Redundant Control Simplified

March 17, 2010
Opto 22 Introduces Snap PAC Redundancy Option Kit

There are many types of industrial machines with operations that require redundant control systems. Redundancy might be dictated by the application, but sometimes it's determined by the design philosophy or organizational approach toward implementing control system architectures. If there's a belief that downtime or interruption to operations is unacceptable, there's probably a need for redundant controllers. These days, more industrial machine builders than ever have to respond to customer requirements that any machine proposed must include a redundant control system.

Opto 22 believes it can help with that. "Our new Snap PAC Redundancy Option Kit (Snap-PAC-ROK) can be used to design an Ethernet-based redundant control system with maximum reliability and distributed intelligence that can recover instantly from failure," says Benson Hougland, vice president of marketing and product development for Opto 22. "It uses standard off-the-shelf components and costs less than traditional redundant controllers."

Using standard Opto 22 S-series Snap PACs and Snap Ethernet I/O, combined with the redundancy kit, designers can configure automation systems with dual programmable automation controllers (PACs) that essentially synchronize and run in parallel. In the event one controller fails or is knocked off-line, the other controller assumes command and continues to execute the control program and the processes it runs with no interruption or restart necessary.

"Snap-PAC-ROK has several features that distinguish it from other redundant systems," says Hougland. "Two identically configured controllers—one active, one backup—are able to swap roles in the redundancy scheme at any time. This offers a unique flexibility and other benefits during system startup and commissioning. Customers can keep their control system online and operating even as they make changes to or completely replace their control program. Once the new program is downloaded to one controller, it becomes active, and the second controller is updated with the new instruction set."

Hougland notes that customers can repeat this process as often as needed, as they test, fine-tune and configure their control program to exact specifications. "Similarly, customers seeking to make their existing system redundant need only to add the redundancy kit and a second S-series controller, which, upon commissioning, is automatically populated with the most current firmware and control program," he adds.

The system doesn't use the traditional architecture in which one controller is the primary and the other is the backup. "With the ROK, the two controllers can be configured in any way," says Hougland. "This provides a lot of flexibility as to which one is running and which one is backup. Changing is a simple software command."

This capability simplifies the lives of maintenance technicians, as well, states Hougland. "If an active controller fails, the other will take over," he says. "The technician can replace the controller and power it up; it will automatically check that it has right firmware and programming and take over. The system will take care of itself and bring itself up to redundancy."

Snap-PAC-ROK is economical. It uses Opto 22's standard controllers, not generally more expensive special redundant controllers, explains Hougland. It communicates over Ethernet using standard Cat. 5 cabling. No proprietary communication protocols, interfaces or custom cables are required.

"You use our standard product to put together a redundant system," says Hougland. "Our research indicates that if you're looking at a redundancy system, you'll see you have to use redundant controllers and will pay a significant premium for that. Overall cost will be significantly increased because you have to buy special controllers. The redundancy option uses standard Ethernet or serial cables. With competitor products, you might need to buy a special link, which also costs more."

The key element in ROK is the Snap PAC Redundancy Arbiter—a stand-alone processor that connects via dedicated RS-485 links to the two controllers and regulates which controller is active and which is in backup mode. The arbiter does this based on status information returned by each controller in response to the arbiter's periodic heartbeat requests. LEDs on the arbiter indicate system health. Hougland says the arbiter is responsible to:

  • Maintain the active/backup status of each controller
  • Manage synchronization of the control program and control data
  • Send heartbeat requests at regular intervals to each controller in order to receive status information
  • Qualify the backup controller, which means that the backup controller is ready to take over as the active controller
  • Instruct the controller to reset, whenever it is disqualified
  • Control the power supply of each controller using its supply-voltage output to drive normally closed relays in the redundant power switch.

The kit also includes a SNAP Redundant Power Switch that allows power restarts to controllers in the event of failure and during firmware upgrades and a Redundancy Option Configuration Utility for configuring redundant systems, including IP addresses, program synchronization and firmware matching and for maintaining and monitoring a functioning system.

Snap-PAC-ROK is available from the Opto 22 website and a worldwide network of authorized distributors as of March 22.

For more information, call 800/321-6786 or browse to