Proc. SPIE. 8052, Acquisition, Tracking, Pointing, and Laser Systems Technologies XXV
KEYWORDS: Field programmable gate arrays, Sensors, Data storage, Actuators, Algorithm development, Mirrors, Detection and tracking algorithms, Computer architecture, Signal processing, Data acquisition
Applied Technology Associates (ATA) is developing a field-programmable gate array (FPGA) based processing platform to transition our state-of-the-art fast-steering mirrors (FSM's) and optical inertial reference units (OIRU's) from the laboratory environment to field operation. This platform offers an abundance of reconfigurable high-speed digital input/output (I/O) and parallel hardware-based processing resources in a compact size, weight, and power (SWaP) form factor with a path to a radiation-hardened version. The FPGA's high-speed I/O can be used to acquire sensor data and drive actuators with minimal latency while the FPGA's processing resources can efficiently realize signal processing and control algorithms with deterministic timing. These features allow high sampling rates between 20 KHz - 30 KHz. This will result in higher open-loop bandwidths in our FSM's and OIRU's. This will subsequently result in improved disturbance rejection in our FSM's and improved base motion jitter rejection in our OIRU's.
This paper briefly presents the embedded system requirements of ATA's FSM's and OIRU's and the FPGA-based computational architecture derived to meet these requirements. It then describes the FPGA cores and embedded software that have been developed to efficiently realize interfacing, signal processing, and data collection tasks. Special attention is given to ATA's high-performance floating-point co-processor and innovative design approach that translates signal processing and control algorithms developed in MATLAB®/Simulink® into their equivalent implementation on the co-processor.
The wavefront control community relies on fast and accurate subsystems for optical tilt correction. New technology
enables large diameter (172 mm), optically-flat (<32 nm rms surface error), highly accurate, fast (500 Hz) steering
mirrors (FSMs) with very low stabilization errors (50 nrad jitter). Applied Technology Associates (ATA) builds and tests
FSMs using Silicon Carbide lightweight optics on very rigid aluminum mounts. Optical encoders provide position
feedback and the mirror control algorithms are embedded in an FPGA processing architecture with fabric-based doubleprecision
arithmetic capability. To characterize mirror performance, ATA integrated a performance verification system
using an xPC MATLAB-based Track Loop Controller to close a 200 Hz optical loop around the FSM. This paper
describes the mirror and FPGA control that enables a new level of FSM stabilization performance and presents both
modeled and measured performance for the system.
Stationary high-bandwidth experiments with a portable lasercom (laser communication) system were performed over a wide range of scintillation indices (< 0.1 to 1) at the Department of Energy’s Nevada Test Site in the summer of 2003. Active alignment was performed with a quad-cell tracking detector at the transmitter transponder and a conical-scan tracking beam at the receiver transponder. During good scintillation conditions, 2-km 10-Gb/s and 11-km 2.5-Gb/s capabilities were demonstrated at error-free bit-error rates over continuous intervals on the order of half an hour. The experimental transponder configuration, which had 2.5-cm transmit-side and 8-cm receive-side aperture diameters, is described and test results are presented. Modifications to the stationary beacon-tracking transponder system that support a semi-autonomous (aided-pointing), mobile, lasercom capability are discussed.
Several applications require agile and accurate positioning of a high power laser beam. Agile positioning is generally accomplished via manipulation of some jointed support apparatus such as a robot arm. The size and weight limitations of high power lasers prevent placement of the laser at the final node of the robot arm. Hughes has developed a continuous, automatic, closed loop, alignment system for robot arm propagation of a high power laser beam based on conical scan tracking. The Hughes design employs a low power, continuous, HeNe laser for control purposes which is co-aligned with the high power beam. The control system receives its continuous feedback from a retro-reflective annulus located at the final node of the robot arm. The system includes a coelostat at each joint enabling complete angular coverage. The type II control system operates with a 30 Hz bandwidth ensuring precise alignment over typical arm velocities and accelerations.