How is a strain gauge used for force measurement?

Short Answer:

A strain gauge is used for force measurement by detecting the small changes in length (strain) that occur in a material when force is applied to it. The strain gauge is bonded to the surface of the object, and as the object deforms due to force, the electrical resistance of the strain gauge changes.

This change in resistance is measured using a Wheatstone bridge circuit, which converts it into a voltage signal. The output voltage is directly proportional to the applied force, making it possible to calculate the amount of force accurately.

Detailed Explanation:

Working of strain gauge for force measurement

A strain gauge is a sensing device that measures strain, which is the deformation or displacement in a material when it is subjected to force or load. Strain is usually very small, but even slight strain causes a measurable change in the electrical resistance of the strain gauge. This property is used to measure force indirectly, since force and strain are related through Hooke’s law.

Construction and Basic Principle:

  • A strain gauge is typically made of a thin metallic wire or foil arranged in a grid pattern and mounted on a flexible backing.
  • It is glued tightly onto the surface of a structural member (like a beam or plate) where the force is to be measured.
  • When a force is applied to the structure, it deforms slightly, and this deformation is transferred to the strain gauge.
  • As the strain gauge stretches or compresses, its length increases or decreases, and its cross-sectional area changes, which results in a change in its electrical resistance.

This change in resistance ΔR\Delta RΔR is related to the strain ε\varepsilonε by the gauge factor (GF):

ΔRR=GF×ε\frac{\Delta R}{R} = GF \times \varepsilonRΔR​=GF×ε

Where:

  • ΔR\Delta RΔR = Change in resistance
  • RRR = Original resistance
  • GFGFGF = Gauge factor (typically around 2)
  • ε\varepsilonε = Strain

Force Measurement Process:

  1. Bonding the Gauge:
    • The strain gauge is bonded securely to the surface of the object that will experience the force.
    • The material should have good elasticity to transmit strain effectively to the gauge.
  2. Applying Force:
    • When a force is applied, the material deforms slightly, either by stretching (tension) or compressing (compression).
    • This deformation changes the length and resistance of the strain gauge.
  3. Change in Resistance:
    • The resistance of the strain gauge increases with stretching and decreases with compression.
    • This change is very small (in milliohms), so it needs to be measured using a sensitive method.
  4. Wheatstone Bridge Circuit:
    • The strain gauge is connected in a Wheatstone bridge, a circuit designed to detect small changes in resistance.
    • As the resistance of the strain gauge changes due to applied force, the balance of the bridge is disturbed.
    • This imbalance produces a voltage output that is proportional to the strain, and therefore to the applied force.
  5. Signal Amplification and Calibration:
    • The voltage signal from the bridge is very small, so it is passed through an amplifier.
    • The system is then calibrated using known forces, so that the voltage output can be translated into an exact force value.

Applications of Strain Gauge in Force Measurement:

  • Load Cells: Used in weighing scales and material testing machines.
  • Structural Monitoring: For bridges, buildings, and aircraft to detect stress.
  • Automotive Industry: In crash testing and suspension testing.
  • Robotics: To measure gripping force or feedback in mechanical arms.
  • Medical Devices: For measuring forces in prosthetics or orthopedic implants.

Advantages:

  • High Accuracy: Capable of detecting very small changes in force.
  • Compact Size: Easily mounted on small surfaces.
  • Wide Range of Measurement: Can measure both static and dynamic forces.
  • Low Cost and Easy Installation
Conclusion:

A strain gauge measures force by detecting the strain (small deformation) in a material caused by the applied force. This strain changes the electrical resistance of the gauge, which is converted into a voltage signal using a Wheatstone bridge circuit. The voltage is then calibrated to represent the actual force. This method is accurate, reliable, and widely used in engineering, structural health monitoring, and industrial measurement systems.