To shape the corrugated panels used in prefabricated buildings, Behlen Industries passes long strips of steel through a series of dies. One of the most difficult parts of the operation is coordinating a step during the shaping process in which a flying shear cuts the panels to length, while punches drive through the metal to produce holes for fasteners. The hole-punching and shearing take place while the continuous steel strips pass through the dies at 8 inches/sec (20 cm/sec).
Plus there is the weight of the shear12,000 lb (5,500 kg)to consider and the need to accelerate it rapidly to match the speed of the steel before it slices through. Control of the process and the equipment isn't trivial.
When Behlen decided to replace its old hardwired controllers, it found plenty of suppliers that offered newer programmable controllers, but these vendors offered little flexibility, and none could integrate a new controller with Behlen's enterprise-wide software. Finally, the company's search for a systems integrator led to Advanced Measurements (Calgary, Alberta, Canada).
At the onset of the project, founder and Chairman Steve Conquergood and his engineering team spent many hours at Behlen, videotaping the production of corrugated panels, making measurements, and attaching sensors to moving equipment to gather acceleration data. "Here's what we faced," says Conquergood. "When the steel reaches a predetermined length, the controller 'fires' the shear. First, a motor must rapidly accelerate the entire shear mechanism to meet the steel's speed. Then, a hydraulically operated guillotine-like blade slices through the metal and quickly retracts. Finally, the entire shear mechanism must move back to its rest position. That action takes place in less than a second within about a foot of space along the production line.
"The punches work similarly, but there's no motion control involved in the application," says Conquergood. "The controller fires a punch, and as it goes through the metal, the moving sheet simply pulls it along on a rail. In a few milliseconds, the punch retracts and a spring pulls the mechanism back to its rest position."
Servo Motors Deliver
"The servo motor for the shear presented us with a challenge," notes Conquergood. "We worked with Danaher Motion to get the best servo motor and motor controller. They also provided the guide rails for the shear. The hydraulics required to operate the shear blade were all in place from the old system, and people at Behlen knew how to take care of that aspect of the system. Our system just had to supply a 'cut' command at the appropriate time to factor in the hydraulic system's performance. So between the capabilities at our company, and those at Danaher and Behlen, we made everything work well together."
The old controller used a quadrature encoder, which produces out-of-phase square waves, to track the position of the moving steel. Summing the counts let the controller "compute" a distance and properly punch holes and cut off sections. The encoder used a rubber wheel to make contact with the moving sheet of metal. Conquergood had doubts about using such a simple sensor, but it had worked well in the old system, so his team adopted it as the sole input of position information.
During the initial investigations, Conquergood worked closely with Behlen's managers and plant operators to ensure he understood their needs as well as their wish list of new features and changes from the old system. "We had to understand things from both an engineer's perspective, and from the operator's perspective. Then we had to carefully document the system requirements and agree on acceptance criteria. If we didn't know the goals that Behlen would accept, we'd put the project at risk. It was a challenge." The project took about four months.
The second lesson learned was that software makes for a flexible control system. Behlen's engineers wanted to avoid errors inherent in operator entry of key manufacturing information. So they chose to obtain much of the needed production data by downloading it from the company's project database, which in turn derived much of its information from design software such as AutoCAD. Then an operator could match the database information with the paperwork for an order and start the automated manufacturing process. Operators still needed to select jobs, review parts requirements, and so on; thus the new system will accept some operator inputs from a touch-screen built into the controller.
"Other system integrators understood how to control things and move things," says Conquergood. "But they couldn't download CAD information and bills-of-materials from a database and adjust manufacturing steps accordingly." The Advanced Measurements team used a combination of a host PC, standard add-in controller cards, and a LabVIEW-RT card and software. Three control processes run simultaneously on the LabVIEW-RT card, independent of the host computer. First, a control loop monitors the quadrature encoder's output and determines where to cut the panel undergoing production. Two other processes control the mechanisms that punch the half-inch holes, and the servo motor and the hydraulic system on the shear. Those three processes run independently in 2-msec, deterministic loops. "The customer gets the reliability of a hardwired PLC system and the flexibility inherent in a system configured with software," says Conquergood.
Functional Testing Key
After Advanced Measurements' engineers assembled the system, they performed func-tional tests in the company's lab to ensure it met Behlen's specifications. A function generator simulated the quadrature encoder's output, and by counting pulses, the software knew when to fire the punches and the shear.
After finishing in-house testing, two mechanical engineers installed the controller at Behlen during a scheduled week-long maintenance shutdown. The in-plant commissioning tests punched and sheared sheet metal to demonstrate the system operated properly and as expected. "It was the advanced software, though, that let us make minor modifications to the system after it arrived at Behlen's plant," says Conquergood.
The third lesson came as an offshoot from the overwhelming use of software rather than hardwired controls to operate the production equipment. The new controller includes a network connection so it can obtain design information. That same network connection, along with some additional software at the controller, makes it easy to transmit production information back to managers. Now, managers can get real-time insight into the quality of the panels produced, inventory use, and equipment scheduling. A company can integrate as many of these data-analysis tasks as it wants into its management systems, says Conquergood. As needs arise for other information, software will let system integrators adapt their programs to meet them.
Contributing writer Jon Titus can be reached firstname.lastname@example.org.