If you recall the old space shuttle launches, you likely remember the painstaking process of moving the shuttle out to the launch pad atop a slow-moving transporter. Though the space shuttle program has come to an end, heavy-duty transporters remain a key component of NASA’s spacecraft assembly and transportation.
Doerfer Companies manufactures the Wheelift Self-Propelled Modular Transporters (SPMTs) used by NASA’s Space Launch System (SLS) program. These SPMTs are put to work in NASA production facilities, such as the Michoud Assembly Facility (MAF) in New Orleans, which houses more than 1,870,000 sq ft of assembly and manufacturing space. At MAF, core stage components for NASA’s SLS rocket program are assembled which, when completed, comprise a rocket capable of carrying the highest payload mass in history. Fully assembled, the SLS measures 322 ft high with a launch weight of 5.5 million pounds and a payload capacity of 77 tons.
Key components of the SLS produced at MAF, such as the tanks for holding liquid oxygen and liquid hydrogen, can weigh hundreds of tons. Large sections of these tanks must be moved throughout the assembly and manufacturing processes at MAF and, when complete, moved out of the manufacturing facility via SPMTs and cranes for packing and shipping to the launch site.
Since NASA began using Doerfer’s Wheelift vehicles—which measure less than 22 inches high, can move loads of more than 500 tons, and pick up heavy materials as low as 20 inches—the space agency has avoided new infrastructure investments at its manufacturing sites. The previous material handling equipment used by NASA did not allow for the rockets to be transported through the assembly facility openings. The Wheelift, however, helps NASA move the components as necessary without having to replace or heavily modify its exterior doors. In addition, getting a Wheelift system up and running does not require months of time for installation and start-up, as the previous systems did. Doerfer says it can prep, load, and run the Wheelift SPMTs on NASA’s plant floor in a week or less.
In many NASA applications of the Wheelift, multiple vehicles are often coordinated to pick up large loads or multiple points of especially heavy loads. Enabling this level of precise coordination among vehicles required Doerfer to develop a control system for the SPMTs that can gather continuous feedback to maintain precise alignment of the load.
“It is challenging to maintain coordination of multiple SPMTs balancing a support fixture over difficult ground conditions and elevation changes on plant floors and pathways to shipping locations,” says John Pullen, principal controls designer at Doerfer Companies. He says Doerfer manages these requirements through the use of heavy-duty servomotors and tires, as well as an advanced automation and control system that can keep up with constantly changing conditions. A PC-based control platform from Beckhoff Automation is a core component of Doerfer’s system to ensure that the Wheelift can “adjust as needed according to feedback from the support fixtures and expertly reposition the SPMTs as needed,” Pullen says.
To coordinate these complex, multiple SPMT maneuvers, Pullen says Doerfer relies on Beckhoff’s TwinCAT 3 automation software to designate one SPMT as the master for the vehicle group, with the others following as slaves in an object-oriented control architecture. Each of the SPMTs operate via the TwinCAT 3 PLC runtime, loaded on Beckhoff CP6201 Panel PCs with an Intel Core2 Duo processor.
“The master PLC, running on one core, 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.”
Highlighting the move toward embedded computing in industry, the newest generation of Wheelift SPMTs created by Doerfer accomplishes these tasks via a DIN rail-mounted Beckhoff CX2030 Embedded PC that features 1.5 GHz Intel Core i7 dual-core CPUs.
“When we began integrating CX2030 ePCs, we increased our software performance because we added so much processor capability,” says Ryan Canfield, controls engineer, Doerfer Companies. “Core isolation in TwinCAT 3 was an important consideration for us, because it allows us to dedicate Core 1 for TwinCAT and assign Core 0 to handle the Windows OS and InduSoft HMI software. This provides considerable benefits for maximizing processor efficiency and making our control software even more robust and responsive.”
To read more about using a CPU’s multiple cores for distinct automation tasks, see this Automation World article and associated video about Beckhoff’s multi-core centralized machine control.
Canfield also notes how important TwinCAT3’s source code and version control features are to the SPMT operation. “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,” he says.
This feature really comes into play with the Wheelift vehicle grouping applications for the NASA assembly projects. With TwinCAT 3, Doerfer has been able to create vehicle objects that are grouped together. “We program Wheelift code using object-oriented extensions of IEC 61131-3 in TwinCAT 3,” Pullen says. “This supports Doerfer in the creation of highly complex features. It’s like giving the objects a starting point and then allowing the parameters to function. The objects can then do the rest on their own. This approach is rather elegant and requires minimal engineering effort.”
“Before taking advantage of the code management tools in TwinCAT 3, software system updates via a large merge [on a project like this] could take nine to 10 hours; today, the update time is well below half an hour,” Canfield adds.
On the networking side of the SPMT’s automation system, EtherCAT serves both as the I/O and drive bus. This allows for sub-millisecond level communication times and flexible connectivity to other bus systems. For example, EL6751 CANopen master terminals are used on the SPMTs to establish connectivity for engine diagnostics and inverters for battery systems, as well as to connect to the radio control interface used by Wheelift operators.
Safety functionality has been added to the SPMT’s I/O system via EL6900 TwinSAFE logic terminals. These I/O terminals have an integrated safety PLC and communicate via EtherCAT. The terminals are connected to hardware e-stop buttons and are also activated by 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.
With as many as 24 axes per vehicle requiring control on an SPMT, motion control for Doerfer’s heavy-duty Uniload wheels are handled by Beckhoff’s AX5000 EtherCAT servo drives and AM3000 servomotors. “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 says. “In addition, the load distribution algorithms programmed by the Doerfer team in TwinCAT 3 distribute the load throughout all of the present cylinders on the Wheelift,” even when driving over bumps or if the load is off-center. In such cases, the Wheelift reacts accordingly and distributes the weight evenly, removing any heavy spots or slip points while transporting materials.
