Digital load shift compensation

• US 4,799,558 A
• Filed: 06/12/1987
• Issued: 01/24/1989
• Est. Priority Date: 06/12/1987
• Status: Expired due to Term

• Current Assignee: Mettler-Toledo Incorporated (Mettler-Toledo International Inc.)
• Original Assignee: Toledo Scale Corp. (Mettler-Toledo LLC)
• Inventors: Griffen, Neil C.
• Primary Examiner(s): Miller, Jr., George H.
• Application Number:
  US07/061,273
• Time in Patent Office: 592 Days
• Field of Search: 177/25.14, 177/211, 177/255
• US Class Current: 177/25.14
• CPC Class Codes: G01G 23/01 Testing or calibrating of weighing apparatus
• CPC Class Codes: G01G 3/1408 the supports being of the column type, e.g. cylindric
• CPC Class Codes: G01G 3/1412 the supports being parallelogram shaped
• CPC Class Codes: G01G 7/06 by electrostatic action

Abstract

A digital weighing scale is compensated for the effects of off-center loading by determining the position of the load and calculating a corrected weight value according to such position. Position sensing transducers are mounted on the load cell to provide information regarding the position of the load on the weighing scale. An expression for corrected weight as a function of load position and magnitude is determined for the particular scale and stored. The expression is used to calculate corrected weight according to the magnitude and position of each weight placed on the scale. The load cell may include a counterforce of substantially any type including single bending beam, dual bending beam or capacitive structure, and the load magnitude and position sensors may be of substantially any type including strain gages and capacitive sensors.

Claims

1. Weighing apparatus compensated for load position comprising a counterforce, transducer means on said counterforce for producing outputs representing the magnitudes of loads applied to said counterforce, load position sensing means mounted on said counterforce to produce outputs representing the positions in at least one of two mutually transverse directions of loads applied to said counterforce, means for storing a mathematical expression for load compensated for load position as a function of load position and magnitude, and means for utilizing said expression and said load position outputs and said load magnitude outputs to provide load indications independent of the positions in at least said one direction of the loads applied to said counterforce.

2. Apparatus as claimed in claim 1 wherein said load position sensing means includes first and second sensing means for producing outputs representing load positions in two mutually transverse directions.

3. Apparatus as claimed in claim 2 wherein said expression is of the form

4. Apparatus as claimed in claim 1 wherein said mathematical expression includes one or more constants determined for the particular weighing apparatus.

5. Apparatus as claimed in claim 1 wherein said counterforce includes at least one member movable in proportion to loads applied to the counterforce, and said transducer means includes means mounted on said movable member responsive to movement of the movable member for providing an indication of the magnitude of the load applied to said counterforce.

6. Apparatus as claimed in claim 1 wherein said counterforce includes a pair of members relatively movable in response to loads applied to the counterforce, and wherein said load position sensing means includes at least one capacitor having a plate mounted on each of said relatively movable members.

7. Weighing apparatus as claimed in claim 6 wherein said relatively movable members include cantilever beams extending parallel to the longitudinal axis of said counterforce.

8. Weighing apparatus as claimed in claim 1 wherein said counterforce includes a pair of members relatively movable in response to loads applied to the counterforce, and wherein said transducer means includes a capacitor having a plate mounted on each of said relatively movable members.

9. Apparatus as claimed in claim 1 wherein said counterforce includes at least one beam member and said transducer means includes at least one strain gage mounted on said beam member to provide an output indicating the magnitude of loads applied to said beam member.

10. Apparatus as claimed in claim 9 wherein said position sensing means includes a first strain gage positioned on a surface of said beam to detect displacement of a load in a direction parallel to the longitudinal axis of said beam and a second strain gage positioned on a surface of the beam to detect displacement of a load in a direction transverse to the longitudinal axis of the beam.

11. Apparatus as claimed in claim 9 wherein said counterforce includes first and second parallel beams and means connecting said beams rigidly together at both ends to form a parallelogram structure.

12. Apparatus as claimed in claim 11 wherein all of said strain gages are mounted on the outer surface of one of said beams, and including means for calculating a load indication compensated for both load position and linearity of the load indication with changing magnitude of the load.

13. Apparatus as claimed in claim 1 wherein said transducer means includes strain gages mounted on said counterforce.

14. Apparatus as claimed in claim 1 including a term in said mathematical expression correcting for nonlinearities in the output of said transducer means, and wherein said means for utilizing said expression provides load indications corrected for said nonlinearities.

15. A method of load shift compensating a digital weighing scale having a load cell for producing a signal representing the magnitude of weights applied to the scale, means for converting said signal to a digital indication of weight and means for displaying said digital weight indication, comprising the steps of providing weight position sensing means on said load cell to produce signals representing the position of the weight in at least one of two mutually transverse directions, developing a mathematical expression for weight corrected for position as a function of the weight magnitude and position, and storing said expression for use in determining a weight indication corrected for position.

16. A method as claimed in claim 15 further comprising the step of including in said mathematical expression a term correcting for nonlinearities in said weight magnitude signal, and storing said expression for determining a weight indication corrected for both position and nonlinearity.

17. A method as claimed in claim 15 including the step of providing first and second weight position sensing means to produce signals representing the position of the weight in two mutually transverse directions.

18. A method of making a compensated weighing scale comprising the steps of providing a counterforce, mounting force transducer means thereon to provide an output representing the magnitude of loads applied to the counterforce, mounting position sensing means on said counterforce to provide output information representing the positions in at least one of two mutually transverse directions of loads applied to the counterforce, determining the response of said force transducer and said position sensing means to loads applied off-center on said counterforce, utilizing said responses to develop an expression for load corrected for the effects of off-center loading of the counterforce as a function of said load magnitude and position information, and storing said expression for use in calculating said corrected load.

19. A method as claimed in claim 18 further comprising the step of including in said expression a term correcting for nonlinearity in the output of said transducer means.

20. A method as claimed in claim 18 wherein said mathematical expression for corrected load includes one or more constants, and including the steps of determining the values of the constants for the particular weighing scale and storing said values for use in calculating corrected loads.

21. A method as claimed in claim 20 wherein the values of said constants are determined by positioning known loads at different locations on said counterforce, determining the responses of said force tranducer means and said position sensing means to said weights and using said responses and said expessions to determine the value of the constants.