Imperial College London

Dr Nicolas Rojas

Faculty of EngineeringDyson School of Design Engineering

Lecturer
 
 
 
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Contact

 

n.rojas

 
 
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Location

 

Dyson BuildingSouth Kensington Campus

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Summary

 

Publications

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58 results found

Lu Q, Baron N, Clark A, Rojas Net al., 2020, The RUTH Gripper: systematic object-invariant prehensile in-hand manipulation via reconfigurable underactuation, Robotics: Science and Systems, Publisher: RSS

We introduce a reconfigurable underactuated robothand able to perform systematic prehensile in-hand manipu-lations regardless of object size or shape. The hand utilisesa two-degree-of-freedom five-bar linkage as the palm of thegripper, with three three-phalanx underactuated fingers—jointlycontrolled by a single actuator—connected to the mobile revolutejoints of the palm. Three actuators are used in the robot handsystem, one for controlling the force exerted on objects by thefingers and two for changing the configuration of the palm.This novel layout allows decoupling grasping and manipulation,facilitating the planning and execution of in-hand manipulationoperations. The reconfigurable palm provides the hand withlarge grasping versatility, and allows easy computation of amap between task space and joint space for manipulation basedon distance-based linkage kinematics. The motion of objects ofdifferent sizes and shapes from one pose to another is thenstraightforward and systematic, provided the objects are keptgrasped. This is guaranteed independently and passively by theunderactuated fingers using a custom tendon routing method,which allows no tendon length variation when the relative fingerbase position changes with palm reconfigurations. We analysethe theoretical grasping workspace and manipulation capabilityof the hand, present algorithms for computing the manipulationmap and in-hand manipulation planning, and evaluate all theseexperimentally. Numerical and empirical results of several ma-nipulation trajectories with objects of different size and shapeclearly demonstrate the viability of the proposed concept.

Conference paper

Baron N, Philippides A, Rojas N, 2020, On the false positives and false negatives of the Jacobian matrix in kinematically redundant parallel mechanisms, IEEE Transactions on Robotics, Vol: 36, ISSN: 1552-3098

The Jacobian matrix is a highly popular tool for the control and performance analysis of closed-loop robots. Its usefulness in parallel mechanisms is certainly apparent, and its application to solve motion planning problems, or other higher level questions, has been seldom queried, or limited to non-redundant systems. In this paper, we discuss the shortcomings of the use of the Jacobian matrix under redundancy, in particular when applied to kinematically redundant parallel architectures with non-serially connected actuators. These architectures have become fairly popular recently as they allow the end-effector to achieve full rotations, which is an impossible task with traditional topologies. The problems with the Jacobian matrix in these novel systems arise from the need to eliminate redundant variables when forming it, resulting in both situations where the Jacobian incorrectly identifies singularities (false positive), and where it fails to identify singularities (false negative). These issues have thus far remained unaddressed in the literature. We highlight these limitations herein by demonstrating several cases using numerical examples of both planar and spatial architectures.

Journal article

Shen M, Clark A, Rojas N, 2020, A scalable variable stiffness revolute joint based on layer jamming for robotic exoskeletons, Towards Autonomous Robotic Systems Conference ( TAROS ) 2020, Publisher: Springer Verlag, ISSN: 0302-9743

Robotic exoskeletons have been a focal point of research dueto an ever-increasing ageing population, longer life expectancy, and adesire to further improve the existing capabilities of humans. However,their effectiveness is often limited, with strong rigid structures poorlyinterfacing with humans and soft flexible mechanisms providing limitedforces. In this paper, a scalable variable stiffness revolute jointis pro-posed to overcome this problem. By using layer jamming, the jointhasthe ability to stiffen or soften for different use cases. A theoretical and ex-perimental study of maximum stiffness with size was conductedto deter-mine the suitability and scalablity of this technology. Three sizes (25 mm,18.75 mm, 12.5 mm diameter) of the joint were developed and evaluated.Results indicate that this technology is most suitable for use in humanfingers, as the prototypes demonstrate a sufficient torque (0.054 Nm) tosupport finger movement.

Conference paper

Wang J, Lu Q, Clark A, Rojas Net al., 2020, A passively complaint idler mechanism for underactuated dexterous grippers with dynamic tendon routing, Towards Autonomous Robotic Systems Conference (TAROS ) 2020, Publisher: Springer Verlag, ISSN: 0302-9743

n the field of robotic hands, tendon actuation is one of themost common ways to control self-adaptive underactuated fingers thanksto its compact size. Either differential or direct drive mechanisms areusually used in these systems to perform synchronised grasping using asingle actuator. However, synchronisation problems arise in underactu-ated grippers whose position of proximal joints varies with time to per-form manipulation operations, as this results in a tendon-driven systemwith dynamic anchor pulleys. This paper introduces a novel passivelycomplaint idler mechanism to avoid unsynchronisation in grippers witha dynamic multi-tendon routing system, such that adequate graspingcontact forces are kept under changes in the proximal joints’ positions.A re-configurable palm underactuated dexterous gripper is used as acase study, with the performance of the proposed compliant idler systembeing evaluated and compared through a contact force analysis duringrotation and translation in-hand manipulation tasks. Experiment resultsclearly demonstrate the ability of the mechanism to synchronise a dy-namic tendon routing gripper. A video summarising experiments andfindings can be found athttps://imperialcollegelondon.box.com/s/hk58688q2hjnu8dhw7uskr7vi9tqr9r5.

Conference paper

Baron N, Philippides A, Rojas N, 2020, A robust geometric method of singularity avoidance for kinematically redundant planar parallel robot manipulators, Mechanism and Machine Theory, Vol: 151, Pages: 103863-103863, ISSN: 0094-114X

Journal article

He L, Lu Q, Abad S-A, Rojas N, Nanayakkara DPTet al., 2020, Soft fingertips with tactile sensing and active deformation for robust grasping of delicate objects, IEEE Robotics and Automation Letters, Vol: 5, Pages: 2714-2721, ISSN: 2377-3766

Soft fingertips have shown significant adaptability for grasping a wide range of object shapes, thanks to elasticity. This ability can be enhanced to grasp soft, delicate objects by adding touch sensing. However, in these cases, the complete restraint and robustness of the grasps have proved to be challenging, as the exertion of additional forces on the fragile object can result in damage. This letter presents a novel soft fingertip design for delicate objects based on the concept of embedded air cavities, which allow the dual ability of tactile sensing and active shape-changing. The pressurized air cavities act as soft tactile sensors to control gripper position from internal pressure variation; and active fingertip deformation is achieved by applying positive pressure to these cavities, which then enable a delicate object to be kept securely in position, despite externally applied forces, by form closure. We demonstrate this improved grasping capability by comparing the displacement of grasped delicate objects exposed to high-speed motions. Results show that passive soft fingertips fail to restrain fragile objects at accelerations as low as 0.1 m/s 2 , in contrast, with the proposed fingertips delicate objects are completely secure even at accelerations of more than 5 m/s 2 .

Journal article

Lu Q, Clark A, Shen M, Rojas Net al., 2020, An origami-inspired variable friction surface for increasing the dexterity of robotic grippers, IEEE Robotics and Automation Letters, Vol: 5, Pages: 2538-2545, ISSN: 2377-3766

While the grasping capability of robotic grippers has shown significant development, the ability to manipulate objects within the hand is still limited. One explanation for this limitation is the lack of controlled contact variation between the grasped object and the gripper. For instance, human hands have the ability to firmly grip object surfaces, as well as slide over object faces, an aspect that aids the enhanced manipulation of objects within the hand without losing contact. In this letter, we present a parametric, origami-inspired thin surface capable of transitioning between a high friction and a low friction state, suitable for implementation as an epidermis in robotic fingers. A numerical analysis of the proposed surface based on its design parameters, force analysis, and performance in in-hand manipulation tasks is presented. Through the development of a simple two-fingered two-degree-of-freedom gripper utilizing the proposed variable-friction surfaces with different parameters, we experimentally demonstrate the improved manipulation capabilities of the hand when compared to the same gripper without changeable friction. Results show that the pattern density and valley gap are the main parameters that effect the in-hand manipulation performance. The origami-inspired thin surface with a higher pattern density generated a smaller valley gap and smaller height change, producing a more stable improvement of the manipulation capabilities of the hand.

Journal article

Liow L, Clark A, Rojas N, 2020, OLYMPIC: a modular, tendon-driven prosthetic hand with novel finger and wrist coupling mechanisms, IEEE Robotics and Automation Letters, Vol: 5, Pages: 299-306, ISSN: 2377-3766

Prosthetic hands, while having shown significant progress in affordability, typically suffer from limited repairability, specifically by the user themselves. Several modular hands have been proposed to address this, but these solutions require handling of intricate components or are unsuitable for prosthetic use due to the large volume and weight resulting from added mechanical complexity to achieve this modularity. In this paper, we propose a fully modular design for a prosthetic hand with finger and wrist level modularity, allowing the removal and attachment of tendon-driven fingers without the need for tools, retendoning, and rewiring. Our innovative design enables placement of the motors behind the hand for remote actuation of the tendons, which are contained solely within the fingers. Details of the novel coupling-transmission mechanisms enabling this are presented; and the capabilities of a prototype using a control-independent grasping benchmark are discussed. The modular detachment torque of the fingers is also computed to analyse the trade-off between intentional removal and the ability to withstand external loads. Experiment results demonstrate that the prosthetic hand is able to grasp a wide range of household and food items, of different shape, size, and weight, without resulting in the ejection of fingers, while allowing a user to remove them easily using a single hand.

Journal article

Lu Q, Liang H, Nanayakkara DPT, Rojas Net al., 2020, Precise in-hand manipulation of soft objects using soft fingertips with tactile sensing and active deformation, IEEE International Conference on Soft Robotics, Publisher: IEEE

While soft fingertips have shown significant development for grasping tasks, its ability to facilitate the manipulation of objects within the hand is still limited. Thanks to elasticity, soft fingertips enhance the ability to grasp soft objects. However, the in-hand manipulation of these objects has proved to be challenging, with both soft fingertips and traditional designs, as the control of coordinated fine fingertip motions and uncertainties for soft materials are intricate. This paper presents a novel technique for in-hand manipulating soft objects with precision. The approach is based on enhancing the dexterity of robot hands via soft fingertips with tactile sensing and active shape changing; such that pressurized air cavities act as soft tactile sensors to provide closed loop control of fingertip position and avoid object’s damage, and pneumatic-tuned positive-pressure deformations act as a localized soft gripper to perform additional translations and rotations. We model the deformation of the soft fingertips to predict the in-hand manipulation of soft objects and experimentally demonstrate the resulting in-hand manipulationcapabilities of a gripper of limited dexterity with an algorithm based on the proposed dual abilities. Results show that the introduced approach can ease and enhance the prehensile in-hand translation and rotation of soft objects for precision tasks across the hand workspace, without damage.

Conference paper

Clark A, Rojas N, 2020, Design and workspace characterisation of malleable robots, IEEE International Conference on Robotics and Automation, Publisher: IEEE

For the majority of tasks performed by traditionalserial robot arms, such as bin picking or pick and place, onlytwo or three degrees of freedom (DOF) are required for motion;however, by augmenting the number of degrees of freedom,further dexterity of robot arms for multiple tasks can beachieved. Instead of increasing the number of joints of a robotto improve flexibility and adaptation, which increases controlcomplexity, weight, and cost of the overall system, malleablerobots utilise a variable stiffness link between joints allowing therelative positioning of the revolute pairs at each end of the linkto vary, thus enabling a low DOF serial robot to adapt acrosstasks by varying its workspace. In this paper, we present thedesign and prototyping of a 2-DOF malleable robot, calculatethe general equation of its workspace using a parameterisationbased on distance geometry—suitable for robot arms of variabletopology, and characterise the workspace categories that theend effector of the robot can trace via reconfiguration. Throughthe design and construction of the malleable robot we exploredesign considerations, and demonstrate the viability of theoverall concept. By using motion tracking on the physical robot,we show examples of the infinite number of workspaces thatthe introduced 2-DOF malleable robot can achieve.

Conference paper

Lu Q, Rojas N, 2019, On soft fingertips for in-hand manipulation: Modelling and implications for robot hand design, IEEE Robotics and Automation Letters, Vol: 4, Pages: 2471-2478, ISSN: 2377-3766

Contact models for soft fingertips are able to precisely computedeformation when information about contact forces and object position is known, thus improving the traditional soft finger contact model. However, the functionality of these approaches for the study of in-hand manipulation with robot hands has been shown to be limited, since the location of the manipulated object is uncertain due to compliance and closed-loop constraints. This paper presents a novel, tractable approach for contact modelling of soft fingertips in within-hand dexterous manipulation settings. The proposed method is based on a relaxation of the kinematic equivalent of point contact with friction, modelling the interaction between fingertips and objects as joints with clearances rather than ideal instances, and then approximating clearances via affine arithmetic to facilitate computation. These ideas are introduced using planar manipulation to aid discussion, and are used to predict the reachable workspace of a two-fingered robot hand with fingertips of different hardness and geometry. Numerical and empirical experiments are conducted to analyse the effects of soft fingertips on manipulation operability; results demonstrate the functionality of the proposed approach, as well as a tradeoff between hardness and depth in soft fingertips to achieve better manipulation performance of dexterous robot hands.

Journal article

Clark A, Rojas N, 2019, Assessing the performance of variable stiffness continuum structures of large diameter, IEEE Robotics and Automation Letters, Vol: 4, Pages: 2455-2462, ISSN: 2377-3766

Variable stiffness continuum structures of large diameters are suitable for high-capability robots, such as in industrial practices where high loads and human–robot interaction are expected. Existing variable stiffness technologies have focused on application as medical manipulators, and as such have been limited to small diameter designs ( $\sim$ 15 mm). Various performance metrics have been presented for continuum structures thus far, focusing on force resistance, but no universal testing methodology for continuum structures that encapsulates their overall performance has been provided. This letter presents five individual qualities that can be experimentally quantified to establish the overall performance capability of a design with respect to its use as a variable stiffness continuum manipulator. Six large diameter ( $>$ 40 mm) continuum structures are developed following both conventional (granular and layer jamming) and novel (hybrid designs and structurally supported layer jamming) approaches and are compared using the presented testing methodology. The development of the continuum structures is discussed, and a detailed insight into the tested quality selection and experimental methodology is presented. Results of experiments demonstrate the suitability of the proposed approach for assessing variable stiffness continuum capability across the design.

Journal article

Cheung YH, Baron N, Rojas N, 2019, Full-rotation singularity-safe workspace for kinematically redundant parallel robots, 20th Towards Autonomous Robotic Systems Conference, Publisher: Springer Verlag, ISSN: 0302-9743

This paper introduces and computes a novel type of work-space for kinematically redundant parallel robots that defines the regionin which the end-effector can make full rotations without coming close tosingular configurations; it departs from the traditional full-rotation dex-terous workspace, which considers full rotations without encounteringsingularities but does not take into account the performance problemsresulting from closeness to these locations. Kinematically redundant ar-chitectures have the advantage of being able to be reconfigured withoutchanging the pose of the end-effector, thus being capable of avoidingsingularities and being suitable for applications where high dexterityis required. Knowing the workspace of these robots in which the end-effector is able to complete full, smooth rotations is a key design aspectto improve performance; however, since this singularity-safe workspaceis generally small, or even non-existent, in most parallel manipulators,its characterisation and calculation have not received attention in theliterature. The proposed workspace for kinematically redundant robotsis introduced using a planar parallel architecture as a case study; the for-mulation works by treating the manipulator as two halves, calculatingthe full-rotation workspace of the end-effector for each half whilst ensur-ing singularity conditions are not approached or met, and then findingthe intersection of both regions. The method is demonstrated ontwoexample robot instances, and a numerical analysis is also carried out asa comparison.

Conference paper

Nanayakkara V, Sornkaran N, Wegiriya H, Vitzilaios N, Venetsanos D, Rojas N, Sahinkaya MN, Nanayakkara Tet al., 2019, A method to estimate the oblique arch folding axis for thumb assistive devices, 20th Towards Autonomous Robotic Systems Conference, Publisher: Springer Verlag, Pages: 28-40, ISSN: 0302-9743

People who use the thumb in repetitive manipulation tasks are likelyto develop thumb related impairments from excessive loading at the base jointsof the thumb. Biologically informed wearable robotic assistive mechanisms canprovide viable solutions to prevent occurring such injuries. This paper tests thehypothesis that an external assistive force at the metacarpophalangeal joint willbe most effective when applied perpendicular to the palm folding axis in termsof maximizing the contribution at the thumb-tip as well as minimizing the pro-jections on the vulnerable base joints of the thumb. Experiments conducted usinghuman subjects validated the predictions made by a simplified kinematic modelof the thumb that includes a foldable palm, showing that: 1) the palm folding an-gle varies from 71.5◦to 75.3◦(from the radial axis in the coronal plane) for thefour thumb-finger pairs and 2) the most effective assistive force direction (fromthe ulnar axis in the coronal plane) at the MCP joint is in the range 0◦<ψ<30◦for the four thumb-finger pairs. These findings provide design guidelines for handassistive mechanisms to maximize the efficacy of thumb external assistance.

Conference paper

Clark AB, Rojas N, 2019, Stiffness-tuneable limb segment with flexible spine for malleable robots, 2019 International Conference on Robotics and Automation (ICRA), Publisher: IEEE

Robotic arms built from stiffness-adjustable, con-tinuously bending segments serially connected with revolutejoints have the ability to change their mechanical architectureand workspace, thus allowing high flexibility and adaptation todifferent tasks with less than six degrees of freedom, a conceptthat we call malleable robots. Known stiffening mechanismsmay be used to implement suitable links for these novel roboticmanipulators; however, these solutions usually show a reducedperformance when bending due to structural deformation. Byincluding an inner support structure this deformation can beminimised, resulting in an increased stiffening performance.This paper presents a new multi-material spine-inspired flexiblestructure for providing support in stiffness-controllable layer-jamming-based robotic links of large diameter. The proposedspine mechanism is highly movable with type and range ofmotions that match those of a robotic link using solely layerjamming, whilst maintaining a hollow and light structure. Themechanics and design of the flexible spine are explored, anda prototype of a link utilising it is developed and comparedwith limb segments based on granular jamming and layerjamming without support structure. Results of experimentsverify the advantages of the proposed design, demonstratingthat it maintains a constant central diameter across bendingangles and presents an improvement of more than 203% ofresisting force at 180°.

Conference paper

Baron N, Philippides A, Rojas N, 2019, A novel kinematically redundant planar parallel robot manipulator with full rotatability, Journal of Mechanisms and Robotics, Vol: 11, Pages: 011008-011008, ISSN: 1942-4302

This paper presents a novel kinematically redundant planar parallel robot manipulator, which has full rotatability. The proposed robot manipulator has an architecture that corresponds to a fundamental truss, meaning that it does not contain internal rigid structures when the actuators are locked. This also implies that its rigidity is not inherited from more general architectures or resulting from the combination of other fundamental structures. The introduced topology is a departure from the standard 3-RPR (or 3-RRR) mechanism on which most kinematically redundant planar parallel robot manipulators are based. The robot manipulator consists of a moving platform that is connected to the base via two RRR legs and connected to a ternary link, which is joined to the base by a passive revolute joint, via two other RRR legs. The resulting robot mechanism is kinematically redundant, being able to avoid the production of singularities and having unlimited rotational capability. The inverse and forward kinematics analyses of this novel robot manipulator are derived using distance-based techniques, and the singularity analysis is performed using a geometric method based on the properties of instantaneous centers of rotation. An example robot mechanism is analyzed numerically and physically tested; and a test trajectory where the end effector completes a full cycle rotation is reported. A link to an online video recording of such a capability, along with the avoidance of singularities and a potential application, is also provided.

Journal article

Bai G, Rojas N, 2019, Self-adaptive monolithic anthropomorphic finger with teeth-guided compliant cross-four-bar joints for underactuated hands, 2018 IEEE-RAS International Conference on Humanoid Robots (Humanoids), Publisher: IEEE

This paper presents a novel approach for modelingone-degree-of-freedom human metacarpophalangeal/ interpha-langeal joints based on a teeth-guided compliant cross-four-barlinkage. The proposed model allows developing self-adaptiveanthropomorphic fingers able to be 3D printed in a singlestep without any accessories, except for simple tendon wiringafter the printing process, using basic single-material additivemanufacturing. Teeth-guided compliant cross-four-bar linkagesas finger joints not only provide monolithic fabrication withoutassembly but also increase precision of anthropomorphic robotfingers by removing nonlinear characteristics, thus reducing thecomplexity of control for delicate grasping. Kinematic analysisof the proposed compliant finger joints is detailed and nonlinearfinite element analysis results demonstrating their advantagesare reported. A two-fingered underactuated hand with teeth-guided compliant cross-four-bar joints is also developed andqualitative discussion on grasping is conducted.

Conference paper

Baron N, Philippides A, Rojas N, 2018, A geometric method of singularity avoidance for kinematically redundant planar parallel robots, Advances in Robot Kinematics 2018, Publisher: Springer, Pages: 187-194

Methods for avoiding singularities of closed-loop robot mechanisms havebeen traditionally based on the value of the determinant or the condition number ofthe Jacobian. A major drawback of these standard techniquesis that the closeness ofa robot configuration to a singularity lacks geometric, physical interpretation, thusimplying that it is uncertain how changes in the robot pose actually move furtheraway the mechanism from such a problematic configuration. This paper presentsa geometric approach of singularity avoidance for kinematically redundant planarparallel robots that eliminates the disadvantages of Jacobian-based techniques. Theproposed method, which is based on the properties of instantaneous centres of rota-tion, defines a mathematical distance to a singularity and provides a reliable way ofmoving the robot further from a singular configuration without changing the poseof the end-effector. The approach is demonstrated on an example robot mechanismand the reciprocal of the condition number of the Jacobian isused to show its ad-vantages.

Book chapter

Porta JM, Rojas N, Thomas F, 2018, Distance geometry in active structures, Mechatronics for Cultural Heritage and Civil Engineering, Editors: Ottaviano, Pelliccio, Gattulli, Publisher: Springer, Pages: 115-136

Distance constraints are an emerging formulation that offers intuitive geometrical interpretation of otherwise complex problems. The formulation can be applied in problems such as position and singularity analysis and path planning of mechanisms and structures. This paper reviews the recent advances in distance geometry, providing a unified view of these apparently disparate problems. This survey reviews algebraic and numerical techniques, and is, to the best of our knowledge, the first attempt to summarize the different approaches relating to distance-based formulations.

Book chapter

Rojas N, Dollar AM, 2017, Distance-based kinematics of the five-oblique-axis thumb model with intersecting axes at the metacarpophalangeal joint, 2017 IEEE RAS/EMBS International Conference on Rehabilitation Robotics, Publisher: IEEE, ISSN: 1945-7901

This paper proposes a novel and simple methodto compute all possible solutions of the inverse kinematicsproblem of the five-oblique-axis thumb model with intersectingaxes at the metacarpophalangeal joint. This thumb model isone of the suggested results by a magnetic-resonance-imaging-based study that, in contrast to those based on cadaver fingersor on the tracking of the surface of the fingers, takes intoaccount muscle and ligament behaviors and avoids inaccuraciesresulting from the movement of the skin with respect to thebones. The proposed distance-based inverse kinematics methodeliminates the use of arbitrary reference frames as is usuallyrequired by standard approaches; this is relevant because thenumerical conditioning of the resulting system of equationswith such traditional approaches depends on the selectedreference frames. Moreover, contrary to other parametrizations(e.g., Denavit-Hartenberg parameters), the suggested distance-based parameters for the thumb have a natural, human-understandable geometric meaning that makes them easier tobe determined from any posture. These characteristics makethe proposed approach of interest for those working in, forinstance, measuring and modeling the movement of the humanhand, developing rehabilitation devices such as orthoses andprostheses, or designing anthropomorphic robotic hands.

Conference paper

Kanner O, Rojas N, Dollar AM, 2017, Between-leg coupling schemes for passively-adaptive non-redundant legged robots, 2017 IEEE International Conference on Robotics and Automation (ICRA), Publisher: IEEE

This paper studies the synthesis of between-leg coupling schemes for passively-adaptive non-redundant legged robots. Highly actuated legged robots can arbitrarily locate their feet relative to their bodies through active control, but often wind up kinematically over-constrained following ground contact, requiring complex redundant control for stable locomotion. The use of passive sprung joints can provide some minimal passive adaptability to terrain, but it is limited to relatively low terrain variability due to practical travel limits. In this paper, using a 4-RR platform as case study, we show that implementing parallel adaptive couplings between legs of a stance platform can yield substantial passive adaptability to rough terrain while still ensuring that the body is fully constrained in stance. This study uses screw theory-based mobility analysis methods to determine the number of constraints required to control the stance platform. Several coupling schemes are then considered and evaluated through a simulation of their stance capabilities over arbitrary terrain. An experimental validation of these simulation results is presented; it demonstrates the viability of the proposed scheme for passive adaptability.

Conference paper

Bircher WG, Dollar AM, Rojas N, 2017, A two-fingered robot gripper with large object reorientation range, 2017 IEEE International Conference on Robotics and Automation (ICRA), Publisher: IEEE

It is very challenging for a robotic gripper to achieve large reorientations with grasped objects without accidental object ejection. This paper presents a simple gripper that can repeatedly achieve large reorientations over π/2 rad through the kinematics of the hand-object system alone, without the use of high fidelity contact sensors, complex control of active finger surfaces, or highly actuated fingers. This gripper is the result of two kinematic parameter search optimizations connected in cascade. Besides the large range of reorientation attained, the obtained gripper also corresponds to a novel topology since ternary joints in the palm are presented. The in-hand planar reorientation capabilities of the proposed gripper are experimentally tested with success.

Conference paper

Rojas N, Thomas F, 2017, Forward kinematics of the general triple-arm robot using a distance-based formulation, 7th IFToMM International Workshop on Computational Kinematics, Publisher: Springer

Distance-based formulations have successfully been used to obtain closure polynomialsfor planar mechanisms without relying, in most cases, on variable eliminations. The methods re-sulting from previous attempts to generalize these techniques to spatial mechanisms exhibit somelimitations such as the impossibility of incorporating orientation constraints. For the first time, thispaper presents a complete satisfactory generalization. As an example, it is applied to obtain a clo-sure polynomial for the the general triple-arm parallel robot (that is, the 3-RPS 3-DOF robot). Thispolynomial, not linked to any particular reference frame, is obtained without variable eliminationsor tangent-half-angle substitutions.

Conference paper

Ward-Cherrier B, Rojas N, Lepora NF, 2017, Model-free precise in-hand manipulation with a 3D-printed tactile gripper, IEEE Robotics and Automation Letters, Vol: 2, Pages: 2056-2063, ISSN: 2377-3766

The use of tactile feedback for precision manipulation in robotics still lags far behind human capabilities. This study has two principal aims: 1) to demonstrate in-hand reorientation of grasped objects through active tactile manipulation; and 2) to present the development of a novel TacTip sensor and a GR2 gripper platform for tactile manipulation. Through the use of Bayesian active perception algorithms, the system successfully achieved inhand reorientation of cylinders of different diameters (20, 25, 30, and 35 mm) using tactile feedback. Average orientation errors along manipulation trajectories were below 5° for all cylinders with reorientation ranges varying from 42° to 67°. We also demonstrated an improvement in active tactile manipulation accuracy when using additional training data. Our methods for active tactile manipulation with the GR2 TacTip gripper are model free, can be used to investigate principles of dexterous manipulation, and could lead to essential advances in the areas of robotic tactile manipulation and teleoperated robots.

Journal article

Kanner OY, Rojas N, Odhner LU, Dollar AMet al., 2017, Adaptive legged robots through exactly-constrained and non-redundant design, IEEE Access, Vol: 5, Pages: 11131-11141, ISSN: 2169-3536

This paper presents a novel strategy for designing passively adaptive, statically stable walking robots with full body mobility that are exactly constrained and non-redundantly actuated during stance. In general, fully mobile legged robots include a large number of actuated joints, giving them a wide range of controllable foot placements but resulting in overconstraint during stance, requiring kinematic redundancy and redundant control for effective locomotion. The proposed design strategy allows for the elimination of actuation redundancy, thus greatly reducing the weight and complexity of the legged robots obtained and allowing for simpler control schemes. Moreover, the underconstrained nature of the resulting robots during swing allows for passive adaptability to rough terrain without large contact forces. The strategy uses kinematic mobility analysis tools to synthesize leg topologies, underactuated robotics design approaches to effectively distribute actuation constraints, and elastic elements to influence nominal leg behavior. Several examples of legged robot designs using the suggested approach are thoroughly discussed and a proof-of-concept of a non-redundant walking robot is presented.

Journal article

Ma RR, Rojas N, Dollar AM, 2016, Spherical hands: toward underactuated, in-hand manipulation invariant to object size and grasp location, Journal of Mechanisms and Robotics, Vol: 8, Pages: 061021-061021-12, ISSN: 1942-4302

Minimalist, underactuated hand designs can be modified to produce useful, dexterous, in-hand capabilities without sacrificing their passive adaptability in power grasping. Incorporating insight from studies in parallel mechanisms, we implement and investigate the “spherical hand” morphologies: novel, hand topologies with two fingers configured such that the instantaneous screw axes, describing the displacement of the grasped object, always intersect at the same point relative to the palm. This produces the same instantaneous motion about a common point for any object geometry in a stable grasp. Various rotary fingertip designs are also implemented to help maintain stable contact conditions and minimize slip, in order to prove the feasibility of this design in physical hand implementations. The achievable precision manipulation workspaces of the proposed morphologies are evaluated and compared to prior human manipulation data as well as manipulation results with traditional three-finger hand topologies. Experiments suggest that the spherical hands' design modifications can make the system's passive reconfiguration more easily predictable, providing insight into the expected object workspace while minimizing the dependence on accurate object and contact modeling. We believe that this design can significantly reduce the complexity of planning and executing dexterous manipulation movements in unstructured environments with underactuated hands.

Journal article

Rojas N, Dollar AM, 2016, Gross motion analysis of fingertip-based within-hand manipulation, IEEE Transactions on Robotics, Vol: 32, Pages: 1009-1016, ISSN: 1552-3098

Fingertip-based within-hand manipulation, also called precision manipulation, refers to the repositioning of a grasped object within the workspace of a multifingered robot hand without breaking or changing the contact type between each fingertip and the object. Given a robot hand architecture and a set of assumed contact models, this paper presents a method to perform a gross motion analysis of its precision manipulation capabilities, regardless of the particularities of the object being manipulated. In particular, the technique allows the composition of the displacement manifold of the grasped object relative to the palm of the robot hand to be determined as well as the displacements that can be controlled-useful for high-level design and classification of hand function. The effects of a fingertip contacting a body in this analysis are modeled as kinematic chains composed of passive and resistant revolute joints; what permits the introduction of a general framework for the definition and classification of nonfrictional and frictional contact types. Examples of the application of the proposed method in several architectures of multifingered hands with different contact assumptions are discussed; they illustrate how inappropriate contact conditions may lead to uncontrollable displacements of the grasped object.

Journal article

Rojas N, Ma RR, Dollar AM, 2016, The GR2 gripper: an underactuated hand for open-loop in-hand planar manipulation, IEEE Transactions on Robotics, Vol: 32, Pages: 763-770, ISSN: 1552-3098

Performing dexterous manipulation of unknown objects with robot grippers without using high-fidelity contact sensors, active/sliding surfaces, or a priori workspace exploration is still an open problem in robot manipulation and a necessity for many robotics applications. In this paper we present a two-fingered gripper topology that enables an enhanced predefined in-hand manipulation primitive controlled without knowing the size, shape, or other particulars of the grasped object. The in-hand manipulation behavior, namely, the planar manipulation of the grasped body, is predefined thanks to a simple hybrid low-level control scheme and has an increased range of motion due to the introduction of an elastic pivot joint between the two fingers. Experimental results with a prototype clearly show the advantages and benefits of the proposed concept. Given the generality of the topology and in-hand manipulation principle, researchers and designers working on multiple areas of robotics can benefit from the findings.

Journal article

Rojas N, Dollar AM, 2016, Classification and kinematic equivalents of contact types for fingertip-based robot hand manipulation, Journal of Mechanisms and Robotics, Vol: 8, Pages: 041014-041014-9, ISSN: 1942-4302

In the context of robot manipulation, Salisbury's taxonomy is the common standard used to define the types of contact interactions that can occur between the robot and a contacted object; the basic concept behind such classification is the modeling of contacts as kinematic pairs. In this paper, we extend this notion by modeling the effects of a robot contacting a body as kinematic chains. The introduced kinematic-chain-based contact model is based on an extension of the Bruyninckx–Hunt approach of surface–surface contact. A general classification of nonfrictional and frictional contact types suitable for both manipulation analyses and robot hand design is then proposed, showing that all standard contact categories used in robotic manipulation are special cases of the suggested generalization. New contact models, such as ball, tubular, planar translation, and frictional adaptive finger contacts, are defined and characterized. An example of manipulation analysis that lays out the relevance and practicality of the proposed classification is detailed.

Journal article

Rojas N, Dollar AM, 2016, The coupler surface of the RSRS mechanism, Journal of Mechanisms and Robotics, Vol: 8, Pages: 014505-014505, ISSN: 1942-4302

Two degree-of-freedom (2-DOF) closed spatial linkages can be useful in the design of robotic devices for spatial rigid-body guidance or manipulation. One of the simplest linkages of this type, without any passive DOF on its links, is the revolute-spherical-revolute-spherical (RSRS) four-bar spatial linkage. Although the RSRS topology has been used in some robotics applications, the kinematics study of this basic linkage has unexpectedly received little attention in the literature over the years. Counteracting this historical tendency, this work presents the derivation of the general implicit equation of the surface generated by a point on the coupler link of the general RSRS spatial mechanism. Since the derived surface equation expresses the Cartesian coordinates of the coupler point as a function only of known geometric parameters of the linkage, the equation can be useful, for instance, in the process of synthesizing new devices. The steps for generating the coupler surface, which is computed from a distance-based parametrization of the mechanism and is algebraic of order twelve, are detailed and a web link where the interested reader can download the full equation for further study is provided. It is also shown how the celebrated sextic curve of the planar four-bar linkage is obtained from this RSRS dodecic.

Journal article

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