Panel PC Control System Lifts Heavy Loads with Precise Positioning

Wheeled transporters that move loads greater than 500 tons are leveraging control technology to combine heavy lifting with intelligent operation and extreme precision. An HMI/industrial PC-based system, plus CANbus networking for Ethernet connectivity, is producing moves as small as .001 inch to facilitate load placement and sub-assembly engagements that previously were not possible.

Combining brute strength with a state-of-the-art control architecture, Wheelift^TM heavy transporters developed by the Doerfer Companies tackle payloads in the 50 to 500+ ton range.  Applications include very heavy products such as transformers, turbine generators, mining machinery and nuclear processes.

While transporter size and capacities are custom-designed to meet individual applications, engineers at Doerfer recently designed three 57 ton-rated transporters to operate both singularly and in tandem.  The units have an 18.5-inch deck height with 6-inches of built-in lift. Operators simply position the flexible transporter(s) beneath a load, raise the deck to lift it, transport it wherever it needs to go and simply set it down.

After looking at PLC-based products and developing early designs that utilized black box PCs, thinking shifted toward a more industrialized approach.  Critical requirements for the new platform included greater reliability, increased accuracy and higher resolution for the tightly integrated hydraulic and electrical servo systems.

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 Wheelift requirements for compact size controls paired with speed, performance and reliability improvements," Lavallee says.

The Wheelift team also wanted an IEC 61131-3 programming environment to handle the complex software for unit's Synchrosteer^® control.  Ron Howell, an electrical engineer at Doerfer, says that TwinCAT PLC software from Beckhoff Automation provided that foundation.  "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," he says.

Howell says they decided to use the CP6201-0001 as a compact, ‘all-in-one' control and display solution which reduced the space taken up by the control system.  The unit features a 12.1-inch screen and an Intel^® Celeron^® M 1.0 GHz processor.

The transporter operates a self-contained system with an on-board engine powering a 480 volt, 3-phase generator which supplies power for the entire system. A three-phase power monitoring module gathers electrical information including phase angles, voltages, watts, current and hertz to continuously monitor power consumption.

Depending on the Wheelift transporter, 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 scale using a traditional PLC system. "Our updates are 1 msec for critical motion functions. There is plenty of room for additional functions when using EtherCAT paired with a 1 GHz processor," he says.

An Ethernet publisher-subscriber methodology is used for high speed communication between Wheelift transporters. For example, if three 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. The master transporter sends commands to the follower transporters, which respond to relay system status.

Real-time Ethernet capabilities allowed Doerfer to tightly synchronize the vehicles for high speed operation. Using CANopen was a critical communications requirement to establish a connection between the Wheelift remote control system 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, Lavallee says.