Through comparison and program demonstration, this paper determines a new type of network-based temperature and humidity monitoring system, that is, the collected measurement results of one or more temperature and humidity sensors are dynamically released in the form of a web page through Internet, and at the same time, any one of the host computers All can issue instructions to the temperature and humidity control device through the network according to the authorization of the administrator to control any node. For this solution, any PC that can be connected to Internet can realize the monitoring and management of the working status of each sensor, and realize the measurement and control network without distance limitation, which is very convenient and flexible.
1 Overall system structure
The overall structure of the system is shown in Figure 1. Each temperature and humidity test control module has a network interface and is connected to a central server through a router. The central server includes the IIS information server system and the SQL database server. The former is responsible for communicating with the online temperature and humidity test node to obtain the temperature and humidity measurement results, and then broadcasts the received temperature and humidity detection data to the customer through the Internet through the website. The received data (test node number, temperature, humidity, measurement time, etc.) are stored in the SOL database. At the same time, the customer can send commands to each test control module through the central server according to different permissions to change a certain temperature and humidity control parameter.
The structure of the temperature and humidity test module is shown in Figure 2. Its structure includes: main controller (MCu), temperature and humidity sensor, digital display and keys, Flash memory, network transmission module, clock device, buzzer. The test control process is: temperature and humidity measurement is performed at regular intervals. If connected to the central server, the results are uploaded to the central server through the network, otherwise the results and current time are stored in the flash on the board; then the humidity measurement The value is compared with the user set value, and different processing is performed according to the result of the comparison. For example, when the test humidity is higher than the set value, the humidity can be reduced by controlling the dehumidification material to absorb moisture. If the set humidity value is reached, the humidity in the cabinet is kept within the set range through a certain feedback control. If the temperature or humidity is higher than the dangerous threshold, the system will alarm through the buzzer.
2 Temperature and humidity test control module
2.1 Selection of temperature and humidity sensors
Since the accuracy of temperature and humidity measurement is the key to determining the performance of the system, here, the digital temperature and humidity sensor SHl75 produced by Swiss Scnsirion is selected to achieve temperature and humidity measurement. The characteristics of the device are: before leaving the factory, each sensor has been accurately calibrated in a very accurate humidity chamber, the calibration coefficients are compiled into corresponding programs and stored in the calibration memory; highly integrated, temperature measurement and humidity measurement are integrated inside , Signal conversion, A / D conversion, heater and other functions; provide two-wire digital serial interface SCK and DATA, simple interface, support CRC transmission check, high transmission reliability; programmable adjustment of measurement accuracy, built-in A / D converter (The resolution is 8 to 12 bits, which can be selected by programming its internal register). The measurement accuracy is high. Due to the integrated temperature and humidity sensor, it can provide temperature compensated humidity measurement values ​​and high-quality dew point calculation functions. Humidity The accuracy is within ± 4%, and the temperature accuracy is within ± 0.4 ° C.
2.2 Test data storage scheme
When the temperature and humidity detection control module is not connected to the control host, the detection data needs to be saved for inspection. Therefore, the system selects SST25VF016B with a storage capacity of 16 Mb, which belongs to NAND type non-volatile flash memory. The memory has the characteristics of fast speed, low cost, and high density. It is widely used in sensor measurement systems. Its read / write operations are in bytes, and the erase operation is in blocks. Read, write, and erase Except for the operation is completed by the command; can not be byte erased, you need to erase an entire block before each rewrite operation; each block is erased a limited number of times, about 100,000 times. According to the needs of the type of data stored in the system, Flash is divided into 2 parts: a part (2 blocks, each 32 kh) stores the system information of the control block, including number, temperature correction information, humidity correction information, humidity control range, etc .; One part (62 blocks, 32 kb each) stores real-time temperature measurement results, including measurement time, temperature, humidity, and whether the record is uploaded. The control algorithm of Fhash erasing and writing is: when starting up, the system reads the system information to obtain various control parameters; then finds the last storage location of the last temperature and humidity record and the usage of each block, and continues to write new measurement information . After the current record block is full, update the block usage record in the system information and find a new block to write. If the stored information reaches the system capacity, select the earliest block of information stored in 62 blocks to erase and reuse it. This design refers to the u-cos file management system, but because the length of the sensor measurement record is fixed, the read and write algorithm of each record is simplified.
2.3 Humidity control algorithm
For the monitoring and control of temperature and humidity, it is difficult to achieve the ideal if only through simple feedback control (that is, the humidity is higher than the set value, the moisture absorption device absorbs moisture, and the moisture will be discharged after saturation, so repeatedly to achieve humidity control) The reason for this is that the operating curves of the temperature control device and the moisture absorption device are usually non-linear, which makes the estimation of the control time uncertain. After repeated research and experiments, this paper uses fuzzy control method in temperature and humidity detection and control system to obtain better results. That is, according to the temperature and humidity measurement values ​​within a period of time in the monitoring node, a fuzzy estimate of the capacity of the moisture absorption device and the temperature control device (divided into 5 categories: strong, strong, medium, weak, weak) is obtained, and according to the humidity and the setting range To obtain the fuzzy estimate of the current humidity value (much greater than, close to, within range, below, far below), and then control the actions of external solid-state relays according to the set fuzzy control rules to achieve good control of temperature and humidity .
2. 4 network interface realization
The network connection between the temperature and humidity detection control module and the central server is realized by Conextop's NePort network serial port conversion module. Its volume is about two standard RJ45 sockets. It has a 10 M / 100 M Ethernet interface and can simultaneously transmit TCP / UDP. Package to multiple data receiving devices, providing 1 to 2 high-speed serial ports, baud rate up to 921 600 b / s. The MCU is connected to NePort through a standard serial port, and NePort is connected to a central server through a router. Each temperature and humidity monitoring system has a unique system code and is connected to the central server through a certain command format. The main functions include: upload temperature and humidity measurement information every 5 s; synchronize the temperature and humidity monitoring system clock and server clock every day; set the temperature and humidity correction information and humidity control information through the network.
3 Design of network monitoring system
3.1 Central server
The central server is equipped with IIS information server and SQL database server, using ASP. The net language realizes the temperature and humidity monitoring system and network service of B / S structure. The advantage of the B / S structure is that any computer connected to the Internet in the system can become a monitoring client, and monitoring and management can be achieved through the IE browser without installing any software. The main functions achieved are: registering and managing each temperature and humidity monitoring system; storing the temperature and humidity data of each temperature and humidity monitoring system into the database and being able to consult and delete it, and generating historical curves and generating reports, etc .; reporting to Internet users through the web page Real-time temperature and humidity measurement information; user registration and authority management, etc.
3.2 AJAX realizes asynchronous transmission of temperature and humidity data
In the traditional B / S structure, when the user needs certain information, the user sends a request to the server through the browser (for example, clicks on a link), and the server sends the webpage pointed by the link to the user-side browser. This process must have User participation. For the sensor network monitoring system, the sensor measurement data is updated in real time, and the new data can only be sent to the browser automatically by the server, instead of the click update method. In addition, the traditional web page transmission process is to completely complete the content of a web page. The server transmits to the client browser, but each time the sensor's network monitoring page is updated is only part of the new measurement data, and the monitoring background screen and the parameter information of each machine are fixed. If the traditional synchronous update method is adopted, a large amount of repeated content will be transmitted, which reduces the response speed of the system. This system selected AJAX technology to overcome these problems.
AJAX technology uses an asynchronous update scheme, which allows the browser to update part of the information in the web page in this data transmission scheme. And only need to update the sensor measurement information every time, reducing the burden of network communication and achieving fast refresh. The working principle of AJAX is equivalent to adding an intermediate layer between the user and the server to make the user operation and the server response asynchronous. In this way, some of the burden of the previous server is transferred to the client, which is beneficial to the idle processing capacity of the client to deal with, reducing the burden on the server and bandwidth, so as to achieve the purpose of saving bandwidth.
The system uses traditional HTMLl static pages to draw out the static background on the page implementing temperature and humidity monitoring, the static content such as the parameters of each temperature and humidity monitoring node, and the background uses JavaSc-ript to create the XMLHttpRequest class to periodically send an HTTP request to the server. It is required to transmit the current temperature and humidity measurement results. After receiving the request, the server obtains the measurement information of each sensor through a web service (Web Service), and returns the result to the requested page. The network transmits only the measurement data that needs to be updated, so the response speed is fast and no user is required participate.
3. 3 VML realizes the temperature and humidity data display
When the browser receives the measurement data returned from the server, it must display it in a clear and friendly interface. In order to reduce the burden on the server, drawing charts (traditional ASP.NET chart implementation method) on the server side should be avoided and should be implemented on the browser side. There are also many solutions implemented, such as AcTIveX, Java applet, SVG, VML, etc. AcTIveX and Java applet are dynamic and interactive multimedia webpages that insert binary components on the webpage, but the binary code is a security threat to the browser, so the general AcTIveX or Java applet needs to be digitally signed before it can be used by users Accepted; In addition, changes in the version of the Microsoft operating system will affect support for AcTIveX and Java applets. SVG and VML are implemented on the browser side through a specific scripting language, and the code is open to the client, so there is no security problem. SVG requires the browser to install the corresponding plug-in to work, and VML is supported by the current mainstream browser IE without any installation, so this system chooses the implementation of VML. VML (Vector Markup Language) is a language for describing graphics with vectors. The graphics can be scaled up and down without loss of quality. In VML, the markup uses XML extensions. Combined with Javascript scripting language, VML can dynamically display graphics. VML avoids the provision of digital signatures by third parties and does not require the installation of new IE plug-ins. The results are shown in Figures 3 and 4. Figure 3 is a graph that plots the temperature and humidity measurement values ​​uploaded by each node. Customers can understand the temperature and humidity changes of each node over a period of time at a glance. Figure 4 shows the real-time temperature and humidity measurement values ​​of multiple nodes while being dynamically updated.
4 Conclusion
This article mainly introduces the development of a new type of network-based temperature and humidity monitoring and transmission system. Its innovative technical characteristics are reflected in: a higher measurement accuracy is achieved by using high-performance temperature and humidity measurement devices; humidity is controlled by fuzzy control technology The regulation reduces the system state switching, and the control effect is good; the Flash file storage scheme completes the saving and erasing of the measurement data. In addition, the network monitoring solution uses AJAX technology to achieve asynchronous transmission of sensor measurement data and improve the response speed of the system; and uses VML to achieve dynamic display of measurement data. This scheme can also be applied to the network monitoring system of other sensors.
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