Design Of New Transducer For AC Excitation Electromagnetic Flow Meter

Electromagnetic flow meter (EMF) , which is based on Faraday's induction law, is an instrument to measure the volume flow of conducting fluid. As a key part of the EMF, the transducer which converts induced electromotive force signals into unified standard current output or digital communication signals. There are two problems for the transducer design. The one is the input signals processing mode in transducer, which is decided by the excitation pattern of front-end sensor, and the other one is the capability of transducer to inhibit and eliminate disturbance.Compared with EMF excited by low frequency rectangle wave designed only for liquid flow measurements, EMF excited by power frequency has wider measurement bandwidth, and also can measure liquid-solid two-phase mixed flow, which cover the shortage of the former. But when power frequency excitation is adopted, it will incur differential interference and in-phase interference, and thus brings a large difficulty in transducer design. The traditional transducer with low integrity has disadvantages such as poor stability and anti-interference, huge size etc., which can not reach the requirements of industry production field.In order to solve the problems mentioned above, a new transducer based on EMF excited by power frequency is designed. The main works in this thesis are as follows. (1) More emphasis is put on how to solve the anti interference, and a new method based on trigonometric function is proposed to eliminate the differential interference and in-phase interference. (2) In order to choose reference signal, innovative adopt transformer as reference signal. When transformer is chose as the source of reference signal, the circuit design is easy and the safety of circuit is improved. (3) Symmetry method is adopted to keep the phase difference constant, solving the problem of the uncertain phase difference between the flow signal and reference signal caused by excitation frequency fluctuation. (4) Divider is used to eliminate the error, which is caused by power voltage fluctuation.The experiment shows that the new transducer's noise immunity, zero point stability and measurement precision are greatly improved, and some intelligent modules such as LCD display, real-time clock, RS-232 interface are also introduced, which makes the instrument convenient and low cost.