| The accurate measurement of open channel flow is the key to improve the utilization rate of water resources in irrigation area and to help the modernization reform of agricultural irrigation.In recent years,our group has been committed to the research of remote automatic metering sluice gate system.The current open channel flow measurement mostly adopts the hydraulic structure measurement method and the traditional velocity area method.The former needs to build structures specially,which is inconvenient to install and use,and the measurement accuracy is relatively low;the latter has no fixed section,which requires high sensor installation accuracy and fixed open channel shape,thus is not suitable for use in different shape channels.In view of the above problems,this paper proposed a method of ultrasonic flow measurement in open channel square cylinder based on transit time difference method,and designed a flowmeter prototype.The flowmeter has standard rectangular section,which can be installed independently at the entrance and exit of small open channel to measure the flow through the square cylinder,or at the back of the flat sluice gate to measure the flow through the gate.In this paper,the types and measuring principles of ultrasonic flow meters were studied.According to the demand of flow measurement in open channel,the transit time difference method was selected finally.The characteristics of velocity distribution in square cylinder of open channel were also studied.Fluent was used to simulate and analyze the flow state of square cylinder in full flow and non full flow state.According to the simulation results,the non-uniform arrangement scheme of transducers and the layered calculation method of flow in square cylinde were determined.The hardware circuit of the flowmeter designed in this paper includes the minimum system circuit with STM32 as the controller,the timing circuit with TDC-GP22 timing chip for transit time measurement,the pulse signal amplification circuit with transducers emitting ultrasonic wave composed of field-effect transistors and high-frequency transformers,and the echo signal processing circuit with fourth-order multi-channel feedback band-pass filter and operational amplifier,sound channel switching circuit with CD4097 multiplexer as the core,as well as temperature acquisition,liquid level measurement and communication module.After the completion of the hardware circuit design,the corresponding software program was compiled,including the main program,initialization configuration,transit time measurement,channel switching,liquid level measurement,data processing and serial communication modules.Finally,the prototype system was tested in the laboratory and the simulative irrigation area,and the experimental results were analyzed.The results show that:(1)the system realized the design of various functional tasks,and can complete the measurement of single channel flow velocity,ultrasonic liquid level and multi-channel open channel flow.(2)The absolute error of flow velocity measurement is less than10mm/s,the relative error is less than 2%,the absolute error of liquid level measurement is not more than 2mm,and the relative error is less than 0.3%.After the correction of K coefficient,the relative error of total flow measurement is less than5%.(3)The system can be applied to environment with the sediment content not higher than 15kg/m~3.It also can be used continuously over a long period of time.As indicated above it has strong adaptability to the complex environment in the field. |
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