Dual networks upgrade plant controls

Aug. 24, 2006
Automotive manufacturers are often slow to embrace new production methods because switching to unproven techniques can pose unacceptable risks.

Several hundred robots at Kuka's Toledo plant weld sheetmetal parts into Jeep Wrangler bodies.

High I/O density Numatics valve terminals interface not only with valve solenoids, but with switches and sensors as well. This let Kuka substantially reduce the number of DeviceNet nodes on the network.

So when DaimlerChrysler teamed with Kuka Flexible Production Systems, based in Sterling Hts., Mich., to supply bodies for Jeep Wranglers, the manufacturer of robots and automation systems viewed it as a unique opportunity to approach the task with a clean slate.

Kuka invested in a new facility near Toledo intending to leap-frog current methods for welding car bodies. "The only restriction on equipment, systems, and processes was to use field-proven technology," says Rod Brown, a controls engineer with Kuka.

"This let us take advantage of best practices used by vehicle manufacturers and Tier 1 suppliers across the U.S. and Europe — in many cases embracing ideas Chrysler simply did not have access to."

A critical factor in the plant's success, says Brown, was upgrading the control and safety networks. The archaic, hardwired networks typical of automotive weld shops are notoriously time-consuming to install, take up a lot of space, are difficult to troubleshoot, and a headache to maintain. Kuka needed something better, and enlisted the engineering expertise of a few key vendors, namely Numatics, Siemens, and Turck, to design and build a state-of-the-art industrial control network.

"We didn't want just suppliers, we wanted partners that would 'own' the networks," emphasizes Brown. This meant getting involved from the start to shape the design, recommend components, supervise commissioning, and work out any bugs. "The automotive experience of these suppliers was essential," he says, "but they also brought knowledge from appliance, medical, and other industries." This let Kuka exploit the best-available practices, regardless of origin. And engineering support was critical, adds Brown. "We didn't have time to worry about suppliers' products."

Kuka decided on Allen-Bradley ControlLogix PLCs from Rockwell Automation and a DeviceNet fieldbus network to control welding and material handling operations. This represented a pragmatic choice that minimized risk, according to Brown. Electricians and line operators are experienced with the systems. And Rockwell products have a long history at Chrysler, which holds a seven year option to purchase the Kuka facility.

Kuka then tapped Numatics and Turck to engineer the network. It may seem counterintuitive to let a pneumatics company handle the control system. But with nearly 3,000 valves and countless sensors operating clamps and tool changers, it made perfect sense to Kuka's engineers.

Of late, pneumatics and electronics are increasingly interdependent. Equipment manufacturers look to suppliers like Numatics not only for valves and cylinders, but for fieldbus nodes, I/O components, and cables and connectors as well, explains the company's Director of Marketing, Mark Madsen. Thus, leading pneumatics companies must have expertise in electronic controls and network technology. Numatics, for instance, even boasts an electronics design staff that can quickly sort out any software and hardware issues that may arise, he adds.

"Turck and Numatics complemented each other well," adds Enrico DeCarolis, Numatics director of technology development. "We provided the hardware and they provided the connectivity, so it made sense from Kuka's perspective to throw the two of us together. And if a problem arose, the source didn't matter — we worked as a team to fix it. This proved to be a very good relationship."

What Brown terms the I/O density of Numatics valve terminals also played an important role in the technical upgrade. With DeviceNet, the number of nodes affects network response time and cost, says Brown. "Thus, one of our main goals was to minimize that number."

The Numatics product houses the fieldbus electronics, as well as the pneumatic circuit, within the valve manifold. Many pneumatic suppliers offer valve terminals with DeviceNet capabilities, Brown notes. But Numatics devices have more I/O capacity than many competing products. The manifolds handle up to 96 inputs and 224 outputs per node, letting it interface not only with valve solenoids, but switches, sensors, and related hardware. Comparable networks Chrysler had used in the past required separate I/O blocks to handle external devices. Now, in many cases, a single valve-manifold DeviceNet node replaces four or five that had been previously required.

The manifolds also offer built-in diagnostics, detecting and isolating faults such as short circuits. The welding fixtures and tooling include a number of clamps, each with sensors sending data to the input modules. It's not uncommon, over time, for a cable to short or a sensor to fail, says DeCarolis. The valve modules can detect a short circuit, cut power to the connector, and immediately signal the fault location to the operator. This is a major timesaver in troubleshooting large and complex production lines.

Numatics played a key role in designing the pneumatics as well. For instance, ISO valves tend to be the defacto standard among automakers. But Numatics' computer modeling determined that its smaller, 26-mm Series 2012 valves provided more than enough flow to power virtually all the clamps and tooling. This cut valve size and weight by about two-thirds, and substantially reduced costs and airflow requirements.

It also made a potentially significant difference in the size of the robots. Pneumatic valves typically control clamps and end effectors on material-handling robots. Compared with Series 2012 units, ISO valves can add 20 to 30 lb to the end of a robot, says Brown. In some cases, this necessitates a larger robot that takes up more space and costs substantially more. "As the structures we handle get bigger and heavier, weight in robotic applications becomes more and more critical," says Brown.

Kuka also took advantage of recent changes in standards for electrical equipment in industrial machinery, installing a safety system that relies on Siemens S7 safety PLCs and a Profisafe Profibus network. According to Brown, it holds substantial advantages over setups in traditional automotive plants, where fencing, E-stop pushbuttons, gates, and light curtains — all hard wired and using many redundant relays — protect workers.

Compared with the traditional hard-wired approach, a safety fieldbus network connects all safety devices via an inexpensive, two-wire cable. This reduces wiring costs, eliminates nearly all relays, and significantly cuts control-panel space requirement. The network installed quickly, and built-in diagnostics accelerated commissioning and pinpoints any faults.

Running DeviceNet for control and a Profibus network for safety, side by side, may seem a bit out of the ordinary, Brown admits. But during the concept stage, Kuka engineers deemed Siemens had the best available safety products based on an open protocol. Ethernet was also considered, but costs and a limited number of products and suppliers at the time killed those efforts. Given recent developments, however, a single, open network for I/O and safety might be a logical choice for future systems, says Brown.

Today at the Kuka Toledo Production Operations assembly plant, several hundred welding, material-handling, gaging, and inspection robots turn sheet-metal parts into Jeep bodies. The plant takes up 30% less floor space than comparable body-welding facilities, requires fewer workers, and is simpler, more reliable, and easier to maintain. And robots with modular end-effectors and tooling offer the flexibility to adapt to future Jeep models, compared with dedicated tooling suited for only a single type of vehicle.

Kuka Flexible Production Systems,

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