These systems were developed in this order (ABS/ASR/ESP), as they are each built on the other's technologies and needed new developments in order to be implemented.
The first system was ABS. ABS stands for Antiblockiersystem or Anti-lock Braking System. It works on the principle by measuring wheel speed sensors at each wheel, the ABS system can tell when a wheel starts to lock up in a braking event. The first action is to close off the pressure to the locking wheel, so that the pressure does not build further. If it still is locked, the pressure to that wheel is reduced by pumping the brake fluid back to the master cylinder (this is the pulsation you feel in the pedal). There are no other pieces to an ABS system - just wheel speed sensors and the hydraulic unit. If your ABS system completely fails, your normal braking system should continue to function correctly.
ASR or Antriebsschlupfregelung or Automatic Slip Reduction (Traction Control) adds integration to the engine management system. ASR only comes on cars with ABS, as it builds on the groundwork of the hydraulic unit. ASR, monitors the wheel speed sensors under acceleration. If wheel spin is detected under acceleration, the first line of defense is to cut engine power through timing or other means. If that is not sufficient, the hydraulic unit motor is turned on, generating pressure in the braking system, and solenoid valves are switched to apply brake pressure to the spinning wheel. So basically it helps the spinning wheel to gain traction with the surface it is on.
ESP or Elektronisches Stabilitaets-Programm or Electronic Stability Program is on another performance level completely. ESP is used to straighten out a car that is experiencing excessive under-steer or over-steer. This is determined through the additional sensors needed for an ESP system: yaw sensor (measures the degree the car is actually pivoting about the center axis of the car), steering position sensor (measures where the driver is intending to steer the car), and lateral accelerometer (measures how much side load the car is in). ESP is only built on a car that already has the ABS/ASR system (if you have ESP, you have ABS and ASR as well). So what ESP does is detect what direction you are steering (where you want to go as indicated by the steering wheel position sensor), and compares that with where you are actually going (using the yaw sensor and accelerometers). If your intended trajectory does not match your actual trajectory, it applies brakes at opposing corners of the car in order to straighten it out. So, if you are turning left, and are in an over-steer condition (the car is actually going way left), the system will apply brakes to the passenger front and driver rear wheels. This will tend to pivot the car to the right, more in line with the intended direction. If you are turning left, and are in an under-steer condition (the car is not turning left enough), the system will apply brakes to the driver front and passenger rear wheels. This will tend to pivot the car to the left, more in line with the intended direction. Again, this happens many times per second. Also, it will only intervene past a certain threshold programmed by the manufacturer. In addition to the brake application, engine power is also modulated to control the spin. As mentioned, the ESP system adds several sensors on to an existing ABS/ASR system, namely the yaw sensor, steering wheel position sensor, and lateral accelerometer. |
