“The auto headlamp follow-up steering system (AFS) combines car steering and other information to control the direction of the lights in real time, reducing blind spots. The driver chip of the stepper motor in this system is indispensable. This article will briefly introduce the realization of the AFS system. Two low-side driver chips NCV7240 and NCV7754 that can drive two stepper motors at the same time are recommended.
The auto headlamp follow-up steering system (AFS) combines car steering and other information to control the direction of the lights in real time, reducing blind spots. The driver chip of the stepper motor in this system is indispensable. This article will briefly introduce the realization of the AFS system. Two low-side driver chips NCV7240 and NCV7754 that can drive two stepper motors at the same time are recommended.
1. Moving headlights
With the development of technology and people’s pursuit of driving safety, auto headlight follow-up steering system (AFS) is also increasingly used on the body of various brands. In the past, the illumination angle of the headlights of most cars was fixed. When the car is driving on a curve, because the beam cannot be deflected, a certain “blind zone” will appear due to the driving angle, which will affect driving to a certain extent. Security. In the case of fixed light, this blind zone is inevitable. If you add a motor to the car’s headlights so that the lights can follow the direction of the car’s turning, this problem can be solved and the driving will be safer. The headlights that turn to follow the steering of the car are the car headlights following the steering system (AFS).
Figure 1 Comparison of No AFS and AFS
When the car is turning, the AFS system uses the steering wheel angle and instantaneous speed of the car as the controller input parameters, and calculates the lamp corner suitable for the vehicle speed and steering according to the designed control to adjust the horizontal direction of the light beam to ensure that the vehicle is turning. Always aim at the road that is about to enter the driver’s field of vision without delay. The body will change its trim angle due to different front and rear loads and acceleration and deceleration. Therefore, it is also necessary to adjust the vertical direction of the headlights according to the front and rear axle sensor signals of the vehicle body to adapt to the lighting requirements of different road gradients.
1. AFS actuator composition
A complete AFS system includes a main controller, a lighting lamp and an actuator that controls the angle of the lighting lamp. The actuator mainly drives two stepping motors to control the angle of the lighting lamp in the horizontal and vertical directions. Among them, the drive chip can use a special chip for a stepper motor, or a multi-channel low-side driver. For example, the automotive-grade eight-channel low-side driver introduced by ON company is very suitable for this system. Each stepper motor uses four low-side channels, so that each eight-channel low-side driver can drive two stepper motors. Each actuator only needs one MCU, and a low-side driver chip can control a front light in the horizontal and vertical directions.
Figure 2 Block diagram of AFS actuator
2. Introduction of low-side driver chip
Figure 3 Typical application circuit
NCV7240/NCV7754 is an eight-channel low-side driver for vehicles, each channel provides up to 600 mA (NCV7754 is 500 mA) drive capability. The main difference between NCV7240 and NCV7754 lies in the driving capability. The driving capability of NCV7754 is slightly weaker, and there is basically no difference in other functions.
Through the SPI port for output control, you can easily report open load (or short-circuit to ground), overload and over-temperature faults. In addition, the output can also be controlled in parallel (in pairs) through the INx pin. Each output driver has current overload protection, and there are two Zener diodes inside to protect the driving transistor from the back electromotive force generated by the motor shut down.
Features and advantages
There are 8 drive channels, high integration, saving circuit board space;
16-bit SPI control can perform 8-bit frame error detection and daisy chain connection;
Support PWM operation;
Low quiescent current in sleep and standby modes, which can meet the quiescent current requirements of automotive modules;
Compatible with 3.3V and 5.0V input levels, suitable for multiple MCU interfaces;
Can report no-load, overload and overheating faults;
NCV7240 provides 8 independent 600mA power transistors, the source is connected to the reference ground pin, and the drain is connected to each pin, thereby generating 8 independent low-side drivers. The output drivers are all located on the same side of the chip to facilitate PCB design. The internal clamping design can limit the transient voltage when switching inductive loads. The overload detection current of each output is 0.6A, and the internal filter is set with a delay time of 3S to avoid misoperation caused by current spikes during normal operation. After the overcurrent time exceeds 3S, the driver will shut down and remain locked. Use SPI to clear the relevant flags to continue working.
The NCV7240 contains eight independent thermal sensors located near each output driver. When the detector detects an overheating event (usually 175°C), the channel will be latched. When the channel is locked, the unaffected channel can also be operated.
After the EN pin of the chip is enabled, the standby mode, input mode, open mode and close mode can be selected for each channel through SPI. It is convenient for MCU to control the drive chip. No-load, over-current and over-heating can be fed back through SPI, which is convenient for MCU to understand the chip status in real time.