Flow Tube Vibration Model And FPGA Digital Drive Technology Of Coriolis Mass Flowmeter

Coriolis mass flow meter is a high-precision flow meter which can directly measure the mass flow of gases and liquids,and it is widely used in petrochemical,chemical,pharmaceutical and food safety fields.As an important part,drive technology plays a key role in the work of Coriolis mass flow meters.The digital driving technology,which uses device to synthesize the driving signal is being adopted by Coriolis mass flowmeter manufacturers because it can maintain the vibration of the flow tube under complicated working conditions such as gas-liquid two-phase flow.However,there are still some problems in digital drive technology that need to be solved.For the problem that how to clarify the vibration characteristics of the flow tube and make the transmitter have a better control of the vibration amplitude of the flow tube,a method combining mechanism analysis and experimental research is proposed to establish a mathematical model of the flow tube vibration system(FTVS).Firstly,the second-order model framework of FTVS is constructed through mechanism analysis.Then the finite length sine wave is used to stimulate the flow tube,and the model parameters are determined according to the free attenuation oscillation signal,the steadystate input signal amplitude and the output signal amplitude.The mathematical model of FTVS under single-phase flow and gas-liquid two-phase flow conditions are built,and the differences of the models are analyzed,which lays the foundation for the control of the flow tube vibration amplitude.For the problem that there are no specific methods to determine the logarithmic error base in digital drive amplitude control and the PI parameters of different flow tubes,the tuning methods of these two types of parameters are designed and implemented.A second-order mathematical model of the flow tube vibration system is established,which is equivalent to an intrinsic gain at steady state,providing a reference for the tuning of the PI parameters.According to the difference of expected values,the logarithmic error of different bases is selected as the input of the subsequent controller so as to meet the requirements for the dynamic characteristics and the steady-state characteristics.The PI controller is used to control the amplitude and the gain of the closed-loop system is used to preset the PI parameters.The final parameters are determined according to the actual dynamic table effect and the steady state effect.When controlling different flow tubes,PI parameters can be adjusted proportionally based on their steady-state characteristics.For the problem that the driver signal update period of DSP-based digital drive is too long,resulting in poor driving effect,an FPGA-based Coriolis mass flowmeter digital drive system is developed,including hardware circuit part and software program part.The circuit part is composed of conditioning circuits,digital drive circuits and other components.The program part is written in Verilog HDL language,including data communication,IIR(infinite impulse response)digital filtering and calculating drive parameter.The water flow calibration experiment of the transmitter based on FPGA drive system is conducted under the single-phase flow condition.The results show that the relative error is less than 0.1% and the repeatability is better than 0.05%.The drive effects are compared between the digit drive based on FPGA and the digit drive based on DSP(digital signal processor)under the gas-liquid two-phase flow condition.The results shows that the effect of FPGA drive is better than DSP drive.