skip to main content

This content will become publicly available on October 1, 2024

Title: Kinematics and Stiffness Modeling of Soft Robot With a Concentric Backbone
Abstract Soft robots can undergo large elastic deformations and adapt to complex shapes. However, they lack the structural strength to withstand external loads due to the intrinsic compliance of fabrication materials (silicone or rubber). In this paper, we present a novel stiffness modulation approach that controls the robot’s stiffness on-demand without permanently affecting the intrinsic compliance of the elastomeric body. Inspired by concentric tube robots, this approach uses a Nitinol tube as the backbone, which can be slid in and out of the soft robot body to achieve robot pose or stiffness modulation. To validate the proposed idea, we fabricated a tendon-driven concentric tube (TDCT) soft robot and developed the model based on Cosserat rod theory. The model is validated in different scenarios by varying the joint-space tendon input and task-space external contact force. Experimental results indicate that the model is capable of estimating the shape of the TDCT soft robot with an average root-mean-square error (RMSE) of 0.90 (0.56% of total length) mm and average tip error of 1.49 (0.93% of total length) mm. Simulation studies demonstrate that the Nitinol backbone insertion can enhance the kinematic workspace and reduce the compliance of the TDCT soft robot by 57.7%. Two more » case studies (object manipulation and soft laparoscopic photodynamic therapy) are presented to demonstrate the potential application of the proposed design. « less
; ; ; ;
Award ID(s):
Publication Date:
Journal Name:
Journal of Mechanisms and Robotics
Sponsoring Org:
National Science Foundation
More Like this
  1. Tendon actuated multisection continuum arms have high potential for inspection applications in highly constrained spaces. They generate motion by axial and bending deformations. However, because of the high mechanical coupling between continuum sections, variable length-based kinematic models produce poor results. A new mechanics model for tendon actuated multisection continuum arms is proposed in this paper. The model combines the continuum arm curve parameter kinematics and concentric tube kinematics to correctly account for the large axial and bending deformations observed in the robot. Also, the model is computationally efficient and utilizes tendon tensions as the joint space variables thus eliminating the actuator length related problems such as slack and backlash. A recursive generalization of the model is also presented. Despite the high coupling between continuum sections, numerical results show that the model can be used for generating correct forward and inverse kinematic results. The model is then tested on a thin and long multisection continuum arm. The results show that the model can be used to successfully model the deformation.
  2. 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;more »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.« less
  3. Soft machines typically exhibit slow locomotion speed and low manipulation strength because of intrinsic limitations of soft materials. Here, we present a generic design principle that harnesses mechanical instability for a variety of spine-inspired fast and strong soft machines. Unlike most current soft robots that are designed as inherently and unimodally stable, our design leverages tunable snap-through bistability to fully explore the ability of soft robots to rapidly store and release energy within tens of milliseconds. We demonstrate this generic design principle with three high-performance soft machines: High-speed cheetah-like galloping crawlers with locomotion speeds of 2.68 body length/s, high-speed underwater swimmers (0.78 body length/s), and tunable low-to-high-force soft grippers with over 1 to 10 3 stiffness modulation (maximum load capacity is 11.4 kg). Our study establishes a new generic design paradigm of next-generation high-performance soft robots that are applicable for multifunctionality, different actuation methods, and materials at multiscales.
  4. INTRODUCTION: Quadriceps tendon autografts have experienced a rapid rise in popularity for anterior cruciate ligament (ACL) reconstruction due to advantages in graft sizing and potential improvement in biomechanics. While there is a growing body of literature on use of quadriceps tendon grafts, deeper investigation into the biomechanical properties of stitch techniques in this construct has been limited. The purpose of this study was to evaluate the performance of a novel suture needle against different conventional suture needles by comparing the biomechanical properties of two commonly used stitch methods, a whip stitch, and a locking stitch in quadriceps tendon. It was hypothesized that the new device would be capable of creating both whip stitches and locking stitches that are biomechanically equivalent to similar stitch techniques performed with conventional needle products. METHODS: This was a controlled biomechanical study. A total of 24 matched pair cadaveric knees were dissected and a total of 48 quadriceps tendons were harvested and tested. All tendon grafts were standardized to the same size. Samples were then randomized into the following groups, keeping the matched pairs together: (Group 1, n=16) consisted of Company W’s novel two-part suture needle design, (Group 2, n=16) consisted of Company A suture, andmore »(Group 3, n=16) consisted of Company B suture. For each group, the matched pairs were categorized into subgroups to be instrumented with either a whip stitch or a locking stitch. Two fellowship-trained surgeons performed all stitching, where they each instrumented 8 tendon grafts per group. For instrumentation, the grafts were clamped to a preparation stand in accordance with the manufacturer’s recommendations for passing each suture needle. A skin marker was used to identify and mark five evenly spaced points, 0.5 cm apart, as a guide to create a 5-stitch series. For Group 1, the whip stitch as well as the locking whip stitch were performed with a novel 2-part needle. For Group 2, the whip stitch was performed with loop suture needle and the locking stitch was krackow with a curved needle. Similarly, for Group 3, the whip stitch was performed with loop suture needle and the locking stitch was krackow with a curved needle (Figure 1). Cyclical testing was performed using a servohydraulic testing machine (MTS Bionix) equipped with a 5kN load cell. A standardized length of tendon, 7 cm, was coupled to the MTS actuator by passing it through a cryoclamp cooled by dry ice to a temperature of -5°C (Figure 2). All testing samples were then pre-conditioned to normalize viscoelastic effects and testing variability through application of cyclical loading to 25-100 N for three cycles. The samples were then held at 89 N for 15 minutes. Thereafter, the samples were loaded to 50-200 N for 500 cycles at 1 Hz. If samples survived, they were ramped to failure at 20 mm/min. Displacement and force data was collected throughout testing. Metrics of interest were total elongation (mm), stiffness (N/mm), ultimate failure load (N) and failure mode. Data are presented as averages plus/minus standard deviation. A one-way analysis of variance (ANOVA) with a Tukey pairwise comparison post hoc analysis was used to evaluate differences between the various stitching methods. Statistical significance was set at P = .05. RESULTS SECTION: For the whip stitch methods, the total elongation was found to be equivalent across all methods (W: 36 ± 10 mm; A: 32 ± 18 mm; B: 33 ± 8 mm). The stiffness of Company A (103 ± 11 N/mm) method was significantly larger than Company W (64 ± 8 N/mm; p=.001), whereas stiffness of whip stitch by Company W was equivalent to Company B (80 ± 32 N/mm). The ultimate failure load was equivalent across all whip stitch methods (W: 379 ± 31 mm; A: 412 ± 103 mm; B: 438 ± 63 mm). For the locking stitch method, the total elongation (W: 26 ± 10 mm; A: 14 ± 2 mm; B: 29 ± 5 mm), stiffness (W: 75 ± 11 N/mm; A: 104 ± 23 N/mm; B: 79 ± 10 N/mm) and ultimate load (W: 343 ± 22 N; A: 369 ± 30 N; B: 438 ± 63 N) were found to be equivalent across all methods. The failure mode for all groups is in Table 1. The common mode of failure across study groups and stitch configuration was suture breakage. However, the whip stitch from Company A and Company B had varied failure modes. DISCUSSION: Products from the three manufacturers were found to produce biomechanically equivalent whip stitches and locking stitches with respect to elongation and ultimate failure load. The only significant difference observed was that the whip stitch created with Company A’s product had a higher stiffness than Company W’s product, which could have been due to differences in the suture material. In this cadaveric quadriceps tendon model, it was shown that when using Company W’s novel two-part suture needle, users were capable of creating whip stitches and locking stitches that achieved equivalent biomechanical performance compared to similar stitch techniques performed with conventional needle products. A failure mode limited solely to suture breakage for methods completed with Company W’s needle product suggest a reliable suture construct with limited tissue damage. SIGNIFICANCE/CLINICAL RELEVANCE: Having a suture needle device with the versatility to easily perform different stitching constructs may provide surgeons an advantage needed to improve clinical outcomes. The data presented illustrates a strong new suture technique that has equivalent performance when compared to conventional needle devices and has promising applications in graft preparation for ligament and tendon reconstruction.« less
  5. This paper introduces a new class of soft reconfigurable robot: balloon animal robots. The balloon animal robot consists of a closed volume inflatable tube which can be reconfigured into structures of varying topology by a collective of simple sub-unit robots. The robotic sub-units can (1) drive along the length of the tube to localize a joint, (2) create pinch points that locally reduce the bending stiffness of the tube to form a joint, and (3) selectively mechanically couple to one another through cable driven actuators to create nodes of the structure. In this work we introduce the hardware necessary to construct the robot, present experiments to guide the hardware design, and formulate an algorithm using graph theory to calculate the number of nodes and node connections needed to form different 2D shapes. Finally, we demonstrate the system with two active nodes and four passive nodes forming multiple 2D shapes from the same hardware.