Entire Workspace Research Articles

Model-based control of redundantly actuated parallel manipulators in redundant coordinates

Model-based control of parallel kinematics machines (PKM) relies on computationally efficient formulations in terms of a set of independent joint coordinates. Since PKM models are commonly expressed in terms of actuator or end-effector coordinates the models are not valid at input- or output-singularities, respectively. Moreover input-singularities limit the motion range of PKM. Actuation redundancy is a means to increase the singularity-free range of motion. However, due to the redundancy only a subset of the actuator coordinates constitute independent coordinates. This subset corresponds to the actuator coordinates of the non-redundant PKM, which does generally not constitute proper minimal coordinates for the entire workspace. Hence a redundantly actuated PKM (RA-PKM) controlled by a model-based controller in terms of minimal coordinates would exhibit the same limitations as the non-redundant PKM. One approach to tackle this problem is to switch between different minimal coordinates, i.e., different motion equations are used within the controller.In this contribution a computed torque and augmented PD control scheme in redundant coordinates is proposed, as an alternative to coordinate switching, and applied to the control of redundantly actuated PKM. That is, no minimal coordinates are selected. This novel formulation is numerically robust and does not suffer from input- or output-singularities. Even more the formulation is always valid except at configuration space singularities. For the redundancy resolution within the inverse dynamics the pseudoinverse of a rank deficient matrix is required, for which an explicit formulation is presented. For both controllers exponential trajectory tracking is shown. Experimental results are reported for a planar 2 DOF RA-PKM.

Particle simulation on the Cell BE architecture

This paper presents two parallel formulations for the Barnes-Hut algorithm on the Cell architecture, which differ in tree distribution and construction phases of the algorithm. In the initial parallelization, the domains are dynamically partitioned and assigned to the synergistic processing elements (SPEs), and SPEs construct local trees of the sub-domains in parallel. The enhanced parallelization scheme provides better clustering of the particles by sequentially constructing the global tree of the entire work space in the power processing element (PPE) and by partitioning the tree into sub-trees that can fit in the Local Store. SPEs operate on the sub-tree data and construct local trees in parallel. Our experimental evaluation indicates that this application performs much faster on the Cell BE compared to the Intel Xeon based system. Specifically, our first and second methods on the Cell BE outperform Intel Xeon by a factor of 5.8 and 7.1 for 8192 particles, respectively.

Design and Prototyping of a Force-Reflecting Hand-Controller for Ultrasound Imaging

This paper presents detailed design, analysis, prototyping, and testing of a novel force-reflecting hand-controller allowing physicians to control a robotic wrist and perform ultrasound examinations on patients in remote locations. The proposed device is a four degree-of-freedom mechanism with a fixed center-of-motion and uses symmetric parallel mechanisms. All movements of the device are kinematically decoupled, i.e., the hand-controller has independent drive systems for each standard ultrasound motion. A technique has been adapted to statically balance the weight of the device over its entire workspace using a single tension spring. The prototype of the device has been constructed and evaluated for ultrasound imaging of kidney and spleen. Maximum and accuracy of the output force are analytically determined and performance of the device in terms of static balancing, static-friction break-away force, and maximum achievable impedances are experimentally evaluated.

Maximally Regular T2R1-Type Parallel Manipulators With Bifurcated Spatial Motion

This paper presents a new family of maximally regular T2R1-type parallel manipulators with bifurcated spatial motion. In each branch, the moving platform has two independent translations (T2) and one rotation (R1) driven by three actuators mounted on the fixed base. The rotation axis is situated in the plane of translation and can bifurcate in two orthogonal directions. This bifurcation occurs in a constraint singularity in which the connectivity between the moving and fixed platforms increases instantaneously, incurring no change in limb connectivity. The Jacobian matrix of the maximally regular solutions presented in this paper is a 3×3 identity matrix in the entire workspace of each branch. This paper presents for the first time a family of maximally regular T2R1-type parallel manipulators with bifurcated spatial motion of the moving platform along with solutions using uncoupled motions.

Ubiquitous Tracking in the Medical Environment

Computer-Integrated Surgery and intra-operative navigation opened a new era in interventional medicine. With the use of surgical navigation systems it has become possible to track tools and visualize the patient's preoperative records in real-time in the desired region, or overlay patient information and critical data. Navigation, as an enabling technology, has facilitated surgical simulation and training of medical students. In the future, we expect the rise of the concept of “ubiquitous tracking”, where all major parts of the medical working environment are tracked and modeled real-time in a virtual environment according to their physical state and position.One of the emerging modalities, electromagnetic tracking's (EMT) main advantage is that it does not required the continuous line-of-sight between the marker and the receiver, while this remains a serious limitation for optical tracking. EMT makes it possible to assist complex interventions, such as bronchoscopy, radiosurgery, heart ablation and beyond 0. In the future, tracking technology should get embedded in the entire operating theater through the combination of EMT sensors and RF ID tags. A fully sensorized operating room (OR) would allow for the better understanding of the procedures, the motion of the tools and the devices, track every movement of the patient, or the posture and the gestures of the surgeon—all without constraining the intervention. This will lead to an enormous improvement in interventional simulation and workflow analysis, and will result in the dramatic improvement of assistive surgical technologies.However, current EMT systems are highly susceptible to the presence of other electronic devices and metallic distortions 0. Ferromagnetic items, such as the surgical tool and instruments (laparoscopic forceps, cutlery, trays, etc.) alter the EM field, therefore the tracking accuracy is highly reduced, and measurements become less reliable. Due to recent advancement in system design, certain materials (e.g., titanium) do not cause significant error any more, but there is still no general solution for metallic sensitivity 0. Further weakness is lower accuracy compared to optical modality and the working volume being significantly limited.In the near future, the research community will be able to overcome most of the technical difficulties, given the right approach and methodology. A few years from now, the primary issues will be solved, including static error correction and susceptibility reduction. Our group is currently focusing on the development of a unified method—a quick calibration protocol—that can assess EM distortions in a given workspace rapidly, and based on what compensation for static distortions of the EM field becomes possible. Next, dynamic distortions have to be measured, modeled and simulated, enabling device-specific tracking in the medical environment 0. Furthermore, all active and passive devices in the OR should be modeled and tracked to support the concept of ubiquitous tracking. Reliable simulation programs will be available for modeling the field and the distortion caused by the individual instruments. The further development of systems will include the support of multi-sensor applications, significant extension of the work space and innovative types of sensors.Within a few decades, we will have accurate soft tissue models available, beyond the current model of the surgical tools. Therefore it will be possible to immediately determine how the magnetic field was affected by the surgical tools. The compensation will be feasible in real time, thus the position of sensors will be highly trusted, and all environmental variables fully tracked and followed.Ultimately, wireless power transmission technologies will overcome cabled ones in these applications 0. The sensors will also be used to transmit and receive power for their functioning; however it will severely influence the EM field, requiring dynamic compensation for distortions. This would leave more space for the physicians, while allowing complete tracking and control of the operating theater.Our paper presents the concept of “ubiquitous tracking” in the medical environment. We enlist the advantages and hinder falls of the technology that will enable complete EMT in the OR. We began to work on the currently most pressing issue, the susceptibility of the EMT systems to the environmental effects. Thorough measurements within the entire workspace have been taken, and we started to develop a model for error assessment and a compact tool for rapid calibration. This shall be a plexiglass calibration bracket that was designed to best measure the possible distortions in a given field. CT breeds and MR fiducials will be incorporated allowing a gold standard calibration to the CAD model.Eventually, the field is advancing towards the concept of fully controlled operating room, where the optical and electromagnetic tracking-based interventions can be done with reasonably high accuracy. Compensating the electromagnetic fields distortions in real-time, tracking can be used to improve minimally invasive procedures and serve as an enabling technology for various medical procedures.

Design and analysis of a totally decoupled 3-DOF spherical parallel manipulator

SUMMARYIn this paper, we propose a unique, decoupled 3 degree-of-freedom (DOF) parallel wrist. The condition required for synthesizing a fully isotropic parallel mechanism is obtained on the basis of the physical meaning of the row vector in the Jacobian matrix. Specifically, an over-constrained spherical 3-DOF parallel mechanism is presented and the modified structure, which avoids the redundant constraints, is also introduced. The proposed manipulator is capable of decoupled rotational motions around the x, y, and z axes and contains an output angle that is equal to the input angle. As this device is analyzed with the Jacobian matrix, the mechanism is free of singularity within its workspace and maintains homogenous stiffness over the entire workspace.

Obstacle avoidance during online corrections

The dorsal visual stream codes information crucial to the planning and online control of target-directed reaching movements in dynamic and cluttered environments. Two specific dorsally mediated abilities are the avoidance of obstacles and the online correction for changes in target location. The current study was designed to test whether or not both of these abilities can be performed concurrently. Participants made reaches to touch a target that, on two-thirds of the trials, remained stationary and on the other third "jumped" at movement onset. Importantly, on target-jump trials, a single object (in one of four positions) sometimes became an obstacle that interfered with the reach. When a target jump caused an object to suddenly become an obstacle, we observed clear spatial avoidance behavior, an effect that was not present when the target jumped but the object did not become an obstacle. This automatic spatial avoidance was accompanied by significant velocity reductions only when the risk for collision with the obstacle was high, suggesting an "intelligent" encoding of potential obstacle locations. We believe that this provides strong evidence that the entire workspace is encoded during reach planning and that the representation of all objects in the workspace is available to the automatic online correction system.

Multifunctional Intelligent Autonomous Parking Controllers for Carlike Mobile Robots

An increasing number of carlike mobile robot (CLMR) studies have addressed the issues of autonomous parking and obstacle avoidance. An autonomous parking controller can provide convenience to a novice driver. However, if the controller is not designed adequately, it may endanger the car and the driver. Therefore, this paper presents a novel multifunctional intelligent autonomous parking controller that is capable of effectively parking the CLMR in an appropriate parking space, by integrating sensor data capable of obtaining the surrounding data of the robot. An ultrasonic sensor array system has been developed with group-sensor firing intervals. A binaural approach to the CLMR has been adopted for providing complete contactless sensory coverage of the entire workspace. The proposed heuristic controller can obtain the posture of a mobile robot in a parking space. In addition, the controller can ensure the ability of the CLMR to withstand collision to guarantee safe parking. Moreover, the CLMR can recognize the parking space and the obstacle position in dynamic environment. Therefore, the proposed controller installed in a car could ensure safe driving. Finally, practical experiments demonstrate that the proposed multifunctional intelligent autonomous parking controllers are feasible and effective.

Measurement and Optimization of Minimally Invasive Intervention Device Design Fitness Using a Multiobjective Weighted Isotropy Index

The recent transition from multiple-port to single-port systems in minimally invasive intervention (MII) procedures has created a need for more flexible, dexterous robotic manipulation devices capable of spanning an entire surgical workspace without the risk of collateral damage. The design of such devices requires a careful balance of the mechanical complexity needed to facilitate clinical functionality and the cost of manufacturing and operating the device. This paper presents a novel metric for measuring the design fitness of kinematically redundant robotic MII devices and for optimizing them to achieve that balance. The proposed fitness metric rewards designs that are conducive to collision avoidance and energy conservation while penalizing those with exorbitant design complexities that adversely affect the economic feasibility of an MII system. The authors’ metric is used here to design a kinematically redundant, single-port MII device capable of accessing the cardiothoracic cavity through a single subxiphoid port and reaching several regions of interest, consistent with procedures such as epicardial ablation and therapeutic substance injection, with minimal physiologic disturbance. The design of this device is determined by a morphological optimization process, which searches a discrete mechanical design parameter space, consisting of linkage parts, part dimensions, and actuator types, using genetic algorithms. The execution of specific surgical maneuvers is simulated for each candidate MII device design, and the design is improved until the fitness metric is maximized. The results of this optimization study demonstrate that redesigning a 20 degree-of-freedom (DOF) MII device using the proposed metric decreased the DOF in the design by 45% while ensuring near-optimal levels of kinematic flexibility. The results also demonstrate the ability of the fitness metric to elucidate the relationship between functionality and complexity and to produce suitable device designs over a broad range of performance and cost goals. The authors conclude that this new design fitness metric, while heuristic in nature, holds the potential to improve both the clinical value and the economy of a wide variety of single-port MII devices, including those used in cardiothoracic surgery.

Dynamic Modeling and Eigenvalue Evaluation of a 3-DOF PKM Module

Due to the structural complexity, the dynamic modeling and quick performance evaluation for the parallel kinematic machines (PKMs) are still to be remained as two challenges in the stage of conceptual design. By using the finite element method and substructure synthesis, this paper mainly deals with the dynamic modeling and eigenvalue evaluation of a novel 3-DOF spindle head named the A3 head. The topological architecture behind the proposed A3 head is a 3-RPS parallel mechanism, which possesses one translational and two rotational capabilities. The mechanical features of the A3 head are briefly addressed in the first place followed by inverse position analysis. In the dynamic modeling, the platform is treated as a rigid body, the RPS limbs as the continuous uniform beams and the joints as lumped virtual springs. With the combination of substructure synthesis and finite element method, an analytical approach is then proposed to formulate the governing equations of motion of system using the compatibility conditions at interface between the limbs and the platform. Consequently, by solving the eigenvalue problem of the governing equations of motion, the distribution of lower natural frequencies of the A3 head throughout the entire workspace can be predicted in a quick manner. Modal analysis for the A3 head reveals that the distributions of lower natural frequencies are strongly related to the mechanism configuration and are axially symmetric due to system kinematic and structural features. The sensitivity analysis of the system indicates that the dimensional parameters of the 3-RPS mechanism have a slight effect on system lower natural frequencies while the joint compliances affect the distributions of lower natural frequencies significantly. The proposed dynamic modeling method can also be applied to other PKMs and can effectively evaluate the PKM's dynamic performance throughout the entire workspace.

Dynamic Dimensional Synthesis of Delta Robot

The dynamic dimensional synthesis of Delta robot widely used for high-speed pick-and-place operations is studied. On the basis of the inverse kinematic and rigid body dynamic formulation, a novel global dynamic performance index is proposed for minimizing the maximum driving torque of a single active joint. The proposed index can explicitly be expressed in terms of dimensional and inertial parameters as well as system configurations, allowing the singular configurations to be easily found. Various geometrical and performance constraints are taken into account in terms of workspace/machine volume ratio, lower and upper bounds of mapping characteristics in terms of velocity, accuracy and actuation rigidity between the joint space and the operation space. The effects of the performance constraints on the feasible domain of design variables and the performance index are investigated in depth via an example, enabling a set of optimized dimensional parameters to be obtained for achieving a good dynamic performance throughout the entire workspace.

Adaptive inverse dynamics control of robots with uncertain kinematics and dynamics

It has been about two decades since the first globally convergent adaptive tracking controller was derived for robots with dynamic uncertainties. However, not until recently has the problem of concurrent adaptation to both the kinematic and dynamic uncertainties found its solution. This adaptive controller belongs to passivity-based control. Though passivity-based controllers have many attractive properties, in general, they are not able to guarantee the uniform performance of the robot over the entire workspace. Even in the ideal case of perfect knowledge of the manipulator parameters, the closed-loop system remains nonlinear and coupled. Thus the closed-loop tracking performance is difficult to quantify, while the inverse dynamics controllers can overcome these deficiencies. Therefore, in this work, we will develop a new adaptive Jacobian tracking controller based on the inverse manipulator dynamics. Using the Lyapunov approach, we have proved that the end-effector motion tracking errors converge asymptotically to zero. Simulation results are presented to show the performance of the proposed controller.

Structural synthesis of maximally regular T3R2-type parallel robots via theory of linear transformations and evolutionary morphology

SUMMARYThe paper presents structural synthesis of maximally regular T3R2-type parallel robotic manipulators (PMs) with five degrees of freedom. The moving platform has three independent translations (T3) and two rotations (R2). A method is proposed for structural synthesis of maximally regular T3R2-type PMs based on the theory of linear transformations and evolutionary morphology. A one-to-one correspondence exists between the actuated joint velocity space and the external velocity space of the moving platform. The Jacobian matrix mapping the two vector spaces of maximally regular T3R2-type PMs presented in this paper is the 5×5 identity matrix throughout the entire workspace. The condition number and the determinant of the Jacobian matrix being equal to one, the manipulator performs very well with regard to force and motion transmission capabilities. Kinematic analysis of maximally regular parallel robots is trivial and no computation is required for real-time control. This paper presents in a unified approach the structural synthesis of PMs with five degrees of freedom with decoupled and uncoupled motions, along with the maximally regular solutions.

Singularity-Free Joint Actuation in Omnidirectional Mobile Platforms With Powered Offset Caster Wheels

This paper presents the analysis of singularity and motion capability of a mobile platform articulated by offset powered caster wheels. Specifically, it presents the analysis of the equations of motion resulting in the sufficient and necessary actuation condition to yield a workspace that is entirely free of singular configurations. This paper shows that powering both the steer and drive joints on two sets of offset caster wheels in a mobile platform guarantees a singularity-free condition throughout the entire workspace. Analysis and discussion on equations of motions that lead to this result are presented.

Method for Parallel Manipulators to Pass through Singular Points with Its Original Configuration

Through the studies of the configuration bifurcation characteristics of a 6-SPS Gough-Stewart manipulator under the action of the single input parameter,and the transform characters of singular points under the disturbances of the other input parameters,a novel method for parallel manipulators to pass through singular points with a persistent configuration is presented. Taking a semi-regular hexagons Gough-Stewart manipulator as an example,the disturbance function for the manipulator to pass through the singular point with a persistent configuration is constructed.Numerical result shows that under the action of the disturbance function,the manipulator can approach the singular point with a very small error and go back with its original configuration.The method presented has solved the problem of the uncertainty of motion at the singular point.In this way,parallel manipulators can give the determinate output in the entire workspace.

Kinematic Design to Improve Ergonomics in Human Machine Interaction

This paper introduces a novel kinematic design paradigm for ergonomic human machine interaction. Goals for optimal design are formulated generically and applied to the mechanical design of an upper-arm exoskeleton. A nine degree-of-freedom (DOF) model of the human arm kinematics is presented and used to develop, test, and optimize the kinematic structure of an human arm interfacing exoskeleton. The resulting device can interact with an unprecedented portion of the natural limb workspace, including motions in the shoulder-girdle, shoulder, elbow, and the wrist. The exoskeleton does not require alignment to the human joint axes, yet is able to actuate each DOF of our redundant limb unambiguously and without reaching into singularities. The device is comfortable to wear and does not create residual forces if misalignments exist. Implemented in a rehabilitation robot, the design features of the exoskeleton could enable longer lasting training sessions, training of fully natural tasks such as activities of daily living and shorter dress-on and dress-off times. Results from inter-subject experiments with a prototype are presented, that verify usability over the entire workspace of the human arm, including shoulder and shoulder girdle.

Structural synthesis of fully-isotropic parallel robots with Schönflies motions via theory of linear transformations and evolutionary morphology

Abstract The paper presents structural synthesis of fully-isotropic parallel robotic manipulators (PMs) with Schonflies motions. The moving platform of a parallel manipulator with Schonflies motions (PMSM) has four degrees of freedom, which are three independent translations and one rotation about an axis of fixed direction. A method is proposed for structural synthesis of fully-isotropic PMSMs based on the theory of linear transformations and the evolutionary morphology. A one-to-one correspondence exists between the actuated joint velocity space and the external velocity space of the moving platform. The Jacobian matrix mapping the two vector spaces of fully-isotropic PMSMs presented in this paper is the identity 4 × 4 matrix throughout the entire workspace. The condition number and the determinant of the Jacobian matrix being equal to one, the manipulator performs very well with regard to force and motion transmission capabilities. The synthesis method proposed in this paper allows us to obtain structural solutions of PMSMs with decoupled and uncoupled motions, along with the fully-isotropic solutions in a systematic manner. Overconstrained/isostatic solutions with elementary/complex and identical/different legs are obtained. Uncoupled and fully-isotropic PMSMs have the advantage of simple command and important energy-saving due to the fact that, for a unidirectional motion, only one motor works as in a serial translational manipulator.

Structural synthesis of fully-isotropic translational parallel robots via theory of linear transformations

Abstract The paper presents a new structural synthesis approach of fully-isotropic translational parallel robotic manipulators (TPMs) based on the theory of linear transformations. A TPM is a 3-DOF (degree of freedom) parallel mechanism whose output link, called platform, can achieve three independent orthogonal translational motions with respect to the fixed base. The manipulators presented in this paper have three legs connecting the moving platform and the base (fixed platform). Only one kinematic pair per leg is actuated by a linear motor situated on the fixed base. A one-to-one correspondence exists between the actuated joint space and the operational space of the moving platform. The Jacobian matrix of fully-isotropic TPMs presented in this paper is the identity 3 × 3 diagonal matrix throughout the entire workspace. The synthesis method proposed in this paper allows us to obtain all structural solutions of fully-isotropic TPMs in a systematic manner. Overconstrained/isostatic solutions with elementary/complex and identical/different legs are obtained. Fully-isotropic TPMs have the advantage of simple command and important energy-saving due to the fact that, for a unidirectional translation, only one motor works as in a serial translational manipulator.

Evaluation of resolution of flexure parallel mechanisms for ultraprecision manipulation

A method for evaluating the resolution of ultraprecision manipulation systems based on the flexure parallel mechanism (FPM) is presented. The resolution of the open-loop system is theoretically determined by the resolution transmission from the actuated joints to the end–effector and the resolution of actuators. The method studies the definition of the resolution indicators that includes the global resolution transmission scale and the uniformity of resolution over the entire workspace of the flexure mechanism. The computational algorithm for the defined resolution indicators is established based on the sampling method. For illustration, the evaluation method is employed to gauge the resolution performance of a two-degree-of-freedom and a three-degree-of-freedom (3-DOF) planar FPM. To demonstrate its application for optimal design, we use this method for the development of a 3-DOF spatial translational FPM. An experiment is carried out to determine the resolution and the repeatability of the developed FPM and verify the proposed evaluation method. The result shows that the method is suitable for the design of FPM for any desired resolution.

A shape design system using volumetric implicit PDEs

Solid modeling based on partial differential equations (PDEs) can potentially unify both geometric constraints and functional requirements within a single design framework to model real-world objects via its explicit, direct integration with parametric geometry. In contrast, implicit functions indirectly define geometric objects as the level-set of underlying scalar fields. To maximize the modeling potential of PDE-based methodology, in this paper we tightly couple PDEs with volumetric implicit functions in order to achieve interactive, intuitive shape representation, manipulation, and deformation. In particular, the unified approach can reconstruct the PDE geometry of arbitrary topology from scattered data points or a set of sketch curves. We make use of elliptic PDEs for boundary value problems to define the volumetric implicit function. The proposed implicit PDE model has the capability to reconstruct a complete solid model from partial information and facilitates the direct manipulation of underlying volumetric datasets via sketch curves and iso-surface sculpting, deformation of arbitrary interior regions, as well as a set of CSG operations inside the working space. The prototype system that we have developed allows designers to interactively sketch the curve outlines of the object, define intensity values and gradient directions, and specify interpolatory points in the 3D working space. The governing implicit PDE treats these constraints as generalized boundary conditions to determine the unknown scalar intensity values over the entire working space. The implicit shape is reconstructed with specified intensity value accordingly and can be deformed using a set of sculpting toolkits. We use the finite-difference discretization and variational interpolating approach with the localized iterative solver for the numerical integration of our PDEs in order to accommodate the diversity of generalized boundary and additional constraints.