Non-linear Control of the Oscillation Amplitude of a Coriolis Mass-Flow Meter

In a Coriolis mass-flow meter (CMFM) the process fluid (of potentially variable density) flows through a pipe undergoing yawing vibration to generate the required transverse Coriolis acceleration. Reliable control of the amplitude of vibration is necessary for this and other applications employing a deliberately oscillating mass-spring-damper where the mass and damping are subject to variation. The method proposed uses positive feedback of the output velocity to cancel the internal damping, with a gain determined by an outer sampled-data amplitude-control loop. The insertion of two non-linearities into the outer loop is shown to induce overall global linearity. Hence a straightforward proportional-plus-integral control design procedure gives results which are uniformly satisfactory over desired amplitudes spanning several decades, unlike naive methods which are effective for only a limited range of amplitudes. A simple desaturation strategy overcomes potential problems caused by power limits in the actuator. A practical example of successful use of the overall controller on a CMFM indicates the improvement over an existing analogue design.