Appeared in print as "NASA loading boom swinging out of control
By Robert J. Sheaf, Jr.
| Photo, NASA |
Several years ago, we were asked to do a five-day hydraulics class for NASA. The class included four astronauts, who were scheduled to travel to the International Space Station, plus several engineers. While attending the class, they expressed a great deal of interest in the circuit that operated horizontal movement of the Space Shuttle’s loading boom. Apparently, the loading boom was having a problem when moving and positioning the Hubble Space Telescope and other heavy cargo during previous missions.
Weight is a mjor factor when loading material into the Shuttle’s cargo bay for a mission, so hydraulic servo controlled motors — which drive ball screws — are used to move and position the boom when on the ground with gravity. A hydraulic system was chosen because it transmits higher power in a much smaller package than other methods. However, in the microgravity of space, weight isn’t a factor, so small electric servomotors engaged the boom instead of the hydraulic system to unload the telescope and other cargo. Because energy consumption is a major concern, this combination of drives works well.
The hydraulic motor had a hollow shaft and when not in use had both ports connected for freewheeling while the dc electric motor would be engaged and disengaged with a clutch attached to the same ball screw.
Their problem only manifested itself when using the hydraulic servomotors. Both systems are controlled by a single handheld joystick control. When an operator commanded the boom to move hydraulically, it would hesitate, then move, and overshoot the commanded position, This caused the boom to oscillate back and forth. The boom has a position feedback sensor, and if the operator worked the joystick just right, he or she could get it to stop oscillating.
The attached drawing is a simplification of the hydraulic servo motor circuit.
What do you think could be causing the problem?
Robert J. Sheaf Jr., is the founder of Certified Fluid Consultants (CFC) and President of CFC-Solar Inc. CFC-Solar provides technical training, consulting, and field services to any industry using fluid power technology. Visit www.cfcsolar.com for more information.
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Solution to May’s mine pump problem
When pressure compensated pumps are compensating, and then a circuit is activated, the demand for flow can be greater than the pump reaction time, causing a momentary pressure drop. In this case, the pressure drop fell below the 1000 psi pressure switch setting. The 1-pt accumulators were specified to provide the instant flow needed during this short period until the pump flow could maintain pressure above the minimum 1000 psi. When the Nitrogen prechange was checked, it was discovered that the accumulator bladder had failed and was causing the problem.
Both pumps had case drain flow meters, and measuring the case drain flow while a pump is compensating at maximum system pressure is a good indicator of the pump’s condition.
If personnel would have checked the flow, they would have found it to be in the normal range and possibly investigated a little further to find the true problem.
When I owned a repair shop in the 1980s, it amazed me how many pumps we received for repair that showed no internal signs of failure.
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