FANUC simple synchronous control: why two axes can differ in position despite receiving the same command
The 15/150-B simple synchronous control definition separates common lag from relative position error. Three position examples show that identical commands, individual following errors and mechanical synchronization accuracy require different assessments.
Contents
Sending the same move command to two servos gives them a shared target, but does not by itself prove that their actual positions remain identical. Analyzing synchronization starts by identifying the synchronization mechanism in use.
Simple synchronization does not include every synchronization check
The FANUC 15/150-MODEL B descriptions manual B-62082E/04, §20.7, page 177 (public mirror of the original manual) explains that simple synchronous control sends the master-axis travel command to both master and slave servo motors. This base function itself does not provide synchronization-error compensation and synchronization-error alarms based on continuous comparison of the two axes' position difference. Master and slave here mean feed axes, not the cutting spindle.
The same manual separately lists a simple-synchronization positional-deviation check in §20.11, page 180. When the difference between the axes' error-counter values exceeds its setting, it can issue an OT alarm, stop motion and output a signal. Distinguish the §20.7 base function from separately configured deviation checking. The base description does not establish that a particular machine has no checking, nor that individual servo axes lack their own protection.
Relating individual following errors to relative position error
An original model assumes aligned axis coordinates in the same reference, no additional compensation and common commanded position C=100.000 mm. Let actual master position be M and slave position S. Define eM=C−M and eS=C−S; the position difference is then d=M−S=eS−eM.
| Case | M | S | eM, eS | d |
|---|---|---|---|---|
| Common lag | 99.950 | 99.950 | 0.050, 0.050 | 0 mm |
| Different lag | 99.970 | 99.960 | 0.030, 0.040 | 0.010 mm |
| Opposite error directions | 100.020 | 99.980 | −0.020, 0.020 | 0.040 mm |
In the first row, the axes agree with each other but neither has reached the target. In the third, each individual error magnitude is only 0.020 mm while the difference is 0.040 mm. Reporting an average error or inspecting only the smaller individual error cannot replace the inter-axis difference.
What the same difference can mean mechanically
As an ideal beam geometry illustration only, supports separated by 2000 mm and offset by 0.040 mm along the feed direction give a small angular skew of approximately 0.040/2000=20 microradians, about 0.001146°. This is not an allowable skew for a particular gantry machine. Real structures also develop torsion, elasticity and internal forces; this angle cannot establish load safety.
Records must share a timestamp and coordinate definition
If M comes from an earlier sample and S from a later one, timing difference during motion can appear as spatial difference. For example, 10 mm/s with a 2 ms offset produces an apparent 0.020 mm difference. This sampling example is independent of the control function above; it is a measurement factor to exclude when comparing records.
Retain the common target, both feedback positions and timestamps separately, then distinguish common from relative error. On a mechanically coupled dual-drive mechanism, do not use this example to move one side independently or alter synchronization parameters. Further control mechanisms are in the OPTION section.