Page 444 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Decisional architecture C HAPTER 14.2

Y max is the maximum allowed lateral acceleration, and 14.2.3.5.2 Reaching an appropriate start
k > 1 is an empirical constant, e.g. k ¼ 1.17 in our location
experiments.
At each time t from the starting time T 0 , the reference A typical situation at the beginning of a parallel parking
!
position p ref is translated along the vector dðS t Þ: n , where manoeuvre is depicted in Fig. 14.2-15. The autonomous
! vehicle A1 is in the traffic lane. The parking lane with
n represents the unit normal vector to the nominal
velocity vector along T ; the reference orientation q ref is parked vehicles B1, B2 and a parking place between them
converted into q ref þ arctan ðvd=vsðs t ÞÞ, and the refer- is on the right-hand side of A1. L1 and L2 are re-
ence velocity V R,ref obtained using the following spectively the length and width of A1, and D1 and D2 are
the distances available for longitudinal and lateral dis-
equation:
placements of A1 within the place. D3 and D4 are the
distðp ðtÞ; p ðt þ DtÞÞ longitudinal and lateral displacements of the corner A13
ref
ref
V R:ref ðtÞ¼ Dt ; (14.2.7) of A1 relative to the corner B24 of B2.
Distances D1, D2, D3 and D4 are computed from data
where dist stands for the Euclidean distance. As shown obtained by the sensorsystems. The length (D1dD3) and
in Fig. 14.2-12, this type of control skill can also be width (D2dD4) of the free parking place are compared
used to avoid a stationary obstacle, or to overtake an- with the length L1 and width L2 of A1 in order to de-
other vehicle. As soon as the obstacle has been detected termine whether the parking place is sufficiently large.
by the vehicle, a value s T,min is computed according to
Equation 14.2.5 and compared with the distance
between the vehicle and the obstacle. The result of this 14.2.3.5.3 Performing the parking manoeuvre
computation is used to decide which behaviour to
apply: avoid the obstacle, slow down or stop. In this During parallel parking, iterative low-speed backward
approach, an obstacle avoidance or overtaking manoeuvre and forward motions with coordinated control of the
consists of a lane changing manoeuvre towards a colli- steering angle and locomotion velocity are performed to
sion-free ‘virtual’ parallel trajectory (see Fig. 14.2-14). produce a lateral displacement of the vehicle into the
The lane changing skill operates the following way: parking place. The number of such motions depends on
the distances D1, D2, D3, D4 and the necessary parking
1. Generate a smooth local trajectory s 1 which connects depth (that depends on the width L2 of the vehicle A1).
s with a collision-free local trajectory s 2 ‘parallel’ to The start and end orientations of the vehicle are the same
T (s 2 is obtained by translating appropriately the for each iterative motion.
involved piece of T ).
Forthei-thiterativemotion(butomittingtheindex‘i’),
2. Track s 1 and s 2 until the obstacle has been overtaken. let the start coordinates of the vehicle be x 0 ¼ xð0Þ;
y 0 ¼ yð0Þ; q ¼ qð0Þ and the end coordinates be
3. Generate a smooth local trajectory s 3 which connects 0
x T ¼ xðTÞ; y T ¼ yðTÞ; q T ¼ qðTÞ, where T is duration
s 2 with T , and track s 3 .
of the motion. The ‘parallel parking’ condition means
14.2.3.5 Parallel parking that
q d < q T < q þ d q ; (14.2.8)
0
0
0
The purpose of the parallel parking SBM is to automat-
ically park the vehicle within an unknown parking area. where d q > 0 is a small admissible error in orientation of
This SBM comprises three main steps (Fig. 14.2-12): (1) the vehicle.
localizing a free parking place, (2) reaching an appropri-
ate start location with respect to the selected parking
place and (3) performing the parallel parking manoeuvre
using iterative backward and forward motions until the
vehicle is parked.

14.2.3.5.1 Finding a parking place
During this step, the vehicle moves slowly along the
traffic lane and uses its range sensors to build a local map
of the environment and detect obstacles. The local map is
used to determine whether free parking space is available
to park the vehicle. If an obstacle is detected during the
motion of the vehicle, another SBM e.g. the trajectory Fig. 14.2-15 Situation at the beginning of a parallel parking
following SBM is activated for avoiding this obstacle. manoeuvre.

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