两线制电磁流量计关键技术研究

Research on key technology of two-wire electromagnetic flowmeter

两线制电磁流量计能量消耗来自4 mA~20 mA电流环路,如果仪表自身耗电大于电流环路等效电流4 mA对应的最大能量值,那么环路将无法输出下限4 mA电流值。因此,一般要求两线制电磁流量计自身功耗(包括传感器在内的全部电路)不大于96 mW(4 mA*24 Vdc),这是两线制仪表的根本原则之一[1]。为确保4~20 mA输出功能正常,通常将4~20 mA电流环路最大取电电流设定为4 mA,对应的最大功率为96 mW。而当电流输出大于4 mA时,除了取电获得的4 mA对应的能量外,其余大于4 mA的那部分能量则被分流而直接浪费。这种特性决定了提供给两线制流量计本身的能量十分有限,励磁电流必然处于较低的水平,最终导致两线制电磁流量计较四线制产品的综合性能总体偏弱。如果依据两线制电磁流量计4~20 mA输出电流状态的变化,动态调整取电状态,输出电流越大,获取能量越多,反之则获取的能量越少。再将获取的能量用来增大流量计励磁电流,则可在一定程度上提升两线制电磁流量计的测量性能,提高仪表测量准确度,增强仪表工况适应能力。

The energy consumption of the two-wire electromagnetic flowmeter comes from the 4~20 mA current loop.If the power consumption of the instrument itself is greater than the maximum energy value corresponding to the current loop equivalent current of 4mA,the loop will not be able to output the lower limit of 4mA current value.Therefore,it is generally required that the power consumption of the two-wire electromagnetic flowmeter(all circuits including sensors)should not be greater than 96mW(4 mA∗24V dc),which is one of the fundamental principles of the two-wire instrument[1].In order to ensure that the 4~20mA output function is normal,the maximum power taking current of the 4~20 mA current loop is usually set to 4mA,and the corresponding maximum power is 96mW.When the current output is greater than 4 mA,in addition to the energy corresponding to 4 mA obtained by power,the rest of the energy greater than 4 mA is diverted and directly wasted.This characteristic determines that the energy provided to the two-wire flowmeter itself is very limited,and the excitation current must be at a lower level,resulting in the overall weak comprehensive performance of the two-wire electromagnetic flowmeter compared with the four-wire flowmeter.If the power-on state is dynamically adjusted according to the change of the 4~20 mA output current state of the two-wire electromagnetic flowmeter,the greater the output current is,the more energy will be obtained,and vice versa.Then use the obtained energy to increase the excitation current of the flowmeter,which can improve the measurement performance of the two-wire electromagnetic flowmeter to a certain extent,improve the measurement accuracy of the instrument,and enhance the adaptability of the instrument to working conditions.