Automotive Electronics
Lighting advances in tomorrow’s vehicles
Electronics remains the fastest growing sector of automotive content and within that space ambient LED applications are fast becoming a key differentiator among automotive manufacturers. As a result it is critical that you choose the right embedded solution
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dvancements made in ambient LED lighting solutions stem from the quest by automotive - OEM interior stylists looking to add value and make their vehicles more appealing to the driver’s senses, while creating a truly positive driving experience and increasing brand awareness. Electronics continues to be the fastest-growing sector of automotive content, over mechanical, pneumatics and hydraulics. With the increasing number of ambient LED applications from cluster backlighting to cupholder lights, to map lights, mood lighting in footwells and door trims, to dome lights, LED solutions, both white and RGB, are differentiators that are being used by automotive OEMs. In fact, research also indicates that lighting can help enhance a driver’s visual senses. As companies look to differentiate it is critical to select the right embedded solution to match the application requirements and enable cost-effective designs with a shorter time to market. In this article well be taking a closer look at: 1. the trade-offs among the methods for controlling LED ambient lighting solutions
2. the use of control algorithms to enable effective binning, dimming and temperature compensation for LED colour
3. creating driver-preferred lighting scenarios by changing the colour and brightness of the LED arrays
With the ever-increasing need for networked lighting modules, implementing best practice for LIN communications in ambient LED lighting solutions can help reduce development costs and cycle times. Because LEDs consume less energy and occupy less space, fuel efficiency is enhanced and the packaging benefits allow stylists to expand their creativity in using lighting to enhance the cabin. Overall, ambient LED lighting solutions
provide OEMs with an opportunity to innovate a comforting atmosphere for both the driver and passenger. Ambient LED lighting is far more than
10 October 2012
just a gimmick. Research on the impact of interior lighting has widely been ignored in the past few years, while the R&D focus has been on exterior lighting. This is changing. With the rapid advancement of LED technology in the past few years, OEMs are in the position to offer not only more colourful interior lighting but also user-adjustable lighting. Drivers are able to adjust lighting to their mood or taste. As a result there is so much more potential in interior lighting design. In future car models, interior lighting is poised to become part of the advanced driver information system. A revolutionary idea it is intended to adapt the interior light according to the driving situation. The interior light colour could be different for the city, rural roads and highways, for example. Red interior light can increase the driver alertness in the city, while a yellow interior light can help the driver to relax when cruising on a highway. Interior lighting could even warn the driver about an imminent accident. In many cases, flicker-free interior lighting is associated with a high-value proposition for car makers to differentiate their products in an increasingly competitive environment.
Driving the LEDs
LEDs must be driven with a source of constant current. Most LEDs have a specified current level that will achieve the maximum brightness for that LED without premature failures. The light output from a LED is proportional to the current passing through it. There are two techniques to control the LED brightness in the circuit. One is to vary the LED drive current. The LED drive current may be controlled using a variable resistor or by using a variable- voltage power supply. Another technique is to apply pulse-width modulation (PWM) or variable-frequency, fixed-duty-cycle PWMs to the LED drive current. The first method has two major disadvantages: as the current is reduced so the LED efficiency may also be reduced. And, in high-power white LEDs, a colour
Components in Electronics Figure 1: Block diagram showing solutions with 3 and 4 PWM channels
shift may take place with the reduction in current level. The PWM dimming technique always drives the LED at full current. Therefore, problems such as reduced efficiency and colour shifts can be eliminated. To create different colour combinations, the duty cycles of the PWM outputs will have to change over time.
Colour control methods Embedded microcontroller technology allows the colour components of a light source to be sensed and actively controlled. Active colour control can be used to create a specific colour of light, or can be used to create a mix. For example, multiple component colours can be blended to create a specific quality of white light. Using three individual colour values for Red (R), Green (G) and Blue (B), plus an additional value for brightness, supports the generation of the desired colour either with three PWM channels or, simpler and more effectively, by three PWM channels -
can be achieved with four channels, each with 10-bit resolution, which is typically found in a low-cost 8-bit MCU. In this four-channel scenario, colour x brightness multiplication occurs in an external hardware circuit. The typical logarithmic gradation required for brightness control can also be realized in a sufficiently precise manner on such a low cost 8-bit MCU, with approximation via a power function (see Figure 1). LED temperature is one of the factors that have a significant impact on the LED’s colour. Hence, temperature has to be compensated. A simple, effective and cost- efficient method for temperature compensation is to use a microcontroller’s on-chip comparator and a low-cost Negative Temperature Coefficient (NTC) thermistor that is positioned close to the LEDs. Another way of gaining control over the LED’s temperature is by measuring its forward voltage drop. The resolution of a microcontroller’s integrated 10-bit analogue-to-digital converter (ADC) is
Figure 2: Block diagram showing LIN transceivers with and without auto addressing
one for each colour - and an additional, separate fourth PWM for brightness. The latter approach can be seen in recent Ford Motor Company models offering mood lighting.
The approach that uses three channels for integrated colour and brightness control typically requires a resolution of 14- bit to 16-bit and a quite powerful microcontroller (MCU). The same quality
sufficient for this task. The forward voltage drop measurement has its benefits, as it eliminates the need for additional external components.
Lighting networks Colour correction, temperature compensation, colour changing, colour mixing, brightness control and the desire by car makers to realise various lighting
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