68 CHAPTER 2 / OBSERVER-BASED VISUAL SERVOING
is obtained, where M =
[M r r r Mr]r
M2 M3 .
Since M and
square, J becomes full
rank if both M and
are invertible and N is full rank. To check the singularity of M, it
is straightforward to see that
det M =
1 x I Yx
1 x 2 Y2
1 x a Y3
Thus M is invertible because any three feature points are not collinear. Since we are
interested in the robot motion only in the nonsingular region,
is invertible. On the other
hand, if p2 + q2 4= 0, the first six rows of N are linearly independent. If p = q = 0, the first
four and the last two rows are linearly independent. Thus N is full rank. Therefore, the image
Jacobian J is proved to be full rank.
Performance improvement by using redundant features is discussed in  and . The
smallest and the largest singular values of the image Jacobian play a central role for
4 NONLINEAR CONTROL LAW
This section introduces a nonlinear controller and a nonlinear observer. An example of a
two-link direct drive robot is also given.
4.1 Controlled Variable
Our goal is to track the object so as to keep the features of the object at the reference features.
The models of robot, object motion, and camera are given by (2.2), (2.3), and (2.6),
respectively. On the basis of these models, it is natural to adopt the features as the controlled
variables, joint angles and joint velocities as the state, and the joint torque as the input. For
a minimum set of features (n =
this selection is appropriate. 2 However, for redundant
features, the system becomes uncontrollable because the features cannot move in
arbitrarily. To resolve this problem, one has to solve nonlinear geometric constraints on the
features that represent the rigidness of the object. Since these constraints are difficult to solve,
we linearize the constraints at the reference point and reduce the dimension of the feature
vector to the dimension of the joint space.
Let ~# be a manifold, that is, the set of all admissible features (2.7), and consider a nominal
that satisfies ~e =
Define a matrix B as follows:
if n~> m, (2.23)
n = m
Note that J is a function of q and p; in this equation, p* and q* are a typical position of the
object and a typical configuration of the robot that satisfy
Scorn(q*)- Sobj(P*)= ra.
matrix B is the image Jacobian at the nominal point if the features are redundant. If the
features are minimum, then B is the identity matrix. The controlled variable is defined by
2There is no other choice.