Large NASA production facilities, such as the Michoud Assembly Facility (MAF) in New Orleans, are among the largest manufacturing facilities in the world, with more than 1,870,000 square feet 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. The most powerful rocket ever built, the SLS will be capable of carrying the highest payload mass in history. When fully assembled, the SLS measures 322 feet high with a launch weight of 5.5 million pounds, and a payload capacity of 77 tons. NASA uses Wheelift® Self-Propelled Modular Transporters (SPMTs), manufactured by Doerfer Companies of Waverly, IA, to move the rocket components.
Key components in the assembly include the liquid oxygen and liquid hydrogen tanks. Representing comparatively smaller rocket components, intertank manufacturing is also handled at MAF. 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.
Operator control of the Wheelift is handled primarily through an intuitive radio interface. 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 equipment over distances as far as 1 to 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.
Four Wheelift SPMTs are each rated for up to 100-ton 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® fluidequalizing suspension technology across every axle in the system, increasing capacities to a virtually unlimited weight. Each of the wheels on the Wheelift automatically holds their own share of the load, and permits omni-directional movement for limitless options for steering and positioning.
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 reposition SPMTs as needed.
Doerfer dynamically sets the SPMTs to work together and share load or torque in different groups within a synchronized system. Hundreds of tons 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.
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 operates via the TwinCAT 3 PLC runtime, loaded on Beckhoff CP6201 Panel PCs with Intel® Core™2 Duo processors. The newest generation of Wheelift SPMTs accomplishes this via DIN rail mounted Beckhoff CX2030 embedded PCs. The HMI devices mounted directly on the Wheelift are Beckhoff CP29xx series multi-touch control panels. EtherCAT serves both as the I/O and drive bus in Wheelift SPMTs for sub-millisecond communication times and flexible connectivity to other bus systems. The terminals are connected to hardware e-stop buttons, and also activate with any loss of radio communications.
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.
For the support fixtures that cradle the rocket sections, the Doerfer team was required to control rotation at ±4 degrees, and the Wheelift held that to ±0.25 degree. Doerfer held the maximum distance between the two fixtures to about 3/8".
Since NASA began using the Wheelift vehicles, which measure less than 22" high, the Agency has successfully avoided major new infrastructure investments at their manufacturing sites. Doerfer is able to prepare the Wheelift SPMTs, load them, and run them on the plant floor for NASA in a week or less.
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