88 CHAPTER 2 / OBSERVER-BASED VISUAL SERVOING
0 10 20
t t l
(a) 30 40 50
i L i i i | ,. i
(b) 0 10 20 30 40 50 60 70
Control input. (a) Feedforward part -
(b) feedback part - K~.
a feedforward control scheme is effective. The results given in this chapter emphasize the
usefulness of object velocity feedforward in the application of vision-based control.
We have introduced a visual feedback controller with a velocity observer. The observer
compensates the delay by estimating the object velocity. Also, the observer provides
intersample information to the joint servo by updating the visual information with the
sampling rate of the joint servo. Thus the problems of slow sampling time and delay of the
vision sensor are resolved. The tracking performance is improved by feedforwarding the
object velocity. Stability of the observer-based control system is presented in a nonlinear
form. Simulations and experiments with a two-link direct drive robot have exhibited the
effectiveness of the observer-based control scheme. A linearized version suitable for industrial
robot control is also presented. Experimental results with PUMP 560 have shown stable and
accurate performance of the observer-based visual servo controller.
 P. K. Allen, A. Timcenko, B. Yoshimi, and P. Michelman. Automated tracking and grasping of a moving object
with a robotic hand-eye system.
IEEE Trans. Robotics and Automation,
[-2] F. Chaumette and A. Santos. Tracking a moving object by visual servoing. In
12th IFA C World Congress, Vol.
9, pages 409-414, Sydney, Australia, 1993.
 P. I. Corke. Video-rate robot visual servoing. In
K. Hashimoto ed., World Scientific, pages
257-283, Singapore, 1993.
 P. I. Corke. Visual control of robot manipulators--a review. In
K. Hashimoto ed., World
Scientific, pages 1-32, Singapore, 1993.