Physical Human-Robot Interactions: Dealing with the Safety-Performance Trade-Off in the Mechanical/Control Co-Design

Prof. Antonio Bicchi

 

 

ABSTRACT

 

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, a concern of paramount importance is 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. The inherent danger to humans of conventional arms can be mitigated by drastically increasing their sensorization and changing their controllers. However, it is well known in the robotics literature that there are intrinsic limitations to what the controller can do to modify the behaviour of the arm if the mechanical bandwidth (basically dictated by mechanism inertia and friction) is not matched to the task. In other words, making a rigid, heavy robot to behave gently and safely is an almost hopeless task, if realistic conditions are taken into account. One alternative approach at increasing the safety level of robot arms interacting with humans is to introduce compliance right away at the mechanical design level. Accuracy in positioning and stiffness tuning would then be recovered by suitable control policies. This approach is clearly closer in inspiration to biological muscular apparatuses than to classical machine-tool design, which has inspired most robotics design thus far In this talk I will discuss the problem of achieving good performance in accuracy and promptness by a robot manipulator under the condition that safety is guaranteed throughout task execution. Intuitively, while a rigid and powerful structure of the arm would favour its performance, lightweight compliant structures are more suitable to safe operation. The quantitative analysis of the resulting design trade-off between safety and performance is one of the objectives of our work. Such analysis 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 our attention on one, the Variable-Stiffness Transmission (VST) approach. Some aspects related to the implementation of the mechanics and control of VST joints will be reported.