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Machine Networking Focus
More control system architectures find speed and precision benefits in networking solutions
Al Presher, Contributing Editor -- Design News, February 27, 2006
Machine control networking solutions are the buzz in the marketplace, but many companies are still on the sidelines in terms of implementing these solutions. Learn from your counterparts how four machinery OEMs are using the latest motion bus technologies for motion, logic, HMI and factory communications to drive the performance of new machinery designs.
Ethernet Brings Edge Banding to Leading Edge
Early fieldbus networking solutions didn't always provide the processing power required for complex, precision machinery. So when Günter Redeker, director for electrical design at IMA (www.ima.de/eng), looked at a pilot project redesign of their Novimat edge banding machines, he wanted to take another step toward the latest in PC technology.
New Machine Concept
These edge banding machines are high-performance systems that apply edges to sheet material such as chipboard or light building board. The Novimat Concept consists of modular and configurable processing units for milling and edge banding, trimming, finishing and smoothing. Several work pieces can be processed at the same time, with the work piece positions monitored and controlled precisely using continuous path control.
New Control Platform
For the redesign, IMA is utilizing EtherCAT technology from Beckhoff (www.beckhoffautomation.com). The older IMA woodworking machines used a Beckhoff PC controller with the DOS operating system, Lightbus automation technology and PC-based visualization and control. Each machine unit featured a Bus Terminal I/O station coupled via Lightbus, servo drives coupled via PROFIBUS for the actual processing, and actuators communicating via CAN.
Redeker says they turned to the new platform because machine installations are very complex with several thousand I/Os and more than 100 axes in total.
"In order to control these machines via fieldbus systems," he says, "we previously used up to four Lightbus strands that collected or transferred a wide range of data with the sampling rate required for the machine. This was the only way to achieve the required performance."
According to Redeker, a good deal of computing power was required for handling the large amount of data. Not only did the processor copy the data from Lightbus into the memory and back again, it also had to sort the input/output data according to the process image. With the EtherCAT solution, "the DMA controller deals with the data traffic between the Ethernet interface and the memory, resulting in a significant improvement for us."
Machine Developments
An additional benefit of the new system is its ability to do continuous path control. Work pieces can be tracked in the machine for precise control during the passage, or for synchronizing parts for flying machining. The work pieces move through the machine with a speed of up to 60 m/min, corresponding to 1 mm of travel and an associated potential deviation per millisecond. At 2 msec for continuous path control, the resulting inaccuracy over a machine length of 60m is already 2 mm which Redeker says is a borderline value, especially since the precision requirements of the machines are continuously increasing.
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He sees long-term potential in a full EtherCAT implementation in the machine, which enables the utilization of the latest, most powerful industrial computer technology. The largest machine lines currently require two computers: one for the user interface and one for real-time control. "In the future, we expect one PC to be sufficient even for these systems. Engineering and service will also be simplified if only one fieldbus has to be dealt with," Redeker adds.
Modeling, Simulation and Control All in One
Automating the process of centering a bearing ring on a rotating spindle in a cylindrical coordinate measurement machine is all about sensing and actuation. But when the goal is accuracies of 1½ to 2 microns, given different bearing sizes, weights and potentially oil on the parts, adapting to friction in the system is non-trivial.
All in One Solution
"With this application, even though it's a simple system, you have a lot of unknowns," says Dr. Thomas Kurfess, director of the International Center for Automotive Research at Clemson University. "Controlling to one micron with a PID controller is tough because of friction. Now it can be done using a standard motion control platform."
Working in conjunction with Timken Company (www.timken.com) and National Instruments (www.ni.com), Kurfess and his team reduced the need for manual labor with skilled people working on an automated system.
"The key is sophisticated software tools that allow us to do modeling, simulation and control all in one package," says Kurfess. "We can model and simulate what we think the system should do, but then it doesn't happen. You ask 'why not?' and update the model."
Need For Efficiency
By designing an active control system to automate the manual centering process, the goal is to reduce centering cycle time. With manual operation, the operator spends 15 percent of the measurement cycle time centering the part.
The automated system consists of a linear slide and a precision spindle including air bearings to improve precision and smooth the motion. An LVDT displacement sensor used as a measurement probe is mounted to the linear slide, along with a fixed pusher contact used to actuate the bearing ring.
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Design Challenges
Designers used the NI Labview Control Design Toolkit to design and analyze the higher-level control loops in the system and program a customized Kalman Filter for the noisy measurement probe output.
"Labview has always been great in terms of signal processing and sampling, but not in terms of control, says Kurfess. "But now Labview Real-time DSP- and FPGA-based targets allow you to rethink your controls approach. Time slicing and the ability to set software priorities is expanding what we can do. Ten years ago, we took a sample and processed it, and then did something on the analog output in terms of control. How fast you could go was dependent upon the computer, and you didn't have a lot of performance guarantees. With the new real-time targets, it's guaranteed and you can set software priorities."
Kurfess adds that using Soft Motion for this application made the system hardware totally transparent, seamless and easy to program. "The real-time controller runs in software, and all you need to do to switch to Soft Motion is change targets. Instead of running the software on the PC, you can change and run it on the DSP target."
Electronic Line-shaft Drives Print Finishing Machines
Centralized control, object-oriented software, digital networking and tightly-coupled, multi-axis servo control is the recipe for success Goss International is using on next-generation Pacesetter® 1100 print stitching solutions.
The new machine design replaces a traditional mechanical line-shaft approach with a sophisticated machine control network including tight synchronization between servos and high speed I/O capture across the entire network from any location.
Engineering Challenges
According to Dr. Atef T. Massoud, senior project engineer for Goss, implementing centralized control and coordinating the large number of axes in the system with a single position reference is what he identified as the major challenges in the project.
Since the Pacesetter® 1100 is a module-based machine that can be custom-configured to accommodate high volume production requirements, and can use from six to 40 feeder stations with a rated speed of 20,000 products per hour, systems can total up to 45 axes. However, the same motion control solution was evaluated and selected for other Goss print finishing machines of up to 94 axes.
For the application, Goss selected the SynqNet network and eXMP controllers from MEI/Danaher Motion (www.motioneng.com). Goss also chose Advanced Motion Controls (www.a-m-c.com) Digiflex series SynqNet Drives, rated up to 1 kW, and motors from Baldor (www.baldor.com). The drives were customized by AMC to include additional I/O.
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Centralized Control
Massoud says they used "object-oriented thinking" to implement the sophisticated multi-thread software architecture required for the machine and to develop the large number of servo axes. "One of the challenges was to implement the system using a centralized control approach," Massoud says. "We needed to create all of the control objects and states for each of the 20 axes commanded by a single eXMP controller."
A feature of MEI's Motion Programming Interface (MPI) is the ability to use and re-use motion programming code across platforms. For this application, Massoud says they defined the set of machine states and commands or functions that the axis can execute for each axis in the system. A particular axis can only operate on its own data and execute a pre-established set of control functions.
"That is how we achieved isolation with independent axes running on the same CPU, because each axis can only operate on its own data," Massoud says. One way the object-oriented approach benefited programming the system in C and maintaining the large axis count system was the ability to re-use code and methods operating on data objects.
Massoud says that, for example, he programmed homing/synchronization sequences and subsequently any axis can use those functions. But if axis 10 is using the homing synchronizing function, it is only operating on its own data. When a function is designed, it is available to all of the servos in the system.
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Communicating via Ethernet
A second challenge for the application was how to communicate the state for each axis to the higher level controller or PLC. With a change from mechanical line shafting to servo control, certain components remain on the Pacesetter® 1100 from previous designs, and one of those components is a PLC used for the machine logic functionality. By adopting Ethernet communication between the PLC and the eXMP, the approach reduced the PLC communication computation load while increasing communication bandwidth and speed. Plus, full diagnostic information could be communicated to the touchscreen HMI on the feeder stations.
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