MRA has extensive experience in anti-collision systems that are SIL-rated and compliant with the emerging use of AS61508 and its related standards.
The SIL design is site specific and incorporates the functional specification of selected components, their diagnostic coverage, safety targets and failure rate calculations deemed necessary by the review process.
The process involves a HAZOP (Hazard and Operability Analysis) risk assessment that will require operators, superintendents, safety experts and machine experts to run through all possible collision scenarios and define the risks are and what controls can be put into place to reduce the risk.
The HAZOP can use the following methods to determine risks and consequences:
- Fault Tree Analysis (FTA)
- Failure mode and Effect Analysis (FMEA)
- Layers of Protection Analysis (LOPA)
- Markov Modelling
Potential risks to be considered are:
- Machine to machine collision
- Machine to stockpile collision
- Machine to end buffer collision
- Machine to conveyor protection
- Machine to bund protection
- Machine to stockyard base layer protection
- Machine to work-area (exclusion zone)
The HAZOP will determine the Safety Integrated Function (SIF) for each task to be performed by the ACS PLC. Once these functions are formalised, a SIL rating will incorporated into a Safety Requirements Specification (SRS) that will define all technical and management activities for the lifecycle of the Safety Instrumented Systems.
Protection methodologies are used to establish Collision Zones with minimum separation distances between machine pairs that can interact with each other. These are updated in real time as each machine operates along the berm and when stationary. The ACS will then restrict and inhibit machine motions should one machine enter within the anti-collision zones of another. The collision zones will restrict machine operation using a low level inhibit and that are escalated for all major motions such as long travel and slew.
These zones include:
- Slow-Down Zones are used to limit the speed of the major motion such as long travel or slew
- Inhibit Zones allow a machine can move at full speed in the opposite direction of the collision risk
- All Motion Stop Zones inhibit all major motions
All anti-collision conditions are alarmed and reported to SCADA. Machine ACS bypasses are used to enable operators to recover the machine into a safe zone of operation, Extensive machine integrity checks are performed on the machine for communications loss to on-board I/O modules; hardware errors; encoder discrepancy; over speed, moving without Instruction; and, failure to detect motion.
These form part of the System Requirement Specification.