Panel PCs handle extreme heavy lifting
Integrated control system helps Wheelift automated guided vehicles move turbine generators, transformers, and other massive payloads.
Global manufacturers in the power, energy, chemical, and construction fields have to manage and efficiently move ever larger products to remain competitive. In Greek mythology, Atlas shoulders the weight of the world and its heavens—making him one of the most famous heavy lifters of all time. Perhaps inspired by the feats of the primordial titan, engineers at Doerfer Companies (TDS Automation) in Waverly, IA, design and build Wheelift heavy transporters to combine brute strength with modern intelligence.
“During the initial engineering phase of our newest Wheelift heavy transporters, we reviewed PLC-based products to meet our demanding requirements, but we were limited by large housing dimensions and few available options,” said Mark Lavallee, a Doerfer controls design manager. Traditional PLCs were ruled out for the application, but a compact panel PC provided the best solution for a central controller, according to those involved.
Wheelift is a dynamic technology and a trademarked name used to describe Doerfer’s wheeled, heavy-capacity automated guided vehicles (AGVs), which are used to transport payloads ranging from 50 to 500+ tons—equal to more than one million pounds. That means Wheelift vehicles meet the heaviest lifting requirements for manufacturers of transformers, turbine generators, mining machinery, nuclear processes, and ships. Wheelift operators position the flexible transporter(s) beneath a load, raise the deck to lift it, transport the load wherever it needs to go, and then set it down.
The Wheelift line is an outgrowth of Doerfer’s existing AGV product offerings, which are well proven for handling payloads in the 600 to 10,000 lb range. While the transporters’ size and capacities are custom-designed to meet individual applications, all vehicles in the line are able to provide precise movement and 6 to 10 inches of lift. For example, Doerfer engineers recently designed three 57-ton-rated transporters to operate both singularly and in tandem. Those units have an 18.5 in. deck height with 6 in. built-in lift. Other examples include single transporters rated at 150 tons with a 21-in. deck height and 6 to 8 in. lift.
Precise, reliable control
“To stay competitive, Doerfer must provide absolute reliability at all times,” said Roy Linden, Wheelift account manager. “Any operational failure is very difficult to recover from due to the extreme loads involved. In addition, manufacturers are making larger modular components that join during final assembly. These demanding applications need a tight, reliable process for safely moving heavy, high value products.”
Doerfer’s Uniload wheel module is designed to meet those demanding requirements. It incorporates five key performance features:
Equalized three-point fluid suspension to minimize peak ground pressure loading values;
Omni-directional steering that provides 100% movement flexibility;
Very low deck profile, which provides increased usability and safety throughout a manufacturing complex;
Precision move capability (moves as small as .001 in.) to facilitate load placement and provide sub-assembly engagements that were previously impossible; and
Built-in lift for self-loading that eliminates additional expense for related machinery and work assignments.
The earliest designs of Wheelift transporters used black box PCs that were programmed in an assembly language. “Unfortunately, only a handful of experts across the U.S. could maintain and update the equipment,” Lavallee said. “That became a compelling reason to shift toward a more industrialized platform.”
Critical control needs
A critical requirement of the new controls platform was its ability to handle the updated hydraulic system.
The new platform had to have greater reliability and increased accuracy and resolution for the tightly integrated hydraulic and electrical servo systems. Beckhoff Automation provided the ideal alternative—a very small, but powerful industrial PC system, said Lavallee: “The platform easily fit our new Wheelift requirements for compact-size controls paired with speed, performance, and reliability improvements.”
The Wheelift team also needed a universally accepted IEC 61131-3 programming environment to handle the complex software of Wheelift’s Synchrosteer control. Beckhoff’s TwinCat PLC software gave them that foundation.
Ron Howell, electrical engineer at Doerfer Companies said, “We use many of the languages defined by PLCopen, and we favor structured text for this application because it is fully supported by TwinCat, along with many other programming languages.”
Howell said Doerfer decided to use the Beckhoff CP6201-0001 panel PC as an all-in-one control and display solution. “This helped to greatly reduce the space taken up by the control system. We also recognized a cost savings by having a powerful processor included in the display. The CP6201-0001 features a 12.1-in. screen and a built-in Intel Celeron M 1.0 GHz processor,” he said.
Craig Schmeiser, Wheelift’s program manager said, “We are always quite conscious of available cabinet space due to the low profile requirements for heavy transporters. We were able to assemble a highly efficient package with Beckhoff PC-based controllers, AX2000 series drives and AM3000 servo motors along with the combined HMI and industrial PC. The streamlined way the controls fit together helped us design a system that is as compact as possible.”
Depending on the Wheelift transporter, 8 to 24 servo axes are individually controlled using one Beckhoff CP6201. “Synchronous, coordinated motion would have been very difficult to accomplish on this scale using traditional PLC systems,” said Howell. “Our updates are 1ms for critical motion functions. There is plenty of room for additional functions when using EtherCat paired with just a 1 GHz processor. Beckhoff provides a scalable PC product line that provides a processing power boost if and when we need it.”
Bus terminals, networked with a BK1120 EtherCat Bus Coupler, are used as the I/O system for communication within each Wheelift transporter.
“The microsecond-level performance of EtherCat is impressive and the equipment is very easy to integrate, since it is based off standard Ethernet technologies,” Howell said. “EtherCat can expertly handle high speed motion control and work in parallel with many other fieldbus networks.”
The Wheelift heavy transporter is a self-contained system with an on-board engine powering a 480 V, three-phase generator, which supplies power for the system, including the Beckhoff components. The Beckhoff KL3403 three-phase power monitoring module gathers electrical information, including phase angles, voltages, watts, current, and hertz. “Knowing Wheelift power consumption at all times helps prevent potential problems before they arise,” said Howell.
One KL3403 terminal lets the Wheelift team monitor the power without converter units. “With large, clunky black boxes, I wouldn’t have been able to monitor power in the Wheelift. With a half-inch-wide KL3403 I/O terminal, I can easily,” Howell said.
Where possible, Wheelift also uses Beckhoff high density KM I/O modules (16-channel) for greater space savings in electrical cabinets. KM I/O modules can pack up to 64 digital inputs or outputs in a compact housing.
An Ethernet publisher-subscriber methodology is used for high speed communication among Wheelift heavy transporters. For example, if three Wheelift transporters are operating in tandem (tied together, each with their own processor), one vehicle can act as a leader with the other two functioning as followers.
Any vehicle can be the lead, depending on where it’s located in the system. That master transporter sends commands to the follower transporters, and the followers respond to relay the system status. “The real-time Ethernet capabilities allowed us to tightly synchronize the vehicles for high-speed operation,” said John Pullen, a Doerfer senior staff designer.
Establishing a connection between the Wheelift remote control system and the Beckhoff system using CANopen was a critical communications requirement. Doerfer received engineering support from Beckhoff to ensure that this interface via CANopen was “a resounding success,” Howell added. An additional benefit of the CP6201 panel PC was the easy integration of a CANopen fieldbus card, which drastically cut back on the required wiring for the system.
“Hardwiring would have required more space than I had available,” Howell stated. “With a traditional PLC, the resulting I/O rack here would have easily taken two or three times the amount of precious space. With this [I/O setup], we keep our cabinet as streamlined and optimized as possible.”
The processing power of the panel PC was more than up to the task of the multi-axis Wheelift systems, Lavallee said. “Even with all the deterministic motion, we’re still only using 27% to 30% of the total PC processing power. The open nature of the system also helps add new features and functionality without artificial roadblocks built in,” he added.
But increased system performance did not increase cost, according to Schmeiser: Beckhoff controls were up to 30% less expensive than the traditional PLC architectures. “Of course, the decision to move to Beckhoff technology was primarily performance-based and the cost reductions were secondary to our decision, but a welcomed benefit,” he said.
End-user feedback for the new Wheelift systems has been very positive. The company has received supportive comments regarding the new system’s ease of use, reliability, and maintainability. Linden attributes this in part to the use of TwinCat software, which “has allowed our programmers to send some very valuable and meaningful data back to the user in formats that are easily understood.”
Looking toward the future, the team is evaluating AX5000 series EtherCat servo drives, whose dual-axis drive variants could further improve cabinet space savings. Regardless, Schmeiser said he plans to use Beckhoff control system on all future Wheelift heavy transporters.
Maybe now old Atlas can take a breather while the Wheelift team helps carry the weight of the industrial world on its capable shoulders.
Edited by Renee Robbins, senior editor , Control Engineering. She can be reached at firstname.lastname@example.org .
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