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Emergency Brake Assist

Last Update:

09/06/2016

Introduction

Emergency Brake Assist system senses situations where emergency braking is necessary and applies the brakes should the driver’s response not be strong or fast enough. The system judges the force and speed with which a brake pedal is depressed and activates emergency braking.

History

  • The credit for implementation of emergency brake assist could go to Mercedes for their implementation in the S and SL class vehicles in 1996 later on going to become standard in all their cars.
  • This was possibly the outcome of a study performed by Daimler Benz in 1992 to study panic stops and crashes. The result showed that 90% of drivers did not apply enough braking pressure.
  • Volvo went on to develop a collision warning with auto brake with radar to sense likelihood of accidents. The system pre-charges the brake so that full braking force is applied on the slightest touch to the brake pedal. The system also had a feature wherein the brakes were applied if the driver did not respond in time.

Emergency Brake Assist system forms part of the Anti-lock Braking System and Electronic Brake Force Distribution system commonly found in advanced cars. It was made mandatory in the European Union countries around 2009 due to its proven results.

Evolution

  • The earliest form of emergency brake was purely mechanical. These earlier units has a pre-set limit set mechanically with a locking mechanism activated by the valve stroke related to the distance the brake pedal is pushed. If the distance exceeds a threshold, the mechanical emergency brake kicks in. The brake booster applies braking, initiated by the brake piston valve.
  • Today’s emergency brake assist systems are electronic, controlled by the car’s ECU obtaining information from sensors and, in turn, activating the ABS and EBD systems.
  • Like the mechanical system, this system too relies on data about the speed with which the brake pedal is depressed and the distance it travels.
  • Unlike mechanical systems, the electronic emergency brake can be programmed to factor in the driver’s braking patterns.

 

EBA circuit

  • Integrated with the car’s ABS and EBD systems that are tied to the car’s ECU, the emergency brake assist has only a few parts but a sophisticated programming interface to help the computer make the decision.
  • These few parts comprise of sensors to sense brake pedal pressure and its rate of acceleration compared to preset algorithms in the computer.
  • The sensors could be as simple as a hall effect sensor or somewhat more sophisticated tri-axial gyroscope and tri-axial accelerometers.
  • Some systems have a travel sensor fitted to the brake pedal. Some may even have a sensor to detect when the foot is lifted off the accelerator working alongside the brake pedal sensor.
  • Advanced systems may have radar sensor and controller, control switch, indicators, brake actuator controller and audio buzzer. All these are interconnected with the computer and the ABS system.

 

How It Works

Normal drivers are not programmed to respond quickly enough in panic situations necessitating emergency stops. They may not apply brake quickly enough or hard enough or not pay attention and are distracted. In such cases, the response time is not fast enough to bring the vehicle to a halt quickly enough to avoid accidents. This is where emergency brake assist systems have proved invaluable.

  • In the usual course the car’s ECU along with the AEB system measures and monitors the driver’s driving and braking patterns to define a reference point.
  • If a panic situation arises and the driver slams down the brake pedal, the ECU interprets it as such and activates the emergency braking procedure.
  • Even if a light pressure is applied, the ECU delivers instructions to the brake booster to apply full braking power through the ABS unit.

In taking this decision the car determines speed of the vehicle and the brake force applied by the driver.

 

Effectiveness

Effectiveness of EBA system and its Challenges

  • Without brakes the vehicle cannot and will not stop in time to prevent an accident that can kill or maim. NHTSA studies found that emergency brake assist reduced the stopping distance to 10 feet and this made all the difference.
  • Emergency brake assist reduces severity of injuries where a collision does take place and has reduced fatalities by 6.5 to 9% according to studies by Page.
  • An ECU controlled emergency brake is quite effective in that it activates the ABS at full force bringing the vehicle to a safe halt without skidding.

The system is not without its challenges.

  • Mostly it is drivers who fail to take full advantage of the technology and understand its advantages as well as limitations.
  • Driver vigilance and input are necessary. Unless brakes are applied fast and with force, the system may not be activated unless “trained” to recognize the braking pattern of the driver.

Because of these inherent drawbacks some manufacturers have gone one up on the system and have introduced intelligent braking that makes use of lasers, radar or video cameras rather than driver input and are, therefore, more effective.

 

Maintain

  • Sensors are critical to the performance of the system. Keeping them free of dust and dirt is something that most drivers can do with ease.
  • Checking brake pad wear, hydraulic lines and the cylinder as well as replacing brake fluid are basic functions within the capabilities of normal drivers.

In any case, it helps if drivers carry out test drivers simulating panic situations at regular intervals to ensure the EBA is working as needed.

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