Question

State function of strain gauge. Give its types and explain working of any one type of it.

Answer 1

Function of strain gauge: -

➢ The strain gauge is a passive, resistive transducer which converts the mechanical elongation and compression into a resistance change.

➢ This change in resistance takes place due to variation in length and crosssectional area of the gauge wire, when an external force act on it.

OR

➢ A Strain gauge is a sensor whose resistance varies with applied force; It converts force, pressure, tension, weight, etc., into a change in electrical resistance which can then be measured

Types of strain gauge: -

The type of strain gauge are as

➢ Wire gauge

1. Bonded strain gauge

2. Unbonded strain gauge

3. Foil type strain gauge

➢ Semiconductor gauge

1. Bonded Resistances wire Strain Gauge

➢ STRAIN is defined as change in length divided by original length

➢ This change in resistance takes place due to variation in length and crosssectional area of the gauge wire, when an external force act on it.

➢ When a strain produced by a force is applied on the wires, L increase and A decrease.

➢ Two main parameters are changes

➢ The change in gauge resistances

➢ The change in length

➢ A resistance wire strain gauge consist of a grid of fine resistance wire. The grid is cemented to carrier which may be a thin sheet of paper Bakelite or Teflon.

➢ The wire is covered on top with a thin sheet of material so as to prevent it from any mechanical damage.

➢ Typical resistances of strain gauges are 120 Ω, 350 Ω and 1000 Ω

OR

2. Unbonded strain gauge

➢ This gauge consists of a wire stretched between two points in an insulating medium such as air.

➢ The wires are of copper nickel, chrome nickel or nickel iron alloys.

➢ The wires are tensioned to avoid buckling when they experience a compressive force

➢ The diameter of the wire used is about 0.003mm, having gauge factor of 2 to 4

➢ The length of wire is 25µm or less

➢ The bridge is balanced with no load applied

➢ At initial preload, the strain & resistances of four arm are normally equal, therefore the o/p voltage of bridge is zero

➢ When an external force is applied, the resistance of the strain gauge changes causing an unbalance of the bridge

➢ The unbalances of the bridge producing an o/p voltage

➢ This voltage is proportional to the strain (applied pressure or force)

➢ A displacement of 50µm can be detected with this strain gauge

OR

3. Foil type strain gauge(

• It consists of following parts: 1. Base (carrier) Materials: several types of base material are used to support the wires. Impregnated paper is used for room temp. applications. 2. Leads: The leads should be of such materials which have low and stable resistivity and also a low resistance temperature coefficient 3. This change in resistance takes place due to variation in length and crosssectional area of the gauge wire, when an external force act on it. 4. This class of strain gauge is only an extension of the bonded metal wire strain gauges. 5. The bonded metal wire strain gauge has been completely superseded by bonded metal foil strain gauges. 6. Metal foil strain gauge use identical material of wire strain gauge 7. These metal foils are used for most general-purpose stress analysis application and for many transducers

OR

4. Semiconductor gauge

1. Semiconductor gauge are used in application where a high gauge factor is desired. A high gauge factor means relatively higher change in resistance that can be measured with good accuracy. 2. The resistance of the semiconductor gauge change as strain is applied to it. The semiconductor gauge depends for their action upon the piezo-resistive effect i.e. change in value of resistance due to change in resistivity. 3. Silicon and germanium are used as resistive material for semiconductor gauges