Definition of axes

A distinction can be made between various types of axes in a control system. One practical classification is between programmable, logical and physical axes.

A physical axis corresponds to a real axis on the machine (translatory or rotary axis). In a control unit a physical axis is represented by software by means of a logical axis (1:1 mapping).

A logical axis is the unified representation of an axis in the axis coordinate system (acs). A logical axis in the control unit provides all the relevant information required for the related physical axis. From the point of view of the CNC, each logical axis has its own parameter set and its own interface to the PLC.

A programmable axis is an axis which can be programmed or commanded by the user in the part program or MDI mode. A programmable axis either corresponds directly to a logical/physical axis (1:1 mapping) or is mapped by kinematic transformation onto logical/physical axes (1:n mapping).

In multi-channel operation, a logical axis may be swapped between channels (“axis swap“). In this case, the same logical axis may occur in different attributes on the NC channels from the point of the view of the programmer, e.g. as a programmable spindle on one channel (speed preset, speed-controlled) and as a programmable C axis on another channel (position preset, position-controlled).

Programmable, logical and physical axes are generally identical (1: 1 mapping) for simple machines (e.g. a 3-axis milling machine with Cartesian linear axes X, Y and Z). However, with complex machine kinematics or industrial robots, kinematic transformation of programmed to logical/physical axes is required (1:n mapping).

Axes are programmed in the CNC program and moved by the CNC. Alternatively, specific motions may also be triggered directly by the PLC.

The PLC interface for axes has a similar structure as the interface for channels. M functions configured as axis-specific M functions also appear on the axis-specific interface and must be acknowledged by this interface. Similarly, this interface contains axis enables and status information for this axis.