Laser Light in The BMW i8 Controlling & E/E Integration Mario Werkstetter, Stefan Weber, Florian Hirth, Christian Amann. Laser diodes for vehicle lighting have been deployed in the BMW i8 for the first time in a series vehicle worldwide. LEDs, and now laser light, on the one hand pose higher challenges for electric/electronics (E/E) systems and, on the other hand, offer new possibilities in the vehicle. This article describes how the development of the laser headlights was supported by the E/E engineers of BMW. The claim of development is to establish a comprehensive solution for the electrical and functional activation, a reliable safety concept and intelligent integration into the vehicle electrical system.
MOTIVATION AND CHALLENGES
Laser light offers additional value for the driver in many ways. It represents the benchmark with regard to illumination of the road and provides innovative functions with additional value for the customer. Therefore, in 2009 BMW initiated the development of a laser light source for automotive use. A detailed system description can be found in the September edition of ATZ [1].
The decisive factor in the case of vehicle lighting systems is the light performance on the road. The high-beam headlights make the toughest demands with regard to the technical lighting variables. Here, the illuminance can be equated to the light intensity, i.e. the visual range, with particular focus on the illuminance and luminous flux in the relevant angular ranges.
The light intensity is idealised proportional to the aperture of the optical system, the number of light sources, the optical system efficiency, as well as the luminance of the light source. However, the first three mentioned parameters can only be optimised to a limited extent, especially due to the constraints with regard to installation space and design. Only the luminance enables an increase in light performance without having to accept compromises or disadvantages in other places. This enables implementation of a significantly more compact system, while simultaneously enhancing efficiency and light intensity. In addition to new design freedoms, a smaller and more efficient optical system also provides the possibility for weight reduction.
LASER LIGHT SOURCE
The first white light sources based on semiconductors with high luminance, were developed, for example, to be used in applications like projectors or medical technology. Based on a comparable principle, BMW and their development partners developed a new Lambertian point light source that meets the requirements of both automotive legislation and application in the vehicle. The result of this development is the laser light source fitted in the BMW i8, 1, the so-called light engine. At an operating temperature of 80 °C, it emits an average luminance of 560 cd/mm² (cd = Candela). In comparison, high-power LEDs are in the range from 40 to 100 cd/mm² [2].
Inside the new light engine, the beams of three blue high-power laser diodes are deflected by special optical elements onto a very small YAG (YAG = Yttrium Aluminium Garnet) phosphor plate, as shown in the cover picture. This converts the blue laser beam into diffused, incoherent, white light. The optical properties of the beam (such as radiation characteristics, spectrum, and colour location) are comparable with a conventional light-emitting diode. A free-form reflector projects the light onto the road.
DEPLOYMENT IN THE BMW i8
For the first time, in the BMW i8, this high-energy light source supplements the optical modules of a full LED headlight. Each light source is optimised for deployment according to its optical properties for the specific application. In this way, the LED modules in the headlight generate homogeneous and wide illumination with high efficiency. The laser-based spot module generates a bundled light distribution in the far field with a very high range of up to 600 m.
With the combination of LED highbeam headlights (100 lx) and laser booster (240 lx), the legal maximum of 344 lx specified in the ECE area of applicability (which corresponds to 215,000 cd, respectively 430.000 cd for both headlights [3]) can be reached. 2 illustrates a comparison of the light performance of low-beam headlights, high-beam headlights, and laser booster.
In addition to the mechanical, geometric, and optical development, the electrical system/ electronics are an important component of modern LEDs and laser headlights. For the activation and control of the light sources at component level and for integration into the vehicle electrical system, there are decisive differences and functional enhancements in comparison to headlight systems with conventional light sources.
CONTROL UNIT AND VEHICLE ELECTRICAL SYSTEM
Nowadays, the headlight is part of the vehicle data network. In the BMW i8, the complete headlight is operated, controlled and connected to the vehicle electrical system by one control unit. The laser-specific functions have also been integrated into the modular control unit. The socalled front light electronics (FLE) are connected via CAN to the system buses and are activated by the central master control unit, i.e. it evaluates the commands of the function master and activates the actuator system in the headlight. 3 shows how the headlight, and thus the FLE, is embedded in the vehicle. As some lighting functions have to be available permanently and/ or already available, when the vehicle is approached or parked, the FLE is supplied with permanent + (terminal 30).
In addition to the development of the light engine, it was also important to optimise the driver stages with regard to energy efficiency, which is why these are designed as switching controllers. A central boost converter boosts the vehicle electrical system voltage to the maximum diodes string voltage. From there, each light module is operated and the current controlled by means of a single buck converter. The two-stage driver topology reliably decouples vehicle electrical system influences from the load, thus stabilising the output capacity and ensuring service life.
4 illustrates the architecture in the headlight. All actuators and sensors are driven and/ or evaluated by that single control unit. The intelligent interplay of the LED modules, laser diodes, and stepper motors provides the features of the adaptive lighting function, e.g. ECO light and freeway lights.
The high-beam function is also controlled adaptively, providing the driver with the optimal light distribution at all times depending on the situation. The activation is linked to a camera-based high-beam assistant, thus also reliably excluding any potential dazzle for other road users. In order to realise an illumination range adapted to the vehicle speed, the laser booster is activated automatically by the system at speeds above 70 km/h.
ACTIVATION AND CONTROL OF A LASER
In the same way as LEDs, the efficiency of laser diodes is being steadily enhanced. The product cycles of diodes are significantly shorter than those of the vehicles. In order to still be able to integrate these increases in luminous power immediately into production, BMW fits a binning resistor on the light source boards. This is read in when the control unit is powered up.
Depending on the measured value, the peak current and a base pulse width modulation (PWM) are selected for this channel in such a way that the homologised and specified light values are reached. The colour temperature emitted by LEDs and all semiconductor-based light sources depends on the current applied to them. The basic brightness and desired colour temperature can be set precisely by a current-controlled switching controller. In order to ensure the same appearance of the light under all environmental conditions, constant current and pulse width modulation are used to set the currently required brightness.
The human eye is sensitive to fluctuations in brightness, especially in the case of moving light sources. This can be seen, for instance, on a passing vehicle with a pulsed light source and a modulation frequency that is too low. [3] describes and quantifies this stroboscopic effect and how it disturbs human beings. In order to reliably rule out this effect, importance was attached to achieving a PWM frequency of 500 Hz in the front light electronics.
In order to make switching on and off as well as the transition between lighting functions high-quality and pleasant, changes in brightness are not executed abruptly, rather by means of a ramp. So that the ramps are perceived by the human eye as step-less, electrical engineers at BMW realise a resolution of the PWM pulse duty factor of 1 %; 0.1 % respectively in the lower brightness range. The non-linear, logarithmic brightness sensitivity of the human eye has also been taken into account, giving the ramp an exponential progression. 5 shows such a switch-on ramp for the laser within 20 ms, together with the resulting optical brightness.
The service life and luminous power of laser diodes, more distinctive than in the case of LEDs, are severely dependent on temperature. In order to maintain the light source in the optimal temperature range, a temperature sensor (NTC) is fitted on the light board, in addition to the binning resistor, and constantly being interpreted during activating of the laser. Depending on the measured temperature, the power output of the laser is regulated to ensure constant luminous power over a broad temperature range. As a supplement to this, the fans are activated and the light sources are cooled as needed. Thus, continuous availability and constant performance of the function is ensured.
PRODUCT SAFETY
As three high-power laser diodes are deployed inside the light engine, the implementation of a reliable product safety concept geared to application in the vehicle is very important in order to ensure the safety of the driver and of road users. To comply with the demanding safety requirements set by BMW itself, a comprehensive, multi-stage safety system has been developed that reliably prevents any theoretically possible risk to humans or animals due to damage or malfunction.
The basis is formed by high mechanical stability of the module, as well as the selection of automotive-qualified materials and components. In addition to the functional operation conditions already mentioned, the control unit checks other requirements before it enables operation of the laser module. This includes, for example, a check of the currently correct installation in the intended vehicle on the basis of the vehicle identification data. Furthermore, the electrical connection and sensor system are checked before the laser module is switched on. If deviations are detected, activation of the laser module is not possible.
During operation, the entire laser system is monitored electrically and optically. Electrical deviations from the target operating point are detected by the monitoring of the voltage and of the regulated current of the three laser diodes. As a supplement, the light exit is optically measured and its plausibility is checked taking account of dedicated characteristic curves in a tight tolerance window.
The front light electronics react to all safety instances within a few milliseconds and, if required, switch the laser module off. The availability of the fundamental LED functions of the headlight, however, is retained in full in the very unlikely event of a fault, and it is possible to continue driving without difficulty.
CONCLUSION
With the development of an innovative laser light engine and its intelligent integration into the vehicle electrical system/ electronics, a new generation of headlights has been implemented in the BMW i8. The very compact installation space and high efficiency provide the customer with a significantly enhanced high-beam headlight range of 600 m, while driving safety and comfort at night is increased perceptibly.
REFERENCES
[1] Amann, C.; Weber, S.: Buck, A.: Laser Light in the BMW i8 – Design und Vehicle Integration. In: ATZ worldwide 119 (2014), No. 9 [2] Hanafi, D.; Erdl, H.; Weber, S.: A new efficient, compact vehicular illumination system using high-power semiconductor laser diodes. In: Proceedings of ISAL (2013), pp. 168-179 [3] ECE-regelung r48, rev. 8, 2013, §6.22.9.4 [4] Bullough, J. D.; Hickcox, K. S.; Klein, T. r.; Lok, A.; Narendran, N.: Detection and acceptability of stroboscopic effects from flicker. In: Lighting research and Technology No. 44(4) (2012), pp. 477-483
DIPL.-ING. (FH) MARIO
WERKSTETTER
is Project Leader Advanced
Development Light Electronics at
BMW AG in Munich (Germany).
DIPL.-ING. STEFAN WEBER
is Project Leader for BMW Laser Light
at BMW AG in Munich (Germany).
DR.-ING. FLORIAN HIRTH
is Project Leader for Front Light ECus
at BMW AG in Munich (Germany).
DIPL.-ING. CHRISTIAN AMANN
is Leader of the Department
Light and Sight at BMW AG
in Munich (Germany).
1 BMW i8, with laser light in a high-beam headlight function from 70 km/h up.
2 Comparison of the light distributions from a bird‘s-eye view of (left) LED low-beam headlights, (mid) LED high-beam headlights, (right) LED high-beam headlights with laser light booster.
3 Diagram of the vehicle electrical system architecture of the front light system with power supply and system bus connection of the headlight control units.
4 E/E architecture in the laser headlight with positioning and connections of the different modules.
Laserlight-head lamp of BMW i8
1 Bi-LED module
2 Daytime running and
parking light with integrated
design element
3 Electronic control unit (FLE)
4 Connector
5 Laser-Lightengine
6 Step motor
7 Fan
8 Daytime running and parking
light with integreted turn indicator
5 representation of a light switch-on ramp: In order to make switching on and off as well as the transition between lighting functions high-quality and pleasant, changes in brightness are not executed abruptly, rather by means of a ramp; peak current (blue), PWM pulse duty factor (red), and optical brightness (grey) over time.
