Jul 24, 2015

Building Highly-Available Automated Systems with Redundancy

Posted by Ray Labbe

Redundancy is something that every company strives for in their automated process, but is one of the hardest functionalities to implement successfully. Companies across the globe are working with solutions comprised of disparate devices, HMI/SCADA systems, historians, and MES systems—each of which has various support for integrating a redundant solution. As an Applications Engineer at Kepware, I have the privilege of answering the question, “How can we implement a redundant automated solution?”

I’m happy to say that by leveraging Kepware’s portfolio of products, redundancy is possible even for customers with unique environments. One of the tools we provide is Media Level Redundancy, an advanced option for KEPServerEX that is supported in Master drivers and drivers that do not require third-party interface cards or library files. Accessed in KEPServerEX through the driver's Device Properties | Redundancy tab, Media Level Redundancy (MLR) was enhanced in the June 2015 release of KEPServerEX version 5.18.

Introducing Kepware’s Enhanced Redundancy Offering

Media Level Redundancy is used to maintain Kepware's KEPServerEX Meeting Room reliable communications with critical components in an automated environment. It accomplishes this by one of two methods: Communication Path Redundancy (which provides multiple network paths to communicate to a critical device) or Device Pair Redundancy (in which identical devices are configured as a redundant unit). KEPServerEX version 5.18 introduces a new operating mode called “Switch on Trigger” that gives the advanced option the ability to determine connection availability by monitoring a tag’s value, quality, or rate of change. These new trigger types provide additional flexibility to design redundant communications and solutions for any automated environment. For example, if redundant devices are intelligently sharing control of a machine, MLR could monitor this status to determine the device with which KEPServerEX should be communicating. Additionally, MLR could monitor a heartbeat item (where supported by devices) to determine if the heartbeat is still active. 

Redundancy may also be necessary when utilizing a tunneling solution, and MLR enables connections to redundant OPC servers with the OPC UA Client driver tunneling solution for KEPServerEX. The new “Switch on Trigger” feature now allows failover to be driven not only by the tunnel connection but also by the status of the devices communicating with the OPC servers—thus forcing the tunnel connection to swap to the secondary OPC server to communicate with devices. This provides the ability to monitor the “downstream” network connection to devices and allows the system to Media Level Redundancy in KEPServerEXfind alternate paths of communication from additional failure conditions.

Although “Switch on Trigger” uses a single tag to evaluate the trigger condition, it can be extended to monitor multiple tags with Advanced Tags for KEPServerEX. For example, a Derived Tag can be created to compute multiple tags’ quality values and provide a “TRUE” condition when some or all of the tags’ quality value is “BAD”. The Derived Tag is then used as the Trigger value, causing MLR to take action when the Derived Tag computes to a “TRUE” condition. Applying this idea to the tunneling solution discussed earlier, you could create a Derived Tag that monitors multiple devices’ states and aggregates these conditions into a single Derived Tag. The tunnel connection could move to the secondary OPC server when two or more devices cease communicating with the primary OPC server, with the Derived Tag providing the resultant condition to MLR.

What's Next?

Redundancy solutions are not one size fits all. Equipment and network designs are unique for every application, and solutions need tools that can be flexible within a variety of environments. Extending Media Level Redundancy with the “Switch on Trigger” functionality further adds capabilities to our product portfolio to help our customers build highly-available automated systems. Learn whether the drivers in your project support MLR in our knowledge base article Do All Drivers Support Media Level Redundancy? and then download the latest version of KEPServerEX to try it in your automated system. 

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