| Guide wave Radar liquid level meter has been developed for three decades,the principle of the measurement already has mature development,not only the old meter with simple function,but the newest one with automatization,rarely make innovative breakthroughs.The subsequent study merely make progress in the way of measurement or signal processing.Therefore,the development of liquid lever meter based on new technology will make it more integrated and intelligent.The design in this paper also based on the principle of time domain reflectometry,optimization measuring circuit,proposing a new design mentality from device selection,scheme option,circuit design,sensor manufacture and so on.Under the premise of meeting the power dissipation and the limited board area,employing redundant design philosophy,in order to expand function later or improve system reliability.This paper mainly aiming at discussing hardware design of the liquid level meter system,implement systematic design techniques,dividing the main system into several subsystems,then,fractionize these subsystem into different circuit block according to their function.As the basis of the liquid level measurement,starting from the realization of UWB narrow pulse,a picosecond pulse generator with digital circuit and step recovery diode is presented,then,for each circuit block's realization,we proceed from a variety of plan,analyze their advantages and disadvantages,and choose the best implementation.To carry out the sampling circuit in a low cost way,equivalent sampling is the only choice.In order to transceiver both signal synchronously without disturbing each other,signal processing circuit abandoned mixer circuit,take a simple alternative solution instead,for the measurement of the time interval between the transmitted signal and the echo,a time to digital converter has been adopted,the calculation of the time difference can be done automatically,which greatly simplifies the complexity of the system.A large numbers of low power consumption devices are used in the system,the power consumption and leakage current as standard to select both the microcontroller and other passive devices.The design of the Power supply circuit is setting about from two point of view,take LDO linear regulator to provide the required power supply when system is battery powered,while using integrated solutions when powered by 24 V DC supply,a single chip provides all voltage levels required by the system,and has better support for HART communications,both two programs will not take up too much board area.The traditional 4 ~ 20 mA current loop with frequency shift keying HART modem added had been selected as the main communication mode,allows transmitting multiple field variable simultaneously,keeping 485 as a backup at the same time.Using STMicroelectronics' s low power products as microcontroller,its rich peripherals and superior performance in addition to meet the basic functions of the system and power requirements,embedded system and protocol stack can also be used to support ethernet communication.Finally,the design method of the sensor as signal carrier is discussed. |
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