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2 Description of the safety function STO
14 Festo – GDCP-CMMS-AS-G2-S1-EN – 1306NH – English
With suspended loads or other external forces, additional measures must be taken to ensure that the
load does not drop (e.g. mechanical holding brakes). In the STO “Safe Torque Off ” state, the standstill
position is not monitored.
The machine must be stopped in a safe manner, e.g. via a safety switching device.
If the STO function is activated with a moving drive, the motor starts after max. 5 ms to coast uncon-
trolled. After that, the automatic brake control is activated.
Note
If uncontrolled coasting can result in a hazard or damage, additional measures are re-
quired.
Note
A clamping unit is actuated by the non-safety-relevant firmware of the motor controller
CMMS-AS-...-G2.
If you use motors with a holding brake, the holding brake is worn each time the STO is switched off
while the motor is running.
Therefore, when using the “STO” function, do not use motors with holding brakes; use clamping units
that permit emergency braking or use automatic locking axis mechanisms.
The holding brakes used by Festo motors are not suitable for active deceleration - only for
holding a position!
Application examples for the STO function are manual manipulations when setting up, equipping and
fault clearance. Use of the integrated solution offers several advantages:
– fewer external components, e.g. contactors.
– less wiring effort and space requirements in the control cabinet.
– and thus lower costs.
Another advantage is the availability of the system. The integrated solution allows the intermediate
circuit of the controller to remain charged. This means no significant waiting time when the system is
restarted.
Note
There is a danger that the drive will advance if there are multiple errors in the
CMMS-AS-...-G2.
Failure of the motor controller output stage during the STO status (simultaneous short
circuit of 2 power semiconductors in different phases) may result in a limited detent
movement of the rotor. The rotation angle / path corresponds to a pole pitch. Example:
Rotating axis, synchronous machine, 8-pin movement
<
45° at the motor shaft.