2013 SSANGYONG NEW ACTYON SPORTS Service Manual

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5. SYSTEM OPERATION
1) Block Diagram of ABS HECU

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2) Basic Theory of ABS Function
To give you a better understanding of the tasks and functions of ABS, we will first look at the physics
principles.
(1) Stopping distance
(2) Brake force on a wheel
The maximum possible brake force on a wheel depends on the wheel load and the adhesion coefficient
between tire and carriageway. With a low adhesion coefficient the brake force, which can be obtained is
very low. You are bound to know the result already from driving on winter roads. With a high adhesion
coefficient on a dry road, the brake force, which can be obtained, is considerably higher. The brake
force, which can be obtained, can be calculated from below formula:
Maximum brake force ▶
FBmax = wheel load FR x coefficient of
frictionMh
The braking process cannot be described
sufficiently accurately with the brake forces
calculated. The values calculated only apply if
the wheel is not locked. In the case of a locking
wheel, the static friction turns into lower sliding
friction, with the result that the stopping distance
is increased. This loss of friction is termed "slip"
in specialist literature.
The stopping distance depends on the vehicle weight and initial speed when braking starts. This also
applies for vehicle with ABS, where ABS always tries to set an optimum brake force on each wheel. As
great forces are exerted between the tires and the carriageway when braking, even with ABS the wheels
may scream and rubber is left on the road. With an ABS skid mark one may be able to clearly recognize
the tire profile. The skid mark of an ABS vehicle does not however leave any hint of the speed of the
vehicle in the case of an accident, as it can only be clearly drawn at the start of braking.

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Slip ▶
The brake slip is the difference between the vehicle speed and the wheel circumference speed. If the
wheel locks, the slip is greatest, that is 100 %. If the wheel is running freely and un-braked, the slip is the
lowest, equal to 0 %. Slip can be calculated from the vehicle speed Vveh and the wheel speed Vw. The
equation for this is:
Vveh = 100 km/h, Vw = 70 km/h
Slip ratio (S) = X 100%
S = 30%Vveh - Vw
Vveh
Typical Slip Curves ▶
For the various road conditions, the friction
coefficients were plotted. The typical course of
the curves is always the same. The only special
feature is shown by the curve for freshly fallen
snow, for this curve increases at 100 % slip. In
a vehicle without ABS, the wheel locks on
braking and therefore pushes a wedge before
it. This wedge of loose surface or freshly fallen
snow means and increased resistance and as
a result the stopping distance is shorter. This
reduction in stopping distance is not possible
with a vehicle with ABS, as the wheel does not
lock. On these surfaces the stopping distance
with ABS is longer than without ABS. The
reason for this is based in physics and not in
the Anti-Lock System.
However, as mentioned before, ABS is not
about the stopping distance, but
maneuverability and driving stability, for the
vehicle with locking wheels without ABS cannot
be steered. Ex)

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KAMM circle ▶
Before we go into the Kamm circle, you should
know that a tire offers a maximum of 100 %
transmissibility. It is all the same for the tire
whether we require 100 % in the direction of
braking or in the direction of the acting lateral
force, e.g. when driving round curves. If we drive
into a curve too fast and the tire requires 100 %
transmissibility as cornering force, the tire cannot
transmit any additional brake force. In spite of the
ABS the car is carried out of the curve. The
relationship between brake force B and cornering
force S is shown very clearly in the Kamm circle.
If we put a vehicle wheel in this circle, the
relationship becomes even clearer. In this
relationship: as long as the acting forces and the
resulting force remain within the circle, the vehicle
is stable to drive. If a force exceeds the circle, the
vehicle leaves the road.
Brake force
When depressing the brake pedal the brake
force increases to the maximum, then the brake
force decreases until the wheel locks.
Cornering force
The cornering force is a maximum when the
wheel is turning freely with zero slip. When
braking the cornering force falls to zero if the
wheel locks (slip 100 %).
ABS operating range
The operating range starts just before the
maximum brake force and ends in maximum, for
the unstable range then begins, in which no
further modulation is possible. The ABS controls
the regulation of the brake pressure so that the
brake force only becomes great enough for a
sufficient proportion of cornering force to remain.
With ABS we remain in the Kamm circle as long
as the car is driving sensibly. We will leave
driving physics with these statements and turn to
the braking systems with and without ABS. -
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- Brake and cornering force ▶

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3) Basic ABS Control
Operation of ABS control unit ▶
Applications of the ABS control unit The signals
produced by the wheel sensors are evaluated
in the electronic control unit. From the
information received, the control unit must first
compute the following variables:
Wheel speed
Reference speed
Deceleration
Slip -
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Reference speed ▶
The reference speed is the mean, I.e. average speed of all wheel speeds determined by simple
approximation.
Simplified ABS control ▶
If, during braking, one wheel speed deviates from the reference speed, the ABS control unit attempts to
correct that wheel speed by modulating the brake pressure until it again matches the reference speed.
When all four wheels tend to lock, all four wheels speeds suddenly deviate from the previously
determined reference speed. In that case, the control cycle is initiated again in order to again correct the
wheel speed by modulating the brake pressure.

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4) ABS Control Pattern
The ABS control is performed by comparing the reference speed with each wheel speed. Firstly, it is
determined whether the vehicle is in the deceleration or acceleration state using the wheel speed
change ratio. Then, a signal is transmitted to the valve.
Finally, the brake pressure is adjusted via the signal.
△V: Vehicle speed
Vref: Vehicle speed reference
Vw: Wheel speed

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5) EBD (Electronic Brake Force Distribution) System
System description ▶
As an add-on logic to the ABS base algorithm, EBD works in a range in which the intervention
thresholds for ABS control are not reached yet.
EBD ensures that the rear wheels are sensitively monitored for slip with respect to the front axle. If slip is
detected, the inlet valves for the rear wheels are switched to pressure hold to prevent a further increase
in pressure at the rear-wheel breaks, thus electronically reproducing
a pressure-reduction function at the rear-wheel brakes.
ABS features an enhanced algorithm which includes control of the brake force distribution between the
front and rear axles. This is called Electronic Brake Distribution. In an unloading car condition the brake
efficiency is comparable to the conventional system but for a fully loaded vehicle the efficiency of the
EBD system is higher due to the better use of rear axle braking capability.
Advantages ▶
Elimination of conventional proportioning valve
EBD utilizes the existing rear axle wheel speed
sensor to monitor rear wheel slip.
Based on many variables in algorithm a
pressure hold, increase and/or decrease
pulsetrain may be triggered at the rear wheels
insuring vehicle stability.
Vehicle approaches the ideal brake force
distribution (front to rear).
Constant brake force distribution during vehicle
lifetime.
EBD function is monitored via ABS safety logic
(conventional proportioning valves are not
monitorable). -
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Ideal distribution
EBD start point
Cut-in point
Fixed distribution 1.
2.
3.
4.

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6. HYDRAULIC CIRCUIT OF ABS
1) Normal Brake Operation (ABS is not working) Mode
If the driver depress the brake pedal so that the ABS does not operate, the hydraulic pressure in the
master cylinder increases through the vacuum booster and it is delivered to the wheel via the normal
open inlet valve. At this moment, the normally-closed outlet valve is closed The speed of the wheel that
hydraulic pressure is delivered reduces gradually.
Solenoid valve Valve Open/Close Pump motor
Inlet valve - Normal open (NO) valve Open
OFF
Outlet valve - Normal close (NC) valve Close