Excitation circuits for coriolis mass flowmeters

An excitation circuit for a Coriolis mass flowmeter, said flowmeter having at least one vibrating flow tube, an electromechanical excitation assembly for vibrating the at least one flow tube at a frequency equal or adjacent to the instantaneous mechanical resonance frequency of the flow tube;

• a tranducer assembly for generating a first transducer signal representing inlet-side vibrations of said measuring tube and a second transducer signal representing outlet-side vibrations of said measuring tube, said excitation circuit comprising:
• an demodulation stage fed by one of the transducer signals or a sum of both transducer signals, said demodulation stage being operable to generate an output signal representing an oscillation amplitude of said vibrating tube,
• a comparison stage fed by the output signal of the demodulation stage, said comparison stage being operable to generate an output signal representing a deviation of said oscillation amplitude of the vibrating tube from an predetermined reference oscillation amplitude for said vibrations;
• an pulse duration modulation stage for modulating said signal fed to the demodulation stage with said output signal from the comparision stage, said pulse duration stage being operable to generate a drive signal for supplying said excitation assembly.

A two-wire Coriolis mass flowmeter which is connected to, is powered exclusively from, and outputs a measurement signal exclusively via a two-wire process control loop, said Coriolis mass flowmeter having at least one vibrating flow tube, an electromechanical excititation assembly for vibrating the at least one flow tube at a frequency equal or adjacent to the instantaneous mechanical resonance frequency of the flow tube, a transducer assembly for generating a first transducer signal representing inlet-side vibrations of said measuring tube and a second transducer signal representing outlet-side vibrations of said measuring tube, and an excitation circuit, said excitation circuit comprising:

• a demodulation stage fed by one of the transducer signals or a sum of both transducer signals, said demodulation stage being operable to generate an output signal representing an oscillation amplitude of said vibrating tube,
• a comparison stage fed by the output signal of the demodulation stage, said comparison stage being operable to generate an output signal representing a deviation said oscillation amplitude of the vibrating tube from an predetermined reference oscillation amplitude of said vibrations;
• an amplitude modulation stage for modulating said signal fed to the demodulation stage with said output signal from the comparison stage, said amplitude stage being operable to generate a drive signal for supplying said excitation assembly.

A two-wire Coriolis mass flowmeter which is connected to, is powered exclusively from, and outputs a measurement signal exclusively via a two-wire process control loop, said Coriolis mass flowmeter having at least one vibrating flow tube, an electromechanical excitation assembly for vibrating the at least one flow tube at a frequency equal or adjacent to the instantaneous mechanical resonance frequency of the flow tube, a transducer assembly for generating a first transducer signal representing inlet-side vibrations of said measuring tube and a second transducer signal representing outlet-side vibrations of said measuring tube, and an excitation circuit, said excitation circuit comprising:

• a demodulation stage fed by one of the transducer signals or a sum of both transducer signals, said demodulation stage being operable to generate an output signal representing an oscillation amplitude of said vibrating tube,
• a comparison stage fed by the output signal of the demodulation stage, said comparison stage being operable to generate an output signal representing a deviation of said oscillation amplitude of the vibrating tube from an predetermined reference oscillation amplitude of said vibrations;
• a pulse duration modulation stage for modulating said signal fed to the demodulation stage with said output signal from the comparison stage, said pulse duration stage being operable to generate a drive signal for supplying said excitation assembly.