CNC scheduler

The real-time part of the CNC controller runs in the GEO task. The GEO task performs the following tasks, among others, in each CNC cycle:

Identifier

Task

Input

Read out axis actual values/status/etc. from the fieldbus

Output

Output new axis command values/status/etc. to the fieldbus

IPO

Interpolation, calculation of new axis command values, channel-specific

CHAN

Display, channel-specific

The CNC scheduler defines the order in which these tasks are executed. Defining the suitable order is dependent on the existing hardware (drives, fieldbus,etc.)

The following task orders are available:

Overview of task order in the cycle
Overview of task order in the cycle

STANDARD

Especially with conventional +-10V drives, it is important to output command values in cycles that are as constant as possible. To avoid fluctuations, the command values calculated in the previous cycle are output to STANDARD directly at the start of the cycle. Then actual values are read in, compensations are calculated and new command values are calculated by IPO for the next cycle.
This order results in a delay between interpolation and the output of command values.

COMPLETE

If the axis parameter P-AXIS-00276 "field_bus_allows_optimised_schedule" is set for all axes, actual values/compensations are first processed and only then are the new command values output. This prevents any delay between interpolation and output.

If the parameter P-AXIS-00276 is not set for all axes, the schedule corresponds to the STANDARD case.

SWITCHED

For digital drives, the order can be further optimised to avoid any delay. In SWITCHED mode, actual values are read in

  1. interpolated
  2. .
  3. Compensations calculated and command values output

Configuration

In TwinCAT, the schedule is set in the "Context" tab of the "CNC" node:

In real-time Linux or Windows Simulator, the P-RTCF-00018 parameter is used for this.