FANUC optional rotary-axis control: how the sign of an absolute command selects direction
Using 30i/31i/32i-B optional rotary-axis control, distinguish rotation direction from endpoint coordinate. Four commands starting at 350° show how the sign can change the arc travelled without changing the absolute endpoint value.
Contents
Seeing C−20.0 in an absolute command does not immediately establish a target circular coordinate of 340°. Under a particular rotary-axis control function, the minus sign can specify direction while 20 gives the absolute value of the endpoint coordinate. The two parts have different roles.
First establish whether the sign has been assigned a direction meaning
The FANUC 30i/31i/32i-B descriptions manual B-64482EN/03, §1.47, page 68 (public mirror of the original manual) specifies this rotary-axis control for absolute commands: the sign gives the direction and the absolute value gives the target position. Rotary-axis roll-over must be active, and the corresponding function must be configured. Roll-over alone does not prove that the machine uses this sign interpretation.
Four original angle examples
Assume one revolution is 360°, current C=350°, and the direction-selection rule above is active. First consider circular geometry without actual axis-travel limits. Every row starts independently from the same 350° position:
| Absolute command value | Selected direction | Endpoint | Travel in that direction |
|---|---|---|---|
| +20.0 | Positive | 20° | +30° |
| −20.0 | Negative | 20° | −330° |
| +340.0 | Positive | 340° | +350° |
| −340.0 | Negative | 340° | −10° |
The first pair shares an endpoint but travels different arcs; the second pair does likewise. A minus sign does not mean choose the shorter route, and a plus sign does not mean add that many degrees. For example, +20.0 moves positively from 350° across zero to 20°, covering only 30°.
How this differs from a purely mathematical negative-angle coordinate
As an ordinary circular position, −20° is equivalent to 340°. Here, however, it is split into negative direction and endpoint 20°. Interpreting the same text by these two rules can therefore produce different targets. This mathematical comparison does not imply that every machine without the function must use a particular path-selection method.
Comparing only final coordinates is insufficient: both first rows display 20°, but that does not establish identical space swept during movement. Whether fixtures, tools, lines and mechanisms permit a given direction and angle depends on the actual structure. A short path across zero and a long path of nearly a full revolution are not interchangeable.
Program migration must check endpoint and direction together
When reviewing postprocessor output, list the command sign, absolute endpoint value and angular difference in the selected direction separately. If the original program relies on this function to express direction, leaving the C-value text unchanged on another configuration does not guarantee the same behaviour. This article provides no enabling parameters and does not cover special cases such as whether an extra full revolution occurs when endpoint and start coincide. Further rotary-axis resources are in the OPTION section.