Intelligent valve control system has gradually replaced the traditional mechanical valve control system, and is widely used in the field of modern industrial control. In order to further improve the control performance of the controller, a new intelligent valve controller is developed based on ARM architecture. The main components and key technologies of the controller are introduced. The controller uses high precision linear adjustable differential transformer as valve position sensor, and uses fast response software programming technology to improve the positioning accuracy and response speed of intelligent valve controller. The test results show that the instrument has the advantages of low power consumption and high reliability, which has important guiding significance for the development of related products. With the rapid development of industrial automation, micro-electronics technology and advanced algorithms are gradually applied to electric valve control. Valve control system begins to develop in the direction of intelligence, efficiency and accuracy, and equipment safety performance [1-4] has been improved. Intelligent valve controller based on ARM adopts hardware circuit which can automatically detect input and output short circuit and circuit breaking function. Software design highlights the idea of ultra-fast response. Optimal design is adopted as a whole, which greatly shortens the response time and reduces the total power consumption of the circuit [5]. The test data show that the time from the change of input instruction to the output of servo current is less than 20 ms, reaching the leading level in China. The new intelligent valve controller system consists of CPU module, power supply, signal detection (LVDT displacement sensor), feedback (output circuit short circuit and circuit breaking detection), output drive and human-computer interaction (upper computer and working lamp), etc. The system structure is shown in Figure 1. The intelligent valve controller manually inputs the valve control signal through the upper computer of the human-computer interaction module, obtains the DC or AC feedback input signal through the signal sampling circuit of the signal detection module, and then enters the CUP module after A/D processing. The output signal is obtained by PI algorithm adjustment, and the servo current output is given by the output driving module, and the CPU is also given by the output driving module. The module transmits the acquired valve indication signal to the upper computer. The power module uses external power supply, and generates 3.3V, 5V, – 15V and 15V voltage through the power conditioning circuit, which is used by CPU, peripheral hardware circuit and drive circuit.
The power module adopts integrated modulation technology, which not only reduces the static current consumption, but also realizes the function of automatically adjusting the output current value according to the load size [6-7]. The human-computer interaction module includes the upper computer and the LED indicator lamp.
It is a window to realize the human-computer information and data interaction. It can complete the functions of setting the initial values of variables and related parameters and real-time display. The hardware circuit of intelligent valve controller is composed of SVP channel card and SVP_CPU card. CPU card provides power access, power isolation and voltage reduction, 485 communication and other functions. As the main part of the circuit, the channel card provides the functions of the main control chip, the output and input circuits of each channel, program burning interface, jumper selection and status indication. The principle of hardware circuit is shown in Figure 2. AC/DC feedback circuit is mainly used to collect the valve position signal of LVDT, which is divided into six-wire AC voltage input signal and two-wire DC current input signal. Among them, the AC voltage input signal collected is relatively weak, which can only be sent to the CPU after rectification, filtering and compensation. For this reason, thermostatic element an AC LVDT feedback conditioning circuit is designed to process the input signal. AC LVDT feedback conditioning circuit, first collects AC input signal through AC full-wave rectification, then adds 5V level compensation, controls the voltage after rectification to 0-10V, then reduces the proportion to 0-2V. Finally, the reduced voltage value is differentiated into AD chip for signal sampling, and the data is exchanged between SPI communication and CORTEX-M3. The primary winding of LVDT must be supplied by AC voltage. The position change of the ferromagnetic core causes the difference of induced voltage between two secondary windings connected in reverse series at the same end to change [8]. The movement of the non-ferromagnetic rod can be determined by detecting the voltage difference. In order to drive LVDT to work properly, a six-wire LVDT excitation circuit is designed. The six-wire LVDT excitation circuit generates sinusoidal wave with a frequency of 1KHZ from the waveform generator chip, which is amplified by the amplifier chip, and finally generates an excitation voltage with an amplitude of <9V to drive the AC LVDT operation. Servo output circuit is used to output high current driving load. Firstly, the output voltage is generated by DA chip, then it is processed by conditioning circuit to positive and negative voltage. Finally, it is used to drive load by op-amp conditioning circuit. Servo output circuit is equipped with an output switch, which can quickly turn off the servo output current and protect the driver when the line breaks down. If the wiring is normal, there will be pulse waveform output when the signal reaches the optocoupler; if the circuit is short-circuited or broken, there will be no pulse output, so as to judge the wiring situation of the feedback circuit, which is the difficulty of the circuit design. In order to detect whether the connection of the servo output circuit is normal, the circuit for detecting the open and short circuit of the servo output current is also designed. This circuit connects the terminals of the servo output circuit to the circuit, reduces the amplitude in proportion by op-amp, improves by voltage compensation, and finally enters the voltage of the AD differential terminal. With this circuit, the voltage difference of the terminals can be detected in real time. Comparing the sampling voltage with the normal voltage, the circuit is judged to be open or short-circuit according to whether the sampling signal exceeds or falls below the normal voltage. In view of the safety of the controller, X5043 device is used to deal with the open-door dog, so that the single-chip computer can reset in time when the controller is in trouble, and take the output shutdown processing at the first time [9].
SUPMAX2000 intelligent valve controller program is mainly composed of system software and host computer software. The system software consists of management module, control module and channel module. The management module mainly completes the task of system monitoring and initialization. It also has the functions of setting and initialization of CPU, peripherals and peripheral A/D, D/A chips, system self-check, card self-calibration and card calibration. Control module is the core and key link of software design, which mainly completes the core tasks of data acquisition and processing, servo output and so on. Communication software includes SPI-compliant communication between bus and channel board CPU, I2C-compliant serial data communication between CPU and off-chip EEROM chip, UART communication in calibration mode, etc. [10-12].
Both of them work in master-slave mode and send and receive data by interruption mode, which not only ensures the real-time transmission of data, but also improves the working efficiency of CPU. Field application requires that the controller can be debugged and used normally. At the same time, it needs to adjust zero and full, and some devices need to be fine-tuned. For this reason, normal mode, debugging mode, self-tuning mode and fine-tuning output mode are set up in the main program. The main program flow is shown in Figure 3. The upper computer realizes the functions of parameter calibration and communication.
Parameter calibration is the key point of software design of host computer. Its working principle is that the current value of input or output is sent to host computer through serial communication. The current value is calibrated by host computer algorithm and then sent to SVP card. The SVP card receives new calibration parameters and converts data again. If the converted data has met the accuracy requirements, then calibration will be carried out. End, or continue the process.
After a certain number of retries, the calibration ends when the accuracy of the parameters meets the requirements, otherwise the SVP card can not be calibrated. The controller communicates with the host computer through RS485. The port number, baud rate and stop bit of RS485 communication need to be configured before communication. When communicating, the data of the controller is read and modified according to different instructions issued by the host computer, and the operation mode of the main program is switched. The state alarm of the controller is displayed on the interface of the host computer. The upper computer software flow chart is shown in Figure 4. In order to meet the needs of field equipment and speed up the operation of the system, the fast acquisition chip AD7190 is adopted, and the sampling time of the measured data is 2 ms. With the processing time of the parameters and the control time of the current output of the controller, the response time of the system from the input of control instructions to the change of the servo output is less than 20 ms, which is far ahead of the domestic level. The response time of this product is compared with that of similar products as shown in Table 1. In order to save hardware resources and controller costs, the A/D configuration is handled flexibly.
In the design process, the unnecessary channels are temporarily closed and the time-sensitive channels, such as polling for output wiring detection, are not strictly required. In the process of storing operation parameters into memory, many checks have been made, and no problems have occurred. Intelligent valve controller is one of the core components of modern process control system. It is of great strategic significance to develop new intelligent valve controller. The intelligent valve controller developed in this paper belongs to a practical new type product, which has excellent ultra-low power consumption performance and high self-detection ability. Its unique rapid response design not only improves the reliability of product operation, but also reduces the production cost, improves the market competitiveness of products, and promotes the domestic new intelligent valve control. The development of the appliance industry has played a positive role.