Diseño de una suspensión magnética para la determinación de densidad de líquidos

Abstract

Density is a derived unit from the base units of mass and length, it is defined as the mass of a body per unit of volume and its unit in the International System of Units (SI) is kg m-3. In the framework of primary metrology, several suspension magnetic systems, used for fluids density determination, have been developed in the last fifty years, allowing the research on physical properties of liquids and gases. The aim of this work is to present the design and development of a magnetic suspension system to be used with a suspension magnetic balance for liquid density determination that will be implemented in the density metrology laboratories at CENAM. For the design of the magnetic suspension, it was necessary to research and choose from different options of systems available by evaluating the impact of the variation on the stability of the suspension related to the variation on the electric current of the bobbin. This study leads to the best selection of the positioning sensor which is important for the feedback of the magnetic suspension control loop. A PID control was designed and the dynamic system was also evaluated in order to obtain the transfer function by identification with least squares approach and some other techniques, with the purpose of a deeper and better knowledge of the system in future work. Results on the stability of the suspension obtained by means of the PID control are shown as well as the impact on the repeatability in measurements of the mass, considering that this will be the main application of the magnetic suspension, where the most relevant input quantity is the repeatability of this system.