Imperial College London

Dr Nicolas Rojas

Faculty of EngineeringDyson School of Design Engineering

Lecturer
 
 
 
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n.rojas

 
 
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Dyson BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
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46 results found

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

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 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, 2018, 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

BOOK CHAPTER

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

JOURNAL ARTICLE

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

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

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 ํ…/ํŸ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 optimizationsconnected 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

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

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, ISSN: 1942-4302

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

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

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, ISSN: 1942-4302

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

Sosa R, Rojas N, Gero JS, Xu Qet al., 2016, Visual divergence in humans and computers, Design Studies, Vol: 42, Pages: 56-85, ISSN: 0142-694X

JOURNAL ARTICLE

Ma RR, Rojas N, Dollar AM, 2016, Towards Predictable Precision Manipulation of Unknown Objects with Underactuated Fingers, Advances in Reconfigurable Mechanisms and Robots II, Editors: Ding, Kong, Dai, Cham, Publisher: Springer International Publishing, Pages: 927-937, ISBN: 978-3-319-23327-7

BOOK CHAPTER

Rojas N, Dollar AM, Thomas F, 2015, A unified position analysis of the Dixon and the generalized Peaucellier linkages, Mechanism and Machine Theory, Vol: 94, Pages: 28-40, ISSN: 0094-114X

JOURNAL ARTICLE

Kanner OY, Rojas N, Dollar AM, 2015, Design of a Passively-Adaptive Three Degree-of-Freedom Multi-Legged Robot With Underactuated Legs, ASME 2015 International Design Engineering Technical Conferences & Computers and Information in Engineering Conference (IDETC/CIE), Pages: V05AT08A062-V05AT08A062

This paper discusses the design of a three degree-of-freedom (3-DOF) non-redundant walking robot with decoupled stance and propulsion locomotion phases that is exactly constrained in stance and utilizes adaptive underactuation to robustly traverse terrain of varying ground height. Legged robots with a large number of actuated degrees of freedom can actively adapt to rough terrain but often end up being kinematically overconstrained in stance, requiring complex redundant control schemes for effective locomotion. Those with fewer actuators generally use passive compliance to enhance their dynamic behavior at the cost of postural control and reliable ground clearance, and often inextricably link control of the propulsion of the robot with control of its posture. In this paper we show that the use of adaptive underactuation techniques with constraint-based design synthesis tools allows for lighter and simpler lower mobility legged robots that can adapt to the terrain below them during the swing phase yet remain stable during stance and that the decoupling of stance and propulsion can greatly simplify their control. Simulation results of the swing phase behavior of the proposed 3-DOF decoupled adaptive legged robot as well as proof-of-concept experiments with a prototype of its corresponding stance platform are presented and validate the suggested design framework.

CONFERENCE PAPER

Tan N, Rojas N, Elara Mohan R, Kee V, Sosa Ret al., 2015, Nested Reconfigurable Robots: Theory, Design, and Realization, International Journal of Advanced Robotic Systems, Vol: 12, Pages: 110-110, ISSN: 1729-8814

JOURNAL ARTICLE

N Rojas, J Borràs, F Thomas, 2015, On quartically-solvable robots, 2015 IEEE International Conference on Robotics and Automation (ICRA), Publisher: Institute of Electrical and Electronics Engineers (IEEE), Pages: 1410-1415, ISSN: 1050-4729

This paper presents a first attempt at a unified kinematics analysis of all serial and parallel solvable robots, that is, robots whose position analysis can be carried out without relying on numerical methods. The efforts herein are focused on finding a unified formulation for all quartically-solvable robots, as all other solvable robots can be seen as particular cases of them. The first part is centered on the quest for the most general quartically-solvable parallel and serial robots. As a result, representatives of both classes are selected. Then, using Distance Geometry, it is shown how solving the forward kinematics of the parallel representative is equivalent to solve the inverse kinematics of the serial representative, thus providing a unified formulation. Finally, it is shown that the position and singularity analysis of these robots reduces to the analysis of the relative position of two coplanar ellipses.

CONFERENCE PAPER

Nansai S, Rojas N, Elara MR, Sosa R, Iwase Met al., 2015, A novel approach to gait synchronization and transition for reconfigurable walking platforms, Digital Communications and Networks, Vol: 1, Pages: 141-151, ISSN: 2352-8648

JOURNAL ARTICLE

Nansai S, Rojas N, Elara MR, Sosa R, Iwase Met al., 2015, On a Jansen leg with multiple gait patterns for reconfigurable walking platforms, Advances in Mechanical Engineering, Vol: 7, Pages: 168781401557382-168781401557382, ISSN: 1687-8140

JOURNAL ARTICLE

Nansai S, Mohan RE, Tan N, Rojas N, Iwase Met al., 2015, Dynamic Modeling and Nonlinear Position Control of a Quadruped Robot with Theo Jansen Linkage Mechanisms and a Single Actuator, Journal of Robotics, Vol: 2015, Pages: 1-15, ISSN: 1687-9600

JOURNAL ARTICLE

N Rojas, A M Dollar, 2014, Characterization of the precision manipulation capabilities of robot hands via the continuous group of displacements, 2014 IEEE/RSJ International Conference on Intelligent Robots and Systems, Publisher: IEEE, Pages: 1601-1608, ISSN: 2153-0858

In robot hands, precision manipulation, defined as repositioning of a grasped object within the hand workspace without breaking or changing contact, is a fundamental operation for the accomplishment of highly dexterous manipulation tasks. This paper presents a method to characterize the precision manipulation capabilities of a given robot hand regardless of the particularities of the grasped object. The technique allows determining the composition of the displacement manifold (finite motion) of the grasped object relative to the palm of the robot hand and defining the displacements that can actually be controlled by the hand actuators without depending on external factors to the hand. The approach is based on a reduction of the graph of kinematic constraints related to the hand-object system through proper manipulations of the continuous subgroups of displacements generated by the hand joints and contacts. The proposed method is demonstrated through three detailed and constructive examples of common architectures of simplified multi-fingered hands.

CONFERENCE PAPER

M R Elara, N Rojas, A Chua, 2014, Design principles for robot inclusive spaces: a case study with Roomba, 2014 IEEE International Conference on Robotics and Automation (ICRA), Publisher: Institute of Electrical and Electronics Engineers (IEEE), Pages: 5592-5599, ISSN: 1050-4729

Research focus on service robots that deals with applications related to healthcare, logistics, residential, search and rescue are gaining significant momentum in the recent years. Their social and economic relevance is more than evident. Yet, while much has been researched about “designing robots” focusing on sensing, actuation, mobility and control of service robots, little work has been done on “design for robots” that looks at designing preferred artefacts or environments for such robots. In this work, we propose a new philosophy of robot inclusive spaces, a cross disciplinary approach that brings together roboticians, architects and designers to solve numerous unsettled research problems in robotics community through design of inclusive interior spaces for robots where the latter live and operate. With a residential floor cleaning robot as a case study, we inductively derived a set of four design principles namely observability, accessibility, activity and safety that guides the realization of an inclusive space for these service robots. Also, the suggested principles are further defined, analysed and validated for their merits in this paper.

CONFERENCE PAPER

V Kee, N Rojas, M R Elara, R Sosaet al., 2014, Hinged-Tetro: A self-reconfigurable module for nested reconfiguration, 2014 IEEE/ASME International Conference on Advanced Intelligent Mechatronics, Pages: 1539-1546, ISSN: 2159-6247

CONFERENCE PAPER

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