METHOD AND APPARATUS FOR NON-INTRUSIVE, CONTINUOUS NOISE MONITORING

Abstract: The extra bandwidth of a fiber optic sensor is suitably appropriated to determine the integrity of a sensor. The present apparatus preferably consists of a means for finding the amplitude of the noise within the chosen frequency range which preferably includes one of several types of signal processors. Once the amplitude of noise is suitably determined from the sensor's output, the computed amplitude is suitably compared to pre-established upper and lower noise bounds. If the noise amplitude falls outside these bounds, a flag or interrupt is set to notify the system processor or other monitoring device of the sensor failure.

Description: METHOD AND APPARATUS FOR NON-INTRUSIVE, CONTINUOUS NOISE MONITORING TECHNICAL FIELD OF THE INVENTION This invention generally relates to a monitoring device for a sensor, and more particularly, to an apparatus and method for continuously monitoring for the possible failure of a fiber optic gyroscope. BACKGROUND OF THE INVENTION Fiber optic gyroscopes (FOG), with few or no mechanical parts, have substantially replaced earlier mechanical gyroscopes. Although the fiber optic gyroscope has substantially eliminated the need to monitor the potential failure of unreliable mechanical components, the performance of the fiber optic gyroscopes must still be monitored to ensure reliability. The prior art methods for monitoring mechanical gyroscope operation are ineffective for modern gyroscope technology and a method for monitoring the performance of modern fiber optic gyroscopes is needed. To determine if the gyroscope attitude read-out is generally accurate, a typical vessel operator performs complicated reasonability tests. When preforming the reasonability tests, the operator usually examines each attitude read-out for the occurrence of substantial changes because normal attitude read-outs, spaced e.g. 0.001 seconds apart, should not substantially change. Thus, to test the performance of a gyroscope, the pilot typically reads and calculates the differential output to determine the existence of substantial changes. Reading an attitude display, however, often requires a separate, time- consuming analysis, using a computer or calculator, for each inquiry. Additionally, if a gyroscope experiences total failure, the monitor often displays a zero output. When reading the zero output, the pilot oftentimes does not know if the readout is indicating a properly- functioning zero attitude shift or if the monitor is malfunctioning - a critical distinction. Therefore, a failure monitor is needed which substantially eliminates the need for reading an attitude display. Fiber optic gyroscopes are optical inertial rotation sensors which typically use the well-known Sagnac effect to detect rotation about an axis. In general, the FOG excites an interferometer from an external optical source and directly measures the light wave's phase shift after the light traverses a closed path (optical fiber). Because the light wave travels in an optical fiber, the effective sensitivity of the gyroscope is multiplied from the use of many turns of the fiber. More particularly, in an interferometric fiber optic gyroscope, an electromagnetic wave (light wave) from a single optical source is split and forced in opposite directions through a coiled optical fiber, with the pair of waves ultimately impinging on a photodetector. The relative properties of the beams are usually not changed by the potential variation in the properties of the fibers (i.e., thermal properties, mechanical properties, etc.) because, according to the principle of reciprocity, if both beams experience the same environmental alterations, both beams will be affected substantially equally. Due to the general theory of relativity, rotation of the gyroscope typically causes the effective optical path length of the first light wave to increase in one rotational direction and decrease in the opposite rotational direction. The second light wave typically experiences the opposite result with respect to its effective optical path length. The effective path length differences between the pair of light waves introduces a phase shift (Sagnac effect) between the waves. If the length of the coiled optical fiber is increased, the phase shift difference between the two beams is increased. This resulting phase shift usually forms a fringe pattern which is detected interferometrically and then used to determine the angular rate of the gyroscope. To accurately measure rotation rates, an alternating bias (dither) is often mechanically or electro-optically introduced into the beams of light, thus affecting the behavior of the beams. A vessel's FOG readout signal will typically not only contain inertial rate information (the rate at which the vessel is turning), but will also contain a signal component directly related to the dithering of the sensor. To obtain an accurate result in low noise systems, the dither signal is often minimized or reduced. Any device which analyzes the noise of a gyroscope system should substantially compensate for the effects of dither. Most of the current systems for monitoring the operation of a gyroscope intrusively alter the gyroscope functions. Intrusive alterations to a gyroscope's functions usually reduce the effectiveness of the gyroscope in its intended operation. Prior art devices may monitor an individual component of the gyroscope, but the prior art monitors often actively affect the noise (U.S. Patent Nos. 5,331,404, 5,469,257 and 5,416,584), input signals (U.S. Patent No. 5,204,631), dither (U.S. Patent Nos. 5,325,173, 5,331,401 and 5,331,402), phase angle (U.S. Patent No. 5,347,359) or amplitude (U.S. Patent No. 5,452,086) in the gyroscope. Additionally, as described in the aforementioned patents, the prior art devices actively affect the gyroscope signals, but do not monitor the health of the gyroscope by analyzing the gyroscope noise. A non-intrusive system for continuously monitoring the failure of a gyroscope's components is needed. Some prior art devices substantially ensure that the gyroscope safely starts-up without continuously monitoring the gyroscope during its operation. A major motive behind these prior art devices is to provide a start-up procedure that substantially results in minimum delay and minimum adverse effects during start-up, but the devices do not monitor a FOG for failures during operation. If a failure occurs during the steady state operation of the device, i.e. subsequent to the start-up, the monitoring capabilities are typically not available. For example, with the prior art monitors, a pilot would typically have to turn the power of the aircraft on and off to recreate the start-up procedure and retest the gyroscopes ~ an obviously undesirable option. The Killpatrick, et al. patent, U.S. Patent No. 5,363,194 discloses a laser gyroscope microprocessor that is limited to providing a start-up control method and apparatus. The '194 patent reveals a system for safely and quickly starting a laser gyroscope. The microprocessor executes tests on the gyroscope and provides a health signal in the timing sequence. If all of the gyroscope's systems are started-up correctly, the system reports an "OK" health status to a process block. The invention executes a built-in test (BIT) function which, if acceptable, sets the gyroscope health to a healthy status. Each BIT routine indicates the health of the specific component which is tested by the microprocessor is connected therefrom. These built-in test functions include reading the read-out intensity monitor, checking the path length, checking the control length controllers, checking dither errors, checking the leg currents, checking the LIM, checking the temperature, etc.. These events are all within the gyroscope and are monitored during a certain time frame. At the end of that time frame, a health status signal is sent. The built-in test functions and self-test functions are executed periodically. Furthermore, after each BIT, data is lost because the device disturbs the gyroscope function. The system does not continue to monitor the gyroscope after the start-up process is complete; thus, the Killpatrick device does not provide continuous monitoring capabilities. Many devices intrusively affect the gyroscope function by substantially altering a gyroscope signal, while other devices intrusively reduce the noise in a gyroscope. The Blake, et al. patent, U.S. Patent No. 5,469,257, discloses a method for reducing noise in the output signals from an optical system. The noise reducer incorporates a device for adjusting the amplitude or the phase of the noise representation signal. During the noise reduction, the receipt of the noise representation signal is delayed. The wave emitted from the source is also delayed in reaching the optical system output. The phase adjustment also intrusively affects the phase of the bias modulation signal. A system is needed that non- intrusively monitors a gyroscope signal without interruption of the data acquisition process. The Moeller, et al. patent, U.S. Patent No. 5,331,404, discloses a low noise fiber gyroscope system which includes excess noise subtraction. The patent discloses a system and method for intrusively subtracting excess noise from a high sensitivity gyroscope system. The resulting signal is polarized, detected and amplified along with a noise signal that is additionally delayed. The excess noise component in the amplified gyroscope signal is reduced by modulating the amplified noise signal with a portion of the amplified gyroscope signal and subtracting the amplitude-modulated noise signal from the amplified gyroscope signal to produce a reduced noise gyroscope signal. The Moeller device also intrusively reduces the noise component of the amplified gyroscope signal by amplitude modulating the component. Other devices intrusively introduce or alter various signals of the gyroscope system. The Kay patent, U.S. Patent No. 5,416,584, discloses intrusive system that injects a sinusoidal noise into the dither of a ring laser gyroscope to prevent dynamic lock-in. The Kay device also intrusively introduces a frequency bias into the ring laser gyroscope light beam. A major objective the Kay device is to reduce lock-in in a ring laser gyroscope, not the monitoring of a gyroscope. The Ogawa, et al. patent, U.S. Patent No. 5,341,210, discloses a digital ramp phase- type optical interference gyroscope. A ramp signal is produced by repeatedly accumulating step values. The step values are sequentially generated corresponding to an input angular rate until the accumulated value exceeds a threshold value. The ramp signal is used to modulate the phases of the light beams to cancel the phase difference between the beams. A subtracting and comparison step is conducted and a correcting signal is added to a reference value to produce the corrected threshold value and the phase difference. Thus, the Ogawa device corrects certain values, namely the threshold values and phase difference, which goes beyond a non-intrusive monitoring function. The Negishi patent, U.S. Patent No. 5,285,257, discloses an optic rotation sensing apparatus providing synchronous detection at a phase at which the AM noise is minimized. A device modulates a phase of light propagating in the sensing loop, while another device depolarizes the light propagating in the sensing loop. The output signal is further controlled and processed. The signal controlling/processing means intrusively controls the phase of the output signal. The Hobbs patent, U.S. Patent No. 5,204,631, discloses a system and method for automatic thresholding of signals in the presence of gaussian noise. A subcircuit is used for setting and adjusting a threshold value signal based on the input signal and the statistics concerning a predetermined acceptable false alarm rate. A second comparator is used for comparing the threshold value signal to the input signal and outputting a signal corresponding to the difference therebetween. The threshold value signal is intrusively adjusted to account for the false alarm rates. The frequency of the false alarm rates is converted to current and an output corresponding to the adjusted threshold value is provided. Again, this prior art device intrusively alters the gyroscope data, i.e threshold value signal. SUMMARY OF THE INVENTION The present failure monitor preferably tests both the optical and electronic circuits of open-loop and closed-loop fiber optic gyroscopes, while being continuously active and transparent to the using system. For example, the present monitor suitably reports a failed light source, fiber splice, optical phase modulator or photo-detector assembly. More particularly, fiber optic gyroscopes typically exhibit spectrally and temporally continuous noise at their output. This noise often exists over the entire bandwidth of the gyroscope, and if the gyroscope's bandwidth is greater than needed to measure its angular inputs, the extra bandwidth is suitably appropriated to a noise monitor for determining the integrity of the gyroscope. The noise monitor preferably consists of a means for determining the amplitude of the noise within a chosen frequency range. In a preferred embodiment, a suitable signal processor determines the amplitude of the noise. During operation, the noise monitor preferably subtracts successive values of the accumulator output from previous values, suitably forms the absolute magnitude of each difference and suitably accumulates the amplitude sum in an accumulator register. Next, the computed amplitude is preferably compared, at pre-determined time intervals, to a pre- established upper and lower noise bound. An accumulated quantity of noise outside the bounds typically indicates either excessively high or low data randomness, and therefore, a failed gyroscope. If the noise amplitude falls outside these bounds, a flag preferably notifies the system processor that a gyroscope failure has occurred. The flag suitably generates and sends an interrupt to the processor, or alternatively, the flag is periodically polled by the system processor. BRIEF DESCRIPTION OF THE DRAWING FIGURES Preferred exemplary embodiments of the present invention will hereinafter be described in conjunction with the appended drawing figures, wherein like numerals denote like elements and: FIG. 1 shows a block diagram of an exemplary apparatus for failure monitoring; FIG. 2 shows a block diagram of a detailed exemplary apparatus for failure monitoring; FIG. 3 shows a schematic graph representing a response curve of the intensity as a function of input rate as it relates to the present invention; and FIG. 4a shows a block diagram of an exemplary apparatus for an open-loop fiber optic gyroscope. FIG. 4b shows a block diagram of an exemplary apparatus for a closed-loop fiber optic gyroscope. DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS A method and apparatus according to various aspects of the present invention substantially continuously monitors a portion of the noise bandwidth outputted from fiber optic sensor 5 to suitably determine the integrity of sensor 5. Fiber optic sensor 5 is any device suitable for sensing a changed signal, including open-loop and closed-loop fiber optic gyroscopes. In a preferred embodiment, sensor 5 is a closed-loop fiber optic gyroscope. While the manner in which the noise monitoring is conducted is described in greater detail below, in general, the sums of input differences of rotational rate values are preferably compared to a fixed upper 90 and lower 100 boundary level to determine if a failure in sensor 5 exists. Referring momentarily to Figure 1 , an exemplary apparatus 1 for noise monitoring according to various aspects of the present invention preferably comprises an amplitude detection device 10, a comparison device 20 and an indicating device 25. Referring momentarily to Figure 2, as described in greater detail below, rate signal 40 that is received by amplitude detection device 10 is suitably compared to preset boundaries 90, 100 in comparison device 20. Indicating device 25 is suitably activated when rate signal 40 falls outside preset boundaries 90, 100. With continued reference to Figure 2, amplitude detection device 10 suitably comprises any detector capable of calculating the amplitude of a noise signal, such as a signal processor and/or the like. Exemplary amplitude detection device 10 in accordance with the present invention preferably includes a light detector 35, an accumulator 50, a subtractor 60, an absolute value circuit 70, a bit accumulator 80, an accumulator register 85 and a clear circuit 87. Detector 35 suitably comprises any detector capable of detecting a light signal and converting the signal to digital words, such as a photodiode light detector and/or the like. In accordance with a preferred embodiment of this invention, detector 35 comprises Epitaxx model ETX 300T. Accumulator 50 suitably comprises any device capable of accumulating signal, such as a gyroscope accumulator and/or the like. Subtractor 60 suitably comprises any device capable of subtracting signal. Absolute value circuit 70 suitably comprises any device capable of calculating the absolute value of signal. Bit accumulator 80 suitably comprises any device capable of accumulating bits of signal. Accumulator register 85 suitably comprises any device capable of registering signal. Clear circuit 87 suitably comprises any device capable of clearing an accumulator. The aforementioned components of the amplitude detection device 10 include various circuits implemented in a gate array, in an application-specific integrated circuit (ASIC), or preferably, in a microprocessor which algorithmically implements the respective functions discussed above. With continued reference to Figure 2, comparison device 20 suitably comprises any device capable of comparing signal to preset levels. Exemplary comparison device 20 in accordance with the present invention preferably includes a comparison device 20 for determining if signal 40 exceeds a fixed upper boundary level 90 and a fixed lower boundary level 100. Boundary levels 90, 100 suitably comprise any rate magnitude capable of establishing a level of comparison, such as a fixed integrated rate noise magnitude and/or the like. With continued reference to Figure 2, indicating device 25 suitably comprises any device capable of notifying a system upon the occurrence of an event. Exemplary indicating device 25 in accordance with the present invention preferably includes failure flag 120. In a preferred embodiment, apparatus 1 monitors a FOG because, due to a FOG's electronics, a FOG typically has high signal resolution thereby allowing a measurably large signal to be extracted from the available bandwidth of sensor 5. Because noise exists in this large output signal, fiber optic gyroscopes exhibit spectrally and temporally continuous noise at their output, such that the noise can be used to monitor the integrity of the gyroscope. The noise exists over the entire bandwidth of the gyroscope such that, if the gyroscope's bandwidth is greater than needed to measure its angular inputs, the extra bandwidth can be appropriated to determine the integrity of the gyroscope. The noise in the output signal typically includes dark current noise (DC current which is not prevalent in a FOG, unless at high temperatures when it can generate shot noise), background radiation noise (not typically a problem unless in satellites), shot noise (noise due to the statistical nature of the photodetection process) and thermal noise (noise generated in the load resistance equivalently located between the two leads of the photodiode; prevalent at low light power). Many of the other operations of a gyroscope contribute to gyroscope noise and are alternatively used to monitor the gyroscope in accordance with the present invention. For example, optical gyroscopes often include a light source which exhibits "excess" noise, which is white noise (broad band), as a salient noise source for monitoring purposes. The source noise in the FOG is suitably controlled because the noise causes a variation in the amplitude of the electromagnetic waves emitted by the source. The variation in the amplitude of the waves consequently results in an apparent variation in the phase shifts between the light waves. Thus, excess noise limits the ultimate sensitivity of the device and causes the performance of the gyroscope system to saturate, not improve, as the source power is increased. A broad band source is needed in a gyroscope for a number of reasons including Rayleigh backscattering and Kerr effect. A broad band source has many different frequencies that are optically interfering (beating) with each other, thus generating relative intensity noise (white band noise) which is a salient noise source for monitoring purposes. Intensity fluctuations in a broadband source also often causes excess noise; however, broadband optical sources with stable spectra are required in fiber optic gyroscopes to minimize coherent Rayleigh backscattering noise and zero rotation drift due to the Ken- effect. For example, intensity noise typically arises in a broadband source because of optical emitters in the source that emit optical frequency light waves which mix with one another to leave intensity fluctuations of relatively low frequency. Alternatively, excess noise (or relative intensity noise) in the system includes flicker noise (1/f noise) or current noise which is generated from electrical circuitry of the source and carrier density fluctuations. The noise in a gyroscope is also increased by a lack of bias stability which often causes polarization and backscatter errors and is a false indication of rotation rate. Consequently, high performance gyroscopes typically require increased bias stability (characterized with a 1/f power spectrum). For example, attitude heading and reference systems require a 1-10 degree bias accuracy, optimum stability is defined in inertial navigation with 0.01-0.001 degree bias accuracy, pointing satellites may require a 0.0001- 0.00001 degree bias accuracy and submarine navigation may require better than 0.00001 degree of bias accuracy. With reference to Figure 2, monitoring device 10 preferably consists of a means for finding the amplitude of the noise within a chosen frequency range. This means preferably consists of a suitable signal processor preferably including solid state processing electronics with digital signal processing integrated circuits. Alternatively, an analog circuitry suitably determines the amplitude of the noise. More particularly, the digital output of an optic gyroscope is suitably converted to an analog voltage and then suitably passed through a bandpass filter of the chosen noise bandwidth. In this way, instead of amplitude, measurements of the filter output are used for comparison. Consequently, analog filtering is typically a simpler method for detector 35 with an inherent analog output. Referring now to Figure 3, a schematic graph preferably represents response curve 200 of the intensity as a function of input rotation rate 40 as it relates to the present invention. The y-axis preferably represents a photodiode light intensity 205 and the x-axis represents input rotation rate 40, such that the phase shift varies linearly with input rotation rate 40. The gyroscope output, however, is not linear because the output of the detector is a cosine function of the phase difference between the two beams, thus having minimums 210 and maximums 220. A properly operating gyroscope typically exhibits a predictable noise amplitude, within definable upper and lower limits. However, the frequency range, within which the noise is suitably monitored, must not be influenced by gyroscope input motion. To monitor noise amplitude, a frequency range at or near the upper end of the gyroscope's inherent bandwidth is preferably chosen. For the fiber optic gyroscope, the bandwidth is preferably determined by the electronics design and is preferably selected to include a band of frequencies at the high end of the response spectrum which are effectively outside the frequency range of all possible mechanical inputs. With continued reference to Figure 3, detector current is preferably insensitive to small phase shift changes near zero input rotation rate 225 where maximum intensity 205 is typically located. Upon implementation of input rotation rate 40, a reduction in intensity 205 occurs. Intensity 205 variations are minimal for input rotation rates 40 around zero input rotation rate 225 because the slope of response curve is about minimum in area 250. Similarly, detector 35 current is insensitive to input rotation rates 40 corresponding to other minimums 210 and maximums 220 of response curve 200. Distinguishing between the input rotation rates 40 in two different directions is also difficult because of the symmetry of the response curve 200. Injecting a bias (bias modulation) into the light waves forces the gyroscope to operate about a finite slope 240 of response curve 200. The FOG is preferably modulated, using demodulator system 400 of Figures 4a and 4b, with a square wave that suitably alternates between maximum slopes 240 of response curve 200. Moreover, gyroscope input rotation rates 40 are preferably spaced symmetrically about the operating point, thus increasing the sensitivity of detector 35. Therefore, the increased sensitivity is preferably a result of the gyroscope operating about finite slopes 240 of response curve 200. More particularly, with respect to Figure 4a, a schematic diagram shows the determination of the rotation rate output 40 from an open-loop fiber optic gyroscope. Light from light source 405 is split by a conventional beam splitter such that the two beams enter the first ends of fiber coil 410 such that the two beams travel in opposite directions. When the two beams leave the second ends of fiber coil 410, modulator 415, located at the second ends of fiber coil 410, modulates the optical phase shift between the two beams before the two beams rejoin at photodetector 420. Modulator 415, in conjunction with bias modulation generator 440, modulates the light beams by generating a bias modulation signal of a selected frequency. The frequency is preferably related to the length of fiber coil 410. For example, 100kHz would be applied to a system with a 1000m length of fiber coil 410. Moreover, in a preferred embodiment, bias modulation generator 440 generates a constant voltage in the form of a square wave with a peak-to-peak signal. After the two modulated beams rejoin, the beams hit photodetector 420 which generates an AC output current, whereby the strength of the AC output current is based on the intensity of the light beams hitting photodetector 420. The output current from photodetector 420 is amplified in a preamplifier 425 then enters demodulator 430. The determination of the direction and rotation of the gyroscope is accomplished by demodulator 430, namely using phase sensitive demodulation. Phase sensitive demodulation is a procedure for reading the amplitude of the sine function of the AC voltage, along with the phase of the AC voltage. The phase of the AC voltage is necessary to determine if the AC voltage is in-phase or out-of-phase with respect to a predetermined reference signal. In a preferred embodiment, the predetermined reference is the direct output signal from bias modulation generator 440. Thus, by comparing the modulated amplitude and phase of the AC voltage to the amplitude and phase of the reference bias modulation signal, respectively, the degree of rotation and direction of rotation is easily determined. With respect to Figure 4b, a schematic diagram shows the determination of the rotation rate output 40 from a closed-loop fiber optic gyroscope. The same components and procedures described above for the open-loop fiber optic gyroscope (see Figure 4a) are included in the closed-loop fiber optic gyroscope of Figure 4b; however, the closed-loop fiber optic gyroscope of Figure 4b adds additional components, namely a first integrator 450, a second integrator 460 and a feedback modulation generator 470. Procedurally, with respect to Figure 4b, demodulator signal from demodulator 430 enters first integrator 450, then enters second integrator 460 and feedback modulation generator 470, thereby, in a steady state, resulting in a feedback modulation waveform. Finally, the ramped feedback modulation waveform enters modulator, along with the signal from the bias modulation generator. In a preferred embodiment, the feedback modulation waveform is a variable quantity representing a ramped "sawtooth" voltage waveform. However, the ramp does not infinitely increase because the waveform resets itself every 2 . Fortunately, a reset of 2 is relatively equal to no reset at all and unnoticed by the system because the system starts at the beginning of a new cycle after each reset. The amplitude of the waveform, measured in optical phase shift, is typically 2 because of the constant relationship between volts and degrees of phase shift. The frequency of the ramped waveform, before it resets itself, is a measure of the input rate. In a preferred embodiment, the ramped voltage waveform is a serrodyne ramp. In an open-loop system (Figure 4a), demodulator 430 output would typically depart from zero (e.g., slightly positive). However, in a closed-loop system (Figure 4b), feedback modulation generator 470 would react to the slight increase in demodulator 430 output by generating the ramped feedback modulation waveform and transmitting it to modulator 415 until the aforementioned demodulator process yields a demodulator 430 output of zero, in other words, a steady state. A demodulator output of zero would be a steady state because the input to first integrator 450 would be zero, thereby first integrator 450 would output a constant finite digital number. Since the output of first integrator 450 is a constant digital number, the output of second integrator 460 is a ramped waveform because the time integral of a constant is a ramped waveform. Consequently, the finite digital number would be rotation rate output 40 of the closed-loop FOG. For example, a steady state would exist if the gyroscope is experiencing a constant rotation rate of about 10 degrees/sec. In a preferred embodiment, modulator 415, demodulator 430, bias modulation generator 440, first integrator 450, second integrator 460 and feedback modulation generator 470 suitably include various circuits implemented in a gate array, in an application-specific integrated circuit (ASIC), or preferably, in a microprocessor which algorithmically implements the respective functions discussed above. Photodetector 420 suitably comprises any detector capable of detecting a light signal and converting the signal to an analog circuit, such as a photodiode light detector and/or the like. In accordance with a preferred embodiment of this invention, photodetector 420 comprises Epitaxx model ETX 300T. Moreover, in a preferred embodiment, any known preamplifier 425 is incorporated into demodulator system 400 after photodetector 420 and before demodulator 430. With reference to Figure 2, rotation rate signal 40, preferably in the form of a digital word of at least 8 bits in width, is suitably sent to gyroscope accumulator 50 whereby gyroscope accumulator 50 suitably accumulates the digital words. The higher bits, which represent greater rate changes, are preferably substantially ignored by gyroscope accumulator 50 because the rate changes are minimal. Subtractor 60 and absolute value circuit 70, respectively, suitably subtract successive rate values of accumulator 50 output from each other and suitably form the absolute magnitude of each difference. Bit accumulator 80 suitably accumulates the sum of the rate value differences in accumulator register 85. The data of accumulator register 85 is preferably sent to comparison device 20. Comparison device 20 suitably compares the data from accumulator register 85 to fixed upper 90 and lower 100 boundary levels (fixed integrated rate noise magnitudes) at predetermined time intervals. Clear circuit 87 preferably clears bit accumulator 80 approximately every 100 milliseconds, based on monitor update clock 110, to allow for another update cycle and a new comparison based on new rate signal 40. Moreover, clear circuit 87 suitably determines the frequency of failure analyses, and subsequent possible reportings. An accumulated quantity of noise outside bounds 90, 100 suitably indicates either excessively high or low signal randomness, and therefore, a failed gyroscope. If the noise amplitude falls outside these bounds, a signal is preferably sent to indicating device 25. Indicating device 25 preferably includes a gyroscope fail flag 120 that suitably notifies the system processor that a gyroscope failure has occurred. The system processor suitably generates an interrupt in the software. Flag 120 suitably generates an interrupt to the processor, but alternatively, flag 120 is periodically polled by the system processor. As described above, noise monitor 1 is substantially continuously active and transparent to the using system because the monitoring is preferably accomplished in a frequency region not affected by external variables. Thus, because the present invention analyzes a sensor's noise, and not the signals, any sensor 5 with a measurable noise amplitude and a definable output noise spectrum can be integrity monitored. In a preferred embodiment, by monitoring the noise, noise monitor 1 suitably tests both the optical and electronic circuits of a fiber optic gyroscope including, inter alia, a failed light source, fiber splice, fiber coupler, optical phase modulator, photo-detector assembly and/or the like. More particularly, because noise is roughly proportional to light power, the dimming of a light source typically results in the reduction of noise. Consequently, modern light sources usually experience greater noise variations, thereby simplifying the detection of altered noise which is indicative of a failed gyroscope. Furthermore, a failed splice often results in a loss of noise which is suitably detected by the present invention. Darkening of the integrated optic chip or the fiber typically results in the reduction of light to the photodiode, thereby resulting in the substantial loss of noise. Failure of the phase modulator to perform its intended function due to a broken electrode wire and/or similar failure also alters noise magnitude such that, if a photodiode malfunctioned, the electronic amplifiers receive substantially different (typically more) noise. Also, failure of the associated electronics to generate the proper bias modulation amplitude typically results in either more or less noise. Therefore, the monitoring of noise is a highly effective and useful tool for determining the integrity of a fiber optic gyroscope.

Claims: It will be apparent to those skilled in the art that the foregoing detailed description of preferred embodiments of the present invention is representative of a method and apparatus for continuous noise monitoring within the scope and spirit of the present invention. Further, those skilled in the art will recognize that various changes and modifications may be made without departing from the true spirit and scope of the present invention. For example, the invention is described with respect to a fiber optic gyroscope, however, the invention applies to any sensing device. Those skilled in the art will recognize that the invention is not limited to the specifics as shown here, but is claimed in any form or modification falling within the scope of the appended claims. For that reason, the scope of the present invention is set forth in the following claims. I claim: 1. An apparatus for continuously monitoring a failure of a sensor, said apparatus comprising: an amplitude detection device for detecting an amplitude of a signal, said signal including data; a comparison device communicating with said amplitude detection device; and, an indicating device communicating with said comparison device. 2. The apparatus of claim 1 , wherein said sensor is a fiber optic sensor. 3. The apparatus of claim 1, wherein said sensor is a closed-loop fiber optic sensor. 4. The apparatus of claim 1 , wherein said amplitude detection device includes: a detector for providing said data; an accumulator communicating with said detector, said accumulator accumulating said data supplied by said detector; a subtractor communicating with said accumulator, said subtractor subtracting successive said data; an absolute value circuit communicating with said accumulator, said absolute value circuit calculating an absolute value of said data; a bit accumulator communicating with said absolute value circuit, said bit accumulator accumulating said data supplied by said subtractor and said absolute value circuit; an accumulator register communicating with said bit accumulator, said accumulator register recording said accumulation of said data supplied by said bit accumulator; a clear circuit communicating with said bit accumulator, said clear circuit periodically clearing said bit accumulator. 5. The apparatus of claim 1, wherein said comparison device includes fixed upper and lower boundary levels, said comparison device compares said accumulator register to said boundary levels. 6. The apparatus of claim 1, wherein said indicating device includes a gyroscope fail flag, said gyroscope fail flag notifying when said data in said accumulator register is outside of said boundary levels. 7. The apparatus of claim 1 further comprising a demodulation device configured to determine a direction and rotation of said sensor, said demodulation device communicating with said amplitude detection device. 8. The apparatus of claim 7, wherein said demodulation device includes: a modulator configured to modulate an optical phase shift between counterpropagating waves; a photodetector configured to generate an AC output voltage, said photodetector in communication with said modulator; a demodulator configured to read the amplitude and phase of said AC voltage, said demodulator in communication with said photodetector; and, a bias modulation generator configured to generate a constant voltage, said bias modulation generator in communication with said modulator and said demodulator. 9. The demodulation device of claim 8 further including: a first integrator; a second integrator in communication with said first integrator; and a feedback modulation generator configured to create a feedback modulation waveform, said feedback modulation generator in communication with said second integrator and said modulator. 10. The demodulation device of claim 9, wherein said feedback modulation waveform includes a ramped voltage waveform which resets itself every 2 . 11. A method for continuously monitoring a failure of a sensor comprising the steps of: providing an amplitude detection device for detecting an amplitude of a signal, said signal including data; providing a comparison device communicating with said amplitude detection device; providing an indicating device communicating with said comparison device; comparing said data; and, indicating a failure. 12. The method of claim 11, wherein said sensor is a fiber optic sensor. 13. The method of claim 11, wherein said sensor is a closed-loop fiber optic sensor. 14. The method of claim 11, wherein said step of providing an amplitude detection device includes: providing a detector for providing said data; providing an accumulator communicating with said detector; accumulating said data supplied by said detector; providing a subtractor communicating with said accumulator; subtracting successive said data; providing an absolute value circuit communicating with said accumulator; calculating an absolute value of said data; providing a bit accumulator communicating with said absolute value circuit; accumulating said data supplied by said subtractor and said absolute value circuit; providing an accumulator register communicating with said bit accumulator; recording said accumulation of said data supplied by said bit accumulator; providing a clear circuit communicating with said bit accumulator; and, periodically clearing said bit accumulator. 15. The method of claim 11 , wherein said step of providing said comparison device comprises providing fixed upper and lower boundary levels, said step of providing said comparison device comprises comparing said accumulator register to said boundary levels. 16. The method of claim 11 , wherein said step of providing an indicating device comprises providing a gyroscope fail flag, said step of providing said gyroscope fail flag comprises notifying when said signal in said accumulator register is outside of said boundary levels. 17. The method of claim 11 further comprising the step of determining a direction and rotation of said sensor using a demodulation device, providing a communication link between said demodulation device and said amplitude detection device. 18. The method of claim 17, wherein said step of determining a direction and rotation of said sensor includes : modulating an optical phase shift between counterpropagating waves using a modulator; generating an AC output voltage; reading the amplitude and phase of said AC voltage using a demodulator; and, generating and sending a constant voltage to said demodulator and said modulator. 19. The method of claim 17 wherein said step of determining a direction and rotation of said sensor further including creating a feedback modulation waveform and sending said waveform to said modulator. 20. The method of claim 17, wherein said feedback modulation waveform includes a ramped voltage waveform which resets itself every 2 .

Inventors: DANKWORT RUDOLF, DANKWORT, RUDOLF
Applicants: HONEYWELL INC, HONEYWELL INC.
Application year: 1998
Main classification: G01C19
IPC codes: G01C19