Safety

Restraint Systems

Seatbelt Systems: State of the Art and Future Trends By Hans-Otto Kock With the current airbag euphoria, the seatbelt is somewhat out of the limelight. In fact, some very effective innovations have been developed in the past decade, such as the pretensioner and the load limiter. The Autoliv group, an international corporation with its head office in Sweden, is very active in this field. With offices and production plant in Braunschweig, Dachau, Döbeln, Elmshorn and Norderstedt, in addition to three crash simulation facilities in Germany, Autoliv is the leading supplier of passenger restraint systems to the German automotive industry. 1

Introduction

Seatbelts came into existence before 1900 – for horse-drawn cabs. And as long ago as 1908, Richard Radtke applied for a patent on a seatbelt with a load limiter. The static three-point belt was invented by a Swede, Nils Ivar Bohlin, in 1959. It was developed into the automatic belt, with requirements as defined in EEG Regulation No. 16. Belts constructed to this standard meet all the strength demands, but are not optimised from the biomechanical point of view. That was normal until the early 90s. But in 1986, Autoliv had marketed the first pretensioner, which gradually became fitted even to the high-volume models. It is largely thanks to Autoliv that a passive restraint system, the seatbelt, has now become an adaptive safety component in conjunction with the airbag.

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The Pretensioner

The pretensioner has the job of keeping the belt webbing in close contact with the body, so that in the event of a crash it can

You will find the figures mentioned in this article in the German issue of ATZ 10/99 beginning on page 780.

start arresting the person’s forward motion as soon as possible. It can take the form of a retractor or buckle pretensioner: both systems are equally well developed and have almost exactly the same effect. The difference is to some extent the given manufacturer’s traditional preference, and perhaps also a case of packaging. Either way, the pretensioner must be capable of producing its full effect within 12 or 15 milliseconds of the start of rapid deceleration. The amount of pretensioning depends on the slack in the belt – a loosely worn belt requires more pretensioning. The pretensioner is activated by a small explosive charge which generally drives a piston connected to a steel cable that transmits the force to the tensioning mechanism. However, Autoliv also produces a rotational pretensioner, Figure 1, in which steel balls contained in a curved tube transmit the gas pressure resulting from the charge to a pinion. This has a higher efficiency than conventional pretensioners, and is also more compact.

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consists of a torsion bar built into the retractor shaft; above a certain force it starts to give, so that the tension in the belt does not rise above a defined limiting value. The torsion rod allows a more linear rise in force than other limiter systems. The maximum force in the shoulder strap can be reduced from about 8 to 12 kN to 4 kN, since the airbag absorbs the remaining kinetic energy of the occupant. The controlled extension of the belt means that a person’s body is restrained more gently, at first by the belt and then by the airbag, so that the risk of injury to the chest and head is reduced. Tests carried out by Autoliv have shown that the head injury criterion (the HIC) is reduced by roughly half when a load limiter is used. In sled tests, the measured load on the chest fell from 50 g to 41 g (g being the acceleration due to gravity). These values depend on the construction of the car, of course. ‘Softer’ structures result in gentler deceleration, while ‘firmer’ structures mean a higher force over a shorter time. The current tendency towards more rigid structures, particularly in the smaller cars, requires ever greater development effort on the part of the suppliers of restraint systems if the crash standards are to be met. Autoliv’s next innovation will therefore be a more sophisticated load limiter in order to implement an even more harmonious force/time progression. Once the seatbelt and the airbag start to act together, the restraining effect is considerably increased, which leads to higher forces on the chest, Figure 2. Thus, the ideal is a design in which the load limiter allows the belt to exert a high force to start with, but reduces the load the moment the person makes contact with the airbag.

Load Limiter

An important factor in the restraint system is the airbag, which has now penetrated almost every sector of the market. The seatbelt and the airbag were each developed independently to restrain and protect the occupant. When used together, the effect is greater than is strictly necessary, so that high forces are exerted on the occupant. Autoliv’s reaction was to develop the seatbelt load limiter. In its present version, this

That requires a degressive load limiter, such as the one Autoliv is currently developing for a client company. The principle is similar to a conventional load limiter, except that the torsion rod is supplemented by a piece of softer steel which absorbs additional energy. After roughly half a turn, the elastic limit is reached, the steel yields, and the lower force of the torsion bar itself applies. Figure 3 shows the torque participations and their variations.

Gegenwärtiger Stand und Trends bei Gurtsystemen

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ATZ worldwide

Restraint Systems

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Webbing

The defined restraint provided by a pretensioner and load limiter cannot be achieved simply with the deformation of the belt webbing. Theoretically, it would be possible to produce a suitable woven fabric, but it would not possess the required uniformity. Even shutting a seatbelt in the door can alter its properties. A load limiter is more versatile – and soon such systems will be adaptive. What is possible is a ‘beltbag’. There are various possibilities: Autoliv has developed and tested an airbag built into the lap portion of the belt. This can achieve a taut belt and side-impact protection for the hips at the same time. The problems are reduced comfort, the wear and tear in everyday use and the reduced compatibility with child seats, so that no great public acceptance can be expected.

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Adaptive Systems

The design of a car is of course subject to tight restrictions, but a ‘smart’ control system for the belt and the airbag offers considerable potential for improvement. The first adaptive restraint systems are expected to be incorporated in series-production cars in the year 2001. That will make it possible to react in a calculated way to particular accident situations and the needs of particular occupants. The point is that present-day specifications are still largely based on the ”50% dummy” (representing a human of median height and weight) and defined accident situations. Not enough account has been taken of the needs of real people, who tend not to weigh precisely 75 kg. The intention in future is to measure continuously the position and type of occupant by means of ultrasonic, infra-red or inductive systems together with weight sensors. An Autoliv ultrasonic sensor, for example, can recognise a person’s position relative to the car. The system is based on three-dimensional pattern recognition; several sensors are arranged in a square above the driver or front-seat passenger. The person’s weight is measured by four load cells in the lower part of the seat frame or on the floor pan. A control unit calculates the weight from the sensor signals, and assigns it to a particular category.

ATZ Automobiltechnische Zeitschrift 101 (1999) 10

Safety

Based on these figures and data from previous accidents, it takes only a few milliseconds to calculate how to set the load limiter in order to arrest the person’s motion with as little risk of injury as possible. A lightweight woman, for example, needs a lower restraining force than a large man if both are to be decelerated by a combination of belt and airbag. To this end, Autoliv developed the two-stage degressive load limiter, Figure 4, which will be capable of reacting flexibly to a real accident and of fulfilling the requirements of the US and European NCAP tests even better than at present.

Autoliv has introduced a number of innovations in recent years to meet the increasing demands of motorists in this respect. For example, the edges of the belt and the chafing resistance have been optimised, and the design of the guides improved. A particularly important factor is how many times the belt changes direction. An unfavourable construction with multiple guides often means that the belt sits on the shoulder at an uncomfortable angle, and is also less effective. New designs of roller guide reduce friction and improve comfort to a considerable extent.

To illustrate what this adaptive system can do: – The current ECE R16 standard lays down that a TNO dummy must not move forward by more than 300 mm. That requires a very high initial setting for the load limiter. After a certain time, the airbag comes into play and the force exerted by the belt needs to be reduced. However, a higher force is required to prevent a heavy person going right through the airbag. – An adaptive load limiter is easy to adjust to a particular type of vehicle while retaining the desired characteristics.

In order to make the belt more comfortable, Autoliv now uses two springs in the retractor: one rewinds the belt when it is unbuckled, while a weaker one comes into play while the belt is being worn. The use of this weaker spring, which exerts a force of only 1 N, is made possible by the pretensioner, which rapidly pulls the belt taut in the event of a crash. When the belt is unfastened, a magnetic switch in the buckle activates the normal spring which retracts the belt with a force of over 2 N. Up to now, the extra cost has meant that the system was used mainly in luxury cars, but it is already starting to penetrate the middle range of the market. A similar effect could be achieved purely mechanically, but it has not yet been possible to solve satisfactorily the problem of switching between the two springs in the practical conditions prevailing in a car.

The Autoliv adaptive load limiter is activated by a pyrotechnical charge. That has the advantage of having very rapid action, so that the electronics can decide during the crash what limiting value to apply, in accordance with the severity of the impact. Figure 5 shows the belt force and pay-out during a dynamic test with switching. The system is fail-safe, meaning that the default value is high enough to ensure sufficient restraint without an airbag. The value is reduced by means of the explosive charge only if necessary. The belt load is limited by means of a two-part torsion rod; one half is always used, while the second comes into play only when a lower deceleration is required. The basic force can be set by choosing a rod of a different diameter – easy to achieve in production thanks to a modular system. Figure 6 shows the adaptive load limiter as a part of the over-all restraint system.

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Improved Comfort

Of course, people are more likely to wear a comfortable seatbelt. Therefore, it is important from a manufacturer’s point of view to improve the comfort of its belts.

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Safety in the Back Seat

The seatbelts fitted to rear seats reflect the relatively recent introduction of a legal requirement for wearing them. They are not as comfortable as they might be; the buckle has a habit of slipping down between the seat and the backrest, and pretensioners and load limiters are the exception here. Autoliv has carried out a study to determine the factors which can influence the load on a dummy. Parameters such as the position of the attachment point and the buckle tongue, the amount of slack, the angle of the seat cushion, the sitting position and the severity of the accident were varied, and weightings evaluated in a Taguchi matrix. It was shown that reduced slack, a steeper seat angle, a load limiter and a pretensioner all have a positive effect. A pretensioner has a considerable effect in reducing the consequences of a sudden impulse, so it would be a good thing if it became standard equipment in the back seat, together with a load

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Safety

Restraint Systems

limiter. That applies particularly to cars with a ‘firm’ body structure.

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Back-seat passengers (as long as they are wearing belts) are unlikely to hit hard or unyielding structures in the event of a head-on collision, so airbags should not be necessary. Any plans to develop rear-seat airbags have so far been thwarted by the lack of support offered by the front seats. It would take considerable design and construction effort to make them suitable for airbags, and climbing into the back of a two-door car would then be very awkward. An important factor is that children usually occupy the back seats, and ‘out of position’ situations are relatively frequent. One possible solution would be airbags that deploy from the seatbelts themselves.

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Development Expertise

It is very important for the manufacturers of restraint systems to possess a high degree of expertise in matching the performance of belts and airbags. Companies offering only one of these products will have a hard job in future because they lack overall systems experience – perfect functioning requires an intimate understanding of the belt/airbag system in addition to knowledge of the body, engine, transmission and ‘packaging’ characteristics of the car. Intensive trials and elaborate computer simulations are needed to optimise restraint systems by a process of fine tuning. Absolute prerequisites are a crash simulation facility and a laboratory in which all kinds of tests and experiments can be carried out. Autoliv also makes use of a modern and effective prototype shop where steel parts can be shaped and machined, dies created and parts injection-moulded. Direct contact with the developers results in a transfer of know-how and above all saves a great deal of time.

– continuation on page 22 –

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