Force Measurement using Strain Gauge

Introduction

Force measurement plays a crucial role in various fields, including engineering, biomechanics, and materials testing. Accurate force measurement is particularly important in the field of Electromagnetic Interference (EMI) Lab experiments. One commonly used tool for force measurement is the strain gauge.

A strain gauge is a device that measures the strain (deformation) experienced by an object when a force is applied to it. This strain is then used to calculate the force applied. In this topic, we will explore the key concepts and principles behind force measurement using strain gauges, as well as the steps involved in solving typical problems and calibrating the system.

Key Concepts and Principles

Strain Gauge

A strain gauge is a sensor that measures the strain (deformation) experienced by an object when a force is applied to it. It consists of a thin wire or foil that is attached to the object and deforms along with it. The change in the electrical resistance of the wire or foil is proportional to the strain experienced by the object.

There are different types of strain gauges, including foil strain gauges and semiconductor strain gauges. Foil strain gauges are the most commonly used type and are made of a thin metal foil that is bonded to the object being measured.

The construction of a strain gauge includes the following components:

1. Backing Material: Provides mechanical support to the gauge.
2. Grid: The thin wire or foil that deforms with the object.
3. Insulating Material: Prevents electrical contact between the grid and the object.
4. Protective Coating: Protects the gauge from environmental factors.

Strain

Strain is the measure of deformation experienced by an object when a force is applied to it. It is defined as the ratio of the change in length (deformation) to the original length of the object. Strain can be categorized into two types: axial strain and shear strain.

Axial strain occurs when a force is applied along the length of an object, causing it to elongate or compress. Shear strain, on the other hand, occurs when a force is applied parallel to the surface of an object, causing it to deform in a shearing manner.

The strain experienced by an object can be calculated using the formula:

$$\text{{Strain}} = \frac{{\text{{Change in length}}}}{{\text{{Original length}}}}$$

Wheatstone Bridge

A Wheatstone bridge is a circuit configuration commonly used in force measurement using strain gauges. It consists of four resistors arranged in a diamond shape, with the strain gauge connected as one of the resistors. The purpose of the Wheatstone bridge is to measure the change in resistance of the strain gauge and convert it into a measurable output voltage.

The components of a Wheatstone bridge include:

1. Strain Gauge: The strain gauge is connected as one of the resistors in the bridge.
2. Excitation Voltage: The voltage source that provides power to the bridge.
3. Output Voltage: The voltage measured across the bridge.
4. Balance Adjustment: A variable resistor used to balance the bridge.

The principle of operation of a Wheatstone bridge is based on the fact that the resistance of the strain gauge changes with strain. When the bridge is balanced (i.e., the output voltage is zero), the resistance of the strain gauge can be used to calculate the applied force.

Calibration

Calibration is an essential step in force measurement using strain gauges to ensure accurate and reliable results. Calibration involves comparing the output of the measurement system with known forces and adjusting the system if necessary.

There are different methods of calibration, including:

1. Deadweight Calibration: In this method, known weights are applied to the system, and the corresponding output readings are recorded. The measured forces are then compared to the known weights to determine the calibration factor.
2. Load Cell Calibration: A load cell is a device that directly measures force. It is used as a reference to calibrate the strain gauge-based force measurement system. The load cell is subjected to known forces, and the output readings are compared to the measurements obtained from the strain gauge system.

Several factors can affect the accuracy of calibration, including environmental conditions (e.g., temperature, humidity), proper installation of the strain gauge, and the stability of the measurement system.

Step-by-Step Walkthrough of Typical Problems and Solutions

Problem 1: Determining the force applied to a structure using a strain gauge

1. Selection and installation of strain gauge on the structure:

   • Choose a suitable strain gauge based on the expected force range and the material of the structure.
   • Clean the surface of the structure and apply adhesive to attach the strain gauge.
   • Ensure proper alignment and avoid any air bubbles or wrinkles in the strain gauge.

2. Wiring and connection of strain gauge to a Wheatstone bridge circuit:

   • Connect the strain gauge to the appropriate terminals of the Wheatstone bridge circuit.
   • Ensure proper wiring and connections to avoid any loose or faulty connections.

3. Measurement and calculation of strain and force:

   • Apply a known force to the structure and measure the corresponding change in resistance of the strain gauge.
   • Use the formula for strain to calculate the strain experienced by the structure.
   • Use the calibration factor obtained from the calibration process to calculate the applied force.

Problem 2: Calibration of a strain gauge-based force measurement system

1. Selection and setup of calibration equipment:

   • Choose a suitable calibration equipment, such as deadweights or a load cell.
   • Set up the calibration equipment according to the manufacturer's instructions.

2. Application of known forces to the system:

   • Apply known forces to the system using the calibration equipment.
   • Record the corresponding output readings from the strain gauge-based force measurement system.

3. Comparison of measured forces with known forces and adjustment of calibration:

   • Compare the measured forces with the known forces applied.
   • Calculate the calibration factor by dividing the known force by the measured force.
   • Adjust the calibration of the strain gauge-based force measurement system based on the calibration factor.

Real-World Applications and Examples

Force measurement in aerospace industry

In the aerospace industry, force measurement using strain gauges is essential for various applications, including:

1. Monitoring structural integrity of aircraft components:

   • Strain gauges are used to monitor the strain experienced by critical components of an aircraft, such as wings and landing gear.
   • This information helps ensure the structural integrity of the aircraft and detect any potential failures or fatigue.

2. Testing and validation of new aircraft designs:

   • Strain gauges are used during the testing and validation phase of new aircraft designs.
   • They help engineers understand the behavior of the aircraft under different loads and conditions.

Force measurement in automotive industry

In the automotive industry, force measurement using strain gauges is used for various purposes, including:

1. Evaluation of vehicle suspension systems:

   • Strain gauges are used to measure the forces experienced by vehicle suspension systems.
   • This information helps engineers optimize the design and performance of the suspension system.

2. Crash testing and impact analysis:

   • Strain gauges are used during crash testing to measure the forces experienced by different parts of a vehicle.
   • This data is crucial for evaluating the safety and performance of the vehicle during impact.

Force measurement in biomechanics

In the field of biomechanics, force measurement using strain gauges is used for various applications, including:

1. Analysis of human movement and muscle activity:

   • Strain gauges are used to measure the forces exerted by muscles during different movements.
   • This information helps researchers understand the mechanics of human movement and optimize performance.

2. Design and evaluation of prosthetics and orthotics:

   • Strain gauges are used to measure the forces exerted on prosthetic limbs or orthotic devices.
   • This data helps in the design and evaluation of these devices to ensure optimal functionality and comfort.

Advantages and Disadvantages of Force Measurement using Strain Gauge

Advantages

1. High accuracy and sensitivity:

   • Strain gauges provide accurate and sensitive measurements of forces.
   • They can detect small changes in strain, allowing for precise force measurement.

2. Wide range of force measurement capabilities:

   • Strain gauges can measure forces ranging from a few grams to several tons.
   • This versatility makes them suitable for a wide range of applications.

3. Relatively low cost compared to other methods:

   • Strain gauges are relatively inexpensive compared to other force measurement methods.
   • This makes them a cost-effective choice for many applications.

Disadvantages

1. Susceptibility to environmental factors:

   • Strain gauges are sensitive to environmental factors such as temperature and humidity.
   • Changes in these factors can affect the accuracy and reliability of the measurements.

2. Complex installation and wiring process:

   • Installing and wiring strain gauges can be a complex and time-consuming process.
   • It requires careful attention to detail to ensure accurate measurements.

3. Limited lifespan and potential for damage or failure:

   • Strain gauges have a limited lifespan and can be prone to damage or failure.
   • Regular maintenance and replacement may be required to ensure reliable measurements.

Summary

Force measurement using strain gauges is an important technique in various fields, including engineering, biomechanics, and materials testing. This topic explores the key concepts and principles behind force measurement using strain gauges, including the working principle of strain gauges, types of strain, the Wheatstone bridge circuit, and the calibration process. It also provides a step-by-step walkthrough of typical problems and solutions, real-world applications, and the advantages and disadvantages of force measurement using strain gauges.

Analogy

Imagine you have a rubber band attached to a door handle. When you pull on the rubber band, it stretches and deforms. The amount of stretch or deformation is similar to the strain experienced by an object when a force is applied to it. Now, imagine if you could measure the amount of stretch in the rubber band using a special device. This device is like a strain gauge, which measures the strain experienced by an object when a force is applied to it.