4.3 Safe Torque Off (STO)
An additional digital input (STO) releases the power output stage of the drive as long as a 24
V signal is applied to this input. If the STO input goes open-circuit, then power will no longer
be supplied to the motor, and the drive will lose all torque and coast to a stop.
The STO safety implementation on the AKD is certified by the IFA (Institut für Arbeitsschutz
der Deutschen Gesetzlichen Unfallversicherung). The safety circuit implementation for real-
izing the safety function "Safe Torque Off" in the drive is suited for SIL2 according to IEC
61508-2 and PLd, Cat.3 according to ISO 13849-1. The subsystems (AKD drives) are totally
described for safety technics with the characteristic data :
Device Operation Mode ISO 13849-1 IEC 61508-2 PFH [1/h] SFF[%]
STO STO single channel PLd, Cat.3 SIL2 0 20
AKD Installation | 4 Approvals
Kollmorgen™ | May 2013 25
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