Abstract:
A vibration control apparatus designed specifically for use on space vehicles includes a stator for mounting in the vehicle, a lower flotor, magnetically levitated on the stator, an upper flotor nested in and magnetically levitated on the lower flotor, and position, orientation and motion sensors carried by the stator and flotors. When any changes in position, orientation or movement, i.e. vibration of apparatus is detected, magnetic force actuators are energized to compensate for such changes to keep a work platform on the upper flotor virtually vibration-free. Moreover, controlled and induced vibration of the work platform and an experiment carried thereby can be effected using the lower flotor as a reaction mass, i.e. without feedback to the vehicle.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a vibration control apparatus, and in particular to a microgravity vibration control apparatus.  
         DISCUSSION OF THE PRIOR ART  
         [0003]    At low frequencies (&lt;0.01 Hertz) space platforms such as the shuttle and the International Space Station (ISS) provide a unique, near ideal free-fall environment, which can be used to conduct material science, fluid physics and crystal growth experiments. Departure from ideal free fall due to atmospheric drag, rotational effects and gravity gradient are of the order of a micro-g (10 −6  g). However, above 0.01 Hz spacecraft vibrations are such that acceleration levels typically exceed 10 −3  g. Experiments conducted on the space shuttle and on MIR have shown that these vibration levels can significantly affect results. Vibrations, which are sometimes referred to as g-jitter, are driven by on-board activities such as attitude control systems, thermal control systems, air conditioning systems, power generation systems, crew activity and the operation of the spacecraft resulting in vibration environments characterized by milli-g (10 −3  g) acceleration levels. On the space shuttle, vibration levels in the frequency band 0.01 Hz to 100 Hz are in the range of 10 −3  g Root Mean Square (RMS), with peaks typically exceeding several milli-g. These are sufficient to cause significant disturbances to experiments that have fluid phases, which includes many material science experiments. The acceleration environment of the International Space Station will likewise not be as clean as originally hoped for, and the ISS will not meet the current vibratory requirements without the use of vibration isolation apparatuses of the type described herein.  
           [0004]    In order to isolate fluid science experiments from spacecraft vibrations, the Canadian Space Agency (CSA) developed a so-called Microgravity Vibration Isolation Mount (MIM), which constitutes a first generation of the present invention. The MIM was operational for more than 3000 hours on the Mir space station between May 1996 and January 1998. A second generation MIM was flown on space shuttle mission STS-85 in August 1997.  
           [0005]    The MIM includes two major components, namely a stator which is fixed to the spacecraft and a flotor on which is mounted an experiment to be isolated. Positions sensing devices track the position and orientation of the flotor with respect to the stator, and accelerometers monitor stator and flotor accelerations. The position sensing devices and accelerometers are used in an active control loop including magnetic actuators for moving the flotor relative to the stator to compensate for even extremely small vibrations of the stator.  
           [0006]    There is a large volume of patent literature relating to vibration isolation and damping systems. Examples of such literature include U.S. Pat. Nos. 2,788,457 (Griest); 3,088,062 (Hudimac); 4,088,042 (Desjardins); 4,314,623 (Kurokawa); 4,432,441 (Kurokawa); 4,585,282 (Bosley); 4,595,166 (Kurokawa); 4,874,998 (Hollis Jr.); 4,710,656 (Studer); 4,724,923 (Waterman); 4,848,525 (Jacot et al); 4,874,998 (Hollis Jr.); 4,929,874 (Mizuno); 4,947,067 (Habermann et al); 5,022,628 (Johnson et al); 5,168,183 (Whitehead); 5,236,186 (Weltin et al); 5,285,995 (Gonzalez et al); 5,368,271 (Kiunke et al); 5,385,217 (Watanabe et al); 5,392,881 (Cho et al); 5,400,196 (Moser et al); 5,427,347 (Swanson et al); 5,427,362 (Schilling et al); 5,445,249 (Aida et al); 5,446,519 (Makinouchi et al); 5,483,398 (Boutaghou); 5,542,506 (McMichael et al); 5,584,367 (Berdut); 5,609,230 (Swinbanks); 5,638,303 (Edberg et al); 5,645,260 (Falangas); 5,718,418 (Gugsch); 5,744,924 (Lee); 5,765,800 (Watanabe et al); 5,844,664 (Van Kimmenade et al); 5,876,012 (Haga et al); 5,925,956 (Ohzeki); 6,031,812 (Liou), and WO 99/17034 (Nusse et al) and WO 00/20775 (Ivers et al).  
         GENERAL DESCRIPTION OF THE INVENTION  
         [0007]    Some fluid phase experiments require controlled and induced vibration of the experiment, with no reaction back to the space vehicle. While a system of the type described above, including a stator and flotor, provides vibration damping, such a system cannot be used to effect such controlled and induced vibration.  
           [0008]    The object of the present invention is to meet the need defined above by providing a vibration control apparatus which can effect controlled and induced vibration of an experiment with no disturbance to the space station. Coincidentally, the apparatus of the present invention is inherently more efficient at damping vibration than a two-stage system.  
           [0009]    Accordingly, the invention provides a vibration control apparatus comprising:  
           [0010]    (a) stator means for mounting on a fixed surface;  
           [0011]    (b) lower flotor means normally spaced apart from said stator means in nesting relationship thereto;  
           [0012]    (c) an upper flotor means normally spaced apart from said lower flotor means in nesting relationship thereto;  
           [0013]    (d) work platform means on said upper flotor means;  
           [0014]    (e) position sensing means associated with said stator means, lower flotor means and upper flotor means for determining the position and orientation of said lower flotor means and said upper flotor means relative to said stator means;  
           [0015]    (f) accelerometer means associated with said stator means, lower flotor means and upper flotor means for determining acceleration of said lower flotor means and upper flotor means with respect to inertial space; and  
           [0016]    (g) vertical and horizontal magnetic force actuator means associated with said stator means, lower flotor means and upper flotor means for imparting motion to said lower flotor means and to said upper flotor means to compensate for vibration of said stator means, whereby vibration of said work platform is minimized. 
       
    
    
     GENERAL DESCRIPTION OF THE DRAWINGS  
       [0017]    The invention is described below in greater detail with reference to the accompanying drawings, which illustrate a preferred embodiment of the invention, and wherein:  
         [0018]    [0018]FIG. 1 is an isometric view of the apparatus of the present invention;  
         [0019]    [0019]FIG. 2 is an exploded, isometric view of the apparatus of FIG. 1;  
         [0020]    [0020]FIG. 3 is an isometric view of a stator used in the apparatus of FIGS. 1 and 2;  
         [0021]    [0021]FIG. 4 is an isometric view from above and the rear of a lower flotor used in the apparatus of FIGS. 1 and 2;  
         [0022]    [0022]FIG. 5 is an isometric view from below and the front of the lower flotor of FIG. 4;  
         [0023]    [0023]FIG. 6 is a schematic cross-section of one side of the apparatus of FIG. 1;  
         [0024]    [0024]FIG. 7 is a partly sectioned, isometric view of the lower flotor of FIGS. 4 and 5;  
         [0025]    [0025]FIG. 8 is a cross section taken generally along line  8 - 8  of FIG. 7;  
         [0026]    [0026]FIG. 9 is a schematic, isometric view of the lower flotor of FIGS. 4, 5 and  7  showing accelerometers used in the flotor;  
         [0027]    [0027]FIGS. 10 and 11 are isometric views of an upper flotor used in the apparatus of FIGS. 1 and 2;  
         [0028]    [0028]FIG. 12 is a schematic cross section of the apparatus of FIG. 1; and  
         [0029]    [0029]FIG. 13 is a schematic, isometric view of coils and magnets used in the apparatus of FIGS. 1 and 2. 
     
    
       [0030]    For the sake of simplicity, various elements have been omitted from most figures of the drawings.  
       DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    Referring to FIGS. 1 and 2, the basic elements of the apparatus include a bottom assembly or stator indicated generally at  1 , a first, lower flotor indicated generally at  2  on the stator  1 , and a second, upper flotor indicated generally at  3 , all of which are formed of aluminum. As shown in FIG. 1, the stator  1  and the flotors  2  and  3  are nested together to form a generally rectangular parallelepipedic body.  
         [0032]    As best shown in FIGS. 2 and 3, the stator  1  includes a housing  5  defined by a top wall  6  on contiguous front wall  7 , side walls  8  and a rear wall  10 , and a removable baseplate  11  closing the bottom of the housing. The housing  5  carries a variety of elements including electronic control components. Connectors  14  and other elements (only a few shown) for coupling the apparatus to a source of power and a control system, neither of which are shown, are mounted in the front wall  7  and in a front cover plate  15  removably mounted on the top wall  6  of the housing  5 .  
         [0033]    A square fence  17  extends upwardly from the top wall  6  of the housing  5 . Circular holes  18  in the centers of side walls  19  and rear end wall  20  of the fence receive position sensing detectors  22  (FIG. 3). Shallow, generally rectangular recesses  23  and  24  in the interior of the front wall  25 , the side walls  19  and the rear wall  20  contain coils  26  and  27  (FIG. 3), which interact with opposed sets of vertical force magnets  29  and  30  (FIGS. 1, 2,  4 ,  6 ,  12  and  13 ), and with horizontal force magnets  31  and  32  in the lower flotor  2  (FIGS. 2 and 4 to  6 ). The coils  26  and  27 , and the magnets  29  to  32  are described hereinafter in greater detail. Rectangular notches  34  are provided at the corners of the fence  17  for accommodating accelerometers  35  (FIG. 7) mounted in the lower flotor  2 .  
         [0034]    As best shown in FIGS.  4  to  7 , the lower flotor  2  includes three parallel fences  37 ,  38  and  39  which are square when viewed from above and concentric with the stator fence  17 . The side walls  40  and the rear wall  41  of the outer fence  37  are vertically aligned with the sides and rear end of the stator top wall  6 . A gap between the front wall  43  of the flotor outer fence  37  and the stator cover plate  15  receives umbilical cords (not shown) extending between the flotors  2  and  3 , and the stator  1 . The umbilical cords carry electrical power and data and control signals between the stator  1  and the flotors  2  and  3 . They can also include video lines for servicing hardware on the upper flotor  3 . The top ends of the outer and intermediate fences  37  and  38  are interconnected by a top wall  44 , and the bottom ends of the intermediate and inner fences  38  and  39  are interconnected by a bottom wall  45 . Thus, as best shown in FIG. 6, the four sides of the lower flotor are crenellated in cross section, defining a pair of square pockets for receiving the stator  1  and the upper flotor  3 .  
         [0035]    A plurality of rectangular openings are provided in the side walls  40  and end walls  41  and  43  of the flotor outer fence  37 . A central hole  49  in the front wall  43  of the outer fence  37  receives a voltage reference module  50  (FIG. 5). Two rectangular holes  52  and  53  in each wall of the outer fence  37  receive the vertical force magnets  29  and horizontal force magnets  31 , respectively, which are mentioned above.  
         [0036]    Two pairs of holes  55  in each wall of the intermediate fence  38  (FIG. 8) receive the magnets  30  and  32 . As will be appreciated from FIGS. 6 and 8, the magnets  29  to  32  in combination with the coils  26  and  27  define Lorentz force actuators for magnetically levitating the lower flotor  2  with respect to the stator  1  which is fixed to a space platform. The eight actuator coils in the stator fence  17  react with the eight magnet assemblies in the outer fence  37  of the lower flotor  2 . It will be noted that the horizontal and vertical force actuators are the same except that the two magnet and coil combinations in each fence are at 90° to each other, i.e. one magnet and coil combination generates a vertical force, and the other combination generates a horizontal force vector. Differential actuator forces can be used to generate torque for controlling rotation about all axes.  
         [0037]    A set of holes  57  near the corners of the fence  37  receive signal conditioning modules  58  (FIGS. 1 and 4) which are connected to the accelerometers  35 . The modules  58  condition data signals from the accelerometers  35  to the control system (not shown) for the apparatus.  
         [0038]    Suitable accelerometers  35  are sold by Honeywell Inc., Minneapolis, Minn., U.S.A. under the trade-mark Q-Flex, specifically Q-Flex QA-3000 accelerometers, which develop an acceleration-proportional output current providing both static and dynamic acceleration measurement. As best shown in FIGS. 7 and 9 there are two accelerometers  35  in each of the corners  59  and  60 , and one in each of the corners  61  and  62  of the lower flotor  2 . Three additional accelerometers in the stator housing  5  act as references for the accelerometers  35  and to three accelerometers  64  (FIG. 11) on the upper flotor  3 .  
         [0039]    Referring to FIG. 9, the accelerometers  35  detect translation and rotation of the flotor  2  about the X, Y and Z axis or vertically, longitudinally and transversely with respect to the stator  1  as indicated by arrows X, Y and Z. Similarly, the accelerometers  64  detect translation and rotation of the flotor  3  about the X, Y and Z axes with respect to the stator  1 . Thus, the accelerometers determine acceleration of the flotors  2  and  3  with respect to inertial space.  
         [0040]    The position sensing detectors (PSDs)  22  mounted in the centers of the side and rear walls  19  and  20 , respectively of the stator fence  17  receive light from collimated light emitting diodes (LEDs)  66  mounted in square, central holes  67  (one shown—FIG. 8) in the side walls and the rear end wall of the intermediate fence  38  of the lower flotor  2 . The PSDs  22  are duo-lateral diodes manufactured by VDT Sensors, Inc., Hawthorne, Calif., U.S.A. which determine the position of the lower flotor  2  with respect to the stator  1  in six degrees of freedom. Suitable LEDs bearing Model No. L2791-02 are available from Hamamatsu Systems Canada Inc., Montreal, Quebec, Canada. These LEDs have a narrow emission angle of ±2° to minimize the size of the light spot on the PSD.  
         [0041]    All four sides of the lower flotor inner fence  39  contain rectangular openings  72  and  73  (FIGS. 2 and 7) for receiving vertical force magnets  74  and horizontal force magnets  75  (FIGS.  4  to  6 ). The magnets  74  and  75  are aligned with coils  77  and  78  mounted in recesses  79  and  80  in a fence  82  defining part of the upper flotor  3 . The magnets  74  and  30 , and the coils  77  also define vertical Lorentz force actuators for magnetically levitating the upper flotor  3  in the lower flotor  2 , and the magnets  75  and  32 , and the coils  78  define horizontal force actuators.  
         [0042]    Referring to FIGS. 1, 10 and  11 , the upper flotor  3  includes a top plate  83  which defines a work platform, and the fence  82  formed by contiguous front wall  84 , rear wall  85  and side walls  86 . An opening  88  in the top plate  83 , providing access to the interior of the flotor  3  and the top of the stator  1  is normally closed by a cover plate  89  (FIGS. 1, 2 and  6 ). The cover plate  89  carries the three accelerometers  64 .  
         [0043]    LEDs  90  (FIGS. 11 and 12) are mounted in square central openings  91  (FIGS. 2 and 10) in the rear and side walls  85  and  86 , respectively of the upper flotor fence  82 . Light from the LEDs is directed inwardly through central holes  93  in the inner fence  39  of the lower flotor  2  to PSDs  94  (FIG. 3) mounted on the top wall  6  of the stator housing  5 .  
         [0044]    Referring to FIGS. 12 and 13, in operation the LEDs  66  and  90  in combination with the PSDs  22  and  94 , and the accelerometers  35  and  64  (FIGS. 9 and 11) provide data signals indicative of the positions, orientation and movement of the flotors  2  and  3  relative to the stator  1 . The signals are processed using an on-board computer (not shown) which generates control signals which are fed to the appropriate force actuators defined by the combinations of magnets and coils in the stator  1 , and the lower and upper flotors  2  and  3 . Vertical force is imparted to the lower flotor  2  using coils  26  in combination with magnets  29  and  30 , and horizontal force is imparted to the flotor  2  using coils  27  in combination with magnets  31  and  32 . By feeding current to the coils  77 , magnetic lines of force are generated in magnets  74  and  30  to move the flotor  3  relative to the flotor  1 . Horizontal movement of the flotor  3  is effected using coils  78  in combination with the magnets  75  and  32 .  
         [0045]    Thus, various combination of coils and magnets can be used to magnetically levitate the flotor  2  with respect to the stator  1  and the upper flotor  3  in the lower flotor  2  compensating for even very minute vibrations in the vehicle carrying the apparatus. The work platform defined by the top plate  83  and the cover plate  89  of the flotor  3  is maintained virtually vibration-free, the apparatus correcting for horizontal and vertical movement of stator  1 , and any roll, pitch or yaw. Moreover, the coil and magnet combinations can be used to induce controlled vibration of the upper flotor  3 , the work platform and an experiment thereon, using the lower flotor as a reaction mass. The controlled vibration is isolated from the vehicle, i.e. there is no vibration of the vehicle as a result of vibration of the experiment