Whether in load cells, pressure sensors, or structural monitoring systems, installing semiconductor strain gauges requires accuracy.
This will go through the steps and specs of installing strain gauges.
What is a Strain Gauge and How Do They Work?
A strain gauge is a sensor that measures strain—usually the deformation of a material when subjected to force.
This change in material shape causes a change in the electrical resistance of the strain gauge.
Using this resistance change the strain gauge gives you data on how much force or strain an object is under.
Key components of how strain gauges work:
- Thin conductive foil: The gauge is a grid pattern of thin foil bonded to the surface of an object.
- Wheatstone bridge: This is the electrical circuit used with strain gauges to detect small resistance changes.
- Gauge factor: This is the ratio between the resistance change and the actual strain experienced by the object.
Semiconductor Strain Gauges
Semiconductor strain gauges (also known as piezoresistive strain gauges) are different from metal types (foil or wire) because they are made from silicon or other semiconducting materials.
These have a much higher gauge factor, and they are much more sensitive to strain than metal strain gauges.
However, this increased sensitivity comes with increased temperature sensitivity, which must be considered during installation.
A typical strain gauge with its zig-zag pattern of conductive strips is a balance of sensitivity and reliability and is suitable for general use.
Key features of semiconductor strain gauges:
- High Sensitivity: Their high gauge factor (around 100) allows for measuring very small strains.
- Temperature Sensitivity: Being semiconducting, these strain gauges are more temperature sensitive than others, so temperature compensation is required during installation.
- Application: For high precision applications like load cells, pressure sensors, force measurements in industries like OEM, mining, and manufacturing.
What Are the Types of Strain Gauges?
Let’s dive into the various types of strain gauges and find the perfect fit for your measurement needs:
- Linear Strain Gauges: Measures strain along a single axis. Simple construction and cost-effective.
- Rosette Strain Gauges: Multiple measuring elements to measure biaxial stress.
- Shear Strain Gauges: For measuring strain caused by torsional force or torque.
- Double Linear Strain Gauges: Two linear gauges in parallel for increased sensitivity.
- Full Bridge Strain Gauges: Four active strain gauges for high accuracy and sensitivity.
Linear Strain Gauges
Linear strain gauges measure strain in one direction, usually along the direction of the main force.
For example, in aircraft component testing, linear strain gauges ensure the structural integrity of critical parts.
Linear strain gauges are part of the design of S-Beam, binocular, reverse bending, and dual beam load cells and pressure transducers.
By placing these gauges correctly, engineers can measure bending, axial, and torsion with high accuracy.
Rosette Strain Gauge
Rosette strain gauges are a type of strain gauge that consists of multiple strain gauges arranged in a specific pattern on a common carrier.
A rosette strain gauge is used to measure strain in multiple directions. This makes it ideal for complex stress analysis, especially in situations where the stress is not just uniaxial, but biaxial or triaxial (in two or three directions).
Shear Strain Gauge
Shear strain gauges are a type of strain gauge specifically designed to measure shear strain, which occurs when an object is subjected to forces that cause parts of it to slide relative to one another.
This type of strain is common in materials experiencing torsional forces, such as twisting or torque.
Double Linear Strain Gauges
Double linear strain gauges are an improved version of linear strain gauges with two parallel linear measuring grids. This configuration is useful for double-sided strain measurement for higher accuracy and reliability.
The parallel arrangement of the measuring grids allows double linear strain gauges to capture more detail of the strain for demanding applications.
Full Bridge Strain Gauges
Full bridge strain gauges are four measuring grids configured in a Wheatstone full bridge. Each measuring grid is offset to the next grid at an angle of 90°, allowing for comprehensive strain measurement.
This is useful for torsion bars, tension or compression bars, and shear stresses in shear beams around the neutral fiber.
Full bridge strain gauges are used in the manufacture of precision force transducers that use the bending beam principle.
Strain Gauge Configurations
Understanding the different strain gauge configurations is necessary when choosing the right setup for your specific needs. Here’s a look at the common configurations and why they matter:
Quarter Bridge:
- What It Is: This setup includes one strain gauge and three fixed resistors.
- Why It’s Relevant: It’s a cost-effective solution for basic strain measurements, making it a good choice if you’re looking for a straightforward and budget-friendly option.
Half Bridge:
- What It Is: This configuration uses two active strain gauges along with additional resistors.
- Why It’s Relevant: It provides better sensitivity and helps manage temperature variations, offering more accurate measurements for applications that need a bit more precision.
Full Bridge:
- What It Is: This setup replaces all four resistors in a Wheatstone bridge with strain gauges.
- Why It’s Relevant: It offers the highest level of accuracy and sensitivity, ideal for high-precision applications such as load cells and force transducers.
Additionally, the strain gauge carrier, typically made of polyimide, plays an important role in the gauge’s performance. It affects how well the gauge performs in different environments, particularly in experimental tests and harsh conditions.
How to Install a Strain Gauge
Now that you know the types and configurations of strain gauges, let’s look at how to install them. Whether you are using resistive foil gauges or more special types, these steps are crucial for a successful installation.
Also, consider products from leading strain gauge manufacturers for high quality and reliability.
Preparation for Installation
Before installing a strain gauge, proper preparation is important to get accurate measurements and a long-lasting bond between the gauge and the material.
Follow these steps to prepare for installation:
Choose the Right Gauge
Make sure you are using the right type of strain gauge for the material and application. The gauge should be able to handle the expected strain and environmental conditions.
Temperature Considerations
Since semiconductor strain gauges are temperature sensitive, consider the operating environment. If there are big temperature fluctuations, choose adhesives and gauge materials that can withstand these conditions or implement temperature compensation.
How to Install a Semiconductor Strain Gauge
Now that the surface is prepared, follow these steps to install the strain gauges:
1. Surface Preparation
- Clean the surface where the strain gauge will be installed. It should be free of any dirt, grease or debris.
- Use a suitable solvent to remove any contaminants and then lightly sand the surface with fine-grit sandpaper to create a smooth and even bonding area.
2. Adhesive Application
- Choose an adhesive that is suitable for semiconductor strain gauges. Epoxy resins are common, and they form a strong and stable bond that can withstand the measured strains.
- Apply the adhesive thinly and evenly to the back of the strain gauge. Do not apply too much adhesive, as it will create an uneven bond and affect the sensor’s performance.
3. Gauge Alignment
- Place the strain gauge on the prepared surface. Correct alignment is important. The gauge must be in the direction of the strain you are measuring.
- Use alignment marks (usually provided on the gauge) to position it correctly.
4. Apply Pressure
- After positioning the gauge, apply firm pressure to bond the gauge to the surface. This will make sure no air bubbles are trapped under the gauge which can affect its accuracy.
- Hold the pressure until the adhesive has partially cured. This can take a few minutes to several hours, depending on the adhesive type.
5. Final Adhesive Cure
- Let the adhesive fully cure before proceeding to the next steps. Following the adhesive manufacturer’s instructions for curing time and conditions, rushing this process can lead to poor bonding and inaccurate readings.
- After all these steps, a post installation inspection is a vital step to detect any possible flaws.
Wiring and Calibration
Proper wiring and calibration of semiconductor strain gauges are critical to get accurate strain measurements.
1. Connecting Wires
Connect the gauge to the measurement equipment using shielded cables to minimize electromagnetic interference.
Make sure the wires are properly secured and insulated to prevent them from coming loose or interfering during operation.
2. Temperature Compensation
Since semiconductor strain gauges are temperature sensitive, temperature compensation is often required. This can be done by using a second strain gauge in a temperature compensating configuration (Wheatstone bridge) or by using external temperature compensation devices that change the supply voltage to compensate for temperature effects.
3. Initial Calibration
After wiring, perform an initial calibration to ensure the gauge is reading correctly. In the Initial Verification (IV) phase, the goal is to adjust the output under no load so that it reads zero.
An ideal strain gauge circuit comprises resistors and gauges with perfectly matched values, resulting in a balanced bridge and a zero output when not loaded. However, in real-world applications, the strain gauge bridge components will have tolerances and may not be perfectly matched, leading to an unbalanced bridge and a non-zero output. This discrepancy is known as the zero offset.
The first step in addressing this issue is to balance the bridge circuit by adjusting a trim resistor within the bridge to bring the output back to, or closer to, zero.
Testing the Strain Gauge
Testing a strain gauge is critical to ensure its accuracy and reliability. The testing process involves applying a known force or strain to the gauge and measuring its output. This output is then compared to the expected value to see if the gauge is accurate.
There are several ways to test strain gauges: calibration, verification, and validation. Calibration is the process of adjusting the gauge to match a known standard.
Verification checks the gauge against predefined criteria, while validation checks the gauge under real-world conditions.
Common Mistakes to Avoid
Even with proper preparation and installation, there are common mistakes that can affect your strain gauge:
- Over-tightening: If you apply too much pressure or tighten the gauge too hard during installation, you can damage the sensor or get inaccurate readings. Always use controlled pressure and follow the manufacturer’s instructions.
- Ignoring Temperature Effects: As mentioned earlier, semiconductor strain gauges are temperature-sensitive. Failing to account for this can lead to wrong measurements. Always implement temperature compensation.
- Inadequate Surface Preparation: Skipping steps in the surface preparation process for applying adhesive unevenly can result in poor bonding and inaccurate strain readings.
Conclusion
By following these steps—surface preparation, gauge positioning and bonding, soldering the connections, and testing the installation—you will get reliable and accurate strain measurements.
Choosing the right type and configuration of strain gauge and proper installation will give you precise, reliable, and suitable results for your application.
Keep Reading About Strain Gauge Types
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