Modular laser gyro

Details Modular laser gyro Modular laser gyro

1997-11-24

2001-03-27

Buczinski, Stephen C. (Department: 3642)

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrometer

C372S094000

Reexamination Certificate

active

06208414

ABSTRACT:

This invention relates generally to laser gyros, and, more particularly, to a modular laser gyro.
RELATED APPLICATIONS
The following issued U.S. Patents and U.S. patent applications are related to the present application and are assigned to the same assignee as the present application.
U.S. Pat. No. 5,225,889, “Laser Gyro Direct Dither Drive”, issued Jun. 6, 1993.
U.S. patent application Ser. No. 07/931,941, entitled “Laser Gyro Microprocessor Start Up Control”, filed Aug. 18, 1992 now U.S. Pat. No. 5,363,194.
U.S. patent application Ser. No. 07/922,612, entitled “Laser Gyro Microprocessor Configuration and Control”, filed Jul. 17, 1992, now U.S. Pat. No. 5,406,369.
U.S. patent application Ser. No. 07/900,403, entitled “Laser Gyro Bias Drift Improvement and Random Drift Improvement”, filed Jun. 18, 1992, now U.S. Pat. No. 5,438,410.
U.S. patent application Ser. No. 08/134,368, entitled “Laser Gyro Microprocessor Based Smart Mode Acquisition and High Performance Mode Hopping”, filed Oct. 1, 1993.
U.S. patent application Ser. No. 07/922,611, entitled “Laser Gyro Built-In Test Method and Apparatus”, filed Jul. 17, 1992, now U.S. Pat. No. 5,390,019.
U.S. patent application Ser. No. 07/805,122, entitled “Laser Gyro Dither Stripper”, filed Dec. 11, 1991, U.S. Pat. No. 5,249,031.
U.S. patent application Ser. No. 08/009,165, entitled “Laser Gyro Single Transformer Power Supply”, filed Jan. 26, 1993, now U.S. Pat. No. 5,371,754.
U.S. patent application Ser. No. 07/902,372, entitled “Laser Gyro Life Prediction”, filed Jun. 16, 1992. U.S. patent application Ser. No. 07/936,155, entitled “Laser Gyro High Voltage Start Module and High Voltage”, filed Aug. 27, 1992, now U.S. Pat. No. 5,299,211.
BACKGROUND OF THE INVENTION
Ring laser angular rate sensors, also called laser gyros, are well known in the art. One example of a ring laser angular rate sensor is U.S. Pat. No. 4,751,718 issued to Hanse, et al., which is incorporated herein by reference thereto. Present day ring laser angular rate sensors include a thermally and mechanically stable laser block having a plurality of formed cavities for enclosing a gap. Mirrors are placed at the extremities of the cavities for reflecting laser beams and providing an optical closed-loop path.
The activation of the laser gyro's various subsystems at start-up may have ramifications for the life of the laser mirrors and other system components. A method is needed to orchestrate the various subsystems at start-up given each subsystem's start-up constraints.
Therefore, it is the motive of this invention to provide a modular laser gyro with a start-up method that provides a synchronized and effective start-up procedure that results in minimum delay and minimum adverse effects.
Laser gyros that utilize microprocessors for their control require that inertial navigation information, control information, test information, and status information be communicated to external systems including an inertial navigation system or a test system. The inclusion of a microprocessor in the laser gyro allows the implementation of new capabilities such as sending autonomous control functions and self testing along with self calibration and self diagnostics. This new capability requires the transmission and reception of a broad spectrum of data, some of which occurs at a high frequency rate.
Therefore it is another motive of this invention to provide a modular laser gyro with an improved communications and control method and apparatus.
Prior art high voltage power supplies for laser gyros used a 2,500 VDC large external power supply placed outside of the laser gyro housing. The external supply required high voltage feed-throughs into the laser gyro housing through a high voltage feed-through connector. The external high voltages also require special cabling and shielding: such high voltage feed-throughs are expensive. Such high voltage feed-through connectors are also difficult to construct while still maintaining a hermetically sealed housing for the laser gyro. Existing high voltage plastic seals may only maintain a vacuum to 10
−6
Torr. In contrast, relatively inexpensive low voltage connector seals may handle a 10
−9
Torr hermetic seal.
It is, therefore, another motive of the invention to provide a modular laser gyro incorporating voltage supply lines that can utilize an inexpensive, hermetic connector.
Associated with such sensors is an undesirable phenomenon called lock-in which has been recognized for some time in the prior art. In the prior art, the lock-in phenomenon has been addressed by rotationally oscillating or dithering such sensors. The rotational oscillation is typically provided by a dither motor. Dither motors of the prior art usually have a suspension system which includes, for example, an outer rim, a central hub member and a plurality of dither motor reeds which project radially from the hub member and are connected between the hub member and the rim. Conventionally, a set of piezoelectric elements serving as an actuator is connected to the suspension system. When actuated through the application of an electrical signal to the piezoelectric elements, the suspension system operates as a dither motor which causes the block of the sensor to oscillate angularly at the natural mechanical resonant frequency of the suspension system. This dither motion is superimposed upon the inertial rotation of the sensor in inertial space. Such dither motors may be used in connection with a single laser gyro, or to dither multiple laser gyros. The prior art includes various approaches to recover inertial rotation data free from dither effects.
It is, therefore, another motive of the invention to provide a modular laser gyro incorporating an improved dither drive and dither stripper which electrically removes (strips) this dither motion from the gyro output.
One technique for maintaining a constant path length is to detect the intensity of one or both of the laser beams and control the path length of the ring laser such that the intensity of one or both of the beams is at a maximum. U.S. Pat. No. 4,152,071 which issued May 1, 1979 to T. J. Podgorski, and is assigned to the assignee of the present invention, illustrates a control mechanism and circuitry as just described. Path length transducers for controlling the path length of the ring laser are well known, and particularly described in U.S. Pat. No. 3,581,227, which issued May 25, 1971 to T. J. Podgorski, U.S. Pat. No. 4,383,763, which issued May 17, 1983 to Hutchings et al and U.S. Pat. No. 4,267,478, which issued May 12, 1981 to Bo H. G. Ljung, et al. All these patents are incorporated herein by reference.
In the aforementioned patents, the beam intensity is either detected directly as illustrated in the aforementioned patents, or may be derived from what is referred to as the double beam signal such as that illustrated in U.S. Pat. No. 4,320,974, which issued on Mar. 23, 1982 to Bo H. G. Ljung, and is also incorporated herein by reference.
Herein “mode” is defined as the equivalent of one wavelength of the laser beam. For a helium-neon laser, one mode is equal to 0.6328 microns which is equal to 24.91 micro-inches.
In path length control systems of the prior art, the path length control finds mirror positioning for which the lasing polygon path length, i.e., the ring laser path length, is an integral number of wavelengths of the desired mode or frequency, as indicated by a spectral line, of the lasing gas. With proper design, the path length control forces the path length traversed by the laser beams to be a value which causes the laser beams to be at maximum power.
As is also known in the prior art, ring laser gyros are subject to small bias drift errors, and noise called random drift errors. Both of these errors may result in significant inaccuracies if the ring laser gyros are operated for extremely long periods of time.
Now referring to
FIG. 56
which shows the results of experiments conducted by Honeywell Inc. of Minneapolis, Minn. which imply the existence of a ring laser gyro bias drift t

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