Design, Modelling and Sensing Possibilities of Magneto-Rheological Based Devices

This thesis has been put in place during the development of an innovative medical device which consists in an intelligent footwear for foot plantar pressure redistribution in diabetic patients. In fact, despite the several sophisticated techniques developed in the last twenty years, diabetes remains one of the first causes of non-traumatic lower limb amputation worldwide. This is mainly due to the combination of peripheral neuropathy, which determines the loss of pain sensation in the lower extremities, and high plantar pressures, both recurrent among diabetic patients. The target application imposes severe constraints for what concerns the system requirements because of the high plantar pressure magnitude and dynamics achieved by diabetic people during walking. Furthermore, the need to maintain the offloading system portable requires at the same time a high level of miniaturisation and a reduced power consumption. Within a so challenging scenario, a regulating principle relying on Magneto-Rheological (MR) fluids, may represent a good solution. In fact, MR-based systems offer as main and common advantages high sustainable loads, high dynamic ranges of operation, low complexity, high reliability and low power consumption. MR fluids are a particular group of smart materials whose rheological properties (mainly the fluid internal yield stress which in turn determines the apparent viscosity of the fluid itself) can be controlled by and external magnetic field. With increasing levels of exciting field higher values of viscosity can be obtained, with the consequent possibility to control the material transition from the liquid to the semi-solid state. The research work presented in this thesis focuses on MR valves, the core element of the offloading system conceived. Nevertheless, the analysis has been conducted in order to be as broad as possible and most of the concepts presented can be extended to all MR-based devices. The development of an enhanced magnetic equivalent circuit to take into account relevant fringing and leakage phenomena is firstly addressed. High accuracy, flexibility and computational efficiency characterise the proposed approach which can be generalised to any axisymmetric structure. Analytical models are developed to describe three MR valves configurations and the analysis steps followed can be used as guidelines to define a design methodology. A dimensioning routine is implemented to shape the valves structures in order to fulfil some imposed design requirements and/or compare the different valves performances. A qualitatively consistent attempt for the dynamic modelling of MR valves is presented through considerations on energy exchanges between the different physical domains involved. This analysis underlined that MR-based systems behave like transducers and their sensing possibilities are demonstrated experimentally. Finally, all the contents addressed contribute to the conception and realisation of a miniature MR soft shock absorber, the basic constitutive element of the variable stiffness sole conceived. The research activities and the related results presented in this thesis do not pretend to definitely clarify and fix all points still open to question. The aim of this work is rather to provide some further elements and concepts to improve the design and modelling of MR- based devices.