Optical: systems and elements – Mirror – Including specified control or retention of the shape of a...
Reexamination Certificate
1998-09-10
2001-09-25
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Mirror
Including specified control or retention of the shape of a...
C359S846000
Reexamination Certificate
active
06293680
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to actuators, and more particularly to an electromagnetic actuator which operates in conjunction with position sensors to provide a large aperture deformable mirror (“DM”).
BACKGROUND OF THE INVENTION
Complex adaptive optics systems previously used piezoelectric actuators to control the DM figure. For example, a 1500 degree-of-freedom segmented mirror using tubular piezoelectric actuators with a 6 &mgr;m stroke was developed by researchers at ThermoTrex. In 1995, FASTTRAC II, a 12 actuator segmented beam combiner for the Multiple Mirror Telescope (MMT), was built using voice coil actuators and capacitor position sensors. In response to astronomical requirements for low-emissivity, large-throw secondary mirrors, prototype adaptive secondary mirrors using 25 voice coil actuators covering a 6-inch square mirror have been built, as well as prototypes with 24-inch diameter deformable mirrors and 60 voice coil actuators. Upgraded MMTs may employ with over 300 actuators. These use fast steering mirrors with voice coil actuators and capacitor position sensors. All of these designs use voice coil actuators with capacitor position sensors to provide feedback to an internal control loop. There remains a need for an actuator which can operate over a longer range and with low maintenance costs.
SUMMARY OF THE INVENTION
The present invention addresses the needs in the prior art by providing an electromagnetically controlled actuator. The actuator is especially useful in applications such as large aperture deformable mirrors. Employment of the actuators with deformable mirrors enables the mirrors to be reliably positioned and replaceable. The invention may employ a simple flat glass substrate gently forced with air pressure against the electromagnetic actuator. The electromagnetic actuator has built-in position sensing. The position sensing may be accomplished with capacitor sensors.
The present invention uses electromagnet actuators coupled with built-in position sensors for closed loop mirror position control. The primary tradeoffs involve the glass substrate thickness against the allowable power dissipation. Both of these variables are easily scaled, leading to a wide design space incorporating manufacturability and performance.
The present invention allows a deformable mirror which may employ multiple different actuators, especially electromagnetic actuators. While piezoelectric, magnetostrictive, or similar actuators offer only the required stroke, electromagnetic actuators can operate over a much longer range and are the preferred actuator. Instead of using an epoxy connection to make up for manufacturing tolerances, a long actuator stroke allows use of a non-rigid attachment. This, in turn, solves the two primary problems encountered in the prior art; the actuator attachment does not use adhesives and the glass figure does not distort from unwanted stresses.
The invention may use a simple flat glass substrate gently forced with air pressure against electromagnetic actuators. This keeps the cost of the mirror optics comparatively low while making the mirror replacement convenient. The invention allows very low maintenance costs over long periods. Electromagnetic actuators have essentially no catastrophic failure modes, and can be driven with low voltage electronics at low power levels. In addition, the mirror is easily replaced if the coating is ever damaged for any reason.
The present invention, based on a flat mirror substrate and long-stroke electromagnet actuators with built-in position sensing, has a number of advantages over more conventional approaches. These advantages include reduced program risk, higher system reliability, improved facility maintainability, and increased optical performance. The control bandwidth and actuator spacing requirements for this DM are good matches for electromagnetic actuators.
The present invention's use of electromagnet actuators provides many advantages over piezoelectric class materials. Piezoelectric-class (PMN or PZT) materials can be made very small, especially since the mirror stroke and total force requirements are not very large. However, since the DM actuator spacing is large, this feature is not important. Being inherently stiff, PMNs or PZTs are also optimal when bandwidths higher than 1 kHz are required. The same stiffness, however, also leads to attachment and residual figure errors. Electromagnetic actuators normally require more mass, and with their moderate electrical inductance, are more difficult to drive at high frequencies. However, since the bandwidth requirement for the DM is only 100 Hz, electromagnetic actuators are ideal.
The back plate for piezoelectric actuators must be very stable and rigid, to prevent loss of mirror stroke. Based on typical DM requirements, electromagnetic actuators are again the optimal choice. For electromagnetic actuators, power dissipation, not stress, limits the stroke. The cost of the auxiliary position sensors and the inner loop control electronics are very small, since these may be mass produced using inexpensive components. Any repairs are infrequent and inexpensive. In other words, manufacturability is designed into the components.
Another advantage of the present invention is that only a simple mirror substrate (flat on both sides) is required, using air pressure to maintain contact between the actuators and the mirror substrate.
This solves the following potential problems. The best mirror coating design can be applied without worrying about damage to a complexly machined substrate, reducing program risk. The finished mirrors can be shipped separately from the mechanical assemblies, and mated in a clean room. This increases mirror lifetime. The force attachments are flexible enough to prevent imprinting or generating high spatial frequency errors. This results in small residual wavefront errors.
Another advantage is the ability to perform all diagnostics, including prototype design and test, on a substitute mirror of similar quality, but without the expensive coatings. In this manner, the finished mirror is held in a protected location, and is only installed during the final tests. This reduces schedule risk by enabling prototype testing early in the program.
Every actuator may be tested during and after final assembly. If an actuator should fail, it is always easily replaced, enhancing maintainability. This testing may be done on the substitute mirror, reducing handling on the final coated mirror. After sufficient testing, the built-in position sensors can be used to verify dynamic range without the use of direct optical interferometry. This further reduces the manufacturing costs and schedule risks. Only after the units are fully completed, tested, and burned-in, need the final coated mirrors be installed.
The mirror substrate is easily replaceable in the event of coating damage, enhancing system maintainability. When the atmospheric pressure difference is released, the mirror can simply be lifted out. The mirror rests against stops on the bottom edge, so the new mirror position will be practically indistinguishable from the original. Depending on the mirror's back side surface quality, recalibration may not be necessary. Closed loop operation can be resumed immediately.
Zero electrical power to the DM assembly results in the free standing shape of the mirror. This may be adequate to meet any open loop flatness requirement, if the mirror's two coatings cancel stresses. If some figure remains, it will be very low order astigmatism and defocus, not higher order, small spatial frequency ripples as occur in piezoelectric designs. To correct this error, the internal control loop is turned on, using preset values at each actuator. Since the invention has essentially no hysteresis, the mirror will be flat.
The invention employs a minimum of precision parts. This reduces risk and cost, since parts can be manufactured at most machine shops, and assembly tasks become routine.
The invention conveniently allows hybrid approaches, m
Fish & Richardson P.C.
Schuberg Darren
Thermotrex Corporation
Treas Jared
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