E
SP
(Electronic Stability Program)
ESP
(Electronic Stability Program)
1.
BASIC
PHYSICS
OF
DRIVING
DYNAMICS
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2.
ESP GENERAL
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3.
JM ESP SYSTEM
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4.
ESP CONTROL MODULE
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5.
HYDRAULIC CONTROL UNIT
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6.
INPUTS AND OUTPUTS
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7.
INPUTS
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8.
OUTPUTS
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9.
DIAGNOSIS & FAILSAFE
--------------------------------------------------------Contents
1. Basic physics of driving dynamics stopping distance
The stopping distance depends on the vehicle weight and the speed at which the vehicle travels when the brakes are first applied.
This also applies to vehicles equipped with ABS. Although ABS attempts to adjust an optimum braking force at each wheel, the forces which take effect between tires and road surface are so high that even wheels equipped with ABS may squeal and leave rubber on the road. The skid mark produced by an ABS brake application may clearly show the tread pattern of the tire.
However, in the event of an accident, the speed at which the vehicle was traveling cannot be concluded from the skid mark of an ABS vehicle because any such mark will be clearly visible at the start of brake application only.
Braking force
Depressing the brake pedal causes the braking force to rise until it reaches a maximum, after which it drops until the wheel locks.
BRAKING FORCE AT A WHEEL
The maximum braking force that can be achieved at any wheel depends upon the load on the wheel and the frictional grip between tire and road surface, which is expressed as the ‘coefficient of adhesion’. If the coefficient of adhesion is low, the braking force that can be achieved is very low. You will probably be familiar with this situation from driving on winter roads. With a high coefficient of adhesion on a dry road, the braking force that can be achieved is substantially higher. The maximum braking force that can be achieved can also be calculated.
MAXIMUM BRAKING FORCE
F
However, the calculated braking forces do no provide a sufficiently accurate description of what happens during braking.
The calculated values are only valid if the wheel does not lock. If a wheel locks, adhesion changes to sliding friction, which exerts less deceleration. In technical literature, this loss of friction is described as ‘slip’.
CORNERING FORCE
The cornering force is highest when the wheel is rolling freely with no slip. Braking causes the cornering force to drop to zero when the wheel locks (slip=100%).
SLIP
Brake slip is the difference between the vehicle speed and the circumferential speed of the wheel. The slip is highest (100%) when the wheel locks and lowest (0%) when the wheel rotates unbraked.
While vehicle driving or braking, complex physical forces occurs in the tire’s contact area with the road. The tire’s rubber elements become distorted and are exposed to partial sliding movements, even if the wheel has not yet locked.
The slip can be calculated from the vehicle speed Vvhc and the wheel speed Vwhl using the following equation:
S = (Vvhc - Vwhl) / Vvhc x 100%
TYPICAL SLIP CURVES
T
On
a vehicle without ABS, the wheel locks when braked, causing a wedge
of loose surface material or new snow to build up in front,
resulting in a higher resistance and a shorter stopping distance.
If
the vehicle is equipped with ABS, the stopping distance cannot be
reduced because the wheel will not lock. On loose surface material
or new snow, the stopping distance of a vehicle with ABS is longer
than that of a vehicle without ABS. This is a physical phenomenon
for which the anti-lock braking system as such cannot be blamed.
However, as already mentioned, ABS is not concerned with stopping
distance, but with steerability and driving stability, permitting
you to steer around an obstacle. A vehicle without ABS is not
steerable when the wheels lock.
ABS WORKING RANGE
The working range starts just before the braking force reaches its maximum and ends when the maximum is reached, because this is the point where the unstable range starts in which control is no longer possible. ABS controls pressure modulation in such a manner that the braking force always stays below a limit where a sufficiently high proportion is still available for cornering. With ABS, only truly reckless driving can move us out of the Kamm circle.
KAMM’S CIRCLE
Before we discuss the Kamm circle, you should know that a tire cannot transmit more than 100% of the forces to which it is subjected. For the tire it is all the same whether you need the 100% in the braking direction or in the effective direction of the lateral force during cornering, for example. If you enter a bend too fast and the tire needs the full 100% which it can transmit as cornering force, it cannot transmit an additional braking force. The car will leave the road in spite of ABS. Kamm’s circle helps us to visualize the relationship between barking force (B) and cornering force (C). To demonstrate our point, we place a road wheel into the circle:
As long as the acting forces and the resultant force (F) stay within the circle, the vehicle is directionally stable. If one force leaves the circle, the vehicle leaves the road.
OVERSTEERING
When the rear tires lose traction before the front tires, a car is oversteering. Recovery from an oversteer situation must be quick since directional control can be lost.
Oversteering causes the tail end of the vehicle to swerve toward the outer side of the band (typical of rear wheel drive vehicles).
UNDERSTEERING
When the front tires lose traction before the rear tires, a car is understeering. Instinctively, a driver will compensate for understeer simply by turning the steering wheel further.
Understeering pushes the front wheels toward the outer verge of the bend (typical of front wheel drive vehicle).
[Oversteering
& Understeering] Increasing
the vehicle’s speed at this point causes the vehicle to move
either outside the original circle due to “Understeering”, or
inside the original circle due to “Oversteering”
SLIP ANGLE
Slip angle is the deviation of a wheel between wheel deflection (steer angle) and actual course.
SIDESLIP ANGLE
Sideslip angle (attitude angel) is the deviation of the vehicle from its longitudinal axis is the direction of travel.
YAW RATE
The yaw rate is a measure of the speed with which a vehicle turns about its vertical axis (swerving).
LATERAL ACCELERATION
Lateral acceleration acts at right angles to the direction of travel and occurs during cornering. It is a measure of the cornering speed.
STEERING ANGLE
The steering angle equals the wheel deflection and represents the course desired by the driver.