Strain Gauges

Definition of Transducers

convert sensored variable into detactable signal form

e.g

mechanical quantity → change in electrical signal (strain gauge)

What type of transducers is strain gauges

device that strain → change in electric resistance

tension→ increase in resistance

Basic equation of resistance

Reason why strain gauge create change in resistance

when strain,

1. there is changes to l and A values due to conservation of volume

2. piezo-resistive effect changes resistivity of material

(increases with tension)

Structure of simple wire strain gauge

wire folded on flexible sheet

change in resistance in active axis is much greater than in passive axis

Definition of gauge factor

Gauge factor changes with different gauge

1.8<G<2.2

Structure of foil strain gauge

• rolling thin foil of the resistive material on thin insulating paper

• cutting away parts of the foil by a photo-etching process, to create the required grid pattern

• adhesive to paper should be creep-free and allow heat dissipation

wire vs foil strain gauge

Foil is better

• larger surface area for adhesion

• accurate reproducibility due to photo-etching technique

• small dimension for localised strain measurement

Derivation of V₀/e=1/4V_sG

Reasons for temperature influencing strain gauge measurements

changes in temperature:

• thermal expansion → change in dimensions of specimen and gauge

• resistivity of gauge

Method of temperature compensation

1. Dummy gauge on separate specimen not under strain

2. Dummy gauge on same specimen perpendicularly

Proof that temperature is compensated for in strain gauge on unstressed specimen

Proof that temperature is compensated for in perpendicular strain gauges

Reason for bridge balancing

resistance of each arm in bridge differ due to

• manufacturing variance,

• temperature difference btw gauges

• static strain in one member

therefore cannot assume that ^^

Method of bridge balancing

Connect a pot (potentiometer) before measurement

adjust pot ^^^, where R_x=R₁+R_pot

Method of semiconductor strain gauges working

• semiconductor material has very large piezo-resistive effect

• reach gauge factor from 100 to 300

• magnitude of piezo-resistive effect determines sensitivity of gauge

Limitations of semiconductor strain gauge

• Strain gauge material has a much smaller elastic limit than metal (4000microstrain compared to 20k microstrain for metal)

• G is not linear at high strain

• G changes significantly with temperature(temp coeff is large)

Method of temperature compensation for semiconductor strain gauges

connect 2 crystal in series but aligned parallel

All the crystal have the same temperature coefficient, temperature changes affect all crystal equally

As R₁ and R₂ increase/decrease same magnitude, and same for R3 and R4, changes in dimension cancels out.

Definition of bulk Modulus [K]