PC-based control simplifies complex, coordinated motion.
Imagine making 10 million steel automobile wheels per year, in several different profiles and sizes, all from single assembly lines. Hayes-Lemmerz AG plants in the U.S. and Germany do just that.
All wheel assemblies contain a disk and rim. Disks start as sheetmetal coil stock that are deepdrawn to shape with progressive shaping and punch-press tools. Rims also begin as coil stock cut to length and bent into circular blanks. Roll stands profile the rim blank into the familiar shape and the ends are welded. The two parts are pressed together, welded, inspected, and then shuttled to a dip prime. The entire process takes just 6 sec, all coordinated over a 600-ft-long production line.
The wheel line, from machine builder Fontijne Grotnes, Netherlands, (www.fontijne.nl) contains 57 servo axes, 42 synchronized to coordinated motion of the 6-sec production cycle. The rest are asynchronous feed axes. A proprietary control and PC-based system once managed the welding process along with downloading programs and generating set values while separate hardware controlled the hydraulics. No more.
A single Windows NT-based PC, running TwinCAT control software from Beckhoff, Minneapolis (www.beckhoff.com), handles the servo axes and 1,500 digital and analog I/Os with an address space of several-thousand bytes. The position controller software cycles through the 57 axes in 3 msec, using about 50% of the PC's CPU capacity — no hardware motion controller is involved. Beckhoff 's Lightbus fiber-optic network pipes set values, drive positions, and hydraulic valve positions, in real time, to and from the TwinCAT central position controller. Math describing the motion is done in PC-based Visual Studio in conjunction with a software PLC. The PLC programmer uses a function block command interface compliant to IEC61131-3.
"This centralized control scheme simplifies complex, coordinated motion," says Gerd Hoppe, president of Beckoff 's Minneapolis operation. "To do the same thing would require a PLC for sequence and several motion controllers, each for four to six axes. Coordinating separate controllers and program debugging would be difficult because subordinate microprocessors communicate with a PLC over a set of registers, a thin and incomplete information channel." Centralized PC control can also eliminate intelligent drives and attendant software, adds Hoppe. Digital speed commands may issue over Sercos while control voltages are sent across a field bus. Encoder data returns to the PC processor by Sercos.
Flexibility is another advantage. The rim-profiling operation, for example, requires that hydraulic controllers switch from position feedback (locates tooling to wheel) to force feedback during forming because excessive force could damage the tooling or overform the metal.
Ditto for the rim-seam welding step. Rim edges must be brought into contact before welding. But simple motion control won't work because of a relatively large tolerance on the cut length. For instance, if the two ends are 0.02 in. from target position because of excess length, a simple motion controller would boost motor current in an attempt to narrow the gap, which could damage the machinery or wheel. But switching the drive from position to force feedback when the two parts contact solves the problem.
A controlled current then fuses (butt welds) the rim edges. Obviously, heating the material to a molten state relaxes it. Holding force constant then would push the ends together and distort the rim. However, toggling back to position control when the controller senses a force loss from the welding operation prevents the unwanted movement. A PC makes such control-mode switching straightforward. In contrast, a typical hydraulics motion controller requires microprocessor development tools for programming.
PC control also streamlines the changeover process. A Visual Basic program, for example, loads wheel DXF files with dimensions from the MCAD program directly into the controller, thereby eliminating manual data entries.