| Electromagnetic water meter(EWM)has many advantages,such as wide measuring range,small starting flow,high measuring accuracy and long service life.It is widely used in urban water industry.EWM is powered by built-in battery.Because of the low power requirement,its excitation current is small.When the inlet flow rate is small,the sensor's induced electromotive force signal is very weak,and the measurement accuracy becomes worse,even cannot be measured.At present,the EWM produced at home and abroad still has the problems of large minimum flow limit value and low accuracy at small flow rate.In order to improve the measurement performance of EWM at small flow rate,EWM sensor's weight function,flow field and magnetic circuit structure are studied and analyzed in this paper.Firstly,using the COMSOL software,the weight function distribution of sensor is obtained,based on the electric field simulation method.And simulation result is compared with the theoretical value,which verifies the reliability of weight function numerical model.Through the simulation comparison of different structures,it can be concluded that weight function distribution of EWM with rectangular shrunk measurement tube is the most uniform,and its dependence of induced voltage on the flow pattern distribution is the lowest,which is beneficial to promote the measurement accuracy.Then the flow field of EWM with rectangular shrunk measurement tube is studied,and pressure loss of rectangular shrunk measurement tube is calculated based using Fluent software.Mathematical model of pressure loss is established by orthogonal test and polynomial step-by-step fitting method.According to the permanent flow rate and national standard of pressure loss requirement,the optimum pipe structure size is obtained,and the prototype is produced.Experimental results show that error between pressure loss' simulation value and experimental value is less than ±2.22%,which verifies the effectiveness of the EWM measurement tube structure design method.Next,based on the optimum measurement pipeline structure,the numerical model of the induced voltage is established using COMSOL software.The magnetic circuit structure of sensor is optimized aiming at improving the intensity of induced electromotive force.Optimum structure parameters of three magnetic circuits which includes yoke,yoke-core and yoke-core-plate are determined,and three prototypes are made.Experimental results show that,error between magnetic field simulated value and measured value for magnetic circuit with yoke-core-plate is less than ±4.63%.Errors between induced voltage simulated values and measured values of three magnetic circuits are ±3.14%,±9.74% and ±8.95% respectively.Experimental results verify the effectiveness of the EWM magnetic circuit structure design method.Finally,three optimized magnetic circuits are assembled with the optimized EWM rectangular shrunk measurement tubes,and water flow calibration experiments are carried out.Experimental results show that,when inlet flow velocity is between 8.84 mm/s and 22.28mm/s,measurement error of the magnetic circuit systems with yoke and yoke-core is less than ±2.51%.Magnetic circuit system with yoke-core-plate has the best measurement performance,and its measurement error is less than ±0.60%,better than accuracy requirement in Class I accuracy measurement. |
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