A rotational gravity gradiometer includes a first member, a second member, a drive member and a motor. The first member is disposed above the second member orthogonal to and centered with respect to the second member. The first member includes support arms extending from the center of the first member. The second member includes a second pair of support arms extending from a center point of the second member. A mass unit is attached at a distal end of the respective first member and second member. A sensor element is attached between each mass unit a connection point of the opposite member for sensing movement of the mass unit. The drive member is coupled to the motor to drive the first member and the second member rotationally. The respective sensor elements generate a signal in response to deflection of the support arm induced by an external mass.

BACKGROUND

The application generally relates to a rotational gravitational gradiometer. The application relates more specifically to a quadrupole responder for an Orthogonal Quadrupole Responder (OQR)-type gravity gradiometer.

A gravity gradiometer measures a gravitational field. Gravity gradiometers can be used, e.g., for controlling and stabilizing the attitude of an artificial earth satellite. The gravity gradiometer may include a gravity gradient member mounted in a gimbal arrangement to have two degrees of freedom. In other gravity gradiometers a flexural pivot and torsion spring may be required.

A gravity gradiometer may measure the Riemann curvature of spacetime produced by nearby masses. By flying a gradiometer in an airplane above the Earth's surface, one can measure subsurface mass variations due to varying geological structure. In an Earth-orbiting satellite, such a gradiometer could measure the gravitational multi-pole moments of the Earth.

SUMMARY

One embodiment relates to a rotational gravity gradiometer including a first member, a second member, a drive member and a motor. The first member is disposed above the second member orthogonal to and centered with respect to the second member. The first member includes a first pair of support arms extending axially from a center of the first member. The second member includes a second pair of support arms extending axially from a center point of the second member. A mass unit is attached to opposite ends of each support arm at a distal end of the respective first member and second member. A sensor element is attached between each mass unit of the first member and a connection point of the second member for sensing movement of the respective mass unit. A sensor element is attached between each mass unit of the second member and the drive member. The drive member is coupled to the motor to drive the first member and the second member rotationally. The respective sensor elements generate a signal in response to deflection of the support arm induced by an external mass.

Another embodiment relates to a rotational gravity gradiometer including a first orthogonal member, a second orthogonal member, and a drive member. Sensor elements for sensing vibration extending from an end mass unit at an end of each orthogonal member. The sensor elements are attached to the second member or drive member beneath the first and second orthogonal members, respectively, to provide a mass spring dampener having a single degree of freedom between each member. The sensor elements generate a signal in response to a deflection of the orthogonal member induced by an external mass.

Yet another embodiment relates to a method for detecting gradients in a gravitational field, including providing a first orthogonal member, a second orthogonal member, and a drive member for rotating the orthogonal members; fixing the first orthogonal member and the second orthogonal member on a common center and having a respective axis orthogonal to each other; rotating the first orthogonal member and the second orthogonal members by the drive member; providing a sensing element connected to opposite ends of each of the first orthogonal member and the second orthogonal and isolating the sensing material from external noise; sensing a movement of one of the first and second orthogonal members relative to one another; generating a signal in response to the sensed relative movement; and determining a gravity gradient in response to the sensed relative movement.

Certain advantages of the embodiments described herein include the ability to detect gradients or variations in the Earth's gravitational field.

Another advantage is the use of an orthogonal gravitational gradiometer as disclosed herein to identify subsurface features such as oil and gas deposits or mineral deposits, etc.

Further advantages include the use of the gradiometer for sensing volcanic activity, natural resource prospecting, space-craft attitude control, ground-based GPS corrections at base stations, airborne/marine gravity surveys, locating meteorites, tracking the melting of ice caps, and similar applications.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Referring toFIGS. 1-5a gravity gradiometer10consists of a first orthogonal member12(FIG. 3) and a second orthogonal member14(FIG. 4). First orthogonal member12is positioned above second orthogonal member14with the axes of orthogonal members12,14generally perpendicular with one another. Each orthogonal member12,14is arranged with a pair of mass units16mounted to mass holder18attached to opposite ends of the respective orthogonal member12,14. Orthogonal member12is formed by a pair of support arms20attached to a hub portion22having a recess24for receiving a centering projection26of adjacent orthogonal member14. Hub portion22connects support arms20in a linear axial relation and provides an axial contact point with orthogonal member14on an axis perpendicular to orthogonal members12,14.

As shown inFIG. 4, second orthogonal member14is arranged below first orthogonal member12and above a drive member28(FIG. 5) that supports a motor30. Orthogonal member14is formed by a pair of support arms32attached to a center connection34having a recess36for receiving a centering projection38of adjacent drive member28. Recess36is positioned opposite center projection26and is axially aligned with center projection26and centering projection38. Orthogonal member14includes a pair of opposing bracket portions40affixed to center connection34. Bracket portion40includes four mounting fingers42for attachment of a sensor element44(FIGS. 1 and 2). In one embodiment sensor element44is a piezo vibration sensor. The piezo vibration sensor may be, e.g., a cantilever-type vibration sensor, which is loaded by mass units16with high sensitivity at low frequencies. Sensor element44detects vibration caused by deflection of support arms20,32. A signal, e.g., a small AC voltage, is generated when the sensor element44moves horizontally back and forth. External electronic equipment may be used to increase or reduce the amplitude of the signal and/or filter noise produced by the sensor in order to condition the signal for processing. Sensor elements44have electrical connection pins at one end to bracket portions40for connecting the wire to sensor element44for transmitting an output signal to, e.g., a printed circuit board. At the opposite end sensor element44is attached to a connector portion46of mass holder18.

When support arms20,32are twisted out of orthogonal alignment, an oscillation is induced in mass units16and support arms20,32in a horizontal plane. Sensor elements44generate an output signal that measures the oscillation amplitude. When placed near an external mass, M, the gradiometer experiences a torque. The torque is generated by the external mass because of a gradient in the gravitational field of M (i.e., because of the spacetime curvature produced by M). The Newtonian forces acting on the two nearest mass units16(m1and m2) to M are greater than the Newtonian forces acting on the more remote mass units16, such that a net torque pulls the two nearest mass units16toward each other, and the remote pair of mass units toward each other. When in operation gravity gradiometer10rotates with an angular velocity about its center, the angular velocity generated by the rotation of motor30. As gravity gradiometer rotates the torques on arm supports20,32oscillate as follows:at ωt=0 net torque pushes (m1and m2) toward each other;at ωt=π/4 net torque is zero;at ωt=π/2 net torque pushes (m1and m2) away from each other.

The angular frequency of the oscillating torque is 2ω. If 2ω is set equal to the natural oscillation frequency of the arms, the oscillating torque drives the arms into resonant oscillation.

Sensor elements44may be made from the piezo laminate of piezo vibration sensors with a laterally bending material for sensor element44. By making the laterally bending material of piezo laminate a sensing element, noise is reduced in the output signal. Sensing element44is attached to connector portion46of mass holder18. By arranging sensor element44in this way, gravity gradiometer10is simplified because a component, sensor element44, performs two functions, the mechanical oscillator for the forced mass-spring-dampener system and the sensing. Also, the arrangement shown isolates sensor element44, since sensor element44is attached only at the ends by two components that are dense, i.e., mass unit16and a structurally sound support arm20,32.

Referring toFIG. 5, drive member28includes centering projection38, projecting from a cross-member48. Cross-member48connects two opposing bracket arms50which also include mounting fingers42for attaching sensor elements44. A connection sleeve52having opening54for receiving motor shaft58is disposed on cross-member48opposite centering projection38. Motor30is powered by control circuitry contained on a printed circuit board56with external electronic equipment (not shown). Motor shaft58may be fixedly attached to sleeve52for driving rotation of drive member28to impart rotation to gravity gradiometer10.

The disclosed arrangement of gravity gradiometer10provides a tiered system: a top arm or first orthogonal member12, a middle arm or orthogonal member14, and drive member28. Sensor elements44extend from end mass units16of a respective support arm20,32, and attach to the tier beneath support arm to form a single degree of freedom, mass-spring-dampener between each tier.

Fingers42projecting from bracket40and bracket arms50attach to the heads of sensor elements44, i.e., the piezo laminate of piezo vibration sensors. From this position wires are routed to above orthogonal member12where a flat planar surface on hub portion22provides a mounting surface to support printed circuit board56containing external electronic equipment or circuits for signal processing, data collection and analysis.

In an alternate embodiment the sensor element44connection from orthogonal member14in the middle tier to drive member28may be omitted, by having the motor to apply frictional torque to orthogonal member14to drive orthogonal member14to the desired rotational velocity. Thus, gravity gradiometer10maintains rotational velocity by inertia and angular momentum, to provide free movement rather than quasi-free movement with the piezo sensor element44. Thus drive member28rotates orthogonal member14and thus the entire gradiometer10. The connection of piezo sensor element44from orthogonal member14to drive member28is needed for the vibration and not the sensing. This set of piezo sensor elements44provides the bottom arm, i.e., orthogonal member14the ability to rotate to external gravitational responses, and still be driven by the driving member. Because driving member28rotates orthogonal member14by friction, orthogonal member14will still be able to swing from the external response, but will rotate to the desired frequency from driving member28.

While the exemplary embodiments illustrated in the figures and described herein are presently preferred, it should be understood that these embodiments are offered by way of example only. Accordingly, the present application is not limited to a particular embodiment, but extends to various modifications that nevertheless fall within the scope of the appended claims. The order or sequence of any processes or method steps may be varied or re-sequenced according to alternative embodiments.