Overview
Integrating flexible pressure sensors based on micro/nanostructures into robots enables sensitive tactile perception. However, conventional symmetric structures such as pyramids and hemispheres typically detect only force magnitude and cannot determine force direction. Tactile feedback is affected not only by normal pressure but also by shear and friction forces, for which direction and magnitude are both important.
According to Mems Consulting, a research team from the Shenyang Institute of Automation, Chinese Academy of Sciences, together with the School of Mechatronic Engineering at Henan Agricultural University, published a paper in Advanced Science titled "Skin-Inspired Capacitive Flexible Tactile Sensor with an Asymmetric Structure for Detecting Directional Shear Forces." The first and corresponding author is Researcher Haibo Yu, and the other corresponding author is Researcher Lianqing Liu.
Design and Fabrication
Inspired by skin structure, the study designed a capacitive flexible tactile sensor that uses an asymmetric microhair (microhair) array as the dielectric layer to detect shear forces and their directions. The asymmetric microhair array was fabricated using two-photon polymerization (TPP) followed by replication. Because of the microhairs' asymmetric geometry, different shear directions produce different deformations. The device can therefore determine both static and dynamic shear directions.
The researchers designed a tilted microhair array (TMHA) as the dielectric layer for the capacitive tactile sensor, drawing inspiration from skin anatomy. Human skin is a multilayer system that includes the epidermis, dermis, subcutaneous tissue, and many mechanoreceptors, as shown in Figure 1a. The tilted microhair structure exhibits biomimetic properties and can be used to fabricate tactile sensors based on TMHA, as shown in Figure 1b.
Figure 1. Biomimetic mechanism, fabrication process, and characterization of the TMHA-based tactile sensor
Simulation of Deformation
The team simulated TMHA deformation to study how the dielectric gap responds under normal pressure and bidirectional shear loads.
Pressure Sensing Characteristics
The TMHA-based capacitive sensor shows a wide pressure response range and high sensitivity at low pressure. The pressure sensing mechanism of the tactile sensor is illustrated in Figure 3c.
Figure 3. Pressure sensing characteristics of the TMHA-based tactile sensor
Directional Shear Sensing
Simulations of the dielectric gap under directional shear loading indicate that signal trends from the TMHA-based capacitive sensor can distinguish shear direction. The shear sensing mechanism of the tactile sensor is shown in Figure 4a.
Figure 4. Shear sensing characteristics of the TMHA-based tactile sensor
Robotic Integration and Applications
The TMHA-based tactile sensor also demonstrated potential for integration into dexterous robotic hands, offering flexibility, sensitivity, and adaptability for human-machine interaction.
To improve perception and accuracy in fine manipulation tasks, the researchers mounted a TMHA-based tactile sensor on a robot thumb as tactile electronic skin (e-skin). The related experiments and results are shown in Figure 6.
Figure 6. Application of a robotic hand instrumented with TMHA-based tactile sensors in grasping experiments
Conclusion
In summary, this study proposed using a TMHA-based tactile sensor to detect both normal pressure and directional shear forces. The asymmetric TMHA structure generates different bending moments under shear forces from different directions. The TMHA-based sensor demonstrated high sensitivity and stability for both pressure and shear sensing. The research also explored integrating TMHA-based sensors into robots and dexterous hands to enhance interaction performance and safety. Robotic grasping experiments demonstrated the sensor's potential for tactile feedback in robotic systems. These TMHA-based tactile sensors offer a new direction for the design of shear force sensors and could be applied in future human-machine interfaces.
