To the Terminal and Step on It

IndustrialNetworking.net

Keywords: Jim Montague, device network and controllers

Device-Level Networks Transport Data to Higher-Level Controllers. Here's How It Gets There and Back

By Jim Montague, executive editor

Big-ticket transportation vehicles can only take you so far. Superhighways, airplanes and trains eventually give way to local streets and homes. The same is true for industrial networks.

Though many users focus on tying industrial networks up to enterprise and administrative summits, it’s becoming increasingly obvious to engineers that they need to focus downward to the I/O points, sensors, switches, valves and other devices to get the data their managers crave. Unfortunately, upper-crust fieldbuses and Ethernet flavors usually can’t be used with production lines, conveyors, drives, sensors and other device-level settings because their data packets are too big and their networking methods are too complex. They need some way to translate, convert signals and link to these lower-level networks and components, and so upper-level fieldbus users are rediscovering the most basic, seemingly unsophisticated networks and pulling them into the limelight.

This is giving new attention and life to lower-level networks and turning them into 10- and 15-year overnight success stories, even though supporters and many engineers know they were in use—hidden in plain sight—all along. Ironically, taking Ethernet to a plant-floor device requires a dedicated line and maybe another for power, which is similar to the disadvantages of using hardwiring.

Finding Flexibility

Installing and maintaining robot cells is difficult, but engineers at ThyssenKrupp Body Stampings (TKBS, tka-as.thyssenkrupp.com) in Cannock Chase, U.K., found that hardwiring islands of pneumatic valves to a controller was becoming unworkable. “Our roadblock to faster, more flexible production was hardwiring because it made installing new robot cells time-consuming and cumbersome. I couldn’t believe we’d persisted with hardwiring for as long as we did,” says Keith Jones, TKBS’ assembly maintenance manager.

The automotive body parts manufacturer evaluated the major fieldbuses and implemented two-wire-in-one-cable Actuator-Sensor Interface (AS-i) because it could control TKBS’ robots and communicate with its HMIs. TKBS also simplified its wiring by adding Kuhnke’s (kuhnke.co.uk) plug-and-play Airbox electro-pneumatic modules with up to four I/O points to replace the valve island and reconfigure its robots quickly. “If a customer like Toyota, Nissan or Saab launches a new or updated model, we suddenly might have high demand for a particular engine mounting bracket or roof support. As a result, we have to mount our robots and turntables on movable bases, so we easily can relocate and adapt production cells.”

Consequently, AS-i and the Airboxes allow TKBS to have 600 I/O points on its physical-level fieldbus, unlike conventional valve islands. Set up in a bi-stable arrangement, each Airbox is clipped onto the AS-i cable and configured to provide two independently programmable 5/2 input valves and four inputs. This electro-pneumatic system sits on one AS-i cable and regulates 60 robot cells, performing mostly presswork and spot/MIG welding tasks. Each cell has one to four GE Fanuc robot arms operating 24/7. Above this physical layer, TKBS uses Profibus to communicate between the robots and welding equipment, while Profinet provides production data at the control layer. AS-i Safety at Work also allows TKBS to add e-stops to the network and enables it to take on Category 4 safety applications.

Lower-Level Network Implementation To-Do List

Inventory application’s existing physical network Evaluate network’s present and future requirements Determine which network methods will work with your I/O nodes Select a suitable gateway type to your higher-level network Justify expenditure to management Install hardware and configure software Assign IP addresses for I/O nodes, and fill in software tags on PLCs as needed Periodically reevaluate and revise network

“Airbox and AS-i mirror our modular design with I/O points easily unhooked and moved as required—just like our robots. As a result, it takes just one month to commission a new work cell, which is a 60% time savings compared to the hardwired valve islands,” says Jones.  

In addition, if any part of the AS-i network develops a problem, it can be pinpointed easily. “We moved away from the valve island solution partly because the adapter cables are effectively black holes when it comes to diagnosing a fault,” adds Jones. “Airboxes clipped onto AS-i diagnose themselves. If we have a problem with the new system, it typically can be fixed in 15-29 minutes. Before, six hours was common.” 

Getting Down

Similar to all industrial networking, device-level networks grew out of the point-to-point hardwired world. However, while most upper-level fieldbuses adopted open architectures and Ethernet, and gained some presence within the industry’s huge 4-20 mA installed base, most device-level networks still face an even-bigger foundation of legacy hardwiring. Many users might be reluctant to give up their cables because there’s so much more wiring to cling to at the device level.

“You can use a bicycle or an 18-wheeler to bring home a loaf of bread,” says Dave VanGompel, network consultant in Rockwell Automation’s portfolio management architecture and software group. “The truck can do the job, but it’s a lot more costly. You just have to decide what trade-offs you can make. Putting in a large TCP/IP system and coordinating 30 nodes carries more overhead than managing 20 motor starters in a control cabinet with a few sensors.”