Functional safety
326
9.
FUNCTIONAL SAFETY
STO safety function
314
DDS
HARDWARE
Ref.1310
9.1 STO safety function
The STO (Safe Torque Off) is the only safety function Fagor Automation
drives AXD/SPD offer according to the standard IEC 61800-5-2.
This safety function may be used to safely disconnect the motor torque and
it is always active.
STO first channel
It is the external main contactor - KM1 that will remove input power to the
power supply (PS, XPS or RPS). To perform the monitoring, the contactor
has to have an N.C. contact (
Normally Closed) that complies with IEC
60947-4-1 or with IEC 60947-5-1.
STO second channel
It is pin 2 “Drive Enable” of connector X2 of the AXD/SPD, that will dis-
able motor torque. The N.C. contact (
Normally Closed) may be monitored
through pins AS1, AS2 of connector X7.
PL d or SIL 2
When a machine requires a PL d or SIL 2, it requires an external safety
controller PL d or SIL 2 that disconnects the motor torque in the two chan-
nels. The safety controller must watch the status of each channel (only
when demanded by the STO) keeping the motor torque off in case of fail-
ure.
Safe state
The safety controller may use the “Torque OFF” state to achieve a safe
state of the machine.
Brake
Requesting the STO safety function while the servo system is moving,
and there is no brake causes the motor to only stop by friction (coasting).
If there are external forces on to the motor (e.g. vertical axis), where an
unwanted movement (e.g. due to gravity) can cause a risky situation, this
must not be used without additional measures to protect against dropping.
A risk analysis of the machine may require a brake to help stop the motor.
Failure reaction and failure reaction time
After a request from STO, if one of the channel fails, its corresponding
feedback contact opens. The safety controller does not allow engaging
the system and consequently, the failure reaction time is zero. Do not mis-
take it with the response time of the STO function itself that could be of
some ms. See
9.7 Response time.
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