Active Vehicle Safety Part 1: Modern ABS

The age-old adage “prevention is better than cure” is perfectly apt when discussing anti-lock braking systems (ABS). Simply put, this system allows a driver to stay in control of the vehicle while applying maximum force on the brake pedal. Crash structures and airbags are valuable safety additions to modern vehicles, but unlike the use of ABS, most people would opt not to test their claimed ability. Today’s anti-lock braking systems employ quick-acting hardware- and software-control algorithms to provide optimum stopping performance under all road conditions. Slamming on the brakes has never been safer.

History

ABS was first developed in the late 1920s for use on airplanes to prevent skidding on runways. The transition to the automotive field took some time, with Royal Enfield experimenting with an early version of ABS on its Super Meteor motorcycle in 1958. Chrysler introduced a computerised ABS called Sure Brake for its 1971 Imperial. Bosch was the first automotive supplier to present a feasible setup for mass production in 1978. Later that year, the system became available first in the Mercedes-Benz S-Class and then in the BMW 7 Series. It weighed a hefty 6,3 kg (some of the company’s latest units weigh less than 1,6 kg). Today, ABS is mandatory on all vehicles sold in the European market. Considering our road-safety record, we can only hope this law is replicated here.

Basic theory

Older readers will be well acquainted with the term “cadence braking” (applied to vehicles that lack ABS), which refers to a method of braking where the driver releases the brake pedal of a vehicle after lock-up to regain wheel rotation in a cyclic manner (brake pumping). The idea is to achieve close to maximum retardation force while still allowing the driver to exercise steering control. Modern ABS achieves the same feat in a far more controlled manner by regulating pressure pulses to each wheel at rates of more than 15 times a second without additional input from the driver (although the pilot may feel the vibration feedback of the pressure pulses through the pressed brake pedal).

Tyres support the vehicle’s mass with a normal force reaction perpendicular to the road surface at the contact patches. The maximum retardation force that is possible is the sum of the normal forces multiplied by the friction coefficient between the tyres and the road (see Theoretical stopping distances and times below). This force is possible only if there is very limited slip between the tyre and road surface (see tyre slip curve above). Excessive slip will reduce the friction coefficient between the tyre and road and degrade stopping performance. This is especially true on wet roads.

ABS allows a wheel to always be close to the optimum friction coefficient (and retardation force) by controlling the wheel-slip percentage during braking.

ABS module

The controller-area-network (CAN) bus allows communication between the ABS module, wheel-speed sensors and engine-control unit (ECU) at speeds of up to 125 Hz. The ABS module fits between the master cylinder and individual brake lines that run to the disc/drum brake assemblies. The unit consist of:

•    A pump that is able to provide additional brake-fluid pressure to each brake line;
•    Eight 2/2-way solenoid valves (two per brake line) that block or allow the brake-fluid pressure through to the disc/drum-brake assemblies (combined with a pressure-release function);
•    A control module with processor and CAN bus connection.

Wheel-speed sensors

Four inductive wheel-speed sensors (one per wheel) are located close to a target disc (perforated at set degree intervals) that spins with the relevant wheel. Each passing metal slat in the perforated disc results in a signal edge that is detected by the ABS unit. By knowing the number of perforations and wheel diameter, the wheel speed can be easily calculated.

Control strategies

The ABS module monitors the individual wheel speeds. When the driver actuates the brakes, the brake booster initially supplies the wheel-brake assemblies with brake-fluid pressure through open 2/2-way solenoid valves. When the ABS module detects that a wheel is about to lock (sharp deceleration above a calibrateable limit that indicates increasing wheel slip), the relevant 2/2-way solenoid valve is closed (pressure freeze) to avoid any further pressure build-up. The next step is to open the pressure release 2/2-way solenoid valve to lessen the brake force on the particular wheel. When the wheel regains speed (little slip), the pressure-release solenoid valve closes and the pressure input valve allows the ABS to supply the specific brake assembly with a brake-fluid pressure pulse in order to regain braking force.

This cyclic behaviour is typical of ABS. The following disturbances to the control loop need to be taken into account by ABS calibration engineers:

•    Different road surfaces (friction coefficients and imperfections) and even a friction coefficient split between the tyres of the vehicle (for example, when two wheels are on dirt);
•    Variations in the fluid-pressure input to the ABS module by the master cylinder that is caused by the driver;
•    Difference in tyre specifications (example, when a space-saver spare wheel is fitted);
•    Varying payload weight;
•    Steering input while braking.

ABS calibration engineers try to achieve the following performance targets:

•    Maintaining directional stability by allowing lateral forces when steering input is detected (even to the detriment of absolute braking force);
•    Stopping in the shortest distance possible;
•    Rapid adjustment to braking force owing to varying friction coefficients of road surfaces;
•    Maintaining driver comfort by minimising vibration feedback.

Future

Owing to stringent safety requirements, the input from the driver is directly connected to the brake assemblies on the wheels through the brake lines. This allows the driver to bring the vehicle to a stop in the event of a total failure of the ABS unit. In future, it will be possible to brake-by-wire, where the input is given digitally to an ABS unit controlling the braking event. Brake-system suppliers are also experimenting with electronic brake actuation that could replace hydraulic brake pistons. This will eliminate the need for a master cylinder, brake lines and brake fluid.

The biggest concern to the enthusiast is the loss of feed-back from the brake pedal. It is quite possible that autonomous braking might eventually even take the driver out of the loop completely.

Electronic brake-force distribution (EBD)

In conjunction with ABS, the possibility exists to cleverly distribute braking force during braking. In older vehicles, a brake-force-proportioning valve controlled the brake bias between the front and rear axles. This mechanical device had to ensure that most of the braking force was sent to the front wheels to prevent the rear wheels from locking (resulting from weight transfer to the front wheels during braking). Today, EBD is capable of analysing grip on all the wheels and so continuously distribute and adjust the braking force (brake-fluid pressure). EBD can also allow controlled vehicle dynamics (example, evasive steering) by regulating brake force on individual wheels.

Theoretical stopping distances and times

As ABS is capable of braking a car at close to the grip limit between the tyre and the road, how would the coefficient of friction influence the theoretical stopping distance and time (ignoring aerodynamic drag)?

Read part 2 of Active Vehicle Safety, Electronic Stability Control here.