Find Center

Setting the Find Reference search type to Center initiates a search sequence positioning the axis in the middle of the forward and reverse limits. Attributes you can specify include the Initial Search Direction and the Final Approach Direction (for when the search encounters each of the limits).

The search is performed at the maximum find velocity specified by Max Find Velocity or Max Find RPM. The velocity of the axis is set to this value during the find operation. The velocity is then restored to its previous value after the search completes.

When the search direction is forward, the axis starts moving in the forward direction. When the limit in that direction is encountered, the direction is reversed to find the reverse limit. After both limits are reached a center position can be calculated from the recorded positions. The axis then proceeds to the calculated center. When the center is found the Wait Reference VI or function returns a true value. A similar search sequence starting in the reverse direction is followed when the initial search direction is reverse.

At the end of each limit of travel, a fine-tuning operation is performed. This provides for a more accurate search. To smoothly approach the edge, this portion of the sequence is executed at a fixed low velocity of 1/4 RPS unless the maximum find velocity is slower. This approach direction feature is used to minimize the effects of motion system windup, backlash, and/or home sensor hysteresis. If the final approach direction is opposite to the direction of travel into the limit (true), the axis slowly backs off the limit until the limit becomes inactive again. If the final approach direction is the same as the direction of travel into limit (false), the axis slowly backs off the limit and re-approaches it. You also can set the active state of the limits in MAX or with the Set Limit Input Polarity VI or function.

If Smart Enable is disabled National Instruments recommends you disable the home input before performing a Find Center. If the home input is enabled and encountered during a Find Center, the search stops at the home input and generates a modal error.

With the Smart Enable feature enabled both limits are enabled and the home input is disabled prior to initiating the search. After the search completes the limits are returned to their previous states. If you use Smart Enable, National Instruments recommends setting the default in MAX to enable the forward and reverse limits and disable the home input.

After the center is located, an offset move is performed. Offset Move specifies the amount and direction of the offset. This offset move is performed at the maximum find velocity specified by Max Find Velocity or Max Find RPM. By default Offset Move is 0, resulting in no offset move.

After the offset move is performed, the position is reset if Reset Position? is true. The position is set to the values indicated by Reset Primary Position and Reset Secondary Position.