Abstract:
The interferometric fiber optic accelerometer is viewed as a mass-spring transducer housed in a sensor case. The sensor case is attached to a moving part whose motion is inferred from the relative motion between the mass and the sensor case. A flexural disk is housed in a sensor case which is accelerated in a direction normal to the plate surface. The plate undergoes displacement resulting in strains on the plate surface. A coil of optical fiber, made to be part of an optical interferometer, is attached to the flexural disk, the strain from the disk is transferred to the fiber thus changing the path length of the fiber interferometer. The interferometer output in demodulated providing the acceleration response. The design of the accelerometer housing is such that it is highly immune to extraneous signals, i.e., dynamic and hydrostatic pressure. Fabrication of the optical fiber coils is accomplished by winding the optical fiber, with a specially designed chuck to form reference and sensing fiber coils which are then bonded to the flexural disk to form the accelerometer.

Description:
This application is a division of U.S. patent application Ser. No. 09/138,018, filed Aug. 21, 1998, now U.S. Pat. No. 6,056,032, which was a division of U.S. patent application Ser. No. 08/845,244 filed Apr. 21, 1997, now U.S. Pat. No. 5,903,349. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention refers generally to an acceleration sensor and more specifically to a high performance fiber optic interferometric acceleration sensor. 
     2. Description of the Related Art 
     An accelerometer is typically viewed as a mass-spring transducer housed in a sensor case with the sensor case attached to a moving part whose motion is inferred from the relative motion between the mass and the sensor case. The relative displacement of the mass being directly proportional to the acceleration of the case and therefore the moving part. 
     One type of accelerometer is a piezoelectric based electronic accelerometer. However, it tends to suffer from several major drawbacks when faced with the continuing stricter demands of the industry. Most higher performance piezoelectric accelerometers require power at the sensor head. Also, multiplexing of a large number of sensors is not only cumbersome but tends to occur at significant increase in weight and volume of an accelerometer array. 
     Another type is the interferometric fiber optic accelerometers based on linear and nonlinear transduction mechanism, circular flexible disks, rubber mandrels and liquid-filled-mandrels. Some of these fiber optic accelerometers have displayed very high acceleration sensitivity (up to 10 4  radians/g), but tend to utilize a sensor design which is impracticable for many applications. For instance, sensors with a very high acceleration sensitivity typically tend to have a seismic mass greater than 500 grams which seriously limits the frequency range in which the device may be operated as an accelerometer and are so bulky that their weight and size renders them useless in many applications. Other fiber optic accelerometers suffer either from high cross-axis sensitivity or low resonant frequency or require an ac dither signal and tend to be bulky (&gt;10 kg) and expensive. For many applications, the fiber optic sensor is expected to have a flat frequency response up to several kHz (i.e., the device must have high resonant frequency), high sensitivity, be immune to extraneous measurands (e.g., dynamic pressure), be lightweight and easily configurable in an array (i.e., easy multiplexing). 
     SUMMARY OF THE INVENTION 
     The object of this invention is to provide an accelerometer that is highly immune to dynamic pressure signals and having excellent directivity that may be produced cheaper than those currently in use. 
     Another objective is to produce an accelerometer that is immune to electromagnetic interference and is of a light weight. 
     Another objective is to produce an accelerometer that may be easily multiplexed to provide for large arrays of accelerometers. 
     These and other objectives are accomplished utilizing an interferometric fiber optic accelerometer mounted on a circular flexural disk. The accelerometer is viewed as a mass-spring transducer housed in a sensor case. The sensor case is attached to a moving part whose motion is inferred from the relative motion between the mass and the sensor case. In this invention, a flexural disk is housed in a sensor case which is accelerated in a direction normal to the plate surface. The plate undergoes displacement resulting in strains on the plate surface. A coil of optical fiber, made to be part of an optical interferometer, is attached to the flexural disk, the strain from the disk is transferred to the fiber thus changing the path length of the fiber interferometer. The interferometer output in demodulated providing the acceleration response. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a center supported flexural disk based fiber optic accelerometer sensor. 
     FIG. 2 shows a flexural disk based fiber optic accelerometer sensor within a housing. 
     FIG. 3 shows a winding spool for winding an optical fiber onto a flexural disk. 
     FIG. 4 shows the winding spool with optical fiber as mounted on the winding machine. 
     FIG. 5 shows the winding spool with optical fiber as mounted on the winding machine with epoxy feed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The high performance accelerometer sensor  10 , as shown in FIG. 1, utilizes a center  14  supported flexural disk  12  approach which provides a sensor  10  design which has low static and dynamic pressure sensitivity while maintaining high acceleration sensitivity. The resonant frequency and acceleration sensitivity of the accelerometer  10  are determined by the geometric shape, the disk  12  material and the type of support  14  provided to the disk  12 . The resonant frequency of the accelerometer  10  is designed to be such that the device is to be operate below its first primary resonance. The acceleration induced strain in the disk  12  is typically measured by high sensitivity interferometric methods. The flexural disk  12  based design along with the high strain sensitivity fiber optic interferometer (e.g., Michelson, Mach-Zehnder, Sagnac, in-line reflectometric, etc.) formed by an optical fiber sensing and reference arm  18  and  22 , respectively, wrapped onto the flexural disk  12  at the heart of the fiber optic acceleration sensor  10 . 
     In the push-pull configuration, the fiber optic acceleration sensor  10  is comprised of two optical fiber coils forming the sensing arm  18  and the reference arm  22  that comprise each leg of the interferometer, preferably a Michelsen type interferometer. The sensor and reference arms  18  and  22 , respectively, are epoxied to the top and bottom, respectively, of the flexural disk  12  using a type of epoxy resin well known to those in the art. The flexural disk  12  is preferably made of aluminum, however other materials may be utilized. The optical fiber forming the sensing arm and reference arm  18  and  22 , respectively, is preferably a single mode optical fiber having a core diameter equal to 6 microns and a cladding diameter equal to 80 microns made by Corning of Corning, N.Y. 
     As the sensor  10  body is accelerated in a direction normal to the disk  12  surface, the disk  12  deflects, thus resulting in strains on the upper and lower surfaces of the disk  12  which are equal in magnitude but 180 degrees out of phase. The acceleration-induced surface strain in the disk  12  changes the path length of the attached fiber  18  and  22  in a push-pull manner resulting in intensity modulation of an optical light from an optical light source  24  at the photodetector (not shown) where it is converted into an electrical signal. The output of the interferometer formed by the sensing and reference arms  18  and  22 , respectively, can be detected by any one of many techniques, e.g., the phase generated carrier technique. 
     The invention described above, by itself, would not only provide an unprotected sensor but will also be extremely sensitive to extraneous measurands. Therefore, it is very important that the flexural disk and the interferometer be housed in an appropriate shell or housing  30 , as shown in FIG.  2 . In certain applications the details of the housing design is of prime significance. It is well known to those skilled in the art that any accelerometer, especially those designed for “deep” underwater applications, should be packaged in such a manner that it can withstand hydrostatic pressures on the order of several hundred pounds per square inch. In addition, certain applications require that the accelerometer be insensitive to acoustic (i.e., dynamic pressure) signals, must be sufficiently stiff so as to not act as a pressure release surface, must not scatter any incident or radiated acoustic fields and must not introduce any unwanted in band resonances. Above all, the sensor is expected to be neutrally buoyant in order to accurately measure the underwater velocity of compliant materials. These opposing sets of requirements introduce severe constraints on the design and construction of the sensor housing  30 . However, achieving these requirements will ensure that a high performance accelerometer sensor mounted on a compliant coating mounting surface will not change any reactive forces on the compliant coating and provide accurate measurement of acoustic signals. 
     Extreme care must be taken to ensure that the housing  30  design does not introduce any resonances in the band of interest. For instance, a simple cylindrical design, with flat lid and base, acting as a type of “pill-box” cover for the flexural disk  12  may introduce unwanted resonances in the frequency band of interest or be acoustically “soft” or both. The preferred housing  30  described herein is such that the lid  32  and the base  34  of the sensor housing  30  introduce no resonances in the frequency band of interest, while maintaining a high degree of acoustic signal isolation from the sensing part  10  (i.e., flexural disk  12  and fiber interferometer formed by the sensing and reference arms  18  and  22 , respectively). FIG. 2 shows a preferred sensor housing  30  design with a tapered thickness lid  32  and base  34 . The “pyramidal” or tapered shape of the lid  32  and base  34  ensures that the housing lid  32  and base  34  have maximum thickness in the center where flexing is maximal. Since the thickness is at a maximum in the middle, the flexing is minimized and the base  34  and the lid  32  introduce no resonances in the band of interest, while maintaining a high degree of acoustic signal isolation from the sensing part  10 . This approach is particularly useful for the center supported flexural disk  12 , as described herein, since the center support  14  of the disk  12  is directly coupled to the center of the base  34  and lid  32 . The pyramidal shape also reduces total sensor  30  weight which helps towards the goal of neutral buoyancy. The overall volume and weight of the total sensor  30  design (flexural disk  12 , fiber interferometer formed by the sensing and reference arms  18  and  22 , respectively, and upper and lower housing  32  and  34 , respectively) determines its neutral buoyancy. 
     Fabrication of an optical fiber coil for the optical fibers  18  and  22  is accomplished using a winding machine (not shown), preferably automatic, of a type well known to those practicing the art is used. At a minimum the winding apparatus should consist of a variable speed spindle with an adjustable chuck to hold a winding spool, an optical fiber length measuring device, an optical fiber storage spool, a tensioning system, and an optical fiber guide system with a V-groove guide wheel positioned about 1.5 inches above the winding spool. 
     The winding spool  40 , as shown in FIG. 3, used to form an optical fiber coil for the optical fibers  18  and  22 , is made of Plexiglass® to facilitate ultraviolet (UV) curing of a bonding agent utilized to affix the coils of the optical fibers  18  and  22  to form either a sensing arm  18  or reference arm  22  coil. The bonding agent should be such that its Young&#39;s modulus approaches that of the flexural disk. The dimensions of the winding spool  40  determine the optical coils thickness and its diameter. The winding spool  40  are split into two parts, one part  42  is an approximately one inch diameter cylinder having a nominal length of approximately 0.4 inches, with a hole  44  bored through the cylinder along its axis. The hole  44  is sized to allow for a slip fit of, preferably, a 8-32 cap headed screw (not shown) approximately 1.5 inches long. One end of the cylinder is made perpendicular to the axis of the cylinder  42  and one side  46  is polished to a fine finish utilizing polishing techniques well known to those skilled in the art. A second part  48  of the spool  40 , similar to the first part  42  but having a raised center portion  52  approximately 0.3 inches in diameter so that when the two parts  42  and  48  of the spool  40  cylinder are mated together, the raised center portion  52  forms a hub around which the optical fiber  18  or  22  is wound to form an optical coil. 
     To facilitate running the optical fiber (not shown) out from the center of the spool  40 , a hole  54 , approximately 0.015 inches in diameter, enters the second part  48  of the spool  40  tangentially with respect to the raised center so that the optical fiber (not shown) can make a smooth transition from the hole  54  to the raised center section  52 . For the first part  42  of the spool  40 , the hole  54  for the optical fiber is right-handed, and for the second part  54  of the spool  40 , the optical fiber hole  54  is left-handed. The hole  54  serves as a bonding agent block and only needs to be approximately 0.1 inches long, the remainder of the hole  54  may be larger. 
     To prepare for the winding of the optical fiber coils, the two parts  42  and  48  of the winding spool  40  are sprayed with a mold release compound of a type that is well known to those skilled in the art, and allowed to dry completely. Winding tension of the optical fiber  62  is set typically to approximately 15 grams. As shown in FIG. 4, a loose end of the fiber  62  is inserted into the 0.015 inch hole  54  of the winding spool  40  and drawn through to a length of approximately 6 inches. The two parts  42  and  48  of the spool  40  are placed together with their polished sides  46  and  56  facing each other. A cap screw  64 , preferably a 8-32 cap head screw approximately 1.5 inches long, is inserted into the center hole  44  and  58  of the spool  40  and a compatible nut  66  threaded onto the cap screw  64  to lock the two parts  42  and  48  together to form one spool  40 . The nut  66  is tightened to a torque level approximating hand-tight. The spool  40  assembly, with the optical fiber  62  attached, is then inserted into the winding machine (not shown) and aligned so that the optical fiber  62  is exiting the 0.015 hole  54  smoothly over the center hub  52  with no sharp bends. The loose end of the optical fiber  62  coming out of the winding spool  40  is then taped to the winding spindle (not shown) with any commercial grade tape. The optical fiber  62  exiting over the hub  52  is then routed over the V-groove guide wheel  68  and any excess slack in the optical fiber  62  removed by engaging the optical fiber tensioning device (not shown). With the optical fiber  62  under tension, the V-groove guide wheel  68  is positioned so that the optical fiber  62  exits approximately midway between the polished faces  46  and  56  on the spool  40 . The spindle (not shown) is kept so that the optical fiber  62  had made a half revolution around the hub  52 . For the first part  52  of the spool  40 , the optical fiber  62  will be coming over the side of the hub  52  closest to the operator. For the second part  48  of the spool  40 , the optical fiber  62  will be behind the hub  52 . This left and right side orientation allows the optical fiber  62  coil leads to exit the same side of the flexural disk  12  when assembled into a complete sensor  10 . 
     As shown in FIG. 5, once the spool  40  has been rotated a half turn, the bonding agent  74  is wicked into the space between the walls  72  of the spool  40  at the top. As the bonding agent  74  wicks into the spool  40 , additional bonding agent  74  is added to the guiding wheel V-groove  68 . This is accomplished by dripping the bonding agent  74  on top while rotating the wheel so that an even distribution of the agent  74  covers the circumference of the V-groove  68 . As the optical fiber  62  plays out over the guide wheel  68 , it carries bonding agent  74  into the spool  40  with it. This insures that all of the optical fiber  62  is coated with the bonding agent  74  as it enters the spool  40  at the start of the winding process. When the bonding agent  74  has wicked half way around the hub  52 , winding may be started. 
     The rotation speed for winding should be set to a rotation rate of approximately 110 RPM. This will ensure that the bonding agent  74  is distributed completely around the hub  52 . Once this distribution of bonding agent  74  completely around the hub  54  has been accomplished, the rotation rate may be lowered. As the optical fiber  62  accumulates in the spool  40 , excess bonding agent  74  will be forced to the outer edge of the spool  40  which can be removed during the winding process with a swab. 
     The optical fiber  62  length should be monitored during the winding process. When the length is within one meter of the desired final total length, the rotation rate should be lowered still further until the desired final length is reached, at which time the winding process is stopped. All excessive bonding agent  74  is then removed from the outer edge of the spool  40  and allowed to cure. The optical fiber coil  18  or  22  is cured, preferably until it can be ensured that the optical fiber coil  18  or  22  will hold its shape and still be easily removed from the spool  40 . When completely cured, the optical fiber coil  18  or  22  should appear slightly yellow in color and be very hard. 
     The invention described herein is not only applicable to underwater acoustics but the principles described are also applicable in such applications as transient signal monitoring, vibration monitoring in aerospace applications, machinery monitoring on ships and industrial complexes, and for general purpose structural monitoring. The fully packaged sensors described herein are all-optical, have a higher sensitivity and lower self-noise than their conventional counterparts, are lightweight, have broad frequency response and can be easily to incorporated into arrays. The accelerometers are highly immune to dynamic pressure signals (i.e., acoustics) and have excellent directivity (i.e., they have extremely low cross-axis sensitivity). Mass production of the described sensor design produces a unit more cheaply than the costs of their conventional counterparts (e.g., piezo-ceramic based accelerometers). 
     It will be understood by those skilled in the art that still other variations and modifications are possible and can be affected without detracting from the scope of this invention as defined in the claims.