FANUC optimum torque acceleration/deceleration: why upward and downward motion need different load assessments
An original gravity-and-friction force table illustrates the four motion states in 30i-A optimum torque acceleration/deceleration. Travel direction, acceleration direction and driving-force direction are not interchangeable.
Contents
A vertical axis moving downward can still require an upward motor force. Braking during descent can also demand more drive margin than accelerating downward. Describing acceleration/deceleration only as “upward is heavy, downward is light” misses the combination of travel and acceleration directions.
Why the original function distinguishes four states
The FANUC 30i/31i/32i-A user manual B-63944EN/03, §20.4, pages 676–678 (public mirror of the original manual) applies optimum torque acceleration/deceleration to rapid traverse. It treats acceleration and deceleration during positive travel, and acceleration and deceleration during negative travel, separately and selects the relevant acceleration curve according to current speed. The applicable acceleration-before-interpolation and configuration conditions must hold; issuing rapid traverse alone does not establish that the function is active.
Draw force directions before assessing load
An original model has a vertically moving mass of 400 kg, positive upward, gravity 9.8 m/s² and sliding friction of magnitude 300 N opposing motion. Ignore balancing cylinders, counterweights and transmission losses. With driving force F positive upward:
F=ma+mg+300·sign(v).
Compare four states, each with acceleration magnitude 1.2 m/s². Here mg=3920 N and the magnitude of ma is 480 N:
| Motion state | Sign of velocity v | Sign of acceleration a | Upward driving force F |
|---|---|---|---|
| Accelerating upward | + | + | 4700 N |
| Decelerating upward | + | − | 3740 N |
| Accelerating downward | − | − | 3140 N |
| Decelerating downward | − | + | 4100 N |
During downward deceleration, acceleration is actually upward: the drive must brake motion while supporting the gravitational load, so force exceeds the 3140 N needed during downward acceleration. Gravity helps reduce speed during upward deceleration. F remains positive in all four cases; “the axis moves downward” does not mean the driving force points downward.
The same acceleration does not imply the same energy state
Mechanical power is Fv. In this model, downward motion has positive F and negative v, so power is negative: energy passes from the moving mechanism back toward the drive. Force margin and the ability to handle braking energy are separate issues. A maximum torque value alone does not prove adequate descent braking.
Available motor torque also varies with rotational speed, and counterweights, balancing pressure and friction change the table's results. This explains why capability is distinguished by direction, acceleration/deceleration state and speed. Linear driving force is not treated here as motor-shaft torque; conversion also requires screw lead, reduction ratio and direction-dependent efficiency.
Use the model to explain behavior, not to generate parameters
If a load peak occurs only during downward deceleration, analyze that phase separately from downward acceleration rather than averaging the whole negative-direction stroke. The model excludes mechanical elasticity, servo regulation and electrical braking limits. It is neither a diagnosis nor a tuning basis. Function curves must follow the machine mechanism and actual capability. Related resources are in the OPTION section.