Optimizing Load Cell Performance

2,878 views

Published on

This paper explores the fundamental characteristics of medium to high capacity load cells and how they are affected by the types and implementation of strain gages.

Published in: Business, Technology
0 Comments
3 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
2,878
On SlideShare
0
From Embeds
0
Number of Embeds
1,343
Actions
Shares
0
Downloads
82
Comments
0
Likes
3
Embeds 0
No embeds

No notes for slide

Optimizing Load Cell Performance

  1. 1. 1 Optimizing Strain GageOptimizing Strain Gage Load CellLoad Cell PerformancePerformance by LaVar Cleggby LaVar Clegg Interface, Inc.Interface, Inc. Western Regional Strain Gage CommitteeWestern Regional Strain Gage Committee Conference September 13, 2011Conference September 13, 2011
  2. 2. 2 SummarySummary  This paper explores the fundamentalThis paper explores the fundamental characteristics of medium to high capacitycharacteristics of medium to high capacity load cells and how they are affected byload cells and how they are affected by the types and implementation of strainthe types and implementation of strain gagesgages  Guidance for successful application ofGuidance for successful application of load cellsload cells
  3. 3. 3 Load Cell Types RepresentedLoad Cell Types Represented Low Profile, center thread Low Profile, flange mount Single Column Hollow column
  4. 4. 4 Performance FeaturesPerformance Features  1. Linearity over measuring range1. Linearity over measuring range  2. Hysteresis (descending loads)2. Hysteresis (descending loads)  3. SEB (why is it useful ?)3. SEB (why is it useful ?)  4. Output symmetry4. Output symmetry  5. Rejection of extraneous loads5. Rejection of extraneous loads  6. Sensitivity to mounting6. Sensitivity to mounting
  5. 5. 5 1. Linearity1. Linearity  A.A. Important because load cell non-Important because load cell non- linearity represents system error when thelinearity represents system error when the instrumentation is linear, as it typically isinstrumentation is linear, as it typically is  B.B. Only smooth calibration curves can beOnly smooth calibration curves can be corrected by compensation in thecorrected by compensation in the instrumentationinstrumentation
  6. 6. 6 Low Profile Linearity ExampleLow Profile Linearity Example  We use a 10,000 lbf low profile type loadWe use a 10,000 lbf low profile type load cell to examine excellent linearitycell to examine excellent linearity behavior over a wide measuring rangebehavior over a wide measuring range
  7. 7. 7 Low Profile ConstructionLow Profile Construction  Shear beam gage shownShear beam gage shown Load Surface Shear beam Gage Base
  8. 8. 8 Compression CalibrationCompression Calibration
  9. 9. 9 10K Compression (cont’d)10K Compression (cont’d)  a. Nonlinearity is relatively low 0.02%FSa. Nonlinearity is relatively low 0.02%FS  b. Calibration points fit a polynomial curveb. Calibration points fit a polynomial curve very closelyvery closely  c. Curve is smooth clear down to zero loadc. Curve is smooth clear down to zero load (tested over 10 to 10,000 lbf range)(tested over 10 to 10,000 lbf range)
  10. 10. 10 Expanded Scale (10X magnified)Expanded Scale (10X magnified)
  11. 11. 11 Comparable behavior in tensionComparable behavior in tension
  12. 12. 12 Expanded Scale (10X magnified)Expanded Scale (10X magnified)
  13. 13. 13 Column Cell LinearityColumn Cell Linearity  a.a. Nonlinearity relativelyNonlinearity relatively large due to the expansionlarge due to the expansion or contraction of theor contraction of the column diameter with loadcolumn diameter with load  b.b. But well-behaved smoothBut well-behaved smooth calibration curves normallycalibration curves normally fitting a 2fitting a 2ndnd degreedegree polynomialpolynomial  c.c. Tension and compressionTension and compression opposite polarity of non-opposite polarity of non- linearitylinearity
  14. 14. 14 2160-1000 kN (225 Klbf) Example2160-1000 kN (225 Klbf) Example Tension NL = - 0.073%FS Compression NL = +0.053%FS
  15. 15. 15 Strain Gage InfluenceStrain Gage Influence  The preceding example load cell wasThe preceding example load cell was made with modified-Karma alloy gagesmade with modified-Karma alloy gages  Constantan alloy gages tend to produceConstantan alloy gages tend to produce higher nonlinearity, about 0.10%FS, in ourhigher nonlinearity, about 0.10%FS, in our experienceexperience  However, a generalization should not beHowever, a generalization should not be made without considering all effects ofmade without considering all effects of geometry and transverse gage factorgeometry and transverse gage factor
  16. 16. 16 Flange Mount Low Profile LinearityFlange Mount Low Profile Linearity  a.a. Nonlinearity relatively low, benefiting from the solidNonlinearity relatively low, benefiting from the solid hubhub  b.b. But well-behaved smooth calibration curves, fit 2But well-behaved smooth calibration curves, fit 2ndnd or 3or 3rdrd degree polynomialdegree polynomial  c.c. Tension – compression symmetry is excellentTension – compression symmetry is excellent
  17. 17. 17 1238 - 250 kN Flange Mount Example1238 - 250 kN Flange Mount Example Tension NL = - 0.008%FS Compression NL = - 0.010%FS
  18. 18. 18 Hollow Column LinearityHollow Column Linearity  a.a. Nonlinearity slightly better than single columnNonlinearity slightly better than single column  b.b. But well-behaved smooth calibration curves, fit 3But well-behaved smooth calibration curves, fit 3rdrd degree polynomialdegree polynomial  c.c. Tension – compression symmetry is goodTension – compression symmetry is good
  19. 19. 19 2350 - 2000 kN Hollow Column2350 - 2000 kN Hollow Column ExampleExample Tension NL = - 0.070%FS Tension NL = - 0.032%FS
  20. 20. 20 2. Hysteresis2. Hysteresis  A.A. Often misunderstoodOften misunderstood  B.B. Descending calibration points are validDescending calibration points are valid only for the particular FS value of a testonly for the particular FS value of a test  C.C. Nevertheless, the measure ofNevertheless, the measure of hysteresis has value as an indicator ofhysteresis has value as an indicator of the range of error to expect from loadthe range of error to expect from load points that do not necessarily ascendpoints that do not necessarily ascend from zerofrom zero
  21. 21. 21 Example of good hysteresis behaviorExample of good hysteresis behavior  Same 10,000 lbf low profile type load cellSame 10,000 lbf low profile type load cell we examined for linearitywe examined for linearity  Descending curve as well behaved asDescending curve as well behaved as the ascending curvethe ascending curve
  22. 22. 22 Smooth descending curveSmooth descending curve  H = +0.03%FS and descending curve closes atH = +0.03%FS and descending curve closes at zero load. Closure requires well behavedzero load. Closure requires well behaved hysteresis and very low creephysteresis and very low creep
  23. 23. 23 Expanded Scale (10X magnified)Expanded Scale (10X magnified)
  24. 24. 24 Many levels of performanceMany levels of performance  In calibrating load cells from manyIn calibrating load cells from many manufacturers around the world, it is seenmanufacturers around the world, it is seen that not all are as well-behaved as thethat not all are as well-behaved as the preceding examples of Interface cellspreceding examples of Interface cells  The differences are in the subtleties ofThe differences are in the subtleties of design and quality controldesign and quality control
  25. 25. 25 interface Example of lower quality load cellExample of lower quality load cell (not an Interface load cell)(not an Interface load cell) A. Nonlinear near zero load B. High hysteresis C.Non-closure of zero return indicates high creep D.Takes a 4th degree polynomial to fit a curve
  26. 26. 26 3. SEB3. SEB  Static Error Band (SEB) is often misunderstood. The SEBStatic Error Band (SEB) is often misunderstood. The SEB output line provides a single slope calibration constant thatoutput line provides a single slope calibration constant that minimizes error on average over a force range.minimizes error on average over a force range. Demonstration of SEB vs Terminal Output -0.060 -0.050 -0.040 -0.030 -0.020 -0.010 0.000 0.010 0.020 0 20 40 60 80 100 Load (%FS) Errorfromstraightline(%FS) Data points Terminal Output Line SEB Output Line Ascending Descending
  27. 27. 27 4. Output Symmetry4. Output Symmetry  Important when both tension and compressionImportant when both tension and compression loadings use the same instrumentation gainloadings use the same instrumentation gain  Generally, low profile shear cells better thanGenerally, low profile shear cells better than column cellscolumn cells  Symmetry Error of our example cells:Symmetry Error of our example cells: Low Profile 10 Klbf 0.01%Low Profile 10 Klbf 0.01% LP Flange 200 kN 0.03%LP Flange 200 kN 0.03% Hollow column 2000 kN 0.05%Hollow column 2000 kN 0.05% Single Column 1000 kN 0.25%Single Column 1000 kN 0.25%
  28. 28. 28 5. Rejection of extraneous loads5. Rejection of extraneous loads  It is desired to measure FzIt is desired to measure Fz  Fx, Fy, Mx, My, Mz are extraneousFx, Fy, Mx, My, Mz are extraneous Fz Fx Fy Mz Mx My
  29. 29. 29 Axial Load vs. Eccentric LoadsAxial Load vs. Eccentric Loads Axial EccentricAxial Eccentric
  30. 30. 30 Method of testing eccentric load sensitivityMethod of testing eccentric load sensitivity  Sensitivity of lessSensitivity of less than 0.1% / inchthan 0.1% / inch is achieved onis achieved on shear low profileshear low profile type cellstype cells A force is applied on a moment arm while monitoring load cell output
  31. 31. 31 Eccentric adjustment exampleEccentric adjustment example 1020-25K Eccentric Load Plot -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0 30 60 90 120 150 180 210 240 270 300 330 360 Position Error(%/inch) Initial Adjusted
  32. 32. 32 6. Sensitivity to mounting6. Sensitivity to mounting  Degrees of reducing installation influenceDegrees of reducing installation influence Basic cell Factory- installed base Factory- installed stud Integral machined stud D e c r e a s i n g m o u n t i n g s e n s i t i v i t y
  33. 33. 33 Preloading large threadsPreloading large threads  Challenging, as in this 10MN wire rope testChallenging, as in this 10MN wire rope test 10 MN (2.2 Million Pound) load cell
  34. 34. 34 Advantage of flange load cellsAdvantage of flange load cells  Screws can be installed with conventionalScrews can be installed with conventional torque wrenches or hydraulic torquetorque wrenches or hydraulic torque wrencheswrenches
  35. 35. 35 Example of mounting sensitive load cellExample of mounting sensitive load cell (not an Interface cell)(not an Interface cell) A. Nonlinear near zero load B. High hysteresis C.Requires hard bearing plates D.No polynomial fit 100 Klbf
  36. 36. 36 Finite Element Analysis of MountingFinite Element Analysis of Mounting  Performance is always dependent upon fixation of the load cell to itsPerformance is always dependent upon fixation of the load cell to its live end and dead end structures. Here in this cutaway view thelive end and dead end structures. Here in this cutaway view the screw clamping force is being analyzed.screw clamping force is being analyzed.
  37. 37. 37 Strain Gage AlignmentStrain Gage Alignment Alignment of gages is critical to rejection ofAlignment of gages is critical to rejection of extraneous loads. Here the gage can be seenextraneous loads. Here the gage can be seen well-aligned with the precisely applied scribe linewell-aligned with the precisely applied scribe line
  38. 38. 38 Tension LinkTension Link  Another type of high capacity load cellAnother type of high capacity load cell  Load is measured between clevis pins inLoad is measured between clevis pins in the ends of the cellthe ends of the cell
  39. 39. 39 Tension Link GagingTension Link Gaging  Strain gages are strategically placed toStrain gages are strategically placed to sense tensile stress in the web sectionsense tensile stress in the web section
  40. 40. 40 ConclusionConclusion  Medium to high capacity load cells areMedium to high capacity load cells are successfully made in a variety of stylessuccessfully made in a variety of styles  The appropriate style is determined by theThe appropriate style is determined by the demands of the applicationdemands of the application  Measurement performance andMeasurement performance and environmental needs can be met throughenvironmental needs can be met through good engineering design andgood engineering design and manufacturing of load cellsmanufacturing of load cells

×