Adaptive cruise control is not only useful, but also an important part of the autonomous driving puzzle. However, not all systems operate in the same way. We put three to the test…
It is early morning outside Bloemfontein on the N1 and the fog is impenetrable. If I drive too slowly, I risk getting hit from behind; up the speed and I might plough into the car in front. These are the kind of conditions where adaptive cruise control (ACC) can be a life-saving safety feature and, fortunately, my Ford Kuga long-termer is equipped with this useful tech.
I set the speed to 80 km/h and the following distance to the maximum setting. Not long after, the Kuga slows and, sure enough, two faint rear lights appear in the grey wall ahead.
While ACC was not invented primarily as a safety net, it is clear that the technology is an important building block on the way to fully autonomous driving. We decided to put systems fitted to three of our long-term vehicles to the test.
Continental manufactures not only tyres, but is one of the main ACC system suppliers, with more than 30 million of its radar sensors sold to auto-motive OEMs. We recently chatted to Norbert Hammerschmidt, Continental’s head of programme management for radar, about the technology. According to him, because it is unaffected by adverse weather conditions (including thick fog), the radar sensor is the most common component used in vehicles fitted with ACC.
Radar: Continental radar sensor tech is now in its fifth generation and is supplied as a package including a processor and software. This sensor operates at 77 GHz and employs the Doppler effect to identify objects with a relative speed difference in a 65° “V” forward from the sensor, up to a distance of 300 metres.
Early sensors could not operate at short distance, which meant that ACC systems did not work in stop/start traffic (usually below 30 km/h). This type of sensor is a cheaper alternative and is still used in some applications (our Ford Kuga long-termer is equipped with it).
Lidar: this is another type of sensor that functions similarly to radar, but uses a laser that produces pulses.
Cameras: Subaru’s innovative Eyesight dual-camera system is the third type of technology used. Our Subaru XV long-termer is equipped with this and it also formed part of our evaluation testing.
Interestingly, in some ACC systems, the radar sensors do not only pick up the immediate vehicle in front, but may also “see” the next vehicle in line as the waves move underneath the first car. This information feeds into the vehicle’s ACC algorithms in case it needs to perform an emergency braking manoeuvre that may cause a chain reaction on the vehicles behind.
These algorithms are developed to identify target vehicles and prevent “false detections” like stationary vehicles next to the road, as well as oncoming traffic in the opposite lane. Steering angle is also one of the inputs used to determine the vehicle’s trajectory when, for example, you may be following another car around a bend. As the sensor is connected to a vehicle’s CAN bus (the network linking all the electronic control units, including the engine control unit), it can control the vehicle’s longitudinal acceleration through accelerating and braking.
Minimum following distances are specified by relevant authorities in the markets where the vehicle is sold, as well as the requirements of the manufacturer. Supplier companies like Continental therefore use resident engineers to help calibrate the system for a specific supplier according to all the legal and client requirements.
Vehicle manufacturers often boast about the power output of their engines, or the size of the infotainment screen, but little information is available regarding the specification of a vehicle’s ACC system. That’s why we put three systems to the test and evaluate the differences between them.
Three of our long-termers proved the perfect test vehicles, a German (Audi Q2), an American (Ford Kuga) and a Japanese (Subaru XV) manufacturer. The Audi and Ford employ radar-based systems, whereas the Subaru utilises the dual-camera setup.
TEST 1: FOLLOWING DISTANCE
The system on each vehicle allows the driver to select the desired distance to the vehicle in front, between a minimum and maximum setting. We measured the minimum and maximum distance (using a Bushnell laser range-finder) at speeds of 60 and 120 km/h (true GPS speed verified with our Racelogics VBox units) with the following results:
From the above results it is clear that the minimum distances are fairly similar, pointing to a safety regulatory requirement aimed at preventing rear-end collisions. By looking at the maximum distances however, it is clear that the Audi’s system in the Q2 has the longest range capability, with a maximum distance at 120 km/h of over 100 m.
TEST 2: DECELERATION RESPONSE
In this test, the subject vehicle’s ACC was set to 120 km/h and was allowed to catch another test vehicle cruising ahead at 60 km/h. The idea was to register the subject vehicle’s ACC system’s braking response to see if it was able to:
• Prevent a collision without driver intervention.
• Adjust the subject vehicle’s speed to match 60 km/h.
All three vehicles were able to slow autonomously, but the response times from each ACC system were vastly different.
Looking at the graphic that depicts vehicle speed against time, it is clear that the Subaru’s system was the most aggressive and took only eight seconds to slow from 120 to 60 km/h. From a driver’s perspective, it was the most uncomfortable experience of the three and a degree of courage and discipline was needed not to intervene, as it felt like the system was leaving the adjustment to the last minute.
The ACC system in the Audi was the only one that allowed the driver to adjust the response, by toggling between comfort and dynamic modes. This did not impact deceleration and in both cases it took just over 12 seconds to slow down.
The Kuga had the most linear response that subjectively felt very comfortable, completing the test in a relaxing 13 seconds.
TEST 3: ACCELERATION RESPONSE
This test placed the emphasis on the ACC system’s ability to accelerate when asked to return to the set speed.
Once again, the vehicle ahead was travelling at a steady 60 km/h, but this time it would move aside, allowing the subject vehicle to accelerate back up to the set speed of 120 km/h.
In this test, the Subaru was the least dynamic and took a leisurely 30 seconds to get back up to 120 km/h. The Ford and Audi (in the comfort setting) took around 23 seconds to complete the task. In its dynamic setting, the Audi accelerated with zest and needed just 13 seconds to blast back up to the original speed.
TEST 4: STOP/START OPERATION
Both the Audi and Subaru successfully completed the city drive test and came to a complete stop behind a stationary vehicle. The Ford Kuga’s ACC system does not function below 30 km/h, which meant it could not complete the city driving test.
On both the Subaru and Audi, if the vehicle is stationary for more than a couple of seconds, the driver has to initiate the ACC system again by touching the accelerator pedal (or pulling the Q2’s cruise control stalk). According to Hammerschmidt, this is to prevent a potential collision with a pedestrian.
Future ACC systems will have two short-range sensors on the vehicle’s front corners to monitor objects crossing its path and allowing the ACC system to pull away by itself in stop/start traffic.
Bushnell laser range-finder
We needed a way to quickly and accurately measure the distance between the subject vehicle (with active ACC) and the vehicle in front. Bushnell kindly provided its Tour-X laser range finder developed for professional golfers to measure the exact distance to the pin. With a range of between 5 and 1 189 metres (and slope technology), it was perfectly suited to our test. The unit’s suggested retail price is R8 999 and you can find more information on this useful piece of kit at bushnellgolf.com…
Author: Nicol Louw
*From the March 2018 issue of CAR magazine