New automatic braking technology fitted to Volvo SUV promises to avoid low-speed collisions. Berenice Baker reports.
If Volvo’s marketing people are to be believed, the front seat of the company’s new SUV, the XC60, might just be the least dangerous place on the planet.
As well as the host of safety features that the luxury car maker is famed for, this slimmed-down 4×4, which goes on sale in the UK in January, also features City Safety, a new piece of automatic braking technology that promises to avoid that most common of accidents: the low-speed collision.
The system, which operates at speeds up to 18mph, uses an optical radar device housed just in front of the rear view mirror to keep a constant watch on the 10m of road directly ahead. An infrared laser is pointed at the traffic ahead and a sensor measures the time it takes this pulse of light to travel back. From this, the system is able to deduce the distance to the car in front, as well as the relative speeds and accelerations.
Martin Distner, one of the engineers behind City Safety, explained that 50 times a second the system will calculate what braking speed is needed to avoid a collision. If the calculated braking force exceeds a given level without the driver responding, the danger of a collision is considered imminent and the system triggers automatic braking.
Distner said City Safety could avoid a collision at relative speeds of up to 9mph and greatly reduce damage at speeds up to 18mph.
The system is specifically designed to identify the rear end of passenger cars or larger vehicles by using the rear reflectors of the back light and the number plate to return a sharp reflection, though it can also detect other objects.
The system communicates with the brakes, initially pre-charging them — closing the gap between the brake pads and the disc — to get a better response. When the vehicles get so close a collision cannot be avoided, it pumps the brakes to avoid the collision. ‘We have a sensor with a limited range of view, which applies the brakes harder at higher speed to avoid a collision. If the relative speed is very low, the intervention will come a lot later than if it’s higher,’ said Distner.
The calculations City Safety uses prevent it activating when pulling up slowly to park, for example. ‘We monitor the situation and recognise that kind of behaviour so City Safety doesn’t activate,’ said Distner. ‘The functionality is only activated when the car is really close to colliding — nothing you would expect to come across in normal driving. We also have a lower speed limit of 2.5mph, below which it won’t activate.’
Over 18mph City Safety is no longer active, but at this point a host of other optional Volvo systems kick in, such as Collision warning with full Auto Brake, which works from 4mph up to full speed.
‘The systems act side by side,’ said Distner. ‘If, during their common speed interval, they are triggered at the same time, the system that applies the most braking will activate — so it’s the system that shouts loudest. There is no way for these systems to jeopardise each other.’
As with any new model and system, the XC60 has been subjected to extensive crash testing. Volvo used dummy inflatable vehicles to test the positive performance of the system in a variety of scenarios. Plus, before the model was launched, City Safety had racked up millions of miles of road testing mounted in taxis, passenger cars and company cars. Each was fitted with computers to record driving information and any system actuation.
While critics may claim that such systems remove control from the driver and ultimately encourage less safe driving, Distner argued that City Safety operates in such specific conditions that there is no danger of the driver becoming used to it.
‘This is a pure support system that operates extremely late,’ he said. ‘City Safety is not a system that you interact with during normal driving. It will only operate when you are in a situation in which you no longer have control over the car because you are far closer to a collision than your normal comfort zone. It’s not a feature that you would run around town testing time after time.’
After City Safety has been activated, control is released back to the driver in one of three ways. If the system avoids a collision with a stationary object, it will bring the car to a standstill and hold it there for 1.5 seconds before releasing full control back to the driver. ‘We chose one and a half seconds so the driver can quickly regain control without feeling the car is parked and he can leave it,’ said Distner.
If the car in front is still moving, the system will brake enough to avoid a collision, continue to roll at the same speed as the car in front, then immediately give control back to the driver.
The third way is called Driver Override. ‘We always believe the driver should be able to regain control if he’d like from the active safety systems,’ said Distner. ‘If you do that, you either steer very abruptly from side to side, or depress the throttle sharply to regain control if you don’t want City Safety on.’
As with all safety-critical systems, it is essential to protect City Safety from hardware and software failures. ‘We have multiple safety and backup systems, so the system is monitored from other parts of the electrical system to ensure it behaves correctly,’ said Distner. ‘We have multiple nodes in the system checking each other. If it doesn’t work, it will be shut down.’
Distner said laser apparatus was selected for City Safety as it is a rapid sensor technology that can accurately detect fast-changing situations in low-speed traffic. ‘When we originally decided to make this system standard, we needed it to be affordable,’ he said. ‘So we chose a technology which fulfilled our requirements but which wasn’t so expensive we couldn’t make it a standard feature.’
The detector was positioned high up behind the windscreen so that very close objects could be detected. It also helps address visibility problems that could be caused by dirty glass by having it under the wiped area of the screen. ‘In that way, we are sure that the driver will keep the windscreen clean with screen wash and wipers,’ said Distner. ‘We found that the sensor is less sensitive than the driver in that respect.’ He added that City Safety is unaffected by fog, but heavy rain or snow may reduce its performance.
Looking forward to future Volvo safety systems, Distner said his team has been working on car-to-car communications systems that help avoid collisions or take over control via information received from other nearby cars.
‘For example, we have a system where if you pass over the centre line into the opposing lane and there is a car coming towards you, it will take the car back into the original lane to help avoid the frontal collision that otherwise would happen,’ he said.