Concept Vehicles

The Mercedes-Benz ESF 2009 Experimental Safety Vehicle: PRE-SAFE, Braking Bag and Communication

Words Chris Danielson | June 10, 2009
For the very first time, inflatable metal side impact protection can be seen in the ESF 2009.
Words Chris Danielson June 10, 2009

PRE-SAFE Structure: It sounds like science-fiction – concealed metallic structures that wait patiently in a collapsed, space-saving state until they are required to go into action. Daimler researchers working together with the gas generator specialists at Autoliv spent two years actively researching such active metal support systems, and tested a variety of applications. For the very first time, inflatable metal side impact protection can be seen in the ESF 2009.

Imagine an inflatable mattress. When it is not needed, it is rolled up flat and e.g. consigned to a shelf in the attic. When inflated, however, it has a highly resistant structure that can easily carry a man weighing 100 kilograms. Inflatable metallic structures work in the same way: when not in use, the metal section is folded together to save space. Once its protective effect is needed, a gas generator just like those used to inflate airbags builds up an internal pressure of 10 to 20 bar within fractions of a second, the metal section is unfolded and the structure has significantly greater stability.

The advantages are obvious, and mainly involve packaging and weight: more stable structures can be accommodated within the increasingly tight installation spaces of an automobile, or weight can be greatly reduced while maintaining the same stability. Using the example of the side impact protection member in the doors of the S-Class, the researchers have calculated that around 500 grams less weight per door would be feasible.

Daimler safety researchers examined various applications for these innovative, crash-responsive metal structures, among them side impact protection, the side skirts and the seat cross-members. These have the advantage of being several centimetres away from the impact zone. The gas generator therefore only needs to be activated when a crash has definitely taken place.

One of the still unsolved problems of these protective members is that unlike the PRE-SAFE® measures already in series production, their active deformation is not reversible. Moreover, the activation of protective members installed well to the outside of the bodyshell that can be inflated by internal pressure requires their deployment before the crash. The pre-crash sensor system must therefore provide highly reliable signals.

Another hurdle is the currently still uncompetitive cost level of the required gas generators in relation to the cost requirements for weight-saving measures. These crash-responsive metal structures are therefore still a thing of the future – but the same was also once true of standard safety features like the airbag, ABS or ESP®.

Braking Bag: a braking parachute for the car

Airbags in cars have previously only been used as a restraint system for the occupants. In the future they might also be a PRE-CRASH- component, activating an auxiliary brake in the vehicle floor and improving both deceleration and compatibility with the other vehicle involved in the accident.

Energy is not only reducible by braking the road wheels: jet fighters and dragsters use braking parachutes, for example. And as early as 1952, Mercedes-Benz was already experimenting with an air-brake at the Le Mans race: when decelerating, the driver was able to move a metal panel on the roof of his racing SL to a vertical position. Even earlier, coachmen used special wheel chocks. These were placed in front of one of both rear wheels on long downhill gradients, and their iron-clad base helped to brake the vehicle during the descent.

This is an old idea that Mercedes safety researchers have revitalised on a similar principle with the Braking Bag, an airbag installed between the front axle carrier and the underbody panelling. If the sensor system concludes that an impact is inevitable, the PRE-SAFE® system not only initiates automatic emergency braking. At the same time the Braking Bag is deployed just before the crash, supporting the car against the road surface by means of a friction coating. The vehicle’s vertical acceleration increases the friction and has an additional braking effect before the impact. The Braking Bag uses the PRE-CRASH sensors in Mercedes-Benz cars, which are already able to initiate preventive occupant protection measures in critical driving situations.

There are several advantages to this unusual auxiliary brake:

  • The rate of deceleration is briefly increased to over 20 m/sec/sec. This scrubs additional energy beyond the potentials of a wheel brake, thereby reducing accident severity.
  • Because the car is raised upwards by up to eight centimetres within a short time, the dive effect that occurs with conventional brakes is substantially compensated. This improves geometrical compatibility with the other party in an accident.
  • This vertical movement also improves the effects of the restraint systems: the seats move towards the occupants by around three centimetres, which enables the belt tensioners to take up more slack. The high deceleration rate before the impact has a “pretensioning” effect on the occupants, so to speak.
  • Downward support for the vehicle during the crash reduces the typical diving motion during a collision.

All in all, the braking airbag has the effect of an additional crumple zone. Mercedes engineers have calculated that even at a low 50 km/h, the additional deceleration has the same effect as lengthening the front end by 180 mm. Initial driving tests in a C-Class have already shown the effectiveness of this new auxiliary brake – though it will still be some time before the Braking Bag becomes another component of the PRE-SAFE® system.

Interactive Vehicle Communication: cars report what their sensors have detected

Cars sometimes know more about their surroundings than their drivers. With the help of intelligent communication systems, vehicles themselves are able to contribute to improved road safety and mobility.

A patch of black ice on the next bend? A bank of fog three kilometres down the road? A new traffic tailback where roadworks are being carried out? What used to come as an unpleasant surprise is far less frightening if the approaching driver receives an up-to-date is warning beforehand. This is a task that will in future be carried out by the other vehicles on the roads at the time – automatically, by radio. This is the basic idea behind Interactive Vehicle Communication.

Cars are nowadays able to collect a great deal of information about the current driving situation, as the numerous sensors, cameras and control units for the dynamic and assistance systems can register e.g. poor weather conditions just as well as sudden braking and avoiding manoeuvres, or broken-down vehicles on the road. There are also other sources of information, for example local police reports. This information can be passed on via additional relay stations (“car-to-x”) such as radio masts at the roadside, stationary nodal points (e.g. traffic centres and overhead gantries) or via the internet. The onboard computer classifies all the reports according to plausibility and relevance. Tailback reports on the radio which are out-of-date or irrelevant to the individual driver will then be a thing of the past.

Mercedes engineers have been working on “Interactive Vehicle Communication” as a technology of the future for more than seven years. The ESF 2009 safety concept vehicle demonstrates the current status of this research: this Mercedes can automatically recognise an approaching police car, for example, and warn its driver by showing a symbol in the display. It is also possible to send and receive warnings of bad weather or obstacles in the road.

The exchange of data between vehicles is via so-called “ad hoc” networks, connections that are spontaneously formed between the vehicles over short distances. These wireless local area networks (WLANs) are self-organising, and require no external infrastructure. Transmission and reception is at a frequency of 5.9 gigahertz, over a distance of up to 500 metres. In fact the achievable communication range is much greater, as oncoming vehicles pass the messages on.

Cars that communicate with each other can do more than just pass on information: linked to modern proximity control systems such as DISTRONIC Plus from Mercedes-Benz, they can help to harmonise the traffic flow and avoid tailbacks by automatically selecting the most suitable vehicle speed when joining a motorway. And collisions can be avoided if onboard sensors recognise an impending accident and automatically regulate the distance.

This technology is currently demonstrating its practicality in the “Safe Intelligent Mobility – Test area Germany” project (simTD), in which Mercedes-Benz and other German manufacturers and suppliers are taking part. Up to 400 vehicles communicate with each other in these, the world’s largest field trials for Interactive Vehicle Communication. simTD is being conducted in the densely populated Frankfurt/Rhine-Main area from autumn 2008 to 2012. Experts expect usable mobile information networks with full coverage to become a possibility when around ten percent of all vehicles have this communications capability.