Coriolis mass-flow meters are used often in process measurements technology for determining mass-flow of a fluid in a section of pipeline. The Coriolis measuring principle is based on allowing the fluid to be investigated to flow through an oscillating measuring tube and evaluating the oscillatory movement. To this end, an oscillation exciter and two oscillation sensors are arranged at the measuring tube. The measuring tube with the fluid form together an oscillatable system, which is normally excited to its resonance frequency. The resonance frequency depends on, among other things, the material and dimensions of the measuring tube. Resonance frequency also depends on the density of the fluid flowing in the measuring tube.
In certain applications, the measuring tube is not excited to the resonance frequency, but, instead, to a neighboring frequency.
The two oscillation sensors register the oscillatory movement of the measuring tube at two locations spaced in the direction of flow and convert the oscillatory movements of the measuring tube into sensor signals. The two sensor signals exhibit the same frequency as the oscillatory movement of the measuring tube. If fluid is flowing through the measuring tube, then the two sensor signals are shifted in phase relative to one another. The phase shift is a measure for the mass-flow of the fluid through this section of pipeline. The sensor signals are evaluated in a measuring circuit portion, in order to determine the value of the mass-flow. This measured value can be presented in a display unit on the Coriolis mass-flow meter. As a rule, sensors, such as Coriolis mass-flow meters, which are used in automation technology, are connected with superordinated units, such as, for example, controllers or control systems, etc. Besides mass-flow, other properties of the fluid, such as, for example, density, can be determined. For this, a frequency evaluation of the oscillatory movement of the measuring tube is necessary.
Various types of Coriolis mass-flow meters are manufactured and sold by the firm Endress+Hauser Flowtec AG.
In the U.S. Pat. No. 4,801,897, an exciter circuit portion for a Coriolis mass-flow meter is described, which is constructed as a type of analog, phase-lag-loop control. The exciter frequency for the measuring tube tunes, in such case, automatically to the resonance frequency of the measuring tube, even in the presence of variable fluid density.
Known measuring circuits work either on an analog basis or digitally. Examples of such measuring circuits are described in greater detail in EP 698783, U.S. Pat. No. 4,895,030, EP 702212, or U.S. Pat. No. 4,529,002.
EP 698783 discloses a measuring circuit for a Coriolis mass-flow meter. Included is an analog control circuit, which controls the two sensor signals to the same amplitude. This amplitude control is of decisive importance for the measuring accuracy of the Coriolis mass-flow meter.
EP 866319 discloses another measuring and operating circuit for a Coriolis mass-flow meter. In the case of this circuit, the two sensor signals are amplified before their further processing, with the amplification factor of the amplifier being variable. In a digital signal processor, the sum and the difference of the two sensor signals are evaluated, as well as one of the sensor signals. Also here, it is essential for the measuring accuracy that the two sensor signals have, following their amplification, the same amplitude.
The amplifiers and pre amplifiers used for the analog amplification of the sensor signals must have a sufficient bandwidth, in order to prevent corruptions of the sensor signals. Due to disturbance signals, however, over-driving of the amplifiers can occur. Such over-driving acts negatively as regards accuracy of measurement. The higher the requirements for accuracy placed on the Coriolis mass-flow meter, the more complex the analog amplifiers have to be, this being reflected in increased price for the amplifiers.
Especially in the case of gas applications, relatively high flow velocities of the fluid occur. Typical values are 50-100 m/s. These high flow velocities mean a relatively high sound level in the measuring tube, which can cause significant disturbance signals.