Daniele Pucci

iCub Whole-Body Control through Force Regulation on Rigid Non-Coplanar Contacts

This paper details the implementation on the humanoid robot iCub of state-of-the-art algorithms for whole-body control. We regulate the forces between the robot and its surrounding environment to stabilize a desired robot posture. We assume that the forces and torques are exerted on rigid contacts. The validity of this assumption is guaranteed by constraining the contact forces and torques, e.g. the contact forces must belong to the associated friction cones. The implementation of this control strategy requires to estimate the external forces acting on the robot, and the internal joint torques. We then detail algorithms to obtain these estimations when using a robot with an iCub-like sensor set, i.e. distributed six-axis force-torque sensors and whole-body tactile sensors. A general theory for identifying the robot inertial parameters is also presented. From an actuation standpoint, we show how to implement a joint torque control in the case of DC brushless motors. In addition, the coupling mechanism of the iCub torso is investigated. The soundness of the entire control architecture is validated in a real scenario involving the robot iCub balancing and making contacts at both arms.

Stability analysis and design of momentum-based controllers for humanoid robots

Envisioned applications for humanoid robots call for the design of balancing and walking controllers. While promising results have been recently achieved, robust and reliable controllers are still a challenge for the control community dealing with humanoid robotics. Momentum-based strategies have proven their effectiveness for controlling humanoids balancing, but the stability analysis of these controllers is still missing. The contribution of this paper is twofold. First, we numerically show that the application of state-of-the-art momentum-based control strategies may lead to unstable zero dynamics. Secondly, we propose simple modifications to the control architecture that avoid instabilities at the zero-dynamics level. Asymptotic stability of the closed loop system is shown by means of a Lyapunov analysis on the linearized systems joint space. The theoretical results are validated with both simulations and experiments on the iCub humanoid robot.

Flight dynamics and control in relation to stall

Control of fixed-wing aircraft at high angles of attack is particularly challenging. In this case, aerodynamic forces can be subjected to strong variations, among which stall is certainly the most critical. This paper tackles flight dynamics and control for aerial vehicles subjected to the stall phenomenon. We propose a class of modeling functions for the lift coefficient, and we investigate the control problem. The equilibria analysis is addressed prior to the control design. We show that the stall phenomenon never forbids the existence of an equilibrium orientation for any reference trajectory, but the uniqueness of this orientation is not in general ensured. Consequently, the equilibrium orientation can be subjected to discontinuities leading to an ill-conditioned control problem. Feedback control laws are derived for reference velocities associated with continuous equilibrium orientations.

Collocated Adaptive Control of Underactuated Mechanical Systems

Collocated adaptive control of underactuated mechanical systems is still a concern for the control community. The main difficulty comes from the nonlinearity of the collocated inverse dynamics with respect to the base parameters, which forbids the direct application of classical adaptive control schemes. This paper extends and encompasses the Slotines adaptive control, which was developed for fully actuated mechanical systems, to stabilize the collocated state space of an underactuated mechanical system. The key point is to define the sliding variable as the difference between the systems velocity and an exogenous state whose dynamics is considered as control input. We first revisit the Slotines result in view of this definition and then show how to extend it to the underactuated case. Stability and convergence of time-varying reference trajectories for the collocated dynamics are shown to be in the sense of Lyapunov. Global well-posedness of the control laws is achieved by means of a new algebraic property of the mass matrix. Simulations, comparisons to existing control strategies, and experimental results on a two-link manipulator verify the soundness of the proposed approach.

Nonlinear control of PVTOL vehicles subjected to drag and lift

The present study extends and encompasses a previous work on the control of thrust-propelled vehicles which focused on vehicles subjected to environmental reaction forces that are reduced to their drag component, as in the case of spherical bodies. Lift forces associated with other body shapes, like winged aerial vehicles, modifies and complicates the control problem significantly. This paper shows the existence of a generic set of drag-and-lift models for which it is possible to recast the initial control problem into the one of controlling a spherical body, thus allowing for the application of control design methods and analyses developed previously. Beside the obtention of more general nonlinear control solutions that apply to a larger class of vehicles and for which (semi) global stability results can be proved, we view this extension as a step to the automatic monitoring of flight transitions between hovering and high-velocity cruising for convertible aerial vehicles.

First Steps in the FTU Migration Towards a Modular and Distributed Real-Time Control Architecture Based on MARTe

The Fusion Advanced Studies Torus (FAST) experiment is being proposed by the Italian laboratories as a European satellite Tokamak that will enhance and facilitate the exploitation of ITER like scenarios and technologies. Its size and complexity is comparable to the largest fusion machine in the world: JET. As such, its real time control system will have to meet basic requirements such as a modular and distributed architecture, where different control subsystems can be easily integrated at different times and can operate either independently or in cooperation with other subsystems. Another important feature, which has to be taken into account, is the transparency regarding both the hardware interfacing and the adopted platform. As a test bed, we are currently planning to upgrade the architecture of the Frascati Tokamak Upgrade (FTU) real-time system in order to improve its flexibility and modularity and have decided to adopt the MARTe package to reach our goal. Currently, there are four systems under development at FTU: the LH-Power system; the gas puffing control system; the ODIN Equilibrium Reconstruction system; and the position and current feedback control system (currently in a design phase). This paper will describe the current status and first results of the previously referred systems integration.

Nonlinear feedback control of axisymmetric aerial vehicles

We investigate the use of simple aerodynamic models for the feedback control of underactuated aerial vehicles flying with large flight envelopes. Thrust-propelled vehicles with a body shape symmetric with respect to the thrust axis are considered. Upon a condition on the aerodynamic characteristics of the vehicle, we show that the equilibrium orientation can be explicitly determined as a function of the desired flight velocity. This allows for the adaptation of previously proposed control design approaches based on the thrust direction control paradigm. Simulation results conducted by using measured aerodynamic characteristics of quasi-axisymmetric bodies illustrate the soundness of the proposed approach.

In situ calibration of six-axis force-torque sensors using accelerometer measurements

This paper proposes techniques to calibrate six-axis force-torque sensors that can be performed in situ, i.e., without removing the sensor from the hosting system. We assume that the force-torque sensor is attached to a rigid body equipped with an accelerometer. Then, the proposed calibration technique uses the measurements of the accelerometer, but requires neither the knowledge of the inertial parameters nor the orientation of the rigid body. The proposed method exploits the geometry induced by the model between the raw measurements of the sensor and the corresponding force-torque. The validation of the approach is performed by calibrating two six-axis force-torque sensors of the iCub humanoid robot.

Highly dynamic balancing via force control

This video shows the latest results in the whole-body control of humanoid robots achieved by the Dynamic Interaction Control Lab at the Italian Institute of Technology. In particular, the control architecture is composed of two nested control loops. The internal loop, which runs at 1 KHz, is in charge of stabilizing any desired joint torque. This task is achieved thanks to an off-line identification procedure providing us with a reliable model of friction and motor constants. The outer loop, which generates desired joint torques at 100 Hz, is a momentum based control algorithm in the context of free-floating systems. More precisely, the control objective for the outer loop is the stabilization of the robots linear and angular momentum and the associated zero dynamics. The latter objective can be used to stabilize a desired joint configuration. The stability of the control framework is shown to be in the sense of Lyapunov. The contact forces and torques are regulated so as to break contacts only at desired configurations. Switching between several contacts is taken into account thanks to a finite-state-machine that dictates the constraints acting on the system. The control framework is implemented on the iCub humanoid robot.

A Whole-Body Software Abstraction Layer for Control Design of Free-Floating Mechanical Systems

In this paper, we propose a software abstraction layer to simplify the design and synthesis of whole-body controllers without making any preliminary assumptions on the control law to be implemented. The main advantage of the proposed library is the decoupling of the control software from implementation details, which are related to the robotic platform. Furthermore, the resulting code is more clean and concise than ad-hoc code, as it focuses only on the implementation of the control law. In addition, we present a reference implementation of the abstraction layer together with a Simulink interface to provide support to Model-Driven based development. We also show the implementation of a simple proportional-derivative plus gravity compensation control together with a more complex momentum-based bipedal balance controller.