Overview

The TwinCAT NC FIFO group provides the facility of supplying externally generated set values to a group of axes via the NC, and thus of driving them synchronously. The set values for the positions of the axes corresponding to a fixed, although selectable, time period (the FIFO cycle time) are present in the form of a FIFO which is kept topped up by the PLC. The FIFO allows the position of a group of axes to be handled synchronously with reference to time. The FIFO contains a fixed number of prescribed reference points, while the NC interpolates position between them and determines the speed. The FIFO's axis positions are

either read out of a large table located in the PLC and calculated offline, or
calculated by a function in the PLC

and continuously passed in each PLC cycle by a PLC program to the NC's FIFO.

Interfaces and Configuration

The user can parameterise the FIFO group via the System Manager or the PLC, and can manipulate the functionality of the FIFO group by means of PLC blocks. He also has available (in the System Manager and through the PLC) a cyclic NC-PLC channel interface. This contains the channel process image, and permits access to a variety of channel variables. Finally, he also has available (in the System Manager and through the PLC) a cyclic NC-PLC axis interface. This contains the axis process image for each FIFO axis, and permits access to a variety of axis variables.

FIFO Parameters and the FIFO

The FIFO group is parameterised by a number of data items. These must be set by the user:

FIFO dimension N = number of FIFO axes,
FIFO length Z = number of FIFO lines,
FIFO cycle time T_f = cycle time for which the FIFO data was generated.

The FIFO is a table, [Z][N]T, with N columns and Z lines. These lines are cyclically refilled after they have been used. At position T[T][A] the FIFO contains the set position value for axis A at time T × T_f (T_f FIFO cycle time).

Sequence and the Generation of Set Values

Preparation phase: Group FIFO and parameterise.
Fill FIFO and run FIFO group.
Prepare FIFO data and refill FIFO.
Stop and reconfigure group.

The generation of set values includes the set position, set speed, set acceleration and the set direction.

Override

The override is used to apply a synchronous online percentage change to the speeds of all the FIFO axes prescribed by the FIFO. There are two logical data channels:

Instantaneous override changes the override instantaneously to the desired value. This means that it is up to the user to modify the override slowly enough that excessive following errors do not result from the change in override.
The PT-2 override, which can be parameterised, and which uses a PT-2 transfer function to modify the override continuously towards the desired speed according to the difference from the current speed.