Deployable space structures, capable of folding for launch and expanding in orbit, are crucial for space exploration. Tape-spring hinges, utilizing stored elastic energy for self-deployment, offer a lightweight solution, but monitoring their complex folding and deployment dynamics presents challenges. Traditional sensing methods, such as optical systems (videometry, laser Doppler vibrometry, digital image correlation), face limitations due to large deformations and varying lighting conditions in space. Metal-based strain sensors and fiber Bragg grating sensors, while stable, often have low gauge factors or introduce structural defects. This research addresses these limitations by proposing a novel soft electronic skin (e-skin) for in-situ monitoring of tape-spring hinges. The e-skin's soft, flexible nature allows it to conform to the significant deformations experienced during folding and deployment, providing accurate and reliable data on various structural parameters.

Existing methods for monitoring deployable structures have limitations. Optical methods like videometry and digital image correlation (DIC) struggle with large deformations and inconsistent lighting, especially in space. While laser Doppler vibrometry is effective for vibration analysis, its application to large-scale deployments is limited. Metal-based strain sensors suffer from low gauge factors, while fiber Bragg grating (FBG) sensors can introduce structural weaknesses and are brittle. The integration of sensors into thin-walled structures also presents challenges in terms of weight and size constraints. This study leverages recent advances in soft electronics to overcome these limitations, offering a more robust and adaptable sensing solution.

A three-layered e-skin was fabricated. The bottom layer is a flexible polyimide (PI) film substrate. The middle layer is a patterned copper (Cu) film providing electrical circuitry. The top layer consists of reduced graphene oxide/single-walled carbon nanotube (rGO/SWNT) thin-film strain sensors and MEMS-based accelerometers and a gyroscope. The rGO/SWNT strain sensors were fabricated using a drop-casting method onto a PDMS mold. The e-skin was bonded to a 6-inch steel tape-spring hinge using adhesive. Finite element analysis (FEA) using Abaqus Standard was conducted to simulate the hinge's behavior under bending, comparing a bare hinge with the e-skin-integrated hinge. Quasi-static folding and deformation experiments involved manually folding the hinge while recording data from the gyroscope and strain sensors. Dynamic deployment experiments involved releasing a 90-degree folded hinge and measuring the resulting acceleration and vibration using the accelerometers. FFT analysis was performed on the acceleration data to determine vibrational frequencies, which were then compared to FEA frequency analysis results.

The e-skin successfully monitored the quasi-static folding and deployment of the tape-spring hinge. The serpentine pattern of the Cu/PI traces in the e-skin accommodated large strains without material failure. The rGO/SWNT strain sensors demonstrated an average gauge factor of 50, accurately measuring both tensile and compressive strains during different folding maneuvers. The gyroscope accurately tracked the rotational motion of the hinge tip. The e-skin also effectively monitored the sliding of the folded hinge region, a critical parameter not easily measurable by other methods. The accelerometers measured the dynamic deployment process, revealing the first two vibrational modes at 90 Hz and 179 Hz, closely matching FEA-predicted frequencies of 86 Hz and 185.9 Hz. The overall mass addition of the e-skin to the tape-spring hinge was only 0.36 ± 0.02 g (10% of the total).

The results demonstrate the feasibility and effectiveness of using a soft e-skin for in-situ monitoring of highly deformable deployable structures. The high gauge factor of the rGO/SWNT strain sensors allows for accurate measurement of even small strain variations, crucial for understanding the hinge's behavior. The integration of multiple sensing modalities provides a comprehensive understanding of the hinge's motion and deformation. The close agreement between experimental results and FEA simulations validates the design and analysis methodology. This approach offers advantages over existing methods by addressing the limitations of optical and rigid sensor systems, paving the way for more reliable and efficient deployable space structures.

This study successfully demonstrated a novel soft e-skin for monitoring the folding and deployment dynamics of tape-spring hinges. The e-skin's ability to accommodate large deformations, its high sensitivity, and its multimodal sensing capabilities offer significant improvements over traditional methods. Future research should focus on the long-term effects of space environments on the e-skin components, exploring protective materials and strategies. Expanding the e-skin to larger sensor networks and integrating it into more complex deployable structures are also promising avenues for future research. The use of modularized soft sensing nodes, local power sources, and soft wireless communication circuitries could address the challenges of scaling up this technology.

The current study focused on a relatively small, 6-inch tape-spring hinge. Further research is needed to investigate the scalability and performance of the e-skin on larger and more complex structures. The long-term durability of the e-skin in the harsh space environment needs further investigation. While protective measures were discussed, comprehensive testing in simulated space conditions is required. The current e-skin design has limitations in terms of size and transmission distance. Future research should focus on developing more scalable and robust communication and power systems.