FIFO Axes
In many applications it is necessary to synchronize two or more axes. Axes can be coupled together in the TwinCAT NC PTP. A master axis is then actively controlled, and the position of one or more coupled slave axes is synchronously controlled by the NC.
The simplest type of coupling is linear coupling with a fixed ratio of transmission (an electronic gearbox).
Some applications require a more complex coupling of master and slave, one which can not be described by a simple mathematical formula. Such a dependency can be described by means of a table that specifies an associated slave position for every master position.
The TwinCAT NC PTP offers the possibility of coupling a slave axis to a master axis by means of a table (electronic camshaft). Here the table contains a certain number of prescribed reference points, and the NC interpolates position and speed between them.
FIFO axes are here collected into a FIFO group. These extend the table concept for any number of sequences that do not usually recur cyclically. In this case, rather than having one master-slave table prescribed in advance, the latest axis positions are constantly "topped up" by a PLC program. Because the FIFO axes are combined into a FIFO group, synchronised axis motions can be implemented.
The present document describes the FIFO groups with the aid of a PLC program that is explained step by step.
NC FIFO axis group
If one or more axes are to be operated in a FIFO group, certain preparatory steps are necessary:
- All axes must be taken to a base position in PTP mode.
- The axes are collected into the FIFO group.
- The FIFO memory is fully or partially filled with axis position data.
- The FIFO group is started, and works through the positions that have so far been entered into to the table, taking them from the FIFO memory.
- While this is taking place the PLC is constantly topping up the FIFO memory with further FIFO data, until the positioning comes to a logical end.
There is more information about this in the TwinCAT FIFO chapter.