Design of Environmental Equipment Control System Based on Man-machine Interface and PLC

introduction

Environmental test equipment is an automatic equipment with the function of simulating one or more comprehensive environmental climate according to different designs, which provides an efficient and reliable way for the realization of various environmental tests. In the process of product design, R & D and manufacturing, enterprises or institutions can conduct various environmental tests on materials, parts or product machines through environmental test equipment, such as high temperature, high humidity, salt fog, sand dust, rain, condensation and other environmental tests, so as to effectively verify whether materials or products meet the quality and reliability required by corresponding standards. Therefore, environmental test equipment is essential for most enterprises and institutions to verify the reliability of materials or products.

As the core component of the environmental test equipment, the control accuracy of the control system directly affects the accuracy and credibility of the environmental test done by the equipment. At present, the application types of environmental test equipment control system are mainly divided into general type and special type. The general control system refers to the combined control mode of traditional man-machine interface (HMI) and programmable controller (PLC), or the combined control mode of industrial PC configuration software and PLC; Special controller refers to the special controller developed for the control characteristics of different environmental test equipment. The field of domestic special-purpose environmental equipment controller is relatively blank compared with foreign countries, which is also the main reason why the accuracy and stability of domestic environmental test equipment are generally inferior to foreign environmental equipment. Therefore, the research and development of special-purpose controller is of great significance to the development of domestic environmental test equipment.

1. Design principle

The environmental equipment control system in this paper is mainly for temperature environmental test equipment, such as high temperature test chamber, high and low temperature test chamber, cold and heat shock test chamber, constant temperature and humidity chamber, damp and heat chamber, etc. The temperature environmental test equipment is usually composed of main box, heating system, refrigeration system, air circulation system and main control system, as shown in Figure 1.

The main control objects of the whole equipment include: refrigeration compressor, electronic expansion valve and solenoid valve to control cooling capacity emission in the refrigeration system; Centrifugal fan of air circulation system; Solid state relay and AC contactor in heating system.

The main measurement parameters of the whole equipment include: temperature and pressure of compressor exhaust and return gas in the refrigeration system, condenser outlet temperature, evaporator inlet and outlet temperature, compressor working current and voltage value; Fan temperature, working current and voltage in air circulation system; The working voltage and current value of the electric heater in the heating system; Temperature and humidity in the box.

Among the above measured parameters, some parameters are not directly related to the control process of the equipment, such as the working voltage and current of compressor and fan, fan speed and wind speed, etc. However, with the rapid development of modern science, technology and industrial information technology, equipment and systems in aerospace, aviation, industrial applications and other fields have higher and higher requirements for reliability, safety and economy, which makes fault prediction and health management (PHM) [3-4] gradually become one of the mainstream development directions of industrial equipment. However, the PHM system needs to be based on the comprehensive monitoring of the operation status of the equipment. When the general PLC control system is used, too many parameter acquisition means the increase of PLC module, which not only improves the manufacturing cost of the equipment, but also makes the volume of the equipment control system bloated. Therefore, this paper proposes a control system based on embedded system. By using embedded system to develop a scheme with high degree of freedom, low cost, strong pertinence, high real-time and high integration, the whole machine operation parameter monitoring of the equipment is realized; It is easier to realize complex algorithm operation and improve the control accuracy and stability of the equipment, such as fuzzy PID control algorithm of the equipment, anti pulse interference average filtering, amplitude limiting average filtering and other digital filtering algorithms.

2. Hardware design of control system

The control system is composed of controller and measurement and control module; The measurement and control module includes I / O module and measurement module, both of which adopt modular design, and carry out modular increase and decrease according to the configuration required by the equipment; The controller only needs to make corresponding software settings or adjustments for devices with different configurations. In this way, the operating parameters of the whole equipment can be collected at low cost; It can also improve the integration of the control system and reduce the volume of the control module. The hardware framework of the control system is shown in Figure 2. After the controller obtains the measurement module and collects the overall parameters of the equipment, it performs I / O and PID control output to the I / O module according to the control settings.

The communication mode between the controller, I / O module and measurement module is based on 485 interface Modbus protocol. Because MODBUS bus is widely used in many fields such as instruments and meters, intelligent high and low voltage appliances, transmitters, programmable controllers, man-machine interfaces, frequency converters, field intelligent devices and so on, it makes the controller, I / O module and measurement module have great scalability and independent possibility to become products.

2.1 controller hardware design

The embedded controller is a hardware development platform based on arm cortex-a9 four core microprocessor. It is mainly responsible for the operation of man-machine interface, logic operation, I / O and PID control in the control system. The exynos4412 processor used in the hardware platform has high-performance data processing capability and relatively complete hardware interface, which provides a good hardware foundation for the construction of Linux embedded system. The functional block diagram of the controller hardware development platform is shown in Figure 3, including onboard WiFi, 3G module, 10m / 100M adaptive network card, 10.1-inch touch LCD, 4-way USB host, etc.

The hardware platform supports starting from EMMC or SD card. EMMC is used to burn the system image. Therefore, after the controller is powered on, it starts from EMMC by default; The SD card startup function can cooperate with USB OTG to quickly upgrade firmware and system software. WiFi, WCDMA, 3G, LAN and other network interfaces are used to connect the controller to the Internet under different conditions, providing hardware support for new applications such as remote control of equipment, remote fault early warning or alarm, expert remote fault diagnosis and so on.

In order to ensure that the controller can realize high-speed, real-time and reliable communication with the measurement and control board, the design of two RS485 communication circuits of the controller is based on adm2483. Adm2483 is an RS485 transceiver with integrated communication isolation, and the maximum communication rate can reach 500kbps. On the premise of ensuring the communication rate and anti-interference ability, it avoids the situation that the optocoupler isolation design needs to occupy a large PCB layout area. Moreover, adm2483 adopts the swing rate limiting design to control the swing rate drop at an appropriate level, which can reduce the bit error caused by improper terminal matching and connector. The interface circuit of the communication module adopts the design of current and voltage limiting. As shown in Figure 5, the voltage stabilizing tubes D1 and D2 and the self recovery fuses Ptc1 and Ptc2 form an effective protection for the interface circuit and improve the electrical reliability of the 485 communication module.

2.2 hardware design of measurement and control module

The hardware block diagram of measurement and control module is shown in Figure 6. With lpc1758 as the core, it is responsible for equipment operation data acquisition, I / O address decoding and I / O control; The relevant temperature, current, voltage, humidity, pressure and other parameters of the equipment are stored in flash after digital filtering by lpc1758 after passing through the acquisition circuit; The controller can communicate with lpc1758 through RS485, read the required parameters for logic operation, and then send the I / O control command to lpc1758 for execution. I / O circuit includes transistor output, relay output and special application output, such as electronic expansion valve control I / O, frequency converter control I / O, etc.

Among them, the single channel 24 bit ADC max1210 of Maxim used in the filter sampling circuit. The ADC integrates analog and reference input buffer amplifiers and provides four GPIO ports, which can be used to control an external 16 channel analog switch, so that the max1210 can effectively collect 16 channel analog signals and reduce the I / O resource burden of lpc1758. The sampling circuit block diagram is shown in Figure 7.

Finally, in order to ensure the accuracy and reliability of I / O circuit, I / O state detection design is added to the hardware circuit. For the output point, the I / O state detection circuit generates the corresponding output sequence signal from the output state. When the command of the controller to change the output state is sent to lpc1758 and executed, the output point state changes, and lpc1758 compares the changed output sequence signal with the output command issued by the controller to ensure the accuracy of the output; For the input point, the corresponding input sequence signal is produced. When the input state changes, lpc1758 can judge whether there is an error in the input circuit by comparing the actual input state with the input sequence signal.

3. Control system software design

In order to meet the requirements of controller multi hardware interface, multi software program application development, multi file operation and system customization, Linux embedded operating system is adopted. The main application programs include man-machine interface program, data processing program and soft PLC program, as shown in Figure 7. The man-machine interface program is generated by the graphical software running in Windows environment through the combination of graphic elements, controls and macro commands. It can be imported into the controller through USB to update the man-machine interface. The data processing program is mainly responsible for equipment industrial control record, PID operation, equipment condition monitoring and other functions. The soft PLC program is developed by German infoteam openpcs software, supports six IEC languages: St, IL, SFC, FBD, LD and CFC, and is responsible for I / O logic operation.

Because the measurement and control module does not need too many application programs and graphical interfaces, UC / OS-II, which is much smaller than that of Linux embedded operating system, is selected. The system has rich functions, including task scheduling, task management, time management, memory management, communication and synchronization between tasks [6]. The main application programs are data acquisition program, I / O detection program and I / O control program. The control flow of the whole I / O is shown in Figure 9. In case of I / O circuit error alarm, the user can choose to power off and restart the equipment or request technical support.

4. Conclusion

The controller and measurement and control module of the embedded environmental test equipment control system designed in this paper have the advantages of low cost, high integration, high precision and strong expansibility. To a certain extent, it can improve the adverse impact of the characteristics of small batch and many varieties of environmental test equipment on design and production. By making full use of the advantages of environmental test equipment produced by our company, the reliability design of control system can be continuously improved through environmental test. The control system has been applied to our company's standardized series of environmental equipment, the control accuracy and stability have reached the domestic advanced level, and the hardware configuration is rich, with good expansion ability.

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