Scientists, astronomers, optical re-searchers, and manufacturers frequently require two-axis reflective optical mechanisms to meet the requirements of robust beamsteering systems. Image tracking, scene scanning, line-of-sight pointing and/or beam stabilization designs often incorporate reduced-performance devices in complex arrangements to constrain costs.
Galvoscanners and piezo motor mechanisms have been employed in the past to accomplish two-axis beamsteering. These devices usually incorporate mirrors attached to separately located and controlled tip-and-tilt structures, which introduce several disadvantages in the realms of beam orthogonality, alignment stability, and heat dissipation, as well as problems with the generally limited area available in optics layouts. Combined tip-and-tilt mechanisms are generally available but tend to be very expensive. Custom systems complete with electronic stabilization filters and reaction masses may increase the price to several hundred thousand dollars.
Ball Aerospace and Technologies Corp. recently introduced a low-cost two-axis fast steering mirror (FSM). The operation of this design, based on technology developed over a 15-year period, depends on a synergistic combination of five components: mirror, suspension, actuators, sensors, and control electronics.
The mirror is available as an aluminum, beryllium, or glass substrate. Metal mirrors are light in weight to reduce inertia and to provide optimal dynamic performance via high-efficiency actuators and high-bandwidth position sensors. The reflective surface of an aluminum mirror is produced by diamond turning; an optical coating is applied to allow for cleaning and to provide selective spectral performance. Beryllium mirrors are nickel-plated and polished. Glass mirrors are user-replaceable, with a variety of flatness and coating options; they provide the least expensive option and greatest flexibility, but with some reduction in performance.
The mechanism's design differs from other technologies in that its unique suspension provides two-axis rotation about the center front surface of the mirror. The FSM can be substituted for a folding mirror to yield dynamic steering and/or angular adjustment in the optical train.
Lorentz-force (voice-coil) actuators are used in pairs to produce diametrically balanced "push-pull" forces on the mirrors' opposite edges.
Passive permanent-magnet armature sections are placed on the mirror; no heat is produced by the motor coils directly on the substrate, and no wiring is required on the moving mechanism. The minimal heat from the highly efficient actuator motor coils is dissipated into the FSM assembly's base.
Mirror position is derived from custom differential eddy-current sensors that achieve exceptional resolution and repeatability. Measurement signals are generated and demodulated in electronics housed in the assembly base. The position sensor output is returned to the control electronics to achieve local high-bandwidth servo control.
Modular electronics provide for two-axis commands from the user. The response to user commands emanates from closed-loop servo control, consisting of the mirror's angular position and feedback into an internal loop with compensation.
In the laboratory test setup shown in the photograph, Ball engineers use two FSMs to validate a laser pointing and tracking hardware design and control algorithm developed to detect and compensate for small deviations in a received optical signal. FSM commands are generated, which null the effects of mechanical jitter or vibration on the optical signal introduced by satellite platform disturbances. Bandwidths above 500 Hz with submicroradian accuracies have been achieved.
The fast steering mirrors are also in use in a Daylight Tracking System located at the Starfire Optical Range in Albuquerque, NM. The mirrors are controlled by a digital system that receives error signals from a high-speed CCD camera. The track bandwidth is variable from 20 to 100 Hz, depending on the camera's coadd factor. The FSM is used to remove line-of-sight angular deviations caused by atmospheric gimbal-mount and seismic disturbances. Closed-loop RMS track errors of less than 1 microradian have been demonstrated.
A typical closed-loop bandwidth and disturbance rejection plot for Ball's Model 3B is shown in the figure. In addition to the Model 3B, three other models are available with varying performance specifications. The most recently added, the Model 3G, features a user-removable and replaceable glass substrate, allowing for easy cleaning or replacement. The notion of disposable optics can easily be entertained with this configuration. Glass substrate mirrors also offer the flexibility of interchangeable elements and reduced cost.
Recent applications for FSMs have been found at the Naval Research Laboratory at Flagstaff, AZ; Boeing North American, Autonetics Electronic Systems Division, Anaheim, CA; Goddard Space Flight Center, Greenbelt, MD; Boeing Company Defense and Space Group, Redstone Arsenal, AL; SVS R&D Systems Inc., Albuquerque, NM; and Boeing North American, Rocketdyne Technical Services Division, Kihei Maui, HI.