| Electromagnetic Flow Meter is an important instrument in industry. It can be divided intoSensor and Converter. Compare with other instrument for flow metering, the EMF has relativelysimple structure that makes it easy for building. Its demand for industrial craft is not high level asothers be. The domestic makers of EMF barely make the Converter them self but some big factories.Key technologies of EMF are almost monopolized by oligarchies such as YOKOGAWA,KROHNE, E+H, and it makes significance for us to develop our one technology or just catch upwith them. The majority of EMF sells now is using analog constant current source as excitationsystem. New type of excitation system called “digital controlled excitation system” has beendesigned in order to make the excitation current programmable and accurately adjustable. This kindof excitation system is based on DC-DC power technology which using PWM wave to adjust thepower of excitation system. Switching Mode Power Supply, is developed in late60’s in19th century.It has higher efficiency than linier one and wastes less energy at converting process. By using thisidea, a novel current source is suggested whose output not only can be stable by feedback loop butalso variable by the duty cycle given. Working in switch mode makes it cost low power rather thananalog one apparently.Three-value-wave-form in low frequency, the excitation wave form is adopted in this work.Zero drift is dramatically inhibited by this type of excitation wave form and the power consumptionis reduced into half due to it, again.The TI’s TMS320C2812is served as micro controller for the Converter. Using485bus tocommunicate the measuring result and command order. The measuring circuit includes TI’sPrecision Instrumentation Amplifier and second order positive feedback Butterworth’s low passfilter. Noises higher than100Hz can be eliminated by it. In order to perform the excitation systembetter, we also designed the DA saw waveform generator. |
PDF Full Text Request