Light Commercial Vehicles – Challenges for Vehicle Stability Control Dr. rer. nat. E. Liebemann Dipl. Ing. T. Führer Dipl. Ing. P. Kröger Robert Bosch GmbH, Chassis Systems Control Germany Paper Number 07-0269 ABSTRACT to see more increase in mobility and in traffic density throughout the world. The progress of crash energy absorbing The electronic stability program (ESP®) is increasingly finding car body design and the standard fitting of airbags significantly acceptance in vans and light commercial vehicles (LCV). improved the passive safety especially combined with the use Nearly all current models, whose gross vehicle weight is of seat belts (Figure 1). generally between 2.8 and 7.5 metric tons, are now available with this active safety system, either as an option or even as But many of the serious accidents happen through loss of standard equipment. control in critical driving situations. When skidding occurs, a side accident is a frequent result. With a reduced protection Many studies have now confirmed that ESP® can prevent a zone for the occupants compared to front crashes, these vehicle from skidding or rolling over in nearly all driving accidents show an amplified severity. Especially with vehicles situations [1, 2]. This is particularly important in the case of of an elevated center of gravity like vans, sport utility vehicles vans, since their design and their use leave them with tighter (SUV) and light commercial vehicles, the loss of control with safety margins. Depending on load, the center of gravity shifts, subsequent skidding may even lead to rollover. and consequently the risk of rollover may increase. Bosch has developed a system specifically for light commercial vehicles Mode of Operation and Benefit of ESP® that automatically adapts its control mechanisms to the current Fatality trend in Europe situation. Vision: accident-free driving; status 2005 Fatalities INTRODUCTION eSafety - 50% 8,100 Worldwide traffic is increasing with more and more vehicles 7,500 EU target 2010 : 6,400 Reduction of fatalities on the road. With further economic growth, we will continue 5,800 5,300 5,400 5,400 by 50% through (-35%) (-28%) (-16%) promotion of 4,400 3,600 4,050 Intelligent Networking for more Safety 3,750 (-24%) intelligent safety 3,200 3,300 functions 2,900 Road safety improvement (-8%) 1,800 2000 2005 Target 2010 350% France Germany Italy Spain UK Traffic volume Source: European Commission 300% Accidents in total All abbreviations within this presentation Chassis Systems Control Injured used for simplification purpose 1 CC/ES | 18.12.2006 | © Alle Rechte bei Robert Bosch GmbH, auch für den Fall von Schutzrechtsanmeldungen. Jede Verfügungsbefugnis, wie 250% Fatalities Kopier- und Weitergaberecht, bei uns. 200% Figure 2. European eSafety initiative of the European Union for 2010 is 150% set to reduce road deaths by 50% relative changes in% 100% Seatbelt ® 50% ABS According to accidentology conducted by VW [1], ESP is Side impact Airbag ESP® airbag 0% considered to avoid 80% of the accidents caused by skidding. ® 1970 1975 1980 1985 1990 1995 2000 2005 VW concludes that the safety benefit of ESP is even greater Source: Statistisches Bundesamt, Bast, Bosch than that of the Airbag. Automotive Technology Confidential | CC/PJ-CAPS | 01.03.2007 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve 4 all rights of disposal such as copying and passing on to third parties. Based on the analysis of Japanese traffic accident statistics, Figure 1. Traffic volume increase and road safety improvement in Toyota [2] estimated that the accident rate of vehicles with Germany from 1970 to 2005 _________________________________________________________________________________________________ Liebemann 1 ESP® is reduced by approximately 50% for severe single car LT are usually equipped with large wheels allowing accidents and reduced by 40% for head-on collisions with remarkable brake sizes with high volume consumption. To other automobiles. The casualty rate of vehicles with ESP® ensure full ESP® and rollover mitigation functions and reduced showed approximately 35% reduction for both types of stopping distance, a special brake system design is required. accidents. Consequently, Bosch develops the ESP®LT with an optimized Although good progress is shown with a reduction of 21% over the first half decade (Figure 2), the European Union will most likely not achieve the objective. Additional efforts will be required to furthermore enhance the road safety. Bosch supports this with ESP® systems for all vehicle segments including Light Commercial Vehicles (LCV) and furthermore with the combination of active and passive safety systems. MAIN SECTION Intelligent safety systems start to support drivers in situations where they are overburdened due to lack of training and driving experience. A study by Prof. Langwieder showed (Figure 3) that in 49% of car-to-car and car-only accidents no braking was applied at all, partial braking was applied in 12% respective 20%, and emergency braking in only 39% respective 31% of the accidents. Intelligent Networking for more Safety Braking Behaviour in Accident Situations 60 Car / Car - Accident Car Only Accident ® ® 49 49 Figure 4. ESP system for Light Trucks. ESP LT with optimized s 50 motor, pump and valves for improved pressure build-up and better 39 40 pressure response time during partial braking and ABS intervention; 31 larger accumulator chamber for large brakes with high volume 30 consumption 20 20 12 % of Interviewed Person 10 motor, pump and valves for improved pressure build-up and better pressure response time during partial braking and ABS 0 No Braking Partial Braking Emergency Braking intervention (Figure 4). Low temperature conditioning is available to ensure full stability performance down to below Source: Prof. Langwieder ( Gesamtverband der Deutschen Versicherungswirtschaft e.V.) Chassis Systems Control -25° C. In addition a larger low pressure accumulator chamber 16 Confidential | CC/PJ-CAPS1 | 02/02/2007 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. W e reserve all rights of disposal such as copying and passing on to third parties. is introduced for excellent ABS performance. Figure 3. Braking behavior of drivers in car to car and car only accidents in Germany Beside typical brake sizes or brake pressure levels, LT and light commercial vehicles share the rather demanding Although partial braking and to a certain extent emergency characteristic of high load and mass variances between empty and fully laden vehicle. Specific measures are mandated to braking can be supported efficiently with brake pre-fill and ® hydraulic brake assist, no braking requires surrounding sensors ensure full braking, traction and ESP performance both for to enable mitigation functions. First safety systems based on the loaded as well as for the empty case. The measure is called e.g. Radar sensors have already been introduced, supporting Load Adaptive Control or LAC. partial braking situations with adapted brake assist and brake pre-fill and no braking situations with automated vehicle LAC – LOAD ADAPTIVE CONTROL deceleration. In particular, vehicles with a tare to gross vehicle mass ratio Still, special emphasize is required to cover the demanding larger than 1.5 such as LCV or LT benefit from LAC. Figure 5 requirements of light commercial vehicles (LCV) and light shows the typical load variation for a passenger car compared trucks (LT). For cargo space optimization, LCVs are usually to a LCV. When the maximum load variance for a car is equipped with comparably small wheels. The resulting typically below 40%, it can reach up to 100% and more for a limitation of brake rotor diameter leads to high pressure ESP® LCV with even stronger relative variations for the Center of applications with challenging durability requirements. Gravity (CoG). _________________________________________________________________________________________________ Liebemann 2 The load configuration has a profound impact on vehicle Yaw Gain vs. vehicle speed with varying rear axle load dynamics. In particular, the load significantly influences: 14 - braking efficiency incl. ABS- and split-µ performance Empty Vehicle - traction efficiency and stability, esp. for vehicles with 12 + 750kg rear axle load + 1200kg rear axle load rear-wheel (RWD) or all-wheel drive (4WD) 10 + 1500kg rear axle load - cornering behavior neutral cornering - rollover tendency 8 The maximum axle loads are important parameters. They are derived from the mass and longitudinal center of gravity. Since 6 the loading platform tends to be behind engine and passenger Yaw Gain [1/s] compartment, payload mainly increases the rear axle load 4 while only having a minor effect on the front axle load. This also means that front-wheel drive (FWD) vehicles may be as 2 influenced by load changes as RWD and 4WD vehicles. 0 0 10 20 30 40 LAC - Load Adaptive Control Vehicle Speed [m/s] Load variation in passenger cars and LCVs Figure 6. Self-steering and cornering behavior of one vehicle under various load conditions. While the empty vehicle shows the expected understeering, a payload of 1500 kg on the rear axle results in oversteering 100% 80% LAC consists of algorithms for the estimations of mass, the Passenger longitudinal shift of CoG and the change of self-steering 60% Car passenger car behavior reflected by the characteristic speed. The estimation LCV relative40% change LCV - Light algorithms are centralized while the resulting adaptations are relative change relative Commercial by their nature decentralized and located in various vehicle Vehicle 20% dynamics modules like the brake slip controller. It is important to note that estimation-based adaptation algorithms need a 0% massmass longitudinallongitudinal centre of gravi ty shift verticalvertical centre of gravity shift shift learning phase, which means that they are never available shift of CoG of CoG immediately after key-on. Since estimations will never be Chassis Systems Control CoG: Center of Gravity CC/MK CC/NE2 | 2/23/2007 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights.