What is Active Cruise Control?

The first stage of autonomous motoring is already with us - and has been for some years

Cruise control which maintains a set distance from the car in front, by juggling throttle and brakes, was a concept long awaited by the time it was invented in the late 1990s.

While much of the credit can go to Japanese companies, Mercedes-Benz popularised the idea with its radar-based Distronic system introduced with the W220 generation S-Class model in 1999. The system didn't always work to perfection, and drivers were frequently required to be more aware of their situation in heavy traffic than they would prefer when using a cruise control system.

But on balance, it's a useful labour-saving device for those familiar with its idiosyncrasies – and it is the foundation stone on which autonomous motoring will establish itself in future.

Back to the beginning

To understand active cruise control, it helps to first understand conventional cruise control. As the name indicates, cruise control adjusts the throttle to maintain a vehicle's speed on the flat and on hills. When we say 'throttle', that's more fuel injected into the combustion chambers to produce more torque.

Basically, conventional cruise control is a hand throttle which can be set from switches usually located on the steering wheel. More commonly seen in cars with automatic transmissions, the cruise control throttling up for really steep grades will prompt the transmission to kick down – without the driver taking any action.

On descents, older systems allow the vehicle to creep above the driver's chosen speed. Newer systems often incorporate a speed limiter, which uses the vehicle's brakes, as well as throttle, to stay at the set speed. It's the system's ability to brake as well as accelerate which separates Active Cruise Control systems (ACC) from conventional cruise control.

Matching varying speeds

And, that ability to moderate speed through an ACC system, allows the car to match speeds with the traffic in front as well as hold speed up or down hills.

Distronic and other ACC systems took the automatic braking function and added computer processing power to adjust the set speed up, or down, as required to match the varying pace of the car ahead.

But for the on-board computer to 'read' the traffic, and observe the closing speed of the car in front, it needs input from one or more sensors, of various types.

Distronic relies on radar, but other systems include LiDAR – essentially a laser system which bounces back off the car in front, just as radio waves do in a radar system.

Another type of ACC system is an optical type, such as Subaru's EyeSight feature, which employs a stereo camera (for depth perception) to determine the distance to the car in front.

Auto stop and go

Car companies are developing ever more sophisticated ACC systems. Many will now bring a car to a full stop behind the car in front – when pulling up at a red light, for instance. Naturally such an ACC system will allow the car to move forward once the vehicle ahead has moved out of the way. Mercedes-Benz calls this technology 'Stop&Go Pilot' – and it will hold the car at a safe distance behind the vehicle ahead at any speed between zero and the speed programmed by the driver.

Another system, introduced to Australia by BMW – Speed Limit Assist – uses optical cameras to recognise speed limit signs and adjust the vehicle's speed to comply with the prevailing limit. It even recognises advisory speed limit signs posted on tighter bends and reduces speed accordingly.

Autonomous emergency braking

All the constituent elements of ACC – sensors, processors, active intervention for throttle and brakes – are also necessary for autonomous emergency braking (AEB), which will bring the car to an emergency halt if the distance to the car ahead is closing rapidly and the driver shows no sign of reacting. But unlike ACC, AEB with pedestrian detection, by necessity, incorporates an optical system.

AEB is the first step on the way to full autonomous motoring – and it couldn't have happened without the decades-long development of cruise control systems.

What is Active Cruise Control?

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