Load CellsLoad CellsLeann ArmbrustLeann Armbrust

Travis WyattTravis WyattSRJC, Engr 45SRJC, Engr 45

Fall 2010Fall 2010

What is a Load Cell?What is a Load Cell?

A load cell is a transducer that is used to A load cell is a transducer that is used to convert a force into electrical signal.convert a force into electrical signal.

The most common The most common type is a type is a strain strain gaugegauge load cell. load cell.

Strain GaugeStrain Gauge

A Strain Gauge is a device A Strain Gauge is a device used to measure the strain used to measure the strain of an object.of an object.

The most common type The most common type of strain gauge consists of strain gauge consists of an insulating flexible of an insulating flexible backing which supports a backing which supports a metallic foil pattern.metallic foil pattern.

What Is It?What Is It?

RR

Strain GaugeStrain GaugeResistanceResistance

==ρρ llAA

Strain Gauge under Strain Gauge under tension.Resistance goes up.tension.Resistance goes up.

Strain Gauge under Strain Gauge under compression.Resistance goes compression.Resistance goes down.down.

Strain GaugeStrain Gauge

The gauge is attached to The gauge is attached to the object by a suitable the object by a suitable adhesive.adhesive.

As the object is deformed, As the object is deformed, the foil is deformed, the foil is deformed, causing its electrical causing its electrical resistance to change.resistance to change.

The resistance change is The resistance change is commonly measured commonly measured using a using a Wheatstone Wheatstone bridgebridge..

How Does It Work?How Does It Work?

Wheatstone BridgeWheatstone Bridge

A Wheatstone Bridge is A Wheatstone Bridge is an electrical circuit.an electrical circuit.

Used in a load cell to Used in a load cell to measure an overall measure an overall change in resistance. change in resistance.

Increases sensitivity Increases sensitivity and reduces the and reduces the affects of affects of temperature.temperature.

What Is It?What Is It?

VV00VVEXEX

RR44

RR33RR22

RR11

++--

Wheatstone BridgeWheatstone BridgeHow Does It Work?How Does It Work?

VV00VVEXEX

RR44

RR33RR22

RR11

++--

Load CellsLoad Cells

HysteresisHysteresis

Effects reduced by material selection.Effects reduced by material selection.

CreepCreep

Adhesive and geometry of gauge.Adhesive and geometry of gauge.

TemperatureTemperature

Wheatstone bridge, additional temperature Wheatstone bridge, additional temperature sensitive resistors in series with the bridge.sensitive resistors in series with the bridge.

Compensation for Compensation for ErrorError

Load CellsLoad Cells

ScalesScales

WeighbridgeWeighbridge

Force GaugesForce Gauges

Torque GaugesTorque Gauges

ApplicationsApplications

S TypeS Type

ButtonButtonCanisterCanister

BeamBeamShearShear

Load CellsLoad CellsTypesTypes

Load Cell ComparisonLoad Cell Comparison

Load Cell Load Cell TypeType

Weight Weight RangeRange

AccuraAccuracy cy

AppsApps StrengthStrength WeaknesWeaknesss

Bending Bending BeamBeam

10-5k lbs10-5k lbs 0.03%0.03%Tanks, Tanks,

platform platform scalesscales

Low cost, Low cost, Simples Simples

constructionconstruction

Strain gages Strain gages exposedexposed

Shear Shear BeamBeam

10-5k lbs10-5k lbs 0.03%0.03%

tanks, tanks, platform platform

scales,off-scales,off-center loadscenter loads

Better sealing Better sealing and protectionand protection

CanisterCanister to 500k lbsto 500k lbs 0.05%0.05%

Truck, tank, Truck, tank, track, track,

hopper hopper scalesscales

Handles load Handles load movementmovement

No horizontal No horizontal load load

protectionprotection

Ring & Ring & PancakePancake

5-500k lbs5-500k lbs Tanks, bins, Tanks, bins, scalesscales

All stainless All stainless steelsteel

No load No load movement movement

allowedallowed

Button & Button & WasherWasher

0-50k lbs0-50k lbs0-200lbs0-200lbs

typ.typ.1%1% Small scalesSmall scales Small Small

inexpensiveinexpensiveLoads must Loads must be centeredbe centered

Strain Gauge Load CellsStrain Gauge Load Cells

Load Cell Comparison Load Cell Comparison Cont.Cont.

Mechanical & Other Load CellsMechanical & Other Load Cells

Load Cell Load Cell TypeType

Weight Weight RangeRange

AccuraAccuracy cy

AppsApps StrengthStrength WeaknesWeaknesss

HydraulicHydraulicUp to Up to

10,000,000 10,000,000 lbslbs

0.25%0.25% Tanks, bins, Tanks, bins, & hoppers& hoppers

Takes high Takes high impacts, impacts,

insensitive to insensitive to temptemp

Expensive Expensive complexcomplex

PneumatiPneumaticc

WideWide HighHigh

food food industry, industry,

hazardous hazardous areasareas

Intrinsically safeIntrinsically safe

Slow Slow response. response. Require Require

clean, dry airclean, dry air

HelicalHelical 0-40k lbs 0-40k lbs 0.2%0.2%Platform, Platform, forklift, forklift,

wheel loadwheel load

Handles off-axis Handles off-axis loads, overloadsloads, overloads

Fiber Fiber OpticOptic

0.1%0.1%Electrical Electrical

transmission transmission cablescables

Immune to Immune to RFI/EMIRFI/EMI

Piezo-Piezo-resistiveresistive

0.03%0.03%

Extremely Extremely sensitive, high sensitive, high signal output signal output

levellevel

High cost, High cost, nonlinear nonlinear

outputoutput

Acknowledgement Acknowledgement

• Michael Wyatt, Mechanical Engineer and Michael Wyatt, Mechanical Engineer and Load Cell Specialist for providing demo Load Cell Specialist for providing demo Load Cell equipment and expertise.Load Cell equipment and expertise.

Special Thanks To:Special Thanks To:

ReferencesReferencesGeneral Info: http://en.wikipedia.org/wiki/Load_cellGeneral Info: http://en.wikipedia.org/wiki/Load_cell

Weighbridge: http://floorscale.com.cn/cheap-weighbridge/Weighbridge: http://floorscale.com.cn/cheap-weighbridge/

Digital Scale: http://www.scales-n-tools.com/Digital Scale: http://www.scales-n-tools.com/

Digital Torque Wrench: http://tohnichi.files.wordpress.com/2010/04/tohnichi-digital-Digital Torque Wrench: http://tohnichi.files.wordpress.com/2010/04/tohnichi-digital-torque-wrenchtorque-wrench

Load Cell Comparisons: http://www.omega.com/prodinfo/loadcells.htmlLoad Cell Comparisons: http://www.omega.com/prodinfo/loadcells.html

Pictures of Load Cells:Pictures of Load Cells:

Shear: http://www.process-controls.com/intertechnology/Revere_Transducers/Shear: http://www.process-controls.com/intertechnology/Revere_Transducers/images/SSB.gifimages/SSB.gif

Beam: http://www.tradeindia.com/selloffer/1616481/Bending-Beam-Load-Cell.htmlBeam: http://www.tradeindia.com/selloffer/1616481/Bending-Beam-Load-Cell.html

S-Type: http://metrology.burtini.ca/img/stype.jpgS-Type: http://metrology.burtini.ca/img/stype.jpg

Button: http://www.appmeas.co.uk/images/cdf-80x80.jpgButton: http://www.appmeas.co.uk/images/cdf-80x80.jpg

Canister: Canister: http://www.omega.co.uk/literature/transactions/volume3/images/loadpic3.jpghttp://www.omega.co.uk/literature/transactions/volume3/images/loadpic3.jpg

Compression/Tension: http://www.ishida.com/technologies/loadcell/html.htmlCompression/Tension: http://www.ishida.com/technologies/loadcell/html.html

Voltage Equation: http://zone.ni.com/devzone/cda/tut/p/id/3642Voltage Equation: http://zone.ni.com/devzone/cda/tut/p/id/3642

Resistivity Equation: http://en.wikipedia.org/wiki/ResistivityResistivity Equation: http://en.wikipedia.org/wiki/Resistivity