Anti-lock Braking System (ABS)

• What is an ABS ?

Anti-lock braking systems (ABS's) are electronic systems that monitor and control wheel slip during vehicle braking. ABS scan improve vehicle control during braking, and reduce stopping distances on slippery (split or low coefficient of friction) road surfaces by limiting wheel slip and minimizing lockup. Rolling wheels have much more traction than locked wheels. Reducing wheel slip improves vehicle stability and control during braking, since stability increases as wheel slip decreases.
 ABS's can be applied to nearly all types of vehicles and can be successfully integrated into hydraulic and air brake systems (including air over hydraulic). This document applies to the ABS's used with air brake systems on commercial vehicles.
 The equipment requirements of FMVSS 121 specify that ABS's on truck-tractors and full trailers must control the brake pressures to at least one front axle and one rear axle. The ABS's on semi-trailers and dollies must control at least one axle of the vehicle. Additionally, the ABS's on tractors must control one of the rear axles with two modulator valves so that the brake pressure on one end of the axle is independent of the brake pressure on the other end. The performance requirements of FMVSS 121 can require an ABS on additional axles.

• How do ABS's works ?

An ABS consists of several key components: electronic control unit (ECU), wheel speed sensors, modulator valves, and exciter rings. Here’s how these components work together:

•  Wheel speed sensors constantly monitor and send electrical pulses to the ECU at a rate proportional to the wheel speed. 
•  When the pulse rates indicate impending wheel lockup, the ECU signals the modulator valve(s) to reduce and/or hold the brake application pressure to the wheel(s) in question.
•  The ECU then adjusts pressure, seeking one which gives maximum braking without risking wheel lockup.
•  When the ECU acts to modulate the brake pressure, it will also (on most vehicles) turn off the retarder (if so equipped) until the risk of lockup is over.
•  The ECU continually checks itself for proper operation. If it detects malfunction/failure in the electrical/electronic system, it will shut down that partof the ABS affected by the problem—or the entire ABS—depending upon the system and the problem. 
•  When this happens, the ABS malfunction lamp lights. 
•  An ABS adjusts brake pressure much faster and more accurately than can drivers. It’s faster because:

  Electronic controls are very fast and ABS modulator valves are physically closer to the brakes than is the driver’s foot brake valve. It is more effective, too, because an ABS can tailor the brake pressure to each wheel or set of wheels to provide maximum braking/stability. Some vehicles also use a traction control system in conjunction with the ABS. Traction control helps the ABS improve vehicle traction by minimizing wheel slip on the drive axle during acceleration. If a wheel on the drive axle starts to slip, the traction control system automatically brakes the wheel slightly, transferring engine torque to the wheels with better traction. If all the drive wheels start to slip, the traction control system may also reduce engine power.Traction control systems are referred to by several different names, depending on the manufacturer. These include: a. Automatic Traction Control (ATC) b. Traction Control (TC) c. Automatic Slip Regulation/Anti-Spin Regulation (ASR)