[Objective] To eliminate the dependence of spacecraft solar-array sun tracking on electronic sun sensors and control electronics by establishing a fully passive, zero-power tracking drive mechanism.
[Methods] The shadow-boundary self-sensing principle is proposed, in which the array's own geometric shadow edge acts as a position-sensitive switch, solar radiation directly actuates shape-memory-alloy (SMA) wires through reverse martensitic transformation, and a zero-net-force equilibrium condition replaces the PID controller in directional decision-making. Analytical relations linking the sensing threshold angle to SMA setback geometry and transformation kinetics are established, together with fatigue-life and vacuum-environment protection models.
[Results] Two complementary configurations are developed. Configuration A (border shadow + dual SMA + one-way ratchet) targets LEO CubeSats with an 80 deg-100 deg tracking window, about 12 parts and zero steady-state power. Configuration B (petal array + SMA auto-clutch) enables deep-space omnidirectional tracking with fault-degradation redundancy, shear-pin overload release and a co-rotating sunshade for self-starting. Complete part inventories, failure-mode mappings and design constraint equations are provided.
[Limitations] This work presents a conceptual design without a prototype; all boundary conditions are derived analytically from published SMA material parameters, and validation depends on thermal-vacuum and microgravity testing. A three-stage verification roadmap (component bench - thermal-vacuum - microgravity) is outlined.
[Conclusions] A complete sense-decide-actuate loop is achievable through purely mechanical and thermodynamic means, without electronic components or actuation power, offering a verifiable zero-power tracking path for LEO CubeSats and long-duration deep-space missions.
