In terms of human-machine interactions, soft grippers offer advantages, but many of them face the challenge of low response time. Bistable structures can improve this characteristic, but predefined structural parameters and grasping modes limit the performance of grippers that use such structures.
Nature of Bistable Structures
In a dynamical system, bistability refers to the system which has two stable equilibrium states. A bistable structure can be resting in either of the two states. A light switch is an example of a mechanical device that is considered bistable. The switch lever can rest in the "on" or "off" position, but not between the two.
Bistable mechanical systems with two local potential energy minima can rest in either of the two equilibrium states without external loadings. A snap-through action may happen under suitable conditions or loading, during which such systems show distinct properties from linear structures.
Over the recent years, efforts have been made to unveil the geometrical configurations of bistable structures from fundamental units to more complex ones. However, such actions were merely from a mechanical perspective, and bistability arises intrinsically from the topology of the systems. Recently, experts have investigated bistable structures in other physical fields such as electric, magnetic, and thermal.
Reprogrammable Bistable Actuators
In a recent initiative, a group of researchers from the Chinese Academy of Sciences (CAS) has introduced a first-of-its-kind soft gripper based on a reprogrammable bistable actuator. Unlike the existing bistable actuators, this pioneering design enables precise control over diverse sensitivities. It also provides several gripping modes and adjustable response speeds with the help of straightforward reprogramming.
The study "Reprogrammable Bistable Actuators for Multimodal, Fast, and Ultrasensitive Grasping" outlines a soft gripper composed of a bistable frame and soft pneumatic bi-directional actuator joined by an unstretchable cable. During the programming process, the pneumatic actuator shortens longitudinally, allowing it to pull the frame through the cable.
This process slowly accumulates strain energies in the lateral plates and reduces the required energy for the fast snap-through of the structure. As a result, the bistable structure demonstrates its reprogrammable nature by adapting to different intermediary states with different sensitivities.
In this study, the scientists analyzed the force-displacement relationship of the frame and predicted trigger forces. The research findings reveal that by reprogramming the sensitivity of the bistable structure, the force required to initiate a fast snap-through can be less than 0.005 times its maximum value.
The researchers also prototyped multiple grippers to highlight the uniqueness of the proposed actuator. This was done by conducting tests that showed swift, multimodal, and empathetic grasping capabilities. Moreover, the gripper also demonstrated the ability to respond to the contact of a swimming fish and capture it within only 0.18 seconds.
According to study lead author Dr. Li Yingtian, their research team has devoted years to exploring the ultra-tunable bistable structure. They are currently conducting ongoing research to investigate the structure's additional applications in bioengineering and robotics to prove their practicability.
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