Constant moment weigh scale with floating flexure beam

Abstract

A weigh scale apparatus in which forces caused by the applied load are transformed into moments of opposite sense and applied to opposite ends of a substantially rigid beam. Deflection of the beam is measured by centrally disposed strain gage resistors to produce an electrical signal quantity proportional to the applied load. The conversion of applied load to moments of opposite sense is accomplished by a pair of spaced parallel pivot members which are suspended by straps between the load receiving pan and the base such that the pivots tend to rotate in opposite directions. The beam is mechanically connected between the pivots such that it is subjected to the bending moment over substantially its entire length. A split scale device is provided by a composite pan, one portion of which is connected to the pivots directly and another portion of which is connected to the pivots through long lever arms to provide moment multiplication. A vibrator device for dithering the pivots is provided. .Iadd.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. Floating beam weigh scale apparatus comprising a base, a load receiving member spaced from the base, first and second pivot members disposed between the base and load receiving .[.members,.]. .Iadd.member, .Iaddend.load transmitting means interconnecting the pivots, the base, and the load receiving member to produce moments in the pivots of opposite sense and about spaced axes, a substantially rigid beam connected to and extending between the pivots, and means carried by the beam for producing a signal related to the bending stress therein over the elastic bending range thereof. 
     
     
       2. Apparatus as defined in claim 1 wherein the transmitting means comprises torsionally compliant straps connected in opposing pairs to the pivots, one such strap at each end being connected between the base and the pivot and another such strap at each end being connected between the pivot and the load receiving member to produce the turning moment of the pivot. 
     
     
       3. Apparatus as defined in claim 1 further including means for dithering the pivots by mechanical vibration thereof. 
     
     
       4. Apparatus as defined in claim 1 wherein the beam comprises a solid member of circular cross section over a substantial portion of the length thereof. 
     
     
       5. Apparatus as defined in claim 4 wherein the beam further comprises a central area having opposing and parallel flat surfaces formed therein substantially in the center of the beam for receiving the means for producing the signal upon elastic deformation thereof. .[.6. Apparatus as defined in claim 1 further including lever arms connected to the pivots and extending longitudinally of the scale and perpendicular to the pivots and means interconnecting at least a portion of the load receiving member to the lever arms for multiplying the turning moment applied to the pivots by the load applied to the load receiving means..]. .[.7. Apparatus as defined in claim 6 wherein the load receiving means comprises a pan having a first surface which is directly connected to the pivot members and a second portion at least partially bounded by the first pan portion and connected to the pivots by way of said lever arms thereby to provide a 
     
     
        split scale apparatus..]. 8. Apparatus as defined in claim 1 wherein the means carried by the beam includes at least one strain gage resistor intimately bonded to the beam and responsive to the bending stress therein 
     
     
        to produce a variable electrical resistive characteristic. 9. Apparatus as defined in claim 8 wherein the beam is configured to define an area of reduced cross section between the ends thereof thus to concentrate the bending stress, the strain gage resistor being disposed on the area of 
     
     
        reduced cross section. 10. Apparatus as defined in claim 9 wherein the flexure area of reduced cross section is substantially in the center of 
     
     
        the beam. 11. Apparatus as defined in claim 1 wherein the base comprises a substantially rectangular assembly of rigid material, the load receiving member comprises a pan and at least one force transmitting rail secured to the pan and extending longitudinally between the spaced pivot members, a pair of laterally extending elements, one of said elements at each end of the longitudinal rail, the force transmitting means comprising straps connected between the laterally extending members and the pivots at each end of the apparatus for producing a turning moment in the pivot when 
     
     
        force is applied to the pan. 12. Apparatus as defined in claim 11 wherein the beam is mechanically connected to the pivot members at the centers 
     
     
        thereof. 13. Apparatus as defined in claim 12 further including means connected between the base and the pan to prevent longitudinal relative 
     
     
        movement therebetween. 14. Apparatus as defined in claim 1 wherein the means for producing the output signal comprises four substantially similar strain gage resistors, two of said resistors being disposed on one side of the beam so as to respond to compression of the beam and the others of said resistors being disposed on the other side of the beam so as to respond to tension in the beam, said four resistors being electrically interconnected into a bridge circuit of such configuration as produce an output signal upon bending of said beam in such a way as to subject two of said resistors to compression forces and the other two of said resistors 
     
     
        to tension forces. 15. Apparatus as defined in claim 14 further including digital display means responsive to the signal produced by said bridge 
     
     
        circuit to produce a visual indication of the magnitude thereof. 16. Apparatus as defined in claim 14 further including at least two temperature responsive compensating resistors connected to opposite terminals of said bridge circuit to compensate the bridge circuit for 
     
     
        temperature variations. 17. In a weigh scale: a load pan, a pair of spaced members, means connecting the pan to said members to produce in said members turning moments of opposite sense and related to loads applied to the pan, a beam of rigid material connected between said spaced members and of substantially greater length than thickness, and means for detecting the extent of bending of said beam as an indication of the 
     
     
        magnitude of the load applied to the pan. 18. Apparatus as defined in claim 17 further including mechanical vibrator means for dithering the 
     
     
        weigh scale. .Iadd.19.  Floating beam weigh scale apparatus comprising: a base;   a load receiving member spaced from the base;   first and second elongated, torsionally rigid pivot members disposed between the base and the load receiving member;   a substantially rigid beam having its ends connected substantially centrally of and to the pivot members so that the beam extends between the pivot members;   load transmitting means spaced from the beam for interconnecting the pivot members, the base and the load receiving member to produce turning moments in said pivot members of opposite sense and about spaced axes, said turning moments being transmitted to the ends of the beam through the lengths of the pivot members to create bending stress in the beam without applying direct forces to the beam; and   means carried by the beam for producing a signal related to the bending stress therein over the elastic bending range thereof..Iaddend. .Iadd.20. Apparatus as defined in claim 19 wherein the load transmitting means comprises torsionally compliant straps connected in opposing pairs to the ends of the pivot members, one such strap of each pair being connected between the base and a pivot members, and another such strap of each pair being connected between the pivot member and the load receiving member, said pairs of straps operative to produce the turning moments in the pivot members. .Iaddend. .Iadd.21. In a weigh scale:   a load pan:   a pair of spaced elongated and torsionally rigid members;   a beam of rigid material having its ends connected substantially centrally of and between said spaced members and being of substantially greater length than thickness;   means spaced from the beam connecting the pan to said members to produce turning moments therein of opposite sense and related to loads applied to the pan, said turning moments being transmitted to the ends of the beam through said members to create bending stress in the beam without applying direct forces to the beam; and   means for detecting the extent of bending of said beam as an indication of the load applied to the pan. .Iaddend.