New Sensors Accurately Measure Force and Strain in Any Direction

Prof. Hao Wu from the Huazhong University of Science and Technology's Flexible Electronics Research Center, State Key Laboratory of Intelligent Manufacturing Equipment and Technology, and School of Mechanical Science and Engineering led a recent study published in Nature Science Reviews.

New Sensors Accurately Measure Force and Strain in Any Direction
The ZPR structure is constructed by combining concave structures (green) and hexagonal structures (purple) in an elaborately designed position, while the strain concentration structure (orange) is embedded in the middle of hexagonal structures. The red dotted wireframe represents a single element of the ZPR structure. The insert in the red wireframe illustrates the local strain distribution of the strain concentration structure under uniaxial stretching, showing the bottom of the wavy structure has the highest tensile strain. Image Credit: ©Science China Press

The past ten years have seen a surge in the exotic sensing capabilities of flexible sensors; however, the lack of independent perception of multi-axial stimuli makes it difficult to measure complex deformation, which typically results from forces or strains from multiple axes.

The primary barrier to independent perception of biaxial stimuli is Poisson's effect of sensing materials. The researchers observe that materials with a zero Poisson's ratio (ZPR), which maintains a constant transverse width under longitudinal strain, may be able to resolve interference problems in the perception of biaxial or multiaxial stimuli.

As elastomers have a nearly 0.5 Poisson's ratio and are incompressible, creating zero Poisson's ratio elastomer membranes is a significant challenge. Professor Wu and his student, Dr. Xin Huang, believed that combining the conventional positive Poisson's ratio (PPR) structure with the negative Poisson's ratio (NPR) structure may effectively achieve zero Poisson's ratio structure.

The team found that the Poisson’s ratio of a hybrid structure was the superposition of the Poisson’s ratio of individual structures.

The feature size of PPR and NPR structures represents the proportion of PPR and NPR on the overall Poisson’s ratio, and the change of feature size and width of the hybrid structure can vary the Poisson’s ratio between positive and negative.

Hao Wu, Professor and Study Principal Investigator, Huazhong University of Science and Technology

The researchers estimated the Poisson’s ratio of the hybrid structure using finite element analysis and determined the ideal parameters to produce the ZPR membrane. The Poisson's ratio of the PDMS membrane with a hybrid structure was 0.07, whereas the Poisson's ratio of the PDMS membrane without a hybrid structure was 0.43. This suggests that the hybrid structure has a mitigating effect on Poisson's ratio.

ZPR membrane-based flexible sensors could both independently and precisely detect uniaxial stimuli and biaxial stimuli. The electric resistance of the sensing units along the stretching direction increased linearly when the flexible sensor was stretched uniaxially, but the resistance of the sensing units perpendicular to the stretching direction showed little increase regardless of the tensile strain. This allowed the flexible sensor to detect biaxial stimuli independently because sensing units along a specific axis only reacted to tensile strain in that axis.

When robotic manipulation and complex deformation are involved, the ZPR flexible sensors can precisely detect force, strain, and motion status. In particular, regardless of the deformation of the objects being grasped, the ZPR flexible sensors can precisely measure the contact forces between rigid manipulators and grasped objects.

The ZPR flexible sensors attached to manipulators’ fingers have two perpendicular sensing units that work together to detect both unexpected collisions with obstacles and normal finger bending. ZPR flexible sensors can also determine the biaxial soft robot’s direction and distance of locomotion.

The exotic sensing capabilities of ZPR sensors have great potential for applications in healthcare, human-machine interfaces, and robotic tactile sensing.

Hao Wu, Professor and Study Principal Investigator, Huazhong University of Science and Technology

Journal Reference:

Huang, X., et al. (2024) Flexible sensors with zero Poisson's ratio. National Science Review.

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