Page 1
Service.
Self-study programme 264
The Brake Assist System
Design and function
Page 2
Accident statistics show that in 1999 alone,
493,527 accidents in Germany were caused by
driver error. Many accidents caused by ignoring
right-of-way, driving on the wrong side of the
road, inappropriate speed, insufficient distance
from other vehicles and so on might have been
prevented had the vehicles been able to brake
faster.
What does this mean?
Studies have shown that many drivers do not
apply the brakes sufficiently in emergency situations due to lack of experience. That means that
the greatest possible braking effect is not
attained because the drivers did not press the
brake pedal hard enough.
Therefore, the brake assist system was developed
to support the driver in critical braking situations.
S264_042
The self-study program presents the design and
function of new technology.
The contents will not be updated.
2
New Warning
Please always refer to the relevant Service literature
for all inspection, adjustment and repair instructions.
Note
Page 3
Table of contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
The hydraulic brake assist system . . . . . . . . . . . . . . . . 8
Design, comparison and function . . . . . . . . . . . . . . . . . 8
Electrical components. . . . . . . . . . . . . . . . . . . . . . . . . . 14
Functional diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
The mechanical brake assist system . . . . . . . . . . . . . 20
Design and function . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Test your knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . .28
3
Page 4
Introduction
Early in
automobile development, the brakes played a
rather subordinate role because the friction in
the drive train was so great that a vehicle was
slowed sufficiently even without the brakes being
used.
Increasing power and speed as well as constantly increasing traffic density led to the consideration in the 20s of how an appropriate brake
system could provide a counterbalance to
greater power and driving performance.
But only after advances in electronics and microelectronics could systems be developed which
could react fast enough in emergency situations.
The ancestor of the electronic brake systems is
the ABS, which, since its introduction in 1978, has
been continuously further developed and
extended by additional functions. These functions
intervene actively in the driving process to
increase driving stability.
Currently, the trend in development is to driver
support systems such as the brake assist system.
The brake assist system supports the driver when
braking in emergency situations to achieve the
shortest possible brake path while maintaining
steering ability.
Mechanical
brakes
Active
wheelspin control
systems
ABS
TCS
EDL
EBD
EBC
ESP
Driver support
systems
Brake assist
system and
future systems
S264_009
4
Page 5
S264_005
What does the brake assist
system do?
To answer this question, let’s first take a look at a
braking manoeuvre without a brake assist system.
A driver is surprised by the car in front of him
braking suddenly. After a momentary shock, he
recognises the situation and applies the brakes.
Perhaps because he has not had to brake in critical situations very often and therefore has no
feel for how hard he must brake, he does not
press the pedal with all his might. Consequently,
the greatest possible brake pressure will not be
developed in the system and valuable braking
distance is lost. The vehicle may not come to a
stop in time.
S264_006
In comparison, let’s look at a car in the same situation but with a brake assist system.
As before, the brakes are not applied with sufficient force. Based on the speed and force with
which the brake pedal is pressed, the brake assist
system detects an emergency. The brake assist
system increases the brake pressure until the ABS
regulation intervenes to prevent the wheels from
locking. This way the greatest possible braking
effect can be achieved and the brake path can
be shortened significantly.
S264_008
5
Page 6
Introduction
Depending on the manufacturer of the wheelspin
regulation system, the developmental goal of a
brake assist system was attained in different
ways. Currently, we can distinguish between two
different types:
- the hydraulic brake assist systems and
- the mechanical brake assist systems.
In hydraulic brake assist systems, like that from
Bosch, the return flow pump of the ABS/ESP
hydraulic system provides pressure, thus the
expression „hydraulic brake assist system“. In this
context, we speak of active pressure development.
The advantage in design is that no additional
components needed to be integrated.
At VOLKSWAGEN, the hydraulic brake assist
system is currently being used in the 2002 Polo,
the 2001 Passat and the D-class vehicle.
6
S264_075 S264_076
Page 7
In the mechanical brake assist systems from Continental-Teves, brake pressure is developed and
an emergency situation is detected by mechanical components in the brake servo.
Both systems make use of existing system components to implement the function of the brake
assist system. Therefore brake assist systems are
currently available only in conjunction with ESP.
The mechanical brake assist system is being used
in the current models of the Golf and Bora.
In this self-study programme, the differences in
design and function between the hydraulic and
mechanical brake assist systems will be
described.
S264_078S264_077
7
Page 8
The hydraulic brake assist system
Design...
The central component in the Bosch brake assist
system is the hydraulic unit with the integrated
ABS control unit and the return flow pump. The
brake pressure sender in the hydraulic unit, the
speed sensors and the brake light switch supply
signals to the brake assist system so that it can
identify an emergency.
Pressure is raised in the brake slave cylinders by
the actuation of certain valves in the hydraulic
unit and the return flow pump for TCS/ESP.
g
h
c
b
a
e
f
d
... Comparison...
The vehicle without a brake assist system attains
the ABS regulation range later than the vehicle
with a brake assist system and consequently has
a longer brake path.
a- Brake servo
b - Brake pressure sensor
c - Brake light switch
d - Hydraulic unit
e - Return flow pump
f - Control unit
g - Brake slave cylinder
h - Speed sensor
Brake pressure (p) in bar
S264_011
Vehicle with brake assist system
Vehicle without brake assist system
S264_001
ABS regulation range
Time (t) in s
8
Page 9
p (bar)
... and Function
The function of the brake assist systems can be
divided into two phases:
Phase 1 Phase 2
S264_012
Brake pressure, brake assist system
Pedal pressure of driver
The brake assist system increases brake
pressure until ABS regulation intervenes.
- Phase 1 - Start of brake assist system
intervention
- Phase 2 - Conclusion of brake assist system
intervention
If the trigger conditions have been fulfilled, the
brake assistance increases the brake pressure.
The ABS regulation range is quickly attained
through this active pressure increase.
t (s)
b
a
Brake slave cylinder
a = Accumulator
b = ESP (brake pressure) switch valve N225
c = ESP high-pressure valve N227
d = Return pressure valve
d
The electronic stability program switch valve
N225 in the hydraulic unit is opened and the
electronic stability program high-pressure valve
N227 is closed. Consequently, the pressure created by the actuation of the return flow pump is
directed immediately to the brake slave cylinders.
c
S264_016
9
Page 10
The hydraulic brake assist system
Phase 1
The function of the brake assist system is to
increase the brake pressure as quickly as possible to the maximum value. The ABS function,
which is supposed to prevent the wheels from
locking, limits the pressure increase when the
locking threshold is reached. That means that
once the ABS intervention has begun, the brake
assist system can not further increase the brake
pressure.
p (bar)
Phase 1
ABS intervention
S264_013
Brake pressure at brake slave cylinder
Pedal pressure of driver
t (s)
When the ABS intervenes, the ESP (brake pressure) switch valve N225 is closed again and the
ESP high-pressure valve N227 is opened. The
discharge from the return flow pump keeps the
brake pressure below the locking threshold.
b
a
a = Accumulator
b = ESP (brake pressure) switch valve N225
c = ESP high-pressure valve N227
d = Return flow pump
d
c
S264_017
10
Page 11
p (bar)
Phase 2
S264_014
Brake pressure at brake slave cylinder
Pedal pressure of driver
Phase 2
t (s)
Brake fluid reservoir
If the driver reduces the pressure on the brake
pedal, the trigger conditions are no longer fulfilled. The brake assist system concludes that the
emergency situation has been resolved and
moves to phase 2. Now the pressure in the brake
slave cylinders is adapted to the driver’s pressure
on the brake pedal. The transition from phase 1
to phase 2 occurs not with a jump but smoothly,
with the brake assist system reducing its contribution to the pressure relative to the reduction of
pressure on the brake pedal. When its contribution finally reaches zero, normal braking function
is restored.
a
The brake pressure is reduced.
a = Accumulator
b = ESP (brake pressure) switch valve N225
c = ESP high-pressure valve N227
d = Return flow pump
The brake assist system also ends its intervention
when the vehicle speed drops below a predefined value. In both cases, brake pressure is
reduced by the actuation of the corresponding
valves. Brake fluid can flow to the accumulator
and is pumped back into the brake fluid reservoir
by the return flow pump.
S264_018
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Page 12
The hydraulic brake assist system
The trigger conditions
An emergency braking situation is identified by
the following trigger conditions, triggering intervention by the brake assist system.
These conditions must be fulfilled:
1. The signal from brake light switch indicating
that the brakes have been applied.
2. The signals from the speed sensors indicating
how fast the vehicle is travelling.
3. The signal from the brake pressure sender
indicating how fast and with what force the
driver has applied the brakes.
The speed and force with which the brakes are
applied are determined using the pressure
development gradient in the brake master cylinder. That means that the control unit determines
the change in current brake pressure via the
pressure sensor in the hydraulic unit over a certain period of time. That is the pressure development gradient.
Hydraulic unit with ABS control unit
Brake light switch
Brake pressure sender
Speed sensors
p (bar)
(km/h)
Actual value
t (s)
Regulative intervention
Specified value
1
x in s
y in bar
z in
km/h
S264_020
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Page 13
The slope of the pressure curve is the determining
factor for brake assist system intervention
p (bar)
S264_021
p (bar)
t
1
t
2
t (s)
The intervention threshold for the brake assist
system is a predefined value depending on the
vehicle speed. If the brake pedal pressure
exceeds this defined value in a period of time,
the brake assist system initiates intervention.
When the change in pressure drops below this
threshold, the brake assist system ends its intervention.
In other words, if the pedal pressure reaches a
certain value within a short period t
, the inter-
1
vention conditions are fulfilled and the brake
assist system intervenes. If the same pedal pressure is attained only after a longer time t
, the
2
curve is flat and the brake assist system does not
intervene. Thus, no intervention occurs if:
- the brake pedal is pressed to slowly or not at
all,
- the change in pressure remains below the
threshold,
- the vehicle speed is to low or
- the driver has applied the brakes with sufficient force.
S264_010
Brake pressure with ABS intervention
Pedal pressure of driver
An experienced driver develops sufficient pressure using the
brake pedal and the brake servo. ABS prevents the wheels
from locking.
t (s)
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Page 14
The hydraulic brake assist system
Electrical components
Brake light switch F
The brake light switch is installed in the pedal
cluster and detects the operation of the brake
pedal.
● How it works
The brake light switch is a classic mechanical
two-position push button.
● How the signal is used
The switch provides one of two signals:
brake pedal pressed or
brake pedal not pressed.
The signal from the brake light switch is used for
the various brake systems, the engine management system and the switching on of the brake
lights.
● Switch failure
The brake assist system is not functional without
the brake light switch signal.
● Self-diagnosis
A switch defect will be detected by self-diagnosis
and saved in the fault memory. If the switch is
renewed, it must be adjusted according to the
workshop manual.
S264_025
Signal:
not
pressed
S264_028
Signal:
pressed
14
S264_029
Page 15
Brake pressure sender G201
If the brake system has ESP, the brake pressure
sender is screwed directly into the hydraulic unit
and senses the current pressure in the brake system.
S264_024
Uniformly distributed charge
● How it works
The heart of the sender is a piezo-electric element. It reacts to changes in pressure with a
change in the charge distribution within the element, producing a measurable change in voltage.
Changes in the sender’s voltage are detected
and evaluated by the control unit.
S264_054
● How the signal is used
As described above, the signal over a period of
time is used to calculate a pressure gradient
which defines the intervention conditions for the
brake assist system.
● Sender failure
Neither the brake assist system nor the ESP is
functional without the signal from the brake pressure sender.
● Self-diagnosis
A sender defect will be detected by self-diagnosis and saved in the fault memory.
S264_026
Irregularly distributed charge
S264_027
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The hydraulic brake assist system
Speed sensors G44 - G47
The speed sensors are inductive sensors which,
using a rotor on each wheel hub as sender
wheel, determine the current rotational speed of
the wheels.
● How it works
The sensor consists of a soft iron core with a permanent magnet and a coil.
The magnetic field which the permanent magnet
creates over the iron core is influenced by the
sender wheel. Changes in the magnetic field
induce measurable voltage in the sensor coil. The
faster the sender wheel passes the coil, the
higher the frequency of the voltage change.
● How the signal is used
The ABS control unit calculates the rotational
speed of each wheel based on the frequency.
The rotational speed of the wheels is used by a
variety of different vehicle systems.
● Sensor failure
Without the speed sensor signal, the brake assist
system cannot calculate the speed-dependent
threshold. The brake assist system is switched off.
● Self-diagnosis
A defect in a speed sensor is detected by selfdiagnosis and saved in the fault memory.
S264_023
a
b
c
U (V)
16
d
S264_022
a - Permanent magnet
b - Soft iron core
c - Coil
d - Rotor
Page 17
Active wheel sensing
There is another type of rotational speed sensors
which are called active sensors and will be used
with increasing frequency for determining wheel
speeds. The term „active“ refers to the required
voltage supply for the sensors, which is not necessary for inductive sensors.
S264_073
Hall voltage
Sensor electronics
Hall-IC
Magnetic track
Supply voltage
S264_074
● How it works
The heart of the sensor is a Hall integrated circuit
(IC).
When current flows through this semi-conductor
chip, a Hall voltage is created. Changes in the
magnetic environment of the sensor cause proportional changes in the Hall voltage because
the resistance in the Hall IC changes.
Depending on the version of the sensor, it can be
paired with either a magnetic sender wheel or a
sender wheel with a magnetic track.
As the sender wheel moves past the sensor, the
magnetic environment and, consequently, the
Hall voltage change.
● How the signal is used
The control unit can determine the rotational
speed based on the frequency of changes in
voltage.
With active sensors, even very low speeds can be
detected.
● Self-diagnosis
A defect in a speed sensor is detected by selfdiagnosis and saved in the fault memory.
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The hydraulic brake assist system
ABS return flow pump V39
During ABS operation, the return flow pump
returns a quantity of brake fluid against the pressure developed by the brake pedal and the
brake servo.
● How it works
It is a double-acting piston hydraulic pump which
can be switched on or off by the ABS control unit.
In this case, „double acting“ means that with
each piston stroke a suction and a discharge
action are performed. With a single-acting piston, the two actions occur consecutively.
The double action is achieved through the
design, which includes working chambers in front
of and behind the piston. When the piston moves
to the left, the front chamber is emptied and
brake fluid is drawn into the back chamber.
When the piston moves to the right, brake fluid is
forced out of the back chamber back into the
suction line. The pre-pressure on the suction side
produces a nearly uniform discharge so that
pressure can be built up quickly. An additional
pump for building up pre-pressure is no longer
necessary.
Discharge side
Suction side
S264_053
Back chamber
Piston
S264_070
● Failure of return flow pump
Without the contribution of the return flow pump,
many brake system functions like, for example
ABS, fail. The brake assist system is likewise nonfunctional.
● Self-diagnosis
A defect in the return flow pump is detected by
self-diagnosis and stored in the fault memory.
18
Front chamber
Suction pressure
Pre-pressure
Discharge pressure
S264_071
Page 19
Functional diagram
A+
SS S
J106 N99
D
F
N101
N100
A+ Battery
D Ignition/starter switch
F Brake light switch
G44
N102
G45 G46 G47 G201
J104
N133
N134
N135
N136
N225
N227
N226
N225 ESP switch valve -1N226 ESP switch valve -2N227 ESP high-pressure valve -1N228 ESP high-pressure valve -2-
a
b
J105 V39
N228
S264_078
G44 Rear right speed sensor
G45 Front right speed sensor
G46 Rear left speed sensor
G47 Front left speed sensor
G201 Brake pressure sender
J104 ABS control unit
J105 ABS return flow pump relay
J106 ABS solenoid valve relay
N99 ABS inlet valve, front right
N100 ABS outlet valve, front right
N101 ABS inlet valve, front left
N102 ABS outlet valve, front left
N133 ABS inlet valve, rear right
N134 ABS inlet valve, rear left
N135 ABS outlet valve, rear right
N136 ABS outlet valve, rear left
SFuse
V39 ABS return flow pump
aCAN high
bCAN low
19
Page 20
The mechanical brake assist system
Design ...
The heart of the Continental-TEVES mechanical
brake assist system is a mechanical switch component in the brake servo.
Mechanical switch component
in brake servo
20
S264_030
Page 21
Vacuum chamber Pressure chamber
S264_031
Switch component
in control housing
The brake servo has a pressure and a vacuum
chamber. When the brakes are not applied, vacuum is created by the intake manifold in both
chambers. The brake force is amplified when,
during brake application, the pressure chamber
is pressurised with atmospheric pressure.
This creates a pressure differential between pressure and vacuum chambers, so that the external
air pressure supports the braking motion.
The mechanical switch component consists of a
locking sleeve with spring, a valve piston and a
ball cage with balls and ball sleeve.
Pushrod to tandem brake master cylinder
Control housing
Brake servo housing
Locking sleeve with spring
Mechanical strip
Ball sleeve
Transfer d is c
Ball cage with balls
Reaction disc
Atmospheric port valve
S264_032
Valve operating
rod with plunger
21
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The mechanical brake assist system
... and Function
As pressure develops in the brake system, the driver feels a counter-pressure in the brake pedal.
The principle of the mechanical brake assist system is to divert this force to the control housing, relieving
the driver physically. The locking mechanism holds the atmospheric port valve open and provides air to
the pressure chamber.
Path of force without brake assist system
Counter-force from
brake system
Atmospheric port valve
S264_033
Pedal force
22
Page 23
Path of force with brake assist system
S264_034
When the brake pedal is pressed with a certain force and a certain velocity, the switch component locks
and the brake assist system intervenes.
Valve piston
Ball housing
Locking sleeve with springBall cage
Switch component in emergency braking
operation
Stop
In this case, the valve piston moves and the balls
are moved inward in the ball cage. Consequently
the locking sleeve can move to its stop. The switch
component is locked.
Because the mechanical events are difficult to
present in a detailed diagram, the individual
steps will be explained in strongly simplified
drawings.
S264_038
23
Page 24
The mechanical brake assist system
Assembly
group
a Valve operating rod,
b Locking sleeve,
c Ball cage, balls,
Parts Colour
valve piston, ball housing,
transfer disc
mechanical stop
control housing
If the brake is applied too slowly, the brake assist
function is not triggered. That means that the
driver feels the full counter-pressure from the
brake system through the brake pedal as counter-force which he must overcome in order to
brake more heavily.
Assembly group (c)
Reaction disc
Assembly group (b)
S264_055
Assembly group (a)
Housing
S264_056
Great counter-force on pedal
24
If the brake pedal is pressed very fast, the brake
assist function is triggered.
The major portion of the counter-force is diverted
through the locking of the assembly groups to the
housing. The driver has to overcome only a very
small force to brake more heavily.
S264_059
Small counter-force on pedal
Page 25
Brake assist system intervention
A relation of two values triggers the mechanical
brake assist system. One is the velocity with
which the brake pedal is pressed and the other is
the force of the brake pedal.
The trigger threshold is presented in the graph. In
the green area above the trigger threshold, the
brake assist system is active.
Example:
1100
1000
900
800
700
600
Application force of brake pedal in N
500
400
300
200
100
0
0100150200250
1
Brake assist system
not active
1 Low application speed at high application force
2 High application speed at low application force
Brake assist system
active
Trigger threshold
2
Application speed of brake pedal in mm/s
S264_082
25
Page 26
The mechanical brake assist system
In Detail
The following, strongly simplified drawings illustrate the movements of the individual parts in relation to
each other.
If the trigger threshold is exceeded, the green
assembly group presses hard into the reaction
disc. Due to its inertia, the light red assembly
group cannot respond so quickly to the fast initial
movement.
The movement of the green assembly group in
relation to the light red group, enables the balls
to roll into the groove in the green group.
Only now can the locking sleeve (dark red) can
slide over the balls, locking the switch component. The balls cannot return to their initial position due to the new position of the locking sleeve.
Reaction disc
Balls
S264_065
S264_066
26
In this position, the counter-forces are diverted,
as previously explained, from the brake system
onto the housing.
Locking sleeve
S264_067
Page 27
Housing
Concluding the brake assist function
If the driver takes his foot from the brake pedal,
both red and the green assemblies move back
together until the stop rests against the housing.
S264_062
Stop
Because the entire mechanism moves further
back within the brake servo, the light red part
now moves in relation to the dark red part. Consequently, the locking sleeve releases the balls.
Locking sleeve releases the balls
Green assembly group again in the initial posi-
S264_063
In the last phase of the movement, the balls are
pressed back into their initial position by the
green assembly group.
The emergency brake assist function is switched
off.
S264_064
tion
27
Page 28
Service
Test in g fu nction
The brake pedal must be pressed with the engine
running and the vehicle stationary so that the
maximum vacuum boost is assured.
The mechanical brake assist system will be activated when the brake pedal is pressed to stop
above the trigger threshold. A click in the brake
servo can be heard when the brake assist system
is triggered. The brake pedal can now be partially released and pressed with a small force.
When the brake pedal is released completely,
the brake assist system must release (no hydraulic pressure in the brake system).
1100
1000
Application force of brake pedal in N
900
800
700
600
500
400
300
200
100
0
0 100 150 200 250
Brake assist system
not active
Brake assist system
active
Trigger threshold
Application speed of brake pedal in mm/s
S264_083
28
Page 29
Test your knowledge
1. What is the function of the brake assist system?
a It prevents the wheels from locking during emergency braking.
b It supports the driver when braking in emergency situations.
c It indicates to the driver how hard he must brake.
d It attains the greatest possible braking effect while maintaining steering ability.
2. In which vehicles is the hydraulic brake assist system currently installed?
aGolf
b Polo 2002
c Passat W8
dLupo 3L
3. The signals of which sensors are used for evaluating the trigger conditions?
a Brake pressure sender
b Engine speed sender
c Speed sensors on wheels
d ABS pressure sender
e Brake light switch
29
Page 30
Test your knowledge
4. Identify the components in the drawing.
a =
b =
c =
a
5. What is the effect of the mechanical brake assist system based on?
a The intake manifold vacuum works against the brake force so that the driver does not feel any
counter-force in the brake pedal.
b The counter-force from the pressure build-up in the brake system is diverted to the control
housing.
b
d
c
d =
30
6. Which conditions must be fulfilled to activate the mechanical brake assist system?
a The application force must be sufficiently great when the application speed is low.
b The application speed must be sufficiently great when the application force is small.
c The activation condition depends entirely on the distance the pedal moves.
Page 31
Notes
6. a, b
5. b
d = Return flow pump
c = ESP high-pressure valve N227
b = ESP (brake pressure) switch valve N225
a = Accumulator
4.
3. a, c, e
2. b, c
1. b, d
Answers:
31
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264
For internal use only © VOLKSWAGEN AG, Wolfsburg
All rights reserved. Technical specifications subject to change without notice.
140.2810.83.20 Technical status 09/01
❀ This paper was produced from
non-chlorine-bleached pulp.
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