Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
  • G01C21/00—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00
  • G01C21/10—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 by using measurements of speed or acceleration
  • G01C21/12—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 by using measurements of speed or acceleration executed aboard the object being navigated; Dead reckoning
  • G01C21/16—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 by using measurements of speed or acceleration executed aboard the object being navigated; Dead reckoning by integrating acceleration or speed, i.e. inertial navigation
  • G01C21/183—Compensation of inertial measurements, e.g. for temperature effects
  • G01C21/188—Compensation of inertial measurements, e.g. for temperature effects for accumulated errors, e.g. by coupling inertial systems with absolute positioning systems
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
  • G01C25/00—Manufacturing, calibrating, cleaning, or repairing instruments or devices referred to in the other groups of this subclass
  • G01C25/005—Manufacturing, calibrating, cleaning, or repairing instruments or devices referred to in the other groups of this subclass initial alignment, calibration or starting-up of inertial devices

Definitions

• the present invention relates to a system and method of correcting the outputs of gyroscopes for force-dependent errors. More specifically, one embodiment of the invention pertains to a calibration method for a gyroscope that utilizes a force-effect model.
• Aircraft inertial navigation relies upon the integration of data throughout a sequence that begins when the aircraft is prepared for takeoff and ends when the aircraft has landed and motion ceased.
• the inertial navigation system (“INS") of an aircraft includes various components, including accelerometers and gyroscopes, that convert the effects of inertial forces into acceleration, velocity and position measurements.
• the accelerometers determine acceleration forces along three orthogonal sensitive axes and this data is converted, through integrations, into the aircraft's velocity and position.
• the gyroscopes that measure the aircraft's attitude also measure that of the accelerometer axes. Data measured by the gyros is employed to resolve accelerometer outputs along the appropriate spatially stabilized axes.
• Gyroscopes may include gimbaled mechanical or electromechanical arrangements, ring laser and fiber optic arrangements, among others, while accelerometers can be of the pendulous mass type and/or employ piezoelectric or silicon technologies. Regardless, each inertial navigation system arrangement is faced, to a greater or lesser extent, with inaccuracies owing to the error peculiarities of its functional components. Because inertial grade instruments are required to measure a very large dynamic range of motions, they typically rely on state-of-the-art technologies. These sensors must be able to measure extremely small quantities.
• a navigation grade accelerometer must measure a few millionths of the standard gravity acceleration, and a gyro must measure a few hundred thousandths of the Earth's rotation rate. Often, it is impossible to precisely identify the sources of minute errors of these magnitudes. Whenever possible, individual error sources should be isolated in order to prevent measurement contamination and to reduce sensitivity to drifts. In particular, gyro error estimation should be rendered insensitive to accelerometer errors since the latter are typically much larger.
• the fast orthogonal calibration technique and other past approaches provided a method of estimating gyro errors independently from accelerometer errors.
• these past techniques did not account for erroneous outputs which gyroscopes have been found to exhibit under force conditions.
• the force-dependent gyroscope error could be a contaminant within the calculations in past approaches of estimating scale factor and orthogonality errors of a gyro triad during a calibration procedure.
• a force-effect model of the effects of acceleration or force on the output of a gyroscope is initially determined in this invention.
• This invention also teaches a method of utilizing rotation sequences to excite and observe the aforementioned force-effect model parameters. These parameters may then be used for calibrating the force-dependent errors.
• a state diagram containing the gyroscope force-effect model parameters is derived from the force- effect model, where moving from one position in the state diagram to another position indicates the gyroscope error which would occur in rotating the gyro triad according to a corresponding motion.
• the present invention provides a novel algorithm for searching for all possible closed loop paths achievable using the state diagram of a two axis rate table with a specified number of rotations in order to separate gyroscope error calibration from the accelerometer calibration. Closed loop paths having minimal lengths and exhibiting larger error sensitivity are chosen to perform error parameter calibration and reduce force-dependent gyroscopic sensitivity.
• Figure 1 is a flow diagram illustrating a method for calibrating a gyro triad according to one aspect of one embodiment of the invention.
• Figure 2 is a flow diagram illustrating a method to perform a novel searching algorithm for according to one aspect of one embodiment of the invention
• Figure 3 is gyro error state diagram matrix for inner and outer gimbal rotations of the gyro triad in accordance with one aspect of one embodiment of the present invention.
• Figure 4 is force-dependent error state diagram matrix for gimbal rotations of the gyro triad in accordance with the one aspect of one embodiment of present invention.
• the inventors of the present invention found that the gyro output is proportional to a force applied along a sensing axis of the gyro and an input frequency.
• a gyro was mounted vertically on the vibration table such that a linear G force could be applied along the gyro-sensing (z) axis.
• the frequency response of the gyro output was obtained by exerting a constant G force sinusoidally swept through a frequency range from 50 to 500 Hz.
• This input/output relationship in the frequency domain can then be converted to the time domain 104 as:
• G B is the G force vector in the body frame, B, measured by the accelerometer.
• Equation (8) can easily be integrated to evaluate the force induced tilt errors
• Gyro tilt errors due to scale factor and misalignment errors can be modeled as: ⁇ C *M* ⁇ B B (9) where M is the 3x3 gyro error parameter matrix defined as
• Rotation sequences are selected which not only to provide the maximum sensitivity and reduce the total test time, but also to separate the gyro errors from those of accelerometer errors in order to prevent cross contamination.
• the present invention makes use of this state diagram technique in order to utilize its same advantages.
• a state diagram is derived from the above-described force-effect model 110.
• the preferred embodiment of the present invention used a low-cost two axis rate table and restricted the rotations to multiples of 90-degrees. In this preferred embodiment, a total of 16 system orientations are possible.
• the corresponding path-dependent gyro tilt errors are computed by integrating Eq. (9) and are presented in a state diagram matrix form as shown in the table in Fig. 1. There are 4 horizontal groups corresponding to the vertical transitions of 90-degree inner gimbal rotations of the gyro triad, and there are also 4 entries in each horizontal group corresponding to the horizontal transitions of 90-degree outer gimbal rotations.
• the first 3 rows are the 3 (x, y, z) tilt errors due to the horizontal (outer gimbal) transition
• the second 3 rows are the 3 errors due to the vertical (inner gimbal) transition.
• the coordinate definition is as follows: x is the roll axis, y is the pitch axis, and z is the yaw/heading axis. When heading is zero, x is in the same direction as the positive outer gimbal axis pointing to north, and z is in the same direction as the positive inner gimbal axis pointing downwards.
• Gyro errors can be estimated independently of the accelerometer errors by selecting a rotation path that always returns to its initial position, otherwise referred to as a closed loop path.
• a closed loop path Depending upon the number of rotations specified, there are only a limited number of possible paths that can satisfy the above condition.
• the time of search increases exponentially with the number of rotations specified.
• the present invention utilizes a novel search algorithm using the process of path elimination 112.
• Figure 2 is a flow diagram illustrating a method to perform a novel searching algorithm according to one aspect of one embodiment of the invention.
• a two-axis rate table with one inner (i) and one outer (o) axis of rotation, is assumed to be used in this algorithm on which a gyro triad may be strapped or attached. It is also assumed that the table could undergo a 90- degree rotation around each axis at each step, so there are 16 (4x4) different possible states, or gyro triad orientations, in total.
• a search path length (Path Length N) is the number of rotations specified to reorient the table or platform on which the gyro triad attaches to return to its original start orientation.
• a counter (Counter k 202) counts the number of steps so far during the search. It starts with a one, goes up to N, and ends with a zero 204 at the end of the search.
• the current state of the search (Current State 202) identifies the current gyro triad orientation during the search, and it is initialized to a default start state. The process of path elimination is implemented to make sure that the Current State will not overlap any of the past states 210 & 214 that the table has gone through so far.
• the Current State transitions into a new state according to the current table rotation step 206, i.e., io condition, which is initialized to an outer axis rotation.
• the Current State is checked to see if it returns to the original start state 216. If it does, it scores a successful closed loop path and will be recorded 218 for future error analysis. If not, or if it is not at the end of the search yet 208, the process will continue to cycle through all the possible inner/outer rotation combinations 212 & 220 & 222.
• the improved algorithm provides increased efficiency by reducing the amount of time required to search for all the possible closed loop paths.
• Table I shows the search results, where path length, N, is the selected number of rotations of the gyro triad; possible paths is the number of possible inner/outer gimbal rotation combinations (io), which amounts to 4 N ; search paths is the number of paths actually searched using the algorithm of the present invention.
• the substantially smaller number of search paths required as compared to the number of possible paths can be seen from Table 1.
• the closed loop paths are the number of desired search results. Table I
• the accumulated gyro tilt errors along those paths are computed using the state diagram developed earlier. Independent paths are preferably selected which would yield simple combinations of error parameters in each of the horizontal tilt observables. In order to be able to observe all the error parameters, we also need to use 45- degree rotations for some parameters to complete the gyro error calibration.
• Gyro calibration results for the two level axes tilt errors accumulated in the course of the aforementioned rotation sequences are shown below in Table II. These errors are thus obtained by measuring the level axes accelerations. In the same table, the errors due to the force-effect accumulated along the same rotations are listed.
• Test 2 190 O720 1-90 Total 900°
• Test 5 190 O180 1180 O180 1-180 O180 1-180 O180 190 Total 1440°
• Test 6 145 O270 1270 O-720 1450 O450 1-45 Total 2250°
• Test 1 O720 Total: 720°
• Test 2 190 O720 1-90 Total: 900° Test 3 O90 1720 O-90 Total 900° Test 4 1180 O180 1+180 O-180 Total 720° Test 5 190 O180 1+180 O-180 190 Total 720° Test 6 145 O720 1-90 O-720 145 Total 1620° Test 7 O90 1180 O90 1-180 O-90 1-180 O-90 1-180 Total 1080° Test 8 190 O90 1180 O90 1180 O-90 1-180 O-90 190 Total 1080° Test 9 0135 1-360 O-90 1360 O-180 1360 O90 1-360 045 Total 1980°
• the method for measuring force-dependent gyroscope sensitivity practiced in accordance with the various embodiments of the present invention provides a novel manner of determining force-dependent errors existing in gyroscope output. By determining and isolating such force-dependent errors, the present invention prevents such force- dependent errors from cross-contaminating scale factor and orthogonality errors of a gyro triad during calibration.
• the present invention provides a method for modeling and estimating force-dependent errors in inertial instruments.
• teachings of the invention which require no hardware in addition to that already utilized in conjunction with an aircraft's INS, one can substantially improve the usefulness of the calibration procedure of constituent instruments, including gyros and accelerometers, regardless of INS configuration.
• the method encompasses and enhances, INS performance based upon strapdown and gimbaled arrangement employing a wide range of device technologies including, but not limited to, ring laser, fiber optics, mechanical, electromechanical and piezoelectric.

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• Engineering & Computer Science (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Automation & Control Theory (AREA)
• Manufacturing & Machinery (AREA)
• Gyroscopes (AREA)
• Force Measurement Appropriate To Specific Purposes (AREA)

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• 2003
  • 2003-03-17 US US10/390,200 patent/US6904377B2/en not_active Expired - Lifetime
  • 2003-11-12 JP JP2004569683A patent/JP4929445B2/ja not_active Expired - Lifetime
  • 2003-11-12 AU AU2003287627A patent/AU2003287627A1/en not_active Abandoned
  • 2003-11-12 WO PCT/US2003/035884 patent/WO2004083782A1/en not_active Ceased
  • 2003-11-12 CA CA002517018A patent/CA2517018A1/en not_active Abandoned
  • 2003-11-12 EP EP03781875A patent/EP1604173B8/en not_active Expired - Lifetime
• 2005
  • 2005-08-30 IL IL170574A patent/IL170574A/en active IP Right Grant

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