Research On Wireless Channel Propagation Characteristics And Wireless Sensor Network Control System In Greenhouse

Wireless sensor networks(WSN)are currently forefront and hot research areas in the international community,involving a crossed subject and highly integrated knowledge,which has been widely used in military defense,industry and agriculture,bio medical,environmental monitoring and so on,and it has a lot of advantages including good flexibility,high reliability and high accuracy,it has great advantages in collecting agricultural information.But the agricultural environment is complicated,especially the environment in greenhouse,which has many environmental factors,including plants,column and metallic materials and so on,they all have a great influence on the wireless signal,the signal has large scale fading due to the change of distance,and small scale fading duo to the scattering.And the greenhouse environment is relatively closed,in rainy days or cold winter,the humidity can reach to 90%,humidity has a great influence on wireless signal too.Therefore,it is necessary to study the characteristics of wireless signal propagation under the humidity environment in greenhouse.At first,in order to solve the problem of WSN planning and deployment in greenhouse under high humidity environment,investigate the effect of high humidity environment,communication distance and the height of the antenna on the wireless signal propagation characteristics,studied on propagation characteristics of 2.4GHz and 433 MHz wireless channel in different humidities and heights,received signal strength indicator and packet loss rate at different humidities and heights were measured through three sets of experiments,and experimental data were analyzed with multivariable linear regression in MATLAB.At the same time,a three-dimensional model which is about humidity,transmission distance and received signal strength indicator was established.And the relationship model between the humidity and the path loss index is established,and the relationship model is brought into the path loss model,thus a new path loss model based on humidity and transmitting distance as independent variables and received signal strength as dependent variable is established for high humidity environment in greenhouse,two models can be used to predict the received signal strength indicator of different transmission distances in high humidity environment effectively,and two models were verified and compared.Besides,studied on propagation characteristics of 2.4GHz and 433 MHz wireless channel in different humidity and heights in two kinds of environments,greenhouse and playground,compared received signal strength indicator and packet loss rate,and made a conclusion that 433 MHz wireless channel has good diffraction,reflection and far transmission distance,it is more suitable to be the fundamental band for building a greenhouse.Chose 433 MHz as the fundamental band,used star network structure,and designed a wireless network base on Modbus communication protocol.Three sensor nodes can collect temperature,humidity,carbon dioxide and other parameters in real time,and transmit it to the sink node,and then the sink node transmit the parameters to PLC by RS-485 bus.Chose temperature priority control strategy to control environmental parameters in Greenhouse,so as to realize comprehensive environmental control of modern greenhouse,which could improve control effect and reduce control cost.The system is running in Jiangsu Academy of Agricultural Sciences,for reasonable scheme design and stable performance,the using effects show that the system can meet the demand of facility agricultural environment.