Monitoring sedimentation of magnetorheological fluids using a vertical axis monitoring system with a low aspect ratio sensor coil

Adaptive energy absorbers (EAs) utilizing magnetorheological fluids (MRFs) are currently being investigated for severe impact or shock mitigation problems because magnetorheological energy absorbers can adjust stroking load to account for severity of impact and payload mass. Utilizing MRFs in such EAs requires a highly stable suspension that maintains a uniform concentration. Suspension stability of MRFs can be studied using a MRF column and an automated vertical axis inductance monitoring system (AVAIMS), where an inductance sensor is translated up and down the vertical MRF column to track the mud-line, defined as the boundary between the clarified fluid at the top of the column and the MRF below. The rate of descent of the mud-line is typically referred to as the sedimentation rate of the MRF. In this paper, a refined inductance sensor is developed that features a low aspect ratio coil to better localize rapid changes in concentration or sedimentation zone boundaries, as well as to improve symmetry of the applied magnetic field in the MRF sample enclosed by the sensor. This low aspect ratio solenoid (LARS) sensor was developed to measure the vertical distribution of the concentration of carbonyl iron particles in the MRF column. Compared with the high aspect ratio solenoid sensor used in our prior work, the results of FEM simulation and sedimentation experiments demonstrate that the LARS sensor was better able to localize rapid changes in concentration or density. Using the AVAIMS with the refined LARS sensor, a method was developed, based on the measured concentration gradient profile, to identify sedimentation zone boundaries in an MRF column, and these results were compared to our prior work using the measured concentration profile only. Sedimentation zone boundaries that are consistent with Kynch's sedimentation theory were obtained using the concentration gradient profile method, such as monotonically descending mud-line, linearly ascending gel-line, and linearly ascending cake-line. Key metrics to characterize the time histories of the sedimentation zone boundaries are measured including mud-line descent rate, gel-line ascent rate, and cake-line ascent rate. Identification of sedimentation zone boundaries using the LARS sensor in conjunction with the concentration gradient profile method exhibited two main advantages including the direct measurement and improved localization of the large concentration change that occurs at a sedimentation zone boundary, and the elimination of sensor bias.