Synthetic vibration isolation system for freefall gravimeter

• US 8,543,350 B2
• Filed: 08/20/2010
• Issued: 09/24/2013
• Est. Priority Date: 08/21/2009
• Status: Expired due to Fees

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Inventors: Hilby, Timothy R.
• Primary Examiner(s): Lau, Tung S
• Assistant Examiner(s): Chang, Stephanie
• Application Number:
  US12/860,804
• Publication Number:
  US 20110046913A1
• Time in Patent Office: 1,131 Days
• Field of Search: 731/520.3, 731/521.8
• US Class Current: 702/141
• CPC Class Codes: G01V 7/14 using free-fall time

Abstract

A gravimeter is disclosed that comprises a base, a reference device coupled to the base and configured to move along a first axis, a falling device configured to free fall from a first position to a second position on a second axis that is parallel to the first axis, a measurement module coupled to the reference device and configured to provide a first signal of the displacement of the reference device relative to the base and provide a second signal of the displacement of the falling device relative to the reference device. A processing unit accepts the first and second signals and computes a displacement of the falling device in inertial space by processing the first and second signals and subtracting the processed first signal from the processed second signal.

Claims

1. A gravimeter, comprising:

a base;

a reference device coupled to the base, the reference device configured to move along a first axis;

a compliant element physically attached to the reference device and the base;

a falling device configured to free fall from a first position on a second axis that is parallel to the first axis to a second position on the second axis;

a measurement module coupled to the reference device, the measurement module configured to provide a first signal comprising a displacement of the reference device relative to the base and provide a second signal comprising a displacement of the falling device relative to the reference device; and

a processing unit configured to accept the filtered first and second signals and compute a displacement of the falling device in an inertial space by processing the first and second signals and subtracting the processed first signal from the processed second signal.

2. The gravimeter of claim 1, wherein the processing unit is further configured to compute an relative acceleration of the falling device in inertial space from a plurality of the computed displacements of the falling device.

3. The gravimeter of claim 1, wherein:

the base comprises a reflective fiducial;

the measurement module comprises an optics module that is coupled to the reference device and a detector module that is coupled to the optics module;

the optics module is configured to accept a beam of laser light, create a reference beam and a measurement beam, direct the measurement beam to the reflective fiducial, capture the reflected measurement beam, and provide an output comprising the reference beam and the reflected measurement beam;

the detector module is configured to accept the output from the optics module, and compute the first signal by comparing the reflected measurement beam to the reference beam.

4. The gravimeter of claim 3, wherein the optics module further comprises an optical frequency shifter coupled to a portion of the accepted laser light beam, wherein the accepted laser light beam has a first frequency, and wherein the optical frequency shifter is configured to shift the portion of the accepted laser light beam from the first frequency to a second frequency.

5. The gravimeter of claim 4, wherein the optics module is further configured to form the measurement beam from the portion of the accepted laser light beam having the first frequency and form the reference beam from the portion of the accepted laser light beam having the second frequency.

6. The gravimeter of claim 1, wherein the processing unit is further configured to compute a displacement of the falling device in an inertial space by processing the first and second signals using a process equivalent to {circumflex over (X)}₂(s)=Y₂(s)−

Y₁(s)·

G(s) wherein;

{circumflex over (X)}₂(s) is a Laplace transform of an relative position of the falling mass in inertial space comprising the Laplace variable s;

Y₁(s) is a Laplace transform of a first distance of the reference device from the base, wherein the first distance is computed from the first signal;

Y₂(s) is a Laplace transform of a distance of the falling device from the reference device, wherein the second distance is computed from the second signal; and

G(s) is a Laplace transform of a filter function.

7. The gravimeter of claim 6, wherein:

the compliant element has a spring rate k;

the reference device has a mass m;

the compliant element and the reference device form a spring-mass system having a first natural frequency ω

₁=√

{square root over (k/m)}; and

G(s) comprises a term

8. The gravimeter of claim 7, wherein the filter natural frequency is less than the first natural frequency.

9. The gravimeter of claim 7, wherein the first natural frequency is greater than 10 Hz.

10. The gravimeter of claim 7, wherein the filter natural frequency is less than 0.5 Hz.

11. A computer-implemented method of measuring the acceleration in inertial space of a free-falling mass, the method comprising the steps of:

measuring, using an axis, a first displacement of a reference device relative to a base during an increment of time;

measuring, using the axis, a second displacement of a free-falling mass relative to the reference device during the same increment of time;

filtering the first displacement;

subtracting, with a processor, the filtered first displacement from the second displacement;

computing, with the processor, the relative displacement of the falling mass in inertial space and storing the relative displacement and time increment;

repeating the above steps of measuring, filtering, subtracting, and computing;

calculating, with the processor, a best-fit estimate of the acceleration of the free-falling mass from the stored relative displacements and time increments made during a single drop;

repeating the drop and repeating the above steps of measuring, filtering, subtracting, computing, repeating, and calculating; and

calculating, with the processor, an overall acceleration of the mass by averaging the best-fit estimates of the acceleration from all the drops.

12. The computer-implemented method of claim 11, further comprising the step of computing, with the processor, a relative displacement position of the falling mass in inertial space using a process equivalent to

{circumflex over (X)}₂(s)=Y₂(s)−

Y₁(s)·

G(s) wherein;

{circumflex over (X)}₂(s) is a Laplace transform of the relative position of the falling mass in inertial space;

Y₁(s) is a Laplace transform of a first distance of the reference device from the base, wherein the first distance is computed from the first signal;

Y₂(s) is a Laplace transform of a distance of the falling device from the reference device, wherein the second distance is computed from the second signal; and

G(s) is a Laplace transform of a filter function.

13. The computer-implemented method of claim 12, wherein G(s) comprises a filter having a 40 dB/decade rolloff at frequencies above a filter natural frequency ω

_h.

14. The computer-implemented method of claim 13, wherein G(s) comprises:

15. The computer-implemented method of claim 11, wherein the steps of measuring the first displacement comprises the steps of:

generating a first measurement beam and a second reference beam of laser light at a first frequency, each beam having a phase;

generating a second measurement beam and a first reference beam of laser light at a second frequency, each beam having a phase, wherein the combination of the second frequency and the first frequency creates a beat frequency;

directing the first and second measurement beams from the reference device to a first reflective fiducial on the base and a reference reflective fiducial that is fixed with respect the origin of the beam, respectively;

capturing the first and second reflected measurement beams that are reflected by the respective reflective fiducials;

directing the first and second reference beams through a first and a second lightwave circuit, respectively, that are a part of the reference device;

combining the first reflected measurement beam with the second reference beam and directing the combined beam to a first detector that generates a first signal related to the characteristics of the combined beam;

combining the second reflected measurement beam with the first reference beam and directing the combined beam to a second detector that generates a second signal related to the characteristics of the combined beam;

calculating, with the processor, the displacement of the reference device relative to the base by comparing the phase of the first and second signals.

16. The computer-implemented method of claim 15, wherein the step of calculating the displacement of the reference device is performed using coherent optical heterodyne interferometry.

17. A non-transitory computer-readable medium having computer-executable instructions stored thereon for execution by a processor to perform a method of measuring the relative position of a falling mass in inertial space, the method comprising the steps of:

measuring a first displacement of a reference device relative to a base during an increment of time, wherein the base comprises a retroreflector;

measuring a second displacement of a free-falling mass relative to the reference device during the same increment of time;

filtering the first displacement;

subtracting the filtered first displacement from the second displacement;

computing the relative displacement of the falling mass in inertial space and storing the relative displacement and time increment;

repeating the above steps of measuring, filtering, subtracting, and computing;

calculating a best-fit estimate of the acceleration of the free-falling mass from the stored relative displacements and time increments made during a single drop;

repeating the drop and repeating the above steps of measuring, filtering, subtracting, computing, repeating, and calculating; and

calculating an overall acceleration of the mass by averaging the best-fit estimates of the acceleration from all the drops.

18. The non-transitory computer-readable medium of claim 17, wherein the method further comprises the step of computing an relative position of the falling mass in inertial space using a process equivalent to {circumflex over (X)}₂(s)=Y₂(s)−

Y₁(s)·

G(s) wherein;

{circumflex over (X)}₂(s) is a Laplace transform of the relative position of the falling mass in inertial space;

Y₁(s) is a Laplace transform of a first distance of the reference device from the base, wherein the first distance is computed from the first signal;

Y₂(s) is a Laplace transform of a distance of the falling device from the reference device, wherein the second distance is computed from the second signal; and

G(s) is a Laplace transform of a filter function.

19. The non-transitory computer-readable medium of claim 18, wherein G(s) comprises: