Glossary · Stopping functions and safe states
Safe Operating Stop (SOS)
Also known as: SOS
German: Sicherer Betriebshalt (SOS)
In drive-based safety, Safe Operating Stop (SOS) is a safety function that maintains a stopped position while actuator power remains available. The drive monitors that the motor does not leave the standstill position beyond a defined tolerance.
- Machine safety
- Risk assessment
In one sentence
Safe Operating Stop (SOS) keeps a drive monitored at standstill with power still applied, so it can hold position against load.
Example
On a CNC machine, the axes are held in SOS while the operator changes a workpiece through the open door in setup mode; if an axis drifts beyond the tolerance, the drive switches to STO.
How it applies
- Machine safety: SOS is the monitored standstill that fits a stop category 2 stop: the motor stays energized and in position control. Because power remains, the risk assessment must justify why that residual energy is acceptable for the task.
- Integrated systems: Define the standstill tolerance and the fault reaction if it is exceeded, usually STO. The drive parameters belong under change control.
- Technical documentation: Tell users that the machine is stopped but not de-energized in SOS. Never describe SOS as a condition for maintenance or cleaning work that requires energy isolation.
- Evidence: Record the achieved performance of the function and its validation, including tests of the monitoring limit.
Safe Operating Stop vs. Safe Torque Off
With SOS the drive actively holds position with power available. With Safe Torque Off no torque-producing energy reaches the motor, so it cannot hold a load without a brake.