2
Description of the safety function STO
16
Festo – GDCP-CMMP-AS-M0-S1-EN – 1412a – English
Note
If one of the control ports STO-A or STO-B is deactivated with an active output stage, it
will result in uncontrolled coasting of the drive.
If uncontrolled coasting can result in a hazard or damage, additional measures are
required.
Note
A clamping unit is actuated by the non-safety-relevant firmware of the CMMP-AS-...-M0
motor controller.
The holding brakes used by Festo motors are not suitable for active deceleration - only for
holding a position!
The safe state can be requested when the power semiconductor (PWM) is activated. The two driver
supply voltage states are detected and analysed in 10 ms cycles. If they are unequal over a prolonged
period, an error message is triggered
è
Section 5.4.2. The safety function presupposes that the two
signals have the same status. Unequal signals are tolerated only during a transition period, the so-
called “discrepancy time”
è
The finite state machine in the CMMP-AS-...-M0 motor controller has its own status in parallel to the
integrated protection circuit. Due to the discrepancy time analysis, this finite state machine may reach
the “Safe status” only with a considerable delay. Accordingly, this state can also be signalled via digital
outputs or a fieldbus only with a considerable delay. The power end stage itself is then, however,
“safely switched off ”. This finite state machine is processed within the 10 ms cycle.
This generally results in a graded response speed as per Tab. 2.2:
Function
Response time
Reaction
Switching time from
high to low
T_STO-A/B_OFF
è
Switching time from
low to high
T_STO-A/B_ON
è
Detection of driver
supply failure
t
Response
125 μs
Activation of the power semiconductor (PWM)
is switched off
Activation of holding
brake
t
Response
10 ms
Activation of the holding brake after detection
of the driver supply failure
Signal analysis and
status display
t
Response
10 ms
Status transitions in the internal finite state
machine, triggering an error message and
showing the status on the display if necessary
Tab. 2.2
Detection and response times of the driver supply voltage
