Abstract:
A system for sensing the load carried in a structural member by attaching a load cell to a neutral axis of bending. Small deflections along an arc of bending create a desirable signal level via mechanical amplification. A load sensor is adapted for mounting on an axle that deviates from a neutral axis when under a load. The load sensor comprises a reaction portion adapted for mounting on the axle and an active portion adapted for mounting to at least two points along the axle. The active portion has an amplifier arm and at least one sensor element. The sensor element is attached at a first end to the reaction portion and the sensor is attached at its second end to the amplifier arm. At least one strain gauge is mounted on the sensor element. At least one flexion web is attached to the reaction portion and to the active portion. When a load is put on the axle, flexion of the axle moves the active portion relative to the neutral axis of the axle, the active portion moves the amplifier arm, causing a curve to be formed in the sensor element, the curve being of sufficient magnitude to be calibrated by the at least one strain gauge.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. Ser. No. 10/954,540 filed Sep. 30, 2004. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable.  
       APPENDIX  
       [0003]     Not Applicable.  
       BACKGROUND OF THE INVENTION  
       [0004]     Installing on-board sensor beams at the neutral axis of bending of a load bearing member such as a truck drive axle or front axle is disclosed by the U.S. Pat. No. 5,327,791 to Walker, 1992.  
         [0005]     The prior art design, with four (4) mounting points, secures a “dog bone” shaped sensor beam sufficiently enough to force it to bend with the load bearing structure. Its neutral axis corresponds to that of the load bearing structure and follows its arc of bending.  
         [0006]     Although very useful in the truck scale industry this technique has limitations. Its size limits its utility. For example: current 12 inch long beams must straddle spring attachment U-bolts to find a suitable installation location. This spacing away from the drive axle housing necessitates tall weld brackets (up to  1¼ inches). There is a need for a new design that would reduce the size (length) of sensor beam to increase utility. An  8 inch long sensor beam would install inboard of U-bolts and outboard of the drive axle banjo.  
         [0007]     Its relatively low output signal limits noise reject and hinders weight resolution. There is a need to increase the output signal of the sensor beam by some mechanical amplification technique. Simply shortening a beam significantly reduces its output signal. However, making a sensor beam longer would increase its output signal but would also further limit its utility.  
         [0008]     There is a need to review hardware being used in sensor beam installation to improve proximity of the sensor to the load bearing member. There is a further need to reduce shear and bending forces on mounting bolts or studs to improve linearity and zero return of installed sensors.  
         [0009]     The aforementioned arc approximates a circular function for the center section of a front axle between the spring mounts. The arc of bending for a drive axle is exponential in character, i.e., deflection increases an amount that is more than proportionate to the distance from the drive axle centerline as that distance increases. In both of the above situations, the deflection available to drive the sensor is only 0.005 inches for prior art sensor beams. It is even less for shorter sensor beams. Because existing neutral axis sensor beams reside in a low deflection zone with respect to load bearing members, there is a need for some multiplying or amplifying factor to facilitate a new design.  
       SUMMARY OF THE INVENTION  
       [0010]     It is in view of the above problems that the present invention was developed. It is therefore the primary objective of the present invention to provide an improved sensor beam with increased utility due to reduced size and increased output signal proportional to load due to mechanical amplifications. The placement of sensors along the no stress neutral axis of bending in primary structures preserves the safety factor of earlier technology. The present invention does not utilize a constant bending center section for the sensor beam and, in fact, performs well in both constant bending and exponential stress fields supported by primary structure.  
         [0011]     The sensor beam is installed by attachment brackets and welding, contiguous with an axle&#39;s neutral axis, which is a reasonably straight line during no load conditions. As load is applied to the axle with the installed sensor beam, an arc of bending in the axle is generated and enters the sensor beam via right side mounting hardware that rotates the right end of the sensor beam clockwise.  
         [0012]     This action moves the center web of the sensor beam upwards and above the arc of bending. The left end of the sensor beam contains the third or reaction mounting point, which pulls the sensor beam left end back down to the arc of bending. The aforementioned differential motion is connected by a sensing element. The sensing element, equipped with four strain gauges, deforms in compound bending thereby activating all four strain gauges proportional to load. A wheatstone bridge wiring, amplifier, power source and digital indicator provide calibration of the sensed load.  
         [0013]     Laboratory testing has revealed that if one end of a sensor beam is released from attachment, the opposite end with its two (2) mounting points will cause the beam to function as a tangent line with respect to the arc of bending. As bending increases, the free end of the sensor beam moves a substantial distance from the arc.  
         [0014]     A reaction point is isolated near the extreme free end of the sensor beam. A void is created adjacent thereto. This member needs to be stabilized dimensionally with two (2) outboard flextures along the sensor&#39;s length. These flextures are soft in bending support, but stiff in torsion or length supports.  
         [0015]     If the reaction point is pulled down to the arc of bending, it is easily noted that a significant displacement disparity exists across the aforementioned void. If the reaction point is released, it will return to the centerline of the sensor beam.  
         [0016]     The last element of the system is a thin connective beam between the reaction point and the remaining outboard end of the beam. This sensing element is flat and straight in the unloaded condition. When the complete assembly is subjected to compliance with an arc of bending, the sensing element will bend into a shallow figure “S” or compound bending pattern. The shape supports four (4) strain gauge locations at four (4) locations, two tension and two compression. When wired into a classic wheatstone bridge circuit, all four gauges contribute to the output signal equally. Three (3) alternate sense element configuration are disclosed as follows:  
         [0017]      FIG. 10  splits the center beam into two (2) elements, top and bottom, with ½ of the element height in each beam. This approach retains the flexibility of the single beam and adds the possibility of a strain gauging pocket that could easily be waterproofed.  
         [0018]     The  FIG. 11  two holes version with an abbreviated center beam is stiffer but exhibits very high output signal.  
         [0019]     The  FIG. 12  hole in the beam creates a top tension bar and a bottom compression bar. By testing it was determined that this is somewhat stiffer than the double reacted bending element but exhibits good output signal. Like  FIG. 10 , it is suitable for easy waterproofing.  
         [0020]     The load cell disclosed and claimed herein may be advantageously deployed in any motor vehicle, including trucks, agricultural vehicles such as grain carts, or it may be used on static moment arms such as building beams, bridge beams, elevator supports or earthquake sensitive structural components. Structural members that may be measured include but are not limited to: truck axles, truck frame members, truck walking beams, conveyor rails and train tracks.  
         [0021]     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is an isometric view of a portion of a truck axle with U-bolt mounted leave spring, depicting the installation of the load sensing system of the present invention.  
         [0023]      FIG. 2  is an enlarged isometric view of sensor beam portion of the system with an exploded view of the mounting hardware and a further enlargement of the sensing element with strain gauge locations.  
         [0024]      FIG. 3  is enlarged plan view of the sensor beam of the system of  FIG. 1 , with dimensional references.  
         [0025]      FIG. 4  is an enlarged cross section of the sensor beam taken along line  4 - 4  of  FIG. 3 , which passes through tapered hole  28 .  
         [0026]      FIG. 5  is an enlarged cross section view of the tapered bushings of the systems.  
         [0027]      FIG. 6  is a front end elevated view of the tapered bushing taken from the left side of  FIG. 5 .  
         [0028]      FIG. 7  is an enlarged cross section view of one of the recessed spacer elements of the system of  FIG. 1 .  
         [0029]      FIG. 8  is a rear end elevated view of the recessed spacer element taken from the right side of  FIG. 7 .  
         [0030]      FIG. 9  is an enlarged side elevation view of the bolt and weld bracket assembly of the system of  FIG. 1 .  FIGS. 4 through 9  constitute an exploded view of the assembly sequence: of one of three attach points for the sensor beam in  FIGS. 2 and 3 .  
         [0031]      FIG. 10  is an enlarged plan view of a sensor beam with all of the attributes of the  FIG. 3  sensor beam except that it suggests an alternate dual sensing element design.  
         [0032]      FIG. 11  is an enlarged plan view of a sensor beam with all of the attributes of the  FIG. 3  sensor beam except that it suggests an alternate short sensing element design.  
         [0033]      FIG. 12  is an enlarged plan view of a sensor beam with all of the attributes of die  FIG. 3  sensor beam except that it suggests an alternate dual sensing element design made possible by a round center cutout versus a rectangular one as in  FIG. 10 .  
         [0034]      FIG. 13  is a table of applied forces versus sensor beam deflection and output signal for a test unit.  
         [0035]      FIG. 14  is a table of applied forces versus sensor beam deflection and output signal for an interim design employing a center hole to form two sensor elements.  
         [0036]      FIG. 15  is a table of applied forces versus sensor beam deflection and output signal for version of the prior art neutral axis sensor disclosed in U.S. Pat. No. 5,327,791.  
         [0037]      FIG. 16  is a functional diagram of four strain gauges wired in a wheatstone bridge with amplification, excitation power and digital display of load being monitored by the sensor beam.  
         [0038]      FIG. 17  is a view showing an isolation slot.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]     The disclosure of U.S. Pat. No. 5,327,791, issued on Jul. 12, 1994, for a, “Vehicle Beam Load Measuring System,” is hereby incorporated by reference. The entire disclosure of U.S. Pat. No. 6,092,838, issued on Jul. 25, 2000, for a, “System and Method for Determining the Weight of a person in a Seat in a Vehicle,” is also hereby incorporated by reference.  
         [0040]      FIG. 1  illustrates the preferred embodiment of a new sensor beam invention for installation on load bearing members of vehicles such as drive axles, front axles, walking beams or frame rails. The  FIG. 1  example places the sensor beam  18  on drive axle housing  10  at the neutral axis of bending  14 , which is also typically a welded seam in said housing. Sensor Beam  18  is located just inboard of spring U-bolt  12  attaching spring  11  and outboard of the circular flange of differential housing  10   a , typically known as the “Banjo.” Sensor beam  18  is physically attached to weld brackets  31 ,  32  and  38 , which attach to housing  10 . The brake plate attachment  10   b  is shown for clarity of placement of sensor beam  18  only.  
         [0041]      FIG. 2  is a perspective view of sensor beam  18  with an exploded view of attachment hardware and further enlargement of sensing element  18   e . Four strain gauges  46 ,  48 ,  50 , and  52 , are located on sensing element  18   e  as noted with gauges  46  and  48  on top of sensing element  18   e  and gauges  50  and  52  on the lower side.  
         [0042]     A sensor beam can be formed from a wide variety of commercially available metals. The depicted embodiment uses 17-4 PH stainless steel bar stock. The length of the depicted embodiment does not exceed 8 inches; the width does not exceed 2 inches, and a stock thickness is as small as ¼ inch.  
         [0043]     Sensor beam  18  has a force driven pair of holes  18   a  at the right hand end. This is the active portion of the load sensor. The center portion of sensor beam  18  is a stiff web  18   b  that extends to the left to connect to sensing element  18   e . This is the amplifier arm.  
         [0044]     The remaining left hand portion of sensor beam  18  is the reaction section  18   c  which contains the third attachment hole.  
         [0045]     The material surrounding the third mounting hole to be stabilized from possible rotation by two long flexture beams emanating from the right end of the sensor beam and created by two machined slots running the length of the flextures. The third mounting hole has the capability and purpose of pulling the left end of the sensor beam down to the arc of bending while forcing the sensing element into compound bending.  
         [0046]     The exploded view of mounting hardware starts on the right with weld, bracket assembly  38 . Tapered bushing  36  slips on to assembly  38  with the small end of the hushing outboard. A tapered hole in the sensor beam passes over tapered bushing  36 . Backup washer  44  passed over the protruding end of assembly  38  with its recessed side against sensor beam  18 . Washer  42  and lock nut  40  complete the assembly. After weld brackets  30 ,  38  and  32  are positioned and welded to housing  10 , three each lock nuts  40  are torqued to a value of 200 inch pounds.  
         [0047]     The present configuration allows a shorter beam to be used, making it more adaptable, quicker and easier to mount. By way of example and comparison to prior designs depicted, sensor beam  18  could be eight inches in length rather than twelve inches in length. Height and thickness of sensor beam  18  need only be appropriately adaptable for mounting on various axles.  
         [0048]      FIG. 3  is a plan view of sensor beam  18 . Horizontal center line  20  corresponds to the neutral axis of bending of load bearing members, like  14  on housing  10 , which sensor beam  18  will be attached to. The loaded axle will deform with its center convex downward, towards the roadway. During loading of housing  10 , neutral axis  14  will bow down in the center, forming an arc below center line  20 . When said arc occurs, hole  28  will follow the arc and move relative to hole  30 . This clockwise rotation of area  18   a  at the intersection of center lines  20  and  24  will cause web  18   b  to rotate its outboard end. In doing so, web  18   b  will cause a force on the right hand end of sensing element  18   e . While the aforementioned motion is occurring at  18   a  and  18   b , area  18   e  is being retained by mounting hole  26 . Area  18   c  provides a reaction point for the left hand end of sensor element  18   e , thereby causing element  18   e  to reverse bend, which means that strain gauge locations  46  and  50  will see tension stress at the gauge interface and strain gauges  46  and  52  will see compression stress at the gauge interface.  
         [0049]     By way of clarification, if sensing element  18   e  were a simple bending beam, being clamped at one end and simply supported at the other end, it would have tension stress all across the upper surface and compression all across the lower surface. Sensing element  18   e  is, however, clamped at both ends.  
         [0050]     Reaction area  18   c  and attachment hole  26  could function without further stabilization, but would have to rely on the relatively weak sensing element  18   e  to keep it from rotating. To provide desirable positional stability to area  18   c , two outer perimeter stabilizing webs  18   f  are added by cutting two slots  18   d . This closing of the outer perimeter of sensor beam  18  adds physical protection and stabilizes the entire system during periods of thermal expansion or contraction along center line  20 .  
         [0051]     Holes  26 ,  28  and  30  lie along center line  20 . They each have a four and one half degree taper with the large diameter inboard to accommodate tapered bushing  36  in  FIG. 2 .  
         [0052]     Holes  28  and  30  that generate the driving force in the system are separated by Dimension X. This dimension defines the active portion of the load sensor. This dimension is ineffective below 1.5 inches and sacrifices output signal above 2 inches. In the depicted embodiment, this dimension is 1.90 inches. Dimension Y is the length of sensing element  18   e . As Dimension Y is increased, the signal level from gauges  46 ,  48 ,  50  and  52 , mounted on sensing element  18   e , goes down. Conversely, as Dimension Y is shortened, signal levels go up. A value for dimension Y above 1.2 to 1.5 inches is counterproductive and a value smaller than 0.5 inches makes installing strain gauges difficult and costly.  
         [0053]     Dimension Z is the distance from the intersection of center lines  20  and  24  to the right hand end of the sensing element  18   e . The ratio of Dimension Z divided by Dimension Y will fall in the range of 3 to 6. This ratio is in fact the amplification factor of this invention.  
         [0054]     Strain gauges  46 ,  48 ,  50  and  52  are generic industry standard load sensors that can be installed by bonding by methods known to those of skill in the art. The part number depicted is: CEA-06-125UN-350. The Manufacturer is Vishay/Measurements Group, Raleigh, N.C., USA.  
         [0055]     In subsequent embodiment descriptions, the stress polarity encountered by each of the four gauges will remain constant. The strain gauge number will move to new and appropriate locations in alternative embodiments depicted in  FIGS. 10, 11 , and  12 .  
         [0056]     The electrical portion of this system is depicted in  FIG. 16 . The source  9 , the amplifier  8  and the digital load indicator  7  are all well known throughout the load sensor/load cell industry as is also the case with wiring and waterproofing techniques. None of the electrical portions is considered claimable and would add nothing to this invention.  
         [0057]      FIG. 4  is a cross section of mounting hole  28  in  FIG. 3 . It has the aforementioned 4.5 degree taper  28   a  that matches the  36   a  taper in  FIG. 5 . The large diameter of taper  28   a  is facing inboard or towards the weld bracket  38  and axle housing  10 .  
         [0058]      FIG. 5  is a cross section of tapered bushing  36 . Taper  36   a  has its large diameter facing inboard as does the taper in sensor beam  18 . Inside diameter  36   c  of hushing  36  closely fits the shank diameter of bolt  34  in assembly  38 .  
         [0059]      FIG. 6  views the large diameter end of the  FIG. 5  hushing. Note the  36   b  slot cut in bushing  36  to allow closure on the already close fit shoulder bolt shank  34 .  
         [0060]      FIG. 7  is a cross section of a backup washer  44 . The recessed portion  46  faces sensor beam  18  at hole  28  and accommodates a portion of bushing  36  that protrudes through hole  28  during torque up.  
         [0061]      FIG. 8  is a plan view of backup washer  44  viewed from the right side of  FIG. 7 . Washer  42  abuts backup washer  44 . Lock nut  32  finishes the sequence of assembly for one of three attach points.  
         [0062]      FIG. 9  is a side view of weld bracket  38 . Shoulder bolt  34  is welded to bracket  38   b  with a small portion of  34 &#39;s head outside of bracket  38   b  at point  34   a . Shank  34   c  has been noted to close fit inside diameter  36   e  in bushing  36 . Thread  34   b  passes through bushing  36 , sensor beam  18  and backup washer  44  with sufficient remaining length to accommodate washer  42  and lock nut  32 .  
         [0063]      FIGS. 4, 5 ,  7  and  9  viewed together constitute an exploded view of one of three sensor beam  18  attachment points.  
         [0064]     An alternative mounting technique utilizes tapered sleeves in tapered mounting holes at specified locations along a sensor beam body to secure the sensor beam to the primary structure. The sensor is mounted along a neutral axis of bending for the primary structure that becomes an arc of bending under applied load. The embodiment reverses the sequence of assembly from prior art, the primary structure of which places the small end of the tapered bushing outboard, thereby facilitating future disassembly. The placement of recessed backing washers outboard further improves the proximity of a sensor beam to the primary structure it is tracking during load defermation.  
         [0065]     The depicted mounting brackets have greatly reduced thickness compared to those required by the prior art. They move the sensor beam closer to primary load bearing structures, thereby reducing bending stresses in the bolts that comprise the sensor piercing portion of the attachment brackets and subsequently improving sensor beam linearity and zero return.  
         [0066]      FIG. 10  is an alternate embodiment of the invention. It shares the fit and general performance of the embodiment of  FIG. 2 , including its amplification factor. The primary variation lies in the configuration of sensing element  18   e . In this embodiment, dual sensing elements are employed with strain gauges  52  and  46  on the inboard side of the upper element and strain gauges  48  and  50  on the inboard side of the lower elements. This facilitates waterproofing.  
         [0067]      FIG. 11  is another alternate embodiment of the invention. It also shares the fit and general performance of the  FIG. 2  design, including amplification factor. The sensing element  18   e  approaches the highest attainable amplification, a factor of 6.  
         [0068]      FIG. 12  is another alternative embodiment of the invention. It also shares the fit and general performance of the  FIG. 2  design, including amplification factor. As in  FIG. 10 , the  FIG. 12  embodiment employs two sensing elements  18   e , but creates them with one centered hole. As with  FIG. 10 , the strain gauges are physically protected and easy to waterproof. The installation costs of applying the strain gauges are kept low with the embodiments depicted in  FIGS. 2, 10  and  12 .  
         [0069]      FIG. 13  represents test data taken from a  FIG. 2  designed sensor beam. A target deflection is 0.0033 inches at the hole  26  end of the sensor beam  18 . This modest deflection was produced with only 16 pounds of dead weight at hole  26 . The demonstrated output of 1585 micro-inches per inch is 150% of that generated by the twelve-inch long sensors noted in U.S. Pat. No. 5,327,791.  
         [0070]      FIG. 14  represents test data taken from an interim design that utilizes the hole that appears in  FIG. 12 , but leaves said hole at the center of the sensor beam without any amplification. At the desired deflection of 0.0033 inches, the applied dead weight required was more than double the  FIG. 13  weight required and the output signal was only 35% of the  FIG. 13  design.  
         [0071]      FIG. 15  represents test data taken from a scaled down “dog bone” sensor beam, that being the industry name for the sensor of U.S. Pat. No. 5,327,791.  
         [0072]     At the desired deflection of 0.003 3 inches at the hole  26  end of an eight-inch long dog bone, the dead weight required was less than 4 pounds, but the output of this sensor beam was only 335 micro-inches per inch or 20% of the  FIG. 10  invention. It was also only about 33% of the output of the original twelve-inch dog bone sensor in U.S. Pat. No. 5,327,791.  
         [0073]     Longitudinal forces along axis  20  may produce unacceptable instability in sensor output at near zero loading. These forces may interfere with the intended sensing direction, that being the normal response of the sensor to vertical displacement of  18   b  with respect to  18   c  referring to  FIG. 17 , and as sensed by  18   e  in compound bending.  
         [0074]     The true neutral axis of bending of the primary structure to which the sensor beam is attached is not always determinable with accuracy, due to variables including but not limited to mounting position, primary structure fabrication, wear and the like. Longitudinal forces could exist in primary structures such as a drive axle housing during normal load reacting service.  
         [0075]     Review of form factors of targeted primary structures discloses the following.  
         [0076]     Drive axles can have slight variations in neutral axis location versus geometric centerline due to unsymmetrical application of bending load to said axle via supporting hardware. The “I” beam web of a front axle has a neutral axis of bending reasonably close to geometric centerline. Other target structures such as walking beams may have a neutral axis that can only be determined by trial and error.  
         [0077]     Testing discloses that longitudinal forces along axis  20  can exist. They are characterized by viewing attach points  28  and  30  as fixed, and reaction attached point  26  as moving left or right due to applied forces. Under such circumstances axial/longitudinal load is applied to the left-hand end of sensing element  18   e  as seen in  FIG. 17 .  
         [0078]     Because sensing element  18   e  is a long column element with very low resistance to buckling, it is likely that when said element is not loaded in its normal shallow compound bending mode with a single zero bending point or inflection point, it could be influenced to pop up or down like a supported diaphragm with high bending in the center and two inflection points outboard of center. This can lead to high inverse bending near the outboard ends of the element where the strain gages are located.  
         [0079]     The isolation slot  18   h  relieves any longitudinal forces from attachment  26 . Isolation slot  18   h  is perpendicular to slots  18   d , making isolation slot  18   h  vertical in  FIG. 17 . Isolation slot  18   h  is located between sensing element  18   e  and reaction point  26 . Isolation slot  18   h  routes longitudinal or axial forces up and down to the outboard stabilizing flextures  18   f  to be reacted to by the combination of attachments  28  and  30 .  
         [0080]     Vertical element  18   g  performs the vertical motion originally accomplished by  18   c  in that its upper and lower ends are still derived by  18   c  and attachment  26 .  
         [0081]     In view of the foregoing, it will be seen that the several advantages of the invention are achieved and attained.  
         [0082]     The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.  
         [0083]     As various modifications could be made in the constructions and methods herein described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.