Proceedings of the 2017 18th International Conference on Advanced Robotics (ICAR) Hong Kong, China, July 2017
Origami-Inspired Bi-Directional Soft Pneumatic Actuator with Integrated Variable Stiffness Mechanism Ajit R. Deshpande
Zion Tsz Ho Tse
Hongliang Ren*
Department of Bioengineering
College of Engineering
University of Louisville Louisville, Kentucky, 40201, USA
[email protected]
The University of Georgia Athens, GA, 30602, USA
[email protected]
Department of Biomedical Engineering National University of Singapore Singapore, Singapore, 119077 *
[email protected]
potential in minimally invasive endoscopic surgery, allowing for flexible, controllable endoscopes which will minimize damage to surrounding tissue while providing the necessary dexterity and strength [10].
Abstract - The field of soft robotics has a wide array of applications, particularly in human-robotic interaction, from medical devices to assembly technology. In this paper, we introduce a novel design for a soft bi-directional pneumatic actuator inspired by the principles of origami. The actuator integrates a variable stiffness application using a layer jamming mechanism (LJM). LJM utilizes the effects of negative pressure on thin layers of material, providing rigidity. Incorporated into an origami bellows structure, the negative pressure causes both contractile action and stiffness, while extensive action is caused by an internal pneumatic chamber, allowing for contractile and extensive force application. Furthermore, the variable stiffness integration improved tensile force application threefold, resistance to outside linear force tenfold, and doubled sheer force resistance. The proposed origamiinspired bi-directional soft pneumatic soft actuator has immense potential to be implemented in complex biomedical applications in the near future.
A key component of soft robotics is the ability to provide flexibility and adaptability to robotic components. To help accomplish this, the use of variable stiffening technology can be applied. Variable stiffness, much like the name implies, is the ability for a material or structure to have controllable rigidity. In soft robotics, there are a variety of methods to induce variable stiffness, including the use of embedded thermoplastics [11] or through the use of a jamming mechanism. Jamming mechanisms stiffen when the material is subjected to a negative pressure environment, “jamming” the material together. However, traditional jamming mechanisms rely on particulate materials such as sand or coffee grounds, which can add considerable weight to the device [12]. A new method termed “layer jamming” has recently been introduced, which demonstrates how thin leaves of material that are stacked together can achieve the same results as previous jamming mechanisms while being substantially lighter [13][14]. From previous literature, the concepts of paper constrained pneumatic actuators and layer jamming mechanisms (LJM) embedded into actuators have been introduced. Several papers have utilized embedded linear layer jamming mechanisms in silicone actuators [15]. Additionally, the use of origami as the basis for pneumatic actuation has recently been demonstrated [16].
Index Terms – origami, soft robotics, variable stiffness, layer jamming mechanism, pneumatic actuation I. INTRODUCTION Origami is a growing field of research, particularly in the biomedical field, largely due to the ability of the structures to compact or enlarge in size, as well as its scalability. Because of these properties, origami principles have been applied to designs used in catheterization [1], drug delivery [2], microfluidics [3], microsurgery [4][5][6], and stent grafts [7]. It has also been shown to provide reinforcement to moveable devices [8]. Furthermore, origami structures can be fabricated quickly through the utilization of 2D and 3D printing technologies [9].
The actuator introduced in this paper presents a novel combination of an LJM with 3D origami structure. Through the stacking and folding of multiple layers of thin material, an LJM retains the form and freedom of movement provided by a single layer origami structure, while providing variable stiffness in a negative pressure environment. This paper will demonstrate this ability through the measurement and comparison of the physical properties of the actuator with an activated and deactivated layer jamming mechanism with multi-layered origami structures.
Soft robotics is a new and quickly growing field of engineering with a wide range of applications. Soft robotics allows for the creation of more compliant components, allowing for increased flexibility, compressibility, and safety in human-robotic interaction. This technology particularly has
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The rest of this paper is organized as follows: Section II covers the materials used in the composition of the device. Section III discusses the methods in which the device was evaluated. Section IV presents the data gathered from testing the device. Section V compiles and interprets the results. Section VI contains the discussion involving the significance and possible applications of the device. II. MATERIALS Construction of the actuator is composed of two main components: the origami layer jamming structure and the internal extensive pneumatic actuator. Extension and contraction are made feasible by ten stacked paper sheets folded into a bellows folding pattern (Fig. 1a), which due to its shape allows for a wide range of movement. The structure is enveloped in an elastic latex cover (Fig 1b), which allows for the formation of a negative pressure environment around the actuator walls. Pneumatic tubing attached to a diaphragm pump is used to provide a negative pressure environment. This negative pressure environment provides the necessary pressure to stiffen the paper walls, causing an increase in rigidity. Additionally, due to the shape of the structure, a contractile force is exerted on the actuator as the latex presses onto the folds.
(b) The completed origami layers (left) and the device with elastic latex cover (right) Fig. 1 (a) The schematic of the origami structure; (b) the origami structure of the layer jamming mechanism, and the completed device with elastic latex cover
To provide an antagonistic extensile force, an elastic pneumatic chamber composed of latex is placed inside the structure and attached to the end plates on both sides of the actuator. Pneumatic tubing is used to provide positive air pressure from a syringe with a fixed volume of air to prevent rupturing. Together, these two components allow for bidirectional linear actuation. III. METHODS To quantify the mechanical properties of the actuator, the device was subjected to an array of tests. These tests were conducted using a Universal Testing Machine (UTM) (Instron 3345, Single Column Testing System, Fig. 2). The mechanical properties evaluated were total extensive/contractile force provided, resistance to change with and without the LJM, and resistance to bending with and without the layer jamming mechanism (LJM). A constant negative pressure environment of -50 kPa was provided through the use of a miniature diaphragm pump supplied by a 12V power source. The extensive force was provided through a 100 mL syringe attached to the internal chamber, allowing for pressurization without the risk of over inflation.
(a) The schematic of the origami structure
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IV. EXPERIMENT RESULTS A. Tensile Test
Fig. 2 Testing set-up on the Instron machine for the tensile and cycle testing (The yellow arrow is indicating the soft robotic LJM actuator)
A. Tensile Test In this test, the actuator was secured in a compressed state of 5 cm onto the Instron machine by attaching endplates to the ends of the actuator and securing the clamps onto the endplates. The Instron machine then proceeded to stretch the device to a given displacement of 55 mm, which correlated with its maximum size. The amount of force needed to stretch the actuator was calculated both with the LJM activated and deactivated. This data was then compiled to generate a relationship between force and displacement (Fig. 3). B. Extension/Contraction Cycle Test In this test, the actuator was secured in a relaxed state of 20 cm in a similar configuration as the tensile test. The Instron machine then cycled through extension and contraction of the device through multiple cycles while collecting the stress and strain on the system. This test was conducted while the LJM was activated and deactivated (Fig. 4). A second cycle test was then conducted where the LJM was activated midway through the procedure to demonstrate the variable stiffness aspect of the device (Fig. 5).
Fig 3 Raw data from tensile test compiled by Instron software into graphs demonstrating (a) the initial comparison run (top), (b) each individual run of the subsequent testing of the deactivated LJM (middle), and (c) activated LJM (bottom)
B. Extension/Contraction Cycle Test
C. Three Point Bending Test To measure the relative resistance to orthogonal forces, a three point bending test was conducted. The device was placed on the appropriate platform and then displaced centrally by a driven wedge. In the interest of preserving the device, the deactivated LJM was displaced 15 cm while the activated LJM was only displaced 10 cm. The forces were then measured corresponding to the displacement of the structure (Fig. 6).
Fig 4. Raw data of stress vs strain compiled by Instron software for cycle test for (a) deactivated LJM (top red), activated LJM (dark red), and zeroed 419
activated LJM (green). (b) The bottom graph demonstrates a cycle test where the LJM is turned on midway through the test
B. Compression/Extension Cycle Test The next test was to determine the variable stiffness resistivity in both extension and compression. In Fig. 4a, the obvious change in slope for the cycle demonstrates the change in elastic modulus, which from Young’s modulus has been calculated as tensile stress over strain. From Fig. 5 there is over a tenfold increase in magnitude for the elastic modulus in the activated LJM compared to the deactivated LJM in both the extensive and contractive phases of the cycle. This result reinforces that it is not just compressive force from the pump causing this change, but a change in the elastic modulus caused by the stiffening of the LJM. In Fig. 4b, the dynamic nature of LJM was demonstrated by the near instant change to the elastic modulus when the vacuum pump was switched on mid cycle. As seen from the graph, the device took less than a cycle to fully stiffen.
Fig 5. Elastic modulus calculated for the activated and deactivated LJM during the cycle testing
C. Three Point Bending Test
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As demonstrated in the force displacement graph (Fig. 6), there is a clear difference in resistance, as the activated LJM takes double the force to be displaced the same amount of distance, which is reinforced by the slope of the line of best fit. This shows that the change in stiffness caused by the activated LJM pertains to a general increase in resistivity and not just linearized in one direction.
C. Three Point Bending Test
VI. CONCLUSION From the tests conducted, there is a clear demonstration of variable stiffness provided by the LJM in the actuator. The use of this integrated variable stiffness mechanism in conjunction with soft pneumatic actuators provides for an effective component that can be utilized in the field of soft robotics. We believe that this device can function in place of a traditional actuator as part of a larger device, allowing for flexibility and compressibility when necessary. Additionally, the ability for the actuator to bend sets it apart from traditional linear pneumatic actuators, while the ability to variably stiffen allows the structure to have a higher mechanical advantage and resistance to unwanted movement compared to other soft actuators. Finally, the paper structure makes the device significantly lighter than silicone soft actuators with an embedded layer jamming mechanism.
Fig. 6 The average force vs displacement graphs for three point bending with corresponding lines of best fit
V. DISCUSSION OVER THE RESULTS A. Tensile Test As seen in the first graph (Fig. 3a), there is nearly a 300% difference for force needed to extend the actuator from a relaxed position between the non-stiffened and stiffened states. The test was then repeated 5 times for both the nonstiffened and stiffened states to determine accuracy. The nonstiffened state was very consistent, taking between 8 N and 9 N to reach 55 mm of displacement. There was a load error in the first test of the LJM activated actuator (as seen in Fig. 3c), though it still roughly followed an expected pattern. The other tests followed a more linear pattern, taking between 24 N and 30 N to reach the target displacement.
The bellows design of the layer jamming mechanism (LJM) does not inherently limit bending. Therefore, it could be used in future research via creation of a directional continuum manipulator through the addition of a wire driven mechanism or through the introduction of a multi-chambered actuator [17]. With this, wide ranging applications can be feasible such as flexible endoscopes, compliant exoskeletons, human-robotic interactions, and assembly technologies.
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[16] Martinez, R. V., Fish, C. R., Chen, X. and Whitesides, G. M. (2012), Elastomeric Origami: Programmable Paper-Elastomer Composites as Pneumatic Actuators. Adv. Funct. Mater., 22: 1376–1384. doi:10.1002/adfm.201102978 [17] Li, Zheng, Hongliang Ren, Philip Chiu, Ruxu Du, and Haoyong Yu. (2016)"A Novel Constrained Wire-driven Flexible Mechanism and Its Kinematic Analysis."Mechanism and Machine Theory
ACKNOWLEDGMENTS Ajit has been supported by the James Graham Brown Foundation, the University of Louisville, and the National University of Singapore. This work is supported by the Singapore Academic Research Fund under Grant R-397-000227-112, NUSRI China Jiangsu Provincial Grant BK20150386 & BE2016077 awarded to Dr. Hongliang Ren. Thanks are due to the members of SINAPSE and the ARC that provided knowledge and assistance on this project, particularly Ghasem Abbasnejad and Hritwick Banerjee for direction, as well as Yeow Bok Seng for his help with conducting prototype evaluations. REFERENCES [1]
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