Find Index

Setting the Find Reference search type to Index initiates a search sequence to find the index (marker) signal of the feedback encoder. Attributes you can specify include the initial search direction, approach velocity percent, and search distance.

The encoder index signal is accurate to one quadrature count and provides a much more repeatable reference than using just a limit or home input edge. Find Index is typically executed after a Find Home, Find Center, or Find Limit operation. With this procedure, the home input need only be accurate enough to repeatedly locate the axis within the same encoder revolution or index period. Then you can use Find Index to find a unique instance of the index.

Note  Find Index is only available on axes with incremental encoder feedback.

When performing a Find Index operation after another find, you might notice inconsistent operation if the index (marker) signal is near to the edge of the home input or limit. In this case, a Find Index operation can sometimes be off by one revolution. To solve this, National Instruments recommends performing an offset move on the previous find to move away from the index (marker) signal, and then perform the Find Index operation.

The search sequence is performed in the specified direction at a fixed low velocity of ¼ RPS unless the maximum find velocity is slower. To guarantee finding the index (if one exists), the length of the move is automatically set to slightly greater than one encoder revolution. If you previously performed a Find Forward Limit or a Find Reverse Limit National Instruments recommends you set the search direction of the Find Index to a direction opposite to the direction of travel into the limit.

Find Index uses the Approach Velocity Percent value set with the Load Reference Parameter VI or function or in MAX throughout the entire search sequence.

Note  If the velocity of the Find Index operation is very high, it is possible the search distance is reported complete before index found reported complete. This results in Index Found status reported as false. To avoid this you can either increase the search distance to allow more time to report the index found and/or decrease the search speed to slow down the search.

In normal operation, Find Index uses high-speed capture circuitry to detect the index. The axis typically overshoots by an amount that varies depending on the velocity, then comes back to the index. In most cases this is not noticeable.

Note  If you are using a linear encoder or if your counts/steps per revolution is set to a value other than the actual counts/steps per revolution, use the Search Distance parameter set with Load Reference Parameter or in MAX to load a custom search distance for a Find Index move.

The Wait Reference VI or function reports the success of a Find Index. Wait Reference also can be used to query if the find operation is complete using a timeout of 0. You also can monitor this status with the Read per Axis Status VI or function. When the index is found the Index Found status is set to True. If the index is not found during the search revolution, the axis comes to a stop and indicates the failure by setting the Index Found status to False. Missing the index is possible for a number of reasons including an incorrectly connected encoder or an incorrect value for counts per revolution. Refer to the motion controller documentation for more information about encoder connections and index phasing.

You can only execute a Find Index on properly configured axes that are presently stopped or killed. Attempting to execute a Find Index while the axis is in motion generates a modal error. Refer to the Errors and Error Handling section of the NI-Motion VI Help or NI-Motion Function Help for more information about errors.

If Smart Enable is disabled National Instruments recommends you disable the home input before performing a Find Index. If you previously performed a Find Home, depending on the settings, you might encounter the home input during a Find Index.

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 index (marker) signal is found, 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 Enable Reset Position is true. The position is set to the values indicated by Primary Reset Position and Secondary Reset Position.