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Smooth as glass

Article-Smooth as glass

Smooth as glass

Process integration and a change from electromechanical to hydraulic actuators let safety-glass processing machinery supplier Tamglass Tempering System Inc. cut production times to speed up shipping schedules. The change greatly simplified the design of a new glass bending system, eliminating 700 parts, reducing labor costs, and cutting assembly time. Motion controllers from Delta Computer Systems (Vancouver, WA) and hydraulic actuators from Parker (Cleveland, OH) were key components in the design.

For suppliers of curved glass panels for automotive, appliance, and architectural uses, the QSB (Quick Set Bend) Bending and Tempering System reportedly makes it easier to form glass with complex J- and S- shapes. Engineers increasingly use such curved glass panels in high-volume products such as automotive rear and side windows to balance both function and form. On the function side, for example, automotive engineers may boost fuel economy using cylindrical glass in place of flat glass to reduce the drag coefficient. While on the form side, vehicle designers can use glass with different radii to create distinctive styling cues.

Tamglass engineers eliminated 700 parts in a glass bending/tempering system by replacing electromechanical actuators with smart hydraulic cylinders. The system redesign also included integrating the oven, bending, and tempering stations into a continuous, computer-controlled process to reduce the cost of making curved glass panels.

The panels are typically produced in a three-stage process. First the glass is heated in an oven to make it pliable. It is then transferred to a molding station where pressure forces the glass to bend. Finally, rapid cooling with forced air tempers the glass.

"In our previous curved glass manufacturing systems," explains Jack Sandlin, Tamglass chief engineer, "the glass was forced to bend on rollers that were positioned mechanically using servomotors. We used clutches to apply power to ball screws, and brakes to hold the rollers in place."

The previous system wasn't as integrated as the new QSB system, which chains the oven, bending, and tempering stations closely into a continuous, integrated, computer-controlled process. Because the glass had further to travel, and more time to cool off in the old system, it had to be heated to higher temperatures to make sure it was still sufficiently pliable to be pressed into shape by the time it reached the bending station.

Consequently, the older system used more energy. "Locating the bending station immediately adjacent to the oven not only saves energy lost from cooling," Sandlin says, "but the new system uses gravity, rather than pressure, to bend the glass, which also saves energy and improves the optical quality of the glass."

To connect all the controllers to the actuators, and tie everything back to the industrial PC, Tamglass engineers chose Profibus, a high band-width, industrially hardened serial interface standard.

More than 100 axes. The simplicity of the new QSB system reduces the re-design and build time for a unit from ten to six months, according to Sandlin. And given that there are more than 100 axes on the machine, the labor savings of mounting a single cylinder instead of a half-dozen components on each really adds up. It also eliminates the hassle of aligning bearing assemblies, mounting motors, ballscrews, clutches, and brakes. The approach eliminates many small components, such as keys, set screws, and couplings.

In the QSB system, the hot glass passes out of the oven, moving on rollers across a curved roller bed, bending as it travels to conform to the shape of the bed. The rollers are mounted on a set of parallel rails (90 rollers on each of seventeen 12-ft rails), with the rolling axis of each roller perpendicular to the length of the rail.

To shape the roller beds to match the precise contour of the desired bend, two hydraulic actuators, one for each end of each rail, individually position each of the 51 roller rails in the system. This requires a total of 102 hydraulic actuators, each with at least a plus or minus 0.003-inch positioning accuracy.

As the glass bends to conform to the shape of the roller bed, it passes into the quench station, where it is sandwiched between two sets of rollers on two more sets of rails (17 roller rails above and 17 below). Air blown through channels mounted next to each roller cools the glass rapidly, tempering it.

"To control the motion of such a large number of axes to such tight tolerances, we needed specialized motion controllers," says Tamglass control engineer John Cecchine. In fact, engineers evaluated four different controller types over a period of six months before ultimately selecting Delta Computer Systems' RMC100 motion controllers. In addition to precise control, Cecchine says, "the controllers had the direct interface with position transducers and variable hydraulic valves that we were looking for, and was the only one that supported failsafe operation should any sensors lose power."

The controllers are mounted in intermediate control cabinets on the machine close to the actuators. "Using position feedback from each of the two actuators on each rail, the controller monitors the difference between the two cylinders while it is traveling, and keeps the position in line with each other," says Cecchine.

Graphical software development. Each controller can manage up to eight motion axes simultaneously, positioning to plus or minus 0.001-inch accuracy. To program, debug, and tune the controllers, Tamglass engineers used RMCWin, a graphical software development package from Delta Computer Systems that converts high-level motion commands into sequences of operations to be performed by the controllers. An industrial PC downloads the motion commands in bursts over Profibus into function tables that are contained in the motion controllers.

The PC performs overall system control and human machine interface (HMI) functions, and stores computer "recipes" that describe how each type of glass product is processed. The PC also controls the heat of the 60-ft-long oven and the speed of the glass traveling through it. "Each piece of glass takes three to five minutes to travel through the oven," Sandlin explains. "The computer tracks from 10 to 50 pieces of glass that may be in the system at any time."

According to Cecchine, computer control makes the glass manufacturing system very flexible. "The types of bends and sizes of glass material can be changed quickly and easily, reducing non-productive time and maximizing throughput."

The controllers support direct Profibus interfaces without requiring any special interface modules. The use of Profibus, a multi-master bus, solves the problem of connecting the industrial computer to multiple motion controllers simultaneously.

Profibus is daisy-chained between the PC and the array of motion controllers. Each motion controller has a unique address on the bus, which the computer references when it writes instruction sequences or interrogates status from that particular controller. "Using Profibus greatly simplifies system wiring compared to I/O-intensive point-to-point wired systems," explains Cecchine.

One of the first installations of the new Tamglass machine is at a glass manufacturer in Sweden. The new QSB system's flexibility allows the company to produce a wide range of glass panel sizes, up to a maximum size of 48 inches by 60 ft. "Fieldbus technology contributed to lower system manufacturing costs and easier system assembly and maintenance," says Sandlin. "It greatly simplifies all of the wiring, so we were able to install and get it up and running faster than any other system we have done."

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