A strain gauge made entirely on a PCB

Structure and Fabrication

• Conductive Tracks: The basic structure of such a strain gauge consists of thin, precisely patterned conductive tracks on the PCB. These tracks are usually made of copper or other suitable conductive materials. The layout and geometry of these tracks are carefully designed to respond to strain in a predictable way. For example, they might be arranged in a serpentine pattern to increase the effective length within a given area on the PCB, which enhances the sensitivity to strain.
• Sensing Element: The conductive tracks act as the sensing element. When the PCB on which the strain gauge is fabricated undergoes mechanical deformation (strain), the length, cross-sectional area, or resistivity of the conductive tracks changes. This alteration in the electrical properties of the tracks leads to a measurable change in electrical resistance, which can be detected and correlated to the amount of strain applied.
• Integrated Circuitry (Optional): In some advanced designs, additional integrated circuitry can be incorporated directly onto the PCB along with the strain gauge. This circuitry might include amplifiers to boost the small signal changes in resistance due to strain, analog-to-digital converters to convert the analog resistance change into a digital signal for easier processing by microcontrollers or other digital systems, and calibration circuits to ensure accurate and consistent measurements.

Working Principle

• Resistance Variation with Strain: The fundamental principle behind the operation of a PCB-based strain gauge is the piezoresistive effect. When the PCB is subjected to mechanical strain, the lattice structure of the conductive material in the tracks is distorted. This distortion causes a change in the resistivity of the material, which in turn leads to a change in the overall resistance of the conductive tracks. The relationship between strain and resistance change is typically characterized by a gauge factor, which is a proportionality constant that relates the fractional change in resistance to the applied strain.
• Signal Detection and Measurement: The change in resistance of the strain gauge is usually detected using a Wheatstone bridge circuit or a similar electrical circuit configuration. The Wheatstone bridge is designed to measure small changes in resistance accurately. By comparing the resistance of the strain gauge (which changes with strain) to the resistance of known reference resistors in the bridge circuit, an unbalanced voltage is generated that is proportional to the strain applied to the PCB. This voltage can then be further processed, either through amplification, filtering, or conversion to a digital format, depending on the requirements of the measurement system.

Advantages

• Cost-Effective: Fabricating a strain gauge on a PCB can be a cost-effective solution compared to traditional strain gauge manufacturing methods. PCBs are mass-produced using well-established manufacturing processes, and integrating the strain gauge onto the PCB can reduce the need for separate manufacturing steps and components. This can lead to lower production costs, especially for applications where a large number of strain gauges are required.
• Miniaturization: PCBs allow for a high degree of miniaturization. A strain gauge made entirely on a PCB can be designed to be very small in size, making it suitable for applications where space is limited, such as in microelectromechanical systems (MEMS), miniature sensors, or portable electronic devices.
• Integration with Other Components: Since the strain gauge is on the PCB, it can be easily integrated with other electronic components and circuitry already present on the board. This enables the creation of more complex and integrated sensor systems. For example, it can be directly connected to a microcontroller for data processing and communication, or integrated with power management circuits to ensure efficient operation.
• Reliability: The manufacturing processes used for PCBs are generally highly reliable. By fabricating the strain gauge on a PCB, it can benefit from the same quality control and reliability measures that are applied to the production of PCBs. This can result in a more reliable strain gauge with consistent performance over time.

Applications

• Structural Health Monitoring: In civil engineering and aerospace industries, PCB-based strain gauges can be used to monitor the structural health of buildings, bridges, aircraft wings, and other large structures. By continuously measuring the strain on critical components of these structures, early signs of damage or excessive stress can be detected, allowing for timely maintenance and repair actions to be taken.
• Biomechanical Measurements: In the field of biomechanics, these strain gauges can be used to measure the strain on bones, muscles, or joints during movement or physical activities. This information can be used to study human locomotion, assess the effectiveness of rehabilitation therapies, or design better prosthetic devices.
• Industrial Automation: In industrial settings, PCB-based strain gauges can be used to monitor the strain on machine parts, such as shafts, gears, or conveyor belts. This allows for predictive maintenance, where potential failures can be predicted based on the measured strain data, reducing downtime and improving productivity.
• Consumer Electronics: In consumer electronics, such as smartphones, tablets, or wearable devices, PCB-based strain gauges can be used to detect user interactions like button presses, bending of the device, or changes in grip pressure. This enables the creation of more intuitive and interactive user interfaces.