For robots to be useful for real-world applications, they must be safe around humans, be adaptable to their environment, and operate in an untethered manner. Soft robots could potentially meet these requirements; however, existing soft robotic architectures are limited by their ability to scale to human sizes and operate at these scales without a tether to transmit power or pressurized air from an external source. Here, we report an untethered, inflated robotic truss, composed of thin-walled inflatable tubes, capable of shape change by continuously relocating its joints, while its total edge length remains constant. Specifically, a set of identical roller modules each pinch the tube to create an effective joint that separates two edges, and modules can be connected to form complex structures. Driving a roller module along a tube changes the overall shape, lengthening one edge and shortening another, while the total edge length and hence fluid volume remain constant. This isoperimetric behavior allows the robot to operate without compressing air or requiring a tether. Our concept brings together advantages from three distinct types of robots—soft, collective, and truss-based—while overcoming certain limitations of each. Our robots are robust and safe, like soft robots, but not limited by a tether; are modular, like collective robots, but not limited by complex subunits; and are shape-changing, like truss robots, but not limited by rigid linear actuators. We demonstrate two-dimensional (2D) robots capable of shape change and a human-scale 3D robot capable of punctuated rolling locomotion and manipulation, all constructed with the same modular rollers and operating without a tether.
For robots to work in conjunction with humans and be useful outside of highly engineered environments, they must be human-safe, robust, adaptable to a variety of scenarios, and capable of moving through diverse types of terrain. These attributes require not only adaptable control algorithms and the collection and processing of rich sensory information but also new forms of reconfigurable, adaptable robotic structures, which are potentially soft in nature.
We present a concept for such a robotic structure: a truss of inextensible, inflatable, constant-length tubes that are manipulated by a collective of interconnected roller modules, allowing for shape change and compliance without a pressure source (Fig. 1A). Pressurized tubes serve as structural elements and the edges of the truss. Each joint in the tubing is formed by a robotic roller module that pinches the tube between cylindrical rollers without creating a seal. The roller modules can be connected to neighboring modules to form a node of a complex two-dimensional (2D) or 3D structure. An electric motor and mechanical transmission then drive these rollers like wheels along the tube, causing the pinch point to translate (Fig. 1B). Edge lengths of the robot are changed not by stretching or contracting the edges but by movement of the roller module along the tube—moving the effective joint and simultaneously lengthening one edge while shortening another (Fig. 1C and movie S1). The sum of all the edge lengths remains constant; therefore, we call the robot an isoperimetric system (constant perimeter). A gap between the rollers ensures that as they move, there is negligible pressure difference between the two edges, leading to a system with constant volume that does not require a pressure source. The individual roller modules are simple and capable of moving along the tube in only one degree of freedom, yet the overall collective is capable of complex behavior.
(A) A large-scale inflated robot that does not require a tether. The robot is composed of a set of identical robotic roller modules that are mounted onto inflated fabric tubes that form the primary structure of the robot. (B) The rollers pinch the fabric tube between rollers, creating an effective joint that can be relocated by driving the rollers. (C) The roller modules actuate the robot by driving along the tube, simultaneously lengthening one edge while shortening another. The roller modules can connect to one another to construct 2D and 3D truss-like structures capable of shape change and locomotion. (D) The roller modules connect to each other at nodes using three-degree-of-freedom universal joints that are composed of a clevis joint that couples two rods, each free to spin about its axis. The arrows indicate how the joints can rotate. (E) The robot locomotes untethered outdoors using a punctuated rolling gait. One face of the robot is highlighted to illustrate the rolling motion.
Our robotic concept is built upon a combination of concepts from collective robots, truss robots, and soft robots. This allows us to realize a unique set of traits, because we exploit advantages while bypassing certain disadvantages of each individual type of robot.
As a collective system of robots, our concept is inherently modular with interchangeable, simple (one degree of freedom) subunit roller modules. However, because our subunits are physically interconnected through a compliant network, the collective achieves complex system-level behavior, capable of applying forces in three dimensions on a large scale. This overcomes a limitation of collective robots that combine together to create structures that can change their shape (1–4)—realizing complex 3D physical interaction while maintaining simplicity at the individual robot level. A related type of collective robotic system uses teams of robots that build passive structures (5–8). The target structure is often truss-like, built by adding passive elements, and sometimes requires that the robots traverse the structure as they build it. Rather than discretely rearranging passive elements within a structure to change its shape, in our concept, the collective continuously deforms passive bodies to change the locations of where the bodies are attached, resulting in very simple robotic subunits.
As a truss-like robot—which has been proposed for intriguing applications like exploring planets (9–12), burrowing underground (13), shoring up rubble (14, 15), and modular robotic systems (16, 17)—our concept is adaptable and customizable. However, because our robot has a compliant structure and moves without requiring linear actuators, it affords robustness that is lacking in other truss-like robots. Ideally, the linear actuators of a truss robot would be lightweight, be robust, have a high extension ratio, and operate untethered. Although certain new actuators meet some of these requirements (18–21), achieving all is challenging. This means that when existing actuators are connected into a truss system, the resulting robot is relatively rigid, slow, heavy, and lacking in robustness to large impacts. Our robot overcomes some of the challenges of conventional truss robots because the structure is composed of lightweight compliant pneumatic beams. Tensegrity robots also overcome the fragility of truss robots, but through a network of compliant cables or compliant beams that create part of their structure (22–28). Tensegrity robots can undergo large shape changes, especially volume changes for deployment, but the fact that typically only a subset of edges change length, and some edges may only support tensile loads, imposes some constraints on the possible shape change. Our robot is not a tensegrity robot, but it incorporates the compliant characteristics that have made tensegrity robots more robust, to enable tough robots that are highly adaptable.
As a soft robot, our concept is inherently human-safe and has a high tolerance to uncertainty in the environment (29–33). However, because it is a constant-volume, isoperimetric system (nodes move, but the total length of the pneumatic structure remains the same), it overcomes a fundamental limitation of pneumatic soft robots—the air supply. Previous methods to provide pneumatic power onboard include carrying a microcompressor (34, 35), carrying a pressurized fluid reservoir (36), using chemical decomposition (37), and using explosive fuels (38, 39). However, each of these is limited: Microcompressors have low flow rates and peak pressures, compressed air in a reservoir has limited overall capacity, and chemical decomposition or burning of a fuel often requires system-level integration and does not easily provide air at useful pressures and rates (40). In contrast, other soft systems use a fixed amount of air within a cavity as a structural element and not as an actuator, requiring no pressure source once the cavity is pressurized (41–46). Some of this work has exhibited direct manipulation of the membrane of an inflated beam to create bending without compressing the air within (42, 43). We built upon this work for our soft, untethered robot, but instead of manipulating a serial robot by deforming the membrane around fixed joints as in (42, 43), we continuously moved the effective joints along the structure, which allowed large, global shape change of a truss-like robot.
Here, we present demonstrations and characterizations of the collective, truss-like, and soft nature of our robots. To highlight the collective and modular nature of the robot, we present three different robots, two 2D robots and one 3D robot, each constructed from identical one-degree-of-freedom roller modules, yet as a collective, capable of complex movement. To demonstrate the truss-like nature of the robot, we show marked shape change of all three of the robots and punctuated rolling locomotion of the 3D robot. To demonstrate and characterize the softness of the robot, we show its robustness to crushing forces, measure its behavior under load, and leverage its compliance to grasp and manipulate objects. Each of these demonstrations is conducted with the robot untethered from a pressure source. Last, we present the models and experiments that inform the mechanical design of the subcomponents of the robots and provide insights into the tradeoffs among our robots, truss robots, and pneumatically actuated robots through theoretical analysis of reachable workspace, efficiency, and speed.
2D collective demonstrating truss-like shape change
We demonstrate the collective and modular nature of the isoperimetric concept by constructing two different 2D robots with the same roller modules (Fig. 2). The first robot is composed of three separate tubes, and the second is composed of a single tube. Robots with multiple tubes are interesting because the modularity is extended to robotic substructures containing multiple roller modules. For example, substructures designed for specific tasks, like grasping or locomotion, could be combined to form a variety of robots. On the other hand, robots with a single tube have fewer constraints on their configuration and larger maximum edge lengths. With both robots, we demonstrate a truss-like shape-changing ability.
(A) A robot, formed from three separate tubes that are routed into triangles and connected together, inflates and springs into shape without intervention. (B) This three-tube robot can change to a variety of shapes. Casters under the roller modules allow motion. (C) A robot composed of a single inflated tube can markedly lengthen its edges, because each edge can exchange material with any other edge. The single-tube design also means that sometimes roller modules must run to pass material through the network, even if the edge lengths immediately connected to it are not changing length. (D) A single active roller module moves, causing one adjacent edge to shorten and the other to lengthen. (E) To lengthen and shorten the two edges adjacent to the passive module, all the active roller modules move in coordination. (F) The single-tube configuration is capable of much larger edge lengths because all other edges can shorten to accommodate the lengthening of two edges.
For the first robot, each of the three individual tubes (3.4 m long and 0.1 m diameter) was routed through two active roller modules before affixing its ends to a passive module that did not contain a motor, creating a triangle. The triangular substructures were then assembled by connecting pairs of roller modules with revolute joints, showing that complex robots can be assembled from multiple simpler robots. The robot could deploy from a small area of 0.85 m2 without human intervention when air was added from an external source (Fig. 2A). After the robot was inflated to an operating pressure of 40 kPa (and an area of 2.9 m2), we removed the tether and drove the roller modules to demonstrate a few feasible shapes: a tall skinny triangle, a hexagon, a square, and a “pincer” shape that could grasp an object (Fig. 2B). It took less than 50 s for the robot to transition among all four of these shapes (movie S2). The minimum length of an edge was 28 cm for this prototype and was fixed by the size of the roller module.
For the second robot, we routed a single tube with a length of 6.8 m through eight active roller modules and a single passive module, as shown in Fig. 2C and movie S3. This single-tube architecture enabled certain behaviors that were not possible with the first, three-triangle architecture, where an edge could only lengthen if another edge in the same triangle shortens. In contrast, when a single tube was used for the entire robot, the material could be exchanged between any two edges in the network. To exchange length between edges that are adjacent, one roller module moved along the tube (Fig. 2D). For edges that are not adjacent, all intermediate powered roller modules must roll to transfer the tube material, even if the edges adjacent to the intermediate roller modules do not change length (Fig. 2E). Because any edge in the robot can contribute length to any other edge, much larger maximum edge lengths could be reached with the single-tube architecture (Fig. 2F), illustrating that the maximum length of an edge depends on the robot architecture.
3D octahedron robot: Truss-like shape change and locomotion
We used the same roller modules from the 2D robots to create a 3D octahedron, formed by connecting four individual triangles, each with a tube length of 3.4 m. As before, a triangle has two active and one passive modules. We demonstrated truss-like 3D shape-changing and locomotion.
The first demonstration of the 3D robot explored its volume change during deployment (Fig. 3A). The structure could compact to a volume of 0.173 m3 when deflated (fitting within a 64 cm–by–71 cm–by–38 cm rectangular prism) and then deploy to an octahedron with a volume of 2.29 m3, increasing by a factor of 13. Next, after untethering the robot, we showed that it is capable of markedly changing its shape, including changing its height by a factor of 2 and moving to an asymmetric configuration where one node extends upward (Fig. 3B and movie S4). Movie S5 shows a simulated robot moving according to our kinematic model (see Materials and Methods) side by side with the real robot moving. Although not a perfect agreement, the character of the robot motion is captured by the simulation. Small errors developed because of imperfections in our current fabrication methods, leading to variations in tube diameter and length. Last, we demonstrated locomotion. The robot could locomote with a punctuated rolling gait at a speed of 2.14 body lengths/minute, or 3.6 m/min (Fig. 3C and movie S6). In the current implementation, each roller module had a battery life of about 23 min under continuous roller movement (see Supplementary Text and fig. S1 for further information).
(A) The robot first inflates from a small package into an octahedron. The octahedron is composed of four individual triangles. (B) The robot can exhibit extreme shape change. A 186-cm-tall human and a 24-cm-diameter basketball are shown for size reference in some images. (C) The robot is also capable of a punctuated rolling gait, beginning with one of the four triangles as a bottom face (t = 0 s) and returning to this configuration (with a different triangle now at the bottom) after two rolling events (t = 28 s).
3D octahedron robot: Compliant behavior and manipulation
The inflated fabric tubes are compliant, a hallmark of soft robots and a property that affords robustness to the structure. To demonstrate this robustness (Fig. 4A and movie S7), we loaded the robot with a wooden pallet before increasing the load until structural failure (Fig. 4A). When the load was removed and external forces were applied to restore the structure to its initial shape, it was again able to support the initial load, undamaged. To quantify the response of the robot under load, we measured force while displacing the top roller module of a single triangle in three different configurations using the experimental setup described in Supplementary Text and fig. S5. The results are shown in Fig. 4B. When an external load was applied to a node of the truss structure, there was a relatively high initial stiffness until the load causes one of the beams to buckle, at which point the force exerted at the node markedly decreases, approaching a zero-stiffness regime. This behavior is like a mechanical fuse: During normal operation, the structure is relatively stiff, allowing functionality; yet, beyond some threshold force, it buckles, limiting damage to itself or the environment. The exact level of the threshold force could be tuned via control of the robot configuration, leveraging existing work on the mechanics of inflated beams (47–49). Because of its relatively high stiffness before buckling, the robot can carry heavy loads without substantial deformation. Figure 4C and movie S8 demonstrate the robot moving a 6.8-kg load over a trajectory. The kinematics model also allows us to predict the forces experienced by the members. Movie S9 shows the predicted axial load on each inflated member, while it changes shape in the presence of an external load similar to the experiment in Fig. 4C.
(A) Overloading the robot causes the robot to collapse. After being restored to its initial configuration, the robot is again able to support the initial load. (B) Load displacement behavior of a single triangle in three different configurations. In all cases, there is a moderate initial stiffness until a critical load is reached and the beam buckles, at which point the force required to maintain a given level of deflection is much lower than the peak value, demonstrating a mechanical fuse–type behavior of the robot. (C) The robot moves a 6.8-kg load over a trajectory.
Different recovery strategies can be invoked after an inflated beam buckles. Occasionally, the beam will recover on its own when the load is removed. This is due to the small but noticeable restoring forces seen in Fig. 4B. If a beam is unable to recover passively, it is possible for active motions of the roller modules to assist in straightening buckled beams (movie S10).
The compliance of the robot allows it to grasp and manipulate objects. We demonstrate this behavior in Fig. 5A, as the robot changed shape to engulf an object (a basketball) before changing shape to pinch the object between two of its edges. The compliant beams bent slightly around the object, increasing the contact area. Once the object was grasped, it changed the shapes of its other faces to pick the object up from the ground. The robot could also manipulate objects “in hand,” leveraging the fact that the edges are composed of continuous tubes that move relative to the nodes. In Fig. 5B, a basketball was placed between two edges of a tube. By driving the roller module closest to the basketball, the tube moved relative to the basketball, causing the ball to rotate within the grasp (movie S11).