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Series Production of Head-Up Displays Manufacturing head-up displays in large numbers is a challenge because of the strict precision standards for the optomechanics. At Continental, the second generation HuD made the transition to series production in 2010. The production process is characterised by a wide vertical range of manufacture.
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autHorS
Tobias Schumm
is Project Manager HuD Development at Audi in Ingolstadt (Germany).
Ralf Worzischek
is Senior Project Manager HuD C7 at Continental Automotive GmbH in Babenhausen (Germany).
Task
The head-up display (HuD) is among the most outstanding passenger car cockpit innovations of recent years. The information displayed in the HuD seems to hover above the hood where they are in the driver’s direct line of sight. The display can be read without taking one’s eyes off the traffic. Due to the long light path, this happens without accommodation work, which makes it less tiring for the eyes. However, manufacturing this type of display is quite challenging. A look at the function principle explains why this so. The driver reads the display, which is flush-mounted deep in the dashboard, via the screen (acting as a mirror in this case) and other mirrors in the HuD. As the front screen is curved and represents a free form in terms of optics, the optomechanic components of the system not only need to be adjusted very precisely to the individual vehicle model and its front screen geometry. In addition, the optical components have to be manufactured to a precision of currently just 30 μm. Further requirements are posed by variants for right-hand and left-hand drive vehicles. After Continental introduced the HuD to the market in 2005, the manufacturing of HuDs in larger numbers for the Audi models A7 and A6 began in 2010. This second HuD generation is much more compact than the first. It is assembled from fewer components and is design-optimized for manufacturing, 1. Instead of the first generation’s four mirrors, for instance, the current HuD design needs only two mirrors. Nevertheless, the full-colour display also has a better performance in as much as it facilitates automatic adjustment of the image at the end of the line. Injection moulding and coating
The linchpin of the HuD’s success is optical quality, which in turn directly depends on the quality of the installed mirrors. Therefore, Continental decided at an early stage to manufacture these key components in-house. The supplier has a long tradition of injection-moulding plastic parts for displays and controls at its Babenhausen site. Still, the particular HuD precision standards required a special process. The injection moulding
❶ Fully assembled second-generation head-up display 04I2011
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2 Detail of the electronically controlled injection moulding machine used for producing mirrors by injection compression moulding
machine uses a specific injection compression moulding process, which in contrast to conventional injection-moulding technology is not controlled hydraulically but via electronics in order to achieve the necessary level of temperature control, 2. Manufacturing the two different HuD mirror sizes (aspherical mirror 1 and aspherical mirror 2) is done in complex tools with numerous heating circuits and an extensive network of sensors. Controlling the process very precisely keeps the inevitable shrinking of the transparent plastic, a cyclic olefin copolymer (COC), from influencing the part’s final dimensions during cooling. When the robot removes the part, all mirror surfaces are finished to a maximum tolerance of 30 μm, 3. Cutting off the sprue is done during part removal. There are eight versions of the injection moulding tool to manufacture both aspherical mirrors for two vehicles and for lefthand and right-hand drive versions. In addition to a manual selection of mirrors, the surface topography is also measured. The precision of the injection compression moulding process is of paramount importance as the mirror surfaces cannot be machined. Furthermore, scrap is to be avoided, as this would impact the machine’s output. As the process is still in the ramp-up phase, the tool is changed roughly every four days of round-the-clock production to manufacture batches of both aspherical mirrors in turn. Production itself
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3 The mirror assumes its final shape within a 30 μm tolerance directly after removal from the injection moulding tool
4 Assembly station of the line, showing the complete first sub-assembly
is minutely planned to have the right part numbers available at the right time for assembling the correct number of HuDs. The unworked mirrors are subsequently transported to an external partner company where the coating is applied. An aluminium coating is applied to the large aspherical mirror (aspherical mirror 2) in an autoclave, which turns the mirror into a regular reflector. Things are a little different with the small aspherical mirror (aspherical mirror 1). Depending on the requirements, this mirror is coated with a multi-layer system, consisting of alternating layers of different materials such as titanium and SiO2 applied by vapour deposition. Therefore, the small aspherical mirror is a dichroic reflector that reflects light within a specific spectral bandwidth, while infrared light can pass through. Assembly process
Due to the sensitive combination of electronics and optomechanics in the HuD, two basic protective measures are taken. Firstly, ESD testing gates in the entry areas neutralise any electrostatic charge that might otherwise damage sensitive electronic components by discharge. No one can enter the production area without confirmation of successfully grounding their shoes. Secondly, roller shutters reduce the amount of dust carried in from outside. By consistently using plastic standard containers for the material in 04I2011
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and out flows, a major source of dust is eliminated from the whole production. The complete supply chain has been integrated into this precaution against dust drag-in and has been part of defining the packaging and permissible particulate concentration. Additional panes are mounted above the HuD assembly lines to prevent dust from falling down onto the lines. In combination with air locks and shielding covers over the assembling stations, the clean room class 100,000 is kept according to ISO class 8. The HuD assembly line for Audi is corridor-shaped with feeding from the outside. Assembly workers move between the two opposite sides of the line as they carry out standard manual assembly steps and proceed from one station to the next. A characteristic feature of this assembly is that every worker can manage every single station. The stations themselves are defined according to HuD sub-assemblies. The line design was developed in-house, following individual HuD sub-assemblies. The assembly begins with a plastic base block, which together with damping elements for a mirror, springs, the aspherical mirror 2 and a visor, forms the first sub-assembly, 4. After that, an inner visor is bolted onto the base block, and the display and light guide are inserted into the optical bench. The bolting-up of the individual components is performed by a pneumatic torque-controlled screwdriver which produces a vibration-resistant friction-welded
joint between the injection-moulded plastic part, which is made from glass-filled polyphenylene sulphide (PPS), and the metal bolt. Specifying the tightening torque, the screw thread and the bore dimensions of this process had to be done precisely because Audi has special requirements for bolting joints. During the subsequent assembly, the light guide and a stepper motor with a shaft that can tilt the support of aspherical mirror 2 are inserted. This motor makes it possible to adapt the visible level of the display later on according to the body height of the driver. In the next step, the light source is inserted. Thermal pads on the rear ensure a good heat transfer to the housing. After the LED board has been mounted and the motherboard has been fixed, the metal housing is bolted on from underneath. Only after this step is the aspheric mirror 1 screwed on. From this moment on, the assembly is a fully functional device, which is now covered with a lid, which is in fact a thin plastic foil. The finished HuD is then passed through a gate and is handed over from assembly to final inspection. Reworking stations are an optional last step of assembly, if the constant visual control mainly of the mirrors reveals a surface defect at this stage. Should this occur, the device is taken apart to the level of re-usable sub-assemblies. During this, all plastic PPS parts which could not be reliably bolted again during a second
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5 Automatic test cell for function testing and for optical adjustment as required
assembly are scrapped, as the conditions for a successful friction weld are no longer fulfilled. Major sub-assemblies and components are scanned during the assembly to warrant traceability of lots. The manufacturing depth at Continental includes all main components of the optomechanics, plus the motherboard, which is manufactured at the Karben site. Purchased parts include components such as the display, the inner visor and the metal housing, of which all the bores and mounting surfaces are machined to the finished state by the supplier.
DOI: 10.1365/s38312-011-0029-5
Final inspection and quality assurance
After final assembly, all HuDs are inspected for function and optical display quality in two fully automated test chambers equipped with robots, 5. For that purpose, the HuD is positioned and activated and all relevant operating states are checked. A camera on the robot arm then films a test image displayed on a block of steel with a polished-in topographic segment of the front screen. Additionally, the achromatic point is measured to ensure a precise colour presentation. If a deviation from the correct geometry of the test grid pattern is detected, an automated adjust-
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ment is carried out. This is possible because the graphic processor of the HuD is capable of performing a real time correction via a look-up table. The image is adjusted by controlled warping of the original image on the display in order to obtain a correct representation on the screen. The complete test log is saved for every HuD. Once the HuD has passed final inspection, an intermediate visor is attached, the foam seal for the connector is applied and a protective cover foil is attached. After that, the device is ready for shipment. On top of the 100 % final inspection of function and optical quality, HuDs are also checked in climate chambers parallel to production. This screening serves to confirm the success of quality-ensuring measures, such as 24-h tempering after injection moulding and spray colouring. By this screening process, which applied to 100 % of all HuDs in the initial rampup phase of production from July 2010 onwards, it is made sure that an HuD will not show any change of image position due to ageing or change within plastic parts even after being exposed to demanding temperature profiles. Integrating the display into the vehicle involves three main assembly steps. First, a mounting bracket for HuD installation is put in place. After that, the controller is
installed and, when the front screen has been glued in place (serving as the last of the mirrors), the system is completed. Now, the HuD can be activated and vehicle-specific settings can be carried out. Finally, the optical quality is checked. If necessary, unwanted effects caused by the integration are automatically corrected. Only by cooperating and networking very closely were Audi and Continental capable of making this system a reality. Summary and outlook
With the second generation HuD, a device that is more compact than the first generation has been developed. It consists of fewer components, weighs only 1.8 kg in the portrayed version for Audi and is easier to manufacture at the same time. The design for manufacturing was aimed, among other things, at generating the displayed image in a way that is tolerant to manufacturing influences. As this target was successfully met, it is possible to move on to large-scale production, which can make the previous premium technology a future optional feature for the upper family car segment and for integration into compact vehicles. Building on the ongoing production for Audi, further applications for other vehicle manufacturers are ahead.
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Tyres, wheels, suspension – how comfortable are our cars? Bernd Heißing | Metin Ersoy (Eds.)
Chassis Handbook
Fundamentals, Driving Dynamics, Components, Mechatronics, Perspectives 2011. XXIV, 591 pp. with 970 fig. and 75 tab. (ATZ/MTZ reference book) hardc. EUR 69,95 ISBN 978-3-8348-0994-0 In spite of all the assistance offered by electronic control systems, the latest generation of passenger car chassis still relies on conventional chassis elements. With a view towards driving dynamics, this book examines these conventional elements and their interaction with mechatronic systems. First, it describes the fundamentals and design of the chassis and goes on to examine driving dynamics with a particularly practical focus. This is followed by a detailed description and explanation of the modern components. A separate section is devoted to the axles and processes for axle development. With its revised illustrations and several updates in the text and list of references, this new edition already includes a number of improvements over the first edition. The contents Introduction - Fundamentals - Driving Dynamics - Chassis Components - Axles in the Chassis - Driving Comfort: Noise, Vibration, Harshness (NVH) - Chassis Development - Innovations in the Chassis - Future Aspects of Chassis Technology The authors Univ.-Prof. Dr.-Ing. Bernd Heißing is director of the Chair for Automotive Engineering at the Technical University of Munich. For almost 15 years, he held a managerial post in chassis development at Audi and is still additionally involved in numerous research projects and participates in congresses on chassis issues. Prof. Dr.-Ing. Metin Ersoy completed his doctorate in Design Systematics at the Technical University of Braunschweig and spent more than 30 years at a managerial level at various companies, including 20 years at ZF Lemförder, where his most recent post was Head of Predevelopment. He is also an honorary professor for chassis technology at the University of Applied Sciences in Osnabrück.
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