Novel Magnet Structures for Mechanically Robust Linear Motors

Abstract

Linear motors form the backbone to many nanometer-precision machine tools, providing precise and smooth force to drive heavy stages at high speeds. In the magnet arrays of linear motors, the permanent magnets must be attached to the backrion to transmit force to the rest of the system. This connection usually is made with an epoxy bond. As machine tools are pressed harder for ever longer service lives, the epoxy's limited fatigue life can be exceeded, potentially leading to a mechanical failure. This poses a serious problem to the growth of the machine tool industry. Interior permanent-magnet (IPM) motors, found in nearly every electric car, offer an interesting approach to solve this problem. In an IPM motor, the magnets are retained in slots in the rotor laminations, providing a robust mechanical constraint.

These steel constraining features can be engineered to have infinite lifetime under the loads present, and reduce or even eliminate the need to use epoxy to retain the magnets. In this thesis, we take inspiration from IPM motors and apply it to linear motors. We suggest two magnet arrays which use novel mechanical geometry to increase mechanical robustness and reliability. Additionally, we use unique magnet structures to increase force production per unit current. First, we propose an array with steel fins in the backiron to mechanically constrain the magnets in shear, to reduce shear stress in the epoxy. This array also uses a Halbach array to increase force production. Using finite element modeling, we predict this array to produce 17% more force per unit amp than the off-the-shelf array. We then propose a second array geometry which has magnets fully constrained inside steel laminations.

Additionally, this array utilizes a novel flux focusing geometry to further increase force production. Using finite-element analysis, we predicted this array to produce 22% more force per unit amp than the off-the-shelf array. We test both of these magnet arrays against an off-the-shelf magnet array and compare performance. We find our first, partially constrained array to provide 25% more force per unit amp that the off-the-shelf array. We find our second, fully constrained array to provide only 7% more force per unit amp that the off-the-shelf array, most likely due to end-effects causing saturation in unexpected ways. We suggest many improvements to our designs in the future work section.