Compliant Robot Research Articles

Numerical Framework and Design Optimization of an Intrinsically Compliant 3-DOF Parallel Robot

Abstract Parallel robots are multiple degrees of freedom (DOFs) systems that are typically used in applications characterized by enhanced accuracy, rigidity, and large force requirements within a compact workspace. In the present research, an intrinsically compliant parallel robot with 3-DOFs, actuated using four pneumatic muscle actuators (PMA), is conceptualized, developed, and analyzed. Despite many benefits, parallel robots also offer certain challenges that arise from the highly coupled and nonlinear motion of their actuators. The small workspace of parallel robots has many singularities and solving a closed-form forward kinematics (FK) for its end-effector motion is complicated. The PMAs can provide intrinsically compliant robotic motions, however, since they are flexible, their unilateral actuation also poses constraints on the achievable DOFs. The present research focuses on analyzing kinematics and dynamics of the developed parallel robot incorporating the stiffness together with force closure analyses besides suggesting design improvements as a consequence of the singularity analysis. Design synthesis and multi-criteria optimization have been performed to obtain a robot design which may provide higher accuracies (near unity condition number), quick response to external wrench (stiffness and rigidity), and reduced actuator force requirements. SPEA2 (Improved Strength Pareto Evolutionary Algorithm) has been implemented to carry out the simultaneous optimization of design objectives and provide Pareto optimal design solutions.

Performance estimation of computed torque control for surgical robot application

In the current paradigm, the development in robotic technology has a huge impact to revolutionize the medical domain. Surgical robots have greater advantages over surgeon such as reduced operating time, reduced tremor, less blood loss, and high dexterity. To perform different operations during surgery a base robot is required with the task-specific end effector. In this paper, the selective compliant assembly robot arm (SCARA) has been considered as the base robot and the complete mathematical modeling of the robot is illustrated. The equation of Kinematics is derived from the D-H notation. SCARA dynamic model is derived from Euler Lagrange. In order to achieve trajectory tracking the Computed Toque Control technique (CTC) applied to the SCARA manipulator. The performance of the CTC technique for trajectory tracking of each joint of the SCARA robot has evaluated in contrast with tuned PD and PID controller. The simulation results were discussed and verified using MATLAB simulation software.

A New Position Detection and Status Monitoring System for Joint of SCARA

High-precision position detection of joint and status monitoring of speed reduction gearbox (SRG) are two key technologies in the research and development of selective compliant assembly robot arm (SCARA). The existing schemes have some problems, such as the large occupation of space, high cost, complex implementation, lack of status monitoring, etc. Therefore, first, in order to get the accurate position information of joint when the SCARA is powered up at the first time, a new combined incremental encoder with unevenly distributed multiple indexing signals and initial position detection method is proposed. Second, in order to get the accurate position information and monitor the operation status of the SRG during the positioning operation, a real-time position detection and status monitoring method based on the transmission error using dual incremental encoder is proposed. The general design principle has been given, and various experimental results have confirmed the effectiveness of the proposed solution. The proposed new combined incremental encoder has the advantage of small occupied space, easy realization, and low cost. The proposed position detection and status monitoring method not only realizes the high-precision detection of the joint position but also provides the key data for the monitoring of the running state of the SRG.

An Active Compliance Control for A Robotic Wound Closure System

Most approaches to medical robotic compliance control rely upon forces, or torques, exerted on the mechanical arm links. The majority of robotic arms allow only position control, and orientation, of the end-effector. This paper uses a soft control technique to develop a position-correction, impedance method that leads to similar responses to the contact environment for simulation. The system is applied to control the contact force between a fixture and the skin in a robotic, vision-controlled, wound closure system. The experimental results show that the algorithm is successful in controlling contact force in fixture placement on different skin models.

Counterexample to Passivity Preservation for Variable Impedance Control of Compliant Robots

Critical technical errors in a recent publication “passivity preservation for variable impedance control of compliant robots” by Emmanouil Spyrakos-Papastavridis, Peter R. N. Childs, and Jian S. Dai are highlighted making its contribution questionable. The counterexample provided gives concrete evidence that the theory and analysis in the paper is incorrect.

Compliant morphing structures from twisted bulk metallic glass ribbons

In this work, we investigate the use of pre-twisted metallic ribbons as building blocks for shape-changing structures. We manufacture these elements by twisting initially flat ribbons about their (lengthwise) centroidal axis into a helicoidal geometry, then thermoforming them to make this configuration a stress-free reference state. The helicoidal shape allows the ribbons to have preferred bending directions that vary throughout their length. These bending directions serve as compliant joints and enable several deployed and stowed configurations that are unachievable without pre-twist, provided that compaction does not induce material failure. We fabricate these ribbons using a bulk metallic glass (BMG), for its exceptional elasticity and thermoforming attributes. Combining numerical simulations, an analytical model based on a geometrically nonlinear plate theory and torsional experiments, we analyze the finite-twisting mechanics of various ribbon geometries. We find that, in ribbons with undulated edges, the twisting deformations can be better localized onto desired regions prior to thermoforming. Finally, we join multiple ribbons to create deployable systems with complex morphing attributes enabled by the intrinsic chirality of our twisted structural elements. Our work proposes a framework for creating fully metallic, yet compliant structures that may find application as elements for space structures and compliant robots.

Design, development and analysis of a conductive fabric based flexible and stretchable strain sensor

This article presents the design, fabrication and characterisation of an elastomeric flexible and stretchable strain sensor using a variable resistance fabric. This provides a viable alternative to the microfluid and nanoparticle based flexible strain sensors involving complex fabrication techniques. A nylon-spandex based stretchable fabric serves as the sensing element which is in turn embedded in an elastomeric substrate. The fabricated sensor is experimentally characterised for determining the linearity, hysteresis, stretchability and gauge factor. The Finite Element Method based simulations to predict the operational force range of the sensor corresponding to its strain sensing range has been validated with the experimental results. These sensors have a wide range of potential applications in the constantly developing field of compliant robotics and mechanisms. In future works, these sensors will be utilized in the biomechanical analysis of human movements to track the various joint parameters.

Lighter and Stronger: Cofabricated Electrodes and Variable Stiffness Elements in Dielectric Actuators

The inherent compliance of soft robots often makes it difficult for them to exert forces on surrounding surfaces or withstand mechanical loading. Controlled stiffness is a solution to empower soft robots with the ability to apply large forces on their environments and sustain external loads without deformations. Herein, a compact, soft actuator composed of a shared electrode used for both electrostatic actuation and variable stiffness is described. The device operates as a dielectric elastomer actuator, while variable stiffness is provided by a shared electrode made of gallium. The fabricated actuator, namely variable stiffness dielectric elastomer actuator (VSDEA), has a compact and lightweight structure with a thickness of 930 μm and a mass of 0.7 g. It exhibits a stiffness change of 183×, a bending angle of 31°, and a blocked force of 0.65 mN. Thanks to the lightweight feature, the stiffness change per mass of the actuator (261× g−1) is 2.6 times higher than that of the other type of VSDEA that has no shared electrode.

Energy Regenerative Damping in Variable Impedance Actuators for Long-Term Robotic Deployment

Energy efficiency is a crucial issue towards longterm deployment of compliant robots in the real world. In the context of variable impedance actuators (VIAs), one of the main focuses has been on improving energy efficiency through reduction of energy consumption. However, the harvesting of dissipated energy in such systems remains under-explored. This study proposes a novel variable damping module design enabling energy regeneration in VIAs by exploiting the regenerative braking effect of DC motors. The proposed damping module uses four switches to combine regenerative and dynamic braking, in a hybrid approach that enables energy regeneration without a reduction in the range of damping achievable. A physical implementation on a simple VIA mechanism is presented in which the regenerative properties of the proposed module are characterised and compared against theoretical predictions. To investigate the role of variable regenerative damping in terms of energy efficiency of longterm operation, experiments are reported in which the VIA equipped with the proposed damping module performs sequential reaching to a series of stochastic targets. The results indicate that the combination of variable stiffness and variable regenerative damping is preferable to achieve the optimal trade-off between task performance and energy efficiency. Use of the latter results in a 25% performance improvement on overall performance metrics (incorporating reaching accuracy, settling time, energy consumption and regeneration), over comparable schemes where either stiffness or damping are fixed.

Miniature Modular Legged Robot With Compliant Backbones

Soft Modular Legged Robot (SMoLBot) is a miniature, foldable, modular, soft-hybrid legged robot with compliant backbones. SMoLBot's body and locomotion mechanisms are folded out of acetate sheets and its compliant connection mechanisms are molded from Polydimethylsiloxane (PDMS). High maneuverability and smooth walking pattern can be achieved in miniature robots if high stiffness kinematic parts are connected with compliant components, providing the robot structural compliance and better adaptability to different surfaces. SMoLBot is exploiting features from origami-inspired robots and soft robots, such as low weight and low cost foldable rigid structures and adaptable soft connection mechanisms made out of PDMS. Each single module in SMoLBot is actuated and controlled by two separate DC motors. This enables gait modification and higher degree of freedom on controlling the motion and body undulation of the robot in turning and rough terrain locomotion. Each module has 44.5 mm width, 16.75 mm length and 15 mm height, which is approximately the same size with two DC motors and a LiPo battery. The comparisons between robots with compliant and rigid backbones demonstrate smoother walking pattern, and approximate decrease in body's roll angle from 12° to 6°, and pitch from 10° to 7°. The independent actuation and control over each leg in n number of modules make SMoLBot an ideal candidate for gait studies. Moreover, the possibility of changing the structural stiffness of the robot with different backbones enables such a compliant modular robot to be used for locomotion optimization studies in miniature scale.

A lobster-inspired articulated shaft for minimally invasive surgery

Novel applications of soft pneumatic actuation in minimally invasive surgery (MIS) are proposed due to its relatively safe robot–environment interactions. Although the inherent compliance of soft robots makes them suitable for surgery, their low force output and complicated system response and behavior may limit their potential as practical MIS instruments. In this paper, three lobster-inspired antagonistic modules are proposed to realize bidirectional translational, bending and rotational motions and variable stiffness in centimeter scale. Their modular design enables flexible combinations of articulated shafts to satisfy end-effector workspace requirements in MIS. Theoretical models are proposed to relate the input pressure, deformation, output force/torque and stiffness, which provide quantitative solutions for independent adjustment on the deformation and stiffness of each module. A series of experimental results show that the proposed modules can deliver sufficient force and torque output for MIS applications, and they can be conveniently assembled into articulated shafts featuring safe actuation, high dexterity, stiffness tuning and reconfigurability.

Real-Time Kinematics of Continuum Robots: Modelling and Validation

Abstract Kinematics of flexible backbone continuum robots is highly non linear and its complexity quickly escalates with the number of sections of the robot, which is usually more than three. This paper introduces a kinematic modelling of actuation and configuration spaces that greatly simplifies the computational requirements compared to the commonly used Piecewise Constant Curvature Kinematics which results in a faster algorithm at a rate proportional to the number of sections. This new algorithm is firstly developed for Twin Pivot Compliant Joint continuum robots but then extended to a general single neutral axis backbone configuration, both achieving a very low error of approximation (0.7% for the prototype developed), which results in several advantages such as the avoidance of highly non-linear functions and singularities, great reduction of computational complexity and an user-friendly graphical representation to help operation and status monitoring of this kind of robots. Moreover, a slender, small diameter and hyper-redundant (175 mm length, 6 mm diameter, 10 Degrees of Freedom) continuum robot prototype is developed and tested in a real-case industrial application for inspection and repair of aero engines in order to validate the proposed model.

Simulation of compensated tool path through virtual robot machining model

Nowadays, industrial robots could be a successful alternative to machine tools for milling of large parts with complex geometry. As it is known, poor accuracy which is most influenced by the stiffness of robot structure is recognized as a limiting factor for successful use of robots in milling tasks. Since there are different sources of error in robots, virtual manufacturing systems provide a useful means for products to be manufactured without the need of physical testing on the shop floor. This paper presents the developed virtual robot machining model, as a part of digital twin, for the simulation of a modified tool path generated by the compensation algorithm for the errors induced by cutting forces due to robot compliance. This part of digital twin includes the robot kinematic model, the Cartesian space robot compliance model, the model of cutting forces and the developed program based on off-line compensation algorithm. Development of such virtual robot machining model is important for the validation of modified tool path before the machining on a real robot. The developed virtual robot machining model is verified via several experiments, where the simulated surfaces are compared with the real machined surfaces generated by tool movement through a modified, i.e., corrected, trajectory on an available industrial robot programmed in G-code.

An Anthropomorphic Robust Robotic Torso for Ventral/Dorsal and Lateral Motion With Weight Compensation

The human torso is known to be highly versatile and dexterous in terms of motion capabilities and dynamic performance. Motivated by the human counterpart, we present the development of a new anthropomorphic robotic torso with comparable workspace, torques, velocities for running, and high level of mechanical robustness. In order to support large loads in ventral and lateral direction a parallel structure of actuation with tendons and a weight compensation via a spring mechanism is introduced into the system. That way, motors can be considerably downsized, which is, in turn, beneficial in terms of the dynamic characteristics of the torso. Robustness to withstand severe impacts is achieved through a clutch-based mechanism in the tendon pulleys. The entire mechanism is experimentally validated in various experiments and will be integrated as component of the compliant humanoid robot David of the DLR.

Minimally Model-Based Trajectory Tracking and Variable Impedance Control of Flexible-Joint Robots

This article presents a novel control scheme that primarily provides a solution to the problem of simultaneously performing stable trajectory tracking and variable impedance control (VIC) of flexible-joint robots. The existing asymptotically stabilizing tracking controllers for compliant robots rely upon several model-based terms and their derivatives. Contrarily, the proposed minimally model-based trajectory tracking control scheme requires knowledge of only the gravitational torque vector model. Although interaction and tracking performance enhancements are achievable through VIC, its operation introduces energy injections, thereby impacting closed-loop stability/passivity. Existing VIC passivity preservation techniques deal exclusively with rigid-joint robots. To this end, the proposed method introduces the minimally model-based trajectory tracking variable impedance control scheme, which ensures passivity/stability when applying VIC to flexible-joint robots. Numerical and experimental results corroborate the controllers' efficacy in terms of realizing VIC interaction and tracking tasks, thus enabling flexible-joint robots to stably replicate biologically inspired behaviors.

Untethered Multimode Fluidic Actuation: A New Approach to Soft and Compliant Robotics.

Fluid actuated soft robots, or fluidic elastomer actuators, have shown great potential in robotic applications where large compliance and safe interaction are dominant concerns. They have been widely studied in wearable robotics, prosthetics, and rehabilitations in recent years. However, such soft robots and actuators are tethered to a bulky pump and controlled by various valves, limiting their applications to a small confined space. In this study, we report a new and effective approach to fluidic power actuation that is untethered, easy to design, fabricate, control, and allows various modes of actuation. In the proposed approach, a sealed elastic tube filled with fluid (gas or liquid) is segmented by adaptors. When twisting a segment, two major effects could be observed: (1) the twisted segment exhibits a contraction force and (2) other segments inflate or deform according to their constraint patterns. Utilizing such effects, various actuation modes could be realized. In this research, four modes of actuation are illustrated: (1) soft actuator and pump actuation, (2) serial actuation, (3) parallel actuation, and (4) agonist and antagonist actuation. Theoretic analysis and experimental studies for the basic actuation principle have been conducted. A case study on an anthropomorphic forearm based on the proposed twisting tube actuation has been developed to showcase the effectiveness of the actuation modes. The studies suggest that the proposed approach has a great potential in both soft and compliant robotics.

Skeleton model-based product low carbon design optimization

With the diversification and customization of product design route, low-carbon design optimization has become a new method to determine the optimal product parameters. Numerical optimization is an effective tool in the analysis of engineering system. As it is crucial to analyze the behaviour of the products with the optimization of complex products, many valuable contributions are obtained. The current researches always focus on optimization design in detail design. However, at the early stage of design process, it could not be obtained with detailed information, and the rough and uncertain information is always gradual in the design process. This paper is devoted to product low carbon design optimization based on the skeleton model under the condition of insufficient information at the early design stage. After the design principle for compliant exoskeleton robotics is discussed, the parameter optimization based on skeleton model under low carbon constraints is then proposed. The exoskeleton robotics with the consideration of compliant mechanism is given as an example which demonstrates that the methodology is helpful to skeleton model-based low carbon optimization approach, which facilitates the development of customized exoskeleton robots.

Locomotion of a Compliant Mechanism With Nonholonomic Constraints

AbstractCompliant mechanisms have been studied extensively as an alternative to traditional rigid body design with advantages like part number reduction, compliance, and multistable configurations. Most of the past research on compliant mechanisms has been restricted to the case where they are subject to holonomic constraints. In this paper, we develop a model of a planar compliant mechanism with nonholonomic constraints as a mobile robot that can move on the ground. The only actuation that is assumed is a torque on the system. It is shown that the dynamics of this system is similar to that of a well-known nonholonomic system, called the Chaplygin sleigh, but with an added degree-of-freedom and an additional quartic potential. The interaction of compliance and the nonholonomic constraint lead to multiple stable limit cycle oscillations in a reduced velocity space that correspond to oscillations about different stable physical configurations. These limit cycle oscillations produce motion of the compliant mechanism in the plane with differing characteristics. The modeling framework in this paper can form the basis for the design of underacted mobile compliant nonholonomic robots or mobile robots that incorporate compliant mechanisms as mechanical switches.

Scalable and Robust Fabrication, Operation, and Control of Compliant Modular Robots.

A major goal of autonomous robot collectives is to robustly perform complex tasks in unstructured environments by leveraging hardware redundancy and the emergent ability to adapt to perturbations. In such collectives, large numbers is a major contributor to system-level robustness. Designing robot collectives, however, requires more than isolated development of hardware and software that supports large scales. Rather, to support scalability, we must also incorporate robust constituents and weigh interrelated design choices that span fabrication, operation, and control with an explicit focus on achieving system-level robustness. Following this philosophy, we present the first iteration of a new framework toward a scalable and robust, planar, modular robot collective capable of gradient tracking in cluttered environments. To support co-design, our framework consists of hardware, low-level motion primitives, and control algorithms validated through a kinematic simulation environment. We discuss how modules made primarily of flexible printed circuit boards enable inexpensive, rapid, low-precision manufacturing; safe interactions between modules and their environment; and large-scale lattice structures beyond what manufacturing tolerances allow using rigid parts. To support redundancy, our proposed modules have on-board processing, sensing, and communication. To lower wear and consequently maintenance, modules have no internally moving parts, and instead move collaboratively via switchable magnets on their perimeter. These magnets can be in any of three states enabling a large range of module configurations and motion primitives, in turn supporting higher system adaptability. We introduce and compare several controllers that can plan in the collective's configuration space without restricting motion to a discrete occupancy grid as has been done in many past planners. We show how we can incentively redundant connections to prevent single-module failures from causing collective-wide failure, explore bad configurations which impede progress as a result of the motion constraints, and discuss an alternative “naive” planner with improved performance in both clutter-free and cluttered environments. This dedicated focus on system-level robustness over all parts of a complete design cycle, advances the state-of-the-art robots capable of long-term exploration.

A High-Payload Proprioceptive Hybrid Robotic Gripper With Soft Origamic Actuators

Proprioception is the ability to perceive environmental stimulations through internal sensory organs. Enabling proprioception is critical for robots to be aware of the environmental interactions and respond appropriately, particularly for high-payload grippers to ensure safety when handling delicate objects. State-of-the-art robotic grippers with soft actuators are typically equipped with pressure sensors for pneumatic regulation and control, but very few utilized them for proprioceptive purposes. This lack of environmental awareness was largely compensated by their inherent compliance and conformity, but also due to the generally limited force capabilities. Targeting at this gap, this work proposes a novel Proprioceptive Origamic Soft Actuator (POSA) joint, and a corresponding hybrid robotic gripper design with high-payload soft origamic actuators and rigid supporting frames, achieving up to 564.5 N actuator output force or 302.4 N finger gripping force at 150 kPa low pneumatic pressure and 3.2 kg self-weight. Despite the substantially higher force capability over state-of-the-art soft grippers, the proposed hybrid gripper could retain the excellent inherent compliance thanks to the novel soft origamic actuators. Moreover, a novel scheme of multi-actuator proprioception is proposed by only using the embedded pneumatic pressure sensors, to enable the hybrid gripper with environmental awareness, achieving real-time position and force estimations of errors at $ 1% and 5.6%, respectively. The principles, design, prototyping, and experiments of the proposed hybrid high-payload gripper were presented in this letter. Combining soft robotic compliance, high payload, and proprioception, the gripper could both hold a pealed grape and crack a walnut, with position and force signals being measured without requiring dedicated sensors.