An impact sensor for cars that can instantly recognise and protect pedestrians in an accident has been developed by engineers in Germany.

If the sensor detects that a pedestrian has been struck, the hood (bonnet) of the car is immediately pushed a few centimetres upwards by a system of hydraulics. This creates a vital buffer zone between the pedestrian and the car’s suspension turrets and engine block. Crash tests have shown this buffer can prevent a pedestrian receiving a serious head injury on impact.

To prevent false alarms popping up the hood, the system distinguishes between pedestrians and other objects on the basis of their mass. It can differentiate the mass of a small child and that of a traffic cone, says Johannes Vetter of Siemens Restraint Systems, the company behind the design.

The key to the system is a set of mirror-coated fibre optics in the car’s fender (bumper). These are broken on impact, meaning that light that leaks out, and the amount of light detected by a sensor at one end decreases. An onboard computer uses this loss and the vehicle’s velocity to calculate the mass of the obstacle struck.

A similar calculation can be made using an accelerometer located in the fender (New Scientist print edition, 30 June 2001). But Vetter claims the optical system is much faster and more reliable.

The car’s hood is tuned to respond only if a pedestrian or something heavier is hit. This should prevent the system activating when the car is struck by something lighter, like a football. The sensor is designed to operate at speeds between 20 and 60 kilometres per hour, at which most pedestrian accidents occur.

Andrew Morris, a researcher at the UK Vehicle Safety Research Centre at Loughborough University, says a number of vehicle manufacturers are hoping to develop similar pedestrian impact systems.

Some use radar or infrared to detect an obstacle, but this has proven unreliable. “They have difficulty determining whether it’s a paper bag blowing across the road, a dog or a pedestrian,” Morris told New Scientist.

A prototype car incorporating the Siemens fender system will go on show at the 60th International Motor Show in Frankfurt, Germany, starting on Thursday.
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