formed by an operational amplifier is used to control the input of D/A, thereby controlling the current value. This system is mainly composed of numerical control part, power supply part and keyboard display circuit. The system principle block diagram is shown as in Fig. 1.
3 Hardware circuit design and software selection
According to the design requirements of the numerical control current source, the system is mainly composed of a control module, a power supply module, a D/A module and a keyboard display module.
3.1 Selection of control module
What this design adopts is PIC16F877A one-chip computer to control. Compared with AT89C51 single-chip microcomputer, PIC16F877A adopts Harvard structure, which can realize single-knuckleization of instructions, has simplified instruction set technology, simple addressing mode, strong I/O port drive capability, I2C and SPI serial bus ports, and peripheral circuits Concise, not only easy to develop, but also save the user's circuit board space and manufacturing costs. The program has strong confidentiality, low power consumption and wide voltage design, which can combine a considerable part of peripheral devices together, easy to use, and improved anti-interference performance.
3.2 Selection of power supply module
The power module generally mainly adopts a full-bridge rectifier plus a capacitor filter circuit, a three-terminal stabilized integrated circuit with an external expansion tube and a switching power supply circuit. The full-bridge rectifier plus capacitor filter circuit is widely used in some less demanding DC current sources. Its driving ability is related to the subsequent filter capacitor. The remarkable feature of this circuit is that it can better meet the transient response of the current. If the load requires continuous large current output, the circuit will be powerless. The external expansion tube of the three-terminal voltage regulator integrated circuit not only uses the good voltage regulation performance of the voltage regulator integrated block, but also has a certain current output. It is widely used in some high-precision linear regulated power supplies, but the effect is poor. The power devices of the switching power supply work in the on-off state, with low power loss and high efficiency. The volume of the matched radiator is greatly reduced, and the volume of the pulse transformer is much smaller than that of the power frequency transformer. Therefore, the current source using the switching power supply has the advantages of high efficiency, small size, and light weight.
Because this design has relatively high requirements for the power supply, it is especially reflected in the requirements for the power and ripple voltage of the power supply. Therefore, here is a switching power supply circuit.
3.3 Selection of D/A conversion module
TLC5615 is a product launched by Texas Instruments in 1999. It is a digital-to-analog converter with a serial interface. Its output is voltage type, and the maximum output voltage is twice the value of the reference voltage. With power-on reset function, that is, reset the DAC register to all zeros. It is a 10-bit high-precision D/A converter with serial input, so the final output voltage after conversion can reach 0V～10V. 10-bit D/A, the resolution is 1/2048, the sampling resistance is 15kΩ, and the resolution of D/A output can achieve a step of 0.01A.
3.4 Software selection
Protel 99 SE software design system is a set of EAD circuit integrated design software platform based on IBM PC compatible computer environment. It has the functions of circuit schematic design, PCB (printed circuit board) design, circuit hierarchical design, report production, circuit simulation and logic device design.
Microchip Company has equipped the PIC series single-chip microcomputer with a powerful software integrated development system Mp lab, which is a complete software package that integrates a variety of single-chip application development tools and software.
This text uses Protel 99 SE software design system to carry on the PCB board design, Mp lab carries on the system simulation.
4 MCU program realization of numerical control current source
The PIC16F877A microcontroller used in this article is a new product developed by Microchip. It has the function of FLASH programming and can directly perform operations such as suspending CPU execution and observing the contents of registers on the microcontroller. It is currently the most widely used PIC microcontroller.
The function to be realized by the MCU program is: the independent keyboard inputs data to the PIC16F877A MCU, and the PIC16F877A MCU processes the obtained data and sends it to the 10-digital-analog converter TLC5615 to control the current.
The C language programming is adopted here, and its advantages are high coding efficiency, intuitive software debugging, convenient maintenance and upgrading, high code repetition rate, and easy cross-platform code transplantation. The main program flow chart is shown as in Fig. 2.
5 System test
This design requires that the output current range is 0.2A-1A, and the voltage across the sampling resistor of the constant current source module is 200mV-2000mV. From the voltage value, the reference voltage of the digital-to-analog conversion module |Vref| is at least 2V (Vref
The test results listed in Table 1 show that the maximum error of the output of this design is when the input current is 32mA, the output current is 33mA, and the error is 1mA. In the question, the absolute value of the output current change is required to be ≤0.1%+1mA of the output current, which is 1.032, so the error value measured by the design has reached the error value specified by the design requirements.
6 Conclusion
The numerical control current source designed in this paper adopts PID algorithm to realize the functions of selectable range, adjustable output, precise stepping, and extremely small ripple current, and can simultaneously display the preset value of the output current and the measured value on the LED. The man-machine interface adopts an independent keyboard and LED display, the control interface is intuitive and simple, and has good man-machine interaction performance. It has the advantages of flexible control, convenient system upgrade, improved reliability of the control system, easy standardization, convenient system maintenance, good consistency, low cost, and convenient manufacturing.
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