(2 2

 G s (s+28hhs + co) (8-2)

Note that the vision control modes were actually position control with feedback of the position

of the fruit rather than the position of the robot joint. Representative open-loop Bode

diagrams for these uncompensated systems are shown in Figures 8.1 and 8.2.

 The selection of a controllers for each of these systems involves choosing a controller

which will meet the previously specified performance criteria while maintaining an adequate

stability margin. If the resonant peak in the plot of the quadratic rises above unity gain, then

the system becomes unstable. This occurrence is comparable to the critical point encirclement on

the Nyquist plot. Therefore, a controller was desired that would guarantee that the resonant

peak of the Bode diagram would never rise above the unity gain. The selection of a controller

that would place the peak far below unity would cause an increase in the stability margin.

However, this guideline would also cause a decrease in the system response rate. For the

fastest system response rate, the compensator was also required to keep the resonant peak from



 log scale

 I G,(jo) I

 Kv,-



 -- -2 slope



 1 \ log scale
 1 co
 rad/sec


Figure 8.1. Bode diagram of the open-loop velocity control system.