As we all know, the round has to go into the square. But not with 'RoboKeeper', a development by the Fraunhofer Institute for Material Flow and Logistics (IML) in Dortmund. In most cases, the artificial goalkeeper emerges victorious from the pitch - and this applies to footballers of all abilities.

The boundary conditions of the project, which was launched in 2005, differ from those of a real soccer match: the goalkeeper kicks from a distance of around 9 m and the goal measures 2 x 4 m². Thomas Albrecht, research associate at Fraunhofer IML and project manager of the 'RoboKeeper' development, explains: "The aim of the development was to keep the balls safe even when they are shot flat into the bottom right or left corner of the goal at a speed of 100 km/h." This corresponds to the maximum distance for the goalkeeper, who stands upright in the middle of the goal when shooting. At this speed, the pure flight time of the ball is around 360 ms, according to Albrecht.

"In this time, the expected trajectory and the point of arrival of the ball in the goal plane must be extrapolated and the complete movement of the goalkeeper, including all acceleration and deceleration processes of the motor, must be completed in order to prevent a hit," says Albrecht. The accuracy requirements are in the range of a few centimetres, as it is not crucial that the goalkeeper figure holds the ball in the middle: "The 'RoboKeeper' only has to hit the ball in such a way that it is deflected and does not land in the goal."

Fast image processing

Two Prosilica area scan cameras from Allied Vision are the "eyes" of the 'RoboKeeper'.

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A fast image processing system serves as the robot goalkeeper's "eyes". Two 'Prosilica' color area scan cameras from Allied Vision Technologies with Gigabit Ethernet interfaces and a resolution of 659 x 493 pixels are mounted on the side above the goal. They focus on the ball and track its trajectory. The ball must stand out from its surroundings in terms of color.

The cameras each deliver up to 90 images per second, which are then evaluated by image processing software developed by Fraunhofer IML on a dual-core processor system from Kontron. If a movement in the direction of the goal is detected during the evaluation of three consecutive images, the first data is transmitted to the motor controller, which is responsible for the movement of the 'RoboKeeper' and positions the Robo goalkeeper.

Based on the calculations and with the help of a high-performance motor, the 'RoboKeeper' only needs around 300 ms to reach the calculated defensive position. It accelerates around 20 times faster than a Formula 1 racing car. The system has seven different difficulty levels and thus adapts to the performance level of the players.

If the system is set up outdoors, the sunlight and therefore the illuminance of the scene varies considerably over the course of the day. For this reason, camera systems equipped with automatic iris control and a video signal-controlled motorized iris were used. The 'CVO GAT23516AC' lenses from Goyo Optical used have a focal length of 3.5 mm and are wide enough to ensure correct ball detection from kick-off to goal.

The image processing setup consisting of the two cameras, the associated motorized lenses and the cables was supplied by Stemmer Imaging.

Fancy a duel?

During the 2018 FIFA World Cup, fans can try their luck on Red Square in Moscow. In addition to the system described above, other versions are also available: a scaled-down version with a shorter shooting distance and reduced goal size, as well as variants for field hockey, ice hockey and handball.