Model, control and performance of a six degree-of-freedom precision pointing and tracking system

In this dissertation, a six degree-of-freedom (6-DOF) precision pointing and tracking system is integrated to demonstrate its laser pointing and tracking capability in ambient laboratory environment. Such a demonstration is intended for potential high accuracy pointing and smooth angular slewing applications in space, such as free-space laser communications among satellites. The key technology adopted is the slotless permanent magnet (PM) self-bearing motor (SBM), which is first utilized to fully provide radial bearing and motoring functionality simultaneously in a novel 6-DOF magnetic actuator.; The precision actuator incorporates two SBMs and one active magnetic bearing (AMB), and thus allows for a complete electromagnetic suspension and precision non-contact pointing. The sensing scheme is critical to the feedback control of the open loop unstable magnetic actuator. Among various sensors used, the unique linear tape encoding strategy uses the same concept as in the SBM and provides high resolution and non-contact measurement for radial and angular displacements simultaneously. To accurately characterize current and negative stiffness gains in the linearized force-current-displacement relation of the large-scale SBM, the analytical force and torque expressions are derived using the Maxwell stress tensor method. A general two-dimensional (2-D) magnetic field analysis in the large effective air gap is conducted and the field components due to thick windings and PMs are formulated in explicit forms. All analytical solutions are validated by the electromagnetic finite element analyses (FEA). An analytical representation of the overall dynamic system is presented for linear controller design. Six decoupled proportional-integral-derivative (PID) controllers are designed and a real-time digital feedback control system is implemented.; Intensive experiments are carried out to evaluate the closed loop performance. The actuator is capable of smooth angular slewing while maintaining good stabilization. Enhanced by a switching algorithm for smooth switches between encoder and position sensing device (PSD) feedback loops, the overall system demonstrates successful acquisition and reacquisition as well as precision pointing and tracking capability of the laser beam. This 6-DOF precision pointing and tracking system achieves a pointing and tracking accuracy below 1 murad with an angular resolution of 754 nrad over a large azimuth range of +/-45° using a single actuator.