Link: https://www.springerprofessional.de/en/optimising-chassis-alignment-using-vehicle-sensors/6115514 SPECIAL ASSEMBLY OPTIMISING CHASSIS ALIGNMENT USING VEHICLE SENSORS The commissioning processes at the End-of-Line (EOL) of vehicle final assembly are characterised by a large amount of manual and therefore cost-intensive work. In a cooperation project with RWTH Aachen University and ZeMA Saarbrücken, Daimler is developing a chassis alignment process that can be executed without the manual insertion and extraction of a steering wheel balancer at the wheel alignment station. 58
AUTHORS DIPL.-WIRT.-ING. CHRISTIAN JANSSEN is Research Assistant at the Laboratory for Machine Tools and Production Engineering (WZL) at RWTH Aachen University (Germany). DR. RER. NAT. HARTMUT PRESTING is Head of Process Analysis and Adjustment Technologies in Production Planning Mercedes-Benz Cars at Daimler AG in Ulm (Germany). DR.-ING. KLAUS JOSTSCHULTE is Head of Validation Technologies in Production Planning Mercedes-Benz Cars at Daimler AG in Ulm (Germany). PROF. DR.-ING. RAINER MÜLLER is Scientific Director of ZeMA ggmbh in Saarbrücken (Germany). OPTIMISATION POTENTIAL IN CHASSIS ALIGNMENT Car production is subject to a great deal of competitive pressure between the manufacturers, therefore leading to a constant need to cut costs. One effective way of doing this is to optimise the production processes [1]. Particularly at the End-of-Line (EOL) of vehicle final assembly, the possible savings are high due to the large number of well-trained workers. At the End-of-Line of final assembly, the vehicles are commissioned, e. g. single systems and functions are checked and/or adjusted. For example, the parking brake and the service brake are checked on the rolling road, while the main headlights and driving assistance systems are often adjusted at a wheel alignment station. ❶ shows an example of a wheel alignment station. After the vehicle is brought to the stand, the wheel geometry (e. g. toe angle, camber angle etc.) is measured with the aid of a laser measuring system and the steering wheel angle is detected with a steering wheel balancer. The data are used to adjust the chassis automatically to the nominal values or the nominal range. In order to increase the efficiency of the EOL, the current project is focusing on increasing the level of automation. Not only the chassis alignment process but also the adjustment process of the main headlights and the alignment of the optional radar sensor and camera have been automated so far. One process that still has to be automated, in order to achieve a fully automated wheel alignment station, is the detection of the steering wheel angle or the insertion of a steering wheel balancer. The current situation is that the steering wheel balancer is mounted on the steering wheel and removed manually. The goal of this project is to detect the current steering wheel angle at the wheel alignment station fully automatically without any manual work at all. DEVELOPMENT OF AN EFFICIENT CHASSIS ALIGNMENT PROCESS In order to develop a chassis alignment process that can be executed without any manual labour for the detection of the steering wheel angle, and to implement this process in the production process, a method has been employed, which has been developed as a part of a dissertation at the chair of assembly systems from the RWTH Aachen to design complex commissioning processes, ❷. In this context, commissioning refers to testing and adjustment processes. Although the different modules of this method are performed in a stringent order, an iterative approach may be necessary due to interdependencies between the modules. For example, when analysing the products, a rough knowledge of the production processes can be beneficial. The practical application of this method is explained in more detail below. PRODUCT ANALYSIS Within the first module, the expectations of the customer are analysed. Furthermore, it examines how the current products technically fulfill the customer s requirements. With the aid of the key characteristic (KC) flow down, components that have a great influence on the fulfillment of the customer s expectations are identified. A top-down approach is em- ❶ Wheel alignment station at the end-of-line of vehicle final assembly 04I2013 Volume 115 59
SPECIAL ASSEMBLY ❷ Method to design complex commissioning processes ployed in order to link the customer s requirements with the assembly group and the elements and their characteristics [2]. The current steering wheel angle is continuously measured with the help of the steering wheel balancer in order to adjust the wheel alignment in such a way that the steering wheel is straight when the vehicle is moving straight ahead, which means that the direction of travel of the front axle corresponds to the wheel alignment geometry, ❸. This condition is required by the customers. In premium segment vehicles in particular, failure to fulfill this expectation will lead to customer complaints. ❹ shows the KC flow down that is used to break down the customer requirements to the component level. α RR α RL Longitudinal axis of the vehicle (and parallel lines) Thrust line (and parallel lines) Wheel center planes α RL : Toe angle rear left α RR : Toe angle rear right α FL : Toe angle front left α FR : Toe angle front right ❸ Wheel alignment geometry and steering system 60 The customer s requirement establishes a direct connection between the vehicle s driving direction and the steering wheel angle. The vehicle s driving direction is predominantly influenced by the wheel alignment, in particular the toe angle values of the wheels at the rear and front axle. While DIN 70 000 defines the toe angle value of the rear axle as the angle between the wheel centre plane and the longitudinal axis of the vehicle (determined by the middle of the front and rear axle), the toe angle value of the front axle is described in DIN 70 027, relative to the thrust line, 3 [3, 4]. The thrust line is defined by the centre planes of the wheels of the rear axle [4]. α FL α FR Steering wheel Steering column Steering track rod Adjustment mechanism Picture source: ZF The toe angles of the front axle can be changed via the adjustment mechanisms located at the steering track rod on the left or right hand side. These adjustment mechanisms also allow an adjustment of the steering wheel angle relative to the toe angles of the front axle. As shown in 3, a steering wheel angle that is not zero leads to a virtual toe angle, which has to be compensated for. Therefore, the steering wheel angle has to be measured continuously. With the aid of the compensation of the current steering wheel angle, the toe angles of the front axle ( VL, VR ) can be determined and adjusted. The components and parts that have been identified in the KC flow down constitute the basis for the following analyses. In order to be able to draw a conclusion about the influence on the customer s requirements, the identified assemblies and components have to be examined. One specific assembly group that makes it rather difficult to fulfill the customer s requirements is, for example, the steering system. Due to their design, vehicle steering systems have a tolerance. This characteristic leads to the fact that one steering wheel angle goes with different toe angles or different driving directions (calculated on the basis of the toe angles of the front axle). ❺ shows a strongly simplified result of an experiment in which the steering wheel was rotated multiple times clockwise and anti-clockwise. As the diagram shows, the change of direction leads to a hysteresis curve. Knowledge of this product characteristic is relevant when evaluating the reproducibility of the results of measuring process. An example of a component that makes a positive contribution to the objective of the project is the steering angle sensor [5]. Steering angle sensors are now available that can measure the current steering wheel angle with a relatively high resolution. In order to evaluate the potential of these sensors, several vehicles were examined in the production process with regard to absolute and repeat accuracy. Since vehicles are products that are subject to continuous change, product analysis focuses not only on the status quo but also on possible future changes. In the presented use case, steer-by-wire systems, for example, could have ramifications on the wheel alignment and accordingly on the measurement of the
steering wheel angle at the wheel alignment station. PRODUCTION ANALYSIS Customer s requirement Steering wheel level while vehicle moving straight ahead In the second module, the framework conditions of the production process are analysed. This refers not only to the production processes that are relevant for fulfilling the customer s requirements but also to the manufacturing resources that are required in order to execute these processes. The wheel alignment process in this use case starts by automatically conveying the vehicle into the wheel alignment station. After the vehicle is centred in the wheel alignment station, the steering wheel balancer is mounted onto the steering wheel and then fixed to the windscreen. Subsequently, the wheel geometry (for example toe and camber angle) is measured by a laser system. On the basis of the continuously measured wheel geometry and steering wheel angle, the front toe angle is adjusted by an automated toe angle adjustment system. Since the steering wheel angle is measured and compensated for, this ensures that the adjustment of the angle is accurate even when the steering wheel is turned. The process analysis yields several manufacturing resources (in particular laser measurement systems, steering wheel balancer and adjustment mechanisms) that have an influence on the customer s requirements (for example measuring accuracy, resolution and minimum possible adjustment steps). In order to be able to draw a conclusion concerning the main influences of the manufacturing resources, their characteristics were examined by means of tolerance analysis. In addition to the wheel alignment process, the assembly and commissioning processes of the steering angle sensor were analysed to estimate the improvement potential of the steering wheel sensor. Product Assembly group Component Wheel geometry (e. g. toe, camber) Rear axle vehicle steering wheel sensor is one possible solution for achieving a fully automated measurement of the steering wheel angle at the wheel alignment station, ❻. A comparison of the results of the product analysis with the results of the production analysis shows that the steering wheel sensor and the steering wheel balancer have a similar repeat accuracy. However, the absolute accuracy of the steering wheel sensor in the observed assembly process is not yet high enough. This challenge can be met, for example, by calibrating the steering wheel sensor, which requires less effort than mounting and dismounting a steering wheel balancer by hand at the wheel alignment station. Another challenge is the fixation of the steering wheel in the wheel alignment station, because, if the steering wheel ❹ Key characteristic flow down of customer s requirements Steering wheel angle [ ] Steering wheel tolerance center Adjustment mechanism sensor is used to detect the steering wheel angle, there is no steering wheel balancer which could be used to fix the steering wheel. With the assumption that a fixation of the steering wheel is not necessary, the developed process plans that the steering wheel sensor is calibrated after its assembly and prior to the wheel alignment station. After the calibration of the sensor, the chassis can be adjusted according to the values gained from the measured data of the steering wheel sensor. IMPLEMENTATION OF THE PROCESS Steering system Front axle Steering wheel Steering wheel sensor In the fourth module, a stepwise secured implementation of the process takes place in the production process. In this use case, there are four stages with the following goals: DEVELOPMENT OF THE COMMISSIONING PROCESS Front axle driving direction [ ] Following the product analysis and the production analysis, the third module deals with the development of a commissioning process. As mentioned above in the product analysis, utilisation of the Steering wheel tolerance ❺ Schematic diagram of the hysteresis curve of steering systems 04I2013 Volume 115 61
SPECIAL ASSEMBLY ❻ Measurement of the steering wheel angle via the steering wheel sensor : 1 st stage: showing that the fixation of the steering wheel balancer at the wheel alignment station is not needed : 2 nd stage: calibration of the steering wheel sensor prior to the wheel alignment station : 3 rd stage: adjustment of the chassis based on the measurement of the steering wheel sensor : 4 th stage: removing the steering wheel balancer. During the first stage, it was shown by experiments in the production area that wheel alignment with an unfixed steering wheel balancer is at least as accurate as adjustment with a fixed steering wheel balancer. Furthermore, in the second stage, it was proven that the required absolute accuracy for adjusting the chassis could be achieved. The third and fourth stages have not been executed yet, but the results gained so far indicate that integration of these two stages into the production process will also be successful. REFERENCES [1] PriceWaterhouseCoopers AG: Kostenmanagement in der Automobilindustrie. Bestandsaufnahme und Zukunftspotenziale. Studie 2007 [2] Whitney, D. E.: Mechanical Assemblies. Their Design, Manufacture and Role in Product Development. Oxford University Press, New York, 2004 [3] Norm DIN 70 000 (Januar 1994). Fahrzeugdynamik und Fahrverhalten. Begriffe [4] Norm DIN 70 027 (August 1992). Straßenfahrzeuge Fahrwerkvermessung. Anzugebende Fahrwerkdaten, Messbedingungen [5] Schutzrecht DE 10 2008 021 521.0 (24.12.2008). Bodenberger, J.; Meier, J.: Einstellverfahren für ein Fahrzeugfahrwerk CONCLUSION By using a method to design complex commissioning processes, a wheel alignment process was developed that allows the steering wheel angle to be measured fully automatically with a vehicle sensor at the wheel alignment station. A basic part of the developed process has already been successfully introduced into the production process. The results so far indicate that the complete concept can be successfully implemented in the production process. With successful implementation, the gap to a fully automated wheel alignment stand can be closed and consequently the efficiency of the End-of- Line of final assembly can be increased. 62