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.
