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#### 了解防抱死系统

Understanding Anti-lock Braking System (ABS) !

is used in almost all modern vehicles.

This system prevents accidents like this,

where you lose control of the steering

as you apply the brakes.

With ABS, you will be able to

steer the vehicle properly

and it also reduces the braking distance.

What is ABS?

How is it able to prevent accidents

due to the braking?

We will explore these questions in this video.

To properly understand ABS,

we first need to understand the principles

of steering and braking.

When you press the brake pedal,

the brake pads on the wheel desks will be activated

and press against the disks.

And this will stop the rotation of the wheels.

As the wheels stop,

this causes slippage between the road and wheels

and the vehicle stops due to this friction.

Now, let’s understand the steering mechanism.

The whole steering mechanism is dependent on

the simple principle of the wheels rolling.

The wheels will always try to

So, this means that the velocity of the wheel at the contact point

will always be zero.

The wheels have two kinds of motion.

They rotate on their axis

and also they move along the direction of the car.

Due to these two kinds of motion,

they have two kinds of velocity,

translational and rotational.

For non-slip rolling conditions,

the sum of the translational and rotational velocities

at the contact point, should be zero.

Just keep this simple principle in mind.

You will very easily be able to

understand the steering mechanism.

In an automobile,

all four wheels should be rolling

at all times to avoid the vehicle slipping.

In fact,

this simple rolling principle is the reason why

when you turn the front wheels.

Let’s examine this.

Assume a hypothetical condition in which

even after turning the wheels,

the car carry straight on.

In this case, the rolling velocity

will be inclined as shown

and the translation velocity remains straight.

Therefore if you add up these two velocities,

the resultant velocity will not be zero.

This will cause the car to slip.

The only way to avoid this is

by making the translation velocity also inclined.

This is possible only when the whole car

makes a turn as shown.

When the car makes a turn

at the instantaneous center shown,

all four wheels can satisfy the condition of rolling.

From this snapshot,

you can clearly see how the velocity at the contact point

of all four wheels becomes zero.

Now let’s analyze the steering mechanism

at the time of braking.

Due to the brake pads being activated,

the wheel stops spinning.

Even though the wheels are turned,

since they are not rotating,

there will be no rotational velocity at all.

In this case, the vehicle does not need to turn

as this has become like a normal case of braking.

The vehicle will no longer react to

the drivers changes to the steering

and will slide in a straight line

which can cause an accident.

Apart from losing control of the steering,

cars without ABS face another big issue-
——在不同光滑程度
braking on surfaces

with different levels of traction.

When the left and right wheels are

on different traction surfaces,

applying the brakes will generate

different frictional forces on the wheels.

This will produce torque.

And the car will go into an uncontrollable spin.

Let’s see how ABS solves these issues.

An anti-lock braking system is very simple.
ABS可以防止
What ABS does is to prevent

the wheels from completely locking up.
ABS系统有速度传感器
The ABS system includes speed sensors

attached to all of the wheels.

When the sensors detect that

a wheel is about to lock up,

a modulator unit partially releases

the brake pad on that wheel.

This way, the wheels will be allowed to

spin intermittently during braking.

You can immediately see the effect of

this intermittent wheel rotation on steering.

Here, the wheels will have a rotational

velocity component.

This will definitely allow the steering to function

and driver will be able to steer the vehicle

even during the braking.

But the big question here is

how does the action of intermittently

releasing the brakes reduce the braking distance.

To get an answer to this question,

we first need to understand

how the coefficient of friction between the tire and the road

varies with respect to slip.

In perfect rolling conditions,

the value of the frictional coefficient

is nearly zero.

However when the wheel is 100% slipping,

sliding friction comes into play.

You can expect the frictional values for

other levels of slippage to vary like this.

But since rubber is a complex material

which has strange behaviors during braking.

The frictional coefficient between the tire and the road

varies as shown.

With respect to slippage,

it has a peak value at

around 12% of slip ratio.

And below and above this,

the frictional coefficient reduces.

During breaking without ABS,

the frictional coefficient

which comes into effect

is predominantly sliding friction.

However, in ABS braking,

the clever algorithm adjusts the break pressure

to keep the slip ratio near to 12%

where the frictional value is at the maximum.

This significantly reduces the braking distance.

Now, back to the huge instability issue of the cars.

By intelligent electronic braking distribution,

which is a subsystem of modern ABS,

one can overcome this issue very easily.
EBD
An EBD.

Yaw rate of the car and

the relative slip of each wheel is measured.

Just by reducing the brake pressure

on the wheels with higher grip,

we will be able to reduce the frictional force

produced by these wheels.

This will keep the yaw torque

and the whole car under control.

We hope this video gave you

a clear conceptual overview of

the workings of ABS.

support our educational activities

on patreon.

Thank you!

ABS系统

Felicity

Tiger_woo