ACTIVE SEAT BELT BUCKLE FOR COMFORT AND SAFETY

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1 ACTIVE SEAT BELT BUCKLE FOR COMFORT AND SAFETY TRW has developed an active restraint system that simplifies the seat belt buckling process and helps to improve occupant safety. The Active Buckle Lifter (ABL) simplifies locating and fastening the belt by lifting the buckle into a convenient position once the passenger is seated, and can help to reduce seat belt slack in dynamic and critical driving situations. The first application of the ABL was developed together with Daimler and will be available on the rear seats of the new Mercedes-Benz S-Class. 32

2 Authors DIPL-ING UWE CLASS is Senior Engineering Manager for Active Restraint Systems and Cognitive Safety Integration at TRW in Alfdorf (Germany). DR RER NAT MARTIN SEYFFERT SEYFFERT is Manager of Advanced Systems and Cognitive Safety Integration at TRW in Alfdorf (Germany). DR-ING ALOIS SEEWALD is Global Director of Research and Development and Integrated Active & Passive Safety Technologies at TRW in Düsseldorf (Germany). MOTIVATION Seat belts are still considered to be the number one lifesaver in cars. A seat belt reminder is therefore assessed in Euro NCAP s rating criteria in order to improve the buckling up rate. Widespread market data also shows that seat belt buckles in modern cars are often perceived as being difficult to reach or positioned too low. For example, on front seats, large or high centre consoles can restrict access to the buckles. On the rear seats, the buckle head is often buried within the seat padding. Buckling up in these cases can also be particularly difficult for elderly occupants or people with reduced mobility. Currently, there is no technology on the market that directly addresses improving the buckling up process. Active seat belt presenters can make it easier to grasp the seat belt; however, the process of inserting the seat belt tongue has remained unchanged. With its improved accessibility, the ABL acts as a buckling up assistant that can complement an acoustic reminder signal. ACTIVE BUCKLE LIFTER FUNCTIONALITY Against this background, TRW has developed an Active Buckle Lifter (ABL) that raises and lowers the seat belt buckle using an electric motor. Depending on the design, it can have up to three functions that are activated in specific driving situations, 1. The comfort function is triggered when the door is opened and causes the buckle head to move upwards. In the buckling up position, the seat belt buckle is more accessible and visible, with the result that the seat belt tongue can be inserted more easily and also acts as a buckling up reminder. After buckling up, the buckle head moves back into its normal position, thereby tightening the belt around the occupant and keeping him or her firmly in the seat, even in relaxed driving situations. The second function is dynamic support, in which the ABL lowers slightly from its normal position in dynamic driving situations, such as quickly going around a bend or dynamic braking manoeuvres. This can help to reduce belt slack around the pelvis and chest, thereby autotechreview December 213 Volume 2 Issue 12 33

3 1 ABL functions helping to keep the occupant in a more controlled seating position. The third function of the ABL is the safety function, which is activated during critical situations such as emergency braking or loss of stability. In this case, the seat belt buckle is quickly and firmly lowered, enabling full reversible tightening, which helps to maintain the proper occupant position for airbag deployment if a crash is deemed to be imminent. Depending on the type and severity of the crash, the ABL can lift and facilitate unbuckling during the post-crash phase. 2 Design of the ABL 34 DESIGN AND PACKAGING The ABL has a 12 V electric motor connected to a spindle nut, which drives the spindle, 2. A steel cable is attached to the nut, which is connected to the seat belt buckle via a deflector. If the motor starts to rotate, the spindle nut moves the steel cable and the seat belt buckle is lifted or lowered depending on the direction of rotation. The design of the ABL encompasses a self-locking system that helps to prevent the forces in the belt system from pulling the seat belt buckle upwards, for example, due to an occupant in the crash phase. To allow for flexible packaging, the housing can be positioned to lie flat or stand upright in the vehicle. On the front seats, the ABL can be placed on the side next to the seat or diagonally under the seat. In the back, it can be integrated into or under the seat upholstery or in the backrest. ECU STRATEGY AND ALGORITHM The electronic control unit strategy of the ABL offers a number of possibilities, 3. There are two independent seat belt buckle systems in operation in the standard variant. The power output, the motor controller and the situation management algorithm (SMA) are integrated into a control unit and ideally installed in an assembly with the actuator. For applications with limited installation space at the seat, the actuator can also be controlled from a central ABL control unit at another location in the car. The cable package required for the ABL mechanics needs an EMC shielding. A third possibility is a master-slave variant in which only the SMA is integrated into an existing control unit. This serves as a master for control unit slaves on the actuators. The algorithm contains all the ABL functions and the minimum triggering

4 levels defined by the vehicle manufacturer. To do this, it retrieves data from existing sensors in the vehicle bus. From the overall sensor image, the SMA detects the situation and introduces a corresponding action. The level of vehicle dynamics is determined on the basis of the single-lane model and evaluates, among other factors, the acceleration, steering angle and brake pressure. The door sensor has proven to be a central triggering criterion for the comfort function, as this allows the buckle to be highly visible when lifted. SYSTEM DESIGN As a comfort feature, the ABL is relatively unobtrusive for the occupant, whereas dynamic assistance requires the buckle to move at a moderate speed and the safety function requires a tightening process that is as quick as possible. The whole system consists of a control unit, actuator, mechanics and electric motor and must therefore be designed in such a way that low forces can be generated slowly and very high forces can be generated quickly. The comfort function of the ABL has a sophisticated design. The goal is to achieve a slow and gentle start of the buckle head motion that the occupant can hardly perceive. A gentle starting phase is followed by a movement at a constant speed that ends with a gentle motion. Technically speaking, this requires very low speeds that are hard to detect. The ABL uses a sensor to detect the motor speed. When the speed is very low, the number of impulses detected by the sensor is insufficient to enable an adequate speed signal to be calculated. To control this, a minimum speed is therefore required, which would be perceived as a jolt when the movement starts. The solution for the ABL consists of starting movements with closed-loop control as opposed to open-loop control. The system only switches to open-loop operation once a specified speed has been reached. TRW has also investigated perceived comfort in studies with test persons in real car environments. The results identified that a medium target lift height of 5 mm should be implemented within a time frame of approximately 1 s. With dynamic support, the ABL can be lowered by up to 4 mm in about 5 ms, until a buckle force of 7 N is reached. With the safety function, the retraction motion of the buckle head takes place within 12 ms with forces of up to 3 N, 4. Power consumption for the comfort function is below 5 A, whereas, for the safety function, it can be more than 25 A for a few milliseconds. In order to create high-quality noise characteristics for the ABL, noise was measured in various operating conditions, such as vibration tests in a laboratory and in a vehicle during the development phase. Acceleration sensors, microphones and three-dimensional visualisation software were used in a semi-anechoic sound chamber in accordance with ISO 3745 to analyse and optimise noise generation. SYSTEM TESTING Validation of the ABL at the component level involves a high number of test cycles depending on the triggering thresholds that are defined. The comfort function must successfully complete around 7, cycles, the dynamic support 1, to 3, cycles and the safety function around 1, cycles in order to represent the lifetime of the system. TRW has also developed a new testing method together with a new test bench that programmes the loads and requirements in realistic situations. In this way, the functions of the ABL and various profiles can be tested without modifications in any sequence, in order to depict realistic ageing of the system. The test bench can apply a force of up to 3 N in less than 1 ms in a controlled manner on up to four ABLs at the same time at temperatures ranging from -35 to +85 C. The series production control unit uses residual bus simulation to receive the information that the algorithm otherwise receives from the vehicle bus. In addition to ageing and system tests, the crash performance of the ABL is also tested. Prior to most accidents, occupants are displaced in their seat due to emergency braking or swerving manoeuvres, and these values must be taken into consideration for a realistic evaluation of active restraint systems. TRW has meas- Separate masters Central ECU Master Slave Central Slave Hall sensors, power, ground, shielding Slave 2 Privater BUS 5 Vehicle ECU as host and master ) ABL Mech. 2) Engine 3) Power Stage 4) Engine controller 5) Situations management algorithm (SMA) 3 Possible system architectures autotechreview December 213 Volume 2 Issue 12 35

5 Buckle force [N] ECU power [A] ured the lateral and frontal occupant displacement in real driving manoeuvres with and without active restraint systems [1]. These values form the basis of the initial condition used for virtual simulations and carriage tests. Occupant displacement [mm] Conventional belt ABL Retraction performance in the safety function ACR 5 Test results for occupant displacement during emergency braking Time [s].15 Time [s].2.2 ❺ shows the differences in the reduction of occupant displacement during emergency braking, in which the ABL safety function achieves similar results to the active control retractor (ACR) seat belt system. ACR and ABL hold the occupant much closer to the normal seat position in those seats for which the passive safety systems are designed. This can reduce the forward displacement of the head by up to about 5 % compared to standard seat belts. OUTLOOK The ABL can be further improved by integrating environmental sensors or GPS data. Retraction of the seat belt into the safety position can then be executed predictively before occupant displacement occurs. If the car is equipped with an automatic emergency braking system (AEB), it can activate the ABL several milliseconds before the brakes are applied. Moreover, the algorithm could warn the driver using belt vibrations if he is cornering too quickly. The ABL can also be combined with an illuminated buckle head in which a low-consumption LED is used to increase the visibility of the seat belt buckle. The ABL-C is an additional TRW development project that will comprise only the comfort function. Extensive packaging studies have already been conducted for this in order to give consideration to limited installation spaces, as is the case with many front seats today. From 214, active safety systems will be given high priority and integrated into Euro NCAP ratings. In the draft ratings of the AEB, active seat belts such as ACR or ABL are being taken into consideration as an essential element of an integrated active-passive safety concept. CONCLUSION The ABL is the first reversible restraint system that combines a comfort feature with a safety function. For the first time, the seat belt buckling up process has been facilitated through a lifting motion of the seat belt buckle. A lowering of the buckle head in dynamic and critical driving situations reduces occupant displacement and thereby helps to enhance safety. The ABL will be available on the rear seats of the new Mercedes-Benz S-Class. This design encompasses the comfort and safety function, and these specifications were developed in close collaboration between TRW and Daimler. Moreover, other car manufacturers are showing great interest in this technology. Reference [1] Mages, M.; Seyffert, M.: Class, U.: Benefit of reversible belt pre-pretensioning for different precrash scenarios reduction of occupant displacement and the effect on injury severity. In 1th International Symposium on sophisticated Car Occupant Safety Systems, Karlsruhe, 21. Read this article on