Strain Gauge Load Cell

1. STRAIN GAUGE LOAD CELL
MANEEB MASOOD(M.Tech.15152010)
Mining Engineering Department
IIT(BHU), Varanasi

2. 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.

3. TYPICAL LOAD CELL

5. COMPRESSION LOAD CELL

6. FOIL STRAIN GAUGE

7. 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.

8. 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).

9. 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.

10.  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.

11. RESISTIVE STRAIN GAGE TYPE ANCHOR BOLT
LOAD CELL

12. RESISTIVE STRAIN GAGE TYPE ANCHOR BOLT
LOAD CELL

13. RESISTIVE STRAIN GAGE TYPE COMPRESSION
LOAD CELL

14. HIGH CAPACITY COMPRESSION LOAD CELL

15. 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.

16.  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.

17.  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.

18. Strain Gauge
Mechanical Force
Electrical
Signal
Signal
Conditioning
Calibration
Readout
• Electrical
• Optical
• Mechanical
• Voltage
• Current
• Potential
Divider
• Wheatstone
Bridge
Change
in
Property

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

20. 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

21. 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.

22. 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

23. WHEATSTONE BRIDGE CALCULATIONS
inout V
RR
R
RR
R
V 









21
2
43
4

24. APPLICATION IN THE FIELD

26. SPECIFICATIONS OF GEOKON MODEL 3000 LOAD
CELL

27. READ-OUT UNIT

28. Amplification and Digitization of Output

29. SENSITIVITY AND ACCURACY
 Sensitivity of load cells is the reciprocal of the
calibration factor(c)
Where λ is gauge factor
E is the modulus of elasticity
v is the Poisson’s Ratio
 AEc
cS
2/)1(
/1




30. 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

31. 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.

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

33. Thank you