For optical systems used in communications and instrumentation applications in space and on the ground, eliminating vibration can be a difficult task. Left Hand Design Corp. (LHDC, Boulder County, CO) received Missile Defense Agency (MDA) funding through a 2002 SBIR Phase II award to develop a suite of optical steering devices designed to improve the vibration immunity of optical systems, allowing them to track moving targets with greater accuracy.
LHDC developed an enhanced fine-steering mirror or fast-steering mirror (FSM). Missile interceptor seekers must rapidly point their optical systems at different targets in a dense cluster, a sufficient challenge without the added problem of vibration caused by other moving components on the vehicle. This task requires a mirror that can be positioned rapidly and precisely. The design improvements LHDC successfully developed for this project can provide benefits for these and a wide variety of commercial and aerospace applications, including image motion compensation for orbiting Earth observation, ground- and space-based astronomy, video cameras, industrial inspection, laser communications, laser surgery, and photolithography
How it Works
Optics in an orbiting telescope can be pointed using a two-axis gimbal and by adjusting the satellite's attitude (its orientation in space). These methods are sufficient for coarse and low-speed positioning, but they cannot provide continuous high-speed compensation in the presence of spacecraft platform vibrations. Such vibrations introduce disabling jitter in the optical signal, but FSMs come to the rescue with their capability of rapidly and accurately stabilizing the line of sight.
The mirror "payload" of the FSM system is typically 15 to 300 millimeters in diameter. An optical-tracking detector and/or inertial sensor senses instantaneous deviations from the ideal pointing angle and provides feedback to motors that rapidly adjust the position of the mirror, bringing the optical path back into precise alignment. The system design is critical, requiring materials with the necessary stiffness, lightness, and thermalexpansion properties, further complicated by stresses introduced when the mirror is attached to its mount. For this reason, LHDC offers mirrors fabricated from a variety of materials chosen for the mission. The company's FSMs are currently flying in a number of military aircraft as well.
For more down-to-earth applications of FSMs, LHDC is receiving attention from customers in the field of line-ofsight optical communications, which use a modulated laser beam to carry the information from transmitter to receiver. Disturbances in the air (thermal, wind, and precipitation) and vibrations in the support structure can cause data dropouts, reducing the transmission bandwidth.
Bandwidth is an important parameter of FSM systems; in this case, the unit hertz (Hz) refers to the highest vibration frequency that the system can compensate for. In orbiting applications, gimbals can achieve bandwidths up to 10 Hz. Typical fine-steering mirrors provide bandwidths up to 1,000 Hz, but LHDC has extended that range to 2.8 kHz.
Another key parameter is acceleration, which permits the FSM to compensate for high-amplitude vibrations. LHDC has achieved accelerations greater than 10,000 radians/second2, more than 10 times higher than typical competing FSMs. In developing its FSM products, LHDC must address the conflicting goals of fine-positioning precision, high acceleration, and high bandwidth while maintaining mirror flatness. Extending the bandwidth (while respecting customer's cost constraints) presents an enormous technical challenge, requiring expertise in motion-control electronics, mechanical design, sensors, and materials and process (M&P) science.
Where it Stands
The company offers two product lines: High- Performance for space and other demanding applications, and Cost-Effective for applications in which cost is more critical than low mass. As of late 2006, LHDC had delivered more than 80 of these systems. These products may be configured for the application.
For more information on LHDC's mirror-steering system, click here . (Source: MDA TechUpdate, Missile Defense Agency, National Technology Transfer Center Washington Operations.)