Abstract:
A gyroscope system comprises a gyroscope block having a plurality of cavities and a plurality of passages that define a path; a plurality of mirrors each located in one of the plurality of cavities; at least one mirror drive coupled to one of the plurality of mirrors and configured to change a position of the respective mirror, wherein the path&#39;s length is changed by the change in the position of the respective mirror; a dither system coupled to the gyroscope block and configured to induce an angular rotation of the gyroscope block; and a controller configured to provide a dither signal indicative of a dither frequency to the dither system and a path length control (PLC) signal indicative of a PLC frequency to the at least one mirror drive. The controller is configured to calculate the PLC frequency as a function of the dither frequency.

Description:
BACKGROUND 
       [0001]    Ring laser gyros (RLGs) are instruments used to measure angular rotation. They include a cavity in which two laser beams travel in counter-rotating (i.e., opposite) directions. The laser beams create an optical interference pattern having characteristics representative of the amount by which the RLG is rotated. The interference pattern is detected and processed to provide the angular rotation measurements. 
         [0002]    RLGs are subject to a phenomenon known as “lock-in” which can degrade their measurement accuracy. One known approach for minimizing lock-in is dithering. Dithering is the mechanical oscillation of the RLG. This function is provided by a dither system which includes a motor for generating the oscillations, and a transducer for generating a signal known as the dither pick-off which is representative of the dither motion amplitude and frequency. RLGs also typically include a path length control (PLC) system which adjusts the path length of the laser beams within the RLG cavity to maintain peak steady state intensity/power. 
       SUMMARY 
       [0003]    In one embodiment, a gyroscope system is provided. The gyroscope system comprises a gyroscope block having a plurality of cavities and a plurality of passages that define a path; a plurality of mirrors each located in one of the plurality of cavities; at least one mirror drive coupled to one of the plurality of mirrors and configured to change a position of the respective mirror, wherein the path&#39;s length is changed by the change in the position of the respective mirror; a dither system coupled to the gyroscope block and configured to induce an angular rotation of the gyroscope block; and a controller configured to provide a dither signal indicative of a dither frequency to the dither system and a path length control (PLC) signal indicative of a PLC frequency to the at least one mirror drive. The controller is configured to calculate the PLC frequency as a function of the dither frequency. 
     
    
     
       DRAWINGS 
         [0004]    Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail through the use of the accompanying drawings, in which: 
           [0005]      FIG. 1  is a block diagram of one embodiment of gyroscope system. 
           [0006]      FIG. 2  is a simplified block diagram depicting an exemplary embodiment of a controller in a gyroscope system. 
           [0007]      FIG. 3  is a flow chart of one embodiment of a method of controlling a path length control (PLC) modulation frequency of a gyroscope. 
       
    
    
       [0008]    In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the exemplary embodiments. 
       DETAILED DESCRIPTION 
       [0009]    In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments. However, it is to be understood that other embodiments may be utilized and that logical, mechanical, and electrical changes may be made. Furthermore, the method presented in the drawing figures and the specification is not to be construed as limiting the order in which the individual acts may be performed. The following detailed description is, therefore, not to be taken in a limiting sense. 
         [0010]      FIG. 1  is a block diagram of one embodiment of a gyroscope system  100 . Gyroscope system  100  includes a gyroscope block  10 . In this embodiment, the gyroscope block  10  is a thermally and mechanically stable triangularly-shaped glass-ceramic block. However, it is to be understood that, in other embodiments, other shapes and materials can be used. The gyroscope block  10  contains a plurality of passages  12 ,  14 , and  16  which in turn contain a gas mixture, such as helium-neon. A cavity  18  interconnects the passages  12  and  16 , a cavity  20  interconnects the passages  14  and  16 , and a cavity  22  interconnects the passages  12  and  14  to form a continuous cavity. The gyroscope block  10  also includes mirrors  24  and  26  positioned adjacent to and in communication with the cavities  20  and  22 , respectively. A partially transmissive output mirror  28  is positioned adjacent to and in communication with the cavity  18 . The mirrors  24 ,  26 , and  28  direct clockwise and counterclockwise traveling laser beams within the glass ceramic block  10  as described in further detail below. A beam combiner  30  is coupled to the partially transmissive output mirror  28 . 
         [0011]    A first anode  32  is mounted on the glass ceramic block  10  between the cavities  18  and  22  and is in communication with the passage  12 . A second anode  34  is similarly mounted on the glass ceramic block  10  between the cavities  20  and  22  and is in communication with the passage  14 . A cathode  36  is mounted on the glass ceramic block  10  between the cavities  18  and  20  and is in communication with the passage  16 . A controller  102  electrically energizes the first and second anodes  32  and  34  and the cathode  36  which results in lasing of the gas mixture to establish clockwise and counterclockwise laser beams within the passages  12 ,  14 , and  16  and the cavities  18 ,  20 , and  22 . 
         [0012]    The clockwise and counterclockwise laser beams are reflected by the mirrors  24 ,  26 , and  28 , and are partially transmitted through the partially transmissive output mirror  28 . The portions of the clockwise and counterclockwise laser beams which are transmitted through the partially transmissive output mirror  28  are combined by the beam combiner  30  and are then directed onto a photodetector  38 . The output of the photodetector  38  is decoded by a conventional logic decoder  40  which provides either a pulse on an output line  42  representing clockwise rotation of the glass ceramic block  10 , or a pulse on an output line  44  representing counterclockwise rotation of the glass ceramic block  10 . 
         [0013]    Gyroscope system  100  also includes a dither system. In this embodiment, the dither system is implemented with radial torsion springs or spokes  46  which are mounted between a central support member or hub  48  and a toroidal rim  50 . The hub  48  can be securely attached to an inertial platform, and the toroidal rim  50  is, in turn, in frictional contact with the gyro block  10 . 
         [0014]    At least one piezoelectric actuator  54  is affixed to at least one of the spokes  46 . A modulation voltage signal provided by the controller  102  is then applied to the piezoelectric actuator  54  such that a torsional stress is imparted to the at least one spoke  46  causing the at least one spoke  46  to flex. Flexure of the at least one spoke causes rotational motion of the toroidal rim  50  and the gyro block  10  relative to the hub  48 . The frequency of the modulation voltage is also referred to herein as the dither frequency. 
         [0015]    In addition, at least one piezoelectric transducer  58  is attached to another one of the spokes  46 . The mechanical oscillation of the spokes  46  constitutes dither and is detected by the piezoelectric transducer  58 . The output of the piezoelectric transducer  58  is coupled to an amplifier  60 . The amplifier  60  generates an output signal indicative of angular rotation of the gyroscope due to the dithering. The signal output from the amplifier  60  is provided to the controller  102 . The controller  102  then determines the dither-induced angular rotation of the gyroscope based on the output of the amplifier  60 . In particular, the controller  102  compares the amplifier output with some fixed reference point when the gyro block  10  is at rest. The amplifier output signal is also referred to herein as the dither angle signal. 
         [0016]    It is to be understood that the dither system shown in  FIG. 1  is provided by way of example and not by way of limitation. For example, another exemplary dither system suitable for use in the gyroscope system  100  is described in U.S. Pat. No. 4,751,718 which is incorporated herein by reference. Furthermore, additional exemplary embodiments of gyroscopes implementing dither systems are described in detail in U.S. Pat. Nos. 6,476,918; 5,225,889; 5,249,031; 5,406,369; and 4,533,248 entitled DITHER CONTROL SYSTEM FOR A RING LASER GYRO, LASER GYRO DITHER DRIVE, RING LASER GYRO DITHER STRIPPER, LASER GYRO MICROPROCESSOR CONFIGURATION AND CONTROL, and RING LASER GYROSCOPE, respectively, all of which are incorporated herein by reference. Thus, other suitable dither systems known to one of skill in the art are used, in other embodiments, in place of the dither system discussed above. 
         [0017]    The gyroscope system  100  also includes a path length control (PLC) system. In the example shown in  FIG. 1 , the PLC system includes two mirror drives  60  and  62  each coupled to the back of one of the mirrors  24 , and  26 , respectively. In this exemplary embodiment, the mirror drives  60 , and  62  are implemented as piezoelectric transducers (PZT). The thickness of the PZT  60 , and  62  is controlled by a modulation voltage signal which is provided to each PZT by the controller  102 . In some embodiments, as the voltage is increased, the thickness of the corresponding PZT is decreased. Decreasing the thickness of the PZT in turn increases the path length since each PZT is on the back of a respective mirror. In such embodiments, decreasing the voltage increases the thickness which in turn decreases the path length. However, it is to be understood that the voltage polarity, PZT position, and PZT operation may be reversed in other embodiments. For example, each PZT can be configured to increase in thickness with increasing voltage and vice versa. The frequency of the modulation voltage signal provided by the controller  102  to PZT  60  and  62  is also referred to herein as the PLC modulation frequency. The modulation voltage signal can be implemented as a sinusoidal, square wave, triangle wave, sawtooth signal, etc. 
         [0018]    It is to be understood that the PLC system described herein is provided by way of example and not by way of limitation. In particular, other PLC systems can be used in other embodiments. For example, U.S. Pat. No. 6,354,964; U.S. Pat. No. 4,152,071; and U.S. Pat. No. 5,400,141 each describe exemplary PLC systems which can be suitably used in the gyroscope system  100 . Each of U.S. Pat. No. 6,354,964; U.S. Pat. No. 4,152,071; and U.S. Pat. No. 5,400,141 entitled SINGLE BEAM SIGNAL BLANKING FOR ENHANCED PATH LENGTH CONTROL IN A RING LASER GYRO, CONTROL APPARATUS, and METHOD AND SYSTEM FOR MINIMIZING ANGULAR RANDOM WALK OR BIAS IN A RING LASER GYROSCOPE THROUGH THE USE OF TEMPERATURE BASED CONTROL, respectively, are incorporated herein by reference. 
         [0019]    The controller  102  is configured to adjust or set the PLC modulation frequency based on the dither frequency to mitigate the effects of the PLC modulation frequency beating with the dither frequency. In particular, the PLC modulation frequency is adjusted such that the beat frequency either does not occur or occurs at a high enough frequency that the performance of the gyroscope system  100  is substantially unaffected by the beat frequency. In some embodiments, the dither frequency is determined based on the dither angle signal. In some such embodiments, the PLC modulation frequency is adjusted in real-time based on variations in the detected dither frequency. In other embodiments, the PLC modulation frequency is selected once based on the detected dither frequency and is not periodically updated. 
         [0020]    Additionally, in some implementations, the dither frequency is obtained from a memory device which has stored thereon the dither frequency to be used by the controller  102 . In such embodiments, the PLC modulation frequency is adjusted based on the dither frequency stored in memory rather than on a detected dither frequency. 
         [0021]    Additionally, the PLC modulation frequency is adjusted as a continuous function of the dither frequency in some implementations. In some such embodiments, the PLC modulation frequency is a fixed multiple of the dither frequency. In particular, in one exemplary embodiment, the PLC modulation frequency is adjusted to be 5.5 times the dither frequency. In other embodiments, the controller  102  adjusts the PLC modulation frequency using a finite number of discrete levels based on the dither frequency. For example, in some embodiments, a fixed number of dither frequencies are used. In such embodiments, a discrete level is correlated with one of the dither frequencies. Thus, if three separate dither frequencies are available, there are three PLC modulation frequencies, each associated with one of the dither frequencies. 
         [0022]    The controller  102  can be implemented using hardware, software, firmware, or any combination thereof. For example, in some embodiments, the controller  102  is implemented using analog circuits known to one of skill in the art to adjust the PLC modulation frequency as a fixed multiple of the dither frequency. In other embodiments, digital circuits involving a processing unit executing an algorithm are used. For example,  FIG. 2  is a simplified block diagram depicting an exemplary embodiment of a controller  202  implemented using digital logic. 
         [0023]    The controller  202  includes a processing unit  204  and a memory  206 . The processing unit  204  also controls the lasing of the gas mixture inside the continuous cavity of the gyro block  10 . In particular, the processing unit  204  outputs a signal to the active current control  216 . Based on the signal from the processing unit  204 , the active current control outputs current to the anodes and cathode to cause the discharge of the gas mixture as described above. 
         [0024]    The processing unit  204  includes or functions with software programs, firmware or other computer readable instructions for carrying out various methods, process tasks, calculations, and control functions, used in adjusting or setting the PLC modulation frequency based on the dither frequency. For example, processing unit  204  can include or interface with hardware components and circuitry such as, but not limited to, one or more microprocessors, memory elements, digital signal processing (DSP) elements, interface cards, and other standard components known in the art. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASIC) and field programmable gate arrays (FPGA). 
         [0025]    These instructions are typically stored on any appropriate computer readable medium used for storage of computer readable instructions or data structures. The computer readable medium can be implemented as any available media that can be accessed by a general purpose or special purpose computer or processor, or any programmable logic device. Suitable processor-readable media may include storage or memory media such as magnetic or optical media. For example, storage or memory media may include conventional hard disks, Compact Disk-Read Only Memory (CD-ROM), volatile or non-volatile media such as Random Access Memory (RAM) (including, but not limited to, Synchronous Dynamic Random Access Memory (SDRAM), Double Data Rate (DDR) RAM, RAMBUS Dynamic RAM (RDRAM), Static RAM (SRAM), etc.), Read Only Memory (ROM), Electrically Erasable Programmable ROM (EEPROM), and flash memory, etc. Suitable processor-readable media may also include transmission media such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as a network and/or a wireless link. In the embodiment shown in  FIG. 2 , a PLC frequency instructions  208  is stored on the memory  206  and executed by the processing unit  204 . 
         [0026]    When executed, the PLC frequency instructions  208  cause the processing unit  204  to calculate a PLC modulation frequency based on the dither frequency. For example, in some embodiments, the dither frequency is read from the memory  206 . In other embodiments, the dither frequency is calculated based on the received dither angle signal, as described above. The processing unit  204  determines what frequency to use for the PLC modulation frequency based on the dither frequency. For example, in some implementations, the PLC modulation frequency is determined by multiplying the dither frequency by a constant value, as described above. After calculating the PLC modulation frequency, the processing unit  204  adjusts the frequency of a PLC pulse width modulator (PWM)  210 . The PLC PWM  210  outputs a signal to the PLC control  212  based on the adjusted frequency. The PLC control  212  then converts the PWM signal from the PLC PWM  210  into a square wave voltage signal which is applied to the PZT coupled to each of the plurality of mirrors in the gyro block  10 . The voltage signal causes the thickness of the PZT to increase or decrease and thereby adjust the path length in the gyro block  10 , as described above. 
         [0027]    The processing unit  204  also controls the duty cycle of a dither PWM  214  which outputs a signal to the dither control  216 . The dither control  216  converts the PWM signal from the dither PWM  214  to a modulation voltage signal which is applied to at least one PZT coupled to a spoke as described above. The processing unit  204  also receives a signal indicative of the dither-induced angular rotation of the gyro block  10  from another transducer, as described above. Based on the received signal, the processing unit  204  calculates the dither frequency. Additionally, in some embodiments, the dither frequency is stored on memory  206  and retrieved by the processing unit  204 . It is to be understood that the processing unit  204  can include other components not shown, such as an analog-to-digital converter (A/D) converter, used in carrying out the various functions of the processing unit  204 . 
         [0028]      FIG. 3  is a flow chart depicting an exemplary embodiment of a method  300  of controlling a PLC modulation frequency of a gyroscope. Method  300  can be implemented by the gyro system  100  above. In particular, method  300  can be implemented, in some embodiments, by the processing unit  204  in the controller  202  above. At block  302 , a dither frequency for the gyroscope is obtained. For example, in some embodiments, the dither frequency is calculated based on a signal received from a transducer coupled to the dither system. In other embodiments, the dither frequency is read from a memory such as an EEPROM. 
         [0029]    At block  304 , the PLC modulation frequency is determined based on the dither frequency. For example, in some embodiments, the PLC modulation frequency is determined based on a continuous function of the dither frequency. In particular, the continuous function can be a fixed multiple of the dither frequency. Additionally, the PLC modulation frequency can be updated periodically while the gyroscope is operating based on the dither frequency. Alternatively, the PLC modulation frequency can be determined once based on the dither frequency and not updated in real time while the gyroscope is operating. Furthermore, in an alternative embodiment, the PLC modulation frequency can be selected from a finite number of PLC modulation frequencies based on the dither frequency, as discussed above. 
         [0030]    At block  306 , a signal indicative of the PLC modulation frequency is provided to one or more mirror drives coupled to one or more mirrors, respectively. The signal controls the mirror drives to adjust the path length within the gyroscope. For example, the signal can be a direct current (DC) voltage signal which controls a piezoelectric transducer to change the position of the respective mirror, as discussed above. 
         [0031]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.