FANUC inter-path superimposed control: why actual slave-axis speed can exceed its own programmed feed
Within same-generation T-series two-path superimposed control, signed-speed and increment examples separate the two contributions to slave-axis motion. Combined speed limits, block-boundary switching and ordinary synchronization protection are treated distinctly.
Contents
In two-path machining, the feed shown in the slave-axis program describes only one component of motion. With inter-path superimposition active, another feed axis's displacement is added. Reviewing only one path can therefore miss actual axis speed and travel.
Superimposition is more than following the master axis
This article uses the FANUC connection manual B-63003EN-1/02, §1.9.4, pages 127–129 (public mirror of the original manual), limited to its T-series two-path context and same-generation 16i/18i-A configurations. Superimposed control lets the slave axis receive its own motion command while adding master-axis motion. “Master axis” here means the feed axis whose motion is superimposed, not the rotating spindle controlled by an S command.
Build the relationship from increments
This original model assumes two parallel linear axes with matching units, scale and positive directions, same-direction superimposition and starting instant t₀. If master-axis increment is Δm and the slave's own program produces Δs, the slave's resulting commanded increment is Δz=Δm+Δs. These are increments over the same time interval, not two absolute-position display values.
During an interval when both components have reached constant speed, v_z=v_m+v_s also holds:
| Master-axis speed | Slave's own speed | Resulting slave speed |
|---|---|---|
| +2400 mm/min | +900 mm/min | +3300 mm/min |
| +2400 mm/min | −900 mm/min | +1500 mm/min |
| +2400 mm/min | −2400 mm/min | 0 |
In the first row, 0.2 seconds produces components of 8 mm and 3 mm, totaling 11 mm. A 3 mm request from the slave's own program does not mean the physical axis travels only 3 mm. In the third row the physical axis can remain stationary, but relative to a workpiece moving with the master axis it still moves at −2400 mm/min.
Apply speed limits to the combined result
With an original assumed mechanical limit of 3000 mm/min, the first row exceeds that limit. Both paths individually being below 3000 does not prove the combination is feasible. Check the signs, magnitudes and acceleration/deceleration phases of simultaneous components, not only each program's F.
The manual accordingly provides dedicated speeds and time constants for the superimposed state. It also explains that switching maximum cutting speed when superimposition starts or ends during automatic operation depends on block boundaries and buffering conditions; it cannot universally be treated as immediate. This article supplies no universal parameter values or M codes.
Do not borrow protection assumptions from ordinary synchronous control
The same section explicitly states that superimposed control does not compensate for or detect loss of synchronization between master and slave. This mode intentionally permits their relative displacement to change, so it cannot be accepted against an ordinary synchronous model that assumes a constant position difference.
Offline review should reconstruct the master component, slave component and combined result on one time axis, then compare physical travel, speed and motion relative to the workpiece separately. These examples omit servo error and are not field operating records. Further technical resources are in the OPTION section.