SSDs in embedded control: cold rolling steel in old European factories

By far, most application stories connected with SSDs revolve around servers and PCs. There’s a vast, hidden market in embedded industrial controls, where PC-like process controllers manage large machinery on a daily basis. Decades ago, these process controllers were simple state machines. In fact, they started as relay-based devices. Now, of course, they’re all processor-based. An article that appeared this week on the Control Engineering Europe Web site and written by Christian Lequeux of C L Consulting SPRL discusses such a process controller used to revamp decades-old cold-rolling steel mills in Europe. These steel mills produce metal sheets, strips, bars, and rods from very large coils of sheet steel. The processes are continuous and to keep the steel flowing, one coil is quickly welded to the trailing edge of a preceding coil before it disappears into the rolling mill’s maw. A loop in the uncoiled sheet provides material while the trailing end of the coil is held in place long enough to make the weld and the welding must be completed and sufficiently cooled before the loop is used up. The resistance welding is done on the fly and involves the fast application of a lot of pressure (as much as 30 tons) and several thousand amps of ac current delivered through thyristors to the welding electrodes. If the weld quality is inadequate or if the timing between the welding operation and subsequent strip feed into the mill equipment is off just a little, the steel strip from the new coil will tear away and tends to recoil and wrap backwards around the steel coil. When this happens, the mill must completely shut down to correct the problem, sometimes for as much as 72 hours. Historically, about 2% of the welds fail in the older mills.

To combat this problem and to increase mill efficiency by as much as 20%, the mills are refurbishing their rolling lines by replacing drive motors, motor controllers, and welding control while keeping the heavy rolling mechanical machinery components rather than completely replace all of the steel-rolling equipment. The article in Control Engineering Europe discusses the use of a National Instruments CompactRIO controller to revamp control circuitry in several mills. The CompactRIO controller is based on an 800MHz Freescale MPC8377 PowerPC processor. These controllers can incorporate as much as 512Mbytes of DDR2 DRAM and an SSD as large as 4Gbytes. The controllers also include an FPGA that can be programmed with sensor and control algorithms using National Instruments’ LabView software. Using an FPGA to implement the control algorithms achieves the nanosecond response times required to meet the desired timing goals for the welding application.

There are few noisier (electrically and mechanically), dirtier, and thermally inhospitable environments than a steel mill, so the use of an SSD in this application isn’t surprising. That’s probably why the article is discussing the use of National Instruments CompactRIO real-time controller, which has an operating temperature range of -40° to 70° C. HDDs likely won’t last very long in such extreme environments. The CompactRIO controllers employ Ethernet connections to send overview information back to a central control PC located in an environmentally benign (at least relatively benign) environment, but most of the time, the controller is on its own.

Over the past four years, replacement of the mill-equipment motors and the revamping of the controllers cut the weld failure rate to 0.05%, which is 10x better than even newer European cold-rolled mills and 40x better than the failure rates observed in the older plants. Consequently, this is just one more application that highlights the benefits of improved control by 21st-century solid-state equipment.