System integration: Wire harness maker finds flexibility with open controls


Motion control, Embedded control

The CX1000 features a 266 MHz Intel Pentium MMX processor and has no moving parts. Source: Beckhoff Automation

A pick and place machine powered by Beckhoff motion control software and embedded PCs establishes communication throughout production line. The Prettl Electric facility located in Greenville, SC specializes in wiring harnesses for the automotive industry. These harnesses are primarily used for anti-lock brake systems (ABS) and emission control systems.

By 2006, Prettl needed to implement a new pick and place segment for the production line that could automatically remove all imperfect wires, ensure error-free quality control and help establish a consistent cycle time for the line's injection molding machine. This project was part of a larger push to utilize automation systems more advanced than those used in the past.

Prettl engineers in Greenville design and build their own machinery as well as production machinery for other Prettl locations. These stand-alone machines usually find their way to Mexico, China, Hungary, Ukraine and Korea.

Ralf Opper, electrical controls engineer for Prettl, is never afraid to try new technologies and work with the best solutions available. With this new pick and place project, Opper sought a controls solution that provided openness and seamless integration into the existing infrastructure of the wiring harness production line. The controls platform had to be extremely adaptable to major change if machine and controls design took a radical shift.

"Our engineering team wasn't sure exactly how the machine would look at the end of the project," Opper said. "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."

Line components talk

Prettl wiring harness production utilizes a heavily modular machine line approach. They maintain short changeover to keep production costs down, decrease production cycles and allow for continuous production improvements. The wire harness orientation, housings and clips need to be perfect every time. Therefore, rigorous checks are made by the production machinery to inspect color, dimension, and shape. Wires with any imperfections are quickly removed from the process for rework or disposal.

Having already experienced positive results in testing, Opper chose to control the new pick-and-place system with a Beckhoff CX1000 DIN rail-mounted Embedded PC and TwinCAT IEC 6 1131-3 compliant software. The CX1000 features a 266 MHz Intel 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 Microsoft Windows CE or XP Embedded as the operating system. TwinCAT software, however, operates independently from Windows in its own kernel to ensure reliable operation in almost any industrial application.

The best programming language often depends on the application. Opper writes most of his code is in Structured Text, but he also works with Ladder, Instruction List, and Sequential Function Chart languages. TwinCAT is multi-lingual, allowing programming in six programming languages, so Opper can use any and all of them with one software license.

"At one point," Opper recalls, "I converted a large amount of Ladder code into Instruction List using TwinCAT.

"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."

The pick and place system is networked via Modbus TCP/IP to the entire wiring harness production line. The injection molding and wire harness clip assembly machines on the line create a digital handshake via Beckhoff I/O modules to the CX1000 on the pick and place machine. 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.

The pick and place machine features BK5120 I/O Bus Couplers and compact Beckhoff AX2500 servo drives that are networked to the CX1000 via CANopen. For remote I/O, Opper also uses Festo CPV1x-VI-IP-8 valve terminals and connects via IP Link to Beckhoff IE1010 8-channel digital inputs in IP67 protection.

As for cost of the Beckhoff IP67 I/O product, "The IE1010 solution was priced around $80, while alternate junction box solutions that would have been required to do the same job would have cost over $200," Opper said. "The Beckhoff IP Link option with the Festo valve terminals was more proof that the overall system is inherently open and flexible. When compared with the effort required to wire junction boxes to multiple PLCs over and over again, IP Link helped Prettl cut remote I/O wiring time by over 50 percent."

Consistent cycle time

The new pick and place machine made an immediate impact on Prettl's overall production. Prettl now keeps a steady and constant feed of wiring harness pallets into their machines. The new system ensures a consistent cycle time for the wiring harnesses, which couldn't be done previously because it was operator dependent. It also ensures that there is always the maximum of six cables in each pallet—an area that had the possibility of error because it relied heavily on operators' sight.

"The pick and place machine has definitely extended its influence to other machines in our line," Opper said. "We can now ensure a constant molding cycle time, which is the most important thing in any injection molding application.

"I can do programming changes on the fly without having to shut down production. With production always running full steam, even during the introduction of a major line component, we can continue to keep Prettl customers well-stocked and completely satisfied."

C.G. Masi , Control Engineering senior editor

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