Strain Gauges
Table of Contents
1 Intro to Strain Gauges
A strain gauge is a sensor whose resistance varies with applied force; it essentially measures strain on an object. When an object deforms, the strain gauge deforms with it, causing its electrical resistance to change. This change in resistance is proportional to the strain experienced.
Strain gauges are used in various applications to measure forces, loads, or stresses impacting different materials or structures.
2 Important Characteristics
Grid Resistance: This refers to the electrical resistance of the metallic grid in the strain gauge, typically measured in ohms. It influences how much the signal can be amplified without noise interference.
Gauge Factor: The ratio of relative change in electrical resistance to the mechanical strain. The gauge factor is critical for calibrating the strain gauge and interpreting its output.
Transverse Sensitivity: This measures the sensitivity of the strain gauge to strain perpendicular to the direction of the main strain. It’s important for applications where multi-directional strain occurs.
Thermal Output Coefficients: These are coefficients that describe how the gauge's output changes with temperature. Different materials have different coefficients, affecting the accuracy in varying thermal conditions.
Stability and Drift: Over time and under varying environmental conditions, the properties of a strain gauge can drift, affecting accuracy. Stability is crucial for long-term measurements.
Hysteresis: The degree to which the strain gauge output is affected by previous load cycles. Lower hysteresis is preferable for repeatable and reliable measurements.
Fatigue Life: Refers to how well a strain gauge can withstand repeated loading and unloading cycles without failure. Important in dynamic applications where the material undergoes frequent or cyclical stress.
Recommended calculator: Micro-Measurements Calculators
3 Mechanical and Electrical Considerations
Surface Preparation: The surface where the gauge will be attached must be clean, smooth, and free of any grease or dirt.
Attaching the Gauge: Adhesive is used to stick the gauge onto the surface. Care must be taken to avoid any air bubbles or wrinkles.
Wiring and Calibration: Connect the gauge to a measuring instrument. The gauge must be calibrated to ensure accurate measurements, adjusting for factors like temperature and material properties.
Measurement: Strain readings are taken as the material undergoes stress, with the changes in resistance measured through an electrical circuit.
Voltage Regulation: Consistent voltage levels must be maintained to ensure that the strain gauge's output remains accurate and stable. Using voltage regulators can prevent fluctuations that might come from battery depletion or other power supply issues.
3.1 Bridge Configurations
Strain gauges are often used in bridge configurations to maximize sensitivity and compensate for external factors like temperature changes. The most common configurations are as follows.
3.1.2 Quarter-Bridge Configuration
Uses one active strain gauge, best for simple, cost-effective measurements where moderate accuracy is acceptable. Good for general applications where the strain is uniform.
3.1.3 Half-Bridge Configuration
Utilizes two gauges; one active gauge measuring strain and another placed perpendicular or at a neutral point to compensate for temperature.
Suitable for bending measurements where temperature compensation is needed without using a full bridge.
3.1.4 Full-Bridge Configuration
Consists of four strain gauges in a Wheatstone bridge arrangement. This configuration provides the highest sensitivity and accuracy. It's ideal for precise measurements and can effectively compensate for temperature and other external influences.
Best for complex applications requiring high accuracy, such aerospace component testing where all factors including temperature and multidimensional strains are critical.
Recommended calculator: Micro-Measurements Calculators
4 Software / Embedded Systems Considerations
Signal Conditioning: The resistance change due to strain is typically small, so signal conditioning, like amplification and filtering, is necessary before digitization. Amplification increases the voltage change for better resolution and sensitivity in the ADC phase.
Analog to Digital Conversion (ADC): The conditioned analog voltage signal is converted into a digital signal by the ADC. The quality and resolution of an ADC affect how finely the changes can be detected and quantified.
Microcontroller Selection: Choose a microcontroller (MCU) that can handle the necessary computation and has sufficient analog inputs if direct connection is required. The MCU should have a high-resolution ADC or support an external ADC for accurate data collection.
Data Acquisition Speed: Ensure that the ADC and microcontroller can handle the sampling rate required for your application. High-speed changes require faster data acquisition to capture transient events accurately.
Noise Reduction: Implementing hardware and software filtering techniques to minimize noise is crucial. This includes using low-pass filters to eliminate high-frequency noise and designing proper shielding and grounding in the circuit layout.
5 Calculating Strain / Stress via ADC
To calculate strain or stress from the ADC signal output of a strain gauge, follow these steps:
5.1 Calculation of Strain
Determine Change in Resistance: First, calculate the change in resistance from the digital output, considering the initial resistance and the characteristics of the amplification. Use the gauge factor (GF) to convert the relative change in resistance (ΔR/R) to strain (ε):
$\epsilon = \frac{\Delta R}{R} \times GF$
- $\epsilon$ = strain.
- $GF$ = Gauge factor.
- $\frac{\Delta R}{R}$ = Relative change in resistance.
- $\Delta R$ = Change in resistance.
- $R$ = Original resistance.
5.2 Calculating Stress (Hooke's Law)
If the modulus of elasticity (E) of the material is known, stress (σ) can be calculated using Hooke's Law:
$\sigma = E \times \epsilon$
- Note recall Hooke's law assumes the material behaves elastically with no plastic deformation.
6 Mechanical Integration
6.1 Installation (Surface Preparation)
Recommended Video: Tutorial: Strain Gage Installation Procedure