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What Do Wearable Stretch Sensors Have To Do With NASA Space Modules?

Here’s a hint — stretch sensors are excellent for monitoring the movement of soft, flexible structures…

If you guessed the answer, well done! Don’t worry if you didn’t, because it’s a tricky one!

Our stretch sensors are mainly used for wearable technology applications, to measure strain for monitoring body movement. NASA has found a new use for the sensors, however: measuring strain for monitoring inflatable space modules.

NASA is developing inflatable, fabric-based structures that may serve as light, large-volume, habitable modules. Inflatable modules would greatly reduce the amount of transport volume and weight required for future space missions. These inflatable structures would expand after being deployed in space to provide an area for astronauts to live and work in. Testing of these inflatable structures is ongoing, and the ability to measure strain is a critical parameter for NASA engineers for both ground testing and in-space structural health monitoring.

Traditional strain gauges and metallic devices do not work well on flexible materials; therefore, NASA has been required to source new technologies for fabric strain monitoring.

In this research paper, released on 09 January 2017, NASA engineers evaluated different methods for measuring strain in inflatable structures. Six devices were tested, as seen in Figure 1 below. One of the methods trialed was the StretchSense Fabric Stretch Sensor.

Commercially available devices were sourced, and six of the top-performing devices were procured for structural evaluation. Five of the devices were based on resistive properties, while the StretchSense Fabric Stretch Sensor uses change in capacitance to measure strain:

  1. High Elongation Foil Strain Gauge
  2. Conductive Paint/RTV
  3. A Conductive Thread Coverstitch (developed by University of Minnesota)
  4. The Conductive Polymer Cord
  5. NanoSonic Metal Rubber (made by NanoSonic In.)
  6. The StretchSense Fabric Stretch Sensor

Figure 1. Fabric Strain Measurement Devices (reproduced from [1])

Three primary tests were conducted:

  • Low-rate tensile tests to evaluate sensitivity to small changes in length
  • Long-term creep tests to record any loss of signal under a sustained load
  • Short-term cyclic tests to identify any hysteresis or timing issues of the resistivity changes over cyclic loading

Important considerations for a strain measurement device for inflatable fabric structures:

  • Ability to measure and stretch to high strains (10-50%)
  • Ability to measure and withstand peak loads during dynamic loading
  • Ability to measure stationary loads over extended periods of time without the loss of signal/creep
  • Ability to be ruggedly adhered to or integrated with a textile and to stay fixed during the entire lifetime of the vehicle or test
  • Ability to be integrated with the textile so that it can be folded and packaged without causing snag hazards or interfering with other components

Conclusions of the testing:

As shown in Table 1 below, of the six devices tested, only the High Elongation Foil Strain Gauge and StretchSense Fabric Sensor made it to the next round for further evaluation by NASA engineers. The StretchSense Fabric Sensor was the top performer in the tests.

Table 1. Summary of results (Reproduced from [2])

The StretchSense Fabric Sensor produced an excellent response when compared to the applied strain, exhibiting:

  • Minimal noise
  • The best creep performance out of any of the sensors
  • Little to no voltage drop or hysteresis in the signal over time
  • Excellent sensitivity, even at very low strains

“The capacitance-based device, compared to the resistance-based devices, offers little to no hysteresis in the signal for creep and cyclic testing. This measure, along with its sensitivity and accuracy, makes the StretchSense device the best choice, out of those evaluated here, for Structural Health Monitoring, for inflatable structures.” — Evaluation of Strain Measurement Devices for Inflatable Structures [1]

See Figure 2 below to see the tensile and creep test data from the StretchSense Fabric Sensor.

Figure 2. StretchSense Fabric Sensor test results (reproduced from [1])

According to NASA, our Fabric Stretch Sensors will undergo further testing on sensitivity to packaging, adhesion to substrate and behavior in a vacuum to determine their suitability for measuring strain for structural health monitoring of inflatable modules. This is exciting because we’re huge NASA fans, and can’t wait to see more test data. Hopefully, we even see our sensors in space one day! If you would like to learn more about our stretch sensors please don’t hesitate to contact us.

References:

  1. Litteken, Douglas A. (2017); Evaluation of Strain Measurement Devices for Inflatable Structures; AIAA SciTech 2017 (Grapevine, Texas), Jan 9-13. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160014024.pdf
  2. Litteken, Douglas A.; Evaluation of Strain Measurement Devices for Inflatable Structures; Presented at AIAA SciTech 2017, Grapevine, Texas, Jan 9-13. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170000369.pdf

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