Distributed computer control system for wood drying

This paper introduces a design method of a distributed computer control system for wood drying with a single-chip microcomputer as the slave controller. The overall block diagram of the system, the hardware structure diagram of the slave, and the software structure diagram of the master and slave are given.
Keywords : distributed control system, single-chip microcomputer, wood drying

1 Introduction
Wood drying technology is a complex process involving many uncertain factors. Wood drying actually controls the moisture content of wood in the wood drying process by changing the temperature and humidity in the wood drying kiln, so that the moisture content of wood is slowly reduced according to certain process requirements to ensure the drying quality requirements of different wood species. Therefore, in the wood processing process, drying technology is an extremely important link. Since the 1980s, some foreign countries with relatively developed wood processing industries have also experienced a process of wood drying control technology from manual, semi-automatic, fully automatic and distributed wood drying computer control systems based on serial communication. Moreover, since the mid-1990s, the domestic market for high-end wood drying controllers has been almost monopolized by developed countries such as Germany and Italy. This article introduces a distributed wood drying computer control system suitable for China's national conditions and for medium and high-end applications. The system has been mass-produced and has achieved good social and economic benefits. 2 Overall block diagram of the system In order to ensure the monitoring and management of multiple wood drying kilns, the control scheme of this design adopts a distributed system composition structure. That is, each drying kiln is equipped with a slave machine that can work independently, and every ten slave machines are equipped with a host machine for centralized monitoring and management, and the master-slave management form is adopted. The overall structure of the system is shown in Figure 1.

The functions and relationships of each part of the system are as follows:
① The host uses Advantech industrial computer as the management computer. It completes parameter setting, data storage, processing and printing functions.
② The slave is the control machine, which uses the 51 series 77E58 single-chip microcomputer to realize the independent control function of each drying kiln. And it can independently realize all control functions under the condition of the host shutdown or failure.
③ The serial communication uses the RS-485 interface to realize one-to-many communication, mainly the host sends the control parameters of each stage of wood drying to the slave, and the slave completes the transmission of various signals and data collected on site to the host for centralized display, data processing and printing of the host.
④ The slave collects the input signals of sensors and transmitters from the input interface, collects the feedback signals of output devices, and realizes the monitoring of sensors and output devices.
⑤ The slave performs continuous or positional control of the temperature, humidity and other signals in the drying kiln through the output interface according to the set parameters and the data collected on site. 3 System working mode The control system uses the wood drying kiln as the monitoring center. The temperature, equilibrium moisture content, wood moisture content and control information and equipment operation status of the wood drying can be monitored and displayed for a single drying kiln or all ten drying kilns according to the user's requirements. The host can set the parameters of the slave. After the host software control module completes the parameter setting or modification confirmation of each drying kiln, the host will send the new parameters to the slave. Various control parameters can also be set through the keyboard of the slave, and the slave can work independently. The slave continuously collects signals such as temperature, equilibrium moisture content, wood moisture content, etc. through the input interface, and makes control decisions based on the setting parameters of the internal control module of the slave to drive the equipment to operate. When accessed by the host, the various data in the drying kiln and the equipment operation feedback information are transmitted to the host in serial mode. After receiving the data from each slave, the host processes the data and displays the current status information of each drying kiln on the monitoring screen, and stores this information in the database in real time so that users can check the historical data at any time. The data within a period of time can be compiled into a report or a graphical curve to realize the analysis and management of the data. 4 Hardware composition of the system Considering the harsh operating environment of the wood drying system, high temperature and humidity, unstable power grid voltage, and weak user maintenance capabilities, the host of the control system uses Advantech industrial computer and is equipped with an RS-232/485 conversion module to achieve communication with each slave. The slave part is based on the 51 series 77E58 microcontroller, and is composed of external data acquisition input circuit, status monitoring circuit, output control circuit, keyboard and LCD display. Figure 2 is the system structure diagram of the slave.

The slave is mainly responsible for data collection and control of the operation of the drying kiln. The microprocessor uses the 77E58 single-chip microcomputer mainly because it contains a 32K program memory, which not only meets the requirements of software capacity, but also simplifies the circuit design. As can be seen from Figure 2, all input and output parts of the system use isolation technology. Even if there is strong interference on site, the system can still work stably and reliably. The basic sensors configured on each slave are: 2-way temperature sensor using PT-100 platinum resistance; 2-way equilibrium moisture content sensor (self-made); 6-way wood moisture content detection. Among them, the accuracy of wood moisture content detection is closely related to the control accuracy and quality of the system. This system uses the resistance method to detect wood moisture content, that is, it uses the data of the corresponding relationship between wood moisture content and its resistivity, density, and temperature, and adopts the research results of the project "Development of Temperature Self-compensation Resistive Wood Moisture Content Measuring Instrument" (Provincial Science and Technology Commission 1994), and further optimizes the curve fitting algorithm of the original wood moisture content to better ensure the detection accuracy. The 6-way wood moisture content test is an independent transmitter unit, which is isolated and input to the single-chip microcomputer.
The actuators of the slave output control mainly include: electric servo valve (continuous adjustment) for kiln temperature control; steam injection valve and moisture exhaust valve (relay output position control mode) for controlling the balance moisture content; the fan control adopts an independent timing reversing control module, and the forward and reverse reversing time of the fan can be set. The status feedback of the control equipment mainly includes: fan working status feedback; steam injection valve and moisture exhaust valve working status feedback, etc. 5 Control system software The whole system adopts a distributed system structure and a master-slave system architecture that can monitor multiple wood drying kilns. Its software system consists of two relatively independent host software and slave software. The host software is written in VB language, and the slave software is written in assembly language. Figures 3 and 4 are the module structure diagrams of the host and slave software. The monitoring platform interface in Figure 3 realizes all centralized management and monitoring functions of the host through 6 sub-modules. The main control module in Figure 4 completes the real-time monitoring and management tasks through the monitoring module, human-computer interaction module and communication module, and all the work is broken down into alarm processing, control decision-making, information display, command processing, parameter setting and information output submodules. Since the wood drying kiln control system is a time-varying, large-lag, nonlinear controlled object, the fuzzy plus PID control method is used in the control decision method to achieve the expected control accuracy requirements.

6 Conclusion
Since the design project was approved in December 1999, it has been promoted and applied to more than 100 sets. Among them, Harbin Beihe Wood Industry has 6 sets, Qitaihe Shuangye Wood Industry has 26 sets, Shandong Penglai Global Wood Industry has 60 sets, etc. The long-term operation technical indicators of the system are: temperature control accuracy is ±1%; equilibrium moisture content control accuracy is ±2%; 6-point moisture content average detection error is ±2%. The promotion and use of this project has very important social and economic benefits in effectively reducing the labor intensity of workers, shortening the wood drying cycle, and improving the quality of China's wood and furniture.

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