Control Strategies Utilizing the Physics of Flux-Pinned Interfaces for Spacecraft

Abstract

Flux-Pinned Interfaces are a developing technology for spacecraft that exploit flux pinning, a phenomenon in superconducting physics, to manipulate the dynamics between two spacecraft modules. Flux pinning occurs in certain types of superconductors which, when cooled below their critical temperature, will resist changes to the distribution of magnetic flux that was present during the temperature transition. The resulting physics passively "pins" a magnetic field source in a six-degree-of-freedom equilibrium relative to the superconductor. By placing a magnetic array on one spacecraft module and a superconductor with this capability on another, it is possible to exploit these passive physics to augment technologies for close-proximity spacecraft operations such as rendezvous, docking, and formation flying. This paper investigates the use of the nonlinearities and specific physics in the flux pinning connection to develop efficient control strategies that will allow spacecraft operators to alter the equilibrium of a flux-pinned space system. Specifically, the dynamic model for flux-pinned spacecraft is presented, a set of design principles for actuators are examined with simulation plots to demonstrate the system response to various stimuli, and two potential control strategies are presented. The paper concludes with an assessment of the actuation strategies and potential advantages and disadvantages that each has to offer in the context of actual spacecraft operations.

Flight-Experiment Validation of the Dynamic Capabilities of a Flux-Pinned Interface as a Docking Mechanism

“…By assuming linear damping and stiffness, the underdamped oscillations are represented by Eq. (9). ( ) is the system energy over time.…”

“…The passivity and the stability of flux-pinned interfaces reduce the sensing and actuation requirements but necessitates more stringent thermal requirements on the spacecraft system to cryogenically cool the superconductors [7] [8]. Electromagnetic actuators can further enhance an FPI by enabling contactless manipulation during close-proximity maneuvers [9].…”

Linearized Dynamics of General Flux-Pinned Interfaces

“…The final matrix expression for linearized force between two magnetic moment dipoles is given in (11). The moment/torque of a magnetic dipole acting on another magnetic dipole at distance ρ is given by (12), also derived by Villani [5]. The same process of linearization is applied to Villani's moment equation to yield (13).…”

“…A flux-pinned interface offers many benefits for robotics applications: passive stability, compliance, absence of mechanical contact, and low mass requirements. Flux-pinned systems can be actively manipulated to control the orientation and position of close-proximity vehicles, while remaining contactless and compliant [12]. Traditional, linear control synthesis may be successful for such systems, but the inherently nonlinear dynamics must be linearized to provide a suitable plant model.…”

Flight Validation of a Multi-Degree-of-Freedom Flux-Pinning Spacecraft Model

“…4 The relative equilibrium of the modules can be altered on-orbit simply by changing the magnetic field in the FPI (for example, by varying currents in an electromagnet). 5 Thus, the natural dynamics of the FPI can provide impact attenuation, automatic docking alignment, and tunable stiff connections between spacecraft. As a result, FPIs offer a robust, flexible solution to proximity operations for spacecraft in a relative distance range that is typically filled with the most risk.…”