Stewart Industrial Automation

Why zeros are mathematically correct but can still cause faults

For compact EOATs with the center of gravity (CoG) very close to the robot flange (e.g., offsets < 5–10 cm), the intrinsic moments of inertia about the CoG (Ixx, Iyy, Izz) are often very small or effectively zero — this is a valid point-mass approximation and is safe for performance.

However, collaborative robots from all major manufacturers (FANUC CR/CRX, Universal Robots, ABB GoFa/SWIFTI, KUKA iiwa, Doosan, etc.) use sensitive torque/force monitoring for human safety. These systems continuously compare measured torque/force from integrated sensors against the expected values based on your entered payload + CoG + inertia.

When inertia is zero and CoG offset is tiny, the robot expects almost no variation in torque. Real-world factors like cable drag, minor asymmetries, sensor noise, mounting tolerance, or gravity vector misalignment create small mismatches — the safety system may interpret this as an error and stop the robot (payload fault, safety stop, or reduced speed).

How to reduce or eliminate these faults (applies to most cobots)

1. Perform payload/tool calibration / zeroing
   Most cobots have a "payload confirmation", "tool calibration", or "sensor zeroing" procedure after power-on or tool change. Always run this with the tool in free space (no contact).
2. Use small non-zero intrinsic inertia values
   Enter conservative values in the inertia fields (about CoG):
   • Ixx ≈ 0.005–0.01 kg·m²
   • Iyy ≈ 0.005–0.01 kg·m²
   • Izz ≈ 0.002–0.005 kg·m²
   These are negligible for dynamics but give the monitoring system more tolerance for real-world variation. Start low and increase slightly if faults persist.
3. Run automatic payload identification (if available)
   Many cobots (FANUC, UR Polyscope, ABB RobotStudio, etc.) offer an auto-identification routine. The robot moves the tool in controlled ways to measure real inertia and CoG — this usually gives the most accurate, fault-free values.
4. Check/adjust safety monitoring sensitivity
   In the safety configuration (DCS on FANUC, Safety Settings on UR, SafeMove on ABB, etc.), increase error margins or vibration tolerances slightly (after proper risk assessment).
5. Other common checks
   • Ensure cables/hoses are balanced and not dragging
   • Verify gravity vector direction and magnitude
   • Confirm the correct payload/tool is active
   • Check mounting orientation/angle is accounted for

Manufacturer-specific notes

• FANUC CR/CRX: Use "Payload Auto Identify" or manual CONFIRM. Small non-zero inertias (0.005+) often solve SYST-325/348 faults.
• Universal Robots: Polyscope auto-compensates well; if faults occur, try "Force Control" tuning or small inertia values.
• ABB GoFa/SWIFTI: Use RobotStudio "Load Identify" or adjust SafeMove tolerance.
• KUKA iiwa / Doosan: Generally more tolerant; use auto-mass/inertia identification routines.

Bottom line: Zero inertia is correct for compact tools, but small non-zero values (0.005–0.02 kg·m²) are a common and safe workaround to prevent nuisance safety stops across all collaborative robot brands. Always follow your robot's manual and perform risk assessments.