G12�Driver�Assistance�Systems
19.�Cruise�Control
121
• Disruptive�and�overly�restrictive�interventions�are�avoided�if�the�driver�himself�would�also�prefer
high�lateral�acceleration.
• Useful�interventions�and�therefore�also�significant�limitations�in�dynamics�are�effected�at�high
speeds.�Most�drivers�find�an�overly�high�lateral�acceleration�in�these�situations�unpleasant,
which�is�why�a�lower�limit�value�is�used.
The�output�variable�for�the�cornering�cruise�control�is�also�used�as�a�set-point�value�for�the�longitudinal
acceleration.
Prioritization�of�the�partial�set-point�value
The�set-point�value�with�the�highest�priority�according�to�the�situation�is�selected�from�the�longitudinal
acceleration�set-point�values�for�the�control�functions�referred�to�above.�Signals�are�filtered�to�prevent
abrupt�jumps�when�adjusting�the�set-point�values.
Estimation�of�disturbance�force
An�acceleration�or�brake�force�must�be�calculated�in�order�to�be�able�to�apply�the�prioritized
longitudinal�acceleration�with�the�assistance�of�the�actuators.�Example:�more�driving�power�is�required
to�achieve�the�same�longitudinal�acceleration�during�an�uphill�journey�than�on�the�flat.�If�the�vehicle
needs�to�decelerate�when�travelling�up�hill,�less�braking�force�is�required�compared�to�driving�on�the
flat.�In�order�to�be�able�to�correctly�calculate�the�forces�required,�the�precise�value�of�the�gradient�and
also�the�weight�of�the�vehicle,�the�rolling�resistance,�the�drag�and�other�acceleration�forces�must�be
known.�As�a�sensor�system�that�can�monitor�all�of�these�disturbances�does�not�exist,�an�estimated
value�is�calculated�instead�by�comparing�the�following�two�variables:
• actual�movement�variables�of�the�vehicle
• anticipated�movement�variables�of�the�vehicle�as�a�result�of�the�driving�power�and�braking
forces�currently�acting�on�the�vehicle.
The�extent�of�the�disturbing�force�determined�using�this�method�is�taken�into�account�during
subsequent�processing�of�the�longitudinal�acceleration�setpoint�value�by�means�of�addition�or
subtraction.
19.2.2.�Activation�of�the�actuators
Driving�power�and/or�braking�forces�must�be�introduced�in�order�to�produce�the�longitudinal
acceleration�calculated�by�the�control�functions�and�thus�compensate�for�disturbing�forces�acting
on�the�vehicle.�To�accelerate�the�vehicle,�a�setpoint�value�for�the�drive�is�usually�transmitted.�In
exceptional�cases�during�downhill�driving�on�a�steep�gradient,�it�may�be�necessary�to�activate�the�brake
to�ensure�the�acceleration�does�not�exceed�a�specific�value.
If�the�vehicle�needs�to�be�decelerated,�the�extent�of�the�contribution�that�may�be�required�by�the�drive
towards�the�deceleration,�and�therefore�the�braking�effect�of�the�engine�and�transmission,�is�initially
determined.�This�value�is�output�to�the�Digital�Motor�Electronics�(DME)�or�and�electronic�transmission
control�(EGS).�If�this�value�cannot�be�achieved�by�the�drivetrain�alone,�the�additional�amount�required�is
transmitted�to�the�Dynamic�Stability�Control�(DSC).
The�vehicle's�brake�lights�are�also�switched�on�if�the�brakes�are�noticeably�actuated�in�order�to
produce�the�desired�deceleration.
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