Connecting Sensors - Actuators Using The 3-Terminal Concept - Siemens SINUMERIK 840D sl Function Manual

Connecting Sensors/Actuators
7.1 Safety–related input/output signals
2. The signals "external STOPs" are processed internally in a special way:
S In order to increase the level of security that a requested "external STOP"
actually takes effect, the STOPs are internally exchanged between the two
channels. Failure of the stop control function in one channel does not cause
an error for these signals (in contrast to the mode changeover signals, e.g.
"SG/SBH active") in the crosswise data comparison.
While other signals can be subject to a forced checking procedure in parallel
and in both channels (and should be – in order to avoid errors being trig-
gered by the crosswise data comparison), the "external STOPs" must be
subject to a forced checking procedure one after the other in both chan-
nels.
7.1.3 Connecting sensors – actuators using the 3–terminal concept
Basic principle for safety–related signal processing
With the 3–terminal concept, three terminals (signals) are required to connect a
sensor or actuator. Faults/errors in the sensors and actuators can be detected in
conjunction with the SPL–crosswise data comparison and forced checking proce-
dure or the forced checking as a result of the process itself. The connecting cables
are generally monitored autonomously by the fail–safe I/O.
The following applies to the safety–related sensor connection:
The following applies to the safety–related actuator connection:
Example of an actuator connection
2 outputs (to control through 2 channels via SGA) and 1 standard test input (for the
forced checking procedure) are required to connect an actuator in a safety–related
fashion. The test input is the feedback signal from the load circuit and is fed from
the power supply voltage of a standard input module. The user should derive this
as directly as possible from the process quantity.
Actuator control, P/M switching:
The actuator is directly controlled using a plus potential (P–switching) and minus
potential (M–switching). If the actuator is not a qualified component (safety compo-
nent or component with fault exclusion), then in the case that the actuator fails, the
user must apply additional cascaded measures in order to bring the process into a
safe condition.
7-190
SINUMERIK 840D sl/SINAMICS S120 SINUMERIK Safety Integrated (FBSI sl) – 05.2008 Edition
2 safety–related inputs + 1 standard test output.
2 safety–related outputs + 1 standard test input.
© Siemens AG 2008 All Rights Reserved
05.08