Gravity gradiometer accelerometers

• US 6,575,029 B1
• Filed: 10/05/1999
• Issued: 06/10/2003
• Est. Priority Date: 06/11/1997
• Status: Expired due to Fees

• Current Assignee: Fugro Finance AG (Fugro N.V.)
• Original Assignee: The Broken Hill Proprietary Company Limited (BHP Group Ltd.)
• Inventors: Turner, Robert John, Adams, Gregory John, OʼKeefe, Graeme Joseph, Goodwin, Graham Clifford, Lee, James Beresford
• Primary Examiner(s): KWOK, HELEN C
• Application Number:
  US09/415,140
• Time in Patent Office: 1,344 Days
• Field of Search: 735/141.6, 735/141.7, 735/141.8, 735/143.1, 735/140.1, 733/82.R, 733/82.G, 733/83
• US Class Current: 73/382.00R
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/132 with electromagnetic counterbalancing means
• CPC Class Codes: G01P 21/00 Testing or calibrating of apparatus or devices covered by the preceding groups
• CPC Class Codes: G01V 7/04 Electric, photoelectric, or magnetic indicating or recording means

Abstract

This invention concerns improvements to gravity gradient instruments (GGI) and in particular to the accelerometers that are paired within these instruments. Accelerometers have a proof mass suspended by a spring within a magnetic field. An internal feedback loop provides a signal related to movement of the proof mass back through a reaction coil retaining the proof mass in the magnetic field, to maintain the proof mass stationary. An external feedback loop adjusts the accelerometer scale factor. The internal feedback loop provides second order compensation to the proof mass and the spring stiffness. In a further aspect the invention is a method of matching accelerometer pairs.

Claims

1. An accelerometer comprising

a magnet means that produces a magnetic field;

a proof mass that is suspended by a spring within said magnetic field and that tends to move in response to an applied acceleration force;

position sensor means for said proof mass and for producing a position signal representative of the position of said proof mass;

a reaction coil adjacent said proof mass and within said magnetic field;

an internal feedback loop that receives said position signal, that provides a feedback signal related to movement tendency of said proof mass, that delivers said feedback signal to said reaction coil to maintain said proof mass stationary and that generates in said reaction coil a current signal which is representative of an acceleration force applied to said proof mass;

an external feedback loop that receives said current signal, that may receive a second corresponding current signal from a second opposing accelerometer which is paired with said accelerometer, and that combines and processes said so received current signals, and that produces a corrective signal;

a relatively small electromagnet means associated with said magnet means for receiving said corrective signal and for responsively adjusting said accelerometer to correct for mismatches between said accelerometer and said second accelerometer;

thereby to adjust an acceleration scale factor and permit said internal feedback loop to provide second order compensation to said proof mass and the stiffness of said spring.

2. The accelerometer of claim 1 wherein said internal feedback loop includes means for high gain to reduce errors in the accelerometer transfer function.

3. The accelerometer of claim 1 wherein said internal feedback loop includes a compensator to provide double pole and double zero compensation as follows:

4. A method of matching accelerometer pairs comprising the steps of:

testing the pair of accelerometers in back to back fixtures on a horizontal shaker which is aligned to a preselected angle;

selecting the accelerometers on the basis of a best scale factor match at 0.5 Hz and a lowest lateral sensitivity at 10 Hz;

applying horizontal excitation at a range of frequencies about 0.25 Hz, and detecting a response with synchronous demodulation at frequencies in said range;

adjusting second order mass compensation to minimize the in phase (I) and the quadrature (Q) components of a signal achieved at 10 Hz;

adjusting second order spring stiffness compensation to minimize the components at 1 Hz; and

iteratively repeating the adjusting steps to achieve matches better than 0.5%.

5. The method of claim 4 wherein a scale factor loop is included that may be adjusted manually while excitation is applied.

6. The method of claim 5 wherein an external feedback loop is included that provides an output signal that is demodulated at the spin frequency Ω

by a demodulator, thereby to correct for mismatches in the strength B of the magnets in the accelerometers.

7. The method according to claim 4 wherein a scale factor loop may be adjusted manually while excitation is applied.

8. The method according to claim 7 wherein an external feedback loop provides an output signal that is demodulated at a spin frequency by a demodulator, thereby to correct for mismatches between the accelerometers.

9. The method according to claim 4 in which at least one of the accelerometers comprises:

a magnet means that produces a magnetic field;

a proof mass that is suspended by a spring within said magnetic field and that tends to move in response to an applied acceleration force;

a position sensor for said proof mass and for producing a feedback signal representative of the position of said proof mass;

a reaction coil adjacent the proof mass and within said magnetic field;

an internal feedback loop that receives said feedback signal and that provides a signal related to movement of said proof mass and that delivers said signal to said reaction coil to maintain said proof mass stationary and to generate a first coil output current signal in said reaction coil that is representative of an acceleration force applied to said proof mass;

an external feedback loop for adjusting said accelerometer that receives said first coil output current signal, that may receive a second coil output current signal from a second accelerometer which opposes but matches said accelerometer, and that combines said received signals to produce an output signal;

a relatively small electromagnet means associated with said magnet means for receiving said output signal and for responsively adjusting said accelerometer to correct for mismatches between said accelerometer and said second accelerometer;

thereby providing the capacity to adjust an acceleration scale factor and permitting said internal feedback loop to provide second order compensation to said proof mass and the stiffness of said spring.

10. The compensation method according to claim 9, wherein the internal feedback loop includes high gain to reduce errors in an accelerometer transfer function.

11. A gravity gradient instrument comprising:

a first, a second, a third and a fourth accelerometer equally spaced around the circumference of a circle with their respective sensitive axes tangential to the circle and arranged in opposing pairs with the first accelerometer opposite the second accelerometer and the third accelerometer opposite the fourth accelerometer;

in use the accelerometers are spun around an axis normal to the circle and passing through the center of the circle;

said instrument further comprising an internal feedback loop for each accelerometer and an external feedback loop common to all the accelerometers to correct for mass ratio mismatch between two paired accelerometers by providing a s² loop gain as a mass compensator.

12. A gravity gradient instrument comprising:

a first, a second, a third and a fourth accelerometer equally spaced around the circumference of a circle with their respective sensitive axes tangential to the circle and arranged in opposing pairs with the first accelerometer opposite the second accelerometer and the third accelerometer opposite the fourth accelerometer;

in use the accelerometers are spun around an axis normal to the circle and passing through the center of the circle;

said instrument further comprising an internal feedback loop for each accelerometer and an external feedback loop common to all the accelerometers to correct for Fo frequency ratio mismatch between two paired accelerometers by providing a s² loop gain as a mass compensator.

13. A gravity gradient instrument comprising:

a first, a second, a third and a fourth accelerometer equally spaced around the circumference of a circle with their respective sensitive axes tangential to the circle and arranged in opposing pairs with the first accelerometer opposite the second accelerometer and the third accelerometer opposite the fourth accelerometer;

in use the accelerometers are spun around an axis normal to the circle and passing through the center of the circle;

said instrument further comprising an internal feedback loop for each accelerometer and an external feedback loop common to all the accelerometers to correct for mass ratio and the Fo frequency ratio mismatch between two paired accelerometers by providing a s² loop gain term as a mass compensator.

14. A gravity gradient instrument according to claim 13 in which one or both of the accelerometers comprises a proof mass suspended by a spring within a magnetic field, an internal feedback loop to provide a signal related to movement of the proof mass back through a reaction coil acting in the magnetic field to maintain the proof mass stationary, an external feedback loop to adjust the accelerometer scale factor, wherein, the internal feedback loop provides second order compensation to the proof mass and a spring stiffness.

15. A gravity gradient instrument according to claim 14, wherein the internal feedback loop includes high gain to reduce errors in an accelerometer transfer function.

16. A gravity gradient instrument according to claim 15, wherein there is a compensator in the internal feedback loop to provide double pole and double zero compensation as follows:

$\frac{m_1<br><br>s^2+k_1}{m_0<br><br>s^2+k_0}$

where;

m₁ is the mass of the proof mass

k₁ is the spring constant of the spring

m₀ is a nominal proof mass and

k₀ is a nominal spring constant

and the nominal proof mass and spring constant represent the accelerometer characteristics to which both accelerometers of a pair are to be matched in order that the two accelerometers are closely matched to each other.

17. An accelerometer apparatus comprising

a magnet means for producing a magnetic field;

a proof mass that tends to move in response to an applied acceleration force;

a spring means for suspending and yieldingly biasing the position of said proof mass in said magnetic field;

a position sensor means for sensing the position of said proof mass and for providing a position signal representative of said proof mass position;

a reaction coil in said magnetic field adjacent said proof mass for retaining said proof mass in a substantially stationary position responsive to a compensating feedback signal received in said reaction coil;

an internal feedback loop for

receiving said position signal from said position sensor,

producing and delivering said compensating feedback signal to said reaction coil, said compensating feedback signal being representative of an acceleration force applied to said proof mass, whereby any tendency towards movement of said proof mass in said magnetic field is opposed, and

producing in said reaction coil a coil current that is representative of an acceleration force applied to said proof mass;

an external feedback loop for adjusting said accelerometer that

receives said coil current,

may receive a second coil current from a second opposing accelerometer which is paired with said accelerometer, and

combines and processes said received coil currents through a summing amplifier, a filter, and a demodulator to produce a corrective output signal; and

a relatively small electromagnet means associated with said magnet means for receiving said corrective output signal and for responsively adjusting said magnet means of said accelerometer to correct for mismatches between said accelerometer and said second accelerometer;

thereby to achieve a matching of said accelerometer and said second accelerometer.

18. The accelerometer of claim 17 wherein said reaction coil is connected to an output port for receiving and passing current flowing through said reaction coil and said output port is connected to said external feedback loop.

19. The accelerometer of claim 17 wherein said internal feedback loop includes a compensator to provide double pole and double zero compensation.

20. The accelerometer of claim 17 which is combination with a second accelerometer that opposes but matches said accelerometer, and that includes a similar combination of components and that generates a current that flows from the reaction coil of the second accelerometer, and the respective output current from each reaction coil are combined in said external feedback loop to produce an output signal that is fed to the respective electromagnet means of each said accelerometer.

21. Apparatus for producing a matched pair of accelerometers in a gravity gradient instrument, said apparatus comprising in combination:

a pair of accelerometers, each said accelerometer having its sensitive axis located perpendicularly and tangentially relative to the perimeter of a circle, and each said accelerometer being in diametrically opposed relationship relative to said circle, each said accelerometer comprising in combination;

a magnet means for producing a magnetic field;

a proof mass that tends to move in response to an applied acceleration force;

a spring means for suspending and yieldingly biasing the position of said proof mass in said magnetic field;

a position sensor means for sensing the position of said proof mass and for providing a position signal representative of said proof mass position;

a reaction coil in said magnetic field adjacent said proof mass for retaining said proof mass in a substantially stationary position responsive to a compensating feedback signal received in said reaction coil and for producing an output signal that is representative of an acceleration applied to said proof mass;

an internal feedback loop for receiving said position signal from said position sensor and for producing and delivering said compensating feedback signal to said reaction coil, said compensating feedback signal being representative of an acceleration force applied to said proof mass, whereby any tendency towards movement of said proof mass in said magnetic field is compensated; and

a relatively small electromagnet means associated with said magnet means for receiving a corrective signal, said electromagnet means being adapted to adjust said accelerometer responsive to said received corrective signal; and

an external feedback loop that is common to both of said accelerometers and that receives said output signal from said reaction coil of each said accelerometer, includes

a summing amplifier that adds together said output signal from each of said reaction coils,

a filter through which the signal from said summing amplifier passes, and

a demodulator that demodulates the signal from said filter at a predetermined spin frequency, thereby to produce said corrective signal, and

feeds said corrective signal to each said electromagnet of said accelerometers;

whereby mismatches between said accelerometers are corrected.

22. The apparatus of claim 21 wherein said internal feedback loop of each of said accelerometers includes means for producing high gain, thereby to reduce errors.

23. The accelerometer of claim 21 wherein said internal feedback loop of each said accelerometer includes a compensator to provide double pole and double zero compensation as follows: