Safety upgrade for stainless steel rolling mill

15 April 2014

A safety PLC from ABB has provided the platform for Sandvik Materials Technology to add zoned safety guarding to a cold rolling mill at its Sandviken plant in Sweden. The mill has been upgraded a number of times throughout its life – including recent changes that added servomotors, and new machine control using ABB's standard AC500 PLC and touchscreen operator interface panels.

The latest upgrade adds safety PLC modules from ABB's AC500-S PLC to the machine control system to enhance the safety of the 20m-long mill with a networked architecture using the PROFIsafe protocol over PROFINET to provide independent control of six separate safety zones. The zoned arrangement means that parts of the machine can remain operational while an operator gains safe access to some other zone – where safety is assured by disabling power. 

Around 50 safety PLC I/O channels are employed to link to gate switch and light curtain guarding, and emergency stop buttons. The I/O also monitors pressure switches to sense that hydraulic power is disabled so that maintenance can take place, and controls power supplies to the motor drives. Safe speed control is another aspect of the safety control provided by the new PLC – to ensure that hands or fingers cannot be trapped between the mill's belt and rollers. 

Sandvik chose ABB's safety PLC for this application because of the good experience with its standard PLC architecture on both this machine and other machinery control projects. The AC500-S safety PLC also offered the possibility of using floating-point numbers, which simplified the safety programming required for this project for tasks such as calculating speed. 

The fact that the standard version of the ABB PLC was already used on the mill simplified the safety upgrade because the safety PLC hardware shares the same look and feel as standard AC500 PLCs, allowing safety and non-safety functions to be mixed in the same system. This compatibility includes identical dimensions to CPU and I/O modules, and the same mounting and wiring scheme. 

The new dual-processor safety CPU module and the safety I/O channels have been integrated alongside the machine's existing AC500 PLC – which controls six DC and AC drives and motors, the DC motors drive the main steel belt and rollers. The AC motors adjust roller position to achieve the desired pressure during strip steel processing. The standard PLC also links with four operator interface panels from ABB's CP600 series.

Programming the safety solution was made simple by the fact that ABB's integrated PLC development tool, Automation Builder, included support for safety PLC programming in its CODESYS-based integrated development environment – and support for the PLCopen Safety Library. ABB also provided its own safety code analyser tool, SCA, which verifies the safety programming rules.

The familiarity of the programming environment and the sophistication of these tools helped Sandvik's developers to quickly understand safety PLC programming concepts. This allowed them to develop the enhanced machine safety architecture in a very short time - meeting the company's tight timescales set for this upgrade project – and quickly bring the enhanced mill into operation.

This project also took place before ABB had actually launched the AC500-S safety PLC. Prior to commercial release, the new PLC was extensively field tested in a large number of pilot applications. Sandvik was one of ABB's pilot customers and developed this safety control system before the product and tools were formally launched. This was made possible by good support from ABB's product development team. Since this application, Sandvik has gone on to use the safety PLC on three other machine refurbishment projects.

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