Summary of "Strain Gauges and load cells part: 2"

Summary of “Strain Gauges and Load Cells Part: 2”

This video lecture provides an in-depth explanation of strain gauges, Wheatstone bridge configurations, load cells, and related sensors, focusing on their working principles, configurations, and practical applications.

Main Ideas and Concepts

1. Review of Wheatstone Bridge with Strain Gauges

Strain gauges (SG) are connected to specific resistors (R1, R2, R3, R4) in a Wheatstone bridge.
Key steps to solve problems involving strain gauges in Wheatstone bridges:
- Identify which resistor each strain gauge corresponds to.
- Determine if the resistance increases (R + ΔR) or decreases (R - ΔR) based on tension or compression forces.
- Assume all resistors initially equal (R), then substitute changes accordingly.
Multiple strain gauges can be used to increase output voltage (sensitivity).
Formula for output voltage ( V_{out} ) depends on resistor configuration and strain gauge changes.
Proper numbering and orientation of resistors and strain gauges are crucial.

2. Effect of Force Direction on Strain Gauges

Elongation (tension) causes resistance to increase.
Compression causes resistance to decrease.
Strain gauges oriented perpendicularly can detect opposite resistance changes, preventing cancellation of output voltage.
Using four strain gauges (full Wheatstone bridge) maximizes voltage output and sensitivity.

3. Load Cells

A load cell is a sensor that converts force into an electrical signal, typically using strain gauges arranged in a Wheatstone bridge.
Force → Mechanical deformation → Strain gauge deformation → Electrical signal.
Load cells can measure tension, compression, or both, depending on design.
Examples:
- Kitchen scales use compression load cells.
- Luggage scales use tension load cells.
Output voltage is very small (millivolts), necessitating amplification for accurate measurement.
Calibration curves convert measured voltage back into force.
Sensitivity of load cells is expressed as mV/V (millivolts output per volt excitation).
Exceeding maximum force capacity can damage the load cell.

4. Strain Gauge Working Principle

Resistance ( R = \rho \frac{L}{A} ) changes with length (L) and cross-sectional area (A).
Tension increases length and decreases area → resistance increases.
Compression decreases length and increases area → resistance decreases.
Orientation of strain gauges relative to force direction determines whether resistance increases or decreases.

5. Types of Strain Gauges and Load Cells

S-type strain gauges: mainly for tension.
Washer-type: mainly for compression.
Cantilever beam load cells: measure bending forces.
Full Wheatstone bridge with four strain gauges maximizes sensitivity.

6. Displacement Sensors

Another form of resistive sensor measuring position changes.
Uses a slider in a voltage divider configuration.
Converts displacement (linear or rotational) into voltage change.
Calibration required to convert voltage output into physical displacement or force.

7. Soil Moisture Sensors (Resistive Sensors)

Measure volumetric water content in soil.
Two probes inserted into soil with voltage applied.
Moist soil conducts electricity better → higher current.
Dry soil has higher resistance → lower current.
Helps optimize irrigation by detecting soil moisture levels.
Typically interfaced with microcontrollers and displays for real-time monitoring.

Methodology / Instructions for Solving Wheatstone Bridge Problems with Strain Gauges

Identify which resistor(s) correspond to each strain gauge.
Determine if each strain gauge experiences elongation (R + ΔR) or compression (R - ΔR).
Assume all resistors have baseline resistance ( R ).
Substitute values into the Wheatstone bridge output voltage formula:

[ V_{out} = V_s \times \frac{R_3 R_2 - R_1 R_4}{(R_3 + R_4)(R_1 + R_2)} ]

Simplify the expression considering ΔR terms.
Interpret the result to find output voltage proportional to strain.
Use multiple strain gauges oriented properly to maximize sensitivity and avoid voltage cancellation.

Practical Applications Highlighted

Kitchen scales (compression load cells).
Luggage scales (tension load cells).
Load cells in industrial and mechanical measurement.
Soil moisture sensors for irrigation control.

Key Terms and Definitions

Strain Gauge: A sensor that changes resistance when deformed by strain.
Load Cell: A sensor that converts force into an electrical signal using strain gauges.
Wheatstone Bridge: An electrical circuit used to measure small changes in resistance.
Sensitivity (mV/V): Output voltage per volt of excitation at full load.
Calibration Curve: Graph or function converting voltage output to physical quantity (force, displacement).
Resistive Sensor: A sensor whose output depends on changes in electrical resistance.

Speakers / Sources Featured

Primary Speaker: The lecturer (unnamed) explaining strain gauges, Wheatstone bridge, load cells, and related sensors.
Molly: A student credited for providing a detailed solution to a Wheatstone bridge problem.
References to previous lectures and students (e.g., mentions of “our history proof” and “fall term” problems).

This summary captures the core lessons, methodologies, and practical insights from the video, suitable for students or engineers learning about strain gauges and load cells.