Development Of Intelligent Two Wire Radio Frequency Admittance Level Transmitter

Level is an important inspected parameter for material consuming and output counting as well as guarantee of continuously producing and safety of equipments.With its construction simply, working dependability, supporting convenience, the traditional level instrument with capacitance is very important in level field. However, the sticky and conductive material will stick on the outside surface of the probe insulation in the level measuring, which is called coating. The coating will result in false level and low accuracy. Therefore, it is important to eliminate the error from coating in the level measuring.This paper develops the RF (radio frequency) admittance level measuring theory, which is based on the advantages and the disadvantages of the traditional capacitance level measuring. RF admittance level measuring principle is discussed in detail, and an principle of the resistive and capacitive components of the coating with equal impedance is deduced out.A conclusion is deduced out, which is "for the vector of and that have already settled measure, only their Point of departure is same alike, the plural resistance for test to find outing only be measured by their range and phase, with other factor irrelevant". And this is the premise of designing level transmitter applied RF admittance level measuring principle. The method of eliminating the coating's effect and getting the true level is given.Finally, the design of hard-ware circuit, and software for getting the true level with two wire RF admittance level intelligent transmitter is described in detail, particularly the turing power circuit, the square wave circuit of fixed phase of 90 degrees and phase sensitive detection circuit.In addition, two wire RF admittance level intelligent transmitter can not only deliver 4～20 mA current signal, but also communicate with the upper computer with RS485 or RS232, which realizes the distribute control system. The technology of single-chip microcomputer is adopted in this system, which enhances the transmitter's precision and strengthens the intelligence and automation of the transmitter.There are the following creative points in this paper. The advantage of foreign RF capacitance level transmitter is absorbed, and a series of the advanced design method of circuit and software arithmetic are adopted. The most intelligent two-wire RF admittance level transmitter is designed,and the true level measuring is realized with the capacity of delivering 4 ～ 20 mA analogue signal and digital signal communication.