OAN551 SENSORS AND TRANSDUCERS

Kurumbapalayam(Po), Coimbatore – 641 107

Accredited by NAAC-UGC with ‘A’ Grade

Approved by AICTE & Affiliated to Anna University, Chennai

Department Of Electrical and Electronics

SNS COLLEGE OF ENGINEERING

Presented by,

Ms. S. Jenita AP/EEE

UNIT III

STRAIN GAUGE LOAD CELL

INTRODUCTION

A load cell is a transducer that is used to create anelectrical signal whose magnitude is directly proportionalto the force being measured. The various types of loadcells include hydraulic load cells, pneumatic load cells andstrain 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/resistancerelationship of electrical conductors.

Any electrical conductor changes its resistance with mechanicalstress, e.g. through tension or compression forces. Theresistance change is partially due to the conductor'sdeformation and partially due to the change in the resistivity ofthe conductor material as a result of microstructural changes.

Operating Principle:

Welded Sensor utilizes bonded strain gages connected inWheatstone bridge circuit. The output is derived fromimbalance 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 ringson the either side of the gauges ensure that the load cell iswater proof. The cable is attached to the cell through awaterproof gland.

A strain relief, in the form of a Kellem’s grip, prevets thecable 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

SignalConditioning

Calibration

Readout

• Electrical

• Optical

• Mechanical

• 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 Configuration

To 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

inVVout

R R R

R

2 1

R2

4 3

R4

APPLICATION IN THE FIELD

SPECIFICATIONS OF GEOKON MODEL 3000 LOAD

CELL

READ-OUT UNIT

Amplification and Digitization of Output

/ 2A

SENSITIVITY AND ACCURACY

Sensitivity of load cells is the reciprocal of the

calibration factor(c)

S 1/c

c (1 )E

Where λ is gauge factor

E is the modulus of elasticity

v is the Poisson’s Ratio

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

Hysteresis

Effects reduced by material selection.

Creep

Adhesive and geometry of gauge.

Temperature

Wheatstone bridge, additional temperature sensitive

resistors in series with the bridge, with a dummy.