Assembly in Action: Motion Control Software Coordinates Entire Line
March 23, 2007
Prettl Group (Pfullingen, Germany) manufactures a wide range of products, including wire harnesses for the automotive industry and control displays for home appliances. The company’s Prettl Electric facility in Greenville, SC, specializes in building wire harnesses used in automotive antilock brake and emission control systems. Engineers at the plant also design and build production machinery that is used in other Prettl locations, in places like Mexico, Hungary and China.
Central to the plant’s success is the use of automated wire harness production equipment to keep costs down and decrease production cycles. The company’s machines employ a modular approach to facilitate short changeovers and allow for continuous production improvements. To ensure that wire harness orientation, housings and clips are perfect every time, the company’s machines and production lines incorporate a rigorous series of checks for wire color, dimension and shape. Wires with any imperfections are quickly removed for rework or disposal.
In 2006, the company decided to create an improved pick-and-place section for its lines that would automatically remove all imperfect wires, ensure error-free quality control and establish consistent cycle times for the line’s injection molding machine. In choosing a controls system for the new machine, engineers wanted something they could seamlessly integrate into the Greenville plant’s existing wire harness production line. The controls platform also had to be adaptable enough to accommodate any new demands that might arise during the development process.
“Our engineering team wasn’t sure exactly how the machine would look at the end of the project,” says Prettl electrical controls engineer Ralf Opper. “At the beginning, we were mainly concerned with a conveyor control system that would take care of feeding wiring harness pallets to our injection molding machine. We decided later that we should also integrate our clip assembly machine into the system and improve communications throughout the line.”
Ultimately, the company decided to go with a CX1000 DIN rail-mounted embedded PC and TwinCAT IEC 6 1131-3 compliant software from Beckhoff Automation (Burnsville, MN). The CX1000 features a 266-megahertz Pentium MMX processor and has no moving parts. Flash memory is used in place of rotating hard drives, and a passive cooling module keeps the device from overheating without using a fan. Users have the option of choosing Windows CE or XP Embedded as the operating system. When TwinCAT software is running on a CX1000, or any other industrial PC, it operates independently from Windows in its own kernel, ensuring reliable operation in almost any industrial application.
“When using the IEC 6 1131-3 programming in TwinCAT, I’m essentially free to program the controls however I want,” Opper says. “With six programming languages to choose from, I can logically select the language according to the complexity and type of project.”
Opper says he also used the system’s conversion option to convert code from one language to another. “At one point, I converted a large amount of LADDER code into Instruction List using TwinCAT,” Opper says. “LADDER gets fairly messy with large programs, and with Instruction List I can keep things better organized in nice blocks of code. We can easily use the same programming platform and efficiently reuse code on future Prettl machine designs.”
With the help of Beckhoff control technology, the pick-and-place system is now networked via Modbus TCP/IP to the entire wiring harness production line. To identify bad parts in the line, Opper created a 3-bit register that shifted quality information to the pick-and-place machine via a Beckhoff BC9000 Ethernet TCP/IP Bus coupler. Opper can also remotely change the system’s programming on the fly through Modbus TCP/IP and establish controller-to-controller communication for all machines in the line through a secure online connection.
Although Beckhoff offers several training courses on TwinCAT programming at various proficiency levels, Opper says he was able to learn the workings of the system on his own with the occasional phone call to Beckhoff technical support. “Once you learn the main concept within TwinCAT and discover all the highly automated features, it’s an intuitive program to use,” Opper says. “The ADS communication to the CX1000 and other devices keeps everything seamless and simple to implement.”
According to Opper, the pick-and-place machine now ensures a consistent cycle time for the wiring harnesses, something that couldn’t be done previously, because it was operator dependent. It also ensures there are never more than six cables on a single production pallet, something else that had to be checked by human operators in the past. “The pick-and-place machine has definitely extended its influence to other machines in our line,” Opper says. “We can now ensure a constant molding cycle time, which is the most important thing in any injection molding application.”
For more on automated control systems, call 952-890-0000 or visit www.beckhoffautomation.com.