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.