Print Press Goes Digital

Printing presses present a big challenge for designers of motion control systems: They run at very high speeds; there are many passes for various colors; and registration has to be very precise or images appear blurry. But the landscape may change now that RG Engineering (Virginia Beach, VA) has recently dropped a digital drive into one of the towers in its new eight-color printing machine and finally found a drive that keeps up to the speed required.

Like many printing press manufacturers, RG has used analog drives to meet strict requirements for speed and precision. Though the company has explored the possibility of switching to digital drives for around five years, they have not matched the speed of analog drives.

"In the torque mode, we need to respond in 86 musec," says David Ellingsworth, engineering vice president at RG. "Our applications are very high bandwidth, so it's important that we have a good response curve."

The problem with digital amplifiers is that they only sample the feedback loop every so often, while analog amplifiers run the loop continuously. This sampling causes delay in the system, and that limits loop gains.

Despite the downside, RG decided to add a digital drive to one of the towers in its new eight-color printing machine. This is a demanding test-since it's easy to tell whether the digital drive matches the speed of the proven analog products.

Ellingsworth sums up the test by saying, "We're working to change our production over to the new digital drives." In some ways, he's not excited about that prospect. "It's hard to change when you've been building equipment with the same line for 12 years. I know in my sleep what that drive can do." But the benefits make that challenge worthwhile.

Too good to pass

For starters, customers will find it much easier to set up and use the equipment. Digital technology also makes it easier for both RG manufacturing personnel and customers to replace motors. "With a digital drive, you can use a variety of motors. From a spare parts standpoint, that's great; we can put one drive on the shelf. We just plug it in, tell it what motor we're using and it's ready," Ellingsworth says.

Another benefit is that there's more of a future for performance improvements with digital products. Analog equipment doesn't get the same level of R&D as digital, which has far more players at every level in the production hierarchy.

That's perhaps the most important for RG's design engineers. The printing machines run huge rolls of papers, measured in weight by tons. Paper widths range up to 72 inches. As the paper rolls unwind, the paper runs through the press at around 2,000 ft/min. Splicers make it possible to run multiple rolls without stopping, so runs can continue for hours.

During this time, the colors must be printed to within 0.003 inch. That's pretty fine considering that the long runs of flexible paper must all be identical for the entire period of the run.

That's particularly difficult in the RG systems, since they eschew the registration marks and reading system employed by most other printing equipment. The company notes that the color movements are caused by differences in the tension of the substrate. "We hold tight tolerances by holding perfect tension," Ellingsworth says.

Among the motors in the system are 8- and 6-inch servos. An ac vector motor is used in place of the disk brakes used in some printing presses. Using the vector, which runs at 6,000 rpm, is efficient from a power standpoint.

It also helps the system hold consistent tension. "Brake pads change with temperature, so you have to keep compensating for that. When you let the brakes go, you also have a tension spike," Ellingsworth says.

That can be catastrophic. "With some runs, when you have a tension spike at the start, you lose the entire run," Ellingsworth says.

Ellingsworth has tried a few different servos, but their role is so critical that most don't work. "We tried one to save money. Now we're removing it and replacing it with Kollmorgen drives," Ellingsworth says.

Servo loops speed up

RG has used Kollmorgen's analog drives for more than a decade, so it's no surprise that it's this division of Danaher Motion that supplies the digital drive. The company's latest drives have a new software technique that lets them match the speed of analog parts.

While a digital drive can't run the loop as fast as an analog drive, it can do something no analog drive can do-speed up the feedback conversion using an observer.

An observer is a software algorithm that runs inside the drive.

It runs a model of the motion system side-by-side with the real system. It compares the output of the real system with the model system and forces the model to follow the actual. By doing this, the observer is able to calculate what's happening in the motor as soon as current is applied, rather than having to wait for the position feedback device. This speeds up the servo loops, allowing higher servo gains and ultimately better performance. By using an observer, the digital drive can more than compensate for the sample delay.

The digital ServoStar line has a number of networking connections, including CANOpen, DeviceNet, and Fieldbus. It also handles from 3 to 70A, which can be a big factor given the many different motors needed in printing equipment. "Some of the printing webs need a lot of torque; others don't need that much," says Joe Anzalone, regional sales manager at Danaher.

The drives also operate at 480 or 240V. That makes it possible to use smaller cables, which can simplify installation and repair.