Unlike parallel robots, for which hundreds of different architectures have been proposed, the vast majority of six-degree-of-freedom (DOF) serial robots have one of two simple architectures. In both architectures, the inverse kinematics can be solved in closed form and the singularities described by trivial geometric and algebraic conditions. These conditions can be readily obtained by analyzing the determinant of the robot’s Jacobian matrix, and provide an in-depth understanding of the robot’s singularities, which is essential for its optimal use. However, for various reasons, robot arms with unorthodox architectures are occasionally designed. Such arms do not have closed-form inverse kinematics and little insight into their singularities can be gained by analyzing the determinant of their Jacobian. One such robot arm for which the conventional singularity analysis approach fails is the new Link 6 collaborative robot by Kinova. In this paper, we study the complex singularities of Link 6 by investigating all possibilities for screw dependencies, deriving a simple equation for each case, and then describing each singularity type using Grassmann line geometry. Twelve different singularity configurations are identified and described with seven relatively simple geometric conditions. Our approach is general and can be applied to other robot arms.

A kinematically redundant (6+3)-degree-of-freedom (DOF) hybrid parallel robot with an axisymmetric workspace is proposed. By arranging the first revolute joint of each leg such that they have the same rotation axis, this robot can achieve an axisymmetric workspace, resulting in a large reachable workspace. In addition, type II singularities, which critically limit the orientational workspace, can be fully avoided by utilizing kinematic redundancy. A gripper mechanism is developed to increase the orientational workspace by exploiting the redundant DOFs. Moreover, the orientational workspace can be further increased by controlling one of the redundant DOFs to keep a certain constant angle. As a result, the proposed hybrid parallel robot achieves a high workspace-to-footprint ratio comparable to that of serial robots. A CAD model of the robot and computer animations are provided to demonstrate the large workspaces and the gripper mechanism. A significant advantage of the proposed robot over serial architectures is that the robot is backdrivable since it uses direct-drive or quasi-direct-drive actuators.

A novel kinematically redundant 6+1-degree-of-freedom (dof) spatial hybrid parallel robot is proposed. Each of the two legs of the robot has a fully parallel structure to minimize the moving inertia by mounting actuators on the base. The kinematic model of each leg and overall robot architecture is developed based on the constraint conditions of the robot geometry. The singularity analysis of legs 1 and 2 reveals that their serial and parallel singularities can be avoided by properly dimensioning the robot and sacrificing the edge of the workspace. In addition, it is shown that the type II (parallel) singularities can be completely avoided, resulting in a large orientational workspace. The gripping mechanism is then introduced which is operated by the redundant degree of freedom of the robot. A CAD model of the robot and a computer animation are provided to demonstrate the positioning and orientation of the robot and the gripping function.

A novel kinematically redundant ($6+3$) -DoF parallel robot is presented in this paper. Three identical 3-DoF RU/2-RUS legs are attached to a configurable platform through spherical joints. With the selected leg mechanism, the motors are mounted at the base, reducing the reflected inertia. The robot is intended to be actuated with direct-drive motors in order to perform intuitive physical human-robot interaction. The design of the leg mechanism maximizes the workspace in which the end-effector of the leg can have a 2g acceleration in all directions. All singularities of the leg mechanism are identified under a simplifying assumption. A CAD model of the (6+3)-DoF robot is presented in order to illustrate the preliminary design of the robot.

Robots need to adapt their behaviour while physically interacting with an operator to guarantee safety and provide intuitiveness. Inferring the intentions of the operator is a challenging problem that can be addressed by introducing sensors, in addition to motor encoders. Also, kinematic redundancy can be used to avoid issues such as singularities or mechanical interference, and the redundant coordinates can be controlled freely. In this work, we propose to use the redundant degrees of freedom to infer the intentions of an operator interacting with a backdrivable kinematically redundant parallel robot, without introducing any additional sensors. The proposed approach is based on the fact that, in mechanically backdrivable robots, the operator can control the redundant degrees of freedom, and this can be sensed using solely motor encoders through the solution of the forward kinematics. This approach is implemented to switch between a position controller and a controller that allows the operator to guide the robot freely thanks to gravity compensation. Experiments are carried out to compare this approach with an existing one and show that it improves intuitiveness during interaction by reducing false mode change detections.

This paper presents reorientation manoeuvres applied to an omnidirectional wheeled robot for impact mitigation during short falls. The proposed robot architecture aims to build upon recent innovations in reorientation robots to attain fast, multi-axis reorientation. Indeed, the use of omnidirectional wheels allows for simplifications to be made with respect to previous mobile robot architectures that make the proposed architecture more efficient for free fall reorientation, while still maintaining free roaming capabilities. To test these improvements, a prototype is built and a free roam and two free fall demonstrations are completed. On the one hand, the free roam demonstration validates that translation along both horizontal axes and rotation about the yaw axis are achieved with the presented prototype. On the other hand, the first free fall demonstration shows that a worst case scenario of a 180-degree reorientation about one axis can be completed in just under 0.45 seconds (one-metre fall) and the second free fall demonstration validates that the prototype is capable of simultaneous reorientation about both the roll and pitch axes. Therefore, the fast, multi-axis reorientation capabilities of the developed prototype are verified.

This article provides a general framework for the use of low-impedance displacement sensors mounted on the links of a serial robot to provide an intuitive physical human–robot interaction. A general formulation is developed to handle the motion guidance problem, i.e., the mapping of the measured motion of the sensors into the required robot joint motions to provide intuitive responsiveness. The formulation is general and can be applied to any architecture of serial robot with any number of displacement sensors each having an arbitrary number of degrees of freedom. Then, the design of a novel three-degree-of-freedom low-impedance displacement sensor is presented as a particularly effective instantiation of the general concept. Partial force balancing is used to reduce the required elastic return action, thereby ensuring the low impedance of the interaction. A prototype of a three-degree-of-freedom displacement sensor is then introduced. Two such sensors are mounted on the links of a custom-built five-degree-of-freedom robot in order to demonstrate the proposed approach. Experimental results are provided and comparisons with other collaborative robots are given. It is shown that the proposed sensors and motion guidance approach yield very intuitive low-impedance interaction involving very low interaction forces.

The time-optimal trajectory planning problem involves minimizing the time required to follow a path defined in space, subject to kinematic and dynamic constraints. Here, we introduce a novel technique, called the bisection algorithm (BA), which is fully implemented in C++ and extends dynamic programming approaches to the problem. These approaches, which rely on dividing the global problem into a series of simpler subproblems, become increasingly advantageous compared to direct transcription methods as the number of problem constraints increases. In contrast to nearly all other dynamic programming approaches, BA does not rely on finding a maximum-velocity curve or explicitly finding acceleration switching points during the trajectory planning process. Additionally, only one forward and one backward integration are used, during which all constraints are imposed. This approach is made feasible through careful control of the numerical integration process and the use of a bisection algorithm to resolve constraint violations during integration. BA is shown to be significantly simpler, faster, and more robust than recently proposed algorithms: a direct comparison is made for a series of paths to be followed by a serial manipulator, subject to kinematic constraints. The wide applicability of BA is then established by solving the time-optimal problem for a parallel manipulator following a complex path, subject to both kinematic and dynamic constraints.

This article presents two distinct manoeuvres allowing an articulated robot in free fall to change its orientation using closed paths in the joint space. It is shown through dynamics simulations that the magnitude of the net rotation is dependent upon the amplitude of the angular displacement of the joints. With realistic joint limitations, the robot, which includes rotary actuators only, can perform a 180°reorientation about its longitudinal axis, similar to the cat righting reflex. The second manoeuvre allows the robot to accomplish rotations of smaller magnitude about a different axis. A physical prototype and a VICON motion tracking system are used to experimentally validate the simulation results. Finally, it is shown that the two manoeuvres, which yield rotations about fixed axes, can be repeated and alternated to enable the robot to reach any arbitrary 3-D orientation.

A novel backdrivable 3-[R(RR-RRR)SR] kinematically redundant (6+3)-degree-of-freedom (DOF) spatial hybrid parallel robot with revolute actuators is proposed for low-impedance physical human–robot interaction. The kinematic model is developed based on the constraint conditions of the robot. It is shown that the type II (parallel) singularities can be completely avoided, thereby yielding a very large translational and orientational workspace. A workspace analysis is presented in order to demonstrate the capabilities of the robot. Mechanisms are then introduced to use the redundant DOF of the robot to operate a gripper with the robot actuators, which are mounted on or close to the base, thus reducing the inertia of the moving parts. The architecture of the robot makes it possible to use direct drive motors, thereby making the robot easily backdrivable and allowing the use of a very simple and effective controller. A prototype of the robot is then designed and built and the large workspace of the robot as well as the effortless physical human–robot interaction are demonstrated. The controller of the robot is then described, including a position control mode and a control mode for physical interaction, which does not require the use of a force/torque sensor or joint torque sensors. Because of its backdrivability and low moving inertia, the robot is particularly well-suited for physical human–robot interaction, as demonstrated in the accompanying videos.

This paper presents a trajectory optimization formulation for planning dynamic trajectories of a six-degree-of-freedom (six-DOF) cable-suspended parallel robot (CSPR) that extend beyond the static workspace. The optimization is guided by low-dimensional dynamic models to overcome the local minima and accelerate the exploration of the narrow feasible state space. The dynamic similarity between the six-DOF CSPR and the three-DOF point-mass CSPR is discussed with the analyses of their feasible force polyhedra. Finally, the transition trajectories of a three-DOF CSPR are used as the initial guess of the translational part of the six-DOF motion. With the proposed approach, highly dynamic motions for a six-DOF CSPR are efficiently generated with multiple oscillations. The feasibility is demonstrated by point-to-point and periodic trajectories in the physics simulation.

Parallel robots with configurable platforms (PRCPs) combine the benefits of parallel robots with additional functionalities such as grasping and cutting. However, some of the theoretical tools used to study classical parallel robots do not apply to parallel robots with configurable platforms. This paper uses screw theory to study the transferable wrenches from the robot's limbs to the configurable platform of an 8-DoF parallel robot. Deriving the transferable wrenches allows one to construct the screw system that is applied to each part of the configurable platform. Based on the analytical expressions of the limb and platform wrenches that have been derived and numerically validated, the mathematical tools that are used to study parallel kinematic structures, such as Grassmann line geometry, can thus be applied to the presented parallel robot with a configurable platform.

This paper investigates the design of a three-degree-of-freedom rotational inertia generator using the gyroscopic effect to provide ungrounded torque feedback. It uses a rotating mass in order to influence the torques needed to move the device, creating a perceived inertia. The dynamic model and the control law of the device are derived, along with those of a comparable concept using three flywheels instead of a gyroscope. Both models are then validated through simulations. Further simulations are conducted to establish motor torque and velocity requirements, and the gyroscopic concept is identified as having the less demanding requirements. The mechatronic design of a prototype of an inertia generator is presented, along with modifications to the dynamic model. Preliminary experimental validations are conducted. As the prototype faces instability issues when using the flywheels at high velocities, they are conducted using 0 RPM initial velocities. The results confirm that it is possible to both reduce and increase the rendered inertia even with current limitations. Finally, improvements for a second version of the prototype are discussed.

This paper presents a novel sensing approach for the physical interaction between a human user and a serial robotic arm. The approach is inspired from the concept of macro-mini robot architecture. The framework is developed for a general multi-degree-of-freedom serial robot and a corresponding impedance control scheme is proposed. In order to illustrate the concept, a five-degree-of-freedom robotic arm was built as well as a six-degree-of-freedom low-impedance sensing device that is used to control the robot. Experimental results are provided.

This paper introduces a novel four-degree-of-freedom (4-DOF) parallel mechanism having 3 translational DOFs and 1 rotational DOF. The mechanism comprises 2 sets of parallelogram linkages, which constrain two of the rotational DOFs of the mechanism. An interesting feature of the mechanism is that it can be driven using 4 parallel sliders mounted on its base. As a result, one of the translational DOFs can be infinitely large. Also, the architecture of the mechanism provides a large rotational DOF in one direction. The kinematic equations of the mechanism are derived and the Jacobian matrices are obtained. The mathematical conditions that lead to singularities are also found. Moreover, a geometric description of the boundaries of the workspace is given, which can be expressed using simple equations. Finally, some design examples are proposed and a prototype is presented.

A novel 3-[R(RR-RRR)SR] kinematically redundant 6+3-degree-of-freedom (dof)spatial hybrid parallel robot with revolute actuators is proposed. The kinematic model is developed based on the constraint conditions of the robot. It is shown that the type II (parallel) singularities can be completely avoided, thereby greatly extending the orientational workspace. Mechanisms are then introduced to use the redundant degrees of freedom of the robot to operate a gripper with the robot actuators, which are mounted on or close to the base. A CAD model of the robot is shown and a computer animation is provided to demonstrate the resulting architecture, which has full 6-dof capabilities and a large orientational workspace.

Robotic devices can be leveraged to raise the abilities of humans to perform demanding and complex tasks with less effort. Although the first priority of such human-robot interaction (HRI) is safety, robotic devices must also be intuitive and efficient in order to be adopted by a broad range of users. One challenge in the control of such assistive robots is the management of the end-effector orientation, that is not always intuitive for the human operator, especially for neophytes. This paper presents a novel orientation control algorithm designed for robotic arms in the context of HRI. This paper aims at making the control of the robot's orientation easier and more intuitive for the user, both in the fields of rehabilitation (in particular individuals living with upper limb disabilities) and industrial robotics. The performance and intuitiveness of the proposed orientation control algorithm is assessed and improved through two experiments with a JACO assistive robot with 25 able-bodied subjects, an online survey with 117 respondents via the Amazon Mechanical Turk and through two experiments with a UR5 industrial robot with 12 able-bodied subjects.

This paper proposes a point-to-point dynamic trajectory planning technique for reaching a series of poses with a six-degree-of-freedom (six-DOF) cable-suspended parallel robot. Each trajectory segment is designed to have zero translational and rotational velocity at its endpoints; transitions between segments have translational and rotational acceleration continuity. This formulation facilitates the synthesis of trajectories that extend beyond the static workspace of the robot. A basis motion is introduced, which is a mathematical function that can be adapted for each coordinate direction along each trajectory segment. Kinematic constraints are satisfied through the selection of the coefficients for this function. Dynamic constraints are imposed by defining feasible regions within the workspace for each segment endpoint, based on the previous endpoint. Spherical linear interpolation (SLERP) is used to produce singularity-free, optimally interpolated rotational trajectory segments. An experimental implementation is presented using a six-DOF prototype and a supplementary video file is included to demonstrate the results.

This paper proposes a dynamic trajectory planning technique for six-degree-of-freedom (6-DOF) cable-suspended parallel robots (CSPRs). First, a passive mechanical system that is equivalent to the CSPR is introduced to provide insight and facilitate the design of trajectories that can extend beyond the robot's static workspace. The tilt-and-torsion angle convention is used to develop the mathematical model and impose restrictions for the rotational component of the trajectories. The dynamic differential equations that govern the translational component of the trajectories are shown to become linear under some conditions. Natural frequencies of an equivalent passive linear system of constant-stiffness springs are, thus, obtained and the set of linear differential equations associated with this system is integrated to produce a general solution for natural, periodic trajectories. This approach is used to produce pure translation trajectories and more complex motion that includes changes in position and orientation. An experimental implementation is also presented using a 6-DOF prototype and a supplementary video file is included to demonstrate the results.

This paper presents a novel robotic hand design that aims at producing a realistic human-robot handshake. A standard characteristic model of the human-palm compliance is developed based on human hand anatomy and an empirical study. Based on this model a realistic palm-compliance rendering is implemented. The backdrivability of the system allows a position-controlled feedback loop which renders human-like agility. An objective and a subjective experimental session, also depicted in the accompanying video, are then presented in order to validate the haptic feedback of the robotic hand in comparison to a real human handshake.

This paper presents an anticipative robot kinematic limitation avoidance algorithm for collaborative robots. The main objective is to improve the performance and the intuitivity of physical human-robot interaction. Currently, in such interactions, the human user must focus on the task as well as on the robot configuration. Indeed, the user must pay a close attention to the robot in order to avoid limitations such as joint position limitations, singularities and collisions with the environment. The proposed anticipative algorithm aims at relieving the human user from having to deal with such limitations by automatically avoiding them while considering the user's intentions. The framework developed to manage several limitations occurring simultaneously in three-dimensional space is first presented. The algorithm is then presented and detailed for each individual limitation of a spatial RRR serial robot. Finally, experiments are performed in order to assess the performance of the algorithm.

Multimodal body-machine interfaces play an important role in providing severely impaired with interaction solutions that can adapt to their functional capacities [1]. This demonstration will allow the visitors to experience an intuitive and wearable control interface, designed for people with upper body disabilities, that translates head motion, shoulder elevation and surface electromyography into appropriate commands for controlling assistive devices, like robotic arms. Visitors will be invited to interact with JACO using the proposed interface, a 6 degree-of-freedom assistive robotic arm developed by Kinova Robotics, within a control task that will consist in moving items or stack objects at specific locations on a table.

This paper presents a new multimodal control interface for people living with upper-body disabilities based on a wearable wireless sensor network. The proposed body-machine interface is modular and can be easily adapted to the residual functional capacities (RFCs) of different users. A custom data fusion algorithm has been developed for emulating a joystick control using head motion measured with a lightweight wireless inertial sensor enclosed in a headset. The wearable network can include up to six modular sensor nodes which can be used simultaneously to read different RFCs including gesture and muscular activity, and translate them into commands. Sensor data fusion is performed inside the sensor nodes in order to free the wireless link and the base station, and decrease power consumption. Requirements of such an interface are established for people using powered-wheelchairs, and a proof of concept system is implemented and used to control an assistive robotic arm. It is shown that the performance of the system compares well to conventional control systems like the joystick controller, while being potentially more suitable for the severely disabled.

In the realm of surface electromyography (sEMG) gesture recognition, deep learning algorithms are seldom employed. This is due in part to the large quantity of data required for them to train on. Consequently, it would be prohibitively time consuming for a single user to generate a sufficient amount of data for training such algorithms. In this paper, two datasets of 18 and 17 able-bodied participants respectively are recorded using a low-cost, low-sampling rate (200Hz), 8-channel, consumer-grade, dry electrode sEMG device named Myo armband (Thalmic Labs). A convolutional neural network (CNN) is augmented using transfer learning techniques to leverage inter-user data from the first dataset and alleviate the data generation burden imposed on a single individual. The results show that the proposed classifier is robust and precise enough to guide a 6DoF robotic arm (in conjunction with orientation data) with the same speed and precision as with a joystick. Furthermore, the proposed CNN achieves an average accuracy of 97.81% on seven hand/wrist gestures on the 17 participants of the second dataset.

Assistive technology (AT) tools and appliances are being more and more widely used and developed worldwide to improve the autonomy of people living with disabilities and ease the interaction with their environment. This paper describes an intuitive and wireless surface electromyography (sEMG) based body-machine interface for AT tools. Spinal cord injuries at C5-C8 levels affect patients' arms, forearms, hands, and fingers control. Thus, using classical AT control interfaces (keypads, joysticks, etc.) is often difficult or impossible. The proposed system reads the AT users' residual functional capacities through their sEMG activity, and converts them into appropriate commands using a threshold-based control algorithm. It has proven to be suitable as a control alternative for assistive devices and has been tested with the JACO arm, an articulated assistive device of which the vocation is to help people living with upper-body disabilities in their daily life activities. The wireless prototype, the architecture of which is based on a 3-channel sEMG measurement system and a 915-MHz wireless transceiver built around a low-power microcontroller, uses low-cost off-the-shelf commercial components. The embedded controller is compared with JACO's regular joystick-based interface, using combinations of forearm, pectoral, masseter, and trapeze muscles. The measured index of performance values is 0.88, 0.51, and 0.41 bits/s, respectively, for correlation coefficients with the Fitt's model of 0.75, 0.85, and 0.67. These results demonstrate that the proposed controller offers an attractive alternative to conventional interfaces, such as joystick devices, for upper-body disabled people using ATs such as JACO.

This paper presents a method to reorient a free-floating serial manipulator using internal motion based on a path planning algorithm using a dynamic model of the manipulator and a potential function. Simulations with the proposed algorithm are performed using a nonlinear optimization technique in order to determine the actuator's velocity trajectories that achieve the reorientation. An example trajectory is presented in which a three-link planar robot starts from a pose in which all the links are aligned and ends with the same joint configuration but with the robot having completed a 180 degrees rotation. For this example, the algorithm finds trajectories that allow the robot to complete approximately 94% of the reorientation. To verify the simulation results against a real robot, a prototype of a planar robot with three bodies and two revolute joints is built. The experiments conducted show that the prototype is able to achieve the prescribed reorientation, even though the control of the orientation was implemented in an open-loop mode.

Recently, robotics has been seen as a key solution to improve the quality of life of amputees. In order to create smarter robotic prosthetic devices to be used in an everyday context, one must be able to interface them seamlessly with the end-user in an inexpensive, yet reliable way. In this paper, we are looking at guiding a robotic device by detecting gestures through measurement of the electrical activity of muscles captured by surface electromyography (sEMG). Reliable sEMG-based gesture classifiers for end-users are challenging to design, as they must be extremely robust to signal drift, muscle fatigue and small electrode displacement without the need for constant recalibration. In spite of extensive research, sophisticated sEMG classifiers for prostheses guidance are not yet widely used, as systems often fail to solve these issues simultaneously. We propose to address these problems by employing Convolutional Neural Networks. Specifically as a first step, we demonstrate their viability to the problem of gesture recognition for a low-cost, low-sampling rate (200Hz) consumer-grade, 8-channel, dry electrodes sEMG device called Myo armband (Thalmic Labs) on able-bodied subjects. To this effect, we assessed the robustness of this machine learning oriented approach by classifying a combination of 7 hand/wrist gestures with an accuracy of ∼97.9% in real-time, over a period of 6 consecutive days with no recalibration. In addition, we used the classifier (in conjunction with orientation data) to guide a 6DoF robotic arm, using the armband with the same speed and precision as with a joystick. We also show that the classifier is able to generalize to different users by testing it on 18 participants.

This paper presents an anticipative robot kinematic limitation avoidance algorithm for collaborative robots. The main objective is to improve the performance and the intuitivity of the physical human-robot interaction. One obstacle to achieve this goal is the management of limitations such as joint position limitation, singularities and collisions with the environment. Indeed, in addition to performing a given principal task, human users must pay a close attention to the manipulator configuration in order to handle the kinematic limitations. The proposed anticipative algorithm aims at relieving the human user from having to deal with such limitations. The algorithm is first presented and detailed for each individual limitation of a planar RR serial robot. The framework developed to manage several limitations occurring simultaneously is then presented. Finally, experiments are performed in order to assess the performance of the algorithm.

This paper introduces a novel architecture of kinematically redundant parallel mechanisms. This family of mechanisms is similar to the well-known Gough-Stewart platform, and it retains its advantages, i.e., the members connecting the base to the moving platform are only subjected to tensile/compressive loads. The proposed architecture exploits kinematic redundancy to avoid singularities and extend the rotational workspace. The novel kinematic architecture is described, and the associated kinematic relationships are developed. Based on the derivation of the Jacobian matrices, it is shown that the singularities of this type of mechanism are governed by the orientation of passive links connecting the redundant legs to the platform. Grassmann geometry is then used to demonstrate that, given some simple geometric assumptions on the architecture, all singularities can be avoided by exploiting the kinematic redundancy. The orientational workspace is then discussed, and a graphical representation is provided for an example architecture comprising nine actuators, whose orientational workspace is shown to be very large. The translational workspace is also studied. Example trajectories are given in order to illustrate the capabilities of the mechanism to produce very large rotation angles without encountering singularities. Computer animations of the trajectories are provided in a multimedia extension of the paper.

This paper proposes a dynamic trajectory planning method for point-to-point motion of three-degree-of-freedom (three-DOF) cable-suspended parallel robots. Natural frequencies as well as associated periodic trajectories that can be obtained from the integration of the dynamic model of an equivalent passive mechanical system are used to design point-to-point trajectories. The trajectories can be used to connect consecutive points in sequence that may lie beyond the static workspace of the robot. The technique ensures zero velocity at each of the target points and continuity of the accelerations. Based on the cable tension constraints, attainable regions can be determined to search for the next target point, while feasible regions of intermediate points are generated in cases for which a given point cannot be directly attained. An example trajectory is performed to illustrate the approach. An experimental implementation is also presented using a three-DOF prototype, and video extensions are provided to demonstrate the results.

This paper presents the design of a locomotion interface for gait simulation and interaction with a virtual environment. The proposed mechanical interface is based on two planar two-degree-of-freedom belt-driven mechanisms which fully decouple the vertical and horizontal motion of two supporting footplates. Force sensors are mounted under the footplates in order to move the end effectors according to the user's intentions. The user's feet are rigidly attached to the footplates but a system of hinges allows an additional unactuated two-degree-of-freedom motion. A lag controller allows the user to move the end-effectors freely and a virtual floor based on a virtual spring and damper restrains the end-effectors' movements so that the user experiences an intuitive contact with the ground. Finally, preliminary experiments are reported in order to demonstrate the capability of the locomotion interface for gait simulation.

This paper introduces a novel family of singularity-free kinematically redundant planar parallel mechanisms that have unlimited rotational capabilities. The proposed mechanisms are akin to conventional three-degree-of-freedom planar parallel mechanisms. By introducing a novel kinematically redundant arrangement, four-degree-of-freedom parallel mechanisms are obtained that can completely alleviate singularities and provide unlimited rotational capabilities. The kinematics of the mechanisms are derived, and the Jacobian matrices are obtained. It is shown that the singularities of this type of mechanism are governed by the orientation of a passive link connecting the redundant leg to the platform and that the latter orientation is easily controlled using the kinematic redundancy, thereby alleviating all direct kinematic singularities. An example mechanism is proposed, and a prototype is demonstrated. Example trajectories that include full cycle rotations are shown. The prototype also illustrates the use of the kinematic redundancy for an auxiliary task, namely grasping.

This paper presents a cable-driven parallel mechanism as a haptic interface and its underlying control method. This human-sized, three-degree-of-freedom mechanism has a tetrahedral architecture, four cables and evolves in three-dimensional space. A brief review of the kinematics of the mechanism is presented. Also, an admittance control law coupled with a closed-loop velocity controller is proposed. The control method is then refined by introducing adaptations for smooth surfaces and sharp edges. This control method is then validated by experimental results. Furthermore, the geometry of the mechanism is identified by a method that does not require any other sensor than the motor encoders.

A control algorithm based on admittance regulation is proposed for human-robot cooperative force amplification. The controller is particularly suitable for physical interaction in both free space and constrained motions thanks to an active transition between the two modes. It is also able to cancel possible high frequency oscillations due to the contact with the environment. A mathematical analysis of the controller is provided and a preliminary experimental validation is performed on a 1-dof test-bench.

This paper presents the concept of a new robotic joint composed of two electric motors as inputs, an epicyclic gearing system for the transmission, and a single output. The proposed joint mechanism has a wider range of speed and torque performances comparatively to a traditional robot joint using a single motor and gearbox. The dynamic equations for the mechanical transmission system are given and a dual-motor joint mechanism is designed and prototyped to test this new concept of robotic joint. Also, the potential advantages of this joint concept for the design of manipulators for which a wide range of performances are desired are discussed. This work is motivated by the development of field robots designed for the operation and maintenance tasks in power distribution lines.

This paper proposes a dynamic trajectory planning approach for planar two-dof redundantly actuated cable-suspended parallel mechanisms. In the recent literature, the global dynamic trajectory planning problem of cable-suspended mechanisms was addressed and some of the characteristic properties of such robots were revealed. In this paper, actuation redundancy is introduced and the dynamic trajectory planning is addressed using straight line periodic trajectories and the application of the antipodal theorem. The results obtained show that introducing actuation redundancy increases the dynamic capabilities of the robots. Special frequencies are revealed that are similar to those encountered with non-redundant mechanisms. Additionally, an alternative architecture is proposed to deal with cable interferences and it is shown that the novel architecture leads to improved dynamic capabilities when compared to the original architecture.

This paper presents two trajectory-planning approaches for the point-to-point motion of planar two-degree-of-freedom (dof) cable-suspended parallel mechanisms. The proposed techniques can be used to plan trajectories that extend beyond the static workspace of the mechanism. Trajectories are specified as a list of target points that must be reached in sequence, with a zero velocity at each of the target points. In the first technnique, polynomial trajectories are designed to connect the target points, while the second approach uses trigonometric functions. Both techniques ensure continuity of the accelerations. Based on the dynamic model of the robot, algebraic inequalities are obtained that represent the constraints on cable tensions. These inequalities are used to determine the feasibility of the planned trajectories. Polynomial trajectories must be discretized in order to verify feasibility, while trajectories that are based on trigonometric functions can be verified globally, based on a set of simple algebraic equations. Example trajectories are given in order to illustrate the approach. An experimental validation is also presented using a two-dof prototype, and two video extensions are provided to demonstrate the results.

Haptic devices have been used to help people to learn or recover specific movements. An interesting application is the handwriting. In this paper we present a haptic device designed to help people to improve their writing skills. Guided by a teacher, who controls one of the mechanisms providing the haptic feedback, a group of students were asked to write few words using their second hand. The test led to some interesting outcomes. A quantitative analysis is then carried out in order to support the considerations uniquely based on the qualitative observation of the results.

This paper presents a 7-DOF partially statically balanced robot that has been developed for physical human robot interaction. The gravity compensation technique uses remote counterweights connected to the robot via a low-pressure hydraulic transmission. Low-friction diaphragm cylinders are used in order to provide very low residual friction. A two-stage balancing bench allows using a unique moving counterweight to adapt the balancing to a payload of up to 10 kg, that is the maximal payload of the gripper. The robot can be easily moved passively while being capable of physically assisting humans in the performance of tasks.

This paper presents a trajectory planning approach for cable-suspended parallel mechanisms. A planar two-degree-of-freedom parallel mechanism is used for the analysis. Based on the dynamic model of the suspended robot, a set of algebraic inequalities is obtained that represents the constraints on the cable tensions. Parametric Cartesian trajectories are then defined and substituted into the constraints in order to obtain global conditions on the trajectory parameters which ensure that the trajectories are feasible. Special frequencies arise from the equations that are akin to natural frequencies of pendulum-type systems. An experimental validation is also presented using a two-dof prototype. The proposed trajectory planning approach can be used to plan dynamic trajectories that go beyond the static workspace of the mechanism, thereby opening novel applications and possibilities for cable-suspended robots.

Robots are currently used in some applications to enhance human performance and it is expected that human/robot interactions will become more frequent in the future. In order to achieve effective human augmentation, the cooperation must be very intuitive to the human operator. This paper presents a variable admittance control approach to improve system intuitivity. The proposed variable admittance law is based on the inference of human intentions using desired velocity and acceleration. Stability issues are discussed and a controller design example is given. Finally, experimental results obtained with a full-scale prototype of an intelligent assist device are presented in order to demonstrate the performance of the algorithm.

This paper presents a planar parallel three-degree-of-freedom underactuated cable-driven robot. The mechanism is first described and a dynamic model is derived. The proposed mechanism does not require any mechanical (e.g. pulleys) or electrical (e.g. actuators) hardware to be mounted on the end-effector. A trajectory planning approach is developed, which is based on the natural frequency of the pendulum-like free motion (unconstrained degree of freedom). Sine-like excitation functions are used and their frequency and phase delay are determined using simulation results. A prototype is then described and experimental results are provided together with a video clip of an example trajectory. The results confirm that the mechanism can be effectively used to perform point-to-point trajectories.

This paper presents the kinematic design of an abduction mechanism for the fingers of an anthropomorphic robotic hand. This mechanism enhances the range of feasible grasps of the underactuated hand without significantly increasing its complexity. Two architectures are investigated: one is purely planar while the other uses a spherical mechanism in conjunction with a planar linkage. The analysis of the planar mechanism between the index finger and the third finger is first assessed, in which the parameters are determined in order to satisfy the constraints on the motion ranges while minimizing the coordination error. Then, the study of the mechanism joining the third finger and the little finger is summarized, followed by the analysis of the spherical mechanism. Finally, prototypes of the fingers' abduction systems using the two proposed architectures are presented.

This paper presents the design, implementation and control of a device intented to mechanically improve the safety of serial robots interacting with humans. The device consists of an electronically adjustable torque limiter placed in series with each actuator, referred to as a Series Clutch Actuator (SCA). By appropriately adjusting the limit torques according to the robot's configuration, the maximum static force that the robot can apply to its environment at the Tool Centre Point (TCP) can be limited to a prescribed safe level. If a limit torque is exceeded, the SCA slips and an emergency stop is triggered while the inertia located upstream from the SCA in the kinematic chain is mechanically disconnected. A method is presented to determine the optimal limit torques that maximize the isotropically achievable force (which can be applied in all directions without triggering any SCA) while satisfying the safe force limit. An approach to optimize the pose of a redundant robot in order to maximize the isotropically achievable force while preserving a safe maximum force threshold is also proposed. The design and fabrication of a torque limiter using a large number of friction discs is presented. Finally, the mechanisms are implemented into a 4-DOF redundant serial arm and preliminary experimental results are presented.

The ability of underactuated hands to grasp small objects is very limited, because the precision grasp is normally unstable. The goal of this paper is to achieve stable precision grasps by means of simple design modifications of the distal phalanges of the fingers. These modifications comprise the curving of the contact area of the distal phalanx, the application of a mechanical limit to prevent hyperextension of the distal phalanx, and the application of a compliant joint between the proximal and distal phalanges. A model is developed to calculate the limits of the finger dimensions in order to achieve stable precision grasps for different object sizes. An experimental setup is used to test the grasp stability and to verify the calculated results. It is concluded that stable precision grasps exist for the combination of concavely curved distal phalanges with a mechanical limit or with a compliant joint, if the limits to the finger dimensions are satisfied.

Providing robots with the capability of sensing their surrounding environment is an important feature that would lead to a more intuitive and safe physical human-robot interaction. This paper proposes a new design of homogeneous flexible and stretchable robot skin based on carbon-black-filled (CBF) silicone and conductive fabric that can sense multiple contact locations as well as applied pressure. CBF silicone has been already used in sensing technology but its piezoresistivity is still largely misunderstood. This particular behavior is investigated in this paper through a set of experiments conducted on isolated sensing cells. Using the results of these experiments, a model describing the variation of the resistivity in the CBF silicone as a function of the applied pressure is proposed. Based on this model, a simple way to accurately estimate the applied pressure in real time is demonstrated. Finally, using this improved knowledge of the behaviour of the CBF silicone, the fabrication of a fully functional sensor array is presented. The proposed design has the particularity of circumventing the well-known problem of cross-talk between sensing cells.

This paper presents a device that significantly improves the safety of ceiling-mounted robots whose end effector orientation remains constant with respect to the vertical direction (e.g. Scara-type robots). The device consists of a three-degree-of-freedom (DOF) parallel mechanism with the Delta architecture on which the revolute actuators have been replaced with torque limiters. The resulting Cartesian force limiting device (CFLD) is implemented as a mechanical connection between the robot and the effector. It is rigid unless excessive forces are applied on the end effector, for example during a collision. The magnitude of force that activates the mechanism is set by properly adjusting the threshold of the torque limiters. Furthermore, a collision can be rapidly detected with a limit switch placed on one of the links of the mechanism and a signal can be sent directly to brakes that will stop the robot, without passing through a controller and thus improving the reliability and reaction-time of the safety system. By mechanically disconnecting the robot from its end effector, the device ensures that the person involved in the collision is only subjected to the inertia of the end effector and thus potential injuries are greatly reduced. This work is the extension of a previous 2-DOF CFLD that was sensitive only to horizontal forces. The new architecture reacts to collisions occuring in any direction and is geometrically optimized for the proposed application. Also, means to achieve gravitity compensation are proposed.

This paper presents a novel planar three-degree-of-freedom pendulum-like underactuated robot. The robot consists of an end-effector with an actuated arm suspended on a cable wound on a reel. The robot can achieve full planar point-to-point motion (position and orientation) with zero-velocity landing by swinging itself as children do on playground swings. The equations of motion of the underactuated cable-driven robot are first developed. Then, the actuated joint trajectory design for swing-up as well as an optimization technique used to control the behaviour of the passive joint are proposed. Finally, a prototype of the robot and its real-time controller are presented with experimental results for point-to-point trajectories. The proposed mechanism constitutes a low-cost solution for applications requiring large workspaces by combining the advantages of cable-driven systems and underactuation and, to the best of our knowledge, this is the first work presenting the real-time control of such a mechanism.

Numerous performance indices have been proposed to compare robot architectures based on their kinematic properties. However, none of these indices seems to draw a consensus among the robotics community. The most notorious indices, which are manipulability and dexterity, still entail some drawbacks, which are mainly due to the impossibility to define a single invariant metric for the special Euclidean group. The natural consequence is to use two distinct metrics, i.e., one for rotations and one for point displacements, as has already been proposed by other researchers. This is the approach used in this paper, where we define the maximum rotation sensitivity and the maximum point-displacement sensitivity. These two indices provide tight upper bounds to the end-effector rotation and point-displacement sensitivity under a unit-magnitude array of actuated-joint displacements. Therefore, their meaning is thought to be clear and definite to the designer of a robotic manipulator. Furthermore, methods for the computation of the proposed indices are devised, some of their properties are established and interpreted in the context of robotic manipulator design, and an example is provided.

This paper presents a design for a cable reel that allows a cable-driven haptic interface to produce rigid impacts with virtual objects in a virtual reality setting. The haptic interface studied in this article has three degrees of freedom (3 DOF) and acts as a sword-fighting simulator. In order to obtain sharp impacts with this interface, an impact generating reel is proposed to transmit forces across the cables to a user holding the end-effector (sword). A prototype is presented in order to demonstrate the concept. As a method of quantifying the credibility of these impacts, an accelerometer was mounted on the end-effector, where the cables are attached in order to measure the vibrations caused by these impacts. These vibrations are compared with the vibrations caused by an impact with a rigid material such as steel in order to classify the stiffness of the impacts generated by the mechanism.

This paper presents preliminary results on the design of a novel architecture of a three-phalanx underactuated finger. An ejection-free underactuated anthropomorphic hand is of the greatest interest for the field of humanoid robots. So far, all underactuated fingers lead to ejection for certain configurations. By using a second system of actuation in a three-phalanx finger, the zone of instability of this finger is reduced and the ejection problem is prevented. Hence, the mathematical model of this finger is first introduced, followed by the choice of transmission ratios to be taken for the two pulley systems. Finally, the zone of instability of this new architecture is presented where it is proved that ejection is avoided.

For physical human-robot interaction, safety and dependability are of utmost importance due to the potential risk a relatively powerful robot poses for human beings. From the control standpoint, it is possible to increase this level of safety by guaranteeing that the robot will never exhibit any unstable behaviour. However, stability is not the only concern in the design of a controller for such a robot. During human-robot interaction, the resulting cooperative motion should be truly intuitive and should not restrict in any way the human performance. For this purpose, we have designed a new variable admittance control law that guarantees the stability of the robot during constrained motion and also provides a very intuitive human interaction. The first characteristic is provided by the design of a stability observer while the other is based on a variable admittance control scheme that uses the force derivative as a way to predict human intention. The stability observer is based on a previous stability investigation of cooperative motion which implies the knowledge of the interaction stiffness. A method to accurately estimate this stiffness online using the data coming from the encoder and from a multi-axis force sensor at the end effector is also provided. The stability and intuitivity of the control law were verified in a user study during a cooperative drawing task with a 3 degree-of-freedom (dof) parallel robot.

Providing contact sensing on the whole body of a robot is a key feature to increase the safety level of physical human-robot interaction. In this paper, a new robot skin capable of sensing multiple contact locations is presented. The motivation behind the proposed design is to produce a relatively inexpensive skin having the capability to provide the spatial location of collisions and also to add compliance to the robot's external cover. The resulting device is a thin flexible sensor sheet made of polyimide films with electrically conductive ink and a pressure sensitive conductive rubber sheet. The problem of internal wire routing is circumvented by the use of conductive ink and a circuit is proposed to minimize the number of output wires. To provide collision absorption and mechanical robustness, the sensor is embedded in different layers of polyurethane using shape deposition manufacturing (SDM). The paper presents the design and the fabrication process of the skin but also some experimental results on the determination of the mechanical properties of the resulting sensor as well as its potential for increasing human safety during human robot interaction.

This paper presents a device that significantly increases the safety level of suspended robots whose end-effector orientation remains constant with respect to the vertical direction (e.g. Scara-type suspended robots). The device is a two-degree-of-freedom (DOF) parallel mechanism with a parallepipedic architecture on which two revolute joints have been replaced with commercially available torque limiters. The device is implemented as a mechanical connection between the robot and the effector. It is rigid unless excessive horizontal forces are applied on the end-effector, for example during a collision. The level of force that activates the mechanism is set by properly adjusting the threshold of the torque limiters. Furthermore, a collision can be rapidly detected with a limit switch placed on one of the links of the mechanism and a signal can be sent directly to brakes that will stop the robot, without passing through a controller and thus improving the reliability and reaction-time of the safety system. By mechanically disconnecting the robot from its end-effector, the device ensures that the person involved in the collision is only subjected to the inertia of the end-effector and thus potential injuries are greatly reduced. A prototype of the proposed device has been built to validate the concept and to study its behaviour for collisions with different velocities and orientations.

A Cable-Driven Locomotion Interface employs two independent cable-driven haptic foot platforms constrained in six degrees of freedom (6-DOF). Its control system and its geometry are designed for performing a wide range of trajectories that could generate cable interferences. This paper presents and analyzes computational methods for determining which cable can be released from an active actuation state while allowing control in a minimal tension state, thereby ensuring that both platforms stay in a controllable workspace. One challaging task is to develop light and fast computational algorithms for hard real time processes included in haptic display applications. Seeing that releasing a cable from an active actuation state might generate discontinuities in tension values in the other cables, this paper proposes collision prediction schemes named Interference Estimated Time of Arrival in order to reduce or completely eliminate such discontinuities.

The intrinsic interaction of a robotic system that includes two 6-degree-of-freedom cable-driven platforms sharing a common workspace might result in cable interferences for random trajectories. This paper presents and analyzes computational methods for geometrically determining and managing these interferences for any trajectory constrained with variable loads. The algorithms considered determine which cable can be released from an active actuation state while allowing control in a minimal tension state, thereby ensuring that both platforms stay in a controllable workspace. The process of managing cable interferences constitutes a challenge as one must take into account the inherent limitations of the workspace, which not only include the possibility of interference itself, but also the geometry of the cable-driven locomotion interface (CDLI), its dynamics, the nonideal behavior of real cables, and the requirement that both platforms must be completely constrained at any time. As releasing a cable from an active actuation state might generate tension discontinuities in the other cables, this paper also proposes collision prediction schemes that are only applied to redundant actuators in order to reduce or completely eliminate such discontinuities. Finally, a simulation of a CDLI embedded as a peripheral in a virtual environment, in which the load applied on each platform comes from the wrench measured under the foot for a natural gait walking, is thoroughly analyzed.

For human-robot cooperation in the context of human-augmentation tasks, the stability of the control model is of great concern due to the risk for the human safety represented by a powerful robot. This paper investigates stability conditions for impedance control in this cooperative context and where touch is used as the sense of interaction. The proposed analysis takes into account human arm and robot physical characteristics, which are first investigated. Then, a global system model including noise filtering and impedance control is defined in a state-space representation. From this representation, a Lyapunov function candidate has been successfully discovered. In addition to providing conclusions on the global asymptotic stability of the system, the relative simplicity of the resulting equation allows the derivation of general expressions for the critical values of impedance parameters. Such knowledge is of great interest in the context of design of new adaptive control laws or simply to serve as design guidelines for conventional impedance control. The accuracy of these results were verified in a user study involving 7 human subjects and a 3-dof parallel robot. In this experiment, the real effective stability frontier was defined for each subject and compared with values predicted using the Lyapunov function.

This paper presents the design and experimental validation of an anthropomorphic underactuated robotic hand with 15 degrees of freedom and a single actuator. First, the force transmission design of underactuated fingers is revisited. An optimal geometry of the tendon-driven fingers is then obtained. Then, underactuation between the fingers is addressed using differential mechanisms. Tendon routings are proposed and verified experimentally. Finally, a prototype of a 15-degree-of-freedom hand is built and tested. The results demonstrate the feasibility of a humanoid hand with many degrees of freedom and one single degree of actuation.

This paper is a contribution to an invited session on the benchmarking of parallel mechanisms. The aim of the session is to compare different existing designs and prototypes of parallel mechanisms using a common set of benchmarking criteria. First, the kinematic architectures of parallel mechanisms of the multipteron family are presented. In addition to the tripteron and the quadrupteron, the pentapteron, a five-degree-of-freedom (dof) parallel mechanism is introduced. Then, the benchmarking criteria are applied to the prototypes of the tripteron (3-dof) and the quadrupteron (4-dof) prototypes. Although the tripteron and quadrupteron parallel mechanisms have been presented elsewhere, their properties, highlighted by the benchmarking analysis presented here are revealed for the first time.

Tensegrity mechanisms have the advantage of being relatively lightweight due to their extensive use of cables and springs. In this work, a novel planar modular 2-DoF tensegrity mechanism that is actuated by cables is introduced. The modular architecture of the mechanism gives it increased flexibility while using cables for the actuation leads to large reachable workspaces. An analysis of the mechanism's statics and kinematics is performed for the case where no external loads are acting

This paper presents a methodology for the analytical determination and representation of the workspace boundaries of symmetrical spherical parallel mechanisms (SPMs). The methodology is based on an intuitive orientation representation which, while not well known, has proven to be very useful for the analysis of symmetrical parallel mechanisms. The latter, previously introduced as "tilt-and-torsion angles," are briefly described. Then, relatively simple analytical expressions are found for the workspace boundaries of general symmetrical SPMs. Next, using these expressions and a simple numerical procedure, a fast algorithm is proposed for representing the so-called constant-torsion workspace. Finally, several examples are provided

The mobile platform of a parallel cable-driven mechanism is connected in parallel to a base by lightweight links, such as cables. Since the cables can only work in tension, the set of poses of the mobile platform for which the cables can balance any external wrench, i.e., for which the platform of the mechanism is fully constrained, is often limited or even nonexistent. Thus, the study and determination of this set of poses, called the wrench-closure workspace (WCW), is an important issue for parallel cable-driven mechanisms. In this paper, the case of planar parallel cable-driven mechanisms is addressed. Theorems that characterize the poses of the WCW are proposed. Then, these theorems are used to disclose the parts of the reachable workspace which belong to the WCW. Finally, an efficient algorithm that determines the constant-orientation cross-sections of these parts is introduced.

This paper proposes a control scheme dedicated to underactuated fingers with the intention of maximizing the capabilities of the latter using tactile and position information at a minimum cost. Tactile sensors are implemented on one prototype of underactuated finger and used to enhance the behaviour of the hand despite its limited number of control signals. First, tactile technology is briefly recalled and discussed. Second, the electronic design of the sensors' controller is presented. Third, a real-time control scheme is introduced, based on a fuzzy force control method. Finally, a slippage prevention technique is presented. Results are discussed based on experimental observations and indicate that the behaviour of underactuated fingers can be substantially enhanced with tactile information and a classic fuzzy control approach.

This paper presents the computation and representation of the Type 2 singularity loci of symmetric spherical parallel mechanisms based on a not-well-known intuitive orientation representation. The latter, previously introduced under the name of the Tilt-and-Torsion angles, is briefly described. Then, to illustrate the approach, the two most basic spherical parallel mechanisms are considered and their Type 2 singularities are fully analyzed for various designs.

Redundancy can, in general, improve the ability and performance of parallel manipulators by implementing the redundant degrees of freedom to optimize a secondary objective function. Almost all published researches in the area of parallel manipulators redundancy were focused on the design and analysis of redundant parallel manipulators with rigid (nonconfigurable) platforms and on grasping hands to be attached to the platforms. Conventional grippers usually are not appropriate to grasp irregular or large objects. Very few studies focused on the idea of using a configurable platform as a grasping device. This paper highlights the idea of using configurable platforms in both planar and spatial redundant parallel manipulators, and generalizes their analysis. The configurable platform is actually a closed kinematic chain of mobility equal to the degree of redundancy of the manipulator. The additional redundant degrees of freedom are used in reconfiguring the shape of the platform itself. Several designs of kinematically redundant planar and spatial parallel manipulators with configurable platform are presented. Such designs can be used as a grasping device especially for irregular or large objects or even as a micro-positioning device after grasping the object. Screw algebra is used to develop a general framework that can be adapted to analyze the kinematics of any general-geometry planar or spatial kinematically redundant parallel manipulator with configurable platform.

In this paper, the design of reactionless 3-degree-of-freedom (DOF) and 6-DOF parallel manipulators is presented. At first, the design and dynamic balancing of a novel 3-DOF parallel mechanism referred to as the parallelepiped mechanism are addressed. Two types of actuation schemes of the mechanism are considered, and the two corresponding mechanical structures are designed. The balancing equations are derived by imposing that the center of mass of the mechanism is fixed and that the total angular momentum is constant with respect to a fixed point. Optimization is performed to determine the counterweights and counter-rotations based on the balancing conditions. Numerical examples of reactionless 3-DOF parallelepiped mechanisms are given. The dynamic simulation software ADAMS is used to simulate the motion of the mechanisms and to verify that the mechanisms are reactionless at all times and for arbitrary trajectories. Finally, the 3-DOF parallelepiped mechanisms are used as legs to synthesize reactionless 6-DOF parallel manipulators.

The paper discusses forward and inverse kinematics of a class of four-degree-of-freedom (DOF), four-legged parallel mechanisms providing three rotational and one translational DOFs. A fully parametric analytical form solution to the inverse-position problem is provided. All working modes of the mechanism are shown and discussed. The equations of the forward-position problem are obtained under different leg arrangements, and a numerical example is provided. New special geometries in the class are proposed, including one suitable for keyhole surgery.

The aim of this paper is to establish a fundamental basis for the analysis of underactuated fingers with a general approach. A new method to obtain the force capabilities of any underactuated fingers will be presented. Force capability is defined as the ability to generate an external wrench onto a fixed object with a given set of phalanges. This method is based on the introduction of two new matrices which completely describe the relationship between the input torque of the finger actuator(s) and the contact forces on the phalanges. Using this tool, one can study the conditions under which certain phalanx forces vanish, and compare different underactuation mechanisms with a rigorous approach.

A reactionless mechanism is one which does not exert any reaction force or moment on its base at all times, for arbitrary trajectories of the mechanism. This paper addresses the synthesis and design of planar and spatial reactionless three-degree-of-freedom (DOF) mechanisms using four-bar linkages without separate counter-rotations. Based on the conditions of dynamic balancing of single planar four-bar linkages developed previously, the suitable architecture and parameters are first determined, and an optimized four-bar mechanism is then designed and prototyped. Planar and spatial 3-DOF reactionless mechanisms are synthesized using the reactionless four-bar linkages. The optimization of the 2-DOF mechanism as the basis mechanism of 3-DOF mechanisms is performed in order to reduce the mass of the mechanism for a given platform. Prototypes of the reactionless planar and spatial mechanisms are designed. With the help of a dynamic simulation software, it is shown that the mechanisms are reactionless for arbitrary trajectories.

3T1R four-degrees-of-freedom (DOF) parallel manipulators (3T1R-PMs) are the parallel counterparts of the 4-DOF SCARA serial robots. In a 3T1R-PM, the moving platform can generate 3T1R motion (also called Schonflies motion), which refers to a rotation about any axis with a given direction in conjunction with 3-DOF translations. A method is proposed for the type synthesis of 3T1R-PMs based on screw theory. The wrench systems of a 3T1R parallel kinematic chain (3T1R-PKC) and its legs are first analyzed. A general procedure is then proposed for the type synthesis of 3T1R-PMs. The type synthesis of legs for 3T1R-PKCs, the type synthesis of 3T1R-PKCs, as well as the selection of actuated joints of 3T1R-PMs, are dealt with in sequence. 3T1R-PKCs with and without inactive joints are synthesized. The phenomenon of dependent joint groups in a 3T1R-PKC is revealed for the first time. Several 3T1R-PMs with identical type of legs are obtained.

This paper studies the force capability of a particular class of underactuated fingers. Force capability is defined as the ability to create an external wrench onto a fixed object. The concept of the underactuation in robotic fingers, with fewer actuators than degrees of freedom (DOF) through the use of springs and the mechanical limits, allows the hand to adjust itself to an irregularly shaped object without complex control strategy and numerous sensors. However, in some configurations, the force distribution in an underactuated finger can degenerate. The finger can no longer apply forces on the object, leading in some cases to the ejection of the latter from the hand. This paper focuses on a 2-DOF finger and studies its ability to seize objects with a secure grasp.

A new class of six-degree-of-freedom (DOFs) spatial parallel platform mechanism is considered in this paper. The architecture consists of a mobile platform connected to the base by three identical kinematic chains using five-bar linkages. Recent investigations showed that parallel mechanisms with such a topology for the legs can be efficiently statically balanced using only light elastic elements. This paper follows up with a workspace analysis and optimization of the design of that parallel mechanism. More specifically, considering a possible industrial application of the architecture as a positioning and orienting device of heavy loads, an optimization procedure for the maximization of the volume of the three-dimensional (3-D) constant-orientation workspace of the mechanism is first presented. As the mechanism could also have great potential as a motion base for flight simulators, we develop here a discretization method for the computation and graphical representation of a new workspace with coupled translational and rotational DOFs. This workspace can be defined as the 3-D space which can be obtained when generalized coordinates x,y and torsion angle /spl psi/ in the tilt-and-torsion angles parametrization are constant. A second procedure is then presented for the maximization of the volume of this second subset of the complete workspace. For both approaches, our purpose is to attempt an optimal design of the mechanism by maximizing the volume of the associated 3-D Cartesian region that is free of critical singularity loci.

The concept of constraint singularity is introduced. This is a phenomenon occurring in parallel mechanisms with reduced freedoms when the screw system, formed by the constraint wrenches in all legs, loses rank.

This paper presents a new type of haptic device using spherical geometry. The basic idea of haptic devices is to provide users with feedback information on the motion and/or force that he or she generates. Haptic devices have several potential applications such as, for example, fine compliant assembly, VR environment simulation, and high-precision teleoperation, especially in hazardous or hostile areas. The particular architecture of the spherical haptic device developed here will be presented and its advantages will be highlighted. Then, its basic kinematic properties, which shall be used for control and geometric optimization purposes, will be reviewed. The design optimization itself will then highlight some important features of the mechanism. The control scheme and real-time force feedback issues will then be presented. Finally, the results obtained with the prototype will be discussed.

CaPaMan (Cassino parallel manipulator) is a three-degree of freedom parallel mechanism that has been designed and built at the Laboratory of Robotics and Mechatronics in Cassino. In the paper a study of the configuration singularities of the CaPaMan manipulator is presented by considering two different methods: an algebraic formulation and a vector analysis. It is shown that a formulation can give singularity related to the failure of the kinematic model at particular configurations of the manipulator. It is also proved that this type of singularity can be avoided by a proper analysis of the problem.

This article discusses the modeling of the dynamics of a class of spatial parallel platform mechanisms. For mechanisms of this class perfect static balancing can be achieved through the use of springs. This means that zero actuator torques are required whenever the mechanism is at rest. Furthermore, only inertial forces have to be sustained when the manipulator is moving. Models for the dynamics are implemented based on two different methods, the principle of virtual work and the method of Neutron-Euler. The results of the dynamic analysis can be used to calculate and compare the required actuator torques of different mechanisms for specific payload and trajectories. The ultimate goal is to use these results to design a prototype with six degrees-of-freedom that only requires very small actuator torques to be used for example in flight simulators.

This paper looks at the control of mechanical impedance of the so-called Gough-Stewart class of parallel platforms. Two methods of compliance control are presented. One is based on global potential energy functions that have previously been applied to controlling serial manipulators and electrodynamically levitated platforms. The second method uses the exponential map to associate finite displacements of the platform from equilibrium with twist displacements. Compliant wrenches are then proportional to the twist displacements. Control of spatial damping is addressed as well, justifying the classification as impedance control. Control algorithms and simulation results are given.

A novel methodology to formulate the closed form solution to the orientation workspace of Gough-Stewart parallel manipulators is presented by using a fictitious four-bar spatial linkage model. The possible mechanical constraints are considered which include the strut length and the passive joint limitations. Several examples are given to illustrate the effectiveness of this approach.

Looks at the control of mechanical impedance of the so-called Gough-Stewart class of parallel platforms. Two methods of compliance control are presented. One is based on global potential energy functions that have previously been applied to controlling serial manipulators and electrodynamically levitated platforms. The second uses the exponential map to associate finite displacements of the platform from equilibrium with screw displacements. Compliant wrenches are then proportional to the screw displacements. Control of spatial damping is addressed as well, justifying the classification as impedance control. Control algorithms and simulation results are given.

The design of gravity-compensated six-degree-of-freedom parallel mechanisms-or manipulators-with revolute actuators is studied. Two methods are studied for the static balancing of these mechanisms, namely, using counterweights and using springs. The first method leads to mechanisms with a stationary global center of mass while the second approach leads to mechanisms whose total potential energy (including the elastic potential energy stored in the springs as well as the gravitational potential energy) is constant. In both cases, the resulting mechanisms are fully compensated for gravity, i.e., the actuators do not contribute to supporting the weight of the moving links in any of the configurations of the mechanisms. The position vector of the global center of mass and the total potential energy of the manipulator are first expressed as functions of the position and orientation of the platform. Then, conditions for static balancing are derived from the resulting expressions. Finally, examples are given in order to illustrate the design methodologies.

In a previous paper by Payeur et al. (1997), probabilistic occupancy modeling has been successfully extended to 3D environments by means of a closed-form approximation of the probability distribution. In this paper, the closed-form approximation is revisited in order to provide more reliable and meaningful models. A merging strategy of local probabilistic occupancy grids originating from each sensor viewpoint is introduced. The merging process takes advantage of the multiresolution characteristics of octrees to minimize the computational complexity and enhance performances. An experimental testbed is used to validate the approach and models computed from real range images are presented.

Polynomial learning networks are proposed in this paper to solve the forward kinematic problem for a planar three-degree-of-freedom parallel manipulator with revolute joints. These networks rapidly learn complex nonlinear functions based on a database mapping. The networks learn the forward kinematics of the manipulator based on examples of the transformation. The obtained networks are then used to follow a test trajectory. For comparison purposes, a neural network approach using backpropagation is also used for this problem. The results show that, in this application, polynomial networks learn much faster and exhibit less error than neural networks.

Probabilistic occupancy grids have proved to be very useful for workspace modeling in 2D environments. Due to the expansion of computational load, this approach was not tractable for mapping a 3D environment in real applications. In this paper, the original occupancy grid scheme is revisited and a generic closed-form function is introduced to avoid numerical computation of probabilities for a range sensor with Gaussian error distribution. Occupancy probabilities are computed and stored in a multiresolution octree for improved performance and compactness. Occupancy models are built in local reference frames and linked to a global reference frame through uncertain spatial relationships that can be updated dynamically. This scheme is used for building a 3D map in a telerobotic maintenance application of electric power lines where perturbations may cause motion of object assembly.

In this paper a new architecture of a planar three-degree-of-freedom (3-dof) parallel manipulator is presented. In the proposed mechanism, the prismatic actuators are fixed to the base which leads to a reduction of the inertia of the moving links and hence makes it attractive, particularly when high speeds are required and electric actuation is considered. After introducing the mechanism, a kinematic analysis is reported. Then, velocity and acceleration equations are derived. Based on the geometry of the manipulator a workspace analysis is performed and a description of the boundaries of the workspace is provided. This manipulator can be used in robotic applications involving the positioning and orientation of a rigid body on the plane with high stiffness or accuracy. Additionally, the mechanism can find applications in motion simulators or other high-precision or high-speed devices.

In this paper, the static balancing of planar parallel manipulators is addressed. Static balancing is defined here as the set of conditions on manipulator dimensional and inertial parameters which, when satisfied, ensure that the weight of the links does not produce any force (or torque) at the actuators for any configuration of the manipulator, under static conditions. These conditions are derived here for planar one-, two- and three-degree-of-freedom parallel manipulators. Subspaces of the parameter space which are associated with statically balanced mechanisms are then defined. It is shown that balancing is generally possible even when the dimensional parameters are imposed, which is a useful property since dimensional parameters are usually obtained from kinematic design or optimization. Finally, examples of balanced planar parallel manipulators are given. Static balancing leads to considerable reduction in the actuator forces (or torques), which in turn leads to less powerful actuators and more efficient designs.

This paper presents a new approach for the computation of generalized positioning inverse kinematic functions for Puma manipulators. This new approach leads to real solutions of the joint variables for any set of Cartesian coordinates located inside or outside the robot's workspace. Therefore, the inverse kinematic functions derived are termed "generalized inverse kinematic functions". These solutions, which produce the minimum distance between the end effector and a Cartesian point to be reached, are particularly useful in tracking operations. Because of the special architecture of the Puma, closed-form solutions are obtained. This leads to simple algorithms and allows for the computation of all solutions.>

This paper presents some results obtained in the development of a three-degree-of-freedom camera-orienting device. The agile eye, as it is referred to, is capable of an orientation workspace larger than that of the human eye. The miniature camera mounted on the end-effector can be pointed within a cone of 140 degrees opening with plus or minus 30 degrees in torsion. The mechanical architecture of the orienting device is based on a spherical three-degree-of-freedom parallel manipulator which leads to high-performance dynamics. A kinematic optimization has been performed in order to determine the dimensional parameters of the prototype which would provide the best overall accuracy. A complete dynamical model of the manipulator has also been derived and programmed, and simulation results have guided the mechanical design. Finally, a prototype has been built and experimented with.>

This article presents a path planning strategy for redundant serial manipulators working in a cluttered environment. Developed in a practical context of telemanipulation, the algorithm, which sacrifices the capability of solving very difficult trajectories for efficiency, allows a human to control a robot at a higher level, in Cartesian space. The model of the environment is provided by a 3D vision system as an occupancy map. An iterative process guides the end effector towards its goal with the help of discrete potential fields, which reduce the number of local minima. The motion of the manipulator is calculated using the velocity inversion of a redundant manipulator, which optimizes the distance to obstacles. The algorithm includes joint limit constraints, collision detection and heuristics for the solution of typical difficult cases, thereby leading to a high success rate. A simulator has been developed to test the algorithms.>

The paper proposes an approach to obtain inverse kinematic functions whose domain is not limited to the manipulator's workspace. As a result, these functions can be used to map points of the Cartesian space that do not belong to the manipulator's workspace. The positioning problem is formulated as a minimization of the distance between the prescribed Cartesian point and the end-effector. Hence, points outside of the workspace of the manipulator are mapped by the inverse kinematic function into joint coordinates that bring the manipulator as close as possible to the prescribed Cartesian location. The formulation is derived for both planar and spatial motion and an extension to problems where the orientation has to be considered is also given. Examples pertaining to 2-DOF and 3-DOF manipulators are solved and both analytical and numerical results are given. The proposed inverse kinematic functions are of great interest for tracking approach and catching operations where the object to be reached or tracked by the manipulator can be outside of the workspace,.>

Novel dexterity indices that can be applied to planar and spatial manipulators are presented. These indices are based on the condition number of the Jacobian matrix of the manipulators, which is known to be a measure of their kinematic accuracy. The formulation of the kinematic equations used leads to dexterity indices that are frame invariant, whereas previous indices are affected by a scaling of the manipulator when both the position and the orientation of the end effector are included in these equations. Two indices are proposed for planar manipulations: the first one is based on a redundant formulation of the velocity equations and the second one on the minimum number of parameters. The corresponding indices are also derived for spatial manipulators. An example is included to demonstrate the invariance of the indices.>

Mechanical systems containing closed-loop kinematic chains can exhibit special types of singularities that result in a loss of controllability of the element of interest (output link or gripper link). Therefore, it is very important for the designer to be able to predict this type of behavior. The approach used by the author is to establish stiffness or conditioning maps of the workspace of the manipulator. A method for obtaining these maps for planar and spatial parallel manipulators is presented. The maps reveal the existence of zones where the stiffness is not acceptable and also help the designer by providing a more accurate representation of the properties of the manipulator.>

The different kinds of singularities encountered in closed-loop kinematics chains are analyzed. A general classification of these singularities in three main groups, which is based on the properties of the Jacobian matrices of the chain, is described. The identification of the singular configurations is particularly relevant for hard automation modules or robotic devices based on closed kinematic chains, such as linkages and parallel manipulators. Examples are given to illustrate the application of the method to these mechanical systems.>