Strain Gauge y, Sandip Nair
[email protected] sandipnair.hpage.com
What is strain gauge? • Principle: - Change in resistance of metallic wire
in response to strain produced • It is used to sense the mechanical strain produced on a body electrical signal • The strain produced is converted into equivalent change in resistance, capacitance or charge
Classification According to the type: • Resistance (all metals) • Semiconductor • Capacitive
Types of strain gauges: • Bonded • Unbonded • Blended metal foil
•
•
deposition • Thin metal deposition
film
by
sputter
Unbonded Strain Gauge • Unbonded strain gauge consists of a piece of wire
stretched in a multiple folds between a pair or more insulated pins fixed to a movable members of a body or even even a sin sin le flexi flexibl blee membe memberr when when strai straine ned d the the wire also gets strained • These are used in preloaded conditions not to allow the ‘strings’ ‘strings’ to go to slack. Materials Materials used- Cu-Ni, Cu-Ni, CrNi or Ni-Fe • Gauge factor : 2 to 4
Bonded Strain Gauge Bonded type is the simplest form consists of wire/ strip of resistance arrangement in grid E = Em E = actual strain Em= measured strain
Bonded Strain Gauge Types of bonded strain gauge: • Flat grid type • Wrap around type • Woven t e
Resistance • It is the obstruction produced on the movement
of electron for the potential difference applied across the conductor • The resistance of a material is given by: -
Where Where
= resisti resistivit vity y of ma materi terial al l = length of the material A = Circular cross sectional area of the material of diameter “d”
Derivation for gauge factor
Differentiating w.r.t. stress σ: -
Dividing by
Derivation continued… A = πd2
As it is a circular cross section
But, Strain = Thus,
Poisson’s ratio =
Derivation continued…
Gauge Factor =
Reference Transducers - D. Patranabis (page-18) • Sensors and Transducers
