KSC is always in need of innovative ways to make handling flight hardware with overhead cranes safer. With this in mind, ASRC Aerospace Corporation’s Advanced Electronics and Technology Development Laboratory and NASA’s Applied Physics Laboratory developed the crane-load contact sensor to help the crane operator know when the crane is aligned with the center of gravity (CG) of the load.
The requirements for the load contact sensor evolved from the need to handle a variety of load sizes and shapes, each with particular and sometimes peculiar handling requirements. For example, the space shuttle external tank is 154 feet long and 28 feet in diameter and weighs over 30 tons. Another example is the space shuttle orbiter maneuvering system (OMS) pod, which provides thrust and maneuvering ability to the Orbiter when in space. The OMS pod is a semicircular cross-sectioned pod 22 feet long and 11 feet wide and attaches to the aft fuselage of the Orbiter.
Each case requires special handling fixtures to accommodate unusual equipment geometry, with placement of the load at odd angles onto oblique mating surfaces complicating the procedure. The lifting points of these loads are not aligned with their CGs, so crane operators have to rely on their expertise and visual observations to align the crane to the load as it is being lifted.
The problem
All space flight hardware components have several things in common: they are expensive; they represent the result of many years of effort by armies of people; and damage to these components (caused by a handling accident) just prior to launch into space can be devastating and could literally end a program before it gets off the ground. Another reason for the high expense of flight hardware is the use of exotic, lightweight materials that are susceptible to external damage by the most seemingly benign occurrence. Furthermore, space flight hardware is hazardous: a variety of toxic propellants, chemicals, ordnance, and high-pressure gas are often loaded on the spacecraft. This increases the risk to the equipment and personnel and poses additional challenges to handling the load, as the CG changes from loading cargo or fluids.
Because of this, the same piece of equipment may need to be handled differently each time it is moved. There are many considerations for each load, each of which can have a serious effect on the hardware, machinery, and personnel involved. Assembly and disassembly of components requires precise spotting of the load and delicate handling of fragile, close-proximity parts. For such operations, an overhead crane with lifting fixture must be directly over the CG of the load so the load will not swing or drift as it is lifted.
To counter this problem of first-time exact alignment, tapered alignment pins provide positive lateral retention of the load to ensure it will not swing as it is lifted just off of its mating surface. Before the load is lifted past the tip of the alignment pins, the hoist is stopped and the overhead crane position can be adjusted until the load is not touching any of the pins. As the load clears the alignment pins, it should not swing. An improperly centered load could move several inches or several feet depending on how severely the overhead crane was misplaced.
In some cases, however, it may appear to the operators that the pins are clear when actually they are not. The load could be rocking on the pins, and this may not be entirely obvious. For this reason, additional verification is necessary.
Design constraints
The first challenge is that the loads are unique flight hardware. Hardware can be lifted and moved, but not modified. For example, strain gauge instruments (or any other instrument) <jedi>cannot be installed on alignment pins. Also, large currents cannot be induced onto fragile flight hardware that is laden with sensitive electronics and loaded with propellants or live ordinance. So a very low power is required to eliminate possible damage to sensitive electronics.
The second challenge is that both the load and support structure are electrically grounded. Thus, a simple check for electrical continuity between the (conductive) load and its support structure, even with the load suspended from the crane and separated from the support structure, will not work.
The sensor
The use of both DC and a high-frequency electrical signal solves these problems. The power level is low enough to not cause concern for the flight hardware’s electronics and ordnance, and the signal source can be placed on the load, near the alignment pin contact point.
The DC signal is used to verify the correct connectivity of the magnetic mount or electrical clips, and the high frequency is used to determine mechanical contact between the load and base. If the load is making contact with the base, this will short-circuit the high-frequency signal and it will be detected by the load contact sensor as in a non-floating condition. That means that the crane is not aligned with the CG of the load.
If the load breaks mechanical contact, and thus electrical contact, with the base, the high frequency signal encounters a high impedance through the crane wires, ground, and building ground. In this case, the load contact sensor detects a floating condition. That means that the crane is aligned with the CG of the load.
This system works well with conductive materials, and the power and frequency of the device can be selected to meet specific equipment needs.
Ease of use and portability are considerations with all equipment. The low-power system is served well with small batteries. A small circuit board was designed to generate and detect the DC and high-frequency signals. Three LEDs were used to indicate very simple functions: yellow for power on, red for a non-floating condition, and green for a floating condition. The yellow and red LEDs will blink if the device detects that its terminals are not properly connected, adding a level of safety for the operation. The sensor is mounted inside an explosion-proof enclosure. Even within this housing, the entire assembly weighs roughly three pounds (1.4kg). Contemplated enhancements include a smaller and lighter housing that still meets the hazardous-environment operation requirement.
Magnetic mounts or an electrical clamp connect the load to its support structure. Magnets can be used to connect to carbon steel items (the magnets are conductive); however, aluminum and stainless steel items require a clamp or some other type of connector. Electrical connection to flight hardware can be a problem. One of the major advantages of this system is that placement of the connectors is not critical since the unit allows for long electrical distances between its two terminals.
Other applications
The load contact sensor has several advantages over other alignment tools. It is small and battery-powered, and the low-wattage system does not affect any other instrumentation. The display provides simple visual indication, easily seen and interpreted directly by people or with a camera. The placement of the sensor onto the component structure does not have to be exact. Equipment modification is not needed as long as there is a conductive material and a ground path.
The load contact sensor may have applications outside the delicate load-handling needs of space flight hardware. Other applications may include remote placement of items where people do not have access. For example, the load contact sensor could be used to place or retrieve objects with a crane in a hot cell for nuclear materials. This could provide a means to ensure exact placement of a load or confirm the correct capture of a load.
KSC’s work to handle a variety of space flight hardware safely is not an ordinary material-handling application. With space flight hardware, even a minor handling accident can be devastating to a program and can threaten the lives of people in a large area. It is hoped the load contact sensor will provide an additional layer of protection between safe handling of the load and having a costly accident.
