Development of a refrigerant central control system based on a single chip microcomputer

       Introduction

       The refrigeration system has become an important configuration of modern automobiles. The amount of refrigerant and the purity of refrigerant in the refrigeration system are the key factors affecting the refrigeration effect. It is necessary to carry out system inspection and refrigerant replacement on schedule. The refrigerant replacement process is divided into four steps: refrigerant recovery, vacuuming, refrigeration oil injection and refrigerant filling. At present, the means of repair and maintenance of automobile refrigeration systems in China are mostly manual operations. The work process is complicated and inefficient, and the system is often damaged due to improper operation.

       The author has completed the development of automated equipment for automobile refrigeration system maintenance, which effectively improves the maintenance efficiency and maintenance quality, realizes the full automatic operation of refrigerant replacement in automobile refrigeration system, is easy to use and has a friendly interface.

       The fully automatic refrigerant filling control system uses the 16-bit single-chip microcomputer MSP430F149 as the core controller, uses the new CPLD device EPF10K10A of ATERA, and realizes external storage data encryption. The whole machine is fully functional and highly reliable.

       2 System structure and control process

       The structure of the fully automatic refrigerant filling system is mainly composed of an embedded control system, an electronic scale, a vacuum pump, a compressor, multiple state detection sensors, an execution switch and a solenoid valve. As shown in Figure 1.

                                                           Figure 1 Refrigerant filling system structure diagram

       The workflow of the automatic control of the refrigerant filling system includes eight items: vehicle information input, filling information query, refrigerant identification and recovery, vacuuming, adding refrigeration oil, adding refrigerant and result printing, as shown in Figure 2.

       (1) Vehicle information input After the refrigerant pipeline of the vehicle to be inspected is correctly connected to the refrigerant filling system, the operator enters the parameters such as the manufacturer, vehicle model, year and month information of the vehicle through the operation panel keyboard. [page]

       (2) Filling information query The control system searches for the refrigerant type, refrigeration oil filling amount, refrigerant filling amount and other control parameters corresponding to the vehicle in the vehicle filling information database based on the input vehicle information.

       (3) Refrigerant identification Send a control signal to start the refrigerant analyzer, and the refrigerant analyzer extracts the vehicle refrigerant sample for type analysis, and submits the identification result to the control system. It is required that the refrigerant type in the refrigerant filling system storage tank is the same as that of the refrigerant vehicle to be filled, otherwise the system will give an alarm prompt, replace the refrigerant storage tank of the same type, and continue the next step.

       (4) Refrigerant recovery Control the corresponding actuator, rely on the pressure in the vehicle refrigerant pipeline itself to recover the refrigerant, and the recovered refrigerant is introduced into the refrigerant storage tank after purification.

       (5) After the vacuum recovery pressure is balanced, control to open the vacuum pump to extract the residual refrigerant gas in the vehicle refrigerant pipeline until it is close to zero pressure. Turn off the vacuum pump and let the vacuum sensor monitor the pressure of the vehicle refrigerant storage chamber. The pressure value changes within a certain period of time to detect whether there is a leak in the refrigerant pipeline. If there is no leak, proceed to the next step. Otherwise, the refrigerant pipeline needs to be repaired first to ensure that the refrigerant pipeline is well sealed.

       (6) Add refrigerant oil According to the control process, add a certain amount of refrigerant oil corresponding to the vehicle to the vehicle refrigeration system.

       (7) Refrigerant filling Add the corresponding amount of the same type of refrigerant to the vehicle.

       (8) Print the filling result The micro printer configured in the system prints the relevant items and parameters of this "filling work" and submits it to the user and archives it. At this point, the whole process of refrigerant filling is completed.
Based on the above system structure and control process, the design of the fully automatic automobile refrigerant filling control system focuses on studying and solving four problems: first, the system works stably and reliably and has strong anti-interference ability; second, the human-machine interface design is friendly, intuitive and easy to operate; third, the embedded vehicle refrigerant filling information database system is realized; fourth, a high-precision electronic scale is developed to ensure the accurate measurement of the refrigerant weight and realize the accurate control of the refrigerant filling amount.

       3 Hardware System Design

       The refrigerant filling system is a "car maintenance" device with a relatively harsh working environment and large electromagnetic interference. In line with the design goals of stability and reliability, simple circuits, and complete functions and novel technology, the hardware system structure of the fully automatic automobile refrigerant filling control system is shown in Figure 3.

       The main components are: MSP430F149 single-chip microcomputer, complex programmable logic device (CPLD), pressure sensor and its conditioning circuit, serial interface LCD display and refrigerant analyzer, keyboard, micro printer, switch input and output control, parallel port E2PROM, etc.

       (1) MSP430F149 single-chip microcomputer M

SP430F149 is the control center of the fully automatic automobile refrigerant filling system. It completes the processing of sensor data results, the detection of switch signals, the recognition of keyboards, the management of the two serial port communications of USART0 and USART1, and the communication and control with CPLD devices. The MSP430F149 microcontroller of Texas Instruments (TI) is a FLASH type microcontroller with built-in 60KB+256 bytes of FLASH and 2KB RAM. The refrigerant filling system uses a liquid crystal display interface. The program contains a large number of interfaces, menus, and animation data. The 60KB FLASH ROM in the MSP430F149 meets the system program's demand for storage space; the 2KB internal RAM ensures temporary data such as data acquisition, processing, and display, as well as a large number of system global variables; the rich I/O resources realize the expansion of the keyboard and the input detection of switch signals, as well as the communication and control of CPLD devices.

       (2) CPLD device expansion The microcontroller is connected to the external memory X28C512, 8-way solid-state relays, and micro printer through the CPLD device. Among them, X28C512 is a 64K parallel port E2PROM, which is used to store the vehicle filling information database. The 8-way solid-state relay sends out drive output signals to control each actuator. The CPLD device uses ALTERA's EPF10K10A, which solves the interface problem between the 3V microcontroller and the 5V working device; the port drive capability of EPF10K10A meets the solid-state relay's requirements for drive current; it realizes the expansion of the microcontroller port line and simplifies the circuit; EPF10K10A contains 6144 Bits RAM, which can realize data encryption of the filling information database.

       (3) Pressure sensor and its conditioning circuit The refrigerant filling system uses a strain gauge pressure sensor to realize the electronic scale function. The sensor detects the weight of the refrigerant tank on the platform and the refrigerant inside it, and controls the refrigerant filling process accordingly, which is expressed by the following formula: Q = (q1+q0) - (q2+q0) In the formula: Q is the amount of refrigerant already filled, q0 is the weight of the refrigerant tank, q1 is the weight of the refrigerant before filling, and q2 is the current weight of the refrigerant.

       The sensor outputs a millivolt strain signal, which is sent to the analog-to-digital conversion chip ICL7135 for A/D conversion after amplification, zeroing, filtering, and other conditioning. The output digital signal is sent to the single-chip microcomputer to complete the pressure signal detection. The single-chip microcomputer performs a table lookup based on the value range of the processing result, realizes segmented linearization, and corresponds the sensor signal to the refrigerant weight. The electronic scale implemented by the refrigerant filling system has a range of 0 to 50 kilograms and an accuracy of 10 grams.

       (4) Serial communication The MSP430F149 microcontroller has two serial communication ports, USART0 and USART1. The refrigerant filling system is connected to three serial communication devices, including the intelligent LCD with serial interface, the refrigerant analyzer and the host computer. The refrigerant analyzer and the host computer reuse the USART1 communication port. The RS232 interface chip uses the 3V-5V compatible MAX3232.

       (5) Keyboard and switch quantity detection are implemented using the P1 and P2 ports of the MSP430F149. The P1 and P2 ports have interrupt functions and can capture trigger events in real time.

       4 Software system design

       The software of the fully automatic automobile refrigerant filling control system includes monitoring programs, interrupt programs, and data processing programs. The main function of the monitoring program is to respond to user service requests in a timely manner, effectively manage the equipment's own hardware and software and human-machine integrated contact equipment, and provide corresponding processing information when the system fails. It mainly includes: system initialization management, keyboard management, display management, communication management, management of various functional modules of the instrument, interrupt management, etc. The interrupt sources used in this system are: serial port interrupt, external interrupt, software timer interrupt. The functions realized by the refrigerant filling control system are: automatic filling, manual filling, electronic scale measurement, filling information database management, system settings, as shown in Figure 4. [page]

       (1) In the automatic filling mode, the refrigerant filling system automatically completes the entire refrigerant filling process; (2) In the manual filling mode, the operator can choose to complete one or more operations of refrigerant identification, recovery, vacuuming, adding refrigeration oil, and filling refrigerant; (3) The electronic scale function can detect the mass of objects on the weighing platform; (4) The data management function realizes the maintenance of the vehicle filling information database, including data query, data browsing, record addition, modification and deletion. In addition, the refrigerant filling system also provides a database update function, which is realized by communicating with the host computer through the control system and writing to the storage chip X28C512 under the control of the single-chip microcomputer; (5) The system setting function includes equipment information setting, user information setting, refrigerant information setting, operating parameter setting, etc.

       In the human-machine interface control of the refrigerant filling system, the software realizes various LCD display function modules and input method programs, which can input numbers, English and Chinese characters. These basic modules are called in the human-machine interface operation. 

       5 Conclusion

       The refrigerant filling control system developed in this paper realizes the fully automatic control of automobile refrigerant filling, improves the maintenance efficiency and repair quality of automobile refrigeration system, and the control system completes various functions and reaches various accuracy indicators with stable performance, high reliability and simple and convenient equipment operation.

参考文献
[1]. MSP430F149 datasheet http://www.dzsc.com/datasheet/MSP430F149.html.
[2]. CPLD datasheet http://www.dzsc.com/datasheet/CPLD_1136600.html.
[3]. EPF10K10A datasheet http://www.dzsc.com/datasheet/EPF10K10A_.
[4]. X28C512 datasheet http://www.dzsc.com/datasheet/X28C512_904921.html.
[5]. ICL7135 datasheet http://www.dzsc.com/datasheet/ICL7135_225194.html.
[6]. RS232 datasheet http://www.dzsc.com/datasheet/RS232_585128.html.
[7]. MAX3232 datasheet http://www.dzsc.com/datasheet/MAX3232_1107769.html.
[8]. MSP430 datasheet http://www.dzsc.com/datasheet/MSP430_490166.html.