Santarossa A (2025)
Publication Language: English
Publication Type: Thesis
Publication year: 2025
URI: https://open.fau.de/handle/openfau/36401 https://doi.org/10.25593/open-fau-2015
Conventional rigid robotic grippers can only securely manipulate a limited range of objects. Therefore, in industrial applications such as factory automation, where the manipulation of diverse objects is required, the gripper needs to be frequently replaced.
Granular grippers represent a considerable step toward highly adaptable manipulators. Their main feature is their ability to reliably grasp objects of different shapes, sizes, and surface properties and even multiple objects at a time without the need for reconfiguration. Typically, a granular gripper comprises a granulate contained in a flexible membrane. In such a state, the granular material can flow when deformed. Thus, when the gripper is pressed onto an object, it deforms, adapting to its shape. If the air is evacuated from the gripper, the pressure difference between the ambient and the interior of the gripper causes the gripper bag to contract and compress the grains. The granulate then jams, i.e., it adopts a mechanically stable state, which makes the gripper rigid. During this transition from a liquid-like to a solid-like state, the gripper pinches the object, applying sufficient force for gripping and lifting it. If air evacuation is ceased, the gripper bag relaxes, the granulate returns to a malleable state, and the object is released.
Despite representing a promising technology, current granular grippers are not yet fully developed and optimized, and their use in industrial applications remains scarce. Since most research on granular grippers has focused on the macroscopic response of these systems when changing its parameters (granular material, material of the membrane, differential pressure), a comprehensive understanding of the particle-scale processes that underline the operation of these grippers is still lacking. Understanding the relation between the macroscopic performance of granular grippers and the microscopic structure and dynamics of the granular material throughout a gripping cycle is essential for improving their functionality and optimizing their design.
This dissertation describes the mechanical phenomena in granular materials underlying the functionality of granular grippers. To that aim, experiments and X-ray imaging are applied. The insights gained are then applied to enhance the operation of granular grippers.
As a first step, a modular granular gripper is developed that allows for automatic holding force measurements suitable for use in X-ray scanners. The apparatus is then used to investigate the effect of particle stiffness on the maximum holding force achieved by granular grippers. A new mode of operation of these grippers is discovered when soft instead of rigid particles are used, which significantly increases the maximum holding force produced by the gripper. A gripper filled with soft particles undergoes a considerable volume reduction due to particle softness, which leads the gripper to press the objects firmly. This leads to large normal forces and, therefore, significant friction between the gripper and the object. Particle softness also improves the gripper's conformation around protrusions of the gripped object, reinforcing geometrical interlocking between the gripper and object.
In a second step, the effect of particle size on the suction mechanism in granular grippers is studied. It is demonstrated that the activation of suction is connected to the size of the particles within the gripper. The gripper closely conforms to the object when small particles are used. In this case, airtight seals can form between the gripper and the object; therefore, suction is activated. If the gripper is filled with large particles, the gripper's bag is not in full contact with the object, leaving gaps between the gripper's bag and the object. If these gaps are not sealed, the pressure within such cavities equalizes with ambient pressure, hindering the suction mechanism from activating.
With these insights, design innovations are introduced to enhance the operation of granular grippers.
First, a granular gripper with an integrated, independent suction mechanism is developed. The new gripper not only enhances the maximum holding force achieved by typical granular grippers but also overcomes their common limitations. It enables the gripping and manipulation of flat and deformable objects, as well as objects larger than the gripper size, which are usually challenging for typical granular grippers. Additionally, the developed gripper allows gripping objects located on unstable grounds or even floating on water, which is challenging for most grippers (granular or others).
Secondly, a new gripper design is introduced, in which air within the gripper is replaced with a density-matched liquid. Since liquid and granulate have the same density, gravity is compensated by buoyancy. This fluidizes the granulate in the unjammed state and significantly reduces the force needed to achieve conformation and successful gripping.
APA:
Santarossa, A. (2025). Experimental research and enhancement of granular gripping systems (Dissertation).
MLA:
Santarossa, Angel. Experimental research and enhancement of granular gripping systems. Dissertation, 2025.
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