Bending beam load cell with torque sensitivity compensation
A bending beam load cell can be compensated for side-to-side off center load sensitivity by simple electrical adjustments if a pair of shear sensing strain gages are bonded to each bending beam midway between axial strain gages used to measure bending strains. The shear sensing strain gages measure torque on the load cell, and are incorporated in bridge circuits that make it possible to vary the amount of torque sensitivity correction by changing the value of a trimming resistor. The bridge circuits also include circuit components for compensation of front-to-back off center load error and for zero adjustment. Four strain gages on each bending beam can be part of a single composite strain gage element, so the shear sensing strain gages do not add any cost to the load cell. Such a load cell can also be hermetically sealed before any compensation of offset load errors is done.
1. A bending beam load cell, comprising:
axially oriented strain gages sensing tension and compression strains connected in a main bridge circuit having two power input terminals and two signal output terminals providing an output signal proportional to a load on the load cell;
a shear sensing strain gage sensing torque on said load cell; and
means for connecting said torque sensing strain gage in series with one of said axially oriented strain gages to compensate for torque sensitivity in said output signal.
2. A bending beam load cell according to claim 1 , wherein a trimming resistor is shunting said one shear sensing strain gage.
3. A bending beam load cell according to claim 1 , wherein at least two adjacent bridge arms contain one axially oriented strain gage and one shear sensing strain gage arranged so that the shear sensing strain gages form a half bridge at one bridge diagonal, and a first trimming resistor is connected across said half bridge.
4. A bending beam load cell according to claim 3 , further comprising a pair of second trimming resistors connected between said power input terminals and one of said output terminals for compensation of front-to-back off center load error.
5. A bending beam load cell according to claim 4 , wherein all of said load cell is hermetically sealed, and said first and second trimming resistors are connected to the strain gage bridge circuit via terminals in said hermetic seal.
6. A strain gage load cell, comprising:
two parallel bending beams arranged to be cantilevered from a base and to have a vertical load act on the free ends of said cantilevered bending beams;
axially oriented strain gages bonded to said bending beams to sense tension and compression strains near the ends of said bending beams and arranged in a main strain gage bridge having a pair of power supply terminals end a pair of signal terminals providing an output signal proportional to load;
strain gages bonded to said bending beams at 45° angles to the axis of each bending beam to sense strains caused by torque on said load cell; and
means for combining signals from said torque sensing strain gages with said output signal to compensate for torque sensitivity in said output signal.
7. A strain gage load cell according to claim 6 , wherein said strain gages oriented at 45° are connected in a separate bridge circuit having an output signal dependent on a side-to-side off center load on said load cell, and means for combining the output signal from said separate bridge with the output signal from said main strain gage bridge.
8. A strain gage load cell according to claim 6 , wherein at least one bridge arm in said main strain gage bridge contains one axially oriented strain gage and one strain gage oriented at 45°, and a trimming resistor is shunting bald one strain gage oriented at 45°.
9. A strain gage load cell according to claim 6 , wherein at least two adjacent bridge arms in said main strain gage bridge contain one axially oriented strain gage and one strain gage oriented at 45° arranged so that the strain gages oriented at 45° form a half bridge at one bridge diagonal, and a first trimming resistor is connected across said half bridge.
10. A strain gage load cell according to claim 9 , further comprising a pair of second trimming resistors connected between said power supply terminals and one of said signal terminals for compensation of front-to-back off center load error.
11. A strain gage loud cell according to claim 10 , wherein all of said load cell is hermetically sealed, and said first and second trimming resistors are connected to the strain gage bridge circuit via terminals in said hermetic seal.
12. A bending beam load cell according to claim 6 , wherein at least two bridge arms adjacent to a power supply terminal in said main strain gage bridge contain one axially oriented strain gage and one strain gage oriented at 45° arranged so that the strain gages oriented at 45° form a half bridge centered at said power supply terminal, and a first trimming resistor connected across said half bridge.
13. A bending beam load cell according to claim 12 , wherein all four strain gages in said two bridge aims are contained in one composite strain gage element.
14. A bending beam load call according to claim 12 , further comprising a pair of second trimming resistors connected between said power supply terminals and one of said signal terminals for compensation of front-to-back off center load error.
15. A bending beam load cell according to claim 14 , wherein all of said load cell is hermetically sealed, and said first and said second trimming resistor are connected to the strain gage bridge circuit via terminals in said hermetic seal.