Analysis and Verification of a Rheological Servomechanism
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This thesis is an investigation into a small hydraulic actuator based on magnetorheological fluids. Magnetorheological fluids are "smart", synthetic fluids with the ability to change their viscosity from liquid to semi-solid state within milliseconds if a sufficiently strong magnetic field is applied. The actuator should be smaller than 5mm in diameter. The thesis first gives a review of three different product development methodologies, before different actuator concepts are investigated and a concept is chosen for further exploration. Mathematical models and simulations in Comsol are used in order to find the optimal geometry and material, and to find the performance of the actuator. Finally, real tests are carried out in order to investigate some of the properties with the models and simulations that was linked to some uncertainty. The main principle for creating a movement of the actuator is pressure distribution caused by addition and subtraction of magnetic fields over the magnetorheological fluid. It seems from the tests that subtraction of magnetic fields has a very little effect on the fluid flow. A theory is that the magnetic fields, instead of getting "subtracted", are deflected and placed parallel to the flow direction. Still, the subtraction does work the intended way, but with an extremely small effect. This can probably be improved by finding ways to lead the deflected field away from the fluid. Even though the dimensions should be quite small (in the order of millimeters), it seems as if there should be no problem in achieving the required magnetic field from an electromagnetic coil without it going into saturation. It is a question, though, how large effect the windings of the coil will have on its size. From the results of the calculations and simulations it seems as if the actuator can achieve both a stiffness of 0.5N/mm and a movement of over 5mm, even if subtraction of magnetic fields does not work the intended way. This is with an actuator diameter of 4mm. A higher stiffness will give a smaller movement, and vice versa. It is recommended to either do more extensive simulations in three dimensions in Comsol, with the permanent magnets and electromagnetic coil incorporated, or to build a full-scale prototype of the actuator.