In 1990 CBW Automation (Ft. Collins, CO) launched the fast retrieval robot for plastic injection-molded parts on the market, a carbon-fiber-reinforced arm that slides between the jaws of the injection mold to grasp the parts at the precise moment they are ejected from the molding machine.
That robot just got faster. After much redesign engineering, CBW released a new control system in June 2003 that decreases the overall cycle time from 800 milliseconds to 550 milliseconds—an impressive 30% savings.
"We're faster than gravity," says Jon Schauer, controls engineer at CBW, who points out that hundreds of milliseconds are wasted waiting for the parts to fall.
"We knew that if our robotic system was going to be competitive in the marketplace, we would have to produce a robot that worked faster than gravity could free-fall the part out of the mold to avoid any delay times."
By using a combination of high-speed technologies, Schauer explains that CBW can offer a system that can reduce the production cycle time by several hundred milliseconds. "It might not sound like much, but when full mold cycle times are approaching the three second mark, just shaving a few milliseconds off each cycle can produce several thousand more parts per day," he says.
The new control system, called Lumera, replaces the previous programmable controller and point-to-point I/O wiring. Lumera uses Bosch Rexroth Indramat servo drives tied into a SERCOS network, while all control is centralized on a Beckhoff TwinCAT industrial PC. Not only is the system faster, but the control cabinets are smaller in size, and it saves a lot more energy in the process.
Adopting the fiberoptic SERCOS communications bus for real-time motion control was another improvement that not only speeds up communication, it greatly reduces the amount of panel wiring. "In the time it takes a technician to build one cable for the electrical-based network, we can have the full SERCOS system cabled and up and running," says Schauer.
The biggest change is in the control system itself. CBW threw out the PLCs, motion controllers, and I/O modules, which occupied a lot of space inside the control panel, and replaced them with a single PC. The new control system uses National Instruments' LabVIEW software for the human machine interface (HMI) and Beckhoff's TwinCat PLC Open 6 1131 software along with Beckhoff's SERCOS distributed field bus components for machine and motion control.
"This allows the best of both controls system types," says Schauer. "By centralizing the controllers on one PC, all of the system control can be located in one place which allows all the variables to be interfaced within one controller without the need for extra communication protocols." The older multi-controller system was slowed down by the communication delays.
"Having the HMI, machine control, and motion control on one controller also reduces the cost of the system so we can offer our customers other features," says Schauer. "And PC controls have other inherent cost savings from the traditional PLC, like the ability to add features such as networking, database and report generation, and printing to the systems. "Also, we can take advantage of cammed synchronous motion that allow two axes to be moving at the same time," he says, which is something they couldn't do before.
At the bottom line for every machine control system is the scan time. For CBW, the PLC scan time—the amount of time it takes to go through one complete control cycle-was 20 milliseconds. With the PC, the same scan is performed in 2 milliseconds, an order of magnitude improvement.
A big challenge for CBW engineers was the IEC 61131 programming package on the Beckhoff TwinCAT system. The industry trend is to adopt open, standard platforms wherever possible. When it comes to programming languages, the IEC 61131 concept is a growing standard widely accepted in Europe but less so in the U.S., where ladder logic programming remains deeply entrenched. TwinCAT includes ladder logic as part of the IEC 61131 standard, but the language is not as refined as U.S. versions. Nevertheless, CBW accepted the challenge and switched to Sequential Function Chart.
Schauer admits the forced change of programming languages was equivalent to culture shock, but says the future is now more secure. "The IEC 61131 software platform should standardize our software development for the future," he says. "We won't need to change software as long as future controllers can be programmed in this standard."
It's Not Running on Windows
But one serious question remains: Most engineers who have any experience with Microsoft Windows know how unreliable it is, so how could CBW choose such a platform with it for its control system?
The truth is, Windows may appear to be running on the Beckhoff computer, but it's not controlling the system. The TwinCAT software includes its own separate real-time operating system that, in effect, bypasses Windows to perform the machine control. Every once in a while, when it is safe to do so, it gives a time slice to Windows so the HMI can run. With PC processors as fast as they are today, most operators would not be aware that Windows is not running all the time.
Faster Cycles: These plots of position vs. time show that engineers reduced overall cycle time from just under 800 mec to about 540 mec. They did so by speeding up communications and synchronizing the motion of multiple axes.
"When CBW did the research to choose a new control system we looked at a lot of PCs as well as PLC options," says Schauer. "We found the Beckhoff system was a good fit. It has not only a total machine control solution but also an open architecture on the software as well as the hardware, with a real-time operating system that runs on the PC's microprocessor and delegates time back to the Windows operating system."
Some engineers would think there is a great risk, moving away from large and well-established control vendors like Siemens and Rockwell, but Schauer doesn't agree. "Beckoff has been doing PC controls for over 20 years and has developed a specialized PC control system that can outperform most control systems on the market today," he says. "But while Beckhoff is specialized, it offers a full industrial controls product line that follows an open architecture on most of its control products. So to stay competitive in the open architecture controls platform marketplace, Beckhoff has found ways to stay competitive though pricing its products eco-nomically while still offering customers like us leading edge technology."
Contributing writer Michael Babb can be reached firstname.lastname@example.org.