Hybrid carbon fiber-textile compliant force sensors for high-load sensing in soft exosuits

Wearable robotic systems which aid or enhance human performance are the subject of substantial ongoing research efforts. Soft exosuits to assist with walking represent one class of system that leverage textiles and apparel to provide a lightweight and nonrestrictive means to interface to the lower extremity and apply assistive joint torques via a cable that applies a force across a biological joint. Current embodiments of the soft exosuit use off-the-shelf load cells attached to the distal end of a Bowden cable to measure and control the delivered force. As these systems evolve, we envision replacing all rigid components with compliant, textile-based components. Here we present a new force sensing concept suitable for exosuit applications. The approach is based on micro-machined carbon fiber composite structures encapsulated in elastomer materials as the transducer element, and high-strength, stiff textiles as the load bearing element. The transduction mechanism uses the Poisson effect in the encapsulating elastomer to create electrical contact between the carbon fiber structures. This method allows the sensor to be sensitive in tension while remaining insensitive to bending deformation. We fabricate a prototype sensor capable of detected forces up to 300 N, yet weighing just 2.15 g. The compliant nature of the materials used in fabrication allow the sensor to be flexible. The sensor output is qualitatively very repeatable, yet exhibits moderate drift at peak load. However this drift begins to stabilize over the test duration. These preliminary results demonstrate the promising potential of this sensor technology for soft exosuit systems.