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This article takes a step to provide humanoid robots with adaptive morphology abilities. We present a systematic approach for enabling robotic covers to morph their shape, with an overall size fitting the anthropometric dimensions of a humanoid robot. More precisely, we present a cover concept consisting of two main components: a skeleton, which is a repetition of a basic element called node, and a soft membrane, which encloses the cover and deforms with its motion. This article focuses on the cover skeleton and addresses the challenging problems of node design, system modeling, motor positioning, and control design of the morphing system. The cover modeling focuses on kinematics, and a systematic approach for defining the system kinematic constraints is presented. Then, we apply genetic algorithms to find the motor locations so that the morphing cover is fully actuated. Finally, we present control algorithms that allow the cover to morph into a time-varying shape. The entire approach is validated by performing kinematic simulations with four different covers of square dimensions and having 3x3, 4x8, 8x8, and 20x20 nodes, respectively. For each cover, we apply the genetic algorithms to choose the motor locations and perform simulations for tracking a desired shape. The simulation results show that the presented approach ensures the covers to track a desired shape with good tracking performances.

Pneumatic grippers are hugely employed in robotic applications. Nonetheless, their control is not easy due to difficulty in managing the pressure inside their air chambers. Pneumatic grippers have often simple structure though the lack of affordable control algorithms complicates their usage. Motivated by these reasons, we wish to deliver a new control architecture for the closed-loop control of pneumatic grippers actuated by pressure regulators. The proposed architecture is composed of a main controller resorting to an optimization algorithm and of a state observer that estimates pressures in both gripper chambers, along with the exerted force. Instead, measured quantities (i.e. physical pressure in the gripper chambers and force recorded by a load cell between the gripper fingers) are used as inputs for the state observer to improve its output. The pneumatic gripper performance benefits from the joint action of the controller and of the state observer, as experimentally demonstrated. The gripper response will be shown for different types of inputs and on different setups.

Jumping can be an effective way of locomotion to overcome small terrain gaps or obstacles. In this paper we propose two different approaches to perform jumps with a humanoid robot. Specifically, starting from a pre-defined CoM trajectory we develop the theory for a velocity controller and for a torque controller based on an optimization technique for the evaluation of the joints input. The controllers have been tested both in simulation and on the humanoid robot iCub. In simulation the robot was able to jump using both controllers, while the real system jumped with the velocity controller only. The results highlight the importance of controlling the centroidal angular momentum and they suggest that the joint performances, namely maximum power, of the legs and torso joints, and the low level control performances are fundamental to achieve acceptable results.

Effective control design of flying vehicles requires a reliable estimation of the propellers' thrust forces to secure a successful flight. Direct measurements of thrust forces, however, are seldom available in practice and on-line thrust estimation usually follows from the application of fusion algorithms that process on-board sensor data. This paper proposes a framework for the estimation of the thrust intensities on flying multibody systems that are not equipped with sensors for direct thrust measurement. The key ingredient of the proposed framework is the so-called centroidal momentum of a multibody system, which combined with the propeller model, enables the design of Extended Kalman Filters (EKF) for on-line thrust estimation. The presented approach tackles the additional complexity in thrust estimation due to uncertainties in the propeller model and the possibly large number of degrees of freedom of the system. For instance, a covariance scheduling approach based on the turbines RPM error is proposed to ensure a reliable estimation even in case of turbine failures. Simulations are presented to validate the proposed algorithm during robot flight. Moreover, an experimental setup is designed to evaluate the accuracy of the estimation algorithm using iRonCub, a jet-powered humanoid robot, while standing on the ground.

Mechanical systems are typically composed of a number of contacting surfaces that move against each other. Such surfaces are subject to friction forces. These dissipate part of the actuation energy and cause an undesired effect on the overall system functioning. Therefore, a suitable model of friction is needed to elide its action. The choice of such a model is not always straightforward, as it is influenced by the system properties and dynamics. In this paper, we show the identification of different friction models and evaluate their prediction capability on an experimental dataset. Despite being state-of-the-art models, some modifications were introduced to improve their performance. A pneumatic gripper was used to collect the data for the models evaluation. Two experimental setups were mounted to execute the experiments: information from two pressure sensors, a load cell and a position sensor was employed for the identification. During the experiments, the gripper was actuated at different constant velocities. Results indicate that all the identified models offer a proper prediction of the real friction force.

Wearable sensors are gaining in popularity because they enable outdoor experimental monitoring. This paper presents a cost-effective sensorised insole based on a mesh of tactile capacitive sensors. Each sensor's spatial resolution is about 4 taxels/cm^2 in order to have an accurate reconstruction of the contact pressure distribution. As a consequence, the insole provides information such as contact forces, moments, and center of pressure. To retrieve this information, a calibration technique that fuses measurements from a vacuum chamber and shoes equipped with force/torque sensors is proposed. The validation analysis shows that the best performance achieved a root mean square error (RMSE) of about 7 N for the contact forces and 2 N m for the contact moments when using the force/torque shoe data as ground truth. Thus, the insole may be an alternative to force/torque sensors for certain applications, with a considerably more cost-effective and less invasive hardware., 21 pages, 12 figures

The paper contributes towards the modeling, identification, and control of model jet engines. We propose a nonlinear, second order model in order to capture the model jet engines governing dynamics. The model structure is identified by applying sparse identification of nonlinear dynamics, and then the parameters of the model are found via gray-box identification procedures. Once the model has been identified, we approached the control of the model jet engine by designing two control laws. The first one is based on the classical Feedback Linearization technique while the second one on the Sliding Mode control. The overall methodology has been verified by modeling, identifying and controlling two model jet engines, i.e. P100-RX and P220-RXi developed by JetCat, which provide a maximum thrust of 100 N and 220 N, respectively., Comment: 8 pages, 12 figures, submitted to RA-L and ICRA

Robotic arms can perform grasping actions thanks to their “dexteorus” part, i.e. the gripper. Among the various categories, nowadays pneumatic grippers became the most employed in industry, as they have low cost and little bulkiness. Despite their simplicity, controlling the force applied by these grippers is not straightforward due to the dependence of such a force on the air pressure in the gripper chambers. As a result, it is still tricky to implement closed-loop force control for pneumatic grippers. This paper intends to deliver a control scheme relying on the force measurement to control pneumatic grippers. The force might be measured through a commercial sensor (e.g. a load cell) and fed back to close the control loop. This includes a calibration which maps the force-pressure relation taking into account both desired force and length of the gripper fingers. The control scheme exploits two different pressure regulators to precisely adjust the air pressure inside the gripper chambers (i.e. opening and closing chambers). To this aim, a quadratic programming algorithm is employed. The control scheme performance revealed to be good: results will be shown in terms of gripper response to sinusoidal and step inputs, along with the pressure-force characterization.

Science fiction has long inspired pioneers of new areas of Engineering. When imagination meets the current needs of civil society, creative thinking then often gets real, and projects aiming at breakthroughs for advancing the scientific state of the art are put in place. Robotics is a scientific field that has always been driven by visionary applications of Engineering, often receiving impetus from the human will of having extended locomotion and interaction capacities. The implementation of Manipulation and Locomotion on robotic platforms, however, remains a big challenge for the Robotics community. The resulting endeavor paved the way to new branches of Robotics aimed at combining Manipulation and Locomotion into single platforms. Aerial Manipulation [1], for instance, unifies Manipulation and Aerial Locomotion by conceiving robots capable of flying while manipulating an object. Humanoid Robotics [2], instead, merges Manipulation and Terrestrial Locomotion since humanoid robots can usually manipulate objects and move around by exploiting contacts with the environment (e.g. walking). This paper presents the challenges towards unifying Manipulation, Aerial, and Terrestrial Locomotion by implementing Aerial Humanoid Robotics. Aerial humanoid robots can then fly, walk, manipulate, and transport objects in the surrounding environment, thus being pivotal for disaster response and opening new branches of applications for humanoid robots. In other words, Aerial Humanoid Robotics unifies Aerial Manipulation [1] and Humanoid Robotics [2]. By doing so, aerial humanoid robots overcome the lack of terrestrial locomotion of aerial manipulators and extend the locomotion capabilities of humanoid robots to the flight case. Aerial humanoid robots can then walk, fly, manipulate and transport objects, thus offering energetically efficient solutions to payload transportation and object manipulation. In fact, a platform implementing Aerial Humanoid Robotics can reach the desired location by flying, thus avoiding challenging terrains; once landed, the platform can move around by walking, and then manipulate objects while standing on two feet. Consequently, aerial humanoid robots are more robust and energetically efficient than classical aerial vehicles accomplishing manipulation tasks. Thanks to their terrestrial locomotion ability, aerial humanoid robots also avoid struggling with flying in confined spaces, where low altitude flight becomes tedious and energetically inefficient.

This paper proposes a control strategy for the stabilization of a jet-powered flying humanoid robot. In particular, the contribution of the paper concerns the design of a control framework capable of tracking a desired robot position and orientation trajectory while flying. Asymptotic stability of the closed loop system is shown by means of a Lyapunov analysis. Simulations are carried out on a model of the humanoid robot iCub to verify the soundness of the proposed approach.

A common approach to the generation of walking patterns for humanoid robots consists in adopting a layered control architecture. This paper proposes an architecture composed of three nested control loops. The outer loop exploits a robot kinematic model to plan the footstep positions. In the mid layer, a predictive controller generates a Center of Mass trajectory according to the well-known table-cart model. Through a whole-body inverse kinematics algorithm, we can define joint references for position controlled walking. The outcomes of these two loops are then interpreted as inputs of a stack-of-task QP-based torque controller, which represents the inner loop of the presented control architecture. This resulting architecture allows the robot to walk also in torque control, guaranteeing higher level of compliance. Real world experiments have been carried on the humanoid robot iCub., IROS 2018

Inspired by the biomechanical and passive properties of human muscles, we present a novel actuator named passive noise rejecting Variable Stiffness Actuator (pnrVSA). For a single actuated joint, the proposed design adopts two motor-gear groups in an agonist-antagonist configuration coupled to the joint via serial non-linear springs. From a mechanical standpoint, the introduced novelty resides in two parallel non-linear springs connecting the internal motor-gear groups to the actuator frame. These additional elastic elements create a closed force path that mechanically attenuates the effects of external noise. We further explore the properties of this novel actuator by modeling the effect of gears static frictions on the output joint equilibrium position during the co-contraction of the agonist and antagonist side of the actuator. As a result, we found an analytical condition on the spring potential energies to guarantee that co-activation reduces the effect of friction on the joint equilibrium position. The design of an optimized set of springs respecting this condition leads to the construction of a prototype of our actuator. To conclude the work, we also present two control solutions that exploit the mechanical design of the actuator allowing to control both the joint stiffness and the joint equilibrium position.

The torso of a humanoid robot is a fundamental part of its kinematic structure because it defines the reachable workspace, supports the entire upper-body and can be used to control the position of the center of mass. The majority of the torso joints are designed exploiting serial or differential mechanisms, while parallel kinematic structures are less used mainly because of their greater design complexity. This paper describes the design and construction of a 4 degrees of freedom (DoF) torso for our new humanoid robot. Three degrees of freedom, namely roll, pitch and heave, have been implemented using a 3 DoF parallel kinematic structure, while the fourth DoF, namely yaw, has been implemented with a rotational joint on top of the parallel structure. The design has been optimized to reduce the cost and the volume of the system. A first prototype of the torso has been constructed and validated with respect to our design requirements. Eventually, experimental tests have been conducted to assess the functionality of the proposed system.

In recent years the robotics field has witnessed an interesting new trend. Several companies started the production of service robots whose aim is to cooperate with humans. The robots developed so far are either rather expensive or unsuitable for manipulation tasks. This article presents the result of a project which wishes to demonstrate the feasibility of an affordable humanoid robot. R1 is able to navigate, and interact with the environment (grasping and carrying objects, operating switches, opening doors etc). The robot is also equipped with a speaker, microphones and it mounts a display in the head to support interaction using natural channels like speech or (simulated) eye movements. The final cost of the robot is expected to range around that of a family car, possibly, when produced in large quantities, even significantly lower. This goal was tackled along three synergistic directions: use of polymeric materials, light-weight design and implementation of novel actuation solutions. These lines, as well as the robot with its main features, are described hereafter.

This paper describes the conception, design, fabrication and testing of a wrist for a new humanoid robot. The wrist is comprised of a three degrees-of-freedom (DOF) parallel mechanism actuated by three motors. The wrist was made to be lightweight and cost-efficient. The design is optimized to fit within the forearm of the robot and is equipped with joint and calibration sensors. It was then tested for consistency in performance and positioning accuracy.

We present a method for humanoid robot walking on partial footholds such as small stepping stones and rocks with sharp surfaces. Our algorithm does not rely on prior knowledge of the foothold, but information about an expected foothold can be used to improve the stepping performance. After a step is taken, the robot explores the new contact surface by attempting to shift the center of pressure around the foot. The available foothold is inferred by the way in which the foot rotates about contact edges and/or by the achieved center of pressure locations on the foot during exploration. This estimated contact area is then used by a whole body momentum-based control algorithm. To walk and balance on partial footholds, we combine fast, dynamic stepping with the use of upper body angular momentum to regain balance. We applied this method to the Atlas humanoid designed by Boston Dynamics to walk over small contact surfaces, such as line and point contacts. We present experimental results and discuss performance limitations., Comment: in 2016 IEEE-RAS 16th International Conference on Humanoid Robots (Humanoids)

Recently, a number of variable impedance actuator designs have been proposed under different motivations (safe human-robot interaction, mechanical robustness and energy storing to cite a few). In a recent paper (Berret et al. (2011)) we observed that none of the available designs seem to reproduce an important characteristic of human muscles, i.e. the ability to open-loop reject disturbances by means of muscle co-activation. Starting form this observation, we recently designed a novel single-joint actuator (nr-VIA) based on the use of non-linear springs in agonist-antagonist configuration. In this paper we discuss some control related characteristics of the proposed design. The theoretical analysis is conducted without specifying the potential energy of the springs. We first design a control law capable of monotonically increasing the joint-stiffness (i.e. disturbance rejection) without changing the joint equilibrium configuration; this result is obtained with minimal requirements on the potential energy of the springs. The same control law is then proven (under more restrictive conditions) to monotonically decrease the sensitivity of the joint equilibrium with respect to the actuation variables, a desirable property when trying to achieve a finer control over joint positioning.

Recently, new trends in robotics have proposed variable stiffness actuators (VSA) as an alternative to the classical actuator design, based on a rigid coupling between motors and actuated joints. These novel technologies ask for novel control and planning strategies. In the present paper we consider the problem of motion planning for a specific class of VSA. This class, named passive noise rejecting VSA (pnrVSA) is capable of rejecting noise without explicitly resorting to feedback. Passive noise rejecting actuators call for new planning and control strategies. In this paper we apply an open-loop control obtained as the solution of a stochastic optimal control problem solved with path integral (PI) approach. Specific focus is on obtaining a feedback-free numerical solution which emphasizes the peculiar characteristics of the pnrVS actuator. The proposed numerical technique is applied to control a two- DOF manipulator equipped with pnrVS actuators. It is shown that stochastic optimal control can successfully simulate an highly unstable task consisting of pushing against an unstable wall in presence of instability and noise. The proposed task is reminiscent of real tasks such as screwing and carving.

In the last decade new actuator designs have been presented trying to introduce at mechanical level the advantages of compliance. Ranging from serial elastic actuators to different designs of variable stiffness actuators, various prototypes have been proposed and implemented on robots, thus allowing performance of novel and challenging tasks. Nevertheless some of these new devices often are affected by the drawbacks related to friction. In particular, static friction due to its discontinuous nature, can produce undesired behaviors that are rather difficult to compensate. In this paper we present a novel kind of passive variable stiffness actuator based on agonist-antagonist configuration. The specific design we adopted improves the capability of the system in mechanically compensating the external disturbances, but on the other hand intensifies the effect of stiction during the cocontraction of the agonist and antagonist side of the actuator. The consequence is the appearance of a set of neutral equilibrium configurations of the output joint that we named “dead-band”. This issue is tackled analytically investigating the propagation and the distribution of the stiction components within the whole system. The result is a condition over the spring potential energies that is exploited to properly design the new non-linear springs. Eventually experimental tests are conducted on the real actuator, showing the effectiveness of our analytical approach.

Recently we proposed a novel principle to design variable stiffness actuators [1]. For a single actuated joint, the principle adopts two motors in agonist-antagonist configuration coupled to the joint via nonlinear springs. Co-activation (the simultaneous activation of both motors) increases both the joint stiffness and the joint passive noise rejection, which is the ability to reduce the effect of noise without relying on explicit feedback loops. In this paper we further explore the properties of this actuator modeling the effect of static frictions (also known as stiction) on the joint equilibrium configuration. In the proposed framework, it is observed that static friction might result in multiple equilibrium configurations for a constant actuation level; this possibility is undesirable given the characteristics of the proposed actuator.We therefore characterize how stiction acting on the two motors influences the equilibrium configurations of the actuated joint. The analysis is conducted without specifying the characteristics of the non-linear springs. This procedural choice allows to give sufficient analytical conditions to guarantee that an increased level of co-activation reduces the effects of stiction on the joint equilibrium. Analytical conditions are then verified on a prototype of the actuator.

One of the most important features for a system capable of working in uncertain and unstructured environments is reliability. Nowadays robots are excellent machines, but are still not able to interact with their surrounding environment as humans or animals do. Recent studies highlight the role played by impedance changes in the human arm during manipulation tasks. In particular the possibility to vary the stiffness of shoulder, elbow and wrist allows humans to interact easily with fast changing environments while rejecting unpredictable noise disturbances [1]. Several studies also showed how the capability of “co-contracting” antagonistic muscles is required to interact with noisy/unpredictable environments. Starting from these premises we recently proposed novel design principles to build actuators with the ability to actively regulate the passive noise rejection (i.e. the ability to cancel the effect of disturbances without explicitly relying on feedback) [2]. In the present paper we implement these principles in the mechanical design of a novel actuator. The actuator is composed of two electric motors in agonist-antagonist configuration. The final design includes also four non-linear springs whose force-displacement characteristic has been customized on the specific application requirements. Validation of the proposed non-linear spring design has been conducted on a prototype and results are reported in this paper. Future works foreseen the integration of the proposed actuators on a two limbed robot with six artificial muscle, (three agonist-antagonist pairs) in a simple and bi-articular configuration.