A Unified Multimodal Interface for the RELAX High-Payload Collaborative Robot.

This manuscript introduces a mobile cobot equipped with a custom-designed high payload arm called RELAX combined with a novel unified multimodal interface that facilitates Human-Robot Collaboration (HRC) tasks requiring high-level interaction forces on a real-world scale. The proposed multimodal framework is capable of combining physical interaction, Ultra Wide-Band (UWB) radio sensing, a Graphical User Interface (GUI), verbal control, and gesture interfaces, combining the benefits of all these different modalities and allowing humans to accurately and efficiently command the RELAX mobile cobot and collaborate with it. The effectiveness of the multimodal interface is evaluated in scenarios where the operator guides RELAX to reach designated locations in the environment while avoiding obstacles and performing high-payload transportation tasks, again in a collaborative fashion. The results demonstrate that a human co-worker can productively complete complex missions and command the RELAX mobile cobot using the proposed multimodal interaction framework.

A Haptic Shared Autonomy With Partial Orientation Regulation for DoF Deficiency in Remote Side.

Orientation regulation permits an autonomous controller to regulate the operators' orientation commands automatically. Although kinds of orientation regulation strategies have been proposed for various purposes, few works have focused on the partial orientation regulation (POR), which requires an autonomous controller to prevent the unreachable rotational motion for safety, while preserving the remaining motions for intuitiveness. However, the POR is deeply demanded for systems with Degree-of-Freedom (DoF) deficiency in remote side. The POR is a more challenging task owing to: First, it is difficult to decompose an orientation into reachable and unreachable components due to the non-linear structure of the rotation group SO(3). Second, it is non-trivial to design a haptic rendering algorithm which can indicate the missing DoF information to human operators. To address the rotational DoF deficiency, we propose a haptic shared autonomy with POR ability, based on the perpendicular curve theory in SO(3). The proposed method can partially regulate the operator's orientation command to discard the unreachable motions and preserve the remaining motions for follower robots. Here the conventional "master" and "slave" are all replaced by "leader" and "follower" to avoid the concern of association to racism and human subjugation. In addition, a haptic rendering algorithm is designed to display correct haptic cues about the missing DoF to operators. The simulation and experimental results validate the effectiveness of the proposed method.

Horizon: A Trajectory Optimization Framework for Robotic Systems.

This paper presents Horizon, an open-source framework for trajectory optimization tailored to robotic systems that implements a set of tools to simplify the process of dynamic motion generation. Its user-friendly Python-based API allows designing the most complex robot motions using a simple and intuitive syntax. At the same time, the modular structure of Horizon allows for easy customization on many levels, providing several recipes to handle fixed and floating-base systems, contact switching, variable time nodes, multiple transcriptions, integrators and solvers to guarantee flexibility towards diverse tasks. The proposed framework relies on direct simultaneous methods to transcribe the optimal problem into a nonlinear programming problem that can be solved by state-of-the-art solvers. In particular, it provides several off-the-shelf solvers, as well as two custom-implemented solvers, i.e. GN-SQP and Iterative Linear-Quadratic Regulator. Solutions of optimized problems can be stored for warm-starting, and re-sampled at a different frequency while enforcing dynamic feasibility. The proposed framework is validated through a number of use-case scenarios involving several robotic platforms. Finally, an in-depth analysis of a specific case study is carried out, where a highly dynamic motion (i.e., a twisting jump using the quadruped robot Spot® from BostonDynamics) is generated, in order to highlight the main features of the framework and demonstrate its capabilities.

NSPG: An Efficient Posture Generator Based on Null-Space Alteration and Kinetostatics Constraints.

Most of the locomotion and contact planners for multi-limbed robots rely on a reduction of the search space to improve the performance of their algorithm. Posture generation plays a fundamental role in these types of planners providing a collision-free, statically stable whole-body posture, projected onto the planned contacts. However, posture generation becomes particularly tedious for complex robots moving in cluttered environments, in which feasibility can be hard to accomplish. In this work, we take advantage of the kinematic structure of a multi-limbed robot to present a posture generator based on hierarchical inverse kinematics and contact force optimization, called the null-space posture generator (NSPG), able to efficiently satisfy the aforementioned requisites in short times. A new configuration of the robot is produced through conservatively altering a given nominal posture exploiting the null-space of the contact manifold, satisfying geometrical and kinetostatics constraints. This is achieved through an adaptive random velocity vector generator that lets the robot explore its workspace. To prove the validity and generality of the proposed method, simulations in multiple scenarios are reported employing different robots: a wheeled-legged quadruped and a biped. Specifically, it is shown that the NSPG is particularly suited in complex cluttered scenarios, in which linear collision avoidance and stability constraints may be inefficient due to the high computational cost. In particular, we show an improvement of performances being our method able to generate twice feasible configurations in the same period. A comparison with previous methods has been carried out collecting the obtained results which highlight the benefits of the NSPG. Finally, experiments with the CENTAURO platform, developed at Istituto Italiano di Tecnologia, are carried out showing the applicability of the proposed method to a real corridor scenario.

Omnidirectional Walking Pattern Generator Combining Virtual Constraints and Preview Control for Humanoid Robots.

This paper presents a novel omnidirectional walking pattern generator for bipedal locomotion combining two structurally different approaches based on the virtual constraints and the preview control theories to generate a flexible gait that can be modified on-line. The proposed strategy synchronizes the displacement of the robot along the two planes of walking: the zero moment point based preview control is responsible for the lateral component of the gait, while the sagittal motion is generated by a more dynamical approach based on virtual constraints. The resulting algorithm is characterized by a low computational complexity and high flexibility, requisite for a successful deployment to humanoid robots operating in real world scenarios. This solution is motivated by observations in biomechanics showing how during a nominal gait the dynamic motion of the human walk is mainly generated along the sagittal plane. We describe the implementation of the algorithm and we detail the strategy chosen to enable omnidirectionality and on-line gait tuning. Finally, we validate our strategy through simulation experiments using the COMAN + platform, an adult size humanoid robot developed at Istituto Italiano di Tecnologia. Finally, the hybrid walking pattern generator is implemented on real hardware, demonstrating promising results: the WPG trajectories results in open-loop stable walking in the absence of external disturbances.

A novel human effort estimation method for knee assistive exoskeletons.

In this work we present a novel method to estimate online the torques at the knee joints with the goal to generate reference signals for knee assistive devices. One of the main advantages of the proposed approach is its reduced sensing requirements, which leads to an ergonomic setup with minimal instrumentation, especially above the knee and of the upper body. Indeed, only the measurement of the forces and torques exchanged between the ground and the user's feet, the posture of the shanks, and the model of the user's shank itself are needed for the estimation of the knee torque. The method does not require information of the state of the upper body and of a possible payload i.e. body pose, mass and center of mass (CoM) location. As a result, a minimalistic sensory system consisting of sensorized shoes and IMUs to track the shanks' orientation are adequate, allowing for an easily wearable and portable setup. The estimation of the knee torques is achieved by imposing an equilibrium condition to the user's shank. Several experiments were performed to test the effectiveness of the proposed estimation method under different body postures and motions (e.g. squat motion and switching foot contacts) and payloads (e.g. by holding weights at different arm postures resulting in variable upper body CoM). Finally an assistive task, conducted with the iT-Knee bipedal system is presented, where the lifted payload changed its CoM location over time.

Horse-like walking, trotting, and galloping derived from kinematic Motion Primitives (kMPs) and their application to walk/trot transitions in a compliant quadruped robot.

This manuscript proposes a method to directly transfer the features of horse walking, trotting, and galloping to a quadruped robot, with the aim of creating a much more natural (horse-like) locomotion profile. A principal component analysis on horse joint trajectories shows that walk, trot, and gallop can be described by a set of four kinematic Motion Primitives (kMPs). These kMPs are used to generate valid, stable gaits that are tested on a compliant quadruped robot. Tests on the effects of gait frequency scaling as follows: results indicate a speed optimal walking frequency around 3.4 Hz, and an optimal trotting frequency around 4 Hz. Following, a criterion to synthesize gait transitions is proposed, and the walk/trot transitions are successfully tested on the robot. The performance of the robot when the transitions are scaled in frequency is evaluated by means of roll and pitch angle phase plots.

On the Kinematic Motion Primitives (kMPs) - Theory and Application.

Human neuromotor capabilities guarantee a wide variety of motions. A full understanding of human motion can be beneficial for rehabilitation or performance enhancement purposes, or for its reproduction on artificial systems like robots. This work aims at describing the complexity of human motion in a reduced dimensionality, by means of kinematic Motion Primitives (kMPs). A set of five invariant kMPs are identified for periodic motions, and a set of two kMPs for discrete motions. It is shown how these two sets of kMPs can be combined to synthesize more complex motion as the simultaneous execution of the periodic and the discrete motions. The results reported are an evidence of the theory of Central Pattern Generators (CPG), showing its effects on the kinematics, and are related to what presented in the literature on the Motor Primitives extracted from EMG signals. Experimental tests with the COmpliant huMANoid (COMAN) were performed to show that the kMPs extracted from human subjects can be used to transfer the features of human locomotion to the gait of a robot.

Feeling through Tactile Displays: A Study on the Effect of the Array Density and Size on the Discrimination of Tactile Patterns.

Tactile arrays are devices that can provide spatially distributed cutaneous signals delivering crucial information during virtual haptic exploration or remote manipulation procedures. Two of the key specifications of a tactile array are the tactor spacing and array size that are believed to directly affect the device performance. In most of the systems developed so far, these two parameters have been chosen by trial and error or by trying to match the tactor density to the spatial resolution in the human fingertip. The objective of this work is to study the effect of tactor spacing and array size on the tactile arrays performance by measuring human tactile discrimination ability. Psychophysical experiments were performed to obtain the differential threshold for discrimination of a ridge angle and the shape recognition performance while exploring edge-based patterns. The patterns were explored through different passive (nonactuated) tactile arrays of vertically moving pins and also directly with the finger. Results indicate that a tactile array of 1.8 mm tactor spacing and 1 cm(2) array size transmits the pattern information with a good level of accuracy. This work shows that tactile devices with low complexity (small number of tactors) are still effective in conveying tactile cues. Moreover, this work provides performance measures that determinate the capabilities of tactile pin arrays to convey accurately tactile information.

A multi-DOF robotic exoskeleton interface for hand motion assistance.

This paper outlines the design and development of a robotic exoskeleton based rehabilitation system. A portable direct-driven optimized hand exoskeleton system has been proposed. The optimization procedure primarily based on matching the exoskeleton and finger workspaces guided the system design. The selection of actuators for the proposed system has emerged as a result of experiments with users of different hand sizes. Using commercial sensors, various hand parameters, e.g. maximum and average force levels have been measured. The results of these experiments have been mapped directly to the mechanical design of the system. An under-actuated optimum mechanism has been analysed followed by the design and realization of the first prototype. The system provides both position and force feedback sensory information which can improve the outcomes of a professional rehabilitation exercise.

A portable rehabilitation device for the Hand.

This paper presents the design of a direct driven under-actuated portable hand exoskeleton for rehabilitation. The design of the proposed Hand EXOskeleton SYStem (HEXOSYS) was driven by multi-objective optimisation strategy and inspiration from the human hand. The optimisation algorithm resulted in the choice of optimum link lengths of the device. The optimisation criteria are based on dexterity, isotropy and exertion of perpendicular forces on the finger digits. Furthermore, a series of experiments on the human hand using appropriate sensory instrumentation guided the selection of actuators thereby resulting in a rehabilitation device which is compatible with the human hand force capabilities. The provision of force as well as position feedback gives quantitative feedback to the therapist and would imply a more efficient rehabilitation process. The first prototype of the device has been designed and realized.