Wireless Environmental Sensor System for Advanced Greenhouse Application
Date Issued
2016
Date
2016
Author(s)
Cheng, Tzu-Heng
Abstract
In this thesis, a novel wireless environmental sensor system for use in advanced greenhouses is presented. The major contribution of this thesis includes the development of RFID sensor tags and the switched-beam circularly polarized (CP) antenna array as the reader antenna, both of which operate in the 902-928 MHz band. The sensor tag module consists of a Texas Instrument MSP430G2553 microcontroller (MCU), Impinj Monza X-2k RFID tag chip with dual interfaces, Silicon Lab Si-7021 integrated temperature and relative humidity sensor, TAOS TSL-45315 light sensor, and RFID tag antenna designed by ourselves. The MCU manages the whole operation procedure, including sensing, data writing, and wireless reading. All the devices in the sensor tag module are compatible with the I2C interface, commonly used in short range communication on PCB board. All the sensors can receive and follow the commands from MCU via I2C interface to measure the environmental information and then send back the data to MCU for it to write the sensing data into the nonvolatile memory (NVM) of the RFID tag chip. Through the RFID Class 1 Gen 2 air interface, RFID reader can wirelessly access the data stored in the tag chip whenever the NVM is not accessed by MCU via I2C interface. Besides, we also proposed two types of RFID tag antennas, one with a compact size and the other having a higher gain. These two antennas are integrated respectively with the sensor tag module, and their performances are verified by comparing the simulated and measured results. The proposed switched-beam CP cavity-backed slot antenna array for use as the RFID reader antenna consists of two parts: the array of radiators and its feeding network. The element radiator is a square slot backed by a metallic cavity and fed by an L-shaped strip to obtain uni-directional CP radiation and lower mutual coupling between adjacent elements. In this thesis, an one dimensional array is implemented considering the limitations imposed by our in-house fabrication process and the operating frequency of the system. The peak gain achieved is about 10 dBi. Our proposed feeding network consists of a microstrip cross junction and two reflection-type phase shifters (RTPSs). By properly choosing the characteristic impedances of the four branches of the cross junction, arbitrary current distribution at the three outputs branches can be obtained to feed into the array elements. The side-lobe level (SLL) of the switched-beam antenna array can readily be improved by implementing a tapered current distribution. The novel RTPS consists of a periodically loaded coupled-line coupler and two identical 2-bit switchable termination circuits, which is also controlled by the TI MSP430G2553 MCU. The RTPS is the key for the automatic sequential beam switching of the array between four predetermined directions.
Subjects
cavity-backed aperture antennas
circular polarization
reflection-type phase shifters
switched-beam antenna arrays
wireless sensor network
Type
thesis
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