Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1999-02-24
2001-06-05
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C342S462000, C342S463000
Reexamination Certificate
active
06242907
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an apparatus and method of determining the orientation of an object and more particularly to an apparatus and method of determining the orientation of an object relative to a magnetic field.
BACKGROUND OF THE INVENTION
Common practice at present for the measurement of angular orientation in three-dimensional space is to use three-axis magnetometers to resolve orientation of an object in a magnetic field such as, for example, the magnetic field of the Earth. The measurement of angular orientation may be employed, for example, in ambulatory systems, electronic guidance and compass systems. Ambulatory systems measure three-dimensional body segment motion such as, for example, fluctuations of movements in persons with degenerative neurological disorders, measuring the level of functional impairment in persons with rheumatic and musculoskeletal diseases, assessing gait and balance deficits in the elderly and persons with neurologic disease, monitoring and documenting the progression of neurological, rheumatic and musculoskeletal diseases and disorders, developing objective outcome measures of the efficacy of rehabilitation programs, surgical interventions, and drug treatments in human diseases and disorders that affect body movement, better understanding the injury mechanisms associated with occupational, recreational and sports activities, and assessing the performance of activities of daily living, as well as occupational, recreational and sports activities in the environments in which they are typically conducted.
A variety of sensors are known in the art for measuring magnetic fields such as, for example, Hall effect sensors, proton quantum interference detectors “SQUID”, fluxgate magnetometers, inductive pickup sensors and magnetoresistive sensors.
Three-axis magnetometers that can be used to estimate orientation with respect to an ambient magnetic field, such as the Earth's magnetic field, are commercially available from several companies. However, the practical use of these systems has been restricted by several significant technical limitations. First, the accuracy of these devices is dependent on the sensitivity and offset of each sensor. Since these devices attempt to directly measure the magnetic field with the magnetic sensors themselves, each sensor must maintain zero and offset values, and have low drift characteristics with temperature change. Also, most magnetic field sensors measure sums of vector components of the imposed magnetic field, which makes the determination of orientation problematic unless precise knowledge of the local magnetic field is available. When employing the Earth's magnetic field, the system software that resolves the angular orientation from the sensor data must have accurate values for the magnitude and direction of the Earth's field vector for every location where the system is used. Databases are available that provide this information for specific locations (latitude and longitude) on the Earth's surface. However, there is no easy way to determine if the Earth's magnetic field is distorted in a particular area, and significant errors will result if the database information is used in such an area.
If the Earth's magnetic field is distorted, if the magnitude and direction of the Earth's magnetic field are unknown, or if the sensor offsets are changed, then the magnetometer must be recalibrated to maintain the accuracy of the measurements. The user can recalibrate by carefully rotating the magnetometer a full revolution about each of its three axes, while monitoring the output of the sensors. The validity of this calibration procedure is predicated on the three sensors having the same sensitivity.
Because of this burden of complexity, as well as the high price for such systems, the three orthogonal sensor system, or three-axis magnetometer, has not seen much practical use except in a tightly controlled laboratory environment. Thus, there is a need in the sensor art for a sensor system that may measure angular positioning in three axes while eliminating the burden of complexity that influences the operation of existing sensor art.
Accordingly, it is an object of the present invention to provide a sensor system which overcomes the above-described disadvantages of prior devices for determining the orientation of an object in a magnetic field.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, a method of determining the orientation of an object relative to a reference magnetic field includes the steps of establishing generally adjacent to the object a modifiable magnetic field. The direction of the modifiable magnetic field is repeatedly changed among a plurality of predetermined, known orientations relative to the object. A resultant magnetic field, including the reference magnetic field and the modifiable magnetic field, is measured adjacent to the object. The step of measuring is directed outwardly from adjacent to the object in predetermined directions that are fixed relative to the object. The method further includes determining when the magnitude of the resultant magnetic field is minimized or maximized indicating that the direction of the modifiable magnetic field is parallel with that of the reference magnetic field. Thereupon, the orientation of the object is correlated to the known orientation of the modifiable magnetic field.
According to a second aspect of the present invention, an apparatus for determining the orientation of an object relative to a reference magnetic field includes first means for establishing generally adjacent to the object a modifiable magnetic field. Second means is provided for changing the direction of the modifiable magnetic field among a plurality of predetermined, known orientations relative to the object. Third means repeatedly measures adjacent to the object a resultant magnetic field, including the reference magnetic field and the modifiable magnetic field, wherein the measuring is directed outwardly from adjacent to the object in predetermined directions that are fixed relative to the object. Fourth means determines when the magnitude of the resultant magnetic field is minimized or maximized indicating that the direction of the modifiable magnetic field is parallel with that of the reference magnetic field, and thereupon correlating the orientation of the object to the known orientation of the modifiable magnetic field.
According to a third aspect of the present invention an apparatus for determining the orientation of an object relative to a reference magnetic field includes a Helmholtz coil system having windings extending in three coordinate axes for generating a modifiable magnetic field when electrical current is passed through the windings. Each of the axes is generally orthogonal relative to each other. A controller directs current to the Helmholtz coil system and repeatedly changes the polarity and magnitude of the electrical current through each of the windings among a plurality of predetermined, known orientations relative to the object, and determines when a resultant magnetic field, including the reference magnetic field and the modifiable magnetic field, is minimized or maximized indicating that the reference magnetic field is parallel to the known orientation of the modifiable magnetic field. At least one magnetic sensor communicates with the controller. The at least one magnetic sensor is to be located adjacent to and in fixed orientation relative to the object for detecting the resultant magnetic field.
REFERENCES:
patent: 5525901 (1996-06-01), Clymer et al.
patent: 5558091 (1996-09-01), Acker et al.
patent: 5600330 (1997-02-01), Blood
patent: 5953683 (1999-09-01), Hansen et al.
Clymer Mark L.
Graves Glenn
Graves Electronics LLC
McCormick Paulding & Huber LLP
Oda Christine
Zaveri Subhash
LandOfFree
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