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Automotive braking systems



Can’t think of a safety system on a vehicle that is more important or basic than this one. What it basically does is slow down the rotation of the wheels so that your car comes to a complete stop. You must have seen kids playing with rolling tires down the driveway. To stop the tire, you just have to grab it and squeeze and the resulting friction will then take care of the rolling. The same thing happens in your car, truck, or SUV. You push down your brake pedal and your braking system resists the rotation of the wheels, thus taking the kinetic energy of your car and converting it to mainly thermal energy through friction. This retardation brings your three-thousand-pound-plus metal machine to rest. The concept is the same, but the equipment, well that’s a bit more complicated.

CLASSIFICATION
Now, the evolution of the braking system has been happening over a long period of time (and still ongoing of course), so it’s a little bit tricky to pinpoint the exact premiere. Based on what we’ve encountered over the years, braking systems can be classified as follows:

● Mechanical Braking System
● Hydraulic Braking System
● Electromagnetic Braking System


MECHANICAL BRAKING SYSTEM
One of the most straightforward braking mechanisms developed ever. It basically comprises various mechanical linkages, levers and other related elements that work together to slow down the rotation of the rotating shaft over which the wheels are mounted. Band brake, the simplest configuration, consists of a metal band lined with wear and heat resistant friction material. Other types include the Disc and Drum Brake Systems which we will discuss in quite some detail.


HYDRAULIC BRAKING SYSTEM
Ingenuity at its best. Remember Pascal’s Law? Well, this one works on that principle. To jog your memory, Pascal’s Law states that, when you apply pressure at any point in a confined fluid (at rest), then that pressure is transmitted equally and undiminished throughout the volume of the fluid. A hydraulic braking system transmits brake-pedal force to the wheel brakes through pressurized fluid, converting the fluid pressure into useful work of braking at the wheels.

The brake pedal relays the driver’s foot effort to the master-cylinder piston, which compresses the brake fluid. This fluid pressure is equally transmitted throughout the fluid to the front disc-caliper pistons and to the rear wheel-cylinder pistons, where this pressure is utilized to rub the friction material against the wheel hub (in case of drum brakes) or against the brake disc (disc brakes of course). The given diagram gives a brief sketch of the components of this system.


ELECTROMAGNETIC BRAKING SYSTEM

This is a clever application of electromagnets. It operates through Electric actuation but produces braking action mechanically. The electromagnetic field produced by the electromagnet is used to stop the rotation of the output shaft.

Basic working of the Electromagnetic Braking System:

1. When the current supply from the battery starts to flow to the coil, the coil becomes an electromagnet &produces a magnetic field.
2. Due to this magnetic field, the armature that rotates about the shaft gets pulled (attracted) towards the stator. Friction comes into play between armature & friction material. Due to this friction, the Armature & shaft rotation stops.
3. Thus, the brake is applied. When the current supply to the coil stops creating the magnetic field. Without the magnetic field, the attraction force on the armature dies off & the stressed spring pulls the armature towards the armature hub. As a result, the brake is released.

Two of the most commonly used brakes are the drum brakes and disc brakes. These can be both Hydraulic and mechanical. Here, we shall discuss their basic structure and work.


DRUM BRAKES
Given is a figure of a commonly used Hydraulic drum brake system. When the driver steps on the brake pedal, the power is amplified by the brake booster (servo system)and changed into hydraulic pressure (oil-pressure) by the master cylinder.
The pressure reaches the brakes on the wheels via tubing filled with brake oil (brake fluid). The delivered pressure pushes the pistons on the brakes of the four wheels. The pistons press the brake linings, which are friction materials, against the inside surfaces of the brake drums which rotate with the wheels. The linings are pressed on the rotating drums, which in turn decelerate the wheels, thereby slowing down and stopping the vehicle.


DISC BRAKES
This is an interesting one. Shown are the parts of a basic Disc Brake Assembly. When the driver steps on the brake pedal, the power is amplified by the brake booster (servo system) and changed into a hydraulic pressure (oil-pressure) by the master cylinder. The pressure reaches the brakes on the wheels via tubing filled with brake oil (brake fluid).
The delivered pressure pushes the pistons on the brakes of the four wheels. The pistons in turn press the brake pads, which are friction material, against the brake rotors which rotate with the wheels. The pads clamp on the rotors from both sides and decelerate the wheels, thereby slowing down and stopping the vehicle.


ANTI-LOCK BRAKING SYSTEM (ABS)
Before jumping into ABS, let’s remind you of the fundamentals of rotational mechanics. Consider a rotating wheel that is also translated with a certain linear velocity (basic rolling motion). Now, ideally, the point of contact wants to stay at rest (pure rolling of course). The situation is depicted in the figure shown. As you can see, the contact point is at rest. Now, you may ask why our car turns if we turn the steering wheel. Well, that’s simple physics. Just think for a moment that the car doesn’t turn when the wheels turn. Then, the point of contact will no longer be at rest. So, turning is the only way.

Now coming to the problematic part. When we apply the brake pedal, the wheels might lock and if they do so, turning the steering wheel won’t turn your car (wheel doesn’t have any rotational velocity; it only has linear velocity so it will continue sliding forward due to the initial inertia). As a result, if you can’t turn you will crash. ABS solves this problem.
Basic working of ABS: ○ The controller (ECU-Electronic Control Unit) reads the signal from each of the speed sensors of the wheel. ○ As the brakes are suddenly applied by the driver, this makes the wheel to decelerate at a faster rate and may cause the wheel to Lock.
○ As the ECU reads the signal which indicates the rapid decrease in the speed of the wheel, it sends a signal to the valve which makes the valve close and the pressure on the brake
pad reduces and prevents the wheel from locking.
○ The wheel again starts to accelerate, again the signal sends to the controller, this time it opens the valve, increasing the pressure to the brake pad and brakes are applied, this again reduces the speed of the wheel and tries to make it stop.
○ This process of applying brakes and releasing them happens 15 times in a second when a driver suddenly applies the brake harder. Due to this, the locking of the wheel is prevented and the skidding of the vehicle is eliminated. During braking with the ABS system, the driver can steer the vehicle and reduces the risk of a vehicle collision.


ELECTRONIC BRAKE-FORCE DISTRIBUTION (EBD)
An extension of the ABS is responsible for diverting equal brakeforce to the respective wheel. The electronic Brake Distribution system uses the vehicle’s ABS to adjust the braking force between all the wheels depending on the tire grip. It automatically adjusts the brake force depending on conditions to make sure that the front and rear axles yield the best braking force possible without the wheel locking up.
This prevents the automobile from rotating due to the different traction on the wheels.
These systems work in tandem with ABS using an electronically controlled valve that diverts hydraulic pressure to the front and rear axles. EBD offers significant braking durability under any conditions and is not dependent on ABS to operate. The hardware of EBD comprises the following components which are: Wheel Speed sensors, load sensors, Brake force modulators /Valves, Electronic Control Unit (ECU).

BRAKING ASSIST (BA)
Also known as Emergency Brake Assist (EBA) is another piece of car braking technology. As the name suggests, it is responsible to assist the driver in emergency stop situations. If you have been through a situation where you had to attempt an emergency stop, you might have felt like jamming the brake pedal all through the floor. However, an average driver isn’t
actually fast enough for a situation like this, where a millisecond of delay can translate to catastrophe. This is where Braking Assist steps in. Brake assist converts that mild pedal pushes to a complete halt. A sensor attached to the brake pedal triggers the brake assist system to detect when the driver is attempting to stop in an emergency situation and applies full braking pressure until the car comes to a complete halt.


REGENERATIVE BRAKING
Here comes my favorite part. This is where I can tell you, “Welcome to the 21st century !!”. If you’re interested in buying an electric or HEV car, you may have heard about regenerative braking. When you step on your petrol or diesel car’s brake pedal, hydraulic fluid pushes brake pads against brake discs on each wheel or drums on older and cheaper models). The resulting friction works to slow the car down, generating heat and wearing away at the material on the pads and discs in the process. Regenerative braking is a way of taking the wasted energy from the process of slowing down a car and using it to recharge the car’s batteries.
Well, how does it work?
The electric motor in your hybrid or electric car runs in two directions - one to drive the wheels and move the car, and the other to recharge the battery. When you lift your foot off the accelerator pedal and onto the brake, the motor swaps directions and starts to put energy back into the battery. When this process kicks in, you can feel the car start to slow down. It’s a different sensation in each car that has this function, because manufacturers can program-in how much regenerative braking occurs when you lift off the pedal. All cars still have normal brakes, so if you push the pedal hard enough then the hydraulic system will kick in to get you stopped quickly (depending on your speed). Again, different cars will have different amounts of force on the pedal needed to get the brakes to kick in.


REFERENCES
For further details you may refer to the following sites:

Report by:

Rishav Nandi
B.E. Mechanical Engineering



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