Today's automobiles often include headlamps with dynamic position control. For high intensity discharge (HID) Xenon lamps, this function is considered critical, and safety regulations in Europe require dynamic control of the vertical position of the main headlamp beam to avoid glare. However, headlamp-positioning systems present a very harsh environment for electronics components. The extremely high ambient temperatures to which they are exposed have a large impact on the design of the microelectronic drivers.
Fortunately, using stepper-motor driver chips, it is possible to design integrated electronic motor-driver circuits for these and similar applications that require only a few passive components. The driver chip gets its instructions as high-level positioning control and diagnostic commands through a LIN, I2C or SPI bus and turns these into PWM signals driving the coils of the stepper motors. Benefits of integrated motor-driver circuits include increased system integration, reduced wire-harness complexity and lower EMC emissions, which result in lower system cost, faster end-product time to market and increased performance.
Traditionally, European halogen headlamp systems were fitted with a manual adjustment to aim the headlamps in the vertical direction. This installation usually consists of an analog servo that drives an actuator containing a geared, brushed-dc motor to a position that corresponds with the manually adjusted position. The feedback of the servo is typically a potentiometer connected to the end-gear of the actuator, and the motor driver is a power op-amp (Figure 1).
This system is relatively inexpensive; however, some OEMs are dissatisfied with this actuator principle. There are several reasons, including quality issues with gears and the potentiometer. Moreover, if a headlamp is incorrectly aimed, this degrades the reputation of the automobile brand and may require a car owner to go to a service station. Even worse, an incorrect headlamp position may cause an accident due to glare or a poorly lit road during night driving.
An alternative approach for the vertical positioning of headlamps is with linear stepper motors. These motors are very robust and they operate in open-loop mode (thereby precluding the need for potentiometer feedback). The linear movement is obtained through a bolt/nut combination. The stepper motor that rotates the bolt has a magnetic rotor that is moved by a control current in the stator coils.
In Asia, Xenon headlamp systems are increasingly equipped with stepper motors (the Xenon headlamp feature has also started to gain market share in the United States), and these linear motors were introduced years ago for Xenon leveling systems. High-volume, high-quality production is currently available, and these linear motors are now also used by some OEMs for halogen-headlamp leveling systems.
In addition to the vertical positioning function previously described, these integrated motor drivers can support other motion-control features emerging in some high-end cars. For example, headlamps with swiveling capability illuminate the relevant part of the road, anticipating a change of direction and thus increasing driver visibility. This motion combination is achieved by rotating the headlamps along two axes. Beam shaping as a function of the environment and car speed can add a third degree of motion control.
In Xenon systems, for example, a full adaptive front-lighting system, or AFS requires actuators for swiveling and beam-focusing functions, in addition to the leveling function, and all functions can be implemented using stepper-motor technology. Stepper motors are preferred over brushed motors due to the frequent operation of these actuators. In addition, halogen swiveling actuators are, due to the stringent quality requirements, also provided with stepper motors.
Because headlamps are safety-critical devices, the motors that control their position must also operate in an autonomous way, turning the lights into a safe position if the communications bus fails. This requirement means that the driver circuit must detect stall conditions without the need for external sensors, and guarantee silent and smooth movement through its micro-stepping modes. These capabilities are, therefore, essential features of any headlamp position control architecture, regardless of whether it is centralized or distributed.
CENTRALIZED ACTUATOR CONTROL
Xenon Headlamp leveling is a typical example of single axis control for vertical headlamp positioning. Although two motors are driven (one for each headlamp), their positioning is synchronized and a microcontroller can drive both stepper motors using the same positioning algorithms. Figure 2 shows an electronic control unit (ECU) based on a microstepping motor driver for bipolar stepper motors. The SW block within the microcontroller (micro) block represents the firmware that implements the motor-positioning algorithm. This algorithm dictates in real time each detailed step that the stepper motor needs to take.
For vehicles with only a single lighting ECU per headlamp, two-axis control (leveling and swiveling) may be required. These systems also use two separate headlamp motors, but the position of each motor is usually different from the other, requiring independent drive control for each motor. The software that drives the two motors in such a system must calculate and execute separate headlamp horizontal positions in real-time, which results in more complex firmware (as indicated by the larger software block in Figure 3). Qualification of the headlamp control system becomes more complex since all different combinations of headlamp target positions and motor speeds, accelerations and decelerations must be tested.
Using smart stepper drivers that contain a positioning state machine — thereby allowing the microcontroller to send high-level commands to the motor driver — reduces the software in the microcontroller. While this reduces the size and complexity of the microcontroller's firmware, it also adds a “smart” component to the driver in order to provide headlamp-positioning intelligence.
Nevertheless, qualification efforts for the microcontroller software are significantly reduced, due to the absence of multiple real-time tasks. Instead, only slow-speed high-level tasks need functional verification. The high-level commands from the microcontroller are translated by the motor-driver chips into low-level timing information to drive each motor, in parallel, to its desired position. This repositioning can even be performed at the requested speed and desired acceleration or deceleration.
Moving beyond this dual-axis control system for only a single headlamp, combining the leveling and swiveling functions of two headlamps effectively means the controller must now deal with three axes of control. While smart stepp-er-motor controllers solve the problem of complex microcontroller software, these systems can require as many as 15 wires per headlamp, and can, therefore, adversely affect the modularity of the hardware.
To understand why this is true, it is important to realize that OEMs often provide different headlamp features, such as swiveling or AFS, as customer-selectable options for the same vehicle model. This means that different ECUs need to be designed by the headlamp system developers, with each ECU equipped with different software that requires varying degrees of qualification. In order to reduce these efforts and also time to market, it makes sense to look at distributed wiring architectures for actuator-motor control based on a local interconnect network (LIN) bus.
DISTRIBUTED ACTUATOR CONTROL
The LIN standard defines a low-cost, serial communication system for automotive distributed electronic systems. LIN complements the existing portfolio of automotive multiplex networks, including those using the controller area network (CAN) protocol. However, the LIN standard targets applications that require networks that do not need excessive bandwidth, performance or extreme fault tolerance.
The LIN standard enables a cost-effective communication network for switches, smart sensors and actuator applications within the vehicle. The communication protocol is based on the SCI (UART) data format, a single-master/multiple-slave concept, and a single-wire (plus ground) 12 V bus.
Figure 4 shows a LIN-based headlamp control system for leveling, swiveling and AFS of two headlamps. The LIN microstepping motor driver is a two-phase driver with a position controller integrated with LIN control/diagnostics. The controller receives high-level positioning instructions through the LIN interface and subsequently drives the motor coils until the desired position is reached. The on-chip position controller is configurable for different motor types, positioning ranges, and parameters (such as those for speed, acceleration, and deceleration). Sensorless stall detection prevents the positioner from losing steps and stops the motor if the system detects a stall condition.
The high abstraction level of the controller's command set reduces the load on the microprocessor in the ECU. Scaling of the application toward different numbers of axes of headlamp motion control is straightforward. Hardware and software designs are extended in a modular way, without severely affecting the demands on the master microcontroller. The architecture in this system is advantageous because only one ECU is used and addition or removal of the optional motors is an easy and inexpensive way to scale the control functions of the system.
Authors Note: More information about headlamp technologies is available from The Motor Vehicle Lighting Council (MVLC) website at http://www.mvlc.info./.
ABOUT THE AUTHOR
Bart De Cock is product manager, automotive products at AMI Semiconductor.
He offers more than a decade of automotive electronics experience to AMI Semiconductor. He began his career within AMIS in 2001.
In 1990, De Cock earned a master degree in electronics engineering from Brussels University. In 2004, he obtained a post-graduate degree from Solvay Business School.
Bart De Cock has had several scientific papers published in ESSIRC, IEEE-JSSC and PAL with an additional number of patent-applications. He has also had several technical articles published in magazines such as Automotive Electronics International and Electronique. De Cock speaks Dutch, English, German, and French.