Imperial College London

Angus B. Clark

Faculty of EngineeringDyson School of Design Engineering

 
 
 
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a.clark17 Website

 
 
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2nd Floor OfficeDyson BuildingSouth Kensington Campus

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Summary

 

Publications

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16 results found

Clark A, Baron N, Orr L, Kovac M, Rojas Net al., 2022, On a balanced delta robot for precise aerial manipulation: implementation, testing, and lessons for future designs, IEEE/RSJ International Conference on Intelligent Robots and Systems, Publisher: IEEE

Using a delta-manipulator for stabilisation of anend-effector to perform precise spatial positioning is a currentarea of interest in aerial manipulation. High speed precisionmovements of a manipulator can cause disturbances to theaerial platform, which hinders trajectory tracking and in somecases could be sufficient to cause a loss of control of the vehicle.In this paper, a statically balanced delta aerial manipulator isdeveloped and evaluated. The system is balanced using threecounter-masses to reduce the force imparted onto the base andthus reduce perturbations to the movement of the drone. Thesystem is thoroughly tested following trajectories while mountedto a force sensor and while on-board an aerial vehicle. Resultsshow that the forces transmitted to the base in all axes arereduced considerably, however improvements in overall flightaccuracy are not observed in aerial settings. Design lessonsto make a balanced delta-manipulator viable for practicalimplementation on an aerial vehicle are discussed in depth.A video summarising the flight testing results is available athttps://youtu.be/fXKnosnVKCk.

Conference paper

Clark A, Rojas N, 2022, Malleable robots: reconfigurable robotic arms with continuum links of variable stiffness, IEEE Transactions on Robotics, ISSN: 1552-3098

Through the implementation of reconfigurability toachieve flexibility and adaptation to tasks by morphology changesrather than by increasing the number of joints, malleable robotspresent advantages over traditional serial robot arms in regardsto reduced weight, size, and cost. While limited in degrees offreedom (DOF), malleable robots still provide versatility acrossoperations typically served by systems using higher DOF thanrequired by the tasks. In this paper, we present the creationof a 2-DOF malleable robot, detailing the design of jointsand malleable link, along with its modelling through forwardand inverse kinematics, and a reconfiguration methodology thatinforms morphology changes based on end effector location—determining how the user should reshape the robot to enablea task previously unattainable. The recalibration and motionplanning for making robot motion possible after reconfigurationare also discussed, and thorough experiments with the prototypeto evaluate accuracy and reliability of the system are presented.Results validate the approach and pave the way for furtherresearch in the area

Journal article

Chappell D, Son HW, Clark AB, Yang Z, Bello F, Kormushev P, Rojas Net al., 2022, Virtual reality pre-prosthetic hand training with physics simulation and robotic force interaction, IEEE Robotics and Automation Letters, Vol: 7, Pages: 1-1, ISSN: 2377-3766

Virtual reality (VR) rehabilitation systems have been proposed to enable prosthetic hand users to perform training before receiving their prosthesis. Improving pre-prosthetic training to be more representative and better prepare the patient for prosthesis use is a crucial step forwards in rehabilitation. However, existing VR platforms lack realism and accuracy in terms of the virtual hand and the forces produced when interacting with the environment. To address these shortcomings, this work presents a VR training platform based on accurate simulation of an anthropomorphic prosthetic hand, utilising an external robot arm to render realistic forces that the user would feel at the attachment point of their prosthesis. Experimental results with non-disabled participants show that training with this platform leads to a significant improvement in Box and Block scores compared to training in VR alone and a control group with no prior training. Results performing pick-and-place tasks with a wider range of objects demonstrates that training in VR alone negatively impacts performance, whereas the proposed platform has no significant impact on performance. User perception results highlight that the platform is much closer to using a physical prosthesis in terms of physical demand and effort, however frustration is significantly higher during training.

Journal article

Yang Z, Clark A, Chappell D, Rojas Net al., 2022, Instinctive real-time sEMG-based control of prosthetic hand with reduced data acquisition and embedded deep learning training, IEEE International Conference on Robotics and Automation

Achieving instinctive multi-grasp control of prosthetic hands typically still requires a large number of sensors,such as electromyography (EMG) electrodes mounted on aresidual limb, that can be costly and time consuming to position,with their signals difficult to classify. Deep-learning-based EMGclassifiers however have shown promising results over traditional methods, yet due to high computational requirements,limited work has been done with in-prosthetic training. Bytargeting specific muscles non-invasively, separating graspingaction into hold and release states, and implementing dataaugmentation, we show in this paper that accurate results forembedded, instinctive, multi-grasp control can be achieved withonly 2 low-cost sensors, a simple neural network, and minimalamount of training data. The presented controller, which isbased on only 2 surface EMG (sEMG) channels, is implementedin an enhanced version of the OLYMPIC prosthetic hand.Results demonstrate that the controller is capable of identifyingall 7 specified grasps and gestures with 93% accuracy, and issuccessful in achieving several real-life tasks in a real worldsetting.

Conference paper

Ranne A, Clark AB, Rojas N, 2022, Augmented reality-assisted reconfiguration and workspace visualization of malleable robots: workspace modification through holographic guidance, IEEE Robotics & Automation Magazine, Vol: 29, Pages: 2-13, ISSN: 1070-9932

Malleable robots are a type of reconfigurable serial robot capable of adapting their topology, through the use of variable stiffness malleable links, to desired tasks and workspaces by varying the relative positioning between their revolute joints. However, their reconfiguration is nontrivial, lacking intuitive communication between the human and the robot, and a method of efficiently aligning the end effector to a desired position. In this article, we present the design of an interactive augmented reality (AR) alignment interface, which helps a malleable robot understand the user’s task requirements, visualizes to the user the requested robot’s configuration and its workspace, and guides the user in reconfiguring the robot to achieve that configuration. Through motion tracking of a physical two degree-of-freedom (2 DoF) malleable robot, which can achieve an infinite number of workspaces, we compute the accuracy of the system in terms of initial calibration and overall accuracy, and demonstrate its viability. The results demonstrated a good performance, with an average repositioning accuracy of 9.64 ± 2.06 mm and an average base alignment accuracy of 10.54 ± 4.32 mm in an environment the size of 2,000 mm × 2,000 mm × 1,200 mm.

Journal article

Lu Q, Baron N, Clark AB, Rojas Net al., 2021, Systematic object-invariant in-hand manipulation via reconfigurable underactuatuation: introducing the RUTH gripper, International Journal of Robotics Research, Vol: 40, Pages: 1402-1418, ISSN: 0278-3649

We introduce a reconfigurable underactuated robot hand able to perform systematic prehensile in-hand manipulations regardless of object size or shape. The hand utilises a two-degree-of-freedom five-bar linkage as the palm of the gripper, with three three-phalanx underactuated fingers—jointly controlled by a single actuator—connected to the mobile revolute joints of the palm. Three actuators are used in the robot hand system in total, one for controlling the force exerted on objects by the fingers through an underactuated tendon system, and two for changing the configuration of the palm and thus the positioning of the fingers. This novel layout allows decoupling grasping and manipulation, facilitating the planning and execution of in-hand manipulation operations. The reconfigurable palm provides the hand with a large grasping versatility, and allows easy computation of a map between task space and joint space for manipulation based on distance-based linkage kinematics. The motion of objects of different sizes and shapes from one pose to another is then straightforward and systematic, provided the objects are kept grasped.This is guaranteed independently and passively by the underactuated fingers using a custom tendon routing method, which allows no tendon length variation when the relative finger base positions change with palm reconfigurations. We analyse the theoretical grasping workspace and grasping and manipulation capability of the hand, present algorithms forcomputing the manipulation map and in-hand manipulation planning, and evaluate all these experimentally. Numericaland empirical results of several manipulation trajectories with objects of different size and shape clearly demonstrate the viability of the proposed concept.

Journal article

Clark AB, Liow L, Rojas N, 2021, Force evaluation of tendon routing for underactuated grasping, Journal of Mechanical Design, Vol: 143, Pages: 1-9, ISSN: 1050-0472

While the modeling analysis of the kinetostatic behavior of underactuated tendon-driven robotic fingers has been largely addressed in the literature, tendon routing is often not considered by these theoretical models. The tendon routing path plays a fundamental role in defining joint torques, and subsequently, the force vectors produced by the phalanges. However, dynamic tendon behavior is difficult to predict and is influenced by many external factors including tendon friction, the shape of the grasped object, the initial pose of the fingers, and finger contact points. In this paper, we present an experimental comparison of the force performance of nine fingers, with different tendon routing configurations. We use the concept of force-isotropy, in which forces are equal and distributed on each phalanx as the optimum condition for an adaptive grasp. Our results show only some of the finger designs surveyed exhibited a partial adaptive behavior, showing distributed force for the proximal and distal phalanxes throughout grasping cycles, while other routings resulted in only a single phalanx remaining in contact with the object.

Journal article

Clark AB, Mathivannan V, Rojas N, 2021, A continuum manipulator for open-source surgical robotics research and shared development, IEEE Transactions on Medical Robotics and Bionics, Vol: 3, Pages: 277-280, ISSN: 2576-3202

Many have explored the application of continuum robot manipulators for minimally invasive surgery, and have successfully demonstrated the advantages their flexible design provides—with some solutions having reached commercialisation and clinical practice. However, the usual high complexity and closed-nature of such designs has traditionally restricted the shared development of continuum robots across the research area, thus impacting further progress and the solution of open challenges. In order to close this gap, this paper introduces ENDO, an open-source 3-segment continuum robot manipulator with control and actuation mechanism, whose focus is on simplicity, affordability, and accessibility. This robotic system is fabricated from low cost off-the-shelf components and rapid prototyping methods, and its information for implementation (and that of future iterations), including CAD files and source code, is available to the public on the https://github.com/OpenSourceMedicalRobots’s repository on GitHub, with the control library also available directly from Arduino. Herein, we present details of the robot design and control, validate functionality by experimentally evaluating its workspace, and discuss possible paths for future development.

Journal article

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, Pages: 25-36, ISSN: 0302-9743

In the field of robotic hands, tendon actuation is one of the most common ways to control self-adaptive underactuated fingers thanks to its compact size. Either differential or direct drive mechanisms are usually used in these systems to perform synchronised grasping using a single actuator. However, synchronisation problems arise in underactuated grippers whose position of proximal joints varies with time to perform manipulation operations, as this results in a tendon-driven system with dynamic anchor pulleys. This paper introduces a novel passively compliant idler mechanism to avoid unsynchronisation in grippers with a dynamic multi-tendon routing system, such that adequate grasping contact forces are kept under changes in the proximal joints’ positions. A re-configurable palm underactuated dexterous gripper is used as a case study, with the performance of the proposed compliant idler system being evaluated and compared through a contact force analysis during rotation and translation in-hand manipulation tasks. Experiment results clearly demonstrate the ability of the mechanism to synchronise a dynamic tendon routing gripper. A video summarising experiments and findings can be found at https://imperialcollegelondon.box.com/s/hk58688q2hjnu8dhw7uskr7vi9tqr9r5.

Conference paper

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, Pages: 3-14, ISSN: 0302-9743

Robotic exoskeletons have been a focal point of research due to an ever-increasing ageing population, longer life expectancy, and a desire to further improve the existing capabilities of humans. However, their effectiveness is often limited, with strong rigid structures poorly interfacing with humans and soft flexible mechanisms providing limited forces. In this paper, a scalable variable stiffness revolute joint is proposed to overcome this problem. By using layer jamming, the joint has the ability to stiffen or soften for different use cases. A theoretical and experimental study of maximum stiffness with size was conducted to determine the suitability and scalablity of this technology. Three sizes (50 mm, 37.5 mm, 25 mm diameter) of the joint were developed and evaluated. Results indicate that this technology is most suitable for use in human fingers, as the prototypes demonstrate a sufficient torque (0.054 Nm) to support finger movement.

Conference paper

Clark A, Rojas N, 2020, Design and workspace characterisation of malleable robots, IEEE International Conference on Robotics and Automation, Publisher: IEEE, Pages: 9021-9027

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, 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

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

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

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, Pages: 3969-3975, ISSN: 2577-087X

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

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