18-10-2012, 05:46 PM
ESP Electronic Stability Program
ESP Electronic Stability.pdf (Size: 62.4 KB / Downloads: 51)
angle can quickly increase drive slip to
such levels that a vehicle without ESP
becomes instable. At some point in this
alternating sequence the vehicle suddenly
ceases to respond to steering inputs
and breaks into a slide. While lateral
acceleration remains virtually constant,
both sideslip angle and yaw rate rise
radically.
Vehicle with ESP (Figure 8, on right)
The ESP electronic stability program
intervenes at an early stage in this
sequence of steering and countersteering
manoeuvres to counter the instability that
threatens right from the outset. ESP relies
on engine intervention as well as individually
controlled braking of all four
wheels to maintain the vehicle’s stability
and steering response. Sideslip angle and
yaw-rate are controlled so that the driver’s
steering demands can be complied with
as far as possible considering the
prevailing physical conditions.
Acceleration/deceleration during cornering
A decreasing-radius curve becomes progressively
tighter as one proceeds. If a
vehicle maintains a constant velocity
through such a curve (as frequently encountered
on Autobahn exit ramps) the
outward, or centrifugal force will automatically
increase at the same time
(Figure 9).
Excessive braking in such a curve is yet
another potential source of radial and
tangential forces capable of inducing
instability during cornering.
This applies to performance-oriented
driving as well as when the driver accelerates
too soon while exiting a curve. In
terms of the physics of vehicle operation,
it produces the same effects as the situation
described above (Figure 10).
A vehicle’s handling response during
simultaneous acceleration and cornering
is determined by testing on the skid pad
(semi-steady-state circulation).
In this test the driver gradually accelerates
on a high-traction surface (mHF = 1.0) and
attempts to stay on a skid pad with a
100 meter radius while circulating at
progressively faster rates; this continues
until the vehicle reaches its cornering
limits.
Vehicle with ESP
Up to a speed of roughly 95 km/h, vehicles
with and without ESP display identical
response patterns. Because this speed
coincides with the vehicle’s stability limit,
the ESP refuses to implement continued
driver demand for further acceleration to
beyond this point. Active engine and
braking intervention reduce the requirement
for steering inputs while preventing
further increases in sideslip angle.
This results in minor deviations from the
initially projected course, which the driver
can then correct with appropriate steering
action. At this point, the vehicle operator
has assumed an active role as an element
within the control loop.