Abstract:
An exemplary distance measuring probe ( 100 ) includes a tube track ( 12 ), a tip extension ( 16 ), a pair of hollow tubes ( 14 ), a pair of air discharge systems ( 115 ), a linear measuring scale ( 18 ), and a displacement sensor ( 19 ). The tip extension is configured to touch a surface of an object ( 50 ). The linear measuring scale and the displacement sensor are respectively fixed relative to one of the tube track and the tip extension. The hollow tubes contain a flux of air, and are configured to cooperatively push the tip extension to move. Each air discharge system ejects part of air in the corresponding hollow tube out of the hollow tube. The linear measuring scale displays values of displacements of the tip extension. The displacement sensor detects and reads the displacement values displayed by the linear measuring scale.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to distance measuring probes for coordinate measuring machines (CMMs) and actuators used in distance measuring probes; and more particularly to a distance measuring probe with an air discharge system using a relatively small, steady measuring force for contact-type distance measuring devices and an actuator providing a relatively small, steady measuring force.  
         [0003]     2. Discussion of the Related Art  
         [0004]     Manufactured precision objects such as optical components (for example, aspherical lenses) and various industrial components need to be measured to determine whether manufacturing errors of the objects are within acceptable tolerance ranges. Such manufacturing errors are differences between design dimensions of the object and actual dimensions of the manufactured object. Measured dimensions of the manufactured object are usually regarded as the actual dimensions. A precision measuring device is used to measure the objects; and the more precise the measuring device, the better. Generally, the precision objects are measured with a coordinate measuring machine (CMM), which has a touch trigger probe that contacts the objects. A measuring force applied to the touch trigger probe of the coordinate measuring machine should be small and steady. If the measuring force is too great, a measuring contact tip of the touch trigger probe is easily damaged and causes a measuring error. If the measuring force is not steady, a relatively large measuring error occurs.  
         [0005]     As indicated above, a contact-type coordinate measuring device is commonly used to measure dimensions of precision objects such as optical components and certain industrial components. A measuring force is provided to the touch trigger probe by the coordinate measuring device. However, if the object has a slanted surface, the contact tip of the touch trigger probe may become bent or deformed by a counterforce acting on the touch trigger probe, thereby causing a measuring error. Therefore, the touch trigger probe is not ideal for measuring precision lenses having slanted surfaces.  
         [0006]     Nowadays, two methods are generally used to reduce a measuring force on the touch trigger probe. In a first method, the contact tip is slantingly arranged so that a component force of gravity acting on the measuring contact tip is regarded as a measuring force. The contact tip is very light, so the measuring force is very small accordingly. However, if a slanted angle of the contact tip changes during measuring, the measuring force changes, which makes the measuring force difficult to control. In a second method, the touch trigger probe is configured with a spring. An elastic force of the spring is regarded as a measuring force. However, when the contact tip moves upward and downward along the surface of the object being measured, a vibration of the upward and downward movement may cause the spring to resonate and deform. Therefore, the measuring force varies with the deformation of the spring. Thus both methods are subject to errors occurring in the measurement results.  
         [0007]     In another kind of probe, a measuring force is provided by an air pump. However, the air pump provides pulsed pressure. Therefore, the air pump cannot provide a small, steady measuring force.  
         [0008]     Therefore, a distance measuring probe employing a relatively small, steady measuring force is desired.  
       SUMMARY  
       [0009]     In one aspect, a distance measuring probe includes at least one tube track, a tip extension, at least one hollow tube, an air discharge system, a linear measuring scale, and a displacement sensor. The tip extension is linearly movable relative to the at least one tube track for touching a surface of an object. The at least one hollow tube is partly received in the at least one tube track and linearly slidable in the at least one tube track. Each of the at least one hollow tube defines a cavity for containing a flux of air, and is configured to be driven by the flux of air to push the tip extension to move. The air discharge system is configured to eject at least part of the flux of air in the at least one hollow tube out of the at least one hollow tube. The linear measuring scale is configured to display values of displacements of the tip extension, and is fixed relative to one of the at least one tube track and the tip extension. The displacement sensor is configured to detect and read the displacement values of the tip extension displayed by the linear measuring scale, and is fixed relative to the other one of the at least one tube track and the tip extension.  
         [0010]     In another aspect, an actuator is provided to drive a tip extension of a distance measuring probe. The actuator includes at least one hollow tube and at least one air discharge system. Each of the at least one hollow tube defines a cavity for containing a flux of air, and is configured to be driven by the flux of air to drive the tip extension to move. The air discharge system is configured to allow at least part of the flux of air in the cavity of the at least one hollow tube be ejected out of the at least one hollow tube.  
         [0011]     Other advantages and novel features will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the distance measuring probe. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
         [0013]      FIG. 1  is a top plan, cross-sectional view of a distance measuring probe in accordance with a first embodiment of the present invention, the distance measuring probe including a pair of hollow tubes and a pair of pipes.  
         [0014]      FIG. 2  is a cross-sectional view of the distance measuring probe of  FIG. 1 , corresponding to line II-II thereof.  
         [0015]      FIG. 3A  is an enlarged, front view of an air discharge system of the distance measuring probe of  FIG. 1 , showing a first embodiment of the air discharge system.  
         [0016]      FIG. 3B  is an enlarged, longitudinal cross-sectional view of a front end portion of one of the hollow tubes of  FIG. 1 , showing the first embodiment of the air discharge system thereof.  
         [0017]      FIG. 4A  is an enlarged, front view of a second embodiment of an air discharge system that can be employed in the distance measuring probe of the first embodiment.  
         [0018]      FIG. 4B  is an enlarged, front view of a third embodiment an air discharge system that can be employed in the distance measuring probe of the first embodiment.  
         [0019]      FIG. 4C  is a cross-sectional view taken along line IVC-IVC of  FIG. 4A  and likewise taken along line IVC-IVC of  FIG. 4B .  
         [0020]      FIG. 5A  is an enlarged, abbreviated view corresponding to one of the hollow tubes and part of a corresponding one of the pipes of  FIG. 1 , showing part of a fourth embodiment of an air discharge system that can be employed in the distance measuring probe of the first embodiment.  
         [0021]      FIG. 5B  is a cross-sectional view corresponding to line VB-VB of  FIG. 5A .  
         [0022]      FIG. 6A  is an enlarged, abbreviated view corresponding to one of the hollow tubes and part of a corresponding one of the pipes of  FIG. 1 , showing a fifth embodiment an air discharge system that can be employed in the distance measuring probe of the first embodiment.  
         [0023]      FIG. 6B  is a cross-sectional view corresponding to line VIB-VIB of  FIG. 6A .  
         [0024]      FIG. 7  is an isometric view of an exemplary application of the distance measuring probe of  FIG. 1 .  
         [0025]      FIG. 8  is a graph showing manufacturing error data obtained by the distance measuring probe of  FIG. 1  measuring a gauge-grade sphere having a radius of 5.5573 mm.  
         [0026]      FIG. 9  is a graph showing manufacturing error data obtained by the distance measuring probe of  FIG. 1  measuring a normal sphere (e.g. a ball bearing) having a radius of approximately 10.0 mm.  
         [0027]      FIG. 10  is a top plan, cross-sectional view of a distance measuring probe in accordance with a second embodiment of the present invention.  
         [0028]      FIG. 11  is a top plan, cross-sectional view of a distance measuring probe in accordance with a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0029]     Referring to  FIG. 1  and  FIG. 2 , these show a distance measuring probe  100  of a first embodiment of the present invention. The distance measuring probe  100  includes a base  102 , a tube track  12 , two hollow tubes  14 , a first tube frame  15 , a tip extension  16 , a second tube frame  17 , a linear measuring scale  18 , a displacement sensor  19 , a pipe holder  110 , and two pipes  111 . In alternative embodiments, the distance measuring probe  100  can include only one hollow tube  14  or more than two hollow tubes  14 . In such cases, there can correspondingly be only one pipe  111  or more than two pipes  111 . The hollow tubes  14  and the first and second frames  15 ,  17  collectively form a movable rack (not labeled).  
         [0030]     The base  102  is substantially a flat rectangular plate. It should be understood that the base  102  may alternatively have any other suitable shape. The tube track  12  is mounted securely onto the base  102 . The tube track  12  has a front end  105  and a rear end  106 . The tube track  12  defines two tube rail channels  13  each extending from the front end  105  to the rear end  106 . The tube rail channels  13  are spaced apart from and aligned parallel to each other. In alternative embodiments, the tube track  12  may define only one tube rail channel  13  or more than two tube rail channels  13 , corresponding to the number of hollow tubes  14 .  
         [0031]     Each of the hollow tubes  14  is a cylinder defining a cavity  142  that extends through the hollow tube  14  from a rear open end of the hollow tube  14  to a front cylinder base  140  of the hollow tube  14 . Each of the hollow tubes  14  is received through the corresponding tube rail channel  13  of the tube track  12 . The open ends of the hollow tubes  14  protrude out from the rear end  106  and are fixed onto the second tube frame  17 . The cylinder bases  140  of the hollow tubes  14  protrude out from the front end  105  and are fixed onto the first tube frame  15 . An outer diameter of the hollow tubes  14  is configured to be smaller than a diameter of the tube rail channels  13 , so that a gap (not labeled) is defined between each hollow tube  14  and the tube track  12 . Air is pumped into the gap between the hollow tubes  14  and the tube track  12 . Thus, an air bearing  113  is formed between each of the hollow tubes  14  and the tube track  12  when the gaps are filled with air. The hollow tubes  14  are made of one of stainless steel, aluminum (Al), titanium (Ti), and carbon steel.  
         [0032]     Each of the pipes  111  is partially inserted into the open end of a corresponding hollow tube  14 . Also referring to  FIG. 6A , an outer diameter of the pipes  111  is smaller than a diameter of the cavities  142  of the hollow tubes  14 , so that a gap  118  is defined between each pipe  111  and the corresponding hollow tube  14 . An air bearing (not labeled) is formed between each pipe  111  and the corresponding hollow tube  14  when air is pumped into the cavities  142  of the hollow tubes  14  via the pipes  111 . Therefore, frictional forces between the hollow tubes  14  and the tube track  12 , and between the pipes  111  and the hollow tubes  14 , are significantly small. The result is that the hollow tubes  14  can move in the tube rail channels  13  smoothly. It should be understood that the gap  118  may be omitted. Alternatively, lubricant can be provided between the pipes  111  and the hollow tubes  14  to reduce frictional forces.  
         [0033]     The pipe holder  110  is fixed on the base  102  behind the second tube frame  17 . The pipe holder  110  is configured to hold the pipes  111  in position. When air is pumped into the cavities  142  of the hollow tubes  14 , an air current inside the cavities  142  creates a pushing force that pushes the hollow tube  14  away from the pipes  111 , thereby driving the tip extension  16  away from the second tube frame  17 . The air pumped into the cavities  142  of the hollow tubes  14  and the tube rail channels  13  may be replaced by any other suitable kind of gas such as oxygen, nitrogen, etc.  
         [0034]     The tip extension  16  is needle-shaped, and has a contact tip  162  that touches an object  50  when the distance measuring probe  100  is used for measuring the object  50 . The tip extension  16  is fixed on the first tube frame  15  so that the tip extension  16  is linearly movable together with the movable rack. The linear measuring scale  18  is fixed on the second tube frame  17  such that it moves (displaces) linearly when the movable rack moves. The displacement sensor  19  is mounted on the base  102  corresponding to the linear measuring scale  18 . The displacement sensor  19  is used for reading displacement values of the linear measuring scale  18 . Alternatively, the positions of the linear measuring scale  18  and the displacement sensor  19  may be exchanged.  
         [0035]     Referring to  FIG. 2 , the distance measuring probe  100  further includes a cover  112  that engages on the base  102  and completely seals the various other components of the distance measuring probe  100 . The cover  112  defines an opening (not labeled) for allowing a part of the tip extension  16  to extend out therefrom. Air is pumped into the gaps between the tube track  12  and the hollow tubes  14  to form the air bearing  113  via a plurality of tubes  114  mounted to the cover  112 .  
         [0036]     The following describes a plurality of exemplary embodiments of an air discharge system  115  of the distance measuring probe  100 . The air discharge system  115  is configured to eject air out of the cavity  142  of each hollow tube  14 .  
         [0037]     Referring to  FIG. 1 ,  FIG. 3A , and  FIG. 3B , a first embodiment of an air discharge system  115   a  includes an air eject hole  116   a  defined in a center of the cylinder base  140  of each hollow tube  14 . The air eject hole  116   a  includes a front cylindrical portion  1160  and a frustum portion  1161 . The frustum portion  1161  intercommunicates the front cylindrical portion  1160  and the cavity  142 . A diameter of the front cylindrical portion  1160  is equal to a smallest diameter of the frustum portion  1161 . The frustum portion  1161  defines a conical frustum shape, with a radius of the conical frustum gradually decreasing from the cavity  142  to the front cylindrical portion  1160 . Thereby, air can flow smoothly out of the hollow tube  14  through the air eject hole  116   a  of the air discharge system  115   a.    
         [0038]     Referring to  FIG. 4A  and  FIG. 4C , a second embodiment of an air discharge system  115   b  includes a central air eject hole  116   b  defined in the cylinder base  140  of each hollow tube  14 , and a plurality of peripheral air eject holes  116   b  defined in the cylinder base  140  and surrounding the central air eject hole  116   b . Each air eject hole  116   b  defines a front cylindrical portion  1163  and a frustum portion  1164 . The frustum portion  1164  intercommunicates the front cylindrical portion  1163  and the cavity  142 . A diameter of the front cylindrical portion  1163  is equal to a smallest diameter of the frustum portion  1164 . The frustum portion  1164  defines a conical frustum shape, with a radius of the conical frustum gradually decreasing from the cavity  142  to the front cylindrical portion  1163 . Thereby, air can flow smoothly out of the hollow tube  14  through the air eject holes  116   b  of the air discharge system  115   b.    
         [0039]     Referring to  FIG. 4B  and  FIG. 4C , a third embodiment of an air discharge system  115   c  includes a plurality of air eject holes  116   c  defined in the cylinder base  140  of each hollow tube  14 . The air eject holes  116   c  are distributed in a regular array. The air eject holes  116   c  may each be configured the same as the air eject holes  116   b . Alternatively, the air eject holes  116   a ,  116   b , and  116   c  may also be configured with cylindrical shaped.  
         [0040]     Referring to  FIG. 5A  and  FIG. 5B , a fourth embodiment of an air discharge system  115   d  includes a plurality of cylindrical air eject holes  117  defined in a sidewall of each hollow tube  14 . In particular, the air eject holes  117  may be defined adjacent to the cylinder base  140  of the hollow tube  14 , in positions where the air eject holes  117  are exposed outside of the first tube frame  15  and always exposed outside of the front end  105  of the tube track  12 . The air eject holes  117  can be arranged in a ring and evenly spaced apart. Each of the air eject holes  117  is cylindrical. Alternatively, each of the air eject holes  117  may be configured to have the same shape as the air eject holes  116   b ,  116   c . The distance measuring probe  100  with the air discharge system  115   d  can tolerate harsh environments, because the cylinder bases  140  of the hollow tubes  14  are completely closed.  
         [0041]     Referring to  FIG. 6A  and  FIG. 6B , a fifth embodiment of an air discharge system  115   e  is constituted by the gap  118  between each hollow tube  14  and the corresponding pipe  111 . Similar to the distance measuring probe  100  with the air discharge system  115   d  of the fourth embodiment, the distance measuring probe  100  with the air discharge system  115   e  can also tolerate harsh environments. Further, the air discharge system  115  of each hollow tube  14  may be selected from any one or more of the above-described first through fifth embodiments. That is, the air discharge system  115  may be selected from any of the group consisting of the air eject hole  116   a  of the first embodiment, the air eject holes  116   b  of the second embodiment, the air eject holes  116   c  of the third embodiment, the air eject holes  117  of the fourth embodiment, and the gap  118  of the fifth embodiment.  
         [0042]     In use, the distance measuring probe  100  is placed near the object  50 . The pipes  111  and the tubes  114  communicate with an air chamber (not shown), and air is pumped into the cavities  142  of the hollow tubes  14  and the gaps between the tube track  12  and the hollow tubes  14 . When the contact tip  162  of the tip extension  16  touches the object  50 , the movable rack together with the tip extension  16  stops moving. When the tip extension  16  and correspondingly the linear measuring scale  18  move from one position to another position, the displacement sensor  19  detects and reads a displacement of the linear measuring scale  18 . That is, a displacement of the tip extension  16  is measured.  
         [0043]     When air is pumped into the cavities  142  of the hollow tubes  14 , air pressure in the cavities  142  pushes air out of the hollow tubes  14  via the air discharge systems  115 . That is, air is continuously pumped into the hollow tubes  14  via the pipes  111  and continuously ejected out of the hollow tubes  14  via the air discharge systems  115 . Part of air pumped into the hollow tubes  14  strikes the cylinder bases  140  of the hollow tubes  14 . Thus, air pressure pushes the hollow tubes  14  to move. The air pressure pushing the hollow tubes  14  is relatively small and steady because air is continuously ejected out of the hollow tubes  14 . That is, an overall measuring force that pushes the tip extension  16  is relatively small and steady. As a result, the tip extension  16  of the distance measuring probe  100  is pushed so that the contact tip  162  gently touches the object  50 . Thus, the contact tip  162  of the tip extension  16  and the object  50  are not easily deformed or damaged, and a precision of measurement is very high. Assuming that an area of an inside end surface of the cylinder base  140  of each hollow tube  14  is constant, then a value of a measuring force pushing the tip extension  16  is determined by an area of the air eject hole  116   a , the air eject holes  116   b , the air eject holes  116   c , the air eject holes  117  or the gap  118  of each hollow tube  14 . For example, in general, the measuring force decreases as the area of the air eject hole  116   a , the air eject holes  116   b , the air eject holes  116   c , the air eject holes  117  or the gap  118  increases. In addition, a pressure inside the cover  112  is kept higher than that of the environment outside the cover  112 , because air ejected out of the air bearings  113  and the hollow tubes  14  fills the cover  112 . Thus, dust and other particles are prevented from entering the cover  112  through any openings thereof.  
         [0044]     In manufacturing precision components such as optical lenses, the optical lenses generally need to be machined again if they do not fall within specified tolerances of shape and dimension. Referring to  FIG. 7 , the distance measuring probe  100  is applied in very high precision equipment for manufacturing optical lenses. The optical lenses are measured on the one piece of equipment immediately after being machined. Therefore, there is no error caused by releasing the optical lenses from machining equipment and reclamping the optical lenses on a measuring machine. Further, much time can be saved. Generally, the manufacturing time can be reduced by as much as ⅕ or even ⅓. The very high precision equipment includes a master actuator that moves the distance measuring probe  100  in at least one direction. That is, the master actuator can be a one-axis actuator, a two-axis actuator, a three-axis actuator, or can be another kind of driving master actuator.  
         [0045]     The distance measuring probe  100  is connected to a processor (not shown). The master actuator of the very high precision equipment, the distance measuring probe  100 , and the processor cooperatively form a coordinate measuring machine. Supposing that a surface of the object  50  (e.g., an optical lens) is manufactured according to predetermined 3D (three-dimensional) coordinate surface values. When the tip extension  16  touches the object  50 , the displacement sensor  19  sends values of the displacements of the tip extension  16  and the movable rack read from the linear measuring scale  18  to the processor. The processor records and manages the values. For example, the processor obtains a distance from one measured point on the surface of the object  50  to a reference point (for example, a z-coordinate distance as a function of x-y coordinates). The distance is then applied to obtain a corresponding point in space of the surface of the object  50 . The point in space is then analyzed and compared with a set of predetermined 3D surface values, in order to calculate manufacturing error values of the object  50 .  
         [0046]      FIG. 8  is a graph showing manufacturing error data obtained by the distance measuring probe  100  measuring a gauge-grade sphere having a radius of 5.5573 mm. The error data shown are obtained by measuring a series of points of a surface of the gauge-grade sphere with a coordinate value in a one-directional axis across a range of about 11 millimeters. It can be seen that most of the manufacturing errors are in the range of ±0.1 microns (μm).  FIG. 9  is a graph showing manufacturing error data obtained by the distance measuring probe  100  measuring a normal sphere (e.g. a ball bearing) having a radius of approximately 10.0 mm. The error data shown are obtained by measuring a series of points of a surface of the normal sphere with a coordinate value in a one-directional axis across a range of about 4.5 millimeters. It can be seen that most of the manufacturing errors are in the range of ±0.04 microns (μm). In each case, the distance measuring probe  100  provides highly accurate manufacturing error data.  
         [0047]     Referring to  FIG. 10 , a distance measuring probe  200  of a second embodiment of the present invention includes a base  21 , two tube tracks  22   a ,  22   b , two hollow tubes  24   a ,  24   b , a first tube frame  25 , a tip extension  26 , a second tube frame  27 , a linear measuring scale  28 , a displacement sensor  29 , and two pipes  211 . In alternative embodiments, the distance measuring probe  200  can include more than two hollow tubes  24   a ,  24   b . In such cases, there can be more than two pipes  211 .  
         [0048]     The tube tracks  22   a ,  22   b  are mounted securely on the base  21 . The tube tracks  22   a ,  22   b  are spaced apart from and parallel to each other. Each tube track  22   a ,  22   b  defines a tube rail channel  23  for receiving the corresponding hollow tube  24   a ,  24   b . An air bearing is formed between the hollow tube  24   a  and the tube track  22   a , and an air bearing  213  is formed between the hollow tube  24   b  and the tube track  22   b . The distance measuring probe  200  is similar in principle to the distance measuring probe  100  of the first embodiment, except that the tube tracks  22   a ,  22   b  are offset from each other. That is, the tube track  22   a  is set at a front portion of the base  21 , and the tube track  22   b  is set at a back portion of the base  21 . The distance measuring probe  200  includes a pair of air discharge systems  215 , which are substantially the same as the air discharge systems  115  of the distance measuring probe  100 . Because the tube tracks  22   a ,  22   b  are offset from each other, the tube tracks  22   a ,  22   b  in combination hold the hollow tubes  24   a ,  24   b  along a greater length as measured along a direction coinciding with an axis of movement of the tip extension  26 , compared with a corresponding length along which the tube track  12  holds the tip extension  16  in the distance measuring probe  100 . Thereby, the tip extension  26  can move very steadily forward and backward with little or no lateral displacement.  
         [0049]     Referring to  FIG. 11 , a distance measuring probe  300  of a third embodiment of the present invention includes a base  31 , two tube tracks  32   a ,  32   b , two hollow tubes  34   a ,  34   b , a tip extension  36 , a tube frame  37 , a linear measuring scale  38 , a displacement sensor  39 , and a pipe  311 .  
         [0050]     The tube tracks  32   a ,  32   b  are mounted securely on the base  31 . The tube tracks  32   a ,  32   b  are spaced apart from and parallel to each other. Each tube tracks  32   a ,  32   b  defines a tube rail channel  33  for receiving the corresponding hollow tube  34   a ,  34   b . An air bearing is formed between the hollow tube  34   a  and the tube track  32   a , and an air bearing  313  is formed between the hollow tube  34   b  and the tube track  32   b . The distance measuring probe  300  is similar in principle to the distance measuring probe  200 , except that no air is pumped into the hollow tube  34   a , and the tip extension  36  is fixed directly to the hollow tube  34   a . Because the tube tracks  32   a ,  32   b  are offset from each other, for reasons similar to those described above in relation to the distance measuring probe  200 , the tip extension  36  of the distance measuring probe  300  can move very steadily forward and backward with little or no lateral displacement. Further, the distance measuring probe  300  is simpler than the distance measuring probe  200  and the distance measuring probe  100 , because only the one hollow tube  34   b  is filled with air.  
         [0051]     Because the distance measuring probes  100 ,  200 ,  300  each has two spaced and parallel hollow tubes  14 ,  24   a ,  24   b ,  34   a ,  34   b , the tip extensions  16 ,  26 ,  36  effectively cannot move in directions other than a direction parallel to axes of the hollow tubes  14 ,  24   a ,  24   b ,  34   a ,  34   b . In typical use of the distance measuring probes  100 ,  200 ,  300 , the hollow tubes  14 ,  24   a ,  24   b ,  34   a ,  34   b , are oriented horizontally. However, the measuring forces of the distance measuring probes  100 ,  200 ,  300  are minimally or not influenced by gravity.  
         [0052]     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.