Fiber optic accelerometer - Patent Review 4671113

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BACKGROUND OF THE INVENTION

This invention relates to the art of accelerometers and more particularly to fiber optic accelerometers.

Heretofore, various types of accelerometers have been used. In one type of accelerometer, a mass is displaced against a spring by the inertial force due to acceleration. The distance which the mass is displaced varies with the acceleration. By monitoring the amount of displacement, the acceleration can be determined. The smaller the acceleration which the prior art acceleration accelerometers are designed to measure, the more sensitive they tend to be to interference. Further, the accuracy of accelerometers is related to the linearity or predictability of the physical changes in response to acceleration. Commonly, it is difficult to maintain the linearity and predictability when measuring relatively small accelerations.

In one prior art fiber optic accelerometer, a mass is positioned in a housing by transverse diaphragms. The mass is suspended between the ends of longitudinally extending tensioned optical fibers. The tensioned optical fibers are secured to the mass and the housing such that under acceleration, the mass elongates one of the fibers and allows the other to contact. Each fiber is mirrored at the end connected to the mass. A laser transmits phase coherent light along both optical fibers toward the mirrored ends. Light reflected from the mirrors is combined and fed to a signal processor or other phase shift detector. The elongation and contraction of the optical fibers caused by acceleration alters the phase relationship of the two reflected light beams. The detected phase shift varies in accordance with the acceleration.

The present accelerometer contemplates a new and improved accelerometer which is capable of measuring small acceleration forces accurately over a wide dynamic range. For example, in the preferred embodiment, the accelerometer is able to measure accelerations on the order of one-millionth the acceleration of gravity.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a two coil, phase modulation fiber optic accelerometer.

In accordance with a more limited aspect of the invention, there is provided an accelerometer having first and second optical fiber coils or lengths acted on by a relatively dense mass, preferably a fluid. The first and second fiber optic lengths are disposed in substantially parallel, planar regions with the mass therebetween. The optical fiber lengths are adapted to be connected with a phase coherent light source, such as a laser, and are adapted for connection with a phase shift detection means. Under acceleration transverse to the parallel planes of the first and second optical fiber coils, the first and second optical fiber coils are subjected to a differential pressure which varies in proportion to acceleration. The pressure causes a corresponding differential length change in the fiber optic lengths which, in turn, causes a shift in the phase relationship of the light passing therethrough. The phase shift varies in proportion to the pressure differential and acceleration.

In accordance with another aspect of the invention, there is provided a fiber optic accelerometer for measuring acceleration along a first axis. First and second lengths of optical fiber are disposed transverse to its first axis. A mass is disposed along the first axis between the first and second optical fiber lengths. A phase coherent light source transmits phase coherent light through the first and second optical fiber lengths. A phase detection means detects a difference or shift in the relative phase of the light transmitted through the first and second fiber optic lengths. An acceleration indictating means transforms the detected phase shift into an indication of acceleration.

A primary advantage of the present invention is that it accurately measures small accelerations.

Another advantage of the present invention is that it is relatively insensitive to interference, particularly electric and magnetic interference.

Yet another advantage of the present invention is that it is strong and stable over a wide range of environmental conditions.

Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various parts and arrangements of parts. The drawings are only for purposes of illustrating a preferred embodiment of the invention and are not to be construed as limiting it.

FIG. 1 illustrates a fiber optic accelerometer in accordance with the present invention;

FIG. 2 illustrates an alternate embodiment of an accelerometer in accordance with the present invention;

FIG. 3 illustrates another alternate embodiment of a fiber optic accelerometer in accordance with the present invention; and,

FIG. 4 illustrates yet another alternate embodiment of an accelerometer in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first fiber optic length A, preferably a first coil and a second fiber optic length B, preferably a second coil, are disposed in substantially parallel planes separated by a distance h. A material C, preferably a fluid, of substantial mass density is disposed between the first and second optical fiber planes.

For light of wave length .lambda. passing through a coil of length l and index of refraction n, the total optical phase shift is given by the relationship:

.phi.=(2.pi.nl)/.lambda. (1)

Under acceleration, the fluid mass subjects the fiber optic lengths to a pressure differential. For a pair of like fiber optic coils of length l, the relative phase shift .delta..phi. is a function of the pressure differential .delta.p, therebetween, i.e.:

.delta..phi.=kl.delta.p (2)

where k is a constant which relates the phase change per meter of optical fiber to the pressure differential. It is known, for example, that for appropriately selected plastic coated optical fibers, a pressure change of 1 dyne/cm..sup.2 produces a phase shift in light of 1 micron wavelength of the order of 10.sup.-4 radians per meter of optical fiber. In this example, the constant is 10.sup.-4 radians/m./dyne/CM..sup.2. For a fluid mass, the pressure differential, .delta.p, is related to the acceleration a, the mass density .rho., and the distance h between the fiber optic lengths, by the relationship:

.delta.p=.rho.ah (3)

Thus, the acceleration a and a phase shift .delta..phi. are related by:

.delta..phi.=(kl.rho.h)a (4)

From these relationships, it can be seen that with a fiber optic length of 10 meters, a mass density .rho. of 1 gram per cubic centimeter, an optical fiber spacing h of 1 centimeter and a wave length .lambda. of 1 micron, each microradian of phase shift corresponds approximately to 10.sup.-3 cm./sec..sup.2, i.e. one-millionth the acceleration of gravity. It is to be appreciated that the sensitivity of the accelerometer can improved by increasing the length of optical fiber, increasing the density of the mass, increasing the spacing between the optical fiber coils, and decreasing the wave length of the coherent light.

With reference to FIG. 1, a source of phase coherent light, such as a laser 10, produces a beam of phase coherent light 12. A laser to fiber optic coupling means 14 such as a microlensed end of a first length of optical fiber 16, causes the laser light to be transmitted along the first optical fiber 16. An optical beam splitter 18, such as an optical fiber to optical fiber coupler, optically couples the first optical fiber with a second optical fiber 20 such that the light is divided approximately equally between the two fibers. In this manner, phase coherent light is transmitted along the first and second optical fibers. The first optical fiber 16 is optically connected with the first optical fiber length or coil A and the second optical fiber 20 is optically connected with the second optical fiber length or coil B. In the preferred embodiment, the first and second optical fiber lengths are close-packed coils. Alternately, the first and second fiber lengths may be arranged in other patterns which enable a relatively long length of optical fiber to be positioned in a relatively small, generally planar region.

A second optical fiber to optical fiber coupling means 22 combines the light carried on the first and second optical fibers. Combining the phase shifted phase coherent beams of light in the two optical fibers produces optical interference which effects the intensity of the combined, modulated light. A pair of output optical fibers 24 and 26 carry the combined, modulated light from the light combining means 22 to a phase detector means 28 which detects shifts in the phase relationship of the phase coherent light transmitted through the first and second optical fibers. The phase detection means includes a pair of photodectectors 30 and 32, such as photodiodes, which sense the light transmitted on the output optical fibers and produce corresponding electrical signals. The electrical signals are processed by a signal processor 34 which detects changes in the electrical signals. An acceleration indicator means 36 is connected with the signal processor for providing a visual indication of the amount of acceleration corresponding to the changes in the electrical signals, hence the relative phase change in the light from the first and second optical fiber lengths.

The fiber optic lengths A and B are positioned in parallel generally planar regions transverse to an axis 38. The planar regions are disposed generally at opposite ends of a housing 40. The housing provides a reservoir for the material or mass means C. Preferably, the material is a fluid and has a relatively high density, e.g. mercury.

With reference to the embodiment of FIG. 2, phase coherent light from the laser 10 is transmitted along first and second optical fibers 16 and 20 to the first and second fiber optic lengths or coils A and B, respectively. The fiber optic coils have mirrored end surfaces 50 and 52 which cause the phase coherent light to be reflected. The optical fiber to optical fiber coupling means 22 combines the reflected and unreflected laser light to provide the photodetector 30 with combined, modulated light. The intensity of the modulated light changes with acceleration in a known manner. The intensity changes in the modulated light intensity detected by the photodectector 30 are translated by the signal processor 34 and the acceleration indicator 36 into an indication of acceleration. The signal processor may, for example, compare the intensity changes with a look-up table that is preprogrammed with experimentally determined intensity/acceleration relationships.

In the embodiment of FIG. 3, an alternate arrangement of determining the acceleration is provided. Under zero or no acceleration, a central controller 60 adjusts a phase modulator 62 on one of the first and second optic fiber lengths until the photodetectors 30, 32 detect a preselected phase relationship and produce corresponding output signals. Under an acceleration to be measured, the controller 60 adjusts the phase modulator 62 and the acceleration indicator 36 by corresponding amounts until the photodetectors 30 and 32 again detect the preselected phase relationship. In this manner, the acceleration produced phase shift is counter balanced by the phase adjustment produced by the phase modulator 62 which in turn is translated into the acceleration indication by the acceleration indicator.

With reference to the embodiment of FIG. 4, the housing 40 is generally tubular being defined by an outer cylindrical wall 70, end walls 72 and 74 and an inner wall 76. The inner and outer walls define a cylindrical cavity 80. The first and second fiber optic lengths include multiple layers of spiral wrapped coils potted in a tetrafluoroethylene or other polymeric acoustooptic coupling material 82. The potting material anchors the fiber optic coils to the end walls to function as support means which supports the two coils and maintains them separated by the constant preselected distance. In the preferred embodiment, the mass means C which fills the cylindrical cavity 80 is mercury.

The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding specification. It is intended that the invention be construed as including all such alterations and modification insofar as they come within the scope of the appended claims or the equivalents thereof.

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