Chapter 7 Safety Function (Safe Torque Off, STO) ASDA A2-E
Revision April, 2015
7-3
7.1.1 Functional Safety Standard and Certificates
Pleasee refer to Chapter 9.
7.2 STO Safety Function
Fault Rate of Safety Function
Item
Definition
Standard
Features
SFF
Safe Failure Fraction
IEC61508
Channel 1: 80.08%
Channel 2: 68.91%
HFT (Type A
subsystem)
Hardware Fault Tolerance
IEC61508 1
SIL
Safety Integrity Level
IEC61508 SIL
2
IEC62061 SILCL
2
PFH
Average frequency of dangerous
failure [h-1]
IEC61508 9.56×10
-10
PFD
av
Probability of Dangerous Failure
on Demand
IEC61508 4.18×10
-6
Category Category
ISO13849-1
Category
3
PL
Performance Level
ISO13849-1
d
MTTF
d
Mean time to dangerous failure
ISO13849-1
High
DC Diagnostic
Coverage
ISO13849-1
Low
Theorem of Safety Function
Two individual hardware circuits control the drive signal of motor current so as to cut off
power supply to avoid generating motor cogging if needed.
See table 1 for the description of theorem
Table
1:
ON
=24V OFF=
0V
Signal
Channel
Status of Opto-Isolator
STO signal
STO_A
~ / STO_A
ON ON
OFF OFF
STO_B
~ / STO_B
ON OFF
ON OFF
Servo Drive Output Status
Ready
STO_B lost
(AL502)
(Torque off)
STO_A lost
(AL501)
(Torque off)
STO Mode
(Torque off)
