Laser Pointing and Tracking Using a Completely Electromagnetically Suspended Precision Actuator

Abstract

S TABLE and precise laser beam positioning is required for reliable laser communications among satellites. A very small angular movement of the source laser corresponds to a significant beamposition errorwhen amplified by the large distance between the transmitting and receiving stations and can thus throw a laser beam far off target. The desired pointing and tracking accuracy is typically on the order of 1 rad rms [1,2]. Many efforts have been taken to improve performances of the pointing and tracking systems for intersatellite optical links. A Stewart platform demonstrated a 1 rad rms pointing error in the ambient laboratory disturbance environment [3]. Passive vibration isolation techniques are usually adopted to reduce adverse effects of vibrations on the precision pointing performance objectives [4]. Magnetic levitation technology has been used in high performance motion control systems, owing to its noncontact nature, active control ability, and feasible multiple degree-of-freedom (DOF) actuation. A 6-DOF hybrid magnetic/fluidic positioning stage achieved a 0.3 nm resolution within a 100 m cube [5]. A 6-DOF magnetically levitated device covered a 300 m travel range in translation and 3.5 mrad in rotation with a 2 nm accuracy in translation and 300 nrad in rotation, and a 5 nm resolution [6]. But these systems require many actuators that add difficulties in controls. Very limited stroke is another major barrier [7]. Magnetic bearings are advantageous in special environments with vacuum or extreme temperature, where lubrication problems associatedwithmechanical bearings cannot be ignored. Self-bearing motors (SBMs) combine magnetic bearing and electric motoring functions into a single electromagnetic actuator to perform both force and torque production, and are attractive to space applications where room and weight are also of great concerns. One particular permanent magnet (PM) SBM design featuring a slotless stator allows for a smoother torque generation and is considered to be a good candidate to solve space-borne precision pointing and tracking problems [8]. To investigate its potential of improving pointing accuracy within a large range, two such slotless PM SBMs were incorporated into a 6-DOF magnetic actuator. Reference [9] showed that six decoupled proportional-integral-derivative (PID) controllers achieved stable levitation and smooth angular slewing of the 6-axis active actuator as a positioning stage, using a tape encoder system as the pointing sensor. In this work, the actuator is integrated into a laser pointing and tracking system setup to demonstrate its capability in tracking an external optical pointing sensor through a laser crosslink. The laser acquisition and reacquisition as well as precision pointing and tracking of the system in azimuth are implemented by a switching control strategy between the large-angle encoders and a small-angle laser position sensing detector (PSD) in rotation control, whereas the other five axes remain closed-loop controlled for magnetic bearing stabilization and precision noncontact pointing. Experimental results show that the system achieves high accuracy laser pointing and tracking over a large azimuth range using the single actuator.