Alberto Cavallo

Effects of hysteresis compensation in feedback control systems

Analyzes the performances of closed-loop control systems when real hysteretic actuators (e.g., Terfenol-D-based devices) are of concern. Shown is a simple but effective strategy, based on the simple idea of pseudo-compensator, for transducers hysteresis compensation. Such a strategy improves the control systems behavior, not only in terms of tracking error reduction but also in decreasing the control signal so as to avoid saturation and harmful stress to the actuator on the one hand and reducing hysteretic energy losses on the other. Experiments are performed on a magnetostrictive actuator used for a micropositioning task.

The DEXMART hand: Mechatronic design and experimental evaluation of synergy-based control for human-like grasping

This paper summarizes recent activities carried out for the development of an innovative anthropomorphic robotic hand called the DEXMART Hand. The main goal of this research is to face the problems that affect current robotic hands by introducing suitable design solutions aimed at achieving simplification and cost reduction while possibly enhancing robustness and performance. While certain aspects of the DEXMART Hand development have been presented in previous papers, this paper is the first to give a comprehensive description of the final hand version and its use to replicate human-like grasping. In this paper, particular emphasis is placed on the kinematics of the fingers and of the thumb, the wrist architecture, the dimensioning of the actuation system, and the final implementation of the position, force and tactile sensors. The paper focuses also on how these solutions have been integrated into the mechanical structure of this innovative robotic hand to enable precise force and displacement control of the whole system. Another important aspect is the lack of suitable control tools that severely limits the development of robotic hand applications. To address this issue, a new method for the observation of human hand behavior during interaction with common day-to-day objects by means of a 3D computer vision system is presented in this work together with a strategy for mapping human hand postures to the robotic hand. A simple control strategy based on postural synergies has been used to reduce the complexity of the grasp planning problem. As a preliminary evaluation of the DEXMART Hand’s capabilities, this approach has been adopted in this paper to simplify and speed up the transfer of human actions to the robotic hand, showing its effectiveness in reproducing human-like grasping.

ATMOSPHERIC RE-ENTRY CONTROL FOR LOW LIFT/DRAG VEHICLES

The re-entry trajectory control of a low lift/drag (L/D) (L/D < 1 sphere-cone shapes) re-entry vehicle is dealt with. The proposed control strategy is based on the linear quadratic regulator (LQR) design technique and on the variable structure systems (VSS) mathematical machinery. The LQR results from the solution of a differential Riccati equation based on the linearization of the equations of motion along a nominal re-entry trajectory. Its objective is the control of the vehicle in the altitude-velocity plane (vertical plane). Terminal control is performed taking into account the range to go in the LQR and by pointing the velocity vector towards the target via a VSS strategy. A single control variable is considered, namely the bank angle, while the angle of attack is kept constant to assure the required L/D. The proposed technique is applied to the assured crew return vehicle re-entry mission in presence of off-nominal aerodynamic and atmospheric and initial conditions. Extreme cases results and a Monte Carlo simulation are presented to test the robustness of the proposed control strategy, which show that the target point is reached with an accuracy of 1 km in more than 50% of the cases and the required 7.6-km maximum error is not exceeded with probability larger than 0.9.

Feedback control systems for micropositioning tasks with hysteresis compensation

This paper proposes the analysis of a control loop system employing a Terfenol-D actuator driving a real mechanical load. The aim of the paper is to analyze the performances of such a system when a strategy of hysteresis compensation is employed. Such strategy has demonstrated its effectiveness in simpler feedback systems (with no mechanical load) by improving the tracking error, decreasing the control signal so as to avoid saturation and harmful stress to the actuator. As a consequence, the algorithm allows also to reduce energy losses in the actuator.

A Sliding Manifold Approach to Satellite Attitude Control

Abstract A sliding manifold based control strategy is proposed for the attitude control of a satellite. By using the theory of singular perturbation a PD feedback control is designed. It exhibits robustness properties with respect to environmental disturbances and plant parametric uncertainties. The resulting control signal remains, after a fast transient, in a neighbourhood of the well-defined equivalent control. Then the control can take into account bounds on the available control signals and avoid the peaking phenomenon of high gain systems. Finally the proposed procedure is applied to the CARINA capsule, and simulations are performed by using the ESA-MIDAS dynamic simulator in presence of environmental disturbances and corrupted Earth-magnetic field measures

Output feedback control based on a high-order sliding manifold approach

In this note, we address the problem of output feedback control of multiple-input-multiple-output plants. The proposed approach is based on a high-order sliding manifold strategy. The resulting controller exhibits strong robustness properties, similar to high-gain control laws, but avoids peaking phenomena, thanks to the adoption of a time-varying sliding surface. Moreover, the dynamic control law is continuous and differentiable, thus avoiding chattering problems.

ROBUST FLIGHT CONTROL SYSTEMS : A PARAMETER SPACE DESIGN

This paper deals with the longitudinal dynamics stabilization, around a trim condition, of an aircraft with unsatisfactory longitudinal stability characteristics. This problem is a single-input control problem because only elevator and canards deflections are used as control inputs, and they are simultaneously driven by the same control signal. To this aim, a new linear parameterization of all compensators stabilizing a given single-input plant is proposed, along with a design method based on a computationally tractable procedure for the robust stability analysis of polynomials with at finely dependent coefficient perturbations. Since scale factor sensor changes are perturbations entering affinely the characteristic polynomials coefficients of the closed-loop system, a controller will be synthesized in order to guarantee that the closed-loop poles lie in a specified domain of the complex plane prescribed by MIL specifications, and to achieve robustness against scale factor sensor failures. An application of the proposed procedure to the F4-E military aircraft is presented.

Buck-boost DC/DC converter for aeronautical applications

In this document a DC/DC converter for power conversion between +28V/+270V voltage levels is presented, and the main characteristics of the converter are discussed. The topological structure is first introduced, outlining the hardware components used and the modular approach. The step-down (or Buck) and step-up (or Boost) modulations are then discussed, focusing on the switching signals to drive the low-voltage and high-voltage bridges of the single modules. The description of the control phase follows, justifying the controller used to steer the output voltage towards the desired value. The firmware realized for the converter management is equipped also with a supervision and protection logic, realized as a composition of finite state machines. Finally, simulative results and real measurements are provided to evidence the correctness of the overall topological choices and of the control system realized for the bidirectional converter, focusing also on the comparison between the expected results and those obtained.

Gray-Box Identification of Continuous-Time Models of Flexible Structures

A noniterative algorithm is proposed to identify the continuous-time state space model of lightly damped flexible structures. This paper is motivated by the need for accurate dynamic models to be used in design of active vibration control systems for, e.g., large space structures and aeronautical structures. A gray-box approach is adopted to preserve the physical meaning of the model parameters and to naturally impose physical constraints to the model, e.g., passivity and reciprocity. The procedure consists of two stages both in the frequency domain; first, a subspace method is applied to estimate the dynamic matrix, then, the input and output matrices are identified by solving a weighted least square problem, both in the colocated and noncolocated case. A numerical case study is carried out to illustrate the features of the algorithm and experimental results are presented by reporting and validating the identified model of a stiffened aeronautical panel in a broad frequency range. The experimental case study serves also for carrying out a comparison between the proposed identification procedure and some of the already existing algorithms.

Limit cycles in control systems employing smart actuators with hysteresis

This paper presents theoretical and experimental results concerning a feedback control system employing a Terfenol-D-based smart actuator. Such magnetostrictive devices exhibit significant hysteresis, which, under some conditions, can generate self-sustained periodic oscillations in the control loop. The paper proposes a general procedure to find these conditions and to compute this periodic solution by exploiting some classical results about describing function analysis and the well-known Preisach operator theory. The theoretical findings are supported by a rigorous mathematical analysis resorting to fixed-point theorems.