Sami Tliba

An event-controlled online trajectory generator based on the human-robot interaction force processing

Purpose – The problem of robotic co-manipulation is often addressed using impedance control based methods where the authors seek to establish a mathematical relation between the velocity of the human-robot interaction point and the force applied by the human operator (HO) at this point. This paper aims to address the problem of co-manipulation for handling tasks seen as a constrained optimal control problem. Design/methodology/approach – The proposed point of view relies on the implementation of a specific online trajectory generator (OTG) associated with a kinematic feedback loop. This OTG is designed so as to translate the HO intentions to ideal trajectories that the robot must follow. It works as an automaton with two states of motion whose transitions are controlled by comparing the magnitude of the force to an adjustable threshold, in order to enable the operator to keep authority over the robots states of motion. Findings – To ensure the smoothness of the interaction, the authors propose to generat...

Dealing with actuator saturation for active vibration control of a flexible structure piezo-actuated

We present a study on the active control of vibrations of a flexible mechanical structure in presence of saturation affecting the closed-loop control signal. The application we consider here is a system composed of a flexible beam with one edge clamped on a movable support and the other kinematically free. Two piezoelectric patches are bonded on the beam: one of them is used as an actuator and the other is used as a sensor. First, we show how we perform a linear ℋ2 synthesis for the design of a controller permitting the active vibration damping with good performances. Then, we analyse how a saturation of the control signal causes a degradation of these performances. The main goal of this work is to design an anti-windup compensator which, combined with the linear controller, permits to recover or to enhance some linear closed-loop performances in presence of this non-linearity.

Control of a Vibrating Axisymmetric Membrane Using Piezoelectric Transducers

Abstract In this paper, the problem of the active vibration control of a thin and flexible disc is addressed. The mechanical structure tackled here is equipped with two piezoelectric circular patches: one of them works as a sensor and the other is used as an actuator. Both are fixed on the disc, one on each side, and centered according to its axis of symmetry. The purpose of this work is to design a controller allowing the active damping of the most vibrating modes in a specified bandwidth. Robustness issues against neglected dynamics are discussed. After describing the problem, we first discuss on the model properties, derived from a finite element analysis, particularly about the structures symmetry consequences. Then, we propose a control method leading to the reduction of several modes of vibration. Numerical simulations are proposed to analyze the modelling and the vibration control efficiency.

Anti-windup augmented controller for active vibration control in a smart flexible structure

In this paper, we propose to explore some recent results on the control of systems with saturations on control input. The purpose is to carry out the active vibration control in a classical smart flexible structure. The practical problem considered here is that of a flexible beam with one edge embedded into a movable support and the other free. This beam is equipped with two piezoelectric patches: one of them is used as an actuator and the other is used as a sensor. For reasons of experimental implementation, the control input is constrained by a symmetric and bipolar saturation at a given level. The main goal of this work is to design an anti-windup augmented controller in order to recover some linear closed-loop performances in the presence of this non-linearity.

H ∞ Controller Design for Active Vibration Damping of a Smart Flexible Structure Using Piezoelectric Transducers

Abstract This paper deals with robust control of a smart flexible structure. The purpose is to reduce the vibrations. The considered structure is plate like. It is equipped with several thin piezoelectric patches. Some of them are used as sensors and the others as actuators. They are optimally positioned and not collocated. The main goal is to reduce the most energetic vibrating modes. By using a state-space representation of a MIMO model of the equipped structure, derived from Finite Elements Modeling and modal analysis, a criterion is proposed to determine the most energetic modes. It is used to obtain a reduced order model for controller design. An H ∞ synthesis including several constraints, such as pole assignment or high-frequencies roll-off specification, is proposed to design a low-order robust control law.

Parallel guiding virtual fixtures: control and stability

Guiding virtual fixtures have been proposed as a method for human-robot co-manipulation. They constrain the motion of the robot to task-relevant trajectories, which enables the human to execute the task more efficiently, accurately and/or ergonomically. When sequences of different tasks must be solved, multiple guiding virtual fixtures are required, and the appropriate guide for the current task must be detected automatically. To this end, we propose a novel control scheme for multiple guiding virtual fixtures that are active in parallel. Furthermore, we determine under which conditions using multiple fixtures is stable. Finally, we perform a pilot study for a real-world application with a humanoid robot.

An Online Trajectory generator-Based Impedance control for co-manipulation tasks

This paper addresses the problem of heavy load co-manipulation in the context of physical human-robot interactions (PHRI). During PHRI, the resulting motion should be truly intuitive and should not restrict in any way the operators will to move the robot such he would like. The idea proposed in this paper consists in considering the PHRI problem for handling tasks as a constrained optimal control problem. For this purpose, we introduce a modified impedance control method named Online Trajectory generator-Based Impedance (OTBI) control. This method relies on the implementation of a specific event controlled online trajectory generator (OTG) combined with control structure allowing a good tracking of the generated trajectory with a desired impedance property of the physical interaction. This OTG is designed so as to translate the human operator (HO) intentions into ideal trajectories that the robot must follow, while enabling the HO to keep authority over the robots states of motion. The key idea of this approach consists in generating a velocity trajectory for the end-effector that stay collinear to the interaction force at every moment. The overall strategy is applied to a two DOF robot.

On Human-Robot Co-Manipulation for Handling Tasks: Modeling and Control Strategy

In this paper, we address the co-manipulation problem for the handling tasks through a viewpoint that we do not think sufficiently explored, even it employs classical tools of robotics. Usually, this problem is tackled through the control objective based on impedance control. We consider that this force is the only exchanged physical signal, showing the robot how to move according to the willingness of the human operator. Indeed, this force gives the desired direction of displacement at every moment, a displacement that should remain under the operators authority. Moreover, for safety purposes, the velocity of displacement should be imposed to avoid hazardous behavior. As a result, dealing with co-manipulation consists in realizing two functions in order to perform a controlled power amplification. The first is the online trajectory generation of an appropriate trajectory. The other lies in the design of a control structure allowing a good trajectory tracking. The overall strategy is illustrated with the co-manipulation problem of a robot that handles a pendulous load.

Co-manipulation with a library of virtual guiding fixtures

Virtual guiding fixtures constrain the movements of a robot to task-relevant trajectories, and have been successfully applied to, for instance, surgical and manufacturing tasks. Whereas previous work has considered guiding fixtures for single tasks, in this paper we propose a library of guiding fixtures for multiple tasks, and propose methods for (1) creating and adding guides based on machine learning; (2) selecting guides on-line based on probabilistic implementation of guiding fixtures; (3) refining existing guides based on an incremental learning method. We demonstrate in an industrial task that a library of guiding fixtures provides an intuitive haptic interface for joint human–robot completion of tasks, and improves performance in terms of task execution time, mental workload and errors.

Vibration damping of a flexible beam with saturated control

This paper contains a study on the vibration control of a flexible beam equipped with a piezoelectric actuator driven by a bounded control signal. This problem is tackled in two stages. First, a linear H∞ synthesis with pole placement constraint is performed for the design of a controller permitting an efficient vibration damping. Then, an anti-windup compensator design is proposed by using two different approaches that are compared, in order to improve closed-loop performances in the saturated operating range. Because of experimental purposes of this work, the amplitude of the dead-zone signal should be bounded. So, among the main sought properties of this compensator, one consists in imposing a bound on the closed-loop dead-zone signal in order to avoid numerical problems that could arise during experimental implementation of the anti-windup compensator.