Sensing force on the body is useful in the design of many on-body systems, including gas-pressurized space suits, for design diagnostics (e.g., determining where an on-body system is exerting potentially dangerous amounts of force) and for in-use monitoring. Mechanical Counter-Pressure (MCP) space suits have advantages over gas suits, and are an example of where measuring on-body force would be necessary. The development of an unobtrusive and practical means of measuring force in an MCP suit has yet to be established. Reasons for the absence of an established method for force-sensing within an MCP suit lie in the difficulties associated with integrating a force sensor, unobtrusively, into the under-layers of the user’s garment. If the sensor introduces pressure points (e.g., is rigid or bulky), it will potentially cause harm to the user. This thesis describes the process of developing a soft and unobtrusive force sensor that avoids the use of a stiff apparatus. Specifically, the criteria for sensor selection and initial characterization of a variety of candidate sensor configurations when exposed to an applied load will be discussed. This thesis focuses on three experiments that were performed. The first experiment featured a commercial piezoresistive flex sensor as well as a custom coverstitched stretch sensor that were adapted to respond to normal forces and evaluated in a laboratory compression test. The flex sensor response displayed considerable noise, particularly evident in recovery artifacts when the load was fully removed from the sensor. The coverstitch sensor, on the other hand, had a more consistent, linear response in relation to the load being applied. Based on the findings and analysis of the first test, a second experiment was performed to examine the accuracy and performance of different lengths and widths of the coverstitched stretch sensor. The findings concluded that the thin 2’’ coverstitched sensor displayed the most promising results in terms of overall correlation with applied load and standard deviation between trials in relation to the other coverstitched samples that were tested. The final experiment extended the findings from the second experiment to test two different support structure substrates, rubber and silicone, each implemented in a topography of small hemispheres in varying size (small-diameter hemispheres, medium-diameter hemispheres, large-diameter hemispheres, and a flat topography). The results of the third experiment showed promise for the flat topography, which exhibited the strongest correlation between sensor response and applied load for both elastomer rubber and silicone substrate materials. Results were less favorable for the more extreme large-diameter hemisphere topography, which exhibited a weaker correlation indicating the larger the diameter the hemisphere was in the substrate material, the weaker the correlation between the load being applied to the sensor that was overlaid on the substrate material and the sensor’s response (resistance readings). The development of an unobtrusive, form-fitted stretch sensor that measures force is a significant step forward for MCP suit design and controllability, as well as for many other domains in which sensing forces on the body is important. The results of this thesis study illustrate the difficulties associated with implementing a flex sensor onto a pliable surface. Additionally, this thesis study illustrates the potential that the coverstitched stretch sensor has for force-sensing applications.
University of Minnesota M.S. thesis. December 2016. Major: Design, Housing and Apparel. Advisor: Lucy Dunne. 1 computer file (PDF); iv, 137 pages.
Berglund, Mary Ellen.
Development of Form-Fitted Body-Worn Force Sensors for Space and Terrestrial Applications.
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