We regularly describe sophisticated networking systems, complex control algorithms, and elegantly structured hierarchical computer systems. But, like an inexplicably non-functioning home appliance, they won't work if they're not plugged in.
Loose connections, sometimes caused by the very machinery to which they're attached, are among a host of wiring and connection problems that can bring to a halt the most carefully developed and hardworking networks.
We surveyed end users, machine builders and OEMs for a list of the most common wiring and connection problems. Here are some, along with their solutions. Each of these problems has numerous variations as well as solutions, many of them vendor-specific.
(1) Vibration/Loose Wiring
Problem: Vibration in industrial applications can cause terminated wires to loosen, resulting in broken or intermittent connections.
Solution: Screw-down and gas-tight connections, such as those seen in spring-clamp and IDC-type terminal blocks, can prevent loosening of wires. Spring-clamp terminal blocks are especially good for stranded-wire applications because the conductor is completely captive within the spring clamp."We used to manufacture our own boards for emergency power-off boxes, and we used a type of connector that was always working itself loose," recalls Darren Willey, chemical systems supervisor in the Boise, Idaho, plant of semiconductor manufacturer Micron Technology. "There was no solid latching of the connector; it just slid on to individual termination points. Plus it was very labor-intensive for us to make all these connections."
Willey said Micron turned to a prefabricated board-level component "with all the connections on the board. Plus the whole component is DIN-rail-mounted — it just snaps in." The spring-loaded insertion keeps constant pressure on the component, ensuring a good connection.
Similar problems were faced by Se-Mar Electric, a West Seneca, N.Y., manufacturer of electrical control panels. "One plant we installed a system in had large, reciprocating equipment, which was always working the connections loose," says Rich DiChristina, quality control supervisor. "We simply used large cage clamps with spring clips. They provide constant positive pressure, and loosening connections were no longer a problem."
(2) Electrical Interface
Problem: Industrial equipment can generate strong electromagnetic fields that degrade analog and even digital signal integrity.
Solution: Route cables in shielded, grounded conduit as far from electromagnetic interference sources as possible.
With lots of robots, welding stations and similar pieces of high-power electrical equipment, "electromagnetic interference (EMI) has been one of our biggest problems," says Terry Shell, network specialist at Toyota Motor Manufacturing North America's Georgetown, Ky., assembly plant. "Signals get distorted. There are lots of collisions on the network. With most of the machinery having Ethernet connections, it's difficult to route wiring without getting EMI."
Toyota instituted a two-pronged defense: Route the cables as far away from sources of EMI as possible and run the cables in conduit. "But make sure the conduit itself is grounded properly," Shell adds. "That made a world of difference."
Shielded cables and grounded conduit alone were not enough. "We also install all our networked cable and conduit 25 ft off the floor and run the connections up to that height," Shell continues. "Putting the cables overhead seemed to be the only way to avoid EMI, plus it keeps the cables off the floor and away from other problems."
You would think ac power lines and a DCS signal would have different enough frequencies that one wouldn't affect the other. That wasn't the case at Eastman Chemical's Longview, Texas, plant. "We had a data highway for the DCS running in conduit next to a cable tray with a 60 Hz ac power supply, and we were getting intermittent interference," says Ray Vincent, principal electrical engineer. "The DCS workstations would go blank. It turned out there was a variable-speed drive on that ac line that was the cause of the problem."
The solution was to put the power line, especially the leads to the variable-speed drive, in conduit also. Generally speaking, "don't put a variable-speed drive anywhere near fieldbus," Vincent warns.