Donut load cell

Current studies on detecting force and slip mainly focus on various methods such as resistive, piezoelectric, triboelectric, and magnetic systems to gauge contact force, from which essential information is gathered to identify slip. For example, Li and colleagues designed a resistive sensor that identifies slip by observing the high-frequency parts of the voltage signal generated as a response to applied force. In another approach, Thomas and his team created a force sensor using a digital micro-barometer, allowing slip detection by monitoring object movement through a network of sensor components. However, these investigations either depend on intricate sensor arrays or utilize devices that generally need over one second for effective slippage detection, and none have specifically targeted the minimum force required for grasping. Moreover, these studies are limited to detecting force and slip on flat surfaces with rigid grippers when holding objects, while detection on flexible surfaces of soft grippers with deformations caused by contact faces challenges due to bending stress affecting sensitive components.
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A common solution is to create sensors that respond only to normal stress while ignoring bending stress. Meng and his team engineered a single-sided electrode along with a stress-absorbing layer to secure the contact area between the resistive layer and the electrode as it bends, producing a bending-insensitive force sensor achieving a sensitivity of 11.74 N−1 across a force range of 1 N. Similarly, Xu’s work involved a force sensing module that remained unaffected by external stretching by utilizing the difference in elastic modulus between the force and stretching sensing modules, which concentrated all bending deformation onto the stretching sensing module when bent. This module showed a sensitivity of 1.30 N−1 with a force range of 58 N. Nevertheless, these studies focus mainly on force detection and overlook slip detection at the bending interface between soft grippers and items, which is vital for handling delicate objects. Additionally, some sensors in these studies have limited sensitivity due to their bending-insensitive designs that absorb stress from the sensing element, whereas others offer high sensitivity but only in a narrow force range of 1 N. As a result, the electrical signals are weak and highly vulnerable to background noise, rendering them ineffective for force and slip detection in soft grippers.
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In their research, the authors created a device made from a hybrid bilayer film of BTO and PDMS, which was developed using a combination of 3D printing and transfer printing techniques, with copper used as the contact material. Chen and colleagues [34] constructed a composite piezoelectric layer by integrating PVDF fibers onto a PDMS film, and this was combined with nylon to create a hybrid nanogenerator, which achieved a sensitivity of 14.98 N−1 within a force range of 2.65 N. Thanks to the mechanisms of tribo-piezoelectric coupling, the generator’s electrical output exceeds the sum of the individual triboelectric and piezoelectric effects [35]. Nevertheless, these investigations have primarily focused on assessing the electrical capabilities of tribo-piezoelectric coupled nanogenerators on flat surfaces, with limited exploration of their performance on curved surfaces.

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