Abstract:
A beam load cell transducer is positioned between a stationary member and moveable member. The stationary member, as well as the moveable member are surrounded by a collar, which collar has an inner surface means to limit the movement of the moveable member of the transducer. By limiting the movement and restraining further movement after a predetermined excessive force is applied, one stops the beam and therefore prevents the beam from fracturing or rupturing. In the unlikely event that the beam does fracture or rupture, then the sleeve acts to hold the entire unit together, thereby maintaining integrity to the transducer.

Description:
FIELD OF THE INVENTION 
   This invention is related to the field of stress measurement devices and more specifically to a safety assembly in case of load cell failure. 
   BACKGROUND OF THE INVENTION 
   Beam load cell stress measurement devices are well-known in the art.  FIG. 1  illustrates a simplified model of a conventional beam load cell  100 . In this model, load cell  100  consists of a fixed first part  110 , a moveable second part  120  and measurement component, referred to as load-beam,  130  therebetween. Measurement component  130  further includes measurement resistors  140  thereon. The resistors may be piezoresistors or other devices. The resistors  140  change resistance value in accordance with the magnitude of an applied force or stress. 
   As force  145  is applied to load cell  100 , the second part  120  moves in accordance with the applied force, and measurement component  130  is stressed in response. Beam type transducers are known in the prior art and it is known how to limit the deflection. See U.S. Pat. No. 4,051,451 entitled, “Beam Type Transducers Employing Dual Direction Limiting and Means”, which issued on Sep. 27, 1977 to A. D. Kurtz et al and is assigned to Kulite Semiconductor Products, Inc., the assignee herein. That patent shows a beam subjected to a transverse force (up and down) to move the beam with a stop above and below the beam. 
   See also U.S. patent application Ser. No. 09/814,903 entitled, “Force Transducer with Environmental Protection” filed Mar. 22, 2001 for A. D. Kurtz et al. and assigned to the assignee herein. This application shows a beam load cell as utilized herein. 
   In the load cell transducer, as shown in  FIG. 1 , as one can ascertain, if unusually large forces are applied in either a pull or a push direction, the beam can fracture and actually come apart. In this manner, it can cause catastrophic failure of the entire system. For example, one may want to measure the stresses applied on an aircraft rudder during a mission or during normal operation. One may use the pressure sensor as part of a servo system, where as the pressure increases, one may want to turn the rudder or apply more or less force on the rudder. If the beam fractures, then the entire sensor becomes inoperable and therefore, there would be no recovery of the control system. However, if the beam is stopped and prevented from rupturing or fracturing, then when normal forces are applied, the sensor will still operate during normal operation and thus, a disaster can be circumvented. In this manner, there is provided a new and improved load cell which employs a stop to prevent the rupture or breakage of a beam employed for measuring forces in either a push or a pull direction. 
   SUMMARY OF THE INVENTION 
   A beam load cell of the type having a stationary member and a moveable member with a beam positioned therebetween, such that when a force is applied to the moveable member, the beam is moved in the direction of the force and for an undesirably large force the beam can rupture. In combination with the load cell there is a stop member to limit the movement of the moveable member in either a push or pull direction and therefore, to limit the movement of the beam. The stop comprises a longitudinal tubular member which surrounds the stationary and moveable members and the beam, and stop means are located on the inner surface of the tubular member to coact with the moveable member for limiting the distance, the movable member travels upon application thereto of an applied push or pull force, the stop means therefore limits the force applied to the beam to prevent it from breaking or rupturing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a cross-sectional view through a longitudinal axis of a conventional prior art load cell; 
       FIG. 2   a  illustrates a cross-sectional view, through a longitudinal axis of a fault-tolerant load cell in accordance with the principles of the invention; 
       FIG. 2   b  illustrates a cross-sectional view, through section A-A of the fault-tolerant load cell shown in  FIG. 2   a;    
       FIGS. 3   a  and  3   b  illustrate enlargements of the engagement means of the fault-tolerance load cell shown in  FIG. 2   a;    
       FIG. 4  illustrates a prospective view of a fault-tolerant load cell in accordance with the principles of the invention; 
       FIG. 5   a  illustrates a cross-sectional view, through a longitudinal axis of another embodiment of a fault-tolerant load cell in accordance with the principles of the invention; 
       FIG. 5   b  illustrates a cross-sectional view, through section A-A of the fault-tolerant load cell shown in  FIG. 6   a ; and 
       FIGS. 6   a,    6   b,    6   c  and  6   d  illustrate cross-sectional views of exemplary fault-tolerance load cells in accordance with the principles of the invention. 
   

   It is to be understood that these drawings are solely for purposes of illustrating the concepts of the invention and are not intended as a definition of the limits of the invention. The embodiments shown in  FIGS. 1 through 6  and described in the accompanying detailed description are to be used as illustrative embodiments and should not be construed as the only manner of practicing the invention. Also, the same reference numerals, possibly supplemented with reference characters where appropriate, have been used to identify similar elements. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2   a  illustrates a cross-sectional view, through a longitudinal axis, of a fault-tolerant load cell  200  in accordance with the principles of the invention. The same reference numerals designate similar parts as depicted in  FIG. 1 . In this case, a fixed first part or stationary member  110  includes recesses  225  that are used to engage and fixedly retain engagement means  220 L. Movable second part  120  similarly includes recesses  230  that are used to engage and slidably retain engagement means  220 R. A cylindrical collar or longitudinal tubular member  210  surrounds the first part  110  and the second part  120  and further includes openings  215 R and  215 L that operates to allow engagement means or pins  220  L and R to pass through collar  210  to be positioned in recesses  225  and  230  respectively. As one can see, the pins  220  are designated as  220 L ( 220  Left) for pins associated with member  110  and as  220 R ( 220  Right) for pins associated with member  120 . This designation is also used for apertures or channels  215 L and  21  SR. As can be seen, recess or aperture  225  is of a smaller width than aperture or recess  230 . Thus, movable member  120  can slide or move within the recess  230  a predetermined distance. The movement of the pin  220 R within the recesses  230  is practically frictionless. If a large force is applied to member  120  in the direction (right) of arrow  245  ( FIG. 2   a ) the pin  220 R will abut against the wall  250  ( FIG. 2   a ) acting as a stop and preventing further movement of member  120 . If a large force is applied in the opposite direction of arrow  245  (left), the pins abuts against the wall  251 , acting as a stop in that direction. 
     FIG. 2   b  illustrates a cross-sectional view, through section A-A, of fault-tolerant load cell  200  in accordance with the principles of the invention. In this view, the insertion of engagement means  220  through channel  215  in collar  210  to engage recesses  225  in first part  110  are more clearly shown. In this case, three engagement means  220  are shown in a conventional orientation of 120 degrees to distribute any load that may be exerted thereon. Although only three pins  220  are illustrated, it would be appreciated that any number of pins may be used. The multiple pins also act as an alignment means to prevent the transducer from responding to off axis loads. The number of pins needed may be determined based on the material properties and thickness of the pin and the holding power necessary to retain the load cell after a failure of component  130 . For example, pin  220  may be selected from materials such as a rigid plastic, nylon, carbon steel, stainless steel, based on the operating conditions and the expected loads that may cause failure. Thus, in light load conditions, plastic or nylon may be suitable materials, whereas in heavier loads or more critical situations, metal or carbon steel composites may be suitable. In conditions having high humidity, stainless steel may be an appropriate material. 
   Engagement means  220  is further illustrated as a pin that is press fit or snap fit into channels  215  L&amp;R. However, it should be appreciated that channel,  215  L&amp;R, recess  225  and  230  may be threaded. In this case, engagement means  220  L&amp;R may be a screw, e.g., a set screw, that is screwed into channel  215  and recess  225  or other threaded device as a bolt. 
     FIG. 3   a  illustrates an enlarged view of engagement of pin  220  through passage  215  in collar  210  to engage recess  225  in first part  110 . In this illustrated case, a tight fit between pin  220  and recess  225  is desired. Accordingly, recess  225  is preferably in the order of 1-2 mils greater than pin  220 . In this case, pin  220  may be snap-fit or press-fit into passage or channel  215  and recess  225 . As noted above, channel  215  and/or recess  225  may be threaded which would allow for the use of a screw or set-screw. 
     FIG. 3   b  illustrates an enlarged view of engagement of a threaded pin  220  through a threaded passage  215  in collar  210  to enter recess  230  in second part  120 . In this case, recess  230  is larger than pin  220  to allow a slip fit between pin  220  and recess  230  as second part  120  is required to move freely in response to the application of force  145 . The force  145  is a pulling force as compared to a force in the opposite direction which is a pushing force (double arrow). A large pulling force  145  can cause the beam to rupture or fracture. Similarly, a large pushing force can bend the beam as in a U-shape and break the beam as well. Once the beam is broken, as seen in  FIG. 1 , there is no coupling between members  110  and  120 . However, the collar keeps the unit together as the pins prevent dislodging of members  110  and  120  and therefore, the sensor will still be in circuit and prevent catastrophic failure. Hence, the width of recess  230  is preferably in the order of 10 mils greater than the width of pin  220 . This is sufficient to enable second part  120  to move unhindered and cause stress in component  130 . For example, assuming at full load second part  120  is designed to move up to 14 mils, then a clearance of 20 mils is needed around pin  220  to allow collar  210  not to limit movement and degrade the measurement. 
   When a larger force  145  is applied, the pin  220  abuts against the wall  250  for a large pull force  145 . For a push force, the pin abuts against wall  251 . Thus, the walls  250  and  251  of the channel  230  engage the pin  220  for a push or pull and stop the movement of the beam in either direction. 
     FIG. 4  illustrates a perspective view of a preferred embodiment of the present invention. In this view, collar or strength bypass element  210  is attached to load cell  100  using two sets of three pins  220 . Pins  220  are set in a series of recesses, shown as holes,  225  on first part  110  and in a recess, shown as groove  230  or part  120 . If the beam  130  breaks apart, the collar  210  will hold the load cell  100  together, through the engagement of pins  220  in holes  225  and groove  230 . Environmental cover  410  is advantageous as it protects load cell and collar  210  from dirt, humidity, etc., but is not necessary for the operation of the invention. One could also use a bellows to surround the beam, as is known. 
   As further noted, if the load beam  130  fails, second part  120  is no longer restrained by load beam  130  and is not coupled to first part  110  and moves or shifts in a manner greater than desired in the prior art. In this case, the movement or shift of second part  120  causes the substantially vertical walls of recess  230  to engage pin  220 . Hence, the integrity of load cell  100  is maintained. For example, when load beam  130  is a single beam measuring 0.270×0.120×0.4 inches of 15-5 steel it can be determined that a load or force of 4455 lbs. can be applied before a failure of the load beam  130  occurs. However, safety collar or assembly  110  incorporating three pins  220  of 3/16 diameter, each having a shear strength of 3600 lbs., may retain the integrity of cell  110  up to a force of 10,800 lbs. However, the main purpose is to stop the movement of the beam when excessive forces are applied and therefore prevent the rupture of the beam. 
   Although the use of holes on first part  110  and a groove on second part  120  is shown, it would be appreciated that recess  225  may also be a groove that allows a tight-fit between pin  220  and recess  225  along the longitudinal axis. Similarly, recess  230  may be an elongated hole or slot that allows for a slip fit between pin  220  and recess  230  in the longitudinal axis. 
     FIG. 5   a  illustrates a cross-sectional view, through a longitudinal axis, of a second embodiment  500  of the present invention. In this embodiment, channel  520  passes though first part  110  and engagement means  510  passes through oppositely opposed channels  215  in collar  210  and through channel  520 . 
     FIG. 5   b  illustrates a cross-sectional view, through section A-A, of the embodiment of the invention shown in  FIG. 5   a . This view more clearly illustrates the passage of pin  510  through first part  110 . Although not shown, it would be appreciated that one or more channels and/or passage  510  may be threaded. In this case pin  520  may include a compatible screw thread. 
     FIGS. 6   a - 6   d  illustrate further exemplary embodiments of the present invention. For example,  FIG. 6   a  illustrates one aspect  600  wherein the second part  120  includes a lip  610  that engages lip  615  on collar  210 . In this case, when the beam component  130  fails, lip  610  and  615  engage to prevent second part  120  from separating from first part  110 .  FIG. 6   b  illustrates another embodiment  620  of the present invention. In this exemplary embodiment lip  630  on second part  120  and lip  635  are angled to create a tapered fit when engaged.  FIG. 6   c  illustrates another embodiment of the present invention, wherein first part  110  is welded, brazened or adhesively engaged to collar  210 .  FIG. 6   d  illustrates another exemplary embodiment of the present invention, wherein first part  110  and collar  210  are threadedly engaged. First part  110  and  210  may be further welded, brazened or adhesively engaged or attached. 
   While there has been shown, described, and pointed out fundamental novel features of the present invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the apparatus described, in the form and details of the devices disclosed, and in their operation, may be made by those skilled in the art without departing from the spirit of the present invention. It is expressly intended that all combinations of those elements that perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated.