| The electromagnetic flowmeter (EMF) based on Faraday's law of electromagnetic induction is a kind of commonly used flowmeter to measure the volumetric flow of conducting fluid. After Michael Faraday proposed this method for the first time in 1832, more than 100 years has passed. Due to great progress in signal processing technique for EMF in the 70's and 80's of 20th century, new types of EMF were developed to adapt different measurement conditions. Because of its prominent advantages, such as high precision, wide measurement range and low pressure loss, the electromagnetic flowmeter is widely used and the market share of EMF is rising constantly. Now the electromagnetic flowmeter is playing an important role in industrial and agricultural production.The advantageous principle of EMF is described by the following: When conducting fluid passes through a magnetic field B, the induced voltage E, which is directly proportional to the mean velocity V of fluid by instrument coefficient K, is generated between the detection electrodes. Supposed that coefficient K is a constant, which depends upon magnetic field intensity, pipe diameter and amplifier's input impedance, the mean velocity of fluid can be measured directly by E. In recent decades, the EMF has already become the most commonly used flowmeter in the applications that meet above assumptions.Although EMF is a preferred instrument in environmental protection and water metering applications, with gradually incremental demands in the fields, such as water supply, drainage, irrigation and sewage discharge monitor, etc., the above assumptions may be no longer satisfied. The instrument coefficient K varies when EMF is used in partially filled pipe. Multi parameters, such as fluid level and velocity, are needed to measure accurately. In recent years, EMF with ability to measure multi-parameter has been one of the research focuses in flow measurement field.In the sensor technology of EMF, such an important characteristic is noticed. In the velocity signal transmitter, a peculiar circuit to measure electric potential in fluid consists of fluid and two detection electrodes contacting the fluid and signal amplifier. The thesis desired to establish additional measurement relation and principle by means of this feather and to solve measurement method and technology when the instrument coefficient K varies. So a novel multi-parameter measurement EMF can be designed.The feature that electrodes of EMF are directly contacted with the conductive fluid is used in this thesis and a parameter-measuring principle based on additional excitement is also put forward. New measurement equations are established by adding new excitement in the electric potential measurement circuit. The implementation method and technology of multi-parameter measurement EMF is studied in the thesis.In order to study how to apply the additional excitement principle to realize multi-parameter measurement completely, two types of additional excitement, which are passive model and active model are studied separately in this thesis. In theory study, the fundamental signal transmission model in the measurement circuit of detection electrodes is studied in the thesis and the basic measurement equations to measure fluid conductance are derived under the condition that additional excitation sources act on the measurement circuit. Accordingly, the origin and feature of the two kinds of excitation sources as well as the measurement modal acting on the electrode measurement circuit are discussed further in this thesis: (1) The fundamental equations to measure fluid conductance with these two kinds of passive additional excitation are derived. It is proved by theoretical analysis and experiment that these two kinds of excitation can be used to detect empty pipe. So by these two kinds of passive exciting signal, the criterion of empty pipe detection and data processing method are discussed. (2) According to characteristic of the electrode measurement circuit, the method and corresponding realization technique of active additional excitation which acts on the electrodes to measure fluid conductance are studied. The model of conductance measurement based on active additional excitation is built and measurement equation of fluid conductance is derived. The working time sequencing and data processing method to realize conductance measurement are designed. By virtue of the relation between conductance and other state parameters, conductivity measurement, empty pipe detection and flow measurement for partially filled pipe are realized, so as to provide theory support, realization means and necessary measurement conditions with multi-parameter measurement of EMF.As a typical application of multi-parameter measurement, both flow and fluid level in partially filled pipe need to be measured simultaneously and multi parameters in the measurement must be processed synthetically. According to non-pressure pipe hydraulics theory, a novel EMF with an arc-electrode sensor which can carry out flow measurement in partially filled pipe is studied. Based on the result of conductivity measurement using additional excitement technique, a model and relation between fluid conductance measured by arc-electrodes and fluid level are researched when the fluid flowing in partially filled pipe. According to this, a new EMF for flow measurement in partially filled pipe is designed. Time sequencing and data processing methods are provided.Based on the research results in this thesis, a new active additional excitement module is designed and a novel multi-parameter EMF prototype is implemented. The experiment platform of multi-parameter EMF flow measurement system which can implement flow measurement in partially filled pipe is established. The following functions are verified in this system: (1) Using traditional two dot-electrode type sensor and active additional excitement, the experiments of fluid conductivity measurement and empty detection are carried out. (2) The contrast experiment in fully filled pipe between standard EMF and new EMF with arc-electrode sensor is done. (3) Flow measurement experiment with the arc-electrode sensor in partially filled pipes is done. The practicability and feasibility of this method are proved by relevant experiment. So the multi-parameter measurement in EMF is also realized in theory and technology. Further researches to be done are put forward in the end of the paper.The multi-parameter EMF designed by this thesis was exhibited and demoed in China Intemational Industrial Fair in November 2006. Its performance reached the current top level in the same area of the world. This novel EMF was paid close attention by insider. |
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