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Others have previously demonstrated controllers for single-linear alternator “displacer-type” Stirling convertors. In those machines, the phase angle and amplitude ratio between the displacer and piston are independently adjusted with forces provided by springs and drive rods. Therefore, piston and displacer amplitudes tend to remain proportional, and instability takes the form of both components over-stroking or under-stroking together. Essentially this amounts to a single degree of freedom vibrational system, which is relatively straight forward to control.
However, for our double-acting convertor, the amplitudes and relative phases of adjacent pistons are not locked together. Instead, the behavior of every piston also influences its neighbors. There are multiple degrees of freedom, requiring increased control strategy sophistication. Pistons cannot be controlled independently but rather must all be controlled simultaneously. A new class of controller is needed.
In addition to supporting possible future NASA radioisotope power system (RPS) missions, a new controller that addresses the unique requirements of our double-acting multi-module convertor would also have immediate benefits for the continued development of Stirling machines similar to ours in the laboratory. The controller would improve both hardware safety and the repeatability of test conditions – leading to increased data quality and confidence in test results.
Under this Phase I project, our team’s core strengths for modeling the device thermodynamics, interdependent dynamics of multiple double-acting pistons, and linear alternators will be applied to create a new end-to-end system model. The system model will be used to evaluate the performance and stability of the preliminary controller design. This work paves the way for a possible Phase II build of controller hardware and demonstration with our existing Stirling machines.