Position Control Requirements

 Position control algorithms were used primarily for two operations. First, the position

 control algorithms were called to return the robot to its home position after a pick cycle was

 completed. This motion was accomplished by setting the position set points to the specified

 home location and calling the position control routines to move the joints to the required

 position. A second use was to stop the joints and hold them stationary while other actions were

 performed. For instance, during the detach phase of a pick cycle, joints 0 and 1 position

 controllers were used to hold the arm stationary while the lip was actuated for girdling the

 fruit.

 During the picking phase, a large error in the joint positions would cause the end-

 effector to fail in its attempt to girdle the targeted fruit. Therefore, the maximum allowable

 error for each joint was the motion of the end-effector which would cause the fruit to be out of

 the picking envelope when the picking mechanism was actuated. For joint 0, the maximum

 steady-state error was determined to be 3 cm at the end-effector. Likewise, the maximum

 steady-state error for joint 1 was 2 cm, and for joint 2, the maximum error was 3 cm. The position

 errors for joints 0 and 1 were influenced by the position of joint 2. For a given angular motion of

 joint 0 or 1, the maximum error at the end-effector was realized when the sliding tube was in

 the extended position. Therefore, the angular position errors allowed for joints 0 and 1 were

 calculated based on the maximum allowable extension of joint 2 (158.3 cm). These values

 translated to maximum steady-state errors of 1.1 degrees and 0.7 degree for joints 0 and 1,

 respectively, and 3 cm for joint 2.

 Dynamic response of the position control algorithms was not as critical as steady-state

 error criteria. These algorithms caused motion only to return the joints to their home positions.

* The home positions could be set to allow some overshoot of the desired position allowing a

 faster response time. Again, experience with the fruit-picking robot suggested that position

 overshoots as high as 15 percent could be allowed for each of the axes without causing any