Tuning of the implemented motion controllers for the citrus-picking robot utilized these
concepts of altering the frequency-response curves and these criteria of a good servo design. For
each category of controller, the following steps were taken to tune the controllers:
1) A frequency-response plot for the open-loop system as specified by the open-loop
dynamic model was plotted for the appropriate system.
2) The lag and lead time constants were chosen to ensure the desired slope at the crossover
frequency.
3) The controller gain was chosen to give the system the highest response possible in the
low frequency range without causing the system to be unstable. For stability, the
resonant peak which occurred at the system natural frequency was kept less than unity.
4) The calculated values of the controller parameters were implemented.
5) The implemented controllers were tested in the closed-loop system.
6) The controller parameters were adjusted to provide the best achievable performance of
the system.
Velocity Controller Tuning
According to the stability criteria, the uncompensated velocity control loop as shown in
Figure 8.1 was unstable due to the fact that the peak was above unity. Therefore, the first
objective of the velocity compensator for each joint was that of lowering the resonant peak to a
value less than unity. Initial choices of the lag or integrating factor, 1/Ti, placed the break
frequency much less than the experimentally determined hydraulic natural frequency (Figure
8.23). The lead factor, 1/rd, was then placed between the crossover frequency and the peak to
cancel the effects of the lag factor and cause the peak to be stabilized. Then, the controller
gain, K, was set to raise the entire curve so that the resonant peak would be just below the unity
line.
By using the worst determined cases of the open-loop transfer function for each joint as a
basis, initial choices of the factors resulted in stable systems that could be tuned on line as
suggested by Palm (1983). The joint was actuated and its reaction was observed. This tuning