Giovanni Tonietti

Fast and "soft-arm" tactics [robot arm design]

This article considered the problem of designing joint-actuation mechanisms that can allow fast and accurate operation of a robot arm, while guaranteeing a suitably limited level of injury risk. Different approaches to the problem were presented, and a method of performance evaluation was proposed based on minimum-time optimal control with safety constraints. The variable stiffness transmission (VST) scheme was found to be one of a few different possible schemes that allows the most flexibility and potential performance. Some aspects related to the implementation of the mechanics and control of VST actuation were also reported.

Design and Control of a Variable Stiffness Actuator for Safe and Fast Physical Human/Robot Interaction

This paper is concerned with the design and control of actuators for machines and robots physically interacting with humans, implementing criteria established in our previous work [1] on optimal mechanical-control co-design for intrinsically safe, yet performant machines. In our Variable Impedance Actuation (VIA) approach, actuators control in real-time both the reference position and the mechanical impedance of the moving parts in the machine in such a way to optimize performance while intrinsically guaranteeing safety. In this paper we describe an implementation of such concepts, consisting of a novel electromechanical Variable Stiffness Actuation (VSA) motor. The design and the functioning principle of the VSA are reported, along with the analysis of its dynamic behavior. A novel scheme for feedback control of this device is presented, along with experimental results showing performance and safety of a one-link arm actuated by the VSA motor.

Compliant design for intrinsic safety: general issues and preliminary design

We describe some initial results of a project aiming at the development of a programmable compliance, inherently safe robot arm for applications in anthropic environments. In order to obtain safety in spite of worst-case situations (such as unexpected delays in teleoperation, or even controller failure), we propose an approach to achieving the compliance by mechanical rather than by control design. We first describe some of the control problems encountered in a typical, large, possibly unknown mechanical compliance, and present the result that shows the possibility to cope with these uncertainties in an adaptive way. Next, we describe the initial development of a new prototype arm under construction in our laboratory. The arm is designed to achieve arbitrary position tracking in 3D with controlled effective compliance at the joints.

Variable Stiffness Actuators for Fast and Safe Motion Control

In this paper we propose Variable Stiffness actuation [1] as a viable mechanical/ control co-design approach for guaranteeing control performance for robot arms that are inherently safe to humans in their environment. A new actuator under development in our Lab is then proposed, which incorporate the possibility to vary transmission stiffness during motion execution, thus allowing substantial motion speed-up while maintaining low injury risk levels.

Physical human-robot interaction: Dependability, safety, and performance

Robots designed to share an environment with humans, such as e.g. in domestic or entertainment applications or in cooperative material-handling tasks, must fulfill different requirements from those typically met in industry. It is often the case, for instance, that accuracy requirements are less demanding. On the other hand, concerns of paramount importance are safety and dependability of the robot system. According to such difference in requirements, it can be expected that usage of conventional industrial arms for anthropic environments will be far from optimal. An approach to increase the safety level of robot arms interacting with humans consists in the introduction of compliance at the mechanical design level. In this paper we discuss the problem of achieving good performances in accuracy and promptness with a robot manipulator under the condition that safety is guaranteed throughout whole task execution. Intuitively, while a rigid and powerful structure of the arm would favor its performance, lightweight compliant structures are more suitable for safe operation. The quantitative analysis of the resulting design trade-off between safety and performance has a strong impact on how robot mechanisms and controllers should be designed for human- interactive applications. We discuss few different possible concepts for safely actuating joints, and focus on aspects related to the implementation of the mechanics and control of this new class of robots.

Adaptive simultaneous position and stiffness control for a soft robot arm

In this paper, an independent joint position and stiffness adaptive control for a robot arm actuated by McKibben artificial muscles is reported. In particular, muscular and dynamic parameters of the system are supposed unknown. Adaptive control performance is tested in a one degree of freedom experimental setup and compared with PID control performance. The adaptive control scheme is then applied to a robot arm that is conceived to perform tasks in anthropic environments. The adaptive control developed is such that performance of the robot arm is very similar to human arm performance. Experimental results are reported.

Optimal Mechanical/Control Design for Safe and Fast Robotics

The problem to ensure safety of performant robot arms during task execution was previously investigated by authors in [1], [2]. The problem can be approached by studying an optimal control policy, the “Safe Brachistocrone”, whose solutions are joint impedance trajectories coordinated with desired joint velocities. Transmission stiffness is chosen so as to achieve minimum–time task execution for the robot, while guaranteeing an intrinsic safety level in case of an unexpected collision between a link of the arm and a human operator. In this paper we extend this approach to more general classes of robot actuation systems, whereby other impedance parameters beside stiffness (such as e.g. joint damping and/or plasticity) can vary. We report on a rather extensive experimental campaign validating the proposed approach.

Safe and fast actuators for machines interacting with humans

This paper describes a new generation of actuators for robotic applications, and more generally for machines that are designed to interact with humans. Such actuators, called variable impedance actuators, are designed to achieve fast motion control while guaranteeing safety of human operators in worst-case impact situation. The fundamental innovation is to implement safety by purely mechanical, passive means, to guarantee intrinsic safety, while active control is used to recover performance. The design concept, which is the subject of a patent application, has led to the experimental implementation of a variable stiffness actuator. The effectiveness of the VSA has been recently validated theoretically and experimentally by authors.

Integrating Two Haptic devices for Performance Enhancement

This paper deals with a new configuration for a haptic system, which is able to simultaneously replicate independent force/displacement and force/area behaviors of a given material. Being force/area information a relevant additional haptic cue for improving softness discrimination, this system allows to extend the range of materials whose rheology can be carefully mimicked. Moreover, according to the Hertz theory, two objects with different curvature radius having the same force/displacement behavior can respond with different contact area to the same applied force. These behaviors can be effectively replicated by the integrated haptic system here proposed enabling and independent control of force/displacement and force/area. The system is comprised of a commercial device (delta haptic device) serially coupled with a contact area spread rate (CASR) device. Two specimens of a material and two of another one, all with different curvature radii, were identified and modeled in terms of force/area and force/displacement. These behaviors were successfully tracked by the integrated haptic system here proposed

A Comparative Dependability Analysis of Antagonistic Actuation Arrangements for Enhanced Robotic Safety

In this paper we introduce an analysis of dependability of an elementary yet critical component of robotic systems designed to operate in environments shared with humans, i.e., the joint-level actuation system. We consider robot joints that implement the variable impedance actuation (VIA) paradigm. The VIA has been demonstrated to be an effective mean to achieve high performance while constantly keeping injury risks to humans by accidental impacts below a given threshold. The paper describe possible implementations of the VIA concept which use the Antagonistic Actuation (AA) in three different arrangements. This study follows a previously reported paper dealing with safety. Here a detailed comparative dependability and performability analysis in front of possible specific failure modes is conducted, whose results provide additional and useful guidelines for design of safe and dependable actuation systems for physical human-robot interaction.