Abstract:
A gyroscope for determining an angular rate output. The gyroscope includes a first demodulator configured to demodulate an angular rate measurement at a first bias modulation frequency to determine the angular rate signal and a second demodulator configured to demodulate the angular rate measurement at a second bias modulation frequency to provide a signal with ARW information. The gyroscope further includes an ARW estimator that provides an output that is proportional to ARW that is then stored in a memory. The second bias modulation frequency is an even order harmonic of the first bias modulation frequency.

Description:
GOVERNMENT INTEREST 
       [0001]    The invention described herein was made in the performance of work under U.S. Government Contract No. N00030-05-C-0063 awarded by the United States Navy. The Government may have rights to portions of this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    An important requirement for a fiber optic gyro (FOG) is the ability to monitor its health status or accuracy for health diagnostics. For most navigation systems including FOGs, angle random walk (ARW) is a major contributor to navigation errors. ARW is measured in units of degrees per root unit time that directly affects angular rate calculations, independent from other types of error (e.g., scale factor or bias error). 
         [0003]    ARW monitors can provide valuable information for health diagnostics. Current systems and methods utilized to monitor ARW indirectly only monitor parameters affecting ARW instead of actual ARW. These indirect ARW determinations can lead to additional support costs as well accuracy issues, including false alarms or false negatives. These systems also require a large number of parameter monitoring devices that negatively increase system size, weight, and power consumption. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention relates to a gyroscope for measuring an angular rate output. In accordance with one aspect of the invention, the gyroscope includes a first component configured to demodulate an angular rate measurement at a first modulation frequency to determine the angular rate output and a second component configured to demodulate the angular rate measurement at a second modulation frequency to determine an ARW output. The gyroscope also includes a memory configured to store the ARW output. 
         [0005]    In accordance with another aspect of the invention, the second modulation frequency is an even order harmonic of the first modulation frequency. 
         [0006]    In accordance with a further aspect of the invention, the gyroscope also includes a filter for filtering an angular rate measurement input of the second demodulator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0008]      FIG. 1  illustrates a functional block diagram of an example gyroscope formed in accordance with an embodiment of the present invention; 
           [0009]      FIG. 2  illustrates a functional block diagram of an example signal processing circuit formed in accordance with an embodiment of the present invention; 
           [0010]      FIG. 3  illustrates a functional block diagram of an example signal processing circuit formed in accordance with an embodiment of the present invention; and 
           [0011]      FIG. 4  illustrates an example filter used in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    The present invention is a gyroscope having a component for measuring angle random walk (ARW) of an angular rate output.  FIG. 1  illustrates a simplified closed-loop architecture of a Fiber Optic Gyroscope (FOG)  10  formed in accordance with an exemplary embodiment of the present invention. The FOG  10  includes a rotation sensing loop having an integrated optics circuit (IOC)  12 . The FOG  10  also includes a photodetection circuit (PDC)  14 , a signal processing circuit (SPC)  16  with an ARW monitor, and an integrated optics drive circuit (IODC)  18 . 
         [0013]    The FOG  10  measures an angular velocity or a velocity about a particular axis of rotation by determining a difference in phase between two beams of light travelling in opposite directions (e.g., clockwise (CW) and counterclockwise (CCW) directions) around fiber optic coils of the IOC  12 . An analog phase output signal from the IOC  12  is communicated to the PDC  14 . The PDC  14  amplifies and converts the analog phase output signal to modulated digital phase shift data. The digital phase shift data of the PDC  14  is then communicated to the SPC  16 . The SPC  16  demodulates, monitors for ARW, integrates, and then communicates the integrated result to the IODC  18 . The IODC  18  converts the signal received from the SPC  16  to analog phase shift data, amplifies it, and then communicates the amplified analog phase shift data to the IOC  12  through a feedback loop. The IOC  12  then utilizes the received analog phase shift data to cancel a phase shift between the two beams of light travelling around the optical coils of the IOC  12 . 
         [0014]      FIG. 2  illustrates an SPC  16 - 1  component of a FOG  20  formed in accordance with an embodiment of the present invention. The SPC  16 - 1  includes an ARW demodulator  24  coupled to an ARW estimator  25 , a rate demodulator  22  that is connected with a rate accumulator  26  and a modulator  28 . The rate demodulator  22 , the rate accumulator  26 , and the modulator  28  provide digital phase shift data to the IODC  18  to cancel phase shifts induced by rotation. 
         [0015]    In an embodiment, both the rate demodulator  22  and the ARW demodulator  24  receive the digital phase shift data from the PDC  14 . The ARW demodulator  24  is biased at a predetermined modulation frequency, such that no adverse rotational rate or mechanical vibrational signals affect the signal being demodulated. By biasing the ARW demodulator  24  at this predetermined frequency the only noise affecting a modulated signal received from an IOC  12 - 1  is related to ARW. The precise selection of the predetermined modulation frequency is critical to determining real ARW, because the frequency band surrounding the bias modulation frequency of the rate demodulator  22  is corrupted by real rotation rates whereas much higher frequency bands are corrupted by mechanical vibrations. 
         [0016]    Depending on the application, the rotation rates can be from baseband to a few hertz or DC to hundreds of hertz. Vibration signals can range from a few hertz to a couple of kilohertz. Acoustic induced signals can range from tens of hertz to several kilohertz. All of these ranges are about the bias modulation frequency, or odd harmonics of the bias modulation frequency. 
         [0017]    At even harmonics frequencies of the bias modulation frequency, a noise measurement of a demodulated signal is essentially void of rotation or vibration signals. Therefore, by selecting a bias modulation frequency within narrow bands surrounding these even harmonics, a demodulated signal can provide real ARW information. In an embodiment, the ARW demodulator  24  is biased at two times the bias modulation frequency of the rate demodulator  22 . In another embodiment, the ARW demodulator  24  is biased at four times the bias modulation frequency of the rate demodulator  22 . 
         [0018]    As shown in  FIG. 2 , the “ARW Output” from the ARW demodulator  24  is not a signal that is proportional to ARW, but rather it&#39;s root-variance (standard deviation) is proportional to ARW. Therefore, to get a signal that is proportional to ARW, an additional function is performed to the output of the ARW demodulator  24 . In one embodiment, the added function could be a standard deviation calculation, or some other similar method that is related to the variance, such as a Fast Fourier Transform based spectral density. This function can reside in either the gyro processor SPC  16 , a system processor (not shown—gyros integrated into a bigger system such as an inertial navigation unit (IMU)) or in a customer&#39;s system (not shown). The ARW estimator  25  performs the function that gives an output that is proportional to ARW. Then the output of the ARW estimator  25  is sent to memory. It is unlikely (but possible) that the output of the ARW demodulator  24  will go directly to memory because the data rates at this point are very high (40 kHz or higher) and therefore would require too much memory. 
         [0019]    In one embodiment, the memory is included in an external health monitoring device (not shown), where the received proportional ARW output is tracked to determine the overall health of the FOG  20 . 
         [0020]      FIG. 3  illustrates an SPC  16 - 2  component of a FOG  30  formed in accordance with another embodiment of the present invention. The SPC  16 - 2  includes an ARW demodulator  34  that is connected to a filter  40 , and a rate demodulator  32  that is connected with a rate accumulator  36 , and a modulator  38 . In this embodiment, the ARW demodulator  34  sends a digital phase shift data to the filter  40 . In one embodiment, the filter  40  includes a band pass filter that is utilized to filter a modulated phase shift data signal from the PDC  14  at a frequency band surrounding the predetermined ARW bias frequency 
         [0021]    The signal from the ARW demodulator  34  is filtered to select out a predetermined frequency band to further reduce any influence from corrupting signals. For example, the ARW monitor may have some corrupting signals at very low frequencies (well below 1 Hz) due to optical glitches caused by the IOC  12 . The filter  40  has a pass band that is optimized to pass only those frequency components that has ARW information void of corrupting signals. 
         [0022]    In another embodiment, the filter  40  includes processing circuitry that facilitates application of a fast Fourier transform (FFT) to transform received data between the time and frequency domains. The bandpass filter  40  or FFT help to reduce unwanted signal components related to real rotation and vibration information and modulation induced errors such as optical glitches from the IOC. 
         [0023]      FIG. 4  illustrates an example filter  40 - 1  that is placed after the ARW demodulator  24 . The filter  40 - 1  includes a filter demodulator  52  that is biased at a predetermined frequency that is void of corrupting signals, such as modulation induced errors. After the filter demodulator  52  is a low pass filter, for example an accumulator  54  that accumulates the filter demodulator output over a predetermined time period. At the end of the accumulation period, the final count of the accumulator is saved into a register  56  and then the accumulator value is reset to zero. This process not only provides low pass filtering of the filter demodulator output, but also decreases the data rate, which may be necessary before the ARW data can be saved in memory and further processed by diagnostic algorithms. Using the accumulator  54  and the register  56  is a very efficient way in terms of processing cycles or FPGA or ASIC gates to do low pass filtering. By filtering with a demodulator followed by a low pass filter, a very narrow frequency band of the ARW demodulator output can be selected, thus greatly improving rejection of corrupting signals. 
         [0024]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.