Short note: Strain gauge

A strain gauge is a resistive transducer whose electrical resistance changes when it is stretched or compressed. It is used to measure strain, and from that, force, pressure, weight and torque.

Principle

When a conductor is stretched, its length increases and its cross-section reduces, so the resistance R = ρL/A increases. The gauge factor is

GF = (ΔR/R)/(ΔL/L) = (ΔR/R)/ε

where ε is the strain. For metal foil gauges, GF is about 2.

Construction

 Backing (plastic film)
  +----------------------+
  |  ____    ____    __  |
  | |    |  |    |  |  | |  <- fine metal-foil grid
  | |    |__|    |__|  | |
  +--o------------------o+
    leads (soldered)

A thin zig-zag grid of constantan foil is bonded to a flexible backing, which is cemented onto the surface being tested. Typical resistance: 120 Ω or 350 Ω.

Application circuit (Wheatstone bridge)

The tiny change in resistance (typically ΔR/R ≈ 0.1 %) is measured using a Wheatstone bridge, followed by an instrumentation amplifier.

           +Vs
        +---+---+
        |       |
        R1      Rg  (active strain gauge)
        |       |
   A ---+       +--- B  -> instrumentation
        |       |          amplifier -> ADC/meter
        R2      R3
        |       |
        +---+---+
           GND

Initially the bridge is balanced (output 0). When the gauge is strained, the bridge becomes unbalanced, and the output is Vo ≈ Vs·ΔR/(4R) for one active arm. The amplifier raises it to a measurable level, which is shown on a meter or sent to an ADC. A dummy gauge in the adjacent arm compensates for temperature changes.

Applications: load cells (weighing machines), pressure gauges, torque and vibration measurement, and testing of bridges and structures.