Abstract:
An accelerometer has a substantially linear strain sensor with a transducer joined to the strain sensor. The transducer has a base that provides rigidity perpendicular to a preferred measurement direction. A plurality of outer flaps are joined to the base supporting the strain sensor. The outer flaps are capable of translating acceleration in the predefined direction to strain in the strain sensor. Two centermost flaps are positioned on either side of the center line of the transducer. Struts are joined between a lower portion of one the centermost flap and an uppermost portion of the nearest outer flap. The struts enhance strain by linking outer flap motion to the centermost flaps. Bridges are joined between each two adjacent outer flaps supporting the strain sensor.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefore. 
    
    
     This application is a continuation in part of the prior filed, co-pending, non-provisional application Ser. No. 11/934,846, filed 5 Nov. 2007. This application repeats a substantial portion of prior filed application Ser. No. 11/934,846, filed 5 Nov. 2007, and adds and claims additional disclosure not present in the prior filed application. 
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates generally to instruments and more particularly to a transducer that can be used with a strain sensor to make an accelerometer. 
     (2) Description of the Prior Art 
     Traditional electrical accelerometers use a magnetic mass supported by a spring in a housing. An induction coil is wound around the interior of the housing. Acceleration is sensed by measuring the electromotive force induced in the induction coil by movement of the magnetic mass. Piezoceramic transducers use a piezoelectric effect to produce a low voltage output in response to force. These sensors usually require a preamplifier to be placed in close proximity. In remote applications, especially those involving arrays of accelerometers, the preamplifiers and telemetry combining signals for transmission to the receive site, can be large and a major factor in the system expense. 
     Fiber optic sensor systems remove the requirement for preamplification and electronic telemetry. Very low sensitivity accelerometers are based on the acceleration causing two optical waveguides to be misaligned, thus varying the intensity of the light signal. Other accelerometers use large coils of tens of meters of optical fiber. The large amount of fiber and the limited bend diameter of the fiber limit these to fairly large sizes. 
     Another type of optical accelerometer uses a fiber optic segment positioned between a base and a ridged reaction mass. Acceleration causes the reaction mass to pinch the fiber optic segment against the base. Acceleration can be measured by measuring the modulation of the light passing through the fiber optic segment. 
     Sometimes it is more convenient to measure strain, and a conventional technique is the use of a force transducer to translate acceleration into strain. Strain sensors include a foil strain gauge having a plurality of foil traces on a backing material. This gauge is mounted on an object to measure elongation of the object. Strain is measured by measuring the increased resistance in the strain gauge. Another linear electrical strain sensor can be constructed by providing mercury or some other conductive material in an elastomeric tube. The elastomeric tube is mounted to the object. Stretching of the tube results in a narrowing of the cross-sectional area which increases electrical resistance in the conductive material. This resistance can be measured giving an indication of strain. 
     A fiber optic sensor can also be used to measure strain. A piece of fiber optic is provided that has a Bragg grating written in the fiber optic at either end of a sensing region. The sensing region is mounted to an object being measured, and elongation of the sensor can be measured by providing light pulses at a known interval in the fiber optic and collecting the reflected pulses. Elongation of the fiber optic sensor region causes a measurable delay in the pulses. 
     U.S. patent application Ser. No. 11/934,850, which is incorporated by reference herein, teaches use of a fiber optic laser to measure strain.  FIG. 1 , below, has additional details concerning the use of fiber laser sensors. 
     As discussed herein, there is provided an accelerometer having a transducer that can be used with a variety of strain sensors. 
     SUMMARY OF THE INVENTION 
     Accordingly, there is provided an accelerometer having a substantially linear strain sensor with a transducer joined to the strain sensor. The transducer has a base that provides rigidity perpendicular to a preferred measurement direction. A plurality of outer flaps are joined to the base supporting the strain sensor. The outer flaps are capable of translating acceleration in the predefined direction to strain in the strain sensor. Two centermost flaps are positioned on either side of the center line of the transducer. Struts are joined between a lower portion of one the centermost flap and an uppermost portion of the nearest outer flap. The struts enhance strain by linking outer flap motion to the centermost flaps. Bridges are joined between each two adjacent outer flaps supporting the strain sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing invention will become readily apparent by referring to the following detailed description and the appended drawings in which: 
         FIG. 1  is a diagram showing a fiber optic laser sensor accelerometer; 
         FIG. 2  is a diagram showing an accelerometer having a transducer constructed according to a first embodiment; 
         FIG. 3  is a diagram showing an accelerometer having a transducer constructed according to a second embodiment; and 
         FIG. 4  is a diagram showing an accelerometer having a transducer constructed according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an accelerometer utilizing a fiber laser sensor. The fiber laser accelerometer  10  includes a fiber laser  12 . Fiber laser  12  can be either a Fabry-Perot type cavity fiber laser or a distributed feedback fiber laser. In a Fabry-Perot type fiber laser, the laser cavity is a length of erbium-doped optical fiber with a Bragg grating written in the fiber core at either end of the laser cavity. In a distributed feedback fiber laser, the fiber laser cavity is a length of erbium-doped optical fiber having a grating written over the full length of the cavity. The distributed feedback fiber laser will have a phase shift at the center of the cavity. A pump laser  14  is provided for transmitting coherent light through optical fiber  16 . Pump laser  14  can be any laser such as a diode laser operating at 980 nm or 1480 nm. Pump laser  14  is joined by fiber  16  to a distributor  18 . Distributor  18  can be a wavelength division multiplexer, circulator or the like. A wavelength division multiplexer operates by providing light at the pump laser wavelength to the fiber laser  12 . Returning light from the fiber laser  12  is at a different frequency and is guided along a different path. A circulator can carry out the same function by transferring light to the next port of the circulator. 
     After coherent light passes through distributor  18 , it is absorbed by the doping material in fiber laser  12 . Absorption of this light causes the doping material to emit photons which are gathered by resonance between the gratings. This causes fiber laser  12  to emit a narrow single mode of light. Fiber laser  12  is mounted to a transducer  20  such that acceleration of the transducer material strains the fiber laser cavity. This strain shifts the emission wavelength of the fiber laser  12 . Transducer  20  can have a wide variety of constructions that will be discussed hereinafter. 
     Fiber laser emission returns along fiber  16  to wavelength division multiplexer  18  where it is separated onto analysis path  22 . An interferometer  24  such as a Mach-Zehnder interferometer is positioned on analysis path  22 . Interferometer  24  converts the shifted emission wavelength of fiber laser into a phase shift of the fiber laser light. Interferometer  24  typically includes a first beam splitter  24 A having a delay loop  24 B on one leg and a modulator  24 C on the second leg. Signals from the delay loop  24 B and modulator  24 C are combined in a second beam splitter  24 D to produce the phase shifted signal. Other interferometer designs can produce the same type of signal. A receiver  26  receives the phase shifted signal. Receiver  26  is capable of demodulating and detecting the signal from the fiber laser by various methods well known in the art. 
       FIG. 2  provides a transducer  20 . Transducer  20  can be used with any linear strain sensor  28 . These include foil resistance strain sensors, conductive liquid strain sensors, optical delay strain sensors, fiber laser strain sensors, or the like. Transducer  20  functions to translate acceleration into strain in strain sensor  28 . 
     Transducer  20  has at least one flap  30 . Flap  30  is attached to a base  32  of the transducer  20  and extends toward a center  34  of strain sensor  28  at an angle. Equal numbers of flaps  30  can be provided on each side of center  34 , and all flaps  30  angle toward the center  34 . Transducer  20  can be made from a polymer material. Transducer  20  allows vertical movement of flaps  30 , as shown, but resists movement in the transverse direction because of its geometry. Movement in the longitudinal direction cannot be controlled without affecting the vertical motion induced by acceleration. The affect of the longitudinal motion tends to cancel out because of the flap arrangement. The volume in between flaps  30  can be a vacuum, gas, liquid, or solid; however, it should allow movement of flaps  30 . A gas, liquid or solid could provide damping, if necessary. In this embodiment, the combined top surfaces  36  of the flaps  30  form an interrupted surface  38  that is higher near center  34 . Sensor  28  is mounted under tension across the top surfaces  36  of the flaps  30  and fixed to the outer two flaps with an adhesive  40  such as ultra-violet cured epoxy. Adhesive  40  should not interfere with sensor  28 . Mounting to the outermost flaps  30  reduces the constraint on the ends of the sensor  28  and yields greater sensitivity to acceleration. 
     Increasing the height of interrupted surface  38  near center  34  allows sensor  28  contact with all of the flap upper surfaces  36 . Friction holds the sensor  28  in place laterally. The body of each flap  30  serves as an inertial mass. When transducer  20  is accelerated away from sensor  28 , flaps  30  move toward sensor  28  and outward from the center  34  due to the angle of the flaps  30 . This causes an unsupported portion  42  of the sensor  28  between the centermost two flaps  30  to be further tensioned. The outer flaps  30  move with the inner flaps  30  and produce additional strain on the fiber portion at center  34 . The flaps  30  have sufficient width in the direction perpendicular to the direction being sensed to ensure that the flaps  30  are much more stiff against motion in that direction. This stiffness in the transverse direction ensures that the accelerometer  10  has good isolation against responding to accelerations in that direction, known as cross-axis isolation. Transducer  20  structure also provides good isolation against longitudinal accelerations in the direction parallel to sensor  28 . In this direction, the half of the flaps  30  on one side of the center  34  move upward and outward, while the half of the flaps  30  on the other side of the center  34  move downward and inward. These two motions tend to cancel, producing little net strain on the center  34  of the sensor  28 . 
     The unsupported fiber portion  40  between the center two flaps  30  can also vibrate in a string mode. This mode is undesirable because it is equally susceptible to acceleration in both directions transverse to the sensor  28 . Thus, it is desirable to minimize this mode and push its resonance to a frequency above that in the sensing range. The response of this mode is controlled by mounting the sensor  28  to the transducer  20  with sufficient tension. 
     A transducer of this form is basically a mass/spring system operated well below resonance. In an idealized mass/spring system the natural or resonant frequency of the system is given by the following equation: 
                     ω   n     =       (     K   M     )       1   2               (   1   )               
where ω n  is the natural frequency, M is the mass, and K is the spring stiffness. A harmonic acceleration can be given by the following equation:
 
 a=a   0  cos ω t   (2)
 
Under these conditions, the displacement of the mass, M, is harmonic with amplitude X given by:
 
                   X   =         Ma   0     K       (     1   -       (     ω     ω   n       )     2       )               (   3   )               
The sensitivity of a sensor based on this system is given by S=X/a 0 . At frequencies well below the resonant frequency, ω n , this is approximated by:
 
                   S   =       (     X     a   0       )     =     M   K               (   4   )               
In the accelerometer, the maximum strain on the sensor  28  is related to the maximum displacement of the flap  30  masses. It can be seen from these equations that both resonant frequency, ω n , and displacement X (or sensitivity S) are directly related to the ratio K/M. If the system is designed for a particular resonant frequency to give a desired bandwidth sensor, the sensitivity is predetermined. In general, it is desired that the sensitivity S be maximized for a particular resonant frequency. These equations indicate that this is not possible for the idealized mass spring system. However, in a non-idealized system not consisting of a point mass, the limitation on displacement refers to the motion of the center of mass.
 
       FIG. 3  shows an embodiment of a transducer  44  having enhanced sensitivity. As above, sensor  28  is fixed in tension to the top of transducer  44 . Sensor  28  is joined to transducer  44  by adhesive  40 . Transducer  44  has a base  46  and is symmetrical about a center line  48 . 
     This embodiment has outer flaps  50  and centermost flaps  52  for translating accelerations perpendicular (upward in the FIG.) to transducer  44  into tensions in sensor  28 . The centermost flaps  52  are positioned on each side of and proximate to center line  48 , and the outer flaps  50  are positioned horizontally along base  46  between the centermost flap  52  and an end of base  46 . A first end  54 A of each flap is joined to base  46  and a second end  54 B of each flap supports sensor  28 . The second ends  54 B of combined flaps form a curved surface for maintaining contact between the flaps and the sensor. The flaps are angled towards the center line  48  of the transducer  44 . 
     The flaps are configured based on their position in transducer  44 . Outer flaps  50  each have a neck  56  joined to a body  58 . Neck  56  acts to reduce the spring constant K of the outer flap  50 . Body  58  acts as a mass. One bridge  60  is joined between each adjacent outer flap  50 . A strut  62  is joined between a lower portion of each centermost flap  52  and an upper portion of the adjacent outer flap  50 . 
     The centermost flaps  52  are designed to be stiff but less massive than outer flaps  50 . The outer flaps  50  have most of their mass away from base  46  to maximize the force that they apply to the centermost flap  52 . Resonance frequency of the outer flaps  50  is maintained by having greater stiffness than the centermost flaps  52 . Stiffness can be adjusted by changing the width of the centermost flaps  52  or by providing voids in the flaps or necks. 
     In operation, when the transducer is accelerated vertically, inertia of outer flaps  50  causes them to move downward toward base  46  and toward center line  48 . The second ends  54 B of outer flaps  50  move together because of linking by bridges  60 . Struts  62  transfer this force to centermost flaps  52 . Each centermost flap  52  acts as a lever arm. A small motion of strut  62  results in a relatively large motion at second ends  54 B of centermost flaps  52 . This movement reduces tension in sensor  28  between centermost flaps  52 . 
       FIG. 4  shows an alternative embodiment of the invention. This embodiment utilizes high density masses  64  positioned in body  58  of outer flaps  50 . Masses  64  can help tailor the resonance frequency of transducer  44 . 
     It is to be understood that the foregoing description and specific embodiments are merely illustrative of the best mode of the invention and the principles thereof, and that various modifications and additions may be made to the invention by those skilled in the art, without departing from the spirit and scope of this invention, which is therefore understood to be limited only by the scope of the appended claims.