In this section, we summarize the scope of the applications of laminar jamming structures, namely their adjustable stiffness, impact response and deformation alternation between discrete and continuous.

Some of the advantages of LJS are that they can be plastically deformed in their vacuumed state, and, if coupled with a soft actuator (e.g., with PneuNets Bending Actuators) can lock in arbitrary shapes after jammed. Below you see a video demonstrating these fundamental characteristics: 

Laminar jamming can also effectively tune the dynamic responses of real-world robotic structures and systems. The impact response of soft structures (i.e., soft substrates with integrated jamming structures) can be tuned to conserve or dissipate energy as desired. For an incoming projectile with high kinetic energy, such as the baseball in the video below, relieving vacuum pressure from the jamming structures allows the energy of the projectile to be maximally preserved and the landing structure to return to equilibrium; on the other hand, applying vacuum pressure to the jamming structures rapidly dissipates the energy of the projectile and preserves the landing structure in a deformed state.

In addition, laminar jamming can be used in structures that simulate joints and can alternate between continuous and discrete deformation. The structures can perform pinch or wrasp graps, depending on the vacuum pressured applied to the LJS. With vacuum is off, the system displays continuous deformation with nearly constant curvature. When vacuum is on, the system exhibits discrete deformation with joints; rigid sections with low curvature are connected by flexible sections with much higher curvature, approximating the kinematics of a traditional articulated manipulator.

Laminar Jamming Structures are composed of stacked layers of flexible sheets, which are very compliant in their natural state. This feature enables the components to conform to complex shapes, allowing them to be used in biomedical applications and in a number of wearables. In addition, the choice of material and number of layers of these structures can be used to tune the desired stiffness and damping ability for specific applications.

The morphology of the LJS - meaning the structure, size and shape - can be easily changed during the fabrication process, allowing fast prototyping and a number of iterations in order to enhance the structure's performance. In addition, jamming structures have the ability to shape-lock, i.e., if they are deformed to a specific shape when they are in their natural, unjammed state and then vacuum is turned on, they can retain the deformed shape and resist static loading. 

Furthemore, a number of other variables can affect the stiffness of the LJS, such as the choice of material, the thickness of layers as well as the total number of layers. For more information, refer to the Design Tool section. 

In this section, we will provide step-by-step instructions on how to fabricate Laminar Jamming Structures in a laboratory environment. Please refer to the Design section or the Design Tool for recommendations on design choices. We will additionally cover alternative methods of fabrication in the Classroom section, which illustrates accessible and fast methods of prototyping LJS.

The main goal in the fabrication of LJS is to obtain a component which is completely airtight - except for a small tube connection - and in which the enclosed material (here referred to as filling) is arranged in layers that are in contact with each other in all areas. The surface contact area  between layers should be maximized, ensuring that the entire structure gets jammed . 

An overview of the process is provided below:

• Set-up: Cut acrylic frame and filling material
• Step 1: Vacuum form TPE layer to the acrylic frame
• Step 2a: Heat press TPE case with filling
• Step 2b: Impulse seal TPE case with filling
• Step 3: Look for leaks and adjust tube connection

In this section, we provide alternative resources for fabrication of Laminar Jamming Structures that could be achieved in an educational setting. Refer first to the documentation in Fabrication: Laboratory for details on the process in a well-resourced laboratory. 

We provide simpler alternatives to two major steps in fabrication:

• Sealing of the airtight bag
• Application of vacuum pressure

Tuning the Impact Response of Drones

This case study describes the design of a Laminar Jamming Structure integrated into drones, here referenced as unmanned aerial vehicles (UAV). The work was carried out by Narang, et al (2018B). 

 

 

Two-Fingered Grasper

This case study describes the design of a two-fingered grasper that can perform pinch or wrasp grasps, according to the pressure applied in its Laminar Jamming Structures. The work was carried out by Narang, et al (2018A).

 

 

 

 

 

In this section, we present some of the physical principles that explain the behavior and mechanical properties of Laminar Jamming Structures when jammed and unjammed. We also provide physical equations that can guide design decisions for the fabrication of LJS. Lastly, we provide a design example of a variable-stiffness problem and some relations between structural characteristics and damping ability. 

Stiffness Calculation

By definition, stiffness is the measurement of the resistance to deformation. In a regular 3-point bending test setup as the one shown on the left, stiffness can be calculated by the relation Applied Load / Deflection, as given in the following formula:

where k = stiffness, F = applied load, d = deflection, E = Young's modulus, L = Distance between supports and I = area moment of inertia.

For an unjammed Laminar Jamming Structure, the moment of inertia is calculated considering that each layer is completely independent, whereas, for a jammed LJS, we can consider that the layers are fully coupled.

This difference in moments of inertia leads to a radically different calculation of stiffness for jammed (kj) and unjammed (k_o) structures, in which the ratio between the two (which is also known as the range) is n².

As it can be seen from the image below, the stiffness of the LJS can be also calculated from the slopes of the linear regions in the force-deflection curve. 

Yield Point and Critical Force

The yield point for a Laminar Jamming Structure is defined as the point after which any deflection will cause the layers of the structure to slip with respect to one another. Therefore it is proportional to the coefficient of friction between layers. Because the structure slips due to shear, the yield occurs when the shear stress in the structure equals the maximum shear stress (τ_max) that can be supported by friction (μP). Note that the maximum shear stress in the rectangular beam is 3V/2A. After yield, the structure becomes much less stiff.

 

 

 

 

 

 

 

 

 

 

Design LJS for Variable Stiffness

If I load my sample with F_stiff Newtons applied in the center and don't want the structure to deflect more than Dstiff mm in the jammed state, I should pick k_j such that:

If I load my sample with Fcompliant Newtons and want it to deflect by at least D_compliant in the unjammed state, I should pick k_o such that:

Make sure your load F_stiff does not exceed F_critical, as the stiffness decreases after yield, as mentioned earlier.

Please note that these calculations were made with the assumption that the applied load was done under a 3-point bending test. You may use/derive different formulas for different loading conditions. 

 

Design LJS for Variable Damping

Damping corresponds to the amount of energy dissipated by your structure. There is not a simple analytical expression to calculate it according to your structure, but damping should scale linearly with μ, P and n. 

If you want to use your Laminar Jamming Structure as an energy dissipation mechanism, make sure that the structure is jammed and that the forces applied are above the yield point.

In order to evaluate Laminar Jamming Structures, we present some empirical tests that may help you quantify and characterize the performance of the jamming structure.

Bending Test

This test was performed in Narang et al (2018).

In 3-point bending testing done in a universal materials testing device (e.g. Instron), important comparisons between properties of Laminar Jamming Structures when vacuum is on or off can be drawn. The structures were placed on the fixture according to the setup (left image) and loaded until a maximum displacement was achieved. Force and displacement measurements were simultaneously recorded.

The mechanical behavior of LJS can be summarized by the image in the right. When vacuum is off, the structure has low bending stiffness, which is proportional to the slope of the curves. When vacuum is on, the structure has three deformation regimes. In pre-slip, the bending stiffness is maximal and constant. After the first critical load, the structure enters the transition regime, in which the layers begin to slip and the bending stiffness decreases. Note that the first critical load corresponds to the yield point. After the second critical load, the structure enters full-slip, in which the layers have slipped at all possible points along their interfaces. The bending stiffness is minimal and constant. When slip occurs, energy is dissipated to friction between the layers, and the structure behaves plastically.

In the experiment above, the effect of the number of layers and of vacuum pressure on the mechanical behavior of the LJS is also displayed, illustrating the different ways in which properties such as stiffness and maximum yield can be tuned.

The 3-point bending test can also be replicated in a classroom setting with calibration weights, for purposes of qualitatively assessing the changes in LJS's stiffness in a simple way. In the video below, you can see a demonstration of this easily replicable test.