Synchronizing a process for highest productivity and quality output often requires precise electronic control of multiple motion axes simultaneously. This is difficult to do if each axis responds to control inputs differently, and yet they all have to produce the same relative motion.
Hagerty Steel & Aluminum is a metals service center. From its modest start in the late 1800s, Hagerty, Hunter & Company served flour mills, grain elevators and distilleries in the Peoria, Illinois, area. In 1906, the company's name became Hagerty Brothers, and in 1928 steel was added to the company's offerings. In 2000, the steel division of Hagerty Brothers was acquired by Liebovich Brothers, a subsidiary of Reliance Steel & Aluminum in Los Angeles. Hagerty is ISO 9001:2008 certified.
Consider a manufacturing line where a sheet of heavy gauge steel up to ½-in thick is being pulled off of a 25-ton coil, is flattened, and then is cut by a hydraulic shear to make sheet stock of a desired length. In the process employed by Hagerty, flattening the steel, which has a memory and initially wants to coil back up, involves passing the steel through a series of rollers—a flattener and a leveler—that bend the material back and forth, followed by pinch rolls that pull the metal tight and push it through the shear. Each stage does a different task and is powered by a different size electric motor. The flattener is driven by a 125-hp dc motor. The leveler is powered by a 300-hp dc motor, and the pinch rolls are powered by a 15-hp dc motor (Figure 1).
“A 15-hp motor can go from zero to full speed a lot quicker than a 300-hp motor,” says Thomas Boon, maintenance manager at Hagerty Steel & Aluminum. “And, unless one takes into account the different drive characteristics, it’s possible to have one of the motors trying to move quickly and one moving sluggishly such that the larger motor will push or pull the smaller one, which can cause it to trip off-line.”
When Hagerty started noticing roll marks on the material in the summer of 2008, it knew it had a problem, explained Boon.
Hagerty’s heavy-gauge, cut-to-length line had been upgraded three times over the past few years, but it wasn’t cutting sheets precisely. “One of our customers requested that we tighten our tolerance, and with the old system it was impossible,” says Boon. To fix the problem, our company, Advanced System Integration & Control (ASIC) of West Chester, Ohio, an integrator with years of experience in machine retrofits, was called in to upgrade the control system.
The upgrade started with the ASIC team getting a thorough understanding of the old machine and the process to be controlled. “There were prints available for the old machine, but they were a combination of drawings from several upgrades that had been done over the years,” explains Mike Hiett, software engineer, ASIC. “We found that the prints didn’t accurately portray the actual wiring that is spread across several control cabinets, so we spent a week tracing the wires and integrating two new dc drives and the motion controller with two existing PLCs and the existing third dc drive.” Then the ASIC team set about to employ a new control strategy for achieving more precise coordination and control of the various motors.
The team needed an electronic motion controller that was capable of performing complex multi-axis synchronization tasks, and, based on experience with an upgrade of another machine at the Hagerty plant, our ASIC engineers knew that an RMC motion controller from Delta Computer Systems could do the job.
The previous application that ASIC did for Hagerty using a Delta motion controller was also a cut-to-length line. It was simpler, in that the entire line is run from a single motor, so there weren’t problems with coordinating the motion of three motors. The motor in that case was a hydraulic motor. The output from the Delta motion controller was fed to a servo control hydraulic valve for controlling the motion. The same encoder that was used on the measuring roll for that line is used on the new Hagerty line to regulate the cut length.