High-speed Visual Feedback Control for Grasping and Manipulation - Visual Servoing via Advanced Numerical Methods - page 46

The hand grasps t weezers

ξ t

The vision system mesures
position of them

2L B

ξ 0

θ 3

θ 1

θ 2

L 2

L 3 (x t , y t , z t )

(x 0 , y 0 , z 0 )

θ w2

2θ

The tweezers grasp a target

θ w1

(a) (b)

Fig. 3.11 (a) System configuration, and (b) visual servo control [5]

manipulation. The object also has to be controlled by the other grasped object. In
order to achieve such a task, visual feedback control is very important [5].

3.3.1 Passive Joint in the Contact Point

As a handled tool, we adopt tweezers. Figure 3.11(a) shows the setup of the experi-
ment. A multifingered hand grasps tweezers with two fingers. The hand manipulates
the tip of the tweezers so that the tweezers pick up the object placed on the table.
The position of hand, tweezers and object are measured by vision system. In this
configuration, the hand and tweezers can be regarded as one mechanism, and the
contact points between the fingers and tweezers are regarded as a kind of passive
joint.
It is possible that the condition at the passive joints may change during manipu-
lation. First, the initial grasp condition of the tweezers may be changed at each trial.
Next, if some external force is added to a part of the tweezers during manipulation,
the rotational moment at the passive joints occurs. When the rotational moments are
larger than the static frictional force, the relative posture of the tweezers to the hand
is also changed. For these reasons, it is necessary to measure the relative positions
in realtime and to control the hand so as to cancel the changes. Visual servoing con-
trol [2] is effective for this problem. If the rate of feedback is sufficiently fast, the
control can be achieved more smoothly.

3.3.2 Visual Servo Control for Tool Manipulation

The hand is controlled by image based visual servoing shown in Figure 3.11(b).
Image based visual servoing is a control method by feedback of the position error in
the image plane. It is robust against the calibration error, and the computation of the
target 3D position is not needed. The fingers are used to control the tweezers grip,
and the 2-axis wrist joints θ w 1 , θ w 2 ] T θ w =[ are used to control the position of the

tip of the tweezers.