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 teams 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. Potential NASA Applications (Limit 1500 characters, approximately 150 words) The controller will be relevant to possible future NASA RPS missions as well as supporting the continued development of double-acting multi-module Stirling machines like ours in the laboratory. NASA previously funded the build and testing of our Stirling convertor and cryocooler, which would both benefit from access to this controller. The stability and adaptive control techniques explored under this project might be relevant to other NASA convertor controllers. Potential Non-NASA Applications (Limit 1500 characters, approximately 150 words) The double-acting multi-module Stirling convertor controller could be adapted for solar thermal power generation applications or applied to support operation of a generator using natural gas or renewable biofuels. As a cryocooler controller, the device could be used for zero-boiloff re-condensation of volatile fuels and other substances or for laboratory and medical cooling systems.
