NASA upgrades cleanroom crane
Overhead cleanroom crane undergoes retrofit to meet lift height requirements of NASA?s Tropical Rainfall Measuring Mission satellite.
By Al Zorn and Dan Sullivan
In mid-1995, officials at NASA`s Goddard Space Flight Center in Greenbelt, MD, were faced with a difficult decision. To stay on schedule, its Tropical Rainfall Measuring Mission (TRMM) satellite required assembly in a Class 100,000 cleanroom outfitted with an overhead crane. Unfortunately, the only crane that could meet the lift height requirements of the TRMM project was in Goddard`s largest cleanroom which was unavailable due to other flight project work already in progress. The Space Flight Center had some decisions to make.
Like other satellites, the TRMM satellite is sophisticated and has state-of-the-art electronics equipment on board. Therefore, contamination control is a high priority. In order to ensure mission success, all systems are checked and tested in a cleanroom environment before being launched into space.
As a facility alternative, Goddard gave consideration to another smaller cleanroom. However, the lift height of the 5-ton (4,500 kg) crane in this facility was insufficient when a Hydraset is used. A Hydraset is a cylindrical hydraulic device 10 inches (24.4 cm) in diameter and 12 inches (30.48 cm) long that hangs directly from the crane`s hoist hook for precise handling and positioning of the lifted load. A Hydraset is often required and used during spacecraft integration to achieve accurate final positioning and alignment of various hardware.
The decision to upgrade
NSI Technology Services (Seabrook, MD), a division of Mantech System Engineering Corp. the Goddard support contractor responsible for safely handling spacecraft and components, was investigating various options to increase the crane hook height. Among other options, converting the existing crane to use flux vector digital controls appeared to be the most promising solution. This option would provide the ultra slow and precise handling capabilities, therefore eliminating the need of the Hydraset. The end result would be additional hook height for the TRMM satellite.
The retrofit
After gathering, analyzing and presenting extensive data and recommendations to Goddard management, approval was obtained. NSI`s Al Zorn, manager of recertification support, and Don Burger, senior engineer, proceeded to specify flux vector crane controls for the crane retrofit.
The flux vector controls they were considering utilize special software and microprocessor-based controls to operate the crane. During operation, the drive receives constant feedback from the motor shaft via an encoder, thus producing a closed-loop system. The drive then uses that information to make a series of split-second calculations and commands to safely control the load at all times. More importantly, flux vector drives offer infinitely variable speeds and micropositioning of loads. In short, flux vector technology offered exactly the kind of performance and accuracy needed for Goddard`s spacecraft integration activities.
Through competitive bidding, the retrofit project was awarded to Electromotive Systems, Inc. (Milwaukee, WI). This project provided a new trolley and motor with festoon power cables, new bridge motors and flux vector drive to control the hoist. Only cleanroom-compatible materials went into the specification of the crane control package. “We basically specified a whole new system — everything but the crane itself,” Zorn says.
The hoist drive on the crane consists of Electromotive Systems` Impulse VG+ flux vector crane control running at 6 HP (4.47 kW), 1,800 rpm, 460 V, NEMA Type C, ac electric motor. The hoist is rated at 10 feet per minute. The trolley is also controlled by an Impulse VG+ flux vector crane control. It is powered by a 0.5 HP (0.37 kW), 1,200 rpm, 460 V, NEMA Type C, ac electric motor. The trolley travels up to 30 feet (76.2 cm) per minute. The bridge motor is mated to the Impulse VG+ flux vector digital drive and uses an encoder for continual feedback.
Each of the Impulse VG+ controls allows Goddard personnel to control the loads at zero speed without setting the electric brake. This control provides precise positioning to 0.002 inches and changes in direction without going through the transition of applying and releasing a mechanical brake. The digital microprocessors allow extremely slow-speed operations, even load float, for extended periods without motor overheating — a problem common in other crane controls.
Operator control of the crane is via a cart-mounted control station comprised of joysticks, pilot light, mushroom-head on/off switch and digital. The control components are housed in a custom-made box that is mounted to a wheeled cart and connected to an overhead festoon system. This allows operation of the crane from any point along the span as the critical spacecraft components are moved into position. The digital readout on the control station is tied into multiple, wall-mounted readouts displaying critical real-time information to crew members.
Because of the cleanroom environment, installation took NSI roughly twice the time to complete the project. Workers had to suit up every time they entered the work area. Retrofit work had to stop and be coordinated around overriding priority activities several times during the installation phase.
Says Burger: “We achieve much better speed control and precision than we ever had with the Hydraset. With the Hydraset, positioning control depends on the agility and expertise of the operator … with a flux vector drive, we all operate the unit equally and can place a load at increments of 2/1,000ths of an inch.”
Additionally, a hoist minimum speed of 0.75 inch (1.9 cm) per minute was surpassed by a final speed of 0.45 inch (1.1 cm) per minute — a 60 percent increase. Goddard`s lifting engineers had never before experienced this type of precise positioning in crane operations.
Al Zorn is manager of recertification support at Mantech Systems Engineering Corp. (formerly NSI Technology Services Corp.) under contract to the NASA Goddard Space Flight Center in Greenbelt, MD. He is in charge of test, inspection, and certification of pressure systems, lifting devices, and equipment. Previously, Zorn spent 32 years with Bethlehem Steel Corp. (Sparrows Point, MD), until he retired in 1990.
Daniel J. Sullivan is vice president of the Advanced Technology Group at Electromotive Systems Inc. in Milwaukee, WI. He has over 30 years experience developing automated and integrated material handling equipment and systems.
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Upgrading to flux vector crane controls gives NASA`s cleanroom crane increased positioning control accurate to 0.002 inches.
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NASA`s 5-ton (4,500 kg) overhead cleanroom crane is used to position delicate and expensive spacecraft components during assembly.
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In the cleanroom, wall-mounted LED readouts provide crew members with critical information from the control station during spacecraft assembly.
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The Impulse VG+ digital drive allows NASA personnel to control loads at zero speed (load float) without setting the electric brake.