DN Staff

May 29, 2009

3 Min Read
Panel PC Control System Lifts Heavy Loads with Precise Positioning

Wheeledtransporters that move loads greater than 500 tons are leveraging controltechnology to combine heavy lifting with intelligent operation and extremeprecision. An HMI/industrial PC-basedsystem, plus CANbus networking for Ethernet connectivity, is producing moves assmall as .001 inch to facilitate load placement and sub-assembly engagementsthat previously were not possible.

Combining brute strength with astate-of-the-art control architecture, WheeliftTM heavy transporters developedby the Doerfer Companiestackle payloads in the 50 to 500+ ton range. Applications include very heavy products such as transformers, turbinegenerators, mining machinery and nuclear processes.

While transporter size and capacities are custom-designed tomeet individual applications, engineers at Doerfer recently designed three 57ton-rated transporters to operate both singularly and in tandem. The units have an 18.5-inch deck height with6-inches of built-in lift. Operators simply position the flexibletransporter(s) beneath a load, raise the deck to lift it, transport it whereverit needs to go and simply set it down.

After looking at PLC-based productsand developing early designs that utilized black box PCs, thinking shiftedtoward a more industrialized approach. Criticalrequirements for the new platform included greater reliability, increased accuracyand higher resolution for the tightly integrated hydraulic and electrical servosystems.

According to Mark Lavallee, a controls design manager for Doerfer,a very small, powerful industrial PC system provided an ideal alternative. "This platform easily fit our new Wheeliftrequirements for compact size controls paired with speed, performance andreliability improvements," Lavallee says.

The Wheelift team also wanted an IEC61131-3 programming environment to handle the complex software for unit'sSynchrosteer(R) control. RonHowell, an electrical engineer at Doerfer, says that TwinCAT PLC software fromBeckhoff Automationprovided that foundation. "We use manyof the languages defined by PLCopen, and we favor Structured Text for thisapplication, because it is fully supported by TwinCAT along with many otherprogramming languages," he says.

Howell says they decided to use theCP6201-0001 as a compact, 'all-in-one' control and display solution which reducedthe space taken up by the control system. The unit features a 12.1-inch screen and an Intel(R) Celeron(R)M 1.0 GHz processor.

The transporter operates a self-contained system with anon-board engine powering a 480 volt, 3-phase generator which supplies power forthe entire system. A three-phase power monitoring module gathers electricalinformation including phase angles, voltages, watts, current and hertz tocontinuously monitor power consumption.

Depending on the Wheelifttransporter, 8 to 24 servo axes are individually controlled using the CP6201. Howell says that the synchronous,coordinated motion would have been very difficult to accomplish on this scaleusing a traditional PLC system. "Our updates are 1 msec for critical motionfunctions. There is plenty of room for additional functions when using EtherCATpaired with a 1 GHz processor," he says.

An Ethernet publisher-subscribermethodology is used for high speed communication between Wheelift transporters.For example, if three transporters are operating in tandem (tied together, eachwith their own processor) one vehicle can act as a leader with the other twofunctioning as followers. The master transporter sends commands to the followertransporters, which respond to relay system status.

Real-time Ethernet capabilities allowed Doerfer to tightlysynchronize the vehicles for high speed operation. Using CANopen was a critical communicationsrequirement to establish a connection between the Wheelift remote controlsystem and Beckhoff controller. "But even with all the deterministic motion,we're still only using 27 to 30 percent of the total PC processing power, Lavalleesays.

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