Giovanni Ulivi

Design of an exact nonlinear controller for induction motors

A novel approach to the control of induction motors is presented. The approach is based on differential-geometric concepts for the control of nonlinear systems. Structural properties of the model are pointed out, and a proper selection of physically meaningful system outputs is indicated which yields, by means of static state-feedback, exact state linearization and input-output decoupling of the closed-loop system. The approach is used to design a controller for motor torque and flux. Simulation results are included. >

An outdoor navigation system using GPS and inertial platform

The use of global positioning system (GPS) in outdoor localization is quite a common solution in large environments where no other reference is available and there are not so demanding positioning requirements. Of course, fine motion without the use of an expensive differential device is not an easy task, even now that available precision has been greatly improved as the military encoding has been removed. In this paper we present a localization algorithm based on Kalman filtering that tries to fuse information coming from an inexpensive single GPS with inertial data and map-based data. The algorithm is able to produce an estimated configuration for the robot that can be successfully fed back in a navigation system. Some experiments show difficulties and possible solutions of this sensor fusion problem.

Fuzzy maps: A new tool for mobile robot perception and planning

An essential component of an autonomous mobile robot is the exteroceptive sensory system. Sensing capabilities should be integrated with a method for extracting a representation of the environment from uncertain sensor data and with an appropriate planning algorithm. In this article, fuzzy logic concepts are used to introduce a tool useful for robot perception as well as for planning collision-free motions. In particular, a map of the environment is defined as the fuzzy set of unsafe points, whose membership function quantifies the possibility for each point to belong to an obstacle. The computation of this set is based on a specific sensor model and makes use of intermediate sets generated from range measures and aggregated by means of fuzzy set operators. This general approach is applied to a robot with ultrasonic rangefinders. The resulting map building algorithm performs well, as confirmed by a comparison with stochastic methods. The planning problem on fuzzy maps can be solved by defining various path cost functions, corresponding to different strategies, and by searching the map for optimal paths. To this end, proper instances of the A* algorithm are devised. Experimental results for a Nomad 200™ robot moving in a real-world environment are presented.

A frequency-domain approach to learning control: implementation for a robot manipulator

A frequency-domain approach to the analysis and design of learning control laws for achieving a desired repetitive behavior in a dynamical system is presented. The scheme uses two separate filters in order to obtain rapid improvement in a specified bandwidth, while cutting off possibly destabilizing dynamic effects that would bar learning convergence. In this way the trade-off between global convergence conditions and approximate learning of trajectories is made explicit. The synthesis is presented for single-input, single-output (SISO) linear systems, but the method is of general application. The proposed learning controller has been used for exact tracking of repetitive trajectories in robot manipulators. In particular, actuator inputs that enable accurate reproduction of robot joint-space trajectories are learned in a few iterations without the knowledge of the robot dynamic model. Implementation aspects are discussed, and experimental results are reported. >

Combinatorial analysis and algorithms for quasispecies reconstruction using next-generation sequencing

BackgroundNext-generation sequencing (NGS) offers a unique opportunity for high-throughput genomics and has potential to replace Sanger sequencing in many fields, including de-novo sequencing, re-sequencing, meta-genomics, and characterisation of infectious pathogens, such as viral quasispecies. Although methodologies and software for whole genome assembly and genome variation analysis have been developed and refined for NGS data, reconstructing a viral quasispecies using NGS data remains a challenge. This application would be useful for analysing intra-host evolutionary pathways in relation to immune responses and antiretroviral therapy exposures. Here we introduce a set of formulae for the combinatorial analysis of a quasispecies, given a NGS re-sequencing experiment and an algorithm for quasispecies reconstruction. We require that sequenced fragments are aligned against a reference genome, and that the reference genome is partitioned into a set of sliding windows (amplicons). The reconstruction algorithm is based on combinations of multinomial distributions and is designed to minimise the reconstruction of false variants, called in-silico recombinants.ResultsThe reconstruction algorithm was applied to error-free simulated data and reconstructed a high percentage of true variants, even at a low genetic diversity, where the chance to obtain in-silico recombinants is high. Results on empirical NGS data from patients infected with hepatitis B virus, confirmed its ability to characterise different viral variants from distinct patients.ConclusionsThe combinatorial analysis provided a description of the difficulty to reconstruct a quasispecies, given a determined amplicon partition and a measure of population diversity. The reconstruction algorithm showed good performance both considering simulated data and real data, even in presence of sequencing errors.

On-line map building and navigation for autonomous mobile robots

The problem of sensor-based robot motion planning in unknown environments is addressed. The proposed solution approach prescribes the repeated sequence of two fundamental processes: perception and navigation. In the former, the robot collects data from its sensors, builds local maps and integrates them with the global maps so far reconstructed, using fuzzy logic operators. During the navigation process, a planner based on the A* algorithm proposes a path from the current position to the goal. The robot moves along this path until one of two termination conditions is verified namely (i) an unexpected obstructing obstacle is detected, or (ii) the robot is leaving the area in which reliable information has been gathered. Experimental results are presented for a Nomad 200 mobile robot.

Stable inversion control for flexible link manipulators

We consider the inverse dynamics problem for robot arms with flexible links, i.e., the computation of the input torque that allows exact tracking of a trajectory defined for the manipulator end-effector. A stable inversion controller is derived numerically, based on the computation of bounded link deformations and, from these, of the required feedforward torque associated with the desired tip motion. For a general class of multi-link flexible manipulators, three alternative computational algorithms are presented, all defined on the second-order robot dynamic equations. Trajectory tracking is obtained by adding a (partial) state feedback, within a nonlinear regulation approach. Experimental results are reported for the FLEXARM robot.

Control experiments on a two-link robot with a flexible forearm

A lightweight robot has been built with the aim of testing advanced control algorithms and demonstrating the engineering feasibility of flexible arm control. The robot is a planar two-link manipulator, with revolute joints and a very flexible forearm. A description of this laboratory facility is given, including mechanical structure, actuators and sensors, and interface electronics. A nonlinear dynamic model of the robot is given, in which link deflection is expressed in terms of orthonormal mode shapes of the associated eigenvalue problem. Simple control algorithms are presented, which are composed of a model-based feedforward term plus a linear feedback. These controllers have been implemented for joint trajectory tracking, and comparative experimental results are reported and discussed.<<ETX>>

Fuzzy logic and autonomous vehicles: experiments in ultrasonic vision

Abstract The opportunities offered by fuzzy logic to build maps for robot navigation are investigated. Characteristics of points of the space (occupied, free, uncertain, etc.) are easily expressed through set theoretical operations. Real-world experiments validate the approach. The experimental set-up is based on modified Polaroid ultrasonic sensors; however, the approach can be easily extended to incorporate other kinds of sensors.

Full linearization of induction motors via nonlinear state-feedback

A novel approach to the control of induction motors is presented, based on differential-geometric concepts for the control of nonlinear systems. Structural properties of the model are pointed out and a proper selection of system outputs is indicated which yields, via static state-feedback, exact state linearization and input-output decoupling of the closed-loop system. The proposed approach has been used to design a controller for motor torque and flux. Simulation tests are included.