STRAIN GAUGE LOAD CELL

MANEEB MASOOD(M.Tech.15152010)Mining Engineering Department

IIT(BHU), Varanasi

INTRODUCTION A load cell is a transducer that is used to create an

electrical signal whose magnitude is directly proportional to the force being measured. The various types of load cells include hydraulic load cells, pneumatic load cells and strain gauge load cells.

The most common type is a strain gauge load cell.

TYPICAL LOAD CELL

COMPRESSION LOAD CELL

FOIL STRAIN GAUGE

WORKING PRINCIPLE The working principle is based on the

strain/resistance relationship of electrical conductors.

Any electrical conductor changes its resistance with mechanical stress, e.g. through tension or compression forces. The resistance change is partially due to the conductor's deformation and partially due to the change in the resistivity of the conductor material as a result of microstructural changes.

Operating Principle: Welded Sensor utilizes bonded strain gages

connected in Wheatstone bridge circuit. The output is derived from imbalance in the bridge circuit as load is sensed by sensor.

DESIGN & CONSTRUCTION The design should be such that it ensures a

uniform strain distribution over the gauge area with the gauges mounted at the maximum strain locations. This is to ensure the highest possible output.

Strain level induced in the gauge(s) at maximum rated load, usually design for a range in the gauge area. This maintains high gauge linearity and fatigue life.

Monolithic construction to improve repeatability and minimize hysteresis(time-based dependence of system’s output on present and past inputs).

DESIGN DETAILS(GEOKON) In the field of Rock Mechanics, the load cells

are basically used are primarily annular design. They are majorly used on tiebacks and rockbolts. They can also be used during pile load tests and monitoring loads in tunnel supports.

Load cells are made from an annulus of high strength steel or aluminium. Electrical resistance strain gauges are cemented around the outside of the annulus and connected to a Wheatstone bridge.

Half the gauges measure vertical strains, half gauges circumferential strain.

An outer shell protects the gauges from damage and rings on the either side of the gauges ensure that the load cell is water proof. The cable is attached to the cell through a waterproof gland.

A strain relief, in the form of a Kellem’s grip, prevets the cable from coming out.

Cables have thick PVC jackets.

RESISTIVE STRAIN GAGE TYPE ANCHOR BOLT LOAD CELL

RESISTIVE STRAIN GAGE TYPE ANCHOR BOLT LOAD CELL

RESISTIVE STRAIN GAGE TYPE COMPRESSION LOAD CELL

HIGH CAPACITY COMPRESSION LOAD CELL

DESIGN DETAILS(ROCTEST)

The load sensing element is a spool of high strength heat-treated steel or aluminum that withstands rough handling and loading.

Electrical resistance strain gauges are bonded to the periphery of the spool. The gauges are mounted in a full bridge configuration that compensates for unevenly distributed loads. High resistance strain gauges are used to minimize cable effects.

The load cells are compensated for temperature variations encountered during normal operations.

A steel housing with O-ring seals covers the spool and protects the strain gauges from mechanical damage and water infiltration.

A plain PVC cable is wired directly to the cell or is connected via a detachable multi-pin connector. On large cells, the cable exit is parallel to the surface of the steel housing to give better clearance.

When force is applied to any metallic wire its length increases due to the strain. The more is the applied force, more is the strain and more is the increase in length of the wire. If L1 is the initial length of the wire and L2 is the final length after application of the force, the strain is given as: ε =(L2-L1)/L1

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

Further, as the length increases, diameter decreases and hence, the resistance decreases.

The input and output relationship of the strain gauges can be expressed by the term gauge factor or gauge gradient, which is defined as the change in resistance R for the given value of applied strain ε.

The resistance change is commonly measured using a Wheatstone bridge.

Strain Gauge

Mechanical Force

Electrical

Signal

Signal Conditioning

Calibration

Readout

• Electrical• Optical• Mechanic

al

• Voltage• Current

• Potential Divider

• Wheatstone Bridge

Change

in

Property

Measurement can be done using a single wire also but we use one or more strain gauges in a Wheatstone’s bridge.

WHY?

What’s the Wheatstone Bridge?

• Wheatstone bridge is an electric circuit suitable for detection of minute resistance

changes, therefore used to measure resistance changes of a strain gage

• The bridge is configured by combining four resistors as shown in Fig.• Initially R1=R2=R3=R4, in this condition no

output voltage is there, e=0

• When one of the Resistances is replaced by strain

Gauge attached to the object whose strain is to be

measured and load is applied, then there is small

change in the resistance of gauge, hence some output

voltage is there which can be related to strain as

From this, strain can be easily determined using the relation

Full Bridge Configuration

To further enhance the sensitivity, all 4

resistances are replaced by strain gauges.

While this system is rarely used for strain

measurement, it is frequently applied to

strain-gage transducers. When the gages at

the four sides have their resistance changed

to R1 + ΔR1, R2 + ΔR2, R3 + ΔR3 and R4

+ ΔR4, respectively, the bridge output

voltage,

e, is

Or

Where K is the Gauge Factor.

Half Bridge ConfigurationTo increase the sensitivity of measurement, two strain gauges are connected in the bridge, this type of configuration is called as Half bridge as shown in fig. and the output voltage and strain can be related as

When gauges are connected to adjacent arms and

When gauges are connected to opposite arms

WHEATSTONE BRIDGE CALCULATIONS

inout VRR

RRR

RV

21

2

43

4

APPLICATION IN THE FIELD

SPECIFICATIONS OF GEOKON MODEL 3000 LOAD CELL

READ-OUT UNIT

Amplification and Digitization of Output

SENSITIVITY AND ACCURACY Sensitivity of load cells is the reciprocal of

the calibration factor(c)

Where λ is gauge factorE is the modulus of elasticityv is the Poisson’s Ratio

AEccS

2/)1(/1

POSSIBLE SOURCES OF ERROR IN STRAIN GAUGE LOAD CELL SIGNALS • Improper Loading and Orientation• Wrapping of bearing plates• Friction between bearing plate and load cell• Cross-sensitivity • Bonding faults • Hysteresis• Effects of moisture• Temperature change

LOAD CELLS COMPENSATION FOR ERROR

HysteresisEffects reduced by material selection.

CreepAdhesive and geometry of gauge.

TemperatureWheatstone bridge, additional temperature sensitive resistors in series with the bridge, with a dummy.

REFERENCES Geokon Manual( Model 3000 Load Cell) Websites: Geokon, Roctest

Thank you