Limitations of Strain Gauge
Strain gauges were invented in the late 1930’s and have been used now for eight decades to measure strain on a substrate. While the strain gauge was cutting edge in 1940, when used in today’s engineering applications it’s limitations can inhibit innovations.
Strain gauges are limiting in the amount of data they can collect, the cumbersome installation process, and bulky wiring. As point sensors, strain gauges can only obtain data about critical points, but if an event occurs between these points, the sensor will not collect the data. The installation process for strain gauges is lengthy, complicated and leaves large margins for errors. When not installed properly, the resulting data will be unusable. For installations with a large number of gauges, the weight and bulk of the lead cables for the sensors becomes difficult to manage. Additionally, the large number of cables required for a strain gauge limits the types of materials that can be tested since the mass of the cables can influence a substrate’s behavior.
Fiber Optic Sensing
Having equal or better accuracy than strain gauges, fiber optic sensing systems offer a more robust strain sensing solution by providing spatially continuous strain data along the entire length of the fiber, having easier installation procedures, and by using virtually weightless sensors and minimal cables.
Some fiber optic sensing systems, like Sensuron’s products, can obtain spatially continuous strain data along the entire length of the fiber. This gives engineers access to the full picture of how the substrate is behaving, rather than looking at individual points. This is analogous to being able to look at an entire photograph rather than attempting to determine what an image looks like while only being able to see ten pixels.
One of the key benefits of fiber optic sensors is the relatively fast installation process. For comparison, it takes approximately the same amount of time to install 3-5 strain gauges is it does to install 10 meters of sensing fiber – the equivalent of approximately 1,000 sensing points with 6 mm gauge spacing. While installing the fiber on a material still takes proper knowledge and training, the process is significantly simplified. In fact, fiber optic sensors are much more resistant to improper installation practices.
The sensing fiber, which typically has a diameter of 195 micron, has a negligible mass, which translates to 100 times the sensors at less than 1% of the total weight of a strain gauge. Additionally, the size of the interrogator (the data acquisition unit for fiber optic sensing) is an order of magnitude smaller than that of traditional data acquisition hardware.
Once installed, fiber optic sensors offer the most accurate solution for long term, lifecycle and fatigue tests. The instrumentation can be portable and ruggedized. Additionally, fiber optic sensing does not suffer from power heating, glue line creep or hardening of gauge filaments. Using fiber optic technology also means immunity to RFI, EMI, corrosive environments and water.
FOS systems have additional advantages over traditional strain measurement methods. For example:
- Fiber optic sensing achieves greater resolution at the micro strain level than traditional gages
- The technology allows for dense sensor arrays and higher spatial resolution
- Sensuron’s fiber optic sensor technology can simultaneously obtain strain, load, temperature, deflection, 3D shape, and pressure measurements – all with one data acquisition unit.
- Sensuron’s technology can also be embedded in composite materials.
With ever-increasing competition, companies are challenged to innovate, to do business faster and safer, while keeping costs to a minimum. Even still, many industries – including automotive, aerospace and medical – are relying on legacy point sensing technologies like strain gauges to meet ever-evolving business demands. Designed to solve the problems of the past, these legacy technologies no longer satisfy the demands of today’s toughest environments. By investing in compact fiber optic sensing solutions, businesses can consolidate several disparate technologies into a single platform while continuously testing, controlling and monitoring the health of such systems to ensure success long into the future.