Remark 1. If a
R
¼ a
L
¼ 1, from equation (2), we know
that v
s
L
¼ v
s
R
¼ 0, it implies a complete slipping, that is, the
wheels of the mobile robot are rotating, while its forward
speed is zero, the mobile robot is uncontrollable, this case is
not considered. When v
s
L
> r!
L
or v
s
R
> r!
R
, that is, a
R
< 0
or a
L
< 0, it indicates decelerated slipping (such as braking
process).
Lateral slipping ratio of the WMR is defined as
35
¼ tan (3)
where is the lateral slipping angle of a mobile robot (see
Figure 1), it is the angle between the velocity of the
mobile robot v and the x axis of a local frame attached
to the mobile robot.
Assumption 2. The lateral slip angle lies in ð0;=2Þ.
Remark 2. If ¼ =2, it implies that mobile robot is in a
state of complete lateral slipping, the mobile robot is
uncontrollable, this case is not considered. That is, lateral
slip ratio is bounded.
From equation (2), the linear velocities of the left and
right wheels of the mobile robot with wheels’ slipping are
given as
v
s
L
¼ r!
L
ð1 a
L
Þ
v
s
R
¼ r!
R
ð1 a
R
Þ
(4)
In coordinate frame F
1
ðx
w
; y
w
Þ, the kinematic mode of
the WMR without wheels’ slipping is described as
_
x
_
y
_
2
4
3
5
¼
cos 0
sin 0
01
2
4
3
5
v
!
(5)
In coordinate frame F
2
ðx
m
; y
m
Þ, a suitable model with
slipping can be written as
_
x
m
¼
r!
L
ð1 a
L
Þþr!
R
ð1 a
R
Þ
2
_
y
m
¼
r!
L
ð1 a
L
Þþr!
R
ð1 a
R
Þ
2
_
¼
r!
R
ð1 a
R
Þr!
L
ð1 a
L
Þ
b
(6)
where b is the distance between two driving wheels. As
shown in Figure 1, the coordinate rotation transformation
from F
2
ðx
m
; y
m
Þ to F
1
ðx
w
; y
w
Þ is given by
x
y
¼
cos sin
sin cos
x
m
y
m
(7)
From equations (6) and (7), in coordinate frame
F
1
ðx
w
; y
w
Þ, the kinematic mode of the differential WMR
with slipping is described as follows
_
x ¼
r!
L
ð1 a
L
Þþr!
R
ð1 a
R
Þ
2
ðcos þ sinÞ
_
y ¼
r!
L
ð1 a
L
Þþr!
R
ð1 a
R
Þ
2
ðsin cosÞ
_
¼
r!
R
ð1 a
R
Þr!
L
ð1 a
L
Þ
b
(8)
where ½x; y;
T
is posture vector of the mobile robot and
is heading angle of the WMR.
Assumption 3. Slipping parameters a
R
, a
L
,and are not
measurable.
Define an auxiliary control input ½v;!
T
, and then the
relationship between auxiliary control input and real con-
trol input ½!
L
;!
R
T
is regarded as
v
!
¼
rð1 a
L
Þ!
L
þ rð1 a
R
Þ!
R
2
rð1 a
L
Þ!
L
þ rð1 a
R
Þ!
R
b
2
6
6
6
6
4
3
7
7
7
7
5
¼ T
!
L
!
R
(9)
where matrix T ¼ r
1 a
L
2
1 a
R
2
ð1 a
L
Þ
b
1 a
R
b
2
6
6
6
4
3
7
7
7
5
, T is a nonsin-
gular matrix. From equation (9), virtual control input
½!
L
;!
R
T
can be obtained as follows
!
L
!
R
¼ T
1
v
!
¼
1
r
1
1 a
L
b
2ð 1 a
L
Þ
1
1 a
R
b
2ð1 a
R
Þ
2
6
6
6
4
3
7
7
7
5
v
!
(10)
Then, equation (8) can be rewritten as
_
x
_
y
_
2
4
3
5
¼
cos þ sin 0
sin cos 0
01
2
4
3
5
v
!
(11)
We can see from equation (8), to handle tracking control
problem of the WMR with unknown slipping parameters a
L
,
a
R
,and, it is the top priority to estimate time-varying slip-
ping parameters online, and then to design tracking controller
on the basis of the estimation of the slipping parameters.
A scheme of the robotic slipping
parameter estimation
Because three slipping parameters a
R
, a
L
, and in equation
(8) cannot be measured directly, it is necessary to estimate
slipping parameters in order to design tracking controller.
Cui et al. 3