In situ monitoring of the iron content in magnethorheological rotary actuators

Abstract

Magnetorheological (MR) fluids are a class of smart materials with versatile applications in various engineering domains. An MR fluid consists essentially of a carrier medium (e.g. synthetic oil) with embedded magnetizable particles (carbonyl iron particles in our case). Upon magnetization, the particles form clusters and similar microstructures affecting the rheological properties of the MR fluid. Apart from the magnetic field, a strong determinant of the viscosity of the fluid is the iron particle concentration. While initially the particle concentration in each fluid is known and the particles are uniformly distributed, previous studies show that the local concentration changes dynamically during operation, especially in areas of high magnetic field strengths, which tend to attract particles. Hence, the viscosity is not only dependent on the magnetic field as desired, but can vary over time. 
This work aims at developing a model that allows to determine the dynamically changing iron content during operation of a MR actuator and therefore builds the foundation for predicting the currently prevailing viscosity, enabling a more precise modeling of the actuator’s behavior. In particular, the iron content is identified by devising a physically inspired equivalent circuit of the coil magnetizing the magnetic circuit. The components of this circuit can be determined from measurements, which, in turn, allow to indirectly determine the iron content from the main inductance of the equivalent circuit, where this inductance turns out to be approximately linearly related to the iron content.