Abstract:
An apparatus and method of measuring the mass of a test specimen located in a microgravity environment. The test specimen is attached to the free end of a cantilevered spring for joint vibration. The natural frequency of vibration of the spring and specimen are measured. The spring constant is calculated and compared with known masses having the same frequency and spring constant. When a match is found, the mass of the test specimen is known.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to a method and apparatus capable of measuring the mass of a specimen by subjecting the mass to a vibrational load and comparing the measured frequency of vibration to frequencies of known masses. In particular, the present invention is directed to an apparatus for determining the mass of a specimen located in a microgravity environment. Such a microgravity environment will exist on the International Space Station (ISS) or any craft in near Earth orbit. 
     A problem confronts scientists when attempting to monitor the mass of a quantity of fluid present in a container located in a microgravity environment. For example, on the ISS it would be difficult to economically monitor the changing mass of a life-sustaining fluid because the mass of the fluid could not be accurately determined due to the microgravity. There is no known approach which utilizes information obtained in a gravity environment to determine the mass of a specimen located in a microgravity environment. Whether the test specimen is in fluid or solid form, the problem of determining the mass microgravity has proven difficult if not impossible. 
     There clearly is a need for a measuring apparatus and method capable of determining the mass of a test specimen maintained in a microgravity environment. There is also a need for a measuring apparatus capable of repeatedly providing accurate readings regardless of changes in the mass or its environment. There is, moreover, a need of a measuring apparatus, i.e., scale, that is compact in size, of minimum weight and as economical as possible to construct. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention recognizes a mass of a test specimen is measured by subjecting the mass to either a constant vibrational load i.e., forced vibration, or by subjecting the mass to a mechanical impulse i.e., free vibration. In either instance, the specimen vibrates at a specific frequency indicative of the mass of the specimen. By performing a number of similar tests in a gravity environment, it is possible to determine the calibration frequencies achieved by a number of specimens of differing mass. By comparing the frequency of the specimen tested in the microgravity environment with the set of pre-determined frequency calibration curves established for known masses, the mass of the test specimen is determined. 
     In another aspect of the present invention, a test specimen located in a microgravity environment and having a mass to be determined is attached to a freely vibrating spring. As the spring is repeatedly vibrated, a sensor measures the frequency of vibration. By calculating the spring constant and referring to a previously determined spring constants for given masses, it becomes possible to determine the mass of the test specimen 
     In another aspect of the invention, a test specimen located in microgravity is contained within a tank attached to a vibrating mount. The tank also includes a pressurized gas separated from the fluid test specimen by a flexible member which may take the form of a bellows or a bladder. The vibrating mount repeatedly oscillates in order to determine the natural frequency of vibration of the mount, from which the spring constant of the mount can be calibrated. This, in turn, may be used to determine the mass of the test specimen. 
     In a yet further aspect of the present invention, a fluid specimen located in a microgravity environment is located within a tank including a bellows and pressurized Nitrogen on the other side of the bellows. The vibrating mount takes the form of a cantilevered beam bolted at one end to a rigid structure. During operation, a sensing element achieves maximum sensitivity in monitoring the frequency of vibration of the vibrating mount, the tank and the fluid test specimen. A device referred to as a “pinger” supplies a mechanical impulse to induce a natural frequency of any object to which the pinger is attached. The pinger serves to oscillate the vibrating mount, thus providing “in situ” readings of the undamped natural vibration frequency of the tank and fluid test specimen. 
     In order to determine the mass of the fluid, the spring constant of the vibrating mount is first calculated with a known mass and measured frequency of natural vibration of the mass. The unknown mass of fluid can then be determined by comparing the frequency of vibration of the test fluid with the frequencies of vibration of known masses. Preferably, known frequencies may be used to create a set of frequency calibration data. Though the spring constant of the of the vibrating mount and tank may vary slightly through differing amplitudes under differing gravity conditions, these minor variations may be easily compensated for in the set of pre-determined calibration curves. 
     In another aspect of the present invention, a method is disclosed for measuring the mass of a test specimen located in a microgravity environment. The specimen may be mounted on the free end of a spring member having an opposite end attached to a rigid member. As the spring and mass are forced to vibrate, a sensor measures the natural frequency of vibration. The spring constant may then be calculated and knowing the frequency of vibration for known masses utilizing a spring with the same constant, it is possible to determine the mass of the test specimen. 
     In another aspect of the present invention, a method is disclosed for measuring the mass of a fluid in a microgravity environment. The fluid may be deposited in a tank on one side of a flexible member which take the form of a bellows or a bladder with a pressurized gas located on the opposite side of the bellows to maintain pressure on the fluid. The tank may be attached to the free end of a cantilevered spring which is forced to vibrate. A sensor is positioned to monitor the frequency of vibration, allowing the spring constant to be determined. This can be compared to the vibration of known masses in a full gravity environment under a spring with the same constant to determine the mass of the fluid. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a measuring device incorporating the principles of the present invention; 
     FIG. 2 a  is a bottom view of a scale formed in accordance with the present invention; 
     FIG. 2 b  is top view of the scale formed in accordance with the present invention and shown in FIG. 2 a;    
     FIG. 2 c  is a cross sectional front view of the scale formed in accordance with the present invention and shown in FIG. 2 a;    
     FIG. 2 d  is a cross sectional side view of the scale formed in accordance with the present invention as taken along axis X—X in FIG. 2 c ; and 
     FIG. 3 is a view of the scale taken along axis A—A of FIG. 2 d  showing the direction of vibration of the scale formed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out the present invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Referring to FIG. 1, the basic concept of the present invention  10  is schematically depicted as utilizing a spring in the form of a cantilevered beam  12  attached at one end to a rigid member  14  and having a “free end”  15  capable of vibrating itself along with an attached mass M. A device  16  periodically induces cantilevered beam  12  and attached mass M to vibrate along a path shown by the arrows Y—Y. A sensor  18  is positioned to measure the frequency of the vibration of beam  12  and mass M. A variety of types of sensors  18  may be utilized, including but not limited to a simple strain gage, an accelerometer and even a proximity probe. The choice of sensor  18  may be made according to the size of the mass M to be measured and/or the desired accuracy of the required measurement. 
     The operation of the apparatus formed in accordance with the present invention utilizes the principle of a free-vibrating system including a mass and a spring, i.e., the beam. The undampened natural frequency f n  (Hz) of the oscillating system is:                f   n     =       1     2                 π              k   m                 (     Equation                 1     )                                
     where 
     m (kg) is the oscillating mass, k is the spring constant. The mass M may be determined as follows: 
     Step 1. The spring constant k must first be calibrated. Spring constant k can be calculated with a known mass m 1  and measured frequency f 1  using the following equation:                f   1     =       1     2                 π              k     m   1                   (     Equation                 2     )                                
     Step 2. The new mass m 2  now can be determined by using the spring constant k calibrated in Step 1 and measured frequency f 2 . By a similar relation as Equation 2:                f   2     =       1     2                 π              k     m   2                   (     Equation                 3     )                                
     The spring constant k may vary slightly through different amplitudes and gravity conditions, but these can be compensated for with the use of calibration curves for k. 
     The “spring” referred to above may take the form of the beam 12 or make take a different form such as a vibrating table. The form of the spring is only limited by the ability to accurately measure its frequency of vibration. By ensuring that the amplitude of the vibration is limited, it is possible to minimize changes in the spring constant k even as the mass M changes. 
     Referring now to FIGS. 2 a - 2   d , one aspect of the present invention will be discussed in greater detail. In particular, FIGS. 2 c  and  2   d  shows an apparatus formed in accordance with the present invention to include a liquid tank  20  enclosing a flexible member which may take the form of bellows  22 . Alternatively, a bladder may be utilized as the flexible member. A first port  24  allows a pressurized gas such as pressurized Nitrogen to be introduced into a portion of tank  20  located on a first side of bellows  22 . A second port  26  extends into a portion of tank  20  located on an opposite side of bellows  20  from port  24 . This may allow a target specimen of fluid to be introduced into a side of tank  20  on an opposite side of bellows  22  from the pressurized nitrogen. During operation, the pressurized nitrogen serves to maintain bellows  22  in contact with the target fluid regardless of any changes in its amount or position where the nitrogen, bellows  22  and specimen are enclosed within the tank  20 . 
     Because of the arrangement of bellows  22 , the pressurized nitrogen as well as the target fluid, bellows  22  is limited to one degree of freedom. Enclosing tank  20  may be fixedly attached to a vibrating mount  30  by bolts  32  or any other conventional fasteners as shown in FIGS. 2 a  and  2   b . Referring further to FIG. 2 a , vibrating mount  30  may function as a spring member having a fork-shaped configuration that may include an elongated base portion  34  and a pair of attached arm portions  36 ,  38  extending parallel to each other on opposite sides of tank  20 . An end  40  of vibrating mount  30  oppositely disposed from arm portions  36 ,  38  may be fixedly attached to a rigid member  42 . Member  42  may comprise any component of sufficient mass so as to be able remain fixed in position when subjected to vibration of vibrating mount  30  and attached tank  20 . As will become readily apparent, vibrating mount  30  functions as a spring in a manner similar to beam  12 . 
     As shown in FIG. 2 a , a vibration sensing element or sensor  44  is located at an opposite end of tank  20  from the elongated base portion  34  of vibrating mount  30 . This assures that sensor  44  will under go maximum vibrational movement. A device such as a “Pinger”  46  may be located anywhere along vibrating mount  30 , but preferably is located adjacent to elongated base portion  34 . During operation, pinger  46  serves to cause vibrating mount  30  and attached tank  20  to repeatedly oscillate. 
     When utilizing a single mass of fluid and a single vibrating mount  30 , it may be assumed that bellows  22  is stationary with regard to the tank  20  reference frame. As previous stated, the natural frequency f n  (Hz) may be calculated from Equation 1. The mass of the target specimen of fluid contained within tank  20  may then be determined. First the spring constant of vibrating mount  30  may be calculated with a known mass by utilizing Equation 2. Then the mass of the fluid in tank  20  may be determined by using the spring constant of vibrating mount  30  determined by Equation 1 and the measured frequency for vibrating mount  30  utilizing Equation 3. 
     In order to better understand the operation of the present invention, the method of measuring the specific mass of a fluid enclosed within tank  20  is determined. The method begins with introducing a quantity of fluid through port  26  into one side of tank  20  and a quantity of pressurized Nitrogen gas through port  24  into the opposite side of tank  20  such that bellows  22  maintains contact with the fluid. Pinger  46  is then activated to start vibrating mount  30 , tank  20  and the fluid vibrating. Sensor  44  measures the frequency of vibration. Referring now to FIG. 3, a realistic value for a spring constant k may then be determined by              k   =       3      E                 I       L   3               (     Equation                 4     )                                
     for a cantilevered beam similar to vibrating mount 30. The moment of inertia I is then determined by                  I     y                 c       =         b   3        h     12       ,           (     Equation                 5     )                                
     5), based on the rectangular cross-section  50  of the vibrating mount  30  as shown in FIG. 3 having the length and height b and h, respectively. In order to determine the Modulus of Elasticity for the particular vibrating mount  30 , it is necessary to know its composition. One well known composition having sufficient flexibility is 6061 Aluminum alloy. For the present example, the values for the apparatus formed in accordance with the present invention are: 
     
       
         Cross-section 50 ( b×h )=0.152×0.076 m  (or 6×3  in ).  
       
     
     
       
         Modulus of Elasticity E of 6061 Alloy=6.83×10 10    Pa  (or 9.9×10 6    psi ).  
       
     
     Effective length L of vibrating mount 30 (the distance from the point of attachment of elongated base portion 34 with the rigid member 42 to the center of mass of tank 20 as projected to the plane of vibrating mount 30)=0.438 m (or 17.25 in) 
     Using the cross sectional measurement (b×h)=(0.152×0.076 m) in Equation 5 yields a value for I yc  of 2.248×10 −5  m 4  (or 54.0 in 4 ). Inserting these values along with the value of L into Equation 4, the resulting k=5.472×10 7  kg/s 2  [N/m] (or 3.125×10 5  lb f /in). With k thus known, the unknown mass m 2  can be readily calculated by measuring the resulting frequency f 2 . Rearranging Equation 3, it becomes          m   2     =       k     4                   π   2          f   2   2         .                            
     Using k=5.472×10 7  kg/s 2  and with a frequency readout of f 2 =166.5 Hz, the mass m 2  is determined to be 50 kg. As stated, the actual dimensions of the measuring apparatus formed in accordance with the present invention may be selected based on the available space as well as the approximate amplitude of vibration that is desired for a range of target specimen to be measured. 
     It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention. For example, any material having a Modulus of Elasticity indicating an ability to vibrate a measurable amount may be employed. The tank  20  may be replaced with any structure capable of supporting the test specimen undergoing vibration. The basic analytical system may change as a result. For example, if a bladder was used in place of the bellows, the basic system will take the form of a mass and two springs in series. Similarly, the test specimen may constitute a fluid, or solid material. In addition, the apparatus is not in any way limited to use in outer space, but has utility in any gravitational environment. The scope of the present invention is set forth in the following claims.