Plant floor diagnostic-device networks

InTech, Jun 2006

The systems integrator necessarily faces the challenge of making various brand name devices and networks fit together.

Integrating different brands of equipment on the plant floor was one of the problems General Motors (GM) wanted to focus on when researching device-level networks.

GM has integrated weld controllers with Alien-Bradley programmable controllers, Namco proximity sensors, and GMF robotic controllers at one facility.

However, GM integrated three completely different suppliers of PLCs, proximity sensors, and robots at another facility.

"Not only did we have to spend a lot of time integrating these different combinations, but they resulted in two distinct control systems," Gary Workman, staff development engineer at GM said. "We wanted to allow each GM facility around the world to choose which suppliers and products best fit their needs. At the same time, we wanted to achieve a level of consistency in designing and operating a control system-no matter what brand of products we were integrating."

Initially, GM researched and monitored the Fieldbus standard as it began its search for the ideal device-level solution.

"After five years of relatively slow progress on the fieldbus front, we realized that we couldn't wait any longer for it to fully evolve," said Workman. "Fieldbus suffered from a number of things, but one of them was a creeping incrementalism in functionality.

"The original charter for Fieldbus was to simply find a digital alternative to the 4 - 20 mA standard, but everyone seemed to have a feature in mind that had to be added to the Fieldbus specification," he said

When first told about DeviceNet, an open communications network based on the Controller Area Network (CAN) technology, GM thought it sounded like what Fieldbus set out to satisfy 10 years ago.

"We knew that our plants, as well as many others, had an immediate need for this level of network, and so we agreed to do some beta site testing," said Workman.

To test the durability of DeviceNet, GM decided to perform a beta test using a tool vital to their manufacturing process.

"We duplicated a machine tool used in our body shop subassembly operation," said Workman. "This tool positions automobile parts for welding operations in an electrically noisy and harsh environment, and we knew that if it survived here, it would survive in our other working environments as well."

The first step was to replace the hardwired I/O. During this process, technicians emptied a 2-inch thick piece of conduit that originally housed the cable for the machine.

The DeviceNet trunk cable, about 3A inch in diameter and capable of withstanding the environmentally harsh conditions, was then routed directly on the machine. The machine tool, which originally used two 9-by-9 inch junction boxes for I/O terminations and one 1 ½-by-2 ½ foot junction box for Remote I/O cards, now utilized only one-third of the space in the third junction box.

This savings was possible by running the trunk cable directly to factory-hardened devices mounted on the tool. "It was very easy to see the wire savings," said Workman. "Imagine replacing a conduit filled with about 50 pairs of wire with a single cable."

Any devices that do not utilize DeviceNet's advanced diagnostic capabilities can still work by wiring them through an I/O module. This preserves manufacturer investments in devices already on the manufacturing floor.

In addition, as new intelligent, addressable devices become available, they connect directly to the taps, eliminating the need for an extra I/O module.

"We eliminated 50 wires that we would have had to install and maintain, we had no trouble integrating devices, and we were able to take advantage of advanced diagnostics that allowed us to react more quickly to potential problems," said Workman. "We are impressed with its diagnostic capability as a master/slave network, and we are looking forward to using this technology as a multimaster network."