Measuring and testing – Gravitational determination
Reexamination Certificate
2001-11-15
2003-09-09
Kwok, Helen (Department: 2856)
Measuring and testing
Gravitational determination
C073S001010
Reexamination Certificate
active
06615660
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a real-time self-compensating gravity gradiometer instrument of the type used to measure local variations in gravity in order to determine the gravity gradient.
Various instruments have been developed to measure gravity, these instruments include gravimeters and gradiometers.
Gravimeters are typically of the uniaxial type and measure the gravity field along the local vertical. A known type of gravimeter uses lasers and a high-precision clock to time a mass (typically, a reflective object) as it falls between two vertically spaced points in an evacuated space. More sophisticated types of these systems as disclosed, for example, in U.S. Pat. No. 5,351,122 to Niebauer et al., use split-beam interferometers to provide increased accuracy.
In contrast, gradiometers measure the differential curvature or ellipticity of gravity equipotential surfaces, the rate of change of the increase of gravity in the horizontal direction, and/or the rate of increase of gravity in the vertical direction.
The above mentioned Niebauer gravimeter can be used to measure the gravity gradient by separating two gravimeters by a known distance “d” with the gravity gradient obtained by:
(
g
1
−g
2
)/
d
Such a multi-gravimeter device is classified as and is referred to as an Absolute Gravity Gradiometer.
Another type of contemporary gravity gradiometer utilizes plural pairs of torque-balance accelerometers that are moved at a constant velocity along an orbital path about a spin axis. Information from each accelerometer at any angular position in the orbit provides information as to the lateral acceleration sensed by the accelerometers.
An exemplary gravity gradiometer suitable for use in the context of the present invention is shown in its basic form in FIG.
9
. This gradiometer is sold by the Lockheed Martin corporation (Niagara Falls N.Y. USA) and is described in more detail in U.S. Pat. No. 5,357,802 issued Oct. 25, 1994 to Hofmeyer and Affleck and entitled “Rotating Accelerometer Gradiometer,” the disclosure of which is incorporated herein by reference.
As shown in
FIG. 9
, the gravity gradiometer instrument GGI includes eight accelerometers
100
mounted at a common radius and equi-spaced about the periphery of a rotor assembly
102
that is rotated at a constant and controlled angular velocity about a spin axis SA. The rotor assembly
102
includes the rotor
104
carried on a support shaft
106
for rotation therewith. The rotor assembly
102
is rotatably mounted in ball bearings
108
and, in turn, carried in a flex-mount assembly
110
. Processing electronics
112
are mounted on the rotor
104
adjacent each accelerometer
100
for processing the respective accelerometer output signal. An inner housing
114
contains the rotor assembly
102
and is designed to rotate with the rotor assembly
102
. An outer housing
116
contains the interior components and includes one or more heaters
118
designed to operate the instrument at some controlled temperature above ambient and also includes a magnetic-field shield
120
. A slip ring assembly
122
at the upper end of the mounting shaft
106
provides the electrical/signal interface with the rotor assembly
102
and the active devices thereon. A shaft encoder
124
at the lower end of the mounting shaft
106
cooperates with an encoder pick-off
126
to provide rotary position information. The output of the encoder pick-off
126
is provided to a computer and speed controller, which, in turn, controls a drive motor
128
at the upper end of the unit to provide a controlled rotary velocity.
The gradiometer includes an internal linear servo-controlled actuator that imparts a 2 Hz sinusoidal acceleration to each accelerometer pair to enable biasing and compensation of various errors including the g
2
rectification error. In addition, the gradiometer is mounted on an external vibration isolation system that a assists in attenuating higher frequency vibration.
Each accelerometer
100
provides a sinusoidally varying analog output that is a function of the acceleration experienced by each accelerometer as the accelerometer orbits the spin axis SA. For a gradiometer having its spin axis SA aligned along the field lines in an ideally uniform and unperturbed gravity field, each accelerometer experiences the same acceleration forces as its proceeds along its orbital path. However, when the local gravity field is perturbed by the presence of one or more masses and/or the spin axis SA is tilted relative to the local vertical field lines, each accelerometer will experience different accelerations throughout its orbit. Gradiometers can be positioned with their spin axis vertical (SAV), their spin axis horizontal (SAH), or at an ‘umbrella’ angle in which the spin axis is tilted 35 degrees from the local vertical. The quantitative output of each rotating accelerometer pair, when summed and differenced, can be used to provide information related to the local gravity gradient field.
A gravity gradiometer of the type described above is known as a “relative” instrument since the torque-balance accelerometers used in the instrument intrinsically do not measure gravity directly as in the case of a ballistic pendulum or a “free-fall” dropping mass device. The instrument only provides an output voltage or a series digital pulses proportioned to the sensed field as the accelerometer pairs move along their circular orbit about the rotation axis. Additionally, a gradiometer configured with relative measurement accelerometers must be calibrated before field deployment.
Calibration of a gradiometer of this type is effected by introducing a precision, constant rotational rate input which creates a centripetal gravity gradient whose magnitude in Eotvos Units (EU) is given by the approximation &THgr;
2
/40. The scale factor of the instrument is determined by dividing the signal output of the instrument in either analog volts or digital pulses per second by the computed magnitude of the acceleration gradient. This requires the instrument to be installed in a gyro-stabilized platform where a precision torquing signal is provided to rotate the gravity gradiometer instrument at programmed rates.
Because of the mechanical and electrical instabilities of the linear torque balance accelerometers contained in the rotating accelerometer gradiometer (including time-dependent changes in materials and electrical circuits), the instrument is subject to ‘drift’ errors by which the instrument goes out of calibration. While, to a certain extent, the drift characteristic for an instrument can be determined and electrically compensated, relative gradiometers used for direct measurement of the gravity gradient must always be monitored (if possible) to determine if they are operating within calibration limits and, of course, re-calibrated periodically.
In contrast, a falling-body gravimeter or gradiometer of the types described above are classified as “absolute” instruments since the measurements are based upon direct application of Newtonian physics (i.e., s=½(g)t
2
) and the output of such an instrument is a measure of a fundamental physical constant. Thus, the falling-body gravimeter and/or gradiometer need not be subject to a stringent calibration procedure as required for a rotating-accelerometer gradiometer as described above.
In order for a gradiometer to support natural resource and/or geophysical information, it must have a signal-determining accuracy at least in the one Eotvos Unit range (i.e., 10
−9
(cm/sec
2
)/cm or 2.54×10
−6
&mgr;/inch) or less with a resolution accuracy in the 1-3 pico-g range. Noise sources can arise from within the instrument itself and from sources outside the instrument, especially in those cases where the instrument is mounted on a moving vehicle (i.e. motor vehicle, ship, or aircraft).
Intra-instrument noise, processing errors, and non-linearity sources can include accelerometer scale-factor variations, control loop non-linearities and instabilities, mechanic
Feinberg Melvin
Grierson Andrew
Kwok Helen
Lockheed Martin Corporation
Walter Wallace G.
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