First Advisor
Jonathan Bird
Term of Graduation
January 2026
Date of Publication
6-1-2026
Document Type
Dissertation
Language
English
Subjects
Actuator, Electromagnetics, Finite element analysis, Magnetic spring, Permanent magnet, Wave energy
Physical Description
1 online resource ( pages)
Abstract
This dissertation investigates new types of magnetic torsion springs with adjustable stiffness for wave energy converter applications. The research has focused on investigating magnetic torsion spring designs that can maximize their energy density and increase their linear stroke length. Unlike mechanical springs, magnetic springs can operate equally well with both positive and negative stiffness, and the stiffness can be adjusted by changing the relative position of the magnets. The efficiency of magnetic springs is high as they exhibit minimal power loss due to their non-contact structure. In contrast, mechanical springs may experience hysteresis and wear-related losses, such as fatigue and contact failure. Magnetic torsion springs also provide overload protection, since they will pole slip rather than catastrophically failing if the peak torque is exceeded. This dissertation investigated a new type of tangential flux torsional magnetic spring that was designed, constructed, and experimentally tested. The linear torque of the spring was created by using a set of large tangentially magnetized inner rotor magnets. Unlike prior published designs, the presented tangential-flux torsional magnetic spring has a highly linear stroke length. However, the spring's maximum angular stroke length was limited to 45°. To maximize the energy density of the tangential-flux torsional spring, a geometric sweep analysis was performed using 3-D finite element analysis, and the measured peak mass-energy density was 8.02 J/kg. The stiffness was experimentally shown to be adjustable via the mechanical translation of the inner rotor relative to the outer rotor, and the measured maximum spring rate was 47.6 Nm/rad. By using a test-bed, the resonance capability of the tangential-flux torsional magnetic spring was also verified by adding a pendulum and a mechanical torsion spring with a 360° stroke length in series with the geared magnetic spring. The mechanical spring and pendulum represented the inertia and intrinsic stiffness of the wave energy converter(WEC), and by changing the stiffness of the magnetic spring, resonance at different frequencies was experimentally demonstrated. To achieve a higher torque capability, a scaled-up version of the tangential-flux torsional spring with a measured peak torque of 763 Nm at 45° was designed for Sandia National Laboratory. To avoid simply enlarging rotor dimensions and causing difficulties in manufacturing and handling giant magnets, the torque was increased by using a unique seven-stage multi-stacking axial design. The design phase employed a finite element analysis energy density geometric sweeping approach that maintained a fixed magnet-to-airgap radius ratio. This approach maintained the maximum energy density while also considering the practical dimensions and assembly difficulties of the magnets. The measured peak energy density for the scaled-up torsion spring was 11.8 J/kg. This dissertation also presents a new type of magnetic lead screw compression spring that consisted of a variable stiffness magnetic compression spring integrated with a magnetic lead screw. The magnetic lead screw converted the linear translation of the magnetic compression spring into angular rotation, thereby amplifying the torsional angle and eliminating the need for a gearbox. This magnetic lead screw compression spring was experimentally shown to achieve a 782° angular stroke length. A scaled-up magnetic lead screw compressive spring design is also presented that was designed to have a 1,013 Nm peak torque and a 510° stroke length. However, because the stroke length for a negative stiffness compression spring is limited, the force requirement becomes excessively large. This then makes the scaled design impractical due to its resulting low energy density and significant mechanical loading requirements. It was concluded that the magnetic lead screw compression spring is more suitable for low torque applications that can operate over a short compressive stroke length. To address the low energy density and stroke length limitations of the prior tested magnetic spring designs, a novel helical magnetic torsion spring inspired by the magnetic lead screw principle was invented. The design used a 4-pole helical magnetic field to create torque with a 130° stroke length. Using 3-D finite element analysis, a peak torque of 1,883 Nm with a 20.4 J/kg mass energy density was calculated. This is a significant energy density improvement over prior published works. The experimentally tested design yielded a torque of 1,353 Nm and an active region energy density of 20.3 J/kg. The helical magnetic spring stiffness can be adjusted by axially translating the inner rotor relative to the outer rotor. However, the stiffness has discrete step changes dictated by the number of axial poles.
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Recommended Citation
Che, Dawei, "Investigation and Design of Adjustable Stiffness Magnetic Springs for Wave Energy Converters" (2026). Dissertations and Theses. Paper 7110.