If you have ever ridden a personal watercraft and released the throttle while attempting to steer, you know what happens: You can’t steer. ABS is also the basis for other safety systems on modern vehicles, such as stability control and traction control. Some applications use ABS for automatic stopping and during distance-based cruise control operation. This system will be even more important when autonomous vehicles become more common.
Many high-end and hybrid vehicles use electronic braking. Most of them do not use a traditional master cylinder to push the brake fluid to the wheels. A pedal travel sensor attached to the brake pedal monitors the driver’s intentions and an electronic pressure modulator sends the fluid to the wheels. This sensor may be a chamber of brake fluid with transducers that convert pressure into electronic signals. The system can send different amounts of pressure to each wheel depending on varying driving conditions such as cornering or panic stopping. This allows for more effective control and a greater level of safety.
The mechanical unit that pulses the brakes during ABS operation is called the modulator. It contains two solenoids in series with the affected wheel. Early systems only controlled the rear wheels. One valve blocked the fluid and the other vented the high-pressure fluid to an accumulator, which released the rear brakes. The system was known as rear wheel antilock (RWAL) or rear antilock brake system (RABS), referring to the fact that only the rear wheels had ABS.
Some of these systems have had issues but most use some form of backup system in the event of system failure. Valves may open in the pressure transducers, allowing fluid to flow to the front calipers for example. The major difference between this system and a conventional brake system is the fact that there is no mechanical connection between the brake pedal and the wheels. The brake pedal input is monitored and sent by electrical signals to the modulator, which usually uses a pump and high-pressure accumulator (HPA) to send the fluid to the wheel using several solenoids.
Some late-model, high-end, and hybrid vehicles use electronic brake systems. These systems generally use an electronic modulator to apply the brakes instead of a conventional master cylinder. This design allows for various pressures to be applied to each wheel relative to operating conditions.
Regenerative braking, also known as “regen” brakes, use magnetism to help slow a vehicle. A motor/generator is typically mounted around the flywheel. It can start the vehicle and also provide braking action while it helps recharge the battery.
When in brake mode, the operation could be described as the opposite of a motor. When the brakes are applied on a vehicle with regen brakes, the drive motor is used as a generator and the magnetism causes the shaft to be more difficult to turn, resulting in braking action. This magnetism is used to recharge the vehicle’s drive battery pack. Vehicles such as the Toyota Prius have used this concept for many years. Because of the lightweight construction of this vehicle, the regen brake system is capable of supplying most of the brake force, so the base brakes often last a long time if the vehicle is driven normally. Some have reported that the brakes lasted the life of the vehicle.
Regenerative braking is used on many hybrid and electric vehicles. Most still use a downsized conventional brake system, but the bulk of the braking is accomplished by using the vehicle’s propulsion motor as a generator during stopping. The generator recharges the batteries during normal stopping. If additional braking is required, the conventional brake system is also applied.
The Possibility of Mandatory Maintenance
As vehicles have progressed and more electronics have been added to replace mechanical functions, they have become more like the systems used in aviation. There, mandatory maintenance is required, even on private aircraft.
When autonomous vehicles are common on the roads, it may be necessary for mandatory maintenance to become a reality in the automotive world. For example, a wire harness leading to a caliper may need to be replaced at a specified time or mileage interval. The decision may not be left up to the owner of the vehicle. ■
A new type of brake system is currently being tested in Europe. Known as Electronic Wedge Braking (EWB), the system uses disc brakes with a sliding caliper that is actuated by electric motors rather than a hydraulic piston. A sliding plate with raised humps on it is moved back and forth by two electric motors. The inboard brake pad also has raised humps, but the valleys of the humps of the pads are in contact with the high spots on the sliding plate. When braking is needed, one of the motors slides the plate, which causes the humps to apply the inboard pad to the spinning rotor. As contact is made, the pad can move with the rotor, which increases brake force by wedging action against the plate. The outboard pad is applied in the usual manner when the caliper slides on its pins or guides.
This design eliminates many expensive, heavy components such as the brake booster, master cylinder, ABS unit, lines, and hoses. The calipers must have a wire harness instead of a brake hose, which could be subject to wear when the suspension moves and steering takes place. It may also become brittle over time or due to cold temperatures and require replacement more frequently than a rubber hose.
This is the main drive battery from a Tesla. Most hybrid and electric vehicles use regenerative braking to slow the vehicle and recharge the battery when the brakes are applied normally. Heavy braking causes the conventional brakes at the wheels to help stop the vehicle.
CHAPTER 2
BRAKE SYSTEM COMPONENTS
The modern automotive and light truck brake system is made of many components working together to achieve effective control while slowing or bringing the vehicle to a safe stop.
When you press the brake pedal in a vehicle with manual brakes, linkage connects the mechanical force to hydraulic pistons, which pressurize sealed systems of lines, hoses, and output pistons with high-pressure fluid that applies the friction material to the spinning rotor or drum to stop the vehicle. Think of a cylinder type of squirt gun. Pressing on the plunger causes water to squirt from the other end. If this end were connected by a hose to another cylinder full of water the same size, the movement of the input plunger would cause equal movement at the output plunger. This movement is used to apply the brake pads or shoes to stop the vehicle. If the output piston diameter were twice that of the input, the output would apply twice the force, but it would move half the distance.
On a vehicle, the cylinder applied by your foot is known as the master cylinder and the cylinder that applies the friction at each wheel is known as a caliper or wheel cylinder. This is referred to as the hydraulic system. No matter how long the lines are, as long as they are full of fluid there will be immediate movement at the output for every movement you make at the input.
The basic principles of hydraulics are indicated here. If water is forced from one into the other, movement takes place that is identical as long as no air is present. Notice that the blue cylinder’s piston is down and the pink cylinder’s piston is up.
When the pink cylinder’s piston is pressed down, the blue cylinder’s piston comes up the exact same amount. No matter how long the brake line or hose connecting the two cylinders is, the same movement takes place if the cylinders are the same diameter and no air is present. If the