7.16 Safe Torque Off (STO)
The STO safety implementation on the AKD is certified. The safety circuit implementation
used for the safety function "Safe Torque Off" in the drive is suited for SIL 2 according to IEC
62061 and PLd / CAT3 according to ISO 13849-1. With AKD-x04807 drives SIL3/PLe is pos-
sible, if both STO-Enable inputs and the corresponding STO-Status signals are used.
AKD-x003 up to AKD-x024
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.
This input is not compatible with IEC 61131-2. You can thus achieve a category 0 stop
(
# 53) by using the STO input without switching a mains contactor.
AKD-x048
Two additional digital inputs (STO-Enable1 and STO-Enable2) release the power output
stage of the drive as long as a 24 V signal is applied to these inputs. If one of the STO inputs
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 inputs are not compatible with IEC 61131-2. You can thus achieve a category 0 stop
(
# 53) by using the STO inputs without switching a mains contactor.
7.16.1 Safety characteristic data
The subsystems (AKD) are described with the following characteristic data:
AKD-x003 up to AKD-x024
Function Operation mode
ISO
13849-1
IEC
62061
PFH
[1/h]
T
M
[Years]
SFF
[%]
STO
single channel
PL d, CAT 3
SIL 2
1.50E-07
20
100
AKD-x048
Function Operation mode
ISO
13849-1
IEC
62061
PFH
[1/h]
T
M
[Years]
SFF
[%]
STO
single channel
PL d, CAT 2
SIL 2
1.88E-07
20
89
STO
dual channel
PL d, CAT 3
SIL 2
5.64E-09
20
87
STO
dual channel with
periodical testing
PL e, CAT 4
SIL 3
5.64E-09
20
87
A very unlikely but possible event can happen, if within a very short time 2 not adjacent
IGBTs will have a short circuit. In such case a movement of a maximum angle of 120° (elec-
trical) can happen. This effect can only happen if the drive is in the function STO. If the total
failure rate of the IGBT is 120 FIT normally for such short circuit 60 FIT will be valid (50:50
model). By such event 2 specific IGBTs have to fail at same time. The calculation shows a
probability of 1.5 * 10
-15
per hour (without common cause failure). Even if the STO function
will be issued for a whole year, this event will only happen every 100 Billion years.
AKD Installation | 7 Technical description and data
Kollmorgen | kdn.kollmorgen.com | October 2017
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