Vibration isolation for coriolis mass-flow meters

Abstract

A Coriolis Mass-Flow Meter (CMFM) is an active device based on the Coriolis force principle for direct mass-flow measurements, with high accuracy, range-ability and repeatability. The working principle of a CMFM is as follows: a fluid conveying tube is actuated to oscillate at a low amplitude. A fluid-flow in the vibrating tube induces Coriolis forces, which are proportional to the mass-flow, and affect the tube motion. Measuring the tube displacement in such a way that the change of its mode shape is determined, allows calculating the mass-flow. For low flows (< 1 kg/h), the Coriolis force induced motion is relatively small compared to motions induced by external vibrations, thus CMFMs designed to be sensitive to low flows are rather sensitive to external vibrations. Mainly external vibrations around the meter’s drive and Coriolis frequencies result in a measurement error. A model-based, quantitative estimation of the expected mass-flow error in response to external vibrations is obtained. The sensitivity to external vibrations is reduced, using passive and active vibration isolation solutions. Passive vibration isolation consists of a mass-spring-damper system between the floor and the measurement device. The performance is insufficient, because the suspension frequency is limited by the maximum stress in the connection tubes and a maximum allowable sag due to gravity. Therefore the passively suspended stage is extended with voice-coil actuators and absolute motion sensors to apply active vibration isolation control. Increased narrow-band vibration attenuation is achieved using loop shaping feedback control. Alternatively, an innovative feed-forward strategy is used, where by the parameters, which are tuned using an adaptive FxLMS technique, are based on the damping and stiffness properties of the stage. Based on the theoretic models, a proof of principle mechanism is realised for validation, whereby attenuation of more than 40 dB is achieved. The performance is limited by sensor noise and for the feedback solution also by the destabilising effect of higher order dynamics. The thesis presents a comprehensive analysis on how vibration isolation can be achieved. Narrow-band active vibration isolation control is successfully implemented in a design of a CMFM, for the first time.