Key Engineering Materials Online: 2014-04-09 ISSN: 1662-9795, Vols. 609-610, pp 801-806 doi:10.4028/www.scientific.net/kem.609-610.801 2014 Trans Tech Publications, Switzerland Design of Intelligent Meteorological System Based on MEMS Wang Hao 1,a, Nie Meng 2,b, Huang Qing an 3,c 123 Southeast University, the Key Laboratory for MEMS of the Education Ministry of China, Nanjing, 210096 a wanghaoseu@163.com, b m_nie@seu.edu.cn, c hqa@seu.edu.cn Keywords: MEMS, Meteorological system, ARM Abstract. Intelligent weather station system based on MEMS sensors is designed. The automatic meteorological system includes a MEMS temperature sensor, MEMS humidity sensor, MEMS pressure sensor, MEMS wind speed sensor and the sensor intelligent control system, etc. The intelligent control system has functions such as precise timing, multiple sensor data automatic acquisition, storage and uploading, which realizes the intelligent control of this weather station system. Introduction Weather information occupies a pivotal position in social life and scientific research fields. Industries, including military affairs, aerospace, agriculture and transportation all need accurate weather information. However, the methods to obtain the weather information are either traditionally mechanical or empirical which can never meet the demand of real-time and accuracy.. The uncertainty of weather information may lead to disastrous consequences. With the development of the MEMS technologies, we are miniaturizing the sensors gradually. New sensors based on the MEMS technologies have been around. Automatic Meteorological Station based on the latest MEMS and integrated circuit technologies will become a trend of the weather stations [1,2]. With the STM32 processor as the control core, the system integration bases on the barometric pressure, temperature and humidity sensors of the latest MEMS technologies. It realizes the weather information collection, processing and uploading by combing the wind speed and direction system. The PC software deals with the final information and displays the result. System Architecture This system falls into four major categories: signal acquisition system, signal processing system, communication system and monitoring system. System diagram is shown in Fig.1. Fig.1 System block diagram The signal acquisition system, mainly composed of weather information sensors, accomplishes the weather information collection within the stipulated time following the instructions of the control core. The signal acquisition system mainly finishes the pretreatment process of the signal All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, USA-09/05/16,02:33:05)
802 Micro-Nano Technology XV collected by the sensors and compressed according to the prescriptive communication protocols. While the communication system is responsible for uploading the processed data to PC side whose software is in charge of showing and post processing data. Hardware Design The block diagram of hardware is shown in Fig.2. Air pressure sensors, temperature sensors and humidity sensors are in charge of gathering data of the temperature and humidity pressure. While the wind speed and direction system is responsible for collecting wind speed and direction information. These sensors communicate with the STM32 control core via the IIC or SPI bus. The TF card and EEPROM serve as storage cells. The EEPROM stores the product and calibration parameters and the TF card stores the weather information in nearly one year. The system communicates with the computer in the RS232 or 485 ways. Once instructions issued by computer to the MCU satisfy the data communication protocol format requirements, it is realizable implement dozens of functions such as baud rate settings, parameter settings and data uploading. Here s a detail of each hardware unit. Fig.2 Hardware block diagram Control Unit. The main control unit is constituted of controllers based on ARM core. Choose STM32 because of its low power, rich peripheral resource and powerful performance. Sensor Unit. Air pressure sensors use integrated sensor based on piezoresistive effect [3], this series of sensors adopt the latest MEMS technologies, they have the advantages such as small size, low power and high distinguishing rate, their relative precision can reach ±0.12hPa and the distinguishing rate can be up to 0.01hPa. They are one of the few sensors which can conformant to requirements of this project. The humidity sensors are based on the humidity sensitive elements [4], these chip sensors containing calibrated digital signals output are complex humidity and temperature sensors.they ensure a high reliability and excellent long-term stability. These sensors adopt capacitive principles of humidity measurement and combine the latest MEMS and Integrated Circuit technologies. The chip has a built-in A/D converter; simple manipulation of output digital signal can obtain the current humidity. These sensors connect on the general I/O interface of MCU directly and communicate with MCU through the means of time sequence simulation. The precision of the humidity sensors is limited by their inherent characteristics.the sensors are optimal and have high integration level and the precision can be as high as ±2%RH. Besides, full digital output increases the ease of use. The thermistor Pt100 is adopted to the temperature sensors. It has a perfect linearity and a wide range of temperature and helps compensate for the weaknesses of Integrated temperature sensors, only work above -40. As the temperature coefficient is around 0.38Ω/, temperature measuring circuits require careful design in order to meet the precision requirement (±0.2 ) of the weather systems. They must have very high sensitivity and extremely strong anti-interference ability. The system adopts 24 bits precise A/D converter as the core of temperature measuring circuits [5]. At the same time, it uses precise Pt100 to sense the temperature.
Key Engineering Materials Vols. 609-610 803 The wind speed and direction system is originally designed by the laboratory, adopting the ultrasound principle. It has many advantages such as high precision, good stability and strong anti-interference ability; it is controlled by single controllers and communicates with MCU via the SPI interface. Storage Cell. The system uses two cards: EEPROM and TF to the store data. The EEPROM is in charge of storing the ID of the system and the key information such as the related product calibration parameters.while the large capacity TF card stores the historical weather information around nearly one year. Real-time Clock. High time precision of weather system claims for reducing the mismatch and temperature drift of the oscillator to a great extent. The clock used in this system just happens to have built-in oscillator and temperature compensation circuit. This kind of architecture decreases the oscillator mismatch maximally and catches up on the temperature drift. Consequently, the timing error of this clock may accurate to ±15sec/month. Heat&Fan. The system need to be placed in aspirated covers to protect against radiation, which is originally designed by the laboratory. One of the key parts is the fan and the way it works is determined by MCU. What is more,the system has various built-in patterns, such as based on time or wind speed. Software Design The system software mainly includes STM32 side and PC control software. The PC control software, wrote in Visual C++, according to the defined agreement, decodes data captured by serial port and provides it to the user after processing data via the friendly human-computer interaction interface and mean while it stores a copy as a backup. STM32 side software is based on the µc/os-ii real-time operating system. The frame work is shown in Fig.3. Fig.3 Software architecture The system sets up tasks of initialization, collection of wind speed and direction, collection of temperature, humidity and pressure. Besides, data storage,data uploading and data command response are also included. The initialization task means mainly initializing and self-testing. Then it reads ID and parameters from EEPROM, meanwhile sets the time and connects to computers. This task execute only once. The collection of wind speed and direction task is executed per 3 seconds driven by the real time clock. Simultaneously, it sends the data to computers. Similarly, the collection task of temperature, humidity and pressure is also executed periodically. It gathers data every 3 seconds, then calculates the average and uploads every 1 minute. The data storage task is mainly mounting file systems. The weather stations support the FAT32 file systems. The weather information is stored in TF cards as text files. The greatest hallmark of the system is the good computer-human interaction. Hence, response is the most important task. This task has the highest priority. The main flowchart is shown in Fig.4.
804 Micro-Nano Technology XV Fig.4 Command response task After receiving the computer command, this task judges its type first, and then calls the appropriate subroutine. The command of upper computers is classified into global device command, specific device and preservation commands. To be specific, it covers ID accessing, system restoration, baud rate setting, parameter settings, time calibration and data uploading. In addition,the system also offer a rich set of extension command such as switches of data uploading both actively and passively as well as the historical data query. Performance Test Whether parameters meet the requirements needs final test. The system test requires specialized devices, such as high-precision temperature and humidity boxes, air pressure calibrating instruments and wind tunnels. Temperature test. Temperature performance tests are accomplished in the temperature and humidity boxes. The measurement temperature ranges from -35 to + 50.The measurement interval is 5 and there are 16 groups in total. The test results are shown in Fig.5. Fig.5 Pt100 test result The test results indicate that temperature measuring circuits have good degree of linearity. The 0.65Ω offset of the compensation resistance makes the difference between test values and theoretical values. Temperature values can be obtained after simply converting the resistance values. Humidity test. The humidity data requires nonlinearity compensation and temperature compensation, the nonlinearity compensation formula reads:
Key Engineering Materials Vols. 609-610 805 RH linear = -2.0468+0.0367 RH+1.5955 10-6 (1) The relative humidity is defined as the ratio between water vapor pressure at certain temperature and saturated water vapor pressure at the same temperature. The saturated water vapor pressure is greatly affected by temperature, so the data after nonlinearity compensation still needs temperature compensation. Its compensation formula reads: RH true = (T( )-25) (0.01+0.00008 RH) + RH linea (2) Finally, adjust zero-drift of the data: RH final = RH liner + α (3) Temperature tests are accomplished in the temperature and humidity boxes,. The measurement ranges from 30% to100%. The temperature t in the temperature and humidity box stays 25 The results are shown in Fig.6. The results show that the measurements matches well with reference values. The maximum error is 1.41%,meeting the requirement of accuracy(±2%). Tests of air pressure and wind. Air pressure performance tests are accomplished in air pressure boxes. The measurement ranges from 550hPa to1050hpa. The results are shown in Fig.7. Fig.6 Humidity sensor test result Fig.7 Pressure sensor test result The wind speed and direction system is originally designed by the laboratory, The relevant compensated circuit and algorithms have been perfect. The performance curves are shown in Fig.8 and Fig.9. Fig.8 Wind speed test result Fig.9 Wind direction test result
806 Micro-Nano Technology XV Test Result. After systematic and perfect test, the final performance indicators are shown in Table1. Table1 Performance index Designation Resolution Index Temperature 0.01[ ] ±0.15[ ] Humidity 0.10[%] 2[%] Pressure [8] 0.01[hPa] 0.3[hPa] Wind speed 0.1[m/s] 0.3[m/s] Wind direction 0.1[ ] 0.3[ ] Power <5[W] Time accuracy <±30[S] The test results show that the system performs well, basically meet the target before design. Summary An intelligent weather station is designed in this paper. Weather Information collection, storage, uploading and provide a series of friendly human-computer interaction command to users, including: time setting, system setting, resetting and historical data query. By appropriate compensation, the system can reach the temperature precision of ±0.2, humidity precision of ±2% and air pressure precision of ±0.3hPa. Furthermore, the power consumption and consistency of the system has made a significant improvement. References [1] Vaisala. Instruetion Manval for Automatic Weather Station Milos 500 Softwares description[z]. (1990) [2] Kevin Keay, Ian Simmonds. The association of rainfall and other weather variables with road traffic volume in Melbourne Australia[J]. Accident Analysis and Prevention. (2005), (37): 109-124 [3] Stmicroelectronics (China) investment co., LTD. STM32 Reference Manual[Z] (2010) [4] Sensirion. SHT75/SHT15/SHT25 Datasheet[EO/OL]. http://www.senrion.com (2008) [5] Bosch. BMP180 Datasheet[EO/OL]. http://www. bosch-sensortec. com (2012) [6] TEXAS INSTRUMENTS. ADS1247 Datasheet[EO/OL](2009)
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