Robots are fabulous machines in the sense that they perform tasks rapidly, precisely and without ever tiring. It is therefore not surprising that they have invaded manufacturing plants, taking part in the fabrication of a large number of products. Reprogrammable, versatile, powerful, robots however possess an important limitation: they are not intelligent. Indeed, despite the developments in artificial intelligence and what science fiction would like us to believe, robots are generally simple machines executing a list of operations described in detail by a human. This explains why industrial robots work only in environments where everything is controlled.

    However, many applications require the adaptability and judgment of humans. Assisted by a robot's force and endurance, humans can accomplish complex tasks in a more productive and ergonomic manner. The research field focussing on the design, control and use of robots interacting with humans is named human-robot collaboration. Figure 1 presents an application which consists in a human lifting a heavy box assisted by two robotic arms.

    Fig. 1: Example of human-robot collaboration (from the Handbook of Robotics).

    Robots designed to physically interact with humans are different from standard industrial manipulators. Indeed, because they are guided by humans, less precision is required for their displacements. On the other hand, they must interact in a safe and intuitive manner. A frequently used method to measure the human's intentions consists in placing a force/torque sensor on the robot. According to the measurements, the robot follows the user in a manner similar to a blind man following his guide dog.

    To illustrate this behaviour, consider Figure 2 where a child is drawing with the assistance of a robot. A force/torque sensor being installed on the pen-holder, the applied forces are measured and interpreted, after which the robot moves in the desired direction. This academic example is useless in practice (it is a lot simpler to draw without any robot!). However, the goal of human-robot collaboration is to offer the possibility to workers to displace bulky objects as easily and precisely as if they were drawing.

    Fig. 2: Child drawing with the help of a robot.