One of the most common mistakes in designing a motorized axis is starting from the motor’s power rating instead of the application’s real load profile. The result is almost always an oversized axis (wasted cost) or an undersized one (following errors, overheating, premature wear).
The parameters to calculate first
- Velocity and acceleration profile required by the machine cycle (not just top speed)
- Load inertia reflected to the motor shaft, including the gear ratio
- Required torque during acceleration/deceleration transients, not just at steady state
- Duty cycle: how much time the axis spends at peak torque versus RMS torque
The load-to-motor inertia ratio
A load-to-motor inertia ratio that’s too high makes the system hard to control precisely, with oscillation and long settling times. Too low a ratio, on the other hand, often points to a motor that’s oversized for the application. The “correct” value depends on the required dynamics and must be checked case by case, not applied as a fixed rule for every machine.
Why tuning matters as much as sizing
Even a correctly sized axis can behave poorly if the control loops (current, velocity, position) aren’t tuned to the real load. Theoretical sizing is the starting point: on-site commissioning and tuning is what determines how the machine actually behaves in production.