Glossary · Functional safety
Sensor subsystem
Also known as: input subsystem, sensor element
German: Sensor-Subsystem
In functional safety, a sensor subsystem is the part of a safety function that detects the process or machine condition requiring action, comprising the sensors and their input circuits up to the logic solver.
- Functional safety
- Integrated systems
In one sentence
A sensor subsystem detects the condition that demands a safety function and passes it to the logic solver.
Example
The sensor subsystem of a high-pressure trip consists of three pressure transmitters in 2oo3 voting, their impulse lines and the safety input cards of the logic solver.
How it applies
- Architecture: Safety functions are commonly split into sensor, logic and final element subsystems. Each subsystem has its own failure data, Hardware fault tolerance (HFT) and diagnostics, which are combined in SIL verification.
- Engineering: Redundant sensors reduce the effect of single failures but must be protected against common causes, such as shared impulse lines or identical calibration errors.
- Maintenance: Calibration, Sensor drift and blocked impulse lines are typical sources of dangerous undetected failures, so proof tests should include the sensing element and not just the signal path.
- Documentation: List every sensor with tag, range, trip setpoint, voting and test method. The documentation team should keep setpoints and tag names identical in the SRS, loop diagrams and maintenance instructions.
Sensor subsystem vs. protective device
In machinery, sensing is often done by a Protective device such as a light curtain, which is itself a certified subsystem. In the process industry, the sensor subsystem is usually assembled from transmitters, switches and input cards selected by the plant.