A Lean Cut

July 18, 2005

7 Min Read
A Lean Cut

Control system architectures are taking a page from lean manufacturing. In the same way that Six Sigma and Kaizen principles are used to analyze manufacturing process steps to eliminate inefficiency, industry experts say that lean automation is finding ways to "collapse" control system architectures, streamline software development, and leverage the power of high-speed networks for automation control.

According to Graham Harris, president of Beckhoff Automation, we can look at lean manufacturing and say the same things about the automation side of the machinery. With automation technology, he says, every point of connection and steps that don't add value to the process can be reduced into one step. "Don't have a separate PC running the HMI. Don't have one controller handling motion and another controller handling sequencing."

Powerful, All-Digital Networking

According to Sal Spada, an industry analyst for ARC Advisory Group, trends that started 10 years ago with the first digital drives are now resulting in more powerful solutions driven by networking technology. "We are seeing a convergence of motion and I/O networks, and a reduction in the number and types of networks in complex machinery," he says.

Mike Miclot, Director of Commercial Marketing for Rockwell Automation's Automation Control and Information Group, says networks have been instrumental in integration of sequential control with motion. Adoption of open technology and motion lagged the sequential area in adoption of networks to replace hard wiring. But the trend to openness now is largely being led in the networking arena.

"The emphasis is not only on sharing a common hardware platform but also the development environment," Miclot says. "Users want to write logic and path planning in one cohesive environment."

In the past, he says, the predominant approach had stand-alone motion controllers tethered together using a network to a PLC doing the sequential operations on the machine. The machine builder had to play systems integrator to get the two to work together. Now, there is a convergence in the environment and a blending of skill sets in the control engineers at the OEM machine builders.

IEC 61131-3 Impact

Many vendors cite the emergence of IEC 61131-3, a programming standard that's been dominant in Europe and now gaining momentum in North America, as being partly responsible for this converging and blending. According to Rick Rey, a product manager for Bosch Rexroth, engineers are attracted to the common programming interface and the ability to program motion and I/O in a standard environment.

"What we see is commonality and open standards in PLC programming," Rey says, "where customers can take logic based on IEC 61131-3 programming languages and PLC Open motion function blocks into a PLC. Using these two open systems, end customers that are using systems based on IEC 61131-3, can take programming modules and import them into different PLC systems."

A stand-up pouch machine implemented a distributed, intelligent control system. Each of the machine's 10 independent servo systems is individually controlled by a servo drive (above) and a master PPC motion controller that synchronizes all operations, including five sealing motions, two cooling motions, and two web motions, followed by a cutoff and a stacking motion. (Image provided by Bosch Rexroth).

Key advantages of IEC 61131-3 are its inherent support for software encapsulation and reusability of code. Many agree that a key issue that new control architectures must continue to address is software solutions that meet the diverse requirements of software developers and maintenance personnel supporting software installations.

IEC 61131-3 provides an ability to create reusable, encapsulated code, which is simple to implement using function blocks. Plus, customers can develop a standard library of reusable function blocks.

"The user can always view the code behind the function block but typically just wants to know the inputs, outputs, and how to set up the capability," says Lisa Scanlan, R&D engineer for ORMEC.

Reusability is vital because the user can take a rotary knife function block and create a second one with little additional effort, Scanlan says. Because the function block is encapsulated, the user doesn't have to worry if it is using the same variables or rename variables to make it work.

ORMEC has developed function blocks for particular applications such as flying shear and rotary knives, or common application needs such as homing and registration control. An entire application can be wrapped into a single function block and provides the user predefined inputs, outputs, and a configuration screen that could be used, say, to set up product recipes.

Centralized or Distributed?

The debate about a trend toward centralized or distributed control quickly becomes a standoff with varying perspectives and the requirements of specific applications. Clearly, as microprocessors have become more powerful in both the main controllers and drives, more computing power is available than ever before and there are a variety of approaches for using that capability. Proponents of centralized architectures cite increases in processing power, the speed of networks, and the advantage of developing a single program as key reasons to use a centralized approach.

"With full bandwidth utilization networks, we're getting PCI bus type speeds on a wire," says Harris of Beckhoff Automation. "You can run 30 axes of servos and hundreds of I/O points and have response times under a millisecond. With the PC, you can run 32 axes of coordinated motion quite easily with response times of a magnitude faster than what you can do with legacy networks. Our question is, 'Why would I bother to put intelligence out at the drives?' Most control engineers will tell you they only want to write one program and keep one set of software maintained, which is a lot better than trying to maintain 10 different programs."

Harris says just the management of your application makes it easier to keep it centralized. And from a code efficiency point of view, it is easier to keep one master program rather than trying to synchronize a bunch of code in distributed, smart devices.

On the other hand, proponents of distributed control say that highly distributed control elements provide speed and autonomy. Says Rockwell's Miclot, "The more you can move control decision making out closer to sensing and actuation, the faster those decisions can be made and the speed of the overall system will improve."

Rick Rey of Bosch Rexroth adds that, in a transfer line application, users can have distributed control at each station and don't need a central control to control the entire line. He says "typically central controls can only handle a limited number of axes, and when you add an axis, you begin to lose performance in the central controller, depending on the number of computing power in the system's DSPs and central CPU. When you distribute the intelligence to the drives so that they are taking care of their own position and velocity loop feedback, even basic I/O with logic in the drive, now the overall central controller overseeing the whole line has more processing power for its own needs."

One additional advantage of distributed intelligence is safety features integrated into the servo drive. These features include safe standstill, safe operational stop, and safe velocity mode that can be implemented in the drives. If, for example, the operator is in safe mode and enters the area and needs to jog the axis, it will only jog at a slow rate of speed defined by a software parameter.

Control System Choices

Whether selecting centralized vs. distributed control, or from among the more than 15 motion control networks, engineers must focus on application expertise and the ability of suppliers to provide specific solutions to problems, Sal Spada says. There are many vertical machine sectors, but machine control with motion embedded in it remains solutions-oriented rather than a commodity business.

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