Figure 1: The Andritz Herr-Voss Stamco leveler, one stage in the Hagerty Steel cut-to-length line. Virtually all the machine hardware was functional; only the motion control system needed to be retrofitted.

Electronic controls and software synchronize motion for steel processes

June 2015 - Synchronizing a process for the highest productivity and quality output often requires precise electronic control of multiple motion axes simultaneously. This is difficult to achieve if each axis is responding to control inputs differently.

Consider a processing line where a sheet of heavy-gauge steel up to 1/2 inch thick is pulled off a 25-ton coil, flattened and then cut by a hydraulic shear to make sheet stock of a desired length. Hagerty Steel & Aluminum Co., East Peoria, Illinois, flattens steel by passing it 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 step represents a different task, each 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 (See Figure 1).

“A 15 hp motor can go from zero to full speed a lot quicker than a 300 hp motor,” said Thomas Boon, Hagerty Steel’s maintenance manager. “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 offline.”

Hagerty Steel’s heavy-gauge cut-to-length line had been upgraded three times over the years, but it was not cutting sheets precisely. “One of our customers requested that we tighten our tolerances but, with the old system, it was impossible,” Boon says. So the company called in Advanced System Integration & Control Inc. (ASIC), West Chester, Ohio, an integrator with many years of experience in machine retrofits, to upgrade the control system.

ASIC’s team started by gaining a thorough understanding of the existing equipment and the processes to be controlled. “There were blueprints available but they were a combination of drawings from several upgrades that had been done over the years. The prints didn’t accurately portray the actual wiring spread across several control cabinets,” says Glenn McIntyre, ASIC’s controls engineering manager. “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.” 

Figure 2. Diagramming machine axes controlled by the Delta RMC 150 motion controller. Not shown is an encoder on each motor that provides position/velocity feedback directly to the RMC 150.

Master and slaves

ASIC then set about to employ a new control strategy for achieving more precise coordination and control of the various motors. Hagerty Steel needed an electronic motion controller that was capable of performing complex multi-axis synchronization tasks. Based on experience with the upgrade of another machine at Hagerty, ASIC’s engineers ascertained that an RMC controller manufactured by Delta Computer Systems Inc., Battle Ground, Washington, could do the job. 

Delta’s RMC 150 eight-axis motion controller has several built-in multi-axis gearing, camming and synchronization functions that enable multiple slave motion axes to produce tightly controlled motion in relation to that of a master axis.

ASIC software engineer Mike Hiett set up the Delta RMC 150 unit to control the three DC motors, each with its own PID control loop yet programmed to function as slaves to the master motion of a measuring roll. The measuring roll, which precisely tracks the length of the steel sheet being cut, was set up as a “virtual axis,” since the roll is not being driven by a motor, but is instead tracking the sheet length.

Through a PLC interface, the machine operator specifies a cut length that translates to a command for the measuring roll to rotate to a specific position that describes a set distance. The motors, which are slaved to this virtual axis, are given velocity commands to drive the steel forward. There’s a rotary encoder on each motor that provides feedback to the motion controller, enabling the RMC to regulate the motor’s RPM and to make sure the motors run together.

At the same time, the RMC performs open-loop control of a pneumatic brake on the supply roll spindle via a proportional valve in response to operator commands to hold tension on the material as it is fed into the cut-to-length line. 

Tuning this complex system was initially challenging for the ASIC team. “Tuning one motor drive with no load is not a hard thing to do,” says Hiett. “But to get the three drives to move in a desired fashion independently of each other in synchronization required a lot of work.”

McIntyre explains the requirements. “As you decelerate to a stop, you want the cut length to fall right in the distance window without having to reverse the motors, because moving backward really slows down the operation of the machine. You want to stop right on the spot.”

While testing the controllers, the unrolled material would initially blow past the set point, then reverse direction and pull past the cut length in the other direction in a never-ending oscillation.

“I’ve tuned a lot of drives, and this tuning is a lot different from what I have done before,” McIntyre notes. “Typically I turn the gains down if there’s an oscillation but, in this case, the oscillations got worse when I did that.”

Figure 3. The Delta RMC 150 multi-axis motion controller can control and synchronize up to eight motion axes simultaneously.

Plotting tools

With help from Delta Computer Systems’ engineers and by analyzing plot information obtained using Delta’s tuning tools, Hiett increased the proportional gains and the system response improved substantially. Additional tuning to reduce rapid changes in velocity got the system to settle down quickly. 

“During a phone and internet collaboration session with the Delta engineer, we used Delta’s plotting tools to display the motion,” Hiett recalls. “We would make adjustments to the control loop gains and then run another piece of steel on the line while Delta’s engineer watched the results via a live Internet connection.” The Delta engineer studied the three real and one virtual axis and suggested re-tuning each axis. 

Hiett says he also employed the “help” index in Delta’s RMCTools software. 

“Delta’s help information is the most useful that I’ve found among motion control vendors.” For example, Delta’s website has sample programs on how to communicate between their product and just about any PLC currently being used in the industry, “including several examples for the PLC that we were using on this project. 

The examples I downloaded easily saved me a full day of software development,” he says.

Hagerty’s Boon was trained to program the Delta controller in the event that software maintenance is required. “I took the Delta RMC technical training in St. Louis,” he says, adding, “I found programming the controller to be easy, and I’m comfortable making software changes when needed.” 

With Hagerty’s controller upgrade, quality issues were resolved. “Whereas the existing line was able to produce steel sheets to within tolerances of only +1/4 inch—and out-of-tolerance material was wasted—we were ultimately able to tune the machine to achieve the required dimension to within 1/1000 of an inch,” McIntyre says. “And we were able to get the system to settle down quickly so our customer could fire the shear more quickly and increase their production rate.” 

The engineers involved in solving Hagerty’s problems suggest other machine operators using outdated motion controls would be well advised to assess the quality and productivity gains that might be obtained by using a multi-axis controller with built-in synchronization capability.