Safety Integrated - Industry - Siemens

Integration of the Readback Signal in an Application
of Category 4 acc. to EN 954-1
Automation Function
Description of the functionality
What is "read back"?
The part of a circuit in which read back is implemented is referred
to as feedback circuit. In this circuit the status of the
contactor auxiliary contacts are evaluated; it is required to use
positively driven contacts. Starting a safety application is only
possible in a closed feedback circuit. If, for instance, a main
contact or a release contact welds at a contactor reactivating
the safety circuit is not possible.
Read back is thus used for monitoring the load to prevent a reactivation
in case of a fault. This also applies to the use of
valves and it is already required from Safety Category 2.
Functional example
In the functional example described here a motor is started
and stopped. Using this example it will be shown
■ how the feedback circuit is integrated in the circuit design
and
■ how it is evaluated using STEP 7
According to the machinery directive each machine has to be
equipped with one or several emergency stop control devices.
This is the reason why an emergency stop functionality is integrated
in this example; however, this functionality is not described
in detail. Explanations on the subject emergency stop
are available in "Safety Integrated Functional Example" 1.
Safety Category 4
In order to meet the requirements of Safety Category 4 it is
obligatory to read back the process signal to the actuator. This
example complies with the requirements of Safety Category 4.
Reaction times
Use the Excel file, which is available for S7 Distributed Safety
V 5.3, for the calculation of the max. reaction time of your
F system. This file is available on the internet:
http://support.automation.siemens.com/WW/view/en/
19138505
Flowchart
The flowchart shown below illustrates the function process of
the example. The acknowledgement in the flowchart is required
■ prior to starting the motor for the first time
■ prior to a start after activating the emergency stop
■ after reintegration (end of a passivation) of the failsafe
input module of the ET 200S
Start
Motor OFF?
Y
Emergency
stop unlocked?
Y
Acknowledgement
required?
N
Readback error?
N
Start command?
Y
Motor ON
Advantages/customer benefits
N
Y
Y
N
Emergency stop
pressed?
Readback error?
Stop command?
■ Wiring reduced to a minimum due to use of failsafe S7-CPU
and distributed I/O. Dieser Vorteil kommt um so mehr zum
Tragen, je mehr Sicherheitsfunktionen realisiert werden
■ Programming the failsafe program with STEP 7 engineering
tools
■ Only one CPU is required, since failsafe and standard program
parts run on a coexistent basis in the CPU
Functional Example No. AS-FE-I-007-V10-EN 245
N
N
N
Y
Y
Y
Motor OFF
G_FB_XX_141
Ex. No.
7