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Simplifying movable-headlamp motor control

Posted: 18 Mar 2008 ?? ?Print Version ?Bookmark and Share

Keywords:CAN bus? LIN? motor control? headlamp? stepper motor?

By Bart DeCock
AMI Semiconductor

Adding dynamic motor control to traditionally fixed headlight sources can result in a safer ride for car occupants. Three types of headlamp motion bring about the safety improvement in automotive headlampsvertical, horizontal, and advanced front-lighting system (AFS).

Automotive manufacturers dynamically control the vertical position of the main headlamp beam to avoid glare in the eye of an oncoming driver. When the vertical light-leveling system is linked to the car's suspension, it maintains correct positioning of the headlight (the beam from the headlamp) for different vehicle loads. More complex systems also include road inclination information to adjust and keep the beam correctly oriented. In all cases, both headlamps receive the same degree of correction.

Headlamps with horizontal swiveling capability provide improved lighting by illuminating the appropriate part of road curves, anticipating a change of direction and thus increasing driver visibility. This motion combination is achieved by rotating the headlamps independently.

Adding another level of complexity, AFS controls the headlight beam based on vehicular steering and suspension dynamics, as well as ambient weather and visibility conditions, vehicle speed, and road curvature and contour. There is also development work underway to use GPS navigation inputs for anticipating changes in road curvature.

In a paper presented at the 2007 annual meeting of the Transportation Research Board (see Reference), the author found that headlamp swivel should be predictive, to aid driver judgment of curve sharpness prior to entering a section of curved road. He also stated that future headlight swivel systems should be able to predict road geometry before arriving at the curve, when coupled with commercial vehicle navigation systems.

Automotive bus
Headlamps are a harsh environment for electronics. The extremely high ambient temperatures to which they are exposed due to self-heating, greatly impact the design of the headlamp motor drivers. In addition, the motors have to operate autonomously, placing the lights into a safe position if the communication system supplying positioning information fails. Using common automotive buses, such as CAN and LIN with programmable stepper-motor-based mechatronic headlamp control systems, supports complex motion control for headlamps and is also useful for other automotive applications.

Headlamp leveling is a typical example for single-axis control. The two motors have synchronized vertical positioning, so a MCU can drive both lamps with the same positioning algorithms. The figure below shows a headlamp leveling system with micro-stepping motor drivers for both right and left headlamps. The "SW" box with the MCU is the firmware used by the MCU for real-time positioning of the headlamp stepping motors.

Two headlamps, each with its own driver, have synchronized vertical motion controlled by a microprocessor with instructions from a CAN bus.

Two headlamps, each with its own driver, have synchronized vertical motion controlled by a microprocessor with instructions from a CAN bus.

Dual-axis control
When you enhance headlamp motion control to combine swiveling and leveling, you now have a situation where each headlamp, independently, must move through two degrees of motion and each uses two motor drivers. The firmware to drive the two headlamps is more complex than the leveling-only scenario, as shown by the larger "SW" box in the figure below. System verification and qualification also become more intricate and time consuming, comprising all possible combinations of speed, acceleration/deceleration, and target headlamp positions.

Controlling vertical and horizontal positioning requires two stepper motors for each headlamp and more complex firmware for the MCU, which now must control a total of four motor drivers.

Using "smart" stepper motor drivers that contain "positioning intelligence"state machines for headlamp positioningallows the MCU to send high-level commands to the motor driver and off-loads some of the firmware the MCU needs.

The job of software qualification is greatly reduced, due to the absence of multiple real-time tasks. Instead, only slow-speed high-level tasks need to be functionally verified. The motor drivers convert high-level commands from the MCU into low-level timing information that drives the motors in parallel to the desired position at the requested speed and desired acceleration or deceleration. The parallelization simplifies software design by making it modular.

Smart micro-stepping motor drivers (below) help alleviate the problem of complex MCU software, such as that required to drive headlamps with leveling, swiveling, and AFS features.

This example of a smart stepper motor driver and controller includes a LIN interface, configurable position controller, and two H-bridges to drive the separate coils of a headlamp positioning motor.

However, the wiring overhead per headlamp of such a system (15 wires) is quite large and the modularity of the headlamp control system is reduced (see figure below).

For headlamps capable of leveling, swiveling, and AFS, you need three drivers per headlamp. The three horizontal orange and white blocks in the center are the smart micro-stepping motor drivers.

Because the various headlamp motion control features are usually car-buyer options, to eliminate the need for different lighting ECUs and different MCU code for various motion feature sets, headlamp motion controller system developers can work with a distributed wiring architecture that uses a LIN-based bus between the MCU and the motor drivers.

Modular distributed networks
As a low-cost, serial communication system for automotive distributed electronic systems, LIN complements the portfolio of automotive multiplex networks, such as CAN, for applications that do not need excessive bandwidth, performance, or extreme fault tolerance, such as headlamp motion control.

The figure below shows a smart-driver motion control system for both headlamps whereby leveling is considered a "standard feature" (for example, for vehicles with Xenon headlamps) and swiveling and AFS are options. The system needs only one ECU and the number of wires to control both headlamps is reduced from 30 to 22. Adding and deleting optional motors is easy and does not significantly affect the ECU hardware or software.

Putting optional features such as swiveling and AFS under LIN control makes the lighting motion control system more modular and reduces wiring harness complexity, cost and weight. The twin orange and white blocks within the ECU box are smart micro-stepping motor drivers, while those four at the far right are LIN-enabled motor drivers.

The LIN-enabled motor drivers in the above configuration are two-phase drivers with a position controller integrated with LIN control and diagnostics. The controller gets high-level positioning instructions from the LIN bus to correctly position the headlamp motors. The on-chip position controller is configurable for different motor types, positioning ranges, and parameters for speed, acceleration, and deceleration. Sensor-less stall detection prevents the positioning logic from losing steps and stops the motor if the system detects a stall condition.

And placing all six headlamp controllers on the LIN bus can further modularize the system and reduce the headlamp-motion wiring harness to 18 wires (below).

Putting all motor controllers on a LIN bus minimizes headlamp control wiring cost and complexity and simplifies vehicle preparation for different headlamp motion control feature sets.

Conclusion
Adding various degrees of motion control to vehicle headlamps gives drivers and passengers a safer, more comfortable ride. Using LIN-enabled smart micro-stepper motor drivers lets designers perform intelligent system partitioning and simplifies complex multi-axis real-time programming, leading to less expensive and faster time-to-market designs, for a larger customer base with access to a desirable, optional safety feature.

About the Author
Bart DeCock
is product manager for automotive products at AMI Semiconductor.

Reference:
Hagiwara, Toru, Investigation of Headlight Swivel-Angle Preference at Curves on Rural Two-Lane Highways, Paper # 07-2139 presented at Transportation Research Board 86th Annual Meeting January 21-25, 2007, Washington, D.C.





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