News

PC-based control and EtherCAT synchronize operation of multiple heavy transporters to move Space Launch System components over long distances

As the world’s most respected and best-known organization for scientific discovery and innovation, NASA is staffed with preeminent engineers of all disciplines – experts who literally conquer the impossible and render it possible. However, even the originator of the much-appropriated term “moonshot” sometimes needs a little help to bring epic plans to fruition and boldly go where no one has gone before. This is why NASA relies on Wheelift® Self-Propelled Modular Transporters (SPMTs) for moving rocket components as they are readied on terra firma so the spacecraft can take over at liftoff to carry astronauts, supplies and satellites out of Earth’s atmosphere. The Waverly, IA location of Doerfer Companies, a leading global supplier of process automation solutions, manufactures the Wheelift heavy transporters used today by NASA. 

Large NASA production facilities, such as the Michoud Assembly Facility (MAF) in New Orleans, are among the largest manufacturing facilities in the world, with over 1,870,000 sq ft of floor space for assembly and manufacturing space that NASA shares with commercial aerospace and U.S. government contractors. A major activity at MAF is the assembly of core stage components for NASA’s Space Launch System (SLS) rocket program.  NASA is working to finalize the SLS, which will be the most powerful rocket ever built, capable of carrying the highest payload mass in history. When fully assembled, the SLS measures 322 ft high with a launch weight of 5.5 million pounds and boasting a payload capacity of 77 tons*. Needless to say, moving these sizeable rocket components around on Earth is no small feat. “This is the world’s most powerful rocket and it can’t go anywhere without Wheelift,” said Chris Bramon, Operations Discipline Lead Engineer (DLE) at NASA Marshall Space Flight Center in Huntsville, Ala. as he explained the importance of the transporters to the SLS program.



 Key components in the assembly include the tank for liquid oxygen (LO2) and the largest section of a rocket, the liquid hydrogen (LH2) tank which holds the fuel. Representing comparatively smaller rocket components, intertank manufacturing is also handled at MAF. The manufacturing of these huge rocket components, which can weigh hundreds of tons, involves welded assemblies containing machined and formed panels and rings. Large sections of the various tanks must be moved throughout assembly and manufacturing processes; when the tanks are complete, they are moved out of the manufacturing facility via SPMT equipment and cranes to be packed and shipped to the launch site, typically by sea on a barge. 

Since January 2014, Wheelift SPMTs have successfully addressed the complex and   extreme material handling needs of NASA, providing the ability to transport rockets and components from site to site. “We even move equipment at the vertical assembly building at Kennedy Space Center in Florida,” states Tom Phillips, Wheelift Business Manager, Doerfer Companies. “In the New Orleans facility, NASA uses the SPMTs for projects other than the direct rocket stages as well. Also part of normal daily work is the movement of large tooling equipment and CNC machinery for manufacturing and assembly processes.”  

NASA rockets hitch a ride with heavy transporters

Taking the heaviest material handling challenges in stride, and moving loads that can weigh over 500 tons is nothing new to Doerfer’s successful Wheelift product line. In addition, Wheelift SPMTs have the ability to pick up heavy materials under very limited heights — as low as 20 inches. Fortunately, operator control of the Wheelift is not rocket science; it is primarily handled through a simple, very intuitive radio interface. If you can operate a simple video game, you have all the basic skills needed to maneuver a Wheelift. Also, instead of buying different specialized vehicles for every project, Wheelift users can buy multiple vehicles of the same type and link them together.

While one Wheelift is more than enough for most applications, NASA applications are much more elaborate. Multiple Wheelift vehicles must work together to pick up large loads and even pick up multiple points of especially heavy loads. In order to safely move such large pieces of the world’s most advanced aerospace equipment over distances as far as 1 – 2 miles, four separate Wheelift SPMTs are coordinated together, holding large fixtures to cradle the NASA rocket components. This required Doerfer to gather continuous feedback from the fixture that supports the various tanks to maintain precise alignment of the load. The four Wheelift SPMTs are each rated for up to 100 tons load capacity, and together move the rocket stage equipment onto a public road, over a levee, and onto a barge. The Wheelift SPMTs and rocket components then travel by barge to the next NASA facility. Wheelift transporters integrate Uniload® fluid equalizing suspension technology across every axle in the system, dramatically increasing capacities to a virtually unlimited weight. Each of the wheels on the Wheelift automatically hold their own share of the load and permit for omni-directional movement for limitless options for steering and positioning.

John Pullen, Principal Controls Designer at Doerfer Companies, explains the automation requirements necessary to successfully pull off this weighty balancing act: “It is challenging to maintain coordination of multiple SPMTs balancing a support fixture over difficult ground conditions such as bumps and other elevation changes in plant floors and pathways. Managing these loads also requires heavy duty servomotors and tires, as well as an advanced automation and control system that can keep up with constantly changing conditions. Through an advanced PC-based control platform from Beckhoff Automation, the engineers at Doerfer ensure that the Wheelift can compensate for this according to feedback from the support fixtures and expertly reposition SPMTs as needed.” 

Computational heavy lifting via PC Control

Doerfer dynamically sets the SPMTs to work together and share load or torque in different groups within a synchronized system. Hundreds of tons’ worth of rocket components must travel over production floors and pathways without damaging the surfaces, as these are likely not reinforced for such extremely heavy loads. The Wheelift system addresses this and other issues, designed such that it does not apply excessive force underneath the SPMTs or across the support fixture. This avoids imbalances and potential damage to the fixture and the fuel tank components in transport. The four SPMTs, working in tandem, must constantly maintain safe velocity and steering centers. “This required a significant architecture change for the Wheelift team, because we needed one controller to coordinate the work of four vehicles,” recounts Phillips. 

Leveraging TwinCAT 3 automation software from Beckhoff, Doerfer has designated one SPMT that works as the “master” for the vehicle group with the others following as slaves in an object-oriented control architecture. Each of the Wheelift SPMTs operate via the TwinCAT 3 PLC runtime, loaded on Beckhoff CP6201 Panel PCs with Intel® Core™2 Duo processor. “The master PLC, running in software on one core which is utilized to about 40 percent, performs all the calculations for each of the vehicle groups,” Pullen adds. “This includes equalizing the torque and steering centers, velocities, load distribution and cylinder height every 10 milliseconds.”  The newest generation of Wheelift SPMTs accomplish this via DIN rail mounted Beckhoff CX2030 Embedded PCs (ePCs) that feature even more powerful 1.5 GHz Intel® Core™ i7 dual-core CPUs. The HMI devices mounted directly on the Wheelift in these cases are Beckhoff CP29xx series multi-touch Control Panels.

“When we began integrating CX2030 ePCs, we increased our software performance because we added so much processor capability. We also expanded our debugging functionality, implicit checks, and timing of the program organizational units (POUs),” says Ryan Canfield, Controls Engineer, Doerfer Companies. “Core isolation in TwinCAT 3 is another important consideration; we dedicated Core 1 for TwinCAT and assigned Core 0 to handle the Windows OS and InduSoft HMI software. This provided considerable benefits for maximizing processor efficiency and making our control software even more robust and responsive.”    

Also key to Doerfer programming efforts for the Wheelift are source code and version control features in TwinCAT 3. “We no longer have to worry about code loss when managing the work of multiple programmers — we can more easily collaborate in teams and merge work from Project A into Project B, for example,” Canfield continues. “We have drastically reduced our manual programming efforts and streamlined our engineering process using source code and version control in TwinCAT 3.” 

Part of the challenge with the NASA projects was the requirement for Wheelift vehicle grouping. Doerfer now has vehicle “objects” in the programming that are grouped together. “We program Wheelift code using object-oriented extensions of IEC 61131-3 in TwinCAT 3. Specifically, we utilize object-oriented programming (OOP) methods and properties with a great success,” Pullen explains. “This supports Doerfer in the creation of highly complex features, but also allows our programmers to be abstracted from the process and fine details of the functionality.  Essentially, it’s like giving the objects a ‘start’ and the parameters to function, and letting those objects do the rest on their own. This approach is rather elegant and requires minimal engineering effort – a game changer for the Wheelift automation software framework.”

Any device, anywhere: EtherCAT sends all the right signals

Adding advanced functionality on the networking side, EtherCAT serves both as the I/O and drive bus in Wheelift SPMTs. This creates the potential for sub-millisecond level communication times and flexible connectivity to other bus systems. For example, EL6751 CANopen Master Terminals are used to establish connectivity for engine diagnostics and inverters for battery systems and to connect to the radio control interface used by Wheelift operators. Adding robust safety functionality to the I/O system on the Wheelift are EL6900 TwinSAFE logic terminals. These system-integrated I/O terminals have an integrated safety PLC and communicate over EtherCAT. The terminals are connected to hardware e-stop buttons, and also activate with any loss of radio communications. When the e-stop is engaged, TwinSAFE forces the drives to ramp to zero speed, and full power is removed, bringing the Wheelift to a halt. 

“EtherCAT is also indispensable for fault monitoring,” Pullen explains. “With the built-in diagnostic capabilities of the EtherCAT system, we can immediately point the operator to any specific cable or device in the event of any service or maintenance need.” In addition, Doerfer has implemented EtherCAT Automation Protocol (EAP) for inter-vehicle communication. “We’re easily getting 10-millisecond communication latencies between the Wheelift vehicles, and that’s all done through EAP,” Pullen adds. Wheelift vehicles in a group communicate wirelessly via radio equipment. The radios build their own network, so vehicles can communicate with any other vehicle, and the operator can easily take control via a handheld interface.

The automation system on the Wheelift, which can have as many as 24 axes per vehicle, must handle highly advanced positioning algorithms to successfully compensate for the movement of extremely heavy loads. Motion control for Doerfer’s heavy duty Uniload wheels on the Wheelift is handled by AX5000 EtherCAT servo drives and AM3000 servomotors from Beckhoff. “With single and dual channel versions of the AX5000 servo drives, the Wheelift can achieve resolution for servo axes and hydraulic axes down to a thousandth of an inch,” Canfield reports. “In addition, the load distribution algorithms the Doerfer team programmed in TwinCAT 3 distribute the load throughout all of the present cylinders on the Wheelift.” This remains true when driving over bumps or if the load is off-center; the Wheelift reacts accordingly and distributes the weight evenly, removing any heavy spots or slip points while transporting materials.  

Covering the first 22 inches of “lift off”

As you might expect, the tolerance requirements set by NASA for the Wheelift systems were exacting in every way. For the support fixtures that cradle the rocket sections, the Doerfer team was required to control rotation at plus or minus 4 degrees, and the Wheelift held that far lower, down to plus or minus 0.25 degrees. In addition, Doerfer held the maximum distance between the two fixtures to about 3/8ths of an inch. These impressive results were accomplished despite incredibly harsh testing conditions, which involved transporting up to 150 tons of material over uneven surfaces that would be difficult even for an unburdened car to drive over. In addition, the turns made in testing were far sharper than what NASA would do in their actual applications. “These outstanding precision results illustrate how the Wheelift team combined their world-class in-house engineering expertise with the Beckhoff portfolio of solutions to exceed the expectations of the world’s preeminent science and technology organization,” says Dennis Sowada, Regional Sales with Beckhoff Automation. “NASA can have full confidence knowing that their equipment is safely handled on the ground so they can take care of business in outer space.”

Since NASA began using the highly-adaptable Wheelift vehicles, which measure less than 22 inches high, the space agency has successfully avoided major new infrastructure investments at their manufacturing sites. With the previous material handling equipment at NASA, these enormous rockets could not be transported through the assembly facility openings, but the Wheelift SPMTs helped NASA avoid replacing or heavily modifying all their exterior doors at these sites. Conservatively speaking, this would have amounted to hundreds of thousands dollars in additional costs. The time and cost savings do not end there – getting a rocket-ready Wheelift system up and running does not require months of time for installation and start-up. In fact, Doerfer is able to prep the Wheelift SPMTs, load and run them on the plant floor for NASA in a week or less. Flexibility and efficiency in the PC-based automation software is just one enabler of this rapid deployment. “Before taking advantage of the time-saving code management tools in TwinCAT 3, software system updates via a large merge could take 9 - 10 hours; today, the update time is well below half an hour,” Canfield reports.  

The Wheelift engineering team at Doerfer takes pride in tackling large-scale heavy transporter projects that other companies would rather not touch, but they do not go it alone. The Beckhoff North America sales and applications engineering teams assist Doerfer every step of the way. “In successfully meeting all of the NASA application requirements, Beckhoff support of the Doerfer Wheelift team has been a crucial factor,” Pullen reports. “Beckhoff sales and application engineers have been very responsive, knowledgeable, and helpful as we solve these extraordinarily challenging projects.” As Doerfer’s Wheelift solution has NASA completely covered on the Earth, the SLS rocket assembly team can focus far higher, including on planned missions to send astronauts to explore an asteroid and eventually, all the way to the planet Mars.

Author:
Shane Novacek
North American Marketing Communications Manager
Beckhoff Automation LLC

For more information:
www.nasa.gov 
www.wheelift.com
www.doerfer.com 
www.beckhoffautomation.com

*Source: NASA Space Launch System (SLS) fact sheet 
http://www.nasa.gov/sites/default/files/atoms/files/sls_fact_sheet_final_508_0307215comp.pdf