Sensitive robots

Robots are commonplace in production halls, but are only allowed to operate in protected areas so as not to endanger humans with their movements. A new cost-efficient, robust force sensor can make robots sensitive to potential collisions.

The arm of the industrial robot steadily approaches the employee, who is so absorbed in his work that he doesn't notice - a risky situation. But as soon as the robot even slightly touches the person, it immediately retracts its steel arm. This vision could soon become reality thanks to a cost-efficient force and torque sensor developed by research scientists at the Fraunhofer Institute for Silicon Technology ISIT in Itzehoe. The sensor sits on the outer joint of the robot's arm. Glued onto a steel plate, the transducer, it can be screwed in between the arm and the grabber. 'We expect our sensors to be far cheaper than conventional force sensors once they enter mass production. This makes them suitable for wide-scale use,' says ISIT head of department Joerg Eichholz. Equipped with the new sensors, robotic assistants would be sufficiently trustworthy to work alongside their human colleagues - something that has been prohibited until now for safety reasons.

The sensor measures the forces and torques exerted by the robot arm. 'It functions in a similar way to a strain gauge: its core element is a long wire through which an electric current flows. If the wire stretches, it becomes longer and thinner - the resistance increases and so less current flows through it,' says Eichholz. 'Our sensor is made from a single square piece of silicon. On each side, we have incorporated bridges carrying electrical resistances.' If the robot arm bumps into an obstacle, the shape of the silicon changes very slightly - by just a few micrometres, to be precise. This causes either more or less current to flow, depending on whether a bridge has been stretched or buckled. Because the sensor consists of just a single piece of silicon, it is less error-prone than its conventional counterparts. Manufacturers normally glue the resistances on separately, which means they are often positioned somewhat inaccurately. 'There is no chance of this happening in the case of our sensor. The resistances are precisely aligned,' says the expert. The system's size can be varied.

The sensor can also help to program a robot. In learning mode, it measures the force with which the employee guides the robot arm. Instead of laboriously entering the coordinates of the movements into the computer, the employee can simply guide the robot by touch and teach it the required motion sequences in this way. The researchers will present a prototype of the sensor at the Sensor + Test trade fair (Hall 12, Stand 688) in Nuremberg from 26 to 28 May.