In Focus: Antilock brake systems

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Antilock brake systems (ABS) prevent a vehicle’s wheels from locking in a panic stopping situation, helping to avoid loss of control. They use inductive sensors, mounted at the wheel ends, which pick up impulses from toothed rings, which rotate with the wheels. Each sensor/wheel combination is kind of an electric generator, consisting of a permanent magnet and coil.

When the teeth on the toothed rings pass a sensor, a magnetic flux picked up by the coil is altered intermittently. This creates an alternating voltage, the strength or frequency of which is proportional to wheel speed.

The wheel speed data then is sent to the vehicle’s electronic control unit (ECU). The ECU is programmed to sense wheel slip (or difference between wheel speed and vehicle speed); when the slip becomes excessive based on the ECU’s programmed lockup parameters, it sends an electrical signal to solenoid or modulator valves that reduce air pressure to one or more brake chamber(s).

The ECU will continue to monitor the slip of the offending wheel until the signal from the sensors equals a rate compatible with normal braking action. When this happens, the ECU sends another signal to the solenoid or modulator valves to restore normal air pressure.

A popular option with ABS is automatic traction control (ATC). ATC uses ABS components to control wheel slip. When a wheel’s speed exceeds that of the vehicle, brakes are applied at that wheel until its speed matches that of the vehicle. And if a driver applies too much power, the ECU communicates with the vehicle’s engine computer telling it to reduce power until the affected wheel is slowed down to an acceptable level.

Antilock systems vary by component configuration, degree of complexity and cost. Most trucks and tractors use the 4S/4M (four sensors, four modulators) system, where the sensors are located on the steering axle wheels and at the rear tandem wheels. This setup especially is helpful when there is a split coefficient of friction (split-mu) between the tires and the road. The system will modulate the wheels on the slipping side to avoid lockup, and will adjust for maximum deceleration on the wheels that are not slipping.

On a single-axle trailer, the 2S/1M system (brakes at all wheels are modulated simultaneously) will meet the minimum performance requirements and provide stability.

On a two-axle trailer, a 4S/2M system is considered optimal for stability and tire-flat-spotting control. With this arrangement, when either the leading or trailing wheel assembly on either side of the trailer approaches lockup, the brakes on the leading and trailing wheels on that side are modulated as a unit.

Recently, Meritor WABCO introduced its Roll Stability Control (RSC) system for tractors and Roll Stability Support (RSS) system for trailers.

RSC is programmed to help prevent rollovers by slowing a tractor when it senses that a critical, lateral-acceleration threshold is exceeded. Intervention comes in three stages: engine power reduction; retarder activation; and service-brake application. RSC uses ABS components, but the optional, automatic traction control component of ABS is integral to RSC, which also requires an extra valve and an acceleration sensor on the ABS circuit.

RSS is an independent system for trailers, and works much like RSC, except a trailer electronically controlled brake system (EBS) and air suspension are required.

Next spring, Bendix will introduce its Electronic Stability Program (ESP), which automatically applies brakes at individual wheel ends to restore stability to a truck or combination vehicle that’s losing control or headed for a jackknife on dry or slippery surfaces. It also provides roll stability by engaging brakes on all axles to reduce a vehicle’s speed when it exceeds a roll threshold on dry surfaces. ESP is built entirely on Bendix’ ABS platform and does not involve electronically controlled braking. Additional components are added to the ABS, such as yaw and steering-angle sensors, but the system retains familiar components and service procedures.

Antilock systems are supplied by Bendix, Haldex, and Meritor WABCO.