There’s a lot of complexities involved in automatic emergency braking (AEB) systems on today’s vehicles. As a continuation to the “Understanding ADAS Systems” article published last month, we’re diving into all things AEB. First, however, is aligning on the correct AEB name. Depending on the OEM, AEBs can take on various names, including:
Collision Mitigation Braking System
Autonomous Emergency Braking
Forward Collision Plus
Forward Collision Warning Plus
Pre Sense Front
Although the names are very different, the way that these systems work is quite similar. Sure, the software and specific hardware may be unique to each OEM and even to a specific model, but the sequence of events that transpire during the application of AEB system is quite common among vehicle makers regardless of the OEM.
The AEB system typically will work in stages to detect a potential collision, provide warnings and avoid a collision or reduce the severity of an impact in the event a collision cannot be avoided entirely. These stages are not independent and will often chronologically overlap as to when they are initiated. These can typically be broken down into four stages:
Pre-Stage and Vehicle Conditioning: This category is first in the sequence and is implemented at the first recognition of a potential collision by the AEB system. By gathering information from sensors and cameras, the system can detect within milliseconds if an emergency braking situation exists. It includes the following actions by the system:
Adjustment of electronically controlled steering and suspension components
Selecting a braking diagram based on the initial sensor/camera information
Pre-charging of the braking system
This means that the AEB system automatically adjusts the steering and/or suspension to tighten it up to limit body roll and weight transfer. This limiting of weight transfer helps to maximize the braking by reducing the amount of weight that moves off of the rear wheels and onto the front wheels. Keeping the weight balanced means that more brake pressure can be applied to the rear without locking up the brakes and causing the ABS to activate.
Limiting body roll will have a similar effect as reducing front to back weight transfer, except here the benefit is seen not so much with braking but with the steering. Sometimes in an emergency braking situation the driver will attempt to avoid the potential collision with a sharp steering maneuver which will involve steering in one direction and immediately steering in the opposite direction. This causes a tremendous amount of force to be applied left and right. Limiting the amount the body can roll during this maneuver reduces the side-to-side weight transfer which can help avoid skidding and loss of vehicle control.
The braking diagram chooses the best braking scenario based on the initial inputs and can change scenarios in milliseconds as the data from the inputs changes. Pre-charging of the brakes puts pressure in the braking system ahead of any braking. This can allow for full pressure to be applied to the brakes even when only light pressure is applied to the pedal by the driver. This will allow shorter stopping times when the driver or the system brakes the vehicle.
Driver warning: There can be a number of different means to bring attention to the driver that a collision may be eminent. These may be independent, or they may be simultaneous. Warnings include:
Visual
Acoustic
Haptic
Steering wheel jolt
Typically, the visual is a warning light or icon that will display on the instrument cluster or a head up display. This is often the first warning the system will give. An acoustic or audible warning is often next. This beep or alarm is often activated along with the visual warning as a way to intensify the importance of the warning to the driver. A haptic (vibration) warning may also be used along with visual and acoustic. Next is a jolt to the steering wheel. This is typically the last warning before braking intervention by the AEB system. Warnings typically continue until automatic braking is applied or until the eminent collision is avoided.
Active brake intervention: The system will apply partial braking to the vehicle if there is zero or insufficient driver braking. Partial braking is typically done in the following stages:
Standard
Extended
The difference in these two stages is that extended braking is roughly double the braking force than that of standard braking. Sufficient driver braking will override the AEB partial braking and often will cause the stop lamps to blink as a warning to other drivers that emergency braking is taking place with the vehicle ahead.
Automatic Emergency braking: This is typically the last resort for AEB, although in some cases it may be the first and only depending upon the situation. For example, if a vehicle suddenly pulled in front from a different lane or pulled out from a side street, it may trigger full braking by the AEB system. Full braking will typically be three times as much braking force as the standard braking with active brake intervention and may apply up to 100% of the braking force of the vehicle.
Again, it is important to remember that these stages do not occur separately but simultaneously to a certain degree and that this overlap in stages will constantly vary depending upon the specific emergency braking situation.
With a better understanding of how AEB works and the speed and complexity of the decision making of the system, consider now the importance of the inputs and how even the slightest misalignment of these inputs could have negative results on the AEB system.
For example, if a front radar sensor or camera is not calibrated correctly and is only off by say one degree, the system may continue to operate and no warning light or DTCs may be set, but that does not mean that the system will be operating as it should or as it was intended. Even one degree off will mean that the radar or camera is not “looking” where it was intended to. This may cause false alerts and unexpected braking or even worse, delayed or non-existent detection of an eminent collision.
Drivers come to expect the AEB to function in a specific and consistent manner. Having a small amount of misalignment of a sensor or camera will inevitably change the manner in which the system works. This will increase the risk for a collision if the AEB does not react as expected, or as it did before.
When dealing with angles and degrees, a change in a degree of measurement does not seem like much. However, the further you move out from the apex of an angle (the point where the two lines meet) the distance between those lines increases.
To visualize this, take a flashlight and shine it on the wall at a close distance (a foot or less). Now move it slightly to the right or left; see how the beam moves only slightly. Now back up 10-15 feet and move the light the same amount to the right or left. Observe how much greater the distance is that the beam of light moves. Now imagine that beam of light is the field of view for a camera or the sensing area for a radar, it should now be clear how even a slight misalignment can make a huge impact.
This means that for a long-range radar, that “sees” out many feet in front of the vehicle, one degree of misalignment at a one-foot distance is only potentially inches off, but at 20 feet out that same degree of error is now off in many inches and potentially even feet and can drastically change the field of view. There is a reason OEMs provide measurement specifications for certain inputs and very specific instructions on how to calibrate those inputs back to the OEM specification.
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