20


 0 10

 Position Setpoint
 0-
 SJoint 1 Position
 o -10

 Position Step: 23.8 deg
 -20 1 I- I .
 0 60 120 180 240 300
 Time (1/60 sec)

 Figure 9.11. Typical closed-loop response of joint 1 to a step change in the position setpoint of
 +23.8 deg.


 12.0 deg which represented a steady-state error of 0.1 deg. In the worst observed trial (Figure

 9.12), the system responded to a negative step with a maximum position of -15.7 deg or 16

 percent overshoot. In this case, the joint settled back to an average value equal to the desired

 position. These results were obtained when the sliding tube was held steady in an extended

 position. For all the trials with different directions and positions of joint 2, a maximum steady-

 state error of 0.14 deg resulted. The steady-state errors from all of the trials averaged to a

 value of 0.1 deg. The maximum overshoot from all trials (16 percent) occurred in the example

 which produced the plots of Figure 9.12. Based on the specified criteria for this controller, the

 actual steady-state errors were all well below the maximum allowable providing adequate

 error margin. On the other hand, the overshoot was desired to be less than 15 percent.

 Adjustments in the controller to compensate for the larger overshoot value caused an

 undesirable sluggish system response. Even though this controller allowed an average

 overshoot of 14 percent, only 1 percent less than the maximum, it provided very good

 performance when used in the picking operation. Therefore, the specified performance

requirements were relaxed to allow its use.