Cogging torque compensation in servo drives
What is cogging torque?
Cogging torque is an undesirable periodic torque fluctuation in permanent magnet motors caused by the magnetic interaction between the rotor magnets and the stator teeth. It occurs even when no current is flowing through the motor and manifests itself as a "jerky" or "stepped" feeling when turning the shaft manually.
Why is this important?
In precision applications—surgical robots, semiconductor handling, optical systems, measurement technology—even small torque fluctuations can cause:
- Positioning error at low speeds
- Vibrations transmitted to the tool or workpiece
- Audible engine noises
- Reduced surface quality during machining or dosing processes
Cogging torque is particularly problematic at low speeds, where the vibration frequency is low enough to directly affect the quality of movement.
How does compensation work?
Software-based cogging compensation actively generates a counter-torque that cancels out the cogging effect. The process consists of two phases:
1. Calibration (one-time recording, approx. 3 minutes):
The motor rotates slowly without load while the drive records the cogging torque profile as a function of the single-turn position. The result is a compensation table that is stored in the drive's non-volatile memory.
- Minimum encoder resolution: 1,024 increments per revolution (4,096+ recommended)
- The motor must be able to rotate freely—no mechanical blockage or significant load.
- Prerequisite: System identification and position auto-tuning must be completed.
2. Active compensation (runtime):
During operation, the drive reads the current rotor position, calculates the corresponding compensation value, and adds it to the torque command. The result is a significantly smoother torque curve.
Compensation is controlled via the Object Dictionary (0x2008): Subindex 1 shows the status, Subindex 2 activates or deactivates compensation.
How does SOMANET implement this?
On SOMANET drives, cogging compensation is configured and recorded via OBLAC Drives. Once recorded, the function can be activated or deactivated at any time—without recalibration.
This function is particularly effective in combination with ACTILINK-JP precision actuators, in which shaft gears amplify any torque fluctuations from the motor side.
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