Monitoring safety relays

A multitude of safety devices guard CSR PGH's brick-making machines around Australia. Co-ordinating the monitoring and control of the safety function of these machines using safety relays was a logistical nightmare until a programmable safety system from Pilz provided the answers.

The machines are large. One of CSR PGH's brick extrusion machines, for example, is capable of extruding at a rate of 90 tonnes per hour and the associated setting machine can handle 24,000 house bricks per hour. Safeguarding such big and highly automated plants is complex - one machine alone has five sets of light curtains and six gates, each with multiple sensors and switches.

This complexity presents engineers like CSR's Mark Murphy with a number of challenges related to design, future proofing, maintenance and production. The solution was to migrate from conventional wiring and relays to a digital system, the Pilz Programmable Safety System (PSS).

In contrast to conventional wiring, the PSS provided a clear division of the control and safety functions, bringing a new transparency and flexibility to CSR PGH's operations.

One of the first priorities was to achieve maximum plant availability so that machinery can be shut down instantly when needed, yet with minimal disruption of the plant.

"Our design is based around maintaining product flow throughout the process line. It is important to keep the extrusion constant as far as possible," Murphy said.

The plant design features safety zones, which allow buffering of product between processes. The Strathpine paver press line, for example, is controlled in eight safety zones. When a safety device is triggered in one zone, production in the seven remaining zones can continue using buffered product.

The zone design effectively buys engineers time and swift troubleshooting is essential to reinstate zones before buffers are exhausted - something that was difficult in the days of using safety relays alone.

Since installing Pilz Programmable Safety Systems (PSS) in four plants around Australia, Murphy says the diagnostic process has been transformed. The Pilz PSSs are linked to the main PLC by a Modbus serial link. The human machine interface (HMI) offers safety system diagnostics at two levels - for operators and for maintenance staff.

Similarly, wiring has been simplified. Data passes directly to the supervisory control and data acquisition (SCADA) systems.

"Using relays, we had been duplicating a lot of wiring to give sufficient feedback to the operators on the status of the safety system," Murphy said.

According to the company, the Pilz SafetyBUSp safe bus system guarantees that all potential faults, including device, wiring and data faults, such as random telegraph repetition, loss/insertion of telegrams or telegram fragments, are detected safely and reported to operators via a single cable.

The move from relays to the Pilz Programmable Safety Systems has brought the added benefit of enhanced safety. Pilz PSSs are BG and TUV certified to category 4, making it easy and economical to safeguard even category 3 hazards to the more stringent category 4 requirement.

With the installation of five new Pilz PSS controlled safety systems, CSR PGH has also reaped training and expandability benefits. Identical functions in the control software simply need to be duplicated, slashing the overall cost of the control technology. Similarly, the clear division between standard and safety technology means changes to the standard controller can be made without compromising the performance of the safety system.

The Pilz system can also be readily replicated in new plants. CSR PGH says it is now adding new Pilz PSSs to its current fleet of five as plant automation levels are increased, with the next scheduled for Auckland, New Zealand this year.