Conduit Between Networks
Fortunately, OPC tunneling eliminates the need for DCOM. An OPC tunneling solution converts the real-time data stream into TCP before sending it across the network, and then turns it back into OPC at the other end. There are several OPC tunneling products on the market, but not all of them provide the robust connectivity that Tschudin needed, combined with the flexibility to configure it for the unique requirements of his system.
Tschudin and I discussed the pros and cons of different approaches. He and Lauterburg had considered using OPC UA, since it is not based on DCOM technology, and thus eliminates the need for OPC tunneling. However, the technology was still in development, and implementation was low. There were no OPC UA servers available for their hardware, and they needed something that would work with their existing system.
Finally, we chose the OPC DataHub from Cogent Real-Time Systems Inc. (www.opcdatahub.com) because it provides a robust tunneling connection and is easy and flexible to configure. We installed one OPC DataHub on the sheet brake's firewalled system computer and configured it as an OPC server for the sheet brake's OPC clients. Then we installed a second OPC DataHub on a utility server on our own firewalled network. This second OPC DataHub was connected to the DCS' OPC server using DCOM. That DCOM connection was relatively easy to configure and reasonably stable because both machines are on the same network, so we didn't have to deal with firewalls there.
Next, we configured the OPC DataHub on the utility server to get data from the DCS OPC server, and then configured a tunneling connection between the two OPC DataHubs (Figure 2). We opened one port on each firewall to allow the connection, and for security reasons, configured the OPC tunnel to be an SSL connection. When the tunnel was activated, the OPC DataHub on the sheet brake side gained a complete copy of all the configured OPC points on the DCS. With a few mouse clicks, we configured the necessary points that the sheet brake system needed, and by lunchtime of the same day, we had a working system.
Of course, there was some fine-tuning involved. Initially, we configured the OPC DataHub to read all the points in ABB's DCS. Since there were a lot of points, that slowed down the start-up time significantly. So, we changed it to connect only to those points that the sheet brake system actually needed. This reduced the start-up time to a fraction of a second. Tschudin and Lauterburg were pleased with the quick work, but needed to test the system for several weeks before deciding to go ahead with the implementation.
The system now has been working for months, and the benefits are clear. "With the vision quality management system connected to our DCS in real time, we can gather important data, such as the machine speed, grade number and grade name, when a particular defect occurs," said Tschudin. "This is particularly helpful when we plan for the use of the paper, such as for facial tissue, paper towels, paper napkins and so on. We have now equipped our second tissue machine line with an identical system and have resolved other problems in the plant by using OPC tunneling."
Bruno Maurer is head of solutions at Logicpark in Thun, Switzerland.