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

8.1.3

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 in the sensors and actuators can be detected in conjunction with the SPL-CDC
and forced checking procedure - or the forced checking as a result of the process itself. The
connecting cables are generally monitored autonomously by the failsafe I/O.
The following applies to the safety-related sensor connection
2 safety-related inputs + 1 standard test output.
The following applies to the safety-related actuator connection
2 safety-related outputs + 1 standard test input.
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 component or component where
faults can be excluded), 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.
Example:
The process quantity, e.g. hydraulic pressure, is switched using a standard valve that is
controlled in a safety-related fashion. A pressure sensor signals the status of the process
quantity. If the valve can no longer switch due to a fault condition, then using a safely controlled
standard contactor, the motor that is generating the pressure, is shutdown. The advantage of
this particular version is that components can be used that are already available as standard.
As to whether this solution can be used, must be confirmed as a result of the risk assessment
(see the following diagram).
Safety Integrated
Function Manual, 12/2017, 6FC5397-4BP40-6BA1
Connecting sensors and actuators
8.1 Safety-related input/output signals
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