Redundant PLCs keep critical processes running

Edited by the Engineeringtalk editorial team Feb 5, 2002

Rockwell Automation has added a redundancy system to its ControlLogix range of PLCs to provide a high availability solution for critical process and other applications.

Rockwell Automation has added a redundancy system to its ControlLogix range of PLCs to provide a high availability solution for critical process and other applications.

Two ControlLogix central processing units (CPUs) are linked via a 1757-SRM system redundancy module, which controls primary/standby status and provides a high-speed fibre-optic datacommunications link between the primary and standby chassis.

The standby unit constantly mirrors the primary unit and automatically takes over in the event that the duty CPU fails, with no interruption in the control of the application.

This facility is especially useful for applications in the process and other industries where reliability and availability of the PLC is vital.

"Many process control applications require lengthy start-up times or involve high value products and cannot tolerate production disruptions", says Richard Sturt, marketing manager at Rockwell Automation.

"ControlLogix redundancy now provides an easy way to increase the availability of the control system, guarding against any disruptions and so reducing total costs of ownership".

The ControlLogix redundancy solution is extremely simple to implement and no special programming or hardware is required.

Users simply have to click on a "redundancy enabled" selection in the RSLogix 5000 configuration software and download to the primary ControlLogix 5555 controller, the Logix does the rest.

The system even crossloads the program to the secondary controller automatically, eliminating the need to maintain two separate programs.

"It has been possible to provide redundancy of PLCs in the past, but it has required a lot of time and effort to set up", says Sturt.

"Now it is simply a matter of selecting an option during routine configuration".

Special versions of hardware are not required for ControlLogix redundancy.

Standard Logix 5555 CPUs, power supplies, chassis, and ControlNet bridges are used.

In operation, the primary and secondary 1756-CNB ControlNet bridges automatically swap addresses when a switchover occurs, seamlessly allowing any external devices to continue working with the new primary controller.

Because of this transparent implementation, ControlLogix redundancy works with any existing ControlNet devices, including Rockwell Automation's I/O offerings, operator interface devices and drives, as well as third-party ControlNet devices.

Other networks, including Ethernet and DeviceNet, can also be used in a redundant system, and communications will be automatically rerouted when switchover occurs.

The fibre link between the primary and standby units provides physical separation between the two units so the standby CPU is able to continue working even if the primary unit is accidentally damaged or destroyed.

In addition to the dual controller capability, redundant power supplies are also available to ensure that a power supply failure will not cause the loss of an entire chassis.

Both AC and DC versions are available to allow simultaneous powering of a chassis from line power and UPS systems.

The redundant power supplies are panel mounted and connect to the standard ControlLogix chassis through a small chassis adapter.