LOAD CELLS Marc Daherg MECH 207 Fall Quarter 2009
Intro To Load Cells A load cell is a transducer that converts force into a measurable electrical output. Many sizes, styles, and types of load cells: Types: Strain Gauge (R/L/C), Hydraulic, Pneumatic Styles: Compression, Tension, S-Beam Load cells measure a wide range of force, from 25 grams to 3,000,000 s Will focus on most common type: resistive strain gauge g load cells M. Daherg, MECH 207, Fall 2009
Physical Principle Ohm s Law R=ρ ρ L/A Combining Ohm s Law with definition of strain ε: R/R = (1+2ν)ε + ρ/ρ = Gε Strain Gauge First term: Under strain, wire changes dimension, and thus the resistance changes. Dominant for metals. Second term: change in resistivity due to the change in the crystal lattice of the material under strain (piezoresistive effect). Dominant in semiconductors (but expensive). Foils/filaments inside the strain gauge g are ~1/1000 th inch diameter, made up of basic metal conductors. M. Daherg, MECH 207, Fall 2009
Load Cell Implementation Change in resistivity under strain is linear when R/R / is less than 1% Small V (mv level) Wheatstone Bridge Circuit is used with a strain gauge as one or more of its resistors: Instrumentation Amplifier Applied force causes small change in resistance in strain gauge g change in output voltage across bridge circuit. Output voltage from bridge circuit is amplified using an instrumentation amplifier, usually to 0-5 or 0-10 V range. Circuitry housed in a mechanical device (the load cell casing itself). Algorithms determine actual force based on output voltage (usually outside load cell).
Common Applications Industrial Weighing Applications Stress Measurement Under Various Traffic Conditions Maintaining Liquid Levels in Tanks Personal Experience: Nozzle Simulator Testing Truck Scales M. Daherg, MECH 207, Fall 2009
Challenges Measurement ringing when subjected to abrupt load changes due to spring-like behavior of the strain gauge. Solution: passive elements (filters). Temperature sensitivity of the strain gauge. Solution: Temperature resistive circuitry. Exceeding the safe load limit of load cells can cause permanent damage. Solution: Taking environmental shock & vibration into account during design. Off-center loading g( (installation & application). M. Daherg, MECH 207, Fall 2009
TYPE OF LOAD CELL Mechanical Cells Hydraulic WEIGHT RANGE Up to 10,000,000 Load Cell Performance Comparison ACCURACY (FS) APPLICATIONS ADVANTAGES DISADVANTAGES 0.25% Tanks, bins and hoppers. Hazardous areas. Pneumatic Wide High Food industry, hazardous areas Strain Gage Cells Bending Beam 10-5,000 Shear Beam 10-5,000 Canister to 500,000 Ring and Pancake Button and washer Other Types 5-500,000 0-50,000 0-200 typ. Helical 0-40,000 0.03% Tanks, platform scales, 0.03% Tanks, platform scales, off- center loads 0.05% Truck, tank, track, and hopper scales Tanks, bins, scales Takes high impacts, insensitive to temperature. Intrinsically safe. Contains no fluids. Low cost, simple construction High side load rejection, better sealing and protection Handles load movements All stainless steel 1% Small scales Small, inexpensive 0.2% Platform, forklift, wheel load, automotive seat weight Fiber optic 0.1% Electrical transmission cables, stud or bolt mounts Handles offaxis loads, overloads, shocks Immune to RFI/EMI and high temps, intrinsically safe Piezoresistive 0.03% Extremely sensitive, high signal output level Expensive, complex. Slow response. Requires clean, dry air Strain gages are exposed, require protection No horizontal load protection No load movement allowed Loads must be centered, no load movement permitted High cost, nonlinear output
Load Cells Price Comparison Transducer Techniques HFG SERIES FORCE GAUGE DIGITAL / COMPACT / PEAK CAPTURE HAND-HELD FORCE GAUGE CAPACITY RANGES: 11 THROUGH 110 LBS. $395 GSO SERIES GRAM LOAD CELL PRECISION GRAM SENSOR LOAD CELL CAPACITY RANGES: 10 THROUGH 1,000 GRAMS. $580 to $625 MDB SERIES LOAD CELL ULTRA PRECISION MINI LOAD CELL CAPACITY RANGES: 2.5 THROUGH 100 LBS. $445 MLP SERIES LOAD CELL MINI LOW PROFILE LOAD CELL CAPACITY RANGES: 10 THROUGH 1,000 LBS. $295 to $395 SLB SERIES LOAD CELL SUB MINATURE COMPRESSION ONLY LOAD BUTTON LOAD CELL CAPACITY RANGES: 25 THROUGH 1,000 LBS. $395 LBO SERIES LOAD CELL LOW PROFILE COMPRESSION ONLY LOAD BUTTON LOAD CELL CAPACITY RANGES: 100 THROUGH 50,000 LBS. $355 to $545 LBC SERIES LOAD CELL LOW PROFILE COMPRESSION ONLY LOAD BUTTON LOAD CELL CAPACITY RANGES: 100 THROUGH 50,000 LBS. $355 to $545 LBM SERIES LOAD CELL LOW PROFILE COMPRESSION ONLY LOAD BUTTON LOAD CELL WITH MOUNTING PROVIDED CAPACITY RANGES: 50 THROUGH 10,000 LBS. $450
DSM SERIES LOAD CELL DUAL STUD MOUNT LOAD CELL CAPACITY RANGES: 50 THROUGH 10,000 LBS. $540 TLL SERIES LOAD CELL ECONOMICAL TENSION LOAD CELL FOR ACCURATE IN-LINE FORCE MEASUREMENT CAPACITY RANGES: 500 THROUGH 50K LBS. $265 to $690 SBO SERIES LOAD CELL PRECISION LOAD CELL CAPACITY RANGES: 50 THROUGH 5,000 LBS. $325 to $525 SWO SERIES LOAD CELL A COMPACT ACCURATE LOAD CELL CAPACITY RANGES: 1,000 THROUGH 50,000 LBS. $395 to $875 HSW SERIES LOAD CELL HERMETICALLY SEALED LOAD CELL CAPACITY RANGES: 1,000 THROUGH 50,000 LBS. $525 to $1,025 LPO SERIES LOAD CELL ULTRA LOW PROFILE LOAD CELL CAPACITY RANGES: 500 THROUGH 20,000 LBS. $575 to $950 LPU SERIES LOAD CELL LOW PROFILE UNIVERSAL TENSION OR COMPRESSION LOAD CELL CAPACITY RANGES: 100 THROUGH 50,000 LBS. $550 to $770 SWP SERIES LOAD CELL PRECISION FATIGUE RESISTANT LOAD CELL CAPACITY RANGES: 1,000 THROUGH 50,000 LBS. $795 to $2,125
Detailed Load Cell Amplifier Diagram The following two pages contain excerpts from the technical specifications for the following load cell: Honeywell Load Cell Model 41
Model 41 Precision Low Profile Load Cell DESCRIPTION Model 41 is a low profile pancake type load cells. These bonded foil, strain gage load cells are engineered to measure loads from 5 to 500,000. The tension/ compression Model 41 is designed with the threaded hole running completely through the center of the cell. Model 41 utilizes two stabilizing diaphragms, which are welded to the sensing member to reduce off-center and side-loading effects. It provides high performance in non-linearity, hysteresis, and repeatability specifications for such applications as tube mills, extruding processes and weighing. Each unit has a welded construction and can be hermetically sealed for added durability. Model 41 load cells are available with optional 0 Vdc to 5 Vdc or 4 ma to 20 ma output. Approved Approved Approved Intrinsically safe amp FEATURES 0.1 % accuracy 5 to 500000 mv/v output (standard); 4 ma to 20 ma and 0 Vdc to 5 Vdc (optional) outputs Double diaphragm design Intrinsically safe available (2N option only) 16 CE approved 17
Model 41 PERFORMANCE SPECIFICATIONS Characteristic Load ranges 18 Non-linearity, 5 to 25 Hysteresis, 50 to 500000 Hysteresis, 5 to 25 Hysteresis, 50 to 500000 Non-repeatability, 5 to 25 Non-repeatability, 50 to 500000 Output (tolerance), 5 to 25 Output (tolerance), 50 to 500000 Measure 5 to 500000 ±0.2 % full scale ±0.1 % full scale ±0.1 % full scale ±0.08 % full scale ±0.1 % full scale ±0.03 % full scale 2 Honeywell Sensing and Control 2 mv/v ±0.5 % full scale 3 mv/v ±0.5 % full scale Operation Compression/tension 13 Resolution Standard calibration Infinite ENVIRONMENTAL SPECIFICATIONS Characteristic Temperature, operating Temperature, compensated Temperature effect, zero Temperature effect, span 5-point calibration: 0 %, 50 % and 100 % of full scale in tension only Measure ELECTRICAL SPECIFICATIONS Characteristic Strain gage type Excitation (calibration) Insulation resistance Bridge resistance (tolerance) Zero balance (tolerance) Shunt calibration data Electrical termination (std) 5 to 5000 Electrical termination (std) 7500 to 500000 Mating connector 5 to 5000 (not incl.) Mating connector 7500 to 500000 (not incl.) -54 C to 121 C [-65 F to 250 F] 15 C to 71 C [60 F to 160 F] 0.002 % full scale/ F 0.002 % full scale/ F Measure Bonded foil 10 Vdc 5000 mohm @ 50 Vdc 350 ohm (nominal) ±1 % full scale Included PTIH-10-6P MS3102E-14S-6P PT06A-10-6S or equiv. (AA111) MS3106A-14S-6S (AA121) MECHANICAL SPECIFICATIONS Characteristic Measure Maximum allowable load 150 % FS 1 Weight Material Less than 200,000 Material greater than or equal to 300000 Deflection Natural frequency RANGE CODES Range Code Available ranges See table 17-4PH stainless steel Carbon steel See table See table Range Code AT 5 DV 10000 AV 10 EJ 15000 BL 25 EL 20000 BN 50 EN 30000 BR 100 EP 50000 CN 250 ER 75000 Available ranges CR 500 ET 100000 CV 1000 FJ 150000 DL 2000 FL 200000 DN 3000 FN 300000 DP 4000 FP 400000 DR 5000 FR 500000 DT 7500 WIRING CODES Connector A B C D E F Unamplified (Std.) (+) excitation (+) excitation (-) excitation (-) excitation (-) output (+) output DEFLECTIONS AND RINGING FREQUENCIES Capacity () Deflection @ full scale (in) Natural ringing frequency (Hz) 5 to 25 0.001 2000 0.8 50 to 1000 0.002 4600 1.5 2000 to 5000 0.002 10000 2.0 7500 to 15000 0.003 6000 8.8 Weight () 20000 to 50000 0.004 8000 11.0 75000 to 100000 0.006 5500 30.9 150000 to 200000 0.010 4500 46.3 300000 to 500000 0.010 4100 130.1
Precision Low Profile Load Cell MOUNTING DIMENSIONS Ranges D mm H mm H*** mm F# ØG mm B.C. ØK mm thru T A mm A* mm B mm B* mm C mm 5, 10, 25 63,5 [2.50] 20,32 [0.80] 44,45 [1.75] 6 50,8 [2.000] 4,83 [0.19] 1/4-28 UNF 20,83 [0.82] 63,5 [2.5] 19,05 [0.75] 22,86 [0.9] 31,75 50, 100, 250, 500, 1000 76,2 [3.00] 25,4 [1.00] 44,45 [1.75] 6 57,15 [2.250] 7,11 [0.28] 3/8-24 UNF 20,83 [0.82] 63,5 [2.5] 19,05 [0.75] 22,86 [0.9] 31,75 2000, 3000, 4000, 5000 88,9 [3.50] 25,4 [1.00] 44,45 [1.75] 6 66,68 [2.625] 8,64 [0.34] 1/2-20 UNF 20,83 [0.82] 63,5 [2.5] 19,05 [0.75] 22,86 [0.9] 31,75 7500, 10000, 15000 139,7 [5.50] 45,7 [1.80] 45,7 [1.80] 8 114,3 [4.500] 10,16 [0.40] 1-14 UNS 31,75 58,42 [2.3] [1.50] [1.5] 50,8 [2.00} 20000, 30000, 50000 152,4 [6.00] 45,7 [1.80] 45,7 [1.80] 8 123,83 [4.875] 13,46 [0.53] 1 1/2-12 UNF 31,75 58,42 [2.3] [1.50] [1.5] 50,8 [2.00} 75000, 100000 228,6 [9.00] 63,5 [2.50] 63,5 [2.50] 12 196,85 [7.750] 16,76 [0.66] 2-12 UN 31,75 58,42 [2.3] [1.50] [1.5] 50,8 [2.00} 150000, 200000 279,4 [11.00] 63,5 [2.50] 63,5 [2.50] 12 241,3 [9.500] 19,81 [0.78] 2 1/2-12 UN 31,75 58,42 [2.3] [1.50] [1.5] 50,8 [2.00} 300000, 400000, 500000 355,6 [14.00] 107,95 [4.25] 107,95 [4.25] 12 298,45 [11.750] 26,16 [1.03] 3 1/2-8 UN 31,75 58,42 [2.3] [1.50] [1.5] ** * Length of load cell with amplified option (see option codes) ** C dimension varies on high ranges. Consult factory *** H dimension with 2n(2N) amplifier TYPICAL SYSTEM DIAGRAM Honeywell Sensing and Control 5
Load Cell Suppliers (listed alphabetically) Cooper Instruments www.cooperinstruments.com Futek www.futek.com Honeywell www.honeywell.com Loadstar Sensors www.loadstarsensors.com Omega www.omega.com RDP Group www.rdpelectronics.com Sensing Systems Corporation www.sensing-systems.com Sensor Land www.sensorland.com Transducer Techniques www.transducertechniques.com References 1) Section 11.18: Strain Gauges, pgs. 430-434. Applied Mechatronics. Smaili, A. & Mrad, F. Oxford University Press, 2008. 2) Load Sensors. Load Star Sensors. www.loadstarsensors.com/technology.html 3) Introduction to Load Cells, Load Cell Home Page, Omega Engineering Technical Reference. www.omega.com/prodinfo/loadcells.html 4) Load Cell Terminology. Transducer Techniques. www.transducertechniques.com/load-cell-terminology.cfm 5) Load Cells. Honeywell Test & Measurement. http://content.honeywell.com/sensing/sensotec/loadcell.asp 6) How It Works Strain Gauge Load Cell. RDP Group. www.rdpelectronics.com/ex/hiw-sglc.htm 7) How They Work: The Strain Gauge. Sensor Land. www.sensorland.com/howpage002.html 8) Load Cell Technology In Practice. Aerocon Aerospace Systems. www.aeroconsystems.com/electronics/load_cell_primer.pdf