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Universal Robotic Gripper based on the Jamming of Granular Material
Eric Brown, Nicholas Rodenberg, John Amend, Annan Mozeika, Erik Steltz, Mitchell R. Zakin, Hod Lipson, Heinrich M. Jaeger
TL;DR
The paper addresses the challenge of gripping unfamiliar objects without the complexity of multi-fingered manipulators. It introduces a vacuum-jammed granular gripper and shows that minute volume changes enable reliable, strong holding through friction, suction, and interlocking.
Problem
Universal grippers must handle unfamiliar objects with varied shapes and surfaces, while multi-fingered designs require many joints, force sensing, and computational control.
Method
A single granular-material mass conforms around an object and, under vacuum, jams into a rigid gripper whose holding force is modeled through friction, suction, and geometric interlocking.
Results
Less than 0.5% volume change reliably gripped a wide range of objects, with 100% success across 10 trials for each tested object group.
Takeaways & Limitations
Gripping strength is governed by the jammed material’s mechanical strength and confining pressure, enabling adaptive gripping without active feedback.
Takeaways & Limitations
Soft targets experience less holding force because interface pinching pressure is limited by the combined deformation of the gripper and target.
Abstract
from arXiv · showhide
Gripping and holding of objects are key tasks for robotic manipulators. The development of universal grippers able to pick up unfamiliar objects of widely varying shape and surface properties remains, however, challenging. Most current designs are based on the multi-fingered hand, but this approach introduces hardware and software complexities. These include large numbers of controllable joints, the need for force sensing if objects are to be handled securely without crushing them, and the computational overhead to decide how much stress each finger should apply and where. Here we demonstrate a completely different approach to a universal gripper. Individual fingers are replaced by a single mass of granular material that, when pressed onto a target object, flows around it and conforms to its shape. Upon application of a vacuum the granular material contracts and hardens quickly to pinch and hold the object without requiring sensory feedback. We find that volume changes of less than 0.5% suffice to grip objects reliably and hold them with forces exceeding many times their weight. We show that the operating principle is the ability of granular materials to transition between an unjammed, deformable state and a jammed state with solid-like rigidity. We delineate three separate mechanisms, friction, suction and interlocking, that contribute to the gripping force. Using a simple model we relate each of them to the mechanical strength of the jammed state. This opens up new possibilities for the design of simple, yet highly adaptive systems that excel at fast gripping of complex objects.
I. RESULTS AND DISCUSSION
The gripper adapts to diverse objects by vacuum-jamming granular material after it conforms around them. Holding force arises from friction, suction, and interlocking, whose contributions depend on contact geometry and jammed-material strength.
- Universal gripping: A 100% success rate was achieved in 10 trials for a wide range of objects using approximately 75 kPa jamming pressure.Objects included bulbs, M&Ms, LEDs, bottle caps, tubing, ear plugs, hardware, and office supplies; failures involved insufficient side coverage or very soft objects.
- Suction: An airtight seal increases holding force by about 10 times relative to porous or powdered surfaces that seal poorly.The seal permits vacuum formation in the gap beneath a smooth sphere.
- Gripping mechanisms: For contact angles above π/4, friction grips porous spheres, suction strengthens smooth-sphere gripping, and interlocking adds force above π/2.Below approximately π/4, gripping strength vanishes; the regimes were isolated with sphere experiments.
- Jamming operation: Vacuum-induced contraction tightens object contact while jamming the granular packing into a rigid state.The packing is initially malleable and conforms to the target before evacuation.
- Mechanistic model: Holding-force models relate friction and suction to pinching stress, while interlocking depends on the jammed material’s stress-strain response.Fits gave µ = 1.04±0.06, σ∗ = 50±4 kPa, and d = 1.07±0.07 mm.
- Jammed strength: A confining pressure of 80 kPa produced δV/V = −0.004, and the corresponding 0.0013 linear strain yielded a 50 kPa compressive stress.Releasing the vacuum reversibly dilated the packing and restored a malleable state.
II. CONCLUSIONS
The gripper reliably adapts to diverse objects with less than 0.5% packing-density change, using friction, suction, and interlocking; jamming predicts performance across key object properties.
- |δV/V | < 0.5% enables reliable adaptation to diverse objects through vacuum-induced jamming and unjamming.The transition changes the gripper from shape-conforming to rigid while requiring no active feedback.
- Friction, suction, and geometrical interlocking together provide versatility and holding strength.Only a fraction of an object's surface needs to be gripped securely.
- The model relates gripping performance to Pjam and granular stress-strain behavior, predicting scaling with object size, roughness, and surface normal angle.The roughness prediction applies to the extent that an airtight seal can form.
- Jamming grippers may suit rapid handling of varied objects when feedback is difficult or expensive.The gripper can adapt autonomously when target shape, material properties, or precise positioning are unavailable.
III. MATERIALS
The experiments used a CRS A465 robotic arm and ground coffee as the granular material for pick-and-place testing.
- Pick-and-place tests used a CRS A465 robotic arm with high-pressure air lines and an embedded solenoid valve.The arm controlled the gripper's vacuum operation.
- Ground coffee was selected because it performed well in jamming hardness tests and had relatively low density.Its low density allowed large grippers to be filled without excessive weight or membrane strain.
V. SUPPLEMENTARY MATERIAL
Supplementary tests characterize resistance to off-axis forces and torques, showing how contact geometry, friction, jamming pressure, and normal force shape holding performance.
- Off-axis force tests applied radial loading at θ = π/2 to a sphere embedded in the evacuated gripper.The sphere had radius R = 19 mm, and the maximum force before failure was designated Fh.
- Torque tests used a string attached to the sphere surface, giving a lever arm equal to the sphere radius.On-axis and off-axis torques were applied at angles π/2 and π, respectively.
- Holding torques from friction are modeled as Th = FfR sin θ for on-axis loading and as an azimuthally averaged expression for off-axis loading.The model uses friction because suction supplies only forces normal to the sphere surface.
- The torque models were simultaneously fit using Ff as the only adjustable parameter.The fitted Ff was about 50% larger than the on-axis-force value because contact area varies with gripper deformation.
- Applying an external normal force can increase sustainable torque by providing additional friction.This configuration is relevant when the gripper is rotated after being pushed onto an object, such as a round door knob.
- Significant holding torques occurred at small contact angles and even when Pjam = 0.Torque-deflection curves were measured with the sphere pushed partially into the gripper.