838 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 10, NO. 6, NOVEMBER 2002
(butno necessary and sufficient)conditions existfor the solution
of this problem. Constructive algorithms are essentially based
on input-output decoupling [22].
The starting point is the definition of an
-dimensional
output
, to which a desired behavior can be assigned.
One then proceeds by successively differentiating the output
until the input appears in a nonsingular way. At some stage,
the addition of integrators on some of the input channels
may be necessary to avoid subsequent differentiation of the
original inputs. This dynamic extension algorithm builds up
the state
of the dynamic compensator (6). If the system is
invertible from the chosen output, the algorithm terminates
after a finite number of differentiations. If the sum of the
output differentiation orders equals the dimension
of
the extended state space, full input–state–output linearization
is obtained.
1
The closed-loop system is then equivalent to a
set of decoupled input–output chains of integrators from
to
.
We illustrate this exact linearization procedure for the uni-
cycle model (1). Define the linearizing output vector as
. Differentiation with respect to time then yields
showing that only affects , while the angular velocity
cannot be recovered from this first-order differential informa-
tion. To proceed, we need to add an integrator (whose state is
denoted by
) on the linear velocity input
being the new input the linear acceleration of the unicycle.
Differentiating further, we obtain
and the matrix multiplying the modified input is nonsin-
gular if
. Under this assumption, we define
so as to obtain
(7)
The resulting dynamic compensator is
(8)
Being
,itis , equal to the output
differentiation order in (7). In the new coordinates
1
In this case,
is also called a flat output [25].
Fig. 3. WMR SuperMARIO.
(9)
the extended system is, thus, fully linearized and described by
the two chains of integrators in (7), rewritten as
(10)
The dynamic compensator (8) has a potential singularity at
, i.e., when the unicycle is not rolling. The occur-
rence of such singularity in the dynamic extension process is
structural for nonholonomic systems [6]. This difficulty must
be obviously taken into account when designing control laws
on the equivalent linear model.
IV. T
ARGET VEHICLE:SUPERMARIO
The experimental validation of the proposed control method
and its comparison with existing controllers has been performed
on our prototype SuperMARIO (Fig. 3).
A. Physical Description
SuperMARIO is a two-wheel differentially driven vehicle.
The wheels have a radius of
cm and are mounted on an
axle
cm long. The wheel radius includes the o-ring used
to prevent slippage; the rubber is stiff enough that point con-
tact with the ground can be assumed. A small passive caster is
placed in the front of the vehicle at 29 cm from the rear axle. The
aluminum chassis of the robot measures 46
32 cm
(l/w/h) and contains two motors, transmission elements, elec-
tronics, and four 12-V batteries. The total weight of the robot
is about 20 kg and its center of mass is located slightly in front
of the rear axle. This design limits the disturbance induced by
sudden reorientation of the caster. Each wheel is driven by an
MCA dc servomotor supplied at 24 V with a peak torque of
0.56 Nm. Each motor is equipped with an incremental encoder
counting
pulses/turn and a gearbox with reduction
ratio
. On-boardelectronics multiplies by a factor
the number of pulses/turn, representing angular increments with
16 bits.