Magnetic Levitation for Spherical Drag-Free Sensor Ground Testing

Testing drag-free proof mass systems for gravitational physics requires a ground test system that simulates space conditions as closely as possible. In this experiment, a magnetic levitation system was built, which can levitate, hold, and spin a hollow sphere in a vacuum to simulate operations in a zero-G environment. An optical shadow sensor consisting of an infrared emitter and a pair of photodiodes is used to determine the position of the sphere. A custom-made electronic board converts the analog signal to digital. The board has a switch to reverse the current direction through the coil in case the sphere becomes magnetized and is attracted to the coil in the vacuum. Digital control code has been developed to create a stable feedback control system to drive the coil and hold the sphere in a stable levitating position. The position of the sphere can be adjusted digitally. The system is designed to achieve initial levitation and to restore the sphere to its levitating position if it drops out of lock. COMSOL Multiphysics[1] was used for thermal and magnetic simulation and system analysis. Importantly, the heat generated by the coil is not easily dissipated in the vacuum. The design is optimized to ensure that the coil can be run for at least two hours without overheating. The sphere is spun using four coils driven by digitally generated sine and cosine functions to produce a rotating magnetic field around its equator. Operation up to 18 Hz is easily achieved. The system can be used to test the three-axis optical sensor system Differential Optical Shadow Sensor (DOSS)[2], which is being developed separately, by rapidly switching to the DOSS sensor while the sphere is levitating. Small magnetic pulses can also be used to simulate thruster perturbations in the plane orthogonal to the support axis.