Although ACC (Adaptive Cruise Control) has been touted as being the closest thing to autonomous vehicle control for years, the fact of the matter is that ACC is nothing more than a small step in that direction, at best. While ACC might improve the driving experience for many drivers across the world, from a mechanics’ perspective, ACC systems are complex, convoluted systems that are sometimes difficult to calibrate, maintain, and diagnose. This article will explain what adaptive cruise control is, how it works, and why it might sometimes be better to refer ACC issues to the dealership, starting with this question-
Previous iterations of cruise control systems could only maintain the speed of a vehicle based on a pre-set speed set by the driver. Adaptive cruise control on the other hand, has the ability to adapt a vehicles’ speed with reference to a lead vehicles’ speed, but within the limits of both a preselected speed and following distance set by the driver and the limitations of the system itself.
However, the lack of a set of standardised and internationally recognised operating protocols has resulted in a multitude of proprietary ACC systems that vary widely not only in design principles, but also in how drivers perceive the desirability (or otherwise) of ACC systems in general. For instance, some systems, such as the one used by Ford, does not work at speeds under 30 km/hour and has great difficulty controlling following distances at high speeds, while the system used by BMW works flawlessly from 30 km/hour all the way up to about 180 km/hour. In addition, some systems cannot bring a vehicle to a full stop without inputs from the driver, while others have the ability to stop and accelerate a vehicle in stop-and-go traffic.
It must be noted that ACC does not work in isolation. In practice, the system depends on inputs from other ADAS (Advanced Driver Assist Systems) such as the proximity warning system, lane departure/lane assist systems(s), traction control, and stability control in varying degrees, depending on the level of sophistication of the ACC system.
Other implicated systems include the throttle control system, transmission control (on automatics) system, the ABS system, and various sensors including the steering angle sensor, wheel speed sensors, vehicle speed sensor, and the brake light switch. All communication between control modules takes place via the CAN bus system, with messages from the ACC usually enjoying priority over most other signals.
In practice though, when the ACC system is in operation the ACC control module represents the heart of the system, in the sense that this control module initiates all control inputs/commands to associated control modules in order to control the vehicles’ speed and following distance relative to a leading vehicle.
Given the above, it should be understood that this article cannot provide detailed information on all ACC systems in use today. This article can only provide a generic overview of the basic features of ACC systems in general, and reference to relevant technical manuals must therefore be made for detailed technical information.
Nonetheless, for the sake of clarity, this article will explain the basic operating principles of ACC at the hand of the various technologies, associated systems, and the relationships between these in terms of ACC system operation, starting with the-
Radar transponder
Modern, i.e., fifth-generation radar systems typically operate at a frequency of 77 GHz, and use the Doppler Effect in a 65 degree-wide “cone” of radiation to detect differences in the relative speeds of the lead and following vehicles, usually out to a distance of about 300 metres or so. On some advanced ACC systems, the radar waves detect not only the lead vehicle, but also the vehicle in front of the lead vehicle, which information is also fed into the ACC control module. This information is used to calculate appropriate braking strategies should both lead vehicles suddenly decelerate.
NOTE: Although radar technology is now the preferred technology in most ACC systems because it is not affected by adverse weather conditions or poor visibility, some manufacturers still use lidar-based systems. In practice, lidar is more cost effective than radar, but since lidar use light waves instead of radio waves, lidar systems are hampered by rain, snow, or mud that are not transparent to light waves. Note also, that one major manufacturer, Subaru, uses a system of two optical cameras that are mounted on the windscreen to detect the lead vehicle.
Adaptive cruise control system control module
The primary function of this control module is to process information received from the radar system to detect not only the presence or absence of a leading vehicle, but also to calculate the relative difference in their speeds. Based on this information, and the following distance and maximum selected by the driver, the ACC control module will communicate with the engine and ABS control modules on a continuous basis to control the vehicles’ speed to match that of the lead vehicle at the selected following distance.
Instrument cluster
When the ACC system is activated, the function of the instrument cluster is to both process and monitor the status of the ACC systems’ various control switches, and then to pass this information along to all implicated control modules and subsystems. Additionally, the instrument cluster also displays all relevant status and warning messages about the ACC system in order to keep the driver informed about the overall status of the ACC system.
Engine control module
If the ACC system is in operation, the engine management system is controlled by the ACC system until the ACC system is deactivated, or the driver intervenes by braking manually to avoid an accident.
ABS control module
On most ACC systems, the ACC control module will control braking via the ABS system up to about 30% or so of the vehicle’s total braking capacity, beyond which audio and visual alarms are activated to alert the driver to apply the brakes manually. However, on some ACC systems the ABS system will respond to inputs from the ACC system to bring the vehicle to a complete stop without any inputs from the driver.
System controls
The ACC system controls are almost invariably mounted on the steering wheel, and commonly include the following switches/buttons-
CAN bus system
All communication between the ACC control module and associated control modules take place via the CAN system, with messages from and to the ACC control module usually enjoying priority over other messages while the ACC system is in operation.
From the above it should be obvious that if the ACC system is to work as intended, all of its associated systems and control modules have to be in perfect working order. Any breaks in communication will automatically deactivate the system, and the driver will see a message on the dashboard that usually says something like “ACC Unavailable”.
While there is no doubt that a fully functional ACC system can improve the driving experience, it has one major drawback, which involves driver expectations. Many drivers are under the mistaken impression that an adaptive cruise control system will drive their vehicles for them, and long experience has taught this writer that many drivers interpret the normal functioning of the ACC system as defects, or sometimes even as attempts by their vehicles to kill them. Two examples of this are given below-
Sudden, violent acceleration
When the ACC system is in normal operation, a vehicle will maintain the distance between itself and the vehicle in front of it fairly accurately by speeding up or slowing down as required. However, when the leading vehicle suddenly leaves the lane it is in, the ACC system might lock on to the next vehicle in the same lane. Since this following distance has now greatly increased, the vehicle might suddenly accelerate in order to achieve the previously set following distance, which is an experience many drivers find highly disconcerting and unpleasant.
Most, if not all car manufacturers are now aware of how this aspect of normal ACC operation affects some drivers, and acceleration rates in the scenario have therefore been tempered somewhat to reduce the sensation some drivers have that their vehicles are trying to kill them.
Sudden, violent braking
On early iterations of ACC, it often happened that a vehicle might slam on the brakes automatically when a vehicle cuts in front of the following vehicle, thereby seriously reducing the distance between the vehicle following, and the vehicle leading.
However, newer iterations use input data from proximity warning and lane departure systems to detect vehicles alongside. This has the effect of forewarning the ACC system that a vehicle might move into the gap between the leading and following vehicles, and based on this input data/information, the ABS system will modulate the braking action to a tolerable level when another vehicle moves into the gap between the leading and following vehicles.
Since most dealer-grade scan tools have the ability to diagnose almost any ACC related defects, actually repairing those issues is often another matter entirely. At the heart of the problem is the fact that on all applications, repairs on the ACC system invariably requires the calibration of the system, which is very difficult, if not impossible to achieve without the required equipment.
Compounding the problem is the fact that on some applications, all the sensors, cameras, and detection equipment that is incorporated into the ACC system have to be calibrated at the same time, and worse, in reference to make and model specific targets under conditions that border on laboratory conditions.
In practice, many factors can upset or disturb the calibration of ACC components, such as minor fender benders that might move a radar transponder out of alignment, hitting a pothole at speed, which might disturb steering angle sensor settings, or even routine brake and suspension system repairs that might upset wheel alignment settings. To put the difficulty of rectifying these issues into some perspective, consider some of the required conditions that must be available to recalibrate a modern ACC system-
Space requirements
Most manufacturers specify that a large, level, indoor area be available. For instance, to calibrate an ACC system on a Honda application, you will need an indoor space that is equipped with non-glare lighting, and that is at least 3.9 metres wide, a minimum of 2 metres high, and that extends at least 7 metres in front of the vehicle. In the case of Lexus applications, the recalibration procedure can be done outdoors, but on a level surface that extends to at least 13.7 metres in front of the vehicle. In both these examples, the area must be free of clutter that can disrupt camera views, and/or metallic objects that can disrupt radar signals.
Required equipment
At a minimum, and apart from a dealer-grade scan tool, you will require a wheel alignment rack that can perform 4-wheel alignments, since almost all AC systems are calibrated to both external targets, and the vehicle’s thrust line. While many new wheel alignment racks now incorporate ADAS calibration equipment, some ACC systems still require additional on-road and static checks to validate/verify the recalibration process.
Vehicle preparation
Preparing a vehicle for an ACC system recalibration is the easy part, but pointless if the other requirements cannot be met. Nonetheless, some specific preparations might include the following-
As a practical matter, it is often impossible for a small, independent shop to perform the required or recommended repairs to adaptive cruise control systems, simply because acquiring the required space and equipment places an impossible financial burden on the operator.
Nonetheless, as experienced technicians, we have the responsibility of always acting in the best interests of our customers, but in this case, the interests of our customers are best served by referring all ACC related issues to the relevant dealership, because if we lack the proper equipment, we cannot verify repairs to this complex system.
