Data processing: vehicles – navigation – and relative location – Navigation – Employing position determining equipment
Reexamination Certificate
2000-02-08
2001-09-18
Cuchlinski, Jr., William A. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Navigation
Employing position determining equipment
C701S221000
Reexamination Certificate
active
06292751
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to methods and apparatus for correcting errors associated with the use of an inertial measurement unit (IMU) to track positions, locations, or movements.
BACKGROUND OF THE INVENTION
Inertial Measurement Units (IMUs) typically use accelerometers and gyros to track accelerations in order to calculate changes in position. Because of inherent errors in the sensors used in IMUs, random errors in the calculated position build with time. One current method of eliminating these errors is to use a second position sensor (e.g., a GPS or an odometer input) along with a Kalman filter to minimize errors from each of the position sensors. A second method is to stop and perform a Zero Velocity Update (ZVU) to re-calibrate the sensors at the beginning of a period of interest.
Most applications using IMUs require good absolute position accuracy over long distances, such as for use in a plane or a tank. Such applications are ideal for use of an IMU coupled with a secondary data source such as a Global Positioning System (GPS) to bound and correct errors in the IMU. A typical IMU operating in conjunction with the GPS may be capable of 10 meters in accuracy relative to a fixed reference point after traversing a distance of 100 miles.
In contrast, some applications, such as to which this invention applies, requires extremely accurate relative position data over short distances. In such applications, it is necessary to determine with high accuracy how far the IMU has moved since the last ZVU was performed.
Neither of the two methods described above are suitable when very high relative positional accuracy is required over very short distances. For example, where a vehicle is used to detect and destroy a land mine, 3 centimeters of accuracy in 6 meters of travel is required. These high levels of accuracy preclude the use of coarse sensors such as an odometer or GPS to eliminate the errors. The nature of these applications also prevents a ZVU from being performed at the beginning of each period of interest (i.e., sensing of a mine or false alarm). Other solutions to the problem of accumulation of errors from an IMU include the use of more expensive sensors with lower noise, and using additional external equipment. As an example, survey quality differential Real Time Kinematic (RTK) GPS can provide highly accurate positional data with the use of a ground based station at a known point, which is disadvantageous in some situations. Thus, to achieve the requisite high levels of accuracy would generally require an IMU using extremely accurate sensors costing approximately $100,000. Therefore, there is a need for methods and apparatus for generating highly accurate position data over short periods of travel using a less expensive IMU.
SUMMARY OF THE INVENTION
An object of the present invention is, therefore, to provide methods and apparatus for generating highly accurate position data over short periods of travel using a relatively inexpensive IMU. This invention utilizes a method of estimating the random velocity errors in order to remove a portion of these errors from the position data.
The invention allows the use of a relatively inexpensive IMU by providing a method of estimating the accumulated error at the beginning and end of the period of interest, and then correcting for these errors. The method employs a ZVU at the end of the period of interest and uses a simple algorithm along with existing data from the IMU to correct for the accumulated errors and thereby provide more accurate position data.
The invention may be employed in a system for detecting and disabling mines as described herein, and may also be used to refine the accuracy of position determination in other applications as well.
The present invention provides several key advantages: (1) it provides highly accurate relative position data over short distances; (2) does not require external or ground-based equipment; (3) does not require stopping for calibration at the beginning of the period of interest; (4) does not require stopping for any length of time to gather data; (5) does not require the use of a second data source; and (6) allows the use of less expensive inertial sensors.
According to the present invention, a conventional IMU is provided for obtaining relative position data. A Zero Velocity Update (ZVU) is performed prior to the commencement of motion to calibrate the IMU. At some time after the commencement of motion a period of interest begins. At the start of the period of interest, the time and position of the IMU is recorded. At a later time, the period of interest ends. At the end of the period of interest, the motion of the IMU is stopped and the time and position of the IMU is again recorded. The velocity indicated by the IMU at the end of the period of interest is also recorded. Because the IMU is now at rest, any velocity indicated by the IMU is the result of errors accumulated during the time of motion of the IMU. The accumulation of error as a function of time is approximated by a function, f(t). From this function, the approximate amount of error at both the beginning and the end of the period of interest can be calculated. This error can then be subtracted from the position data for both the beginning and ending points of interest to minimize the error in the indicated relative change in position between these points.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
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BAE Systems
Cuchlinski Jr. William A.
Fulbright & Jaworski
Marc-Coleman Marthe
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