Please keep your distance

A lot of congestion on our motorways is the result of us driving too close to the guy in front. The reasoning is this: the car in front slows down, the driver behind sees the brake lights and because he’s too close has to brake harder, this goes back along the line of traffic until someone has to come to a stop.

So what to do about it? Adaptive cruise control could be the answer. The principle is based on current cruise control where you set your speed and then let your feet relax. The major difference with ACC (automatic cruise control) is that the car automatically adjusts its speed to maintain a safe distance from other vehicles in the same lane.

Currently European vehicle manufacturers are leading the market for the adoption of ACC systems. The reason for this is partly due to the Prometheus programme, a European initiative aimed at producing a future intelligent transport system. Two companies participating in this programme, Lucas Varity and Thomson CSF, have come with an ACC system. The key components are the disc brakes, ABS, smart braking system, drive by wire engine control, radar headway sensor, digital signal processor and longitudinal controller. ACC links three electronic technologies sensing, control and actuation.

The radar sensing utilises a millimetre wave signal which is produced by using monolithic microwave integrated circuit technology. The radar is low power so non polluting with a range of around 150m. Hidden behind the grille or bumper, the radar sensor will detect any obstacles in the lane ahead and transmit this information to a signal processing unit which interprets the input. This target resolution is then transmitted to the longitudinal controller which translates the data to activate the brake or throttle dependent on the view of the lane ahead.

Control Engineering

The digital signal processor employs high performance devices to convert the complex signals produced by the microwave components into information about obstacles in front of the radar. These algorithms are able to determine to a very high resolution the velocity of those obstacles relative to the ACC vehicle. This permits very smooth control of the vehicle speed to achieve a safe headway distance.

The longitudinal controller controls the behaviour of the vehicle using braking and acceleration. It is derived from initial data obtained during step tests. This produces a model which is then implemented in Simulink to allow experimentation with several control strategies. This allows the rapid evaluation of different control strategies selecting the most appropriate in the laboratory, avoiding time consuming road trials. A controller based on headway is then simulated and its performance analysed. Once satisfactory performance is obtained in the modelling process the controller is implemented in C code and programmed in an ECU. Safety is also a major concern so techniques such as failure mode effects analysis, fault tree analysis and hazard analysis are used to ensure a safe, robust system.

Actuation

Situated under the bonnet, as an integral part of the system is the proportional brake booster. An electronic control unit is integrated within the brake booster which enables the electronic instructions to be converted into brake pressure.

The brake booster controls hydraulic pressure in a split hydraulic system with pressure in each circuit. Release switches within the booster are used to detect driver’s application of the brakes. There are two switches within the booster. If the driver applies pressure to the pedal, these switches are used to disable the electrical actuation.

The other critical part of the system is the link to engine speed. This is controlled by an electronic control unit connected to an engine controller using so-called drive-by-wire technology.

ACC will appear on high-end cars soon, and most European vehicle manufacturers are actively developing systems which will start appearing on higher volume platforms from 2000.

The North American market will require an additional capability from the ACC system enabling the detection and warning of stationary objects. ACC alert systems will appear on the market from 2002/03 onwards and are expected to rapidly achieve high volume since cruise control is already standard in the USA.

The major car companies in Asia, Japan and Korea, are also planning ACC systems for both the export and home markets. Volumes here are expected to grow steadily during the early part of the next decade.

Future ACC systems with collision warning will have the capability to warn the driver either audibly or through a haptic throttle. Lucas Varity/Thomson CSF are currently in advanced development stages of stationary object detection which will be a key feature of the system. Also, emergency braking and a stop-go function are features which the research and development plans are focused on.