Automatic Braking Systems and How They Work

11.15.19

I first heard of automatic braking systems (ABS) while I was a sophomore in high school, taking my drivers' education course. My driving instructer took me and a friend to the long road across the street from the school and had us push the accelerator down to the floor until we reached about 60-70 mph. He then instructed us to slam on the brakes so that we could feel how the car's ABS took over. This system replaced the manual cadence brakeing that individual's used to develop a skill for, and is a great example of how controls help us automate a miriad of tasks.

Automatic Braking Systems consist of three components, a central control unit, two or more hydraulic valves, and four wheel speed sensors. The control unit measures how fast each wheel is moving, and if the wheel speed is slower than it should be compared to the overall vehicle (which means the wheel is skidding) then the brakes are lessened on that wheel. This applies vice-versa to wheels moving faster than the overall vehicle. To increase/decrease the speed of an individual wheel the ABS modulates the hydraulic pressure to the wheel's brakes through a valve. It's important to note that the electronic control system will not activate when the difference in wheel speed is below a certain value, such as what happens when a car is turning; the outer wheels turn faster than the inner wheels. This valve can apply/release brake pressure up to 15 times a second.

Expanding off of ABS, many cars today used an advanced electronic stability control unit. These involve a least two sensors in addition to those used by ABS; a gyroscopic sensor and a stteering wheel angle sensor. If the the gyroscopic sensor measures that the car direction doesn't match the steering wheel direction the electronic stability control unit will modulate up to three wheels' braking systems to right the car's direction.

Now for a more detailed look at the various components. The speed sensors transfers mechanical work to electrical signals that the control unit can process. When the wheel rotates around the hall effect sensor, it creates a magnetic field and the fluctuations in this field generates an electric current. This electric current is carried to the control unit. The valves have three positions; one where the valve is completely open and the pressure from the main brake cylinder goes directly to the brake, one where the valve is closed and isolates the brake from the main cylinder, and one where the valve is halfway open and releases brake pressure. The pump in an ABS is used to ramp the pressure back up in the hydraulic brakes after the valve is moved from an open to closed position.

The algorithm that is activated when the ABS is engaged can be as simple as this. First if the ABS senses that the wheels are decelerating at a much faster rate than the car is (the car is about to skid) then the pressure is released on the brakes slightly so that they can accelerate a bit. It then adds more pressure on to continue braking and the cycle begins again. This lets drivers across the world (and me!) maintain the ability to steer, even in slippery conditions.