Automotive Engineer: What are the advantages and disadvantages of ACC adaptive cruise technology?

Note from Leiphone.com: The author of this article, Mr. He, is an automotive engineer and Leiphone.com has been authorized to reproduce the article.

Cruise is divided into fixed speed cruise and adaptive cruise

Cruise control

As the name implies, cruise control means that the car cruises at a certain speed without the driver's operation (stepping on the accelerator). Cruise control requires a certain speed to enter (this can be calibrated, for example, some cars can only enter when the speed is greater than 50). There will be an initial speed (for example, 50kmh) when entering cruise control, and the speed can be adjusted by buttons (increase, decrease, fast increase, fast decrease). There are also a series of conditions for exiting cruise control, such as exiting cruise control when stepping on the brakes, and exiting cruise control when the car itself is not in a suitable state (there is a problem with a component).

Cruise control is relatively simple (actually the logic is quite complicated), it just does not have complex sensors, processors, etc.

Cruise control is only applicable when the road conditions are good, such as highways with few cars.

Adaptive cruise control

Adaptive cruise control (ACC) is relatively intelligent and can generally enter cruise control at lower speeds. In addition to highway conditions, it can also be used in urban conditions and stop-and-go roads.

The car's sensors (radar) will send instructions to the actuators (throttle, brake, gear) based on the driving status (distance and speed) of the vehicle in front and the car itself, after calculation and judgment by the ECU, to determine its own driving status, whether to accelerate or decelerate, or exit cruise.

The most basic function of adaptive cruise control is to keep the vehicle moving longitudinally. When there is a risk of collision, the vehicle will alert the driver and perform active braking intervention.

As for the advantages, it is obvious that ACC can reduce the driver's driving fatigue to a certain extent.

Composition of ACC system

First of all, there are sensors. Currently there are radars (long-range radars), ultrasonic ranging sensors, infrared ranging sensors, etc.

The sensor is equivalent to the "eyes". For the eyes, the key is to identify the vehicle ahead in the lane and exclude the influence of vehicles in the adjacent lanes.

After the eyes receive the signal, it is passed to the ACC cruise control system in the car's brain, which will query a series of rules and regulations to decide how the car should drive. These rules and regulations are the control strategies in ACC.

The safe distance model (meaning the appropriate distance between the car and the car in front) is one of the main control strategies of the ACC system. It cannot be too large, otherwise it will cause protests from the vehicle behind you. It cannot be too small, otherwise there will be a risk of rear-end collision. The safe distance is a function of the minimum stopping distance and the current vehicle speed.

PS: The safe distance is not actually a fixed unit of length, but the so-called TTC, time to collision, which means the time required for two vehicles to rear-end each other assuming the current relative speed is maintained.

After the brain tells the car what to do, it's up to the actuators, which are the car's "hands and feet."

The actuators include throttle, brake, and gear. The car is controlled through the actions of these mechanisms.

Rather than talking about the shortcomings, it is better to talk about the development of ACC.

Development of ACC

Compared with other automotive electronic control technologies, ACC is still relatively immature. The difficulty lies in its adaptability to road conditions.

As a driving assistance system, adaptive cruise control cannot be as intelligent as a human being, and cannot distinguish all road conditions and respond accordingly. Currently, ACC is mainly used on roads with better road conditions (highways or elevated roads), and is used to judge target vehicles in the main lane.

However, the monitoring of side lanes, multiple target vehicles, and prediction of vehicles with the intention of merging lanes are still insufficient. For example, if the vehicle in front suddenly enters a curve, the vehicle may think that the safe distance is too large, misjudge and suddenly accelerate into the curve.

The following is an analysis of some typical road conditions:

1. If there is no vehicle ahead, ACC will cruise at a certain speed (the cruising speed is within the speed limit you set);

2. When a vehicle appears within the radar monitoring range, if the speed is too high, the car will slow down and follow the vehicle in front at a certain speed to maintain a safe distance; if the vehicle in front cuts out of this lane again, the car will automatically accelerate to the set speed.

As shown in the following figure (video screenshot), there is no car in the lane ahead, and the speed is 80km/h:

In Figure 2 below, a vehicle appears in the lane ahead and the vehicle speed decreases:

3. When the front car changes direction, the car will change the following target;

4. ACC stop-and-go function (if any) can be activated at low speeds or even when the car is stationary, which is useful in stop-and-go urban driving conditions;

The system can maintain the distance from the vehicle in front at low speeds and brake the car until it stops. After a few seconds, if the vehicle in front starts, ACC will automatically start.

If the stop time is longer, the driver can enter cruise mode again by simply pressing the pedal lightly.

PS: To realize ACC with stop-and-go function, the assistance of camera is usually needed, because although radar has strong ability to identify targets, it is very susceptible to clutter interference, and the image recognition function of camera is still needed to confirm the target. Mobileye's products can even realize ACC with only camera, of course, it will be useless in cloudy, rainy and snowy days.

At the same time, after following a vehicle to a stop, the strategy of most manufacturers is that the driver must confirm before starting again, which can be confirmed by pressing a button or stepping on the accelerator.

Map example: The following figure shows the city driving condition, and the vehicle speed is 25km/h:

As shown in the figure below, when waiting for a red light, the car can brake automatically and the speed drops to 0. After the front car starts, this car automatically follows and starts:

5. When entering a curve, the car will adjust its speed according to the curve conditions. Long-range radar has a small field of view and may lose the target if the curve radius is too large, so the current highest level of ACC only has performance requirements for curve radii of more than 150m.

Summary: ACC, as an intelligent driving technology, will be the future direction of automobile development, just like driverless driving. However, machines are always machines and cannot completely replace humans. Even the most intelligent driving is only auxiliary driving and cannot be completely relied upon or trusted.

Dear experienced drivers, please use it with caution.