Strain Gauge w635q

Strain Gauge By, 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 • Strain gauge converts mechanical strain into 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 • Piezoelectric

Types of strain gauges: • Bonded • Unbonded • Blended metal foil • Thin metal film by vacuum deposition • Thin metal film by sputter deposition

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 of a body or even a single flexible member when strained, the wire also gets strained • These are used in preloaded conditions not to allow the ‘strings’ to go to slack. Materials used- 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 type

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 = resistivity of material 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

But, Strain = Thus,

As it is a circular cross section

Poisson’s ratio =

Derivation continued…

Gauge Factor =

Reference • Sensors and Transducers - D. Patranabis (page-18)