For companies running cold rolling mills that produce metal strip for a multitude of uses—from razor blades, to cans for foodstuffs, to automobile fenders—monitoring the flatness of the mill’s output product is very important. Without careful control, the strip-stock can be elongated more on one edge than on the other or more at the middle of the strip than at the edges. The flatness errors, such as rippled edges on the strip, usually are visible only when tension is removed after rolling. Although such a defective strip often is unsuited to its intended uses and represents an economic loss to companies that run rolling mills, a nonflat strip can cause much worse problems. In extreme cases, the strip, which moves through the mill at speeds in excess of 1,000 ft/min (11.3 mph), can break, causing the mill to crash with a resulting loss of production and equipment damage costing many thousands of dollars.
Although experienced operators know how to tease mills into producing acceptable strip-stock, only the most highly skilled operators can accomplish the job without online flatness monitoring. Moreover, until recently, electronic shape-measurement systems were economically suited only to large rolling mills.
Considering the cost of sensors, electronics, cabling, and software, the price for such monitoring systems could exceed $1 million, which might have been appropriate for mills with price tags of $15 million or more excluding the cost of the shape-monitoring systems. But million-dollar measuring systems are wholly inappropriate for small mills that can cost as little as $3 million.
A step that further reduces the operator’s required skill level is the use of closed-loop control of the strip flatness. Automatic flatness-control (AFC) systems use the information provided by online shape-monitoring systems as input. In some cases, AFC is essential because only closed-loop controls can manipulate the mill’s adjustments quickly enough to maintain high-quality output since no human operator can react fast enough.
Simple in Concept; More Difficult to Execute
The basics of rolling-mill construction are not difficult to understand. Nevertheless, the characteristics that enable production of high-quality material are not trivial and can be subtle.
To reduce the thickness of the metal fed into them, mills squeeze the metal between pairs of horizontal rolls (Figure 1). In the mill business, the rolls that do the squeezing are known as work rolls. The metal pushes back with considerable force, which causes the work rolls to deflect or bow outward along their axes.
Contacting the first intermediate rolls are six (second) intermediate rolls with diameters approximately three times the work roll diameter. Around the second intermediate rolls are eight sets of backup bearings with diameters as much as five times that of the work rolls.
Mills whose output strip is significantly thinner and longer than their input strip usually pass the strip through several stacks, each of which is called a stand. Alternately, a single stand is used, and the strip is rolled back and forth in a series of passes. As the strip passes through the several stands or after several passes on a reversing mill, it grows progressively thinner. The ratio of the output-strip-to-input-strip length is called the elongation.
There are many ways to make real-time measurements of the elongation of metal strips moving at high speed. All approaches involve measuring tension distribution, which also is a measure of stress, strain, and elongation in the strip. If the strain is constant across the width of the strip, the elongation also is constant, and the strip will be flat when tension is removed. For best results, strain must be measured at 20 or more points across the width of the strip.
To measure strain, it is common to rely on a special sensor roller. As it travels along, the strip wraps over this roller and presses down on it. The sensor roller, located slightly downstream from the mill stand, only measures tension distribution.
Within the measurement roller, in most cases, are load cells, which may be magnetostrictive or strain-gauge varieties. The excitation voltage traditionally has been delivered to the sensors via slip rings. Similarly, slip rings traditionally have carried the sensor outputs to the signal-conditioning circuits.
A newer and more complex arrangement eliminates the slip rings by embedding sophisticated signal-conditioning circuits within the measurement roller. These circuits digitize the measured quantities and transmit the data via optical couplers to additional stationary electronics nearby. To completely eliminate slip rings, the power for the embedded sensors and signal-conditioning circuits can be magnetically coupled into the roller.
Ingenious But Problematic
Ingenious as they are, instrumented rollers have problems, among which high cost is paramount. Spare-parts provisioning and maintenance present additional challenges.
In contrast, the measurement roller used by UEI and Sendzimir contains no sensors or electronics and is supported along its length, typically at intervals of 2 in. or less, by assemblies that resemble old-fashioned upside-down roller skates—the kind with a wheel at each of the four corners. Except for a minuscule amount of up-and-down travel, these assemblies remain stationary; each one sits atop a load cell that measures the force exerted by the skate it supports.
As it passes over the measurement roller, the metal strip that the mill produces exerts a downward force on the roller, which transmits the force further down through the skate-like assemblies to a bank of load cells. Each load cell is connected to one channel of an eight-channel strain-gage signal-conditioning module mounted within a UEI PowerDNA Cube LAN-connected distributed data acquisition unit. The outputs of the multiple load cells represent a profile of the strain across the strip under test (Figure 3). A system for a 48-in.-wide mill typically uses 24 load cells and three eight-channel strain-gage signal-conditioning modules.
The hardware includes quick-disconnect screw-terminal blocks for wiring to the load cells’ strain-gage bridges. In addition, instead of keeping the load cells fully energized at all times, which invites problems with thermal drift, especially in the poorly controlled environment of the shop floor, the hardware applies full excitation to a load cell only when its output is being measured. The result is high measurement resolution because of the high excitation voltage without the usual offset-drift problems since, most of the time, the excitation voltage is zero and the average power dissipated in the load cells is greatly reduced.
On a four-high mill, one whose stands each have four rolls one above the other, the bearings that support the backup rolls that support the work rolls are mounted on jackscrews whose axes are vertical. Adjusting both of the jackscrews associated with a work roll varies the thickness of the output strip.
Changing only one jackscrew raises or lowers one end of the work roll, correcting the simplest kind of flatness problem in the strip-stock: elongation that varies linearly from one edge of the strip to the other.
The strip being processed is, of necessity, at least a little bit narrower than the work rolls. In addition, the distance between the work roll support bearings is somewhat greater than the roll width.
The strip exerts an outward force on the work rolls which causes bowing. The use of backup rolls mitigates the bowing but does not eliminate it.
Reducing the strip thickness by rolling generates heat, which causes the mill rolls to expand. Moreover, the temperature rise varies across the width of the work roll.
Generally, the hottest part of the strip is at its midpoint. This uneven heating opens the possibility of using liquid coolant to control the thickness variations along the width of the strip; by controlling the amount of coolant delivered to the strip, the strip temperature and the strip deformation can be controlled, resulting in improved stock flatness at the mill output.
The coolant has an additional salutary effect on the rolling process: It lubricates the work rolls. This method most commonly is used on four-high mills. Hydraulic cylinders mounted between the roll chocks also can be used to bend the rolls and control the strip flatness.
About the Author
Dennis Kraplin is director of hardware engineering at UEI. He has an advanced master degree in computer science and 10 years of experience designing data acquisition devices. Mr. Kraplin is co-author of the patents that cover PowerDNA Intellectual Property and the DAQBIOS Real-Time Protocol. United Electronic Industries, 611 Neponset St., Canton, MA 02021, 781-821-2890, e-mail: [email protected].