Active seat-belt buckle is a new innovation for the rear seats which is currently under development by safety pioneer Mercedes-Benz and will shortly go into series production in one of the Stuttgart manufacturer’s luxury-segment models. An electric motor extends and retracts the seat belt buckle automatically. In this way, the belt slack in the area of the pelvis and thorax can be reduced and passengers are secured more firmly in sideways and lengthways directions. Active seat-belt buckle fits in with Mercedes-Benz’s integrated safety concept. In keeping with an all-embracing approach to safety, this concept covers all phases of automotive safety: from safety during driving through safety in hazardous situations and protection in the event of an accident to minimisation of the consequences of accidents.
The innovation is typically Mercedes-Benz, improving safety and comfort in one. Fastening seat belts in the rear is also made much simpler: the seat belt buckle emerges from the upholstery when the rear doors are opened and is provided with an illuminated insertion slot. Simplifying use of the seat belt in this manner may increase the percentage of rear passengers who buckle up. The innovation also makes it easier to assist children or people who require help in fastening their seat belts.
Active seat-belt buckle is also integrated in the PRE-SAFE® preventive safety system. This means that whenever PRE-SAFE® becomes active in critical driving situations or when a critical distance from other objects is detected, reversible belt tensioning is also effected in the rear via the active seat-belt buckle function. The restraint system and the passengers are thus better prepared for a possible accident situation. Active seat-belt buckle complements the pyrotechnic belt tensioning which has featured as standard on all outer rear seats in Mercedes-Benz vehicles for many years now. This pyrotechnic system is triggered in the event of actual impact.
The Mercedes safety experts have developed active seat-belt buckle using detailed computer models of the human body, as dummies are only able to simulate the human pelvic system along very rough lines.
“Active seat-belt buckle is another element of our ‘PRE-SAFE®’ concept, which is intended to provide our customers with optimum protection in real accident scenarios,” stresses Prof. Dr. Ing. Rodolfo Schöneburg, Head of Passive Safety and Vehicle Functions at Mercedes-Benz Cars. “After all, we aim to offer a high standard of safety not only in all model series, but also in all seats.” Active seat-belt buckle has also been developed with an eye on new markets in which the level of occupied rear seats is up to 30 percent – markedly higher than in Europe. “In the rear of premium saloons in particular, passengers tend to sit in particularly relaxed positions in the generous space which is available,” explains Schöneburg. “Active seat-belt buckle as part of PRE-SAFE® unlocks the full potential of the on-board restraint systems.”
Integration in the vehicle’s PRE-SAFE® system
Active seat-belt buckle is a moving belt buckle which can be extended by 70 millimetres and retracted by 40 millimetres via an electric motor. The belt buckle is attached via a cable to a spindle nut which runs on a spindle, translating the electric motor’s rotary movement into a linear movement.
The following functions are carried out by means of this movement:
Seat belt reminder/easier fastening of the seat belt: Active seat-belt buckle is provided with an illuminated insertion slot, attracting the occupants’ attention to the buckle. Particularly in the dark, the belt buckle is easier to find and to connect with the seat belt tongue. In addition, the belt buckle is extended by 70 millimetres when occupants enter the rear of the vehicle, to facilitate fastening of the seat belt. In extended state the belt buckle is more readily accessible and it is easier to insert the belt tongue. Both functions are activated when the doors are opened by passengers entering the rear of the vehicle.
Minimised belt slack: after the belt has been inserted, the belt buckle returns to its original position. This reduces any belt slack in the area of the pelvis and thorax, and the belt fits correctly in the pelvis area.
PRE-SAFE® function for rear occupants: Active seat-belt buckle is fully integrated in the vehicle’s PRE-SAFE® system. Whenever PRE-SAFE® becomes active in critical driving situations or when a critical distance from other objects is detected, reversible belt tensioning is also effected in the rear via the active seat-belt buckle function. The restraint system and the passengers are thus better prepared for a possible accident system. In contrast to its extended position to facilitate fastening of the seat belt, the belt buckle is retracted by approx. 40 millimetres. This reversible function increases the belt tensioning on the occupant by up to 80 millimetres. In addition, the belt buckle branches off at a lower point at the occupant’s hip. This reduces the risk of the pelvis pushing through under the belt. Securing the occupants more effectively in this way reduces the stress to which they are exposed in the event of an accident. Active seat-belt buckle is set to replace the pyrotechnic belt tensioning which has featured as standard in Mercedes-Benz vehicles for many years now. This pyrotechnic system is triggered in the event of actual impact.
POST-SAFE functionality: to facilitate the rescue of occupants and to assist rescue personnel or the occupants themselves in unbuckling the seat belts, the belt buckle is extended once again as soon as the doors are opened by rescue personnel or the occupants themselves after the system has detected a crash and verified that the vehicle is stationary, provided that the on-board electrical system is intact. The raised belt buckle position allows better access from inside the vehicle, while illumination of the buckle draws attention to the belt’s opening mechanism.
In keeping with Mercedes-Benz’s integral safety concept, active seat-belt buckle covers all four areas of automotive safety – “Safe driving”, “In the event of danger”, “In an accident” and “After an accident”.
Virtual human model as an unconventional development tool
The Mercedes-Benz safety experts have developed active seat-belt buckle with the aid of virtual human models, which provide a clearer picture of what happens to a vehicle’s occupants in an accident than crash-test dummies. These digital models simulate not only the human body’s outer form, but also its internal structures, such as bones and soft tissue.
“All the crucial biological features of humans – joints, muscles, tendons, ligaments, bones – can only be simulated in very rough terms with dummies,” explains Dr Hakan Ipek, expert for virtual human models at Mercedes-Benz. “Some seated positions, such as when a rear passenger is dozing and the belt does not pass over the pelvis in the correct manner, simply cannot be recreated with a dummy,” he adds by way of example.
With virtual human models, the biomechanical characteristics of the human body are simulated in detail on a computer, enabling examination of the stress to which the model is exposed in a virtual crash test.
In 1981 Mercedes-Benz was the world’s first automobile manufacturer to present the airbag and belt tensioner as restraint systems to the public in a series-production car. These two milestones in passive safety were premiered in a W 126-series S-Class Saloon at the 1981 Geneva Motor Show. At Mercedes-Benz, this began the introduction of the modern airbag as a passive safety feature into the entire passenger car range, with the airbag and belt tensioner already becoming available as optional extras for all Mercedes-Benz cars in 1982. By 1992 a driver airbag was standard equipment in all Mercedes-Benz models, followed by a front passenger airbag as a standard safety feature in 1994, and afterwards Mercedes-Benz realised numerous other applications for its airbag technology.
Mercedes-Benz research on the airbag began in 1966, with practical trials starting in 1967. This was one of the company’s responses to the large increase in accident figures in the 1960s. In previous years Mercedes-Benz had already set standards in passive safety with innovations such as the safety bodyshell with a rigid passenger cell and crumple-zones at the front and rear. The airbag also became a decisive component in the brand’s continuous commitment to improved vehicle safety.
Research on the new restraint system was given even more impetus by plans in the United States of America to prescribe an automatic occupant protection system for all cars from 1969. Airbags were seen as a very promising technology with which the new legal requirements could be met. The principle of the airbag as protection for the driver and front passenger during an accident was already patented in the 1950s. The principal pioneers were the German Walter Linderer (Patent DE 896312 of 6 October 1951) and the American John W. Hedrik (Patent US 2649311 of 18 August 1953).
For more than ten years the “inflatable container in a folded state, which automatically inflates in the event of danger” (Linderer’s description of his invention in the patent application) now became an object of research with the aim of bringing it to series production maturity. The work of Mercedes-Benz engineers and their colleagues working for other automobile manufacturers and suppliers during the 1960s was initially basic research. Equipment with which the idea from the 1950s might be realised did not exist at the time.
Particularly the necessary sensor systems and gas generation continued to present major challenges to the developers. When American manufacturers began to deliver their first test fleets with compressed-air operated airbags, these restraint systems – which were conceived as an alternative to seat belts – sometimes led to serious injuries and in a few cases even fatalities. For this reason the general introduction of airbags in all passenger cars in North America, which had initially been urgently demanded, was repeatedly postponed.
Meanwhile Mercedes-Benz in Stuttgart was working on an airbag technology that was different in many respects. The safety specialists at Mercedes-Benz opted for a gas generation system based on chemical propellants rather than pressurised gas. Neither was the airbag developed as a stand-alone restraint system, but instead as a feature working together with the seat belt. This was expressed in the internationally used abbreviation SRS, which stands for “Supplemental Restraint System”. As early as 1970, Mercedes-Benz commented on its accident research findings in a letter to a German motoring magazine: “The effectiveness of the airbag system in combination with a lap-belt and head restraint during a frontal or rear-end collision can be described as good.”
From 1967 onward, practical testing of gas generation systems continued with chemicals similar to the solid fuels used for rockets. In contrast to gas-filled cartridges, this form of propellant proved to be a reliable and rapid gas generator. The resulting gas mixture mainly consisted of nitrogen, inflating the airbag made from special woven material within fractions of a second so that it could gently cushion passengers as they were thrown forward by the impact in an accident.
The central findings from these early trials were formulated in patent no. DE 2152902 C 2, which the then Daimler-Benz AG filed on 23 October, 1971. This patent application was the key document for all subsequent airbag development at Mercedes-Benz. It already contained the operating principle of the new technology as it was to be implemented in series production ten years later: sensors register the particularly severe deceleration common to collisions and activate the airbag mechanism. This ignites a propellant charge (at the time consisting of sodium azide = hydrazoic acid salt, potassium nitrate = nitric acid salt and sand), which explodes to form mainly gaseous nitrogen and a little water and oxygen.
As the tests showed, the airbag really only achieved its full effectiveness in combination with the seat belt. And it was the seat belt that played a decisive role in the second innovation presented by Mercedes-Benz in Geneva in 1981 – the belt tensioner. Initially this was developed and made available for the front seat passenger, but by 1984 the belt tensioner already became standard equipment for the front seats of all Mercedes-Benz passenger cars. Like the airbag, the belt tensioner is activated by pyrotechnics: during an accident, the control unit fires a propellant charge which tightens the seat’s three-point inertia-reel seat belt within milliseconds. This eliminates the slack between the occupants torso and the seat belt, the occupant is held firmly in the seat by the belt and the forces generated by the kinetic energy of the collision are immediately transferred to the belt. From 1995 onward, belt tensioners were combined with belt force limiters in all models to adapt the action of the restraint system to individual requirements. In 2002 an electronic belt tensioner was added to the pyrotechnical belt tensioner with the introduction of the preventive occupant protection system PRE-SAFE®.
The evolution undergone by the airbag was even more extensive: the driver airbag became standard equipment in all Mercedes-Benz passenger cars in 1992, and in 1994 it was followed by the front passenger airbag. As airbag modules became smaller and smaller thanks to the work of the engineers, it was possible to incorporate them in other areas of a vehicle to achieve comprehensive protection in the event of a side impact as well. In 1993 Mercedes-Benz presented a sidebag as a study, and in 1995 the sidebag became available as an optional extra, initially in the E-Class. The windowbag became standard equipment from 1998, at first in the S-Class. In 2001 the head/thorax sidebag was introduced for the roadsters in the Mercedes-Benz SL-Class. Adaptive airbags form part of the safety philosophy PRO-SAFE™ in the W 221-series S-Class. And in 2010 the experimental safety vehicle ESF 2009 from Mercedes-Benz showed a totally new form of airbag: the Braking Bag is accommodated in the vehicle’s underbody, and is activated just before a collision. It has a friction coating to support the vehicle against the road surface, providing additional deceleration prior to the impact.
Production of the first S-Class models with an airbag and belt tensioner began at the Sindelfingen plant in 1980; after its presentation in Geneva, the 126 series went on sale with the new feature in July 1981. The combination of a driver airbag and front passenger belt tensioner was initially restricted to the S-Class, and was priced at DM 1525.50 as an optional extra for the Saloon and Coupé. By way of comparison, the flagship model in the S-Class at the time, the model 500 SEL, cost DM 61,505.90 in July 1981. In the second half of 1981 the Coupé with the same engine, the C 126-series model 500 SEC, was to be had for DM 73,902.
In the first year after introduction of the airbag and belt tensioner, 2636 S-Class customers already decided in favour of the new safety feature. The airbag was on its way to resounding success. More than twelve million Mercedes-Benz vehicles were equipped with the new airbag technology in the first 25 years after its presentation, with other car brands immediately following suit. The optional extra of 1981 became a standard restraint system that acts together with other passive safety features and has since saved thousands of lives in accidents.