Measuring and testing – Speed – velocity – or acceleration – Fluid
Reexamination Certificate
1999-10-29
2001-06-12
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Fluid
C073S523000
Reexamination Certificate
active
06244113
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods and apparatus for measuring acceleration, and more particularly relates to measurement of microgravity acceleration.
BACKGROUND OF THE INVENTION
Microgravity accelerations experienced onboard a space vehicle during flight are vector quantities, which comprise a magnitude and a direction, resulting from numerous forces acting on the vehicle. These accelerations have many sources, including residual gravity, drag, orbiter rotation, vibration from equipment and crew activity. The equivalent acceleration vector at any location in the orbiter is a combination of many different sources and, thus, a complex vector quantity changing over time.
Many experiments conducted in microgravity conditions are extremely sensitive to slight changes in microgravity acceleration. For example, experiments involving gravity-dependent fluid phenomena, such as buoyancy or sedimentation, and experiments involving crystal growth, can be greatly affected by microgravity accelerations. Even acceleration as low as 1 micro-g continuously acting in the same direction could affect certain classes of experiments. As a result, measurement of microgravity accelerations is often necessary to successfully conduct and evaluate such experiments.
However, measurement of the residual quasi-steady g-vector, which is generally the vector sum of aerodynamic drag and gravity gradient accelerations, has been a difficult task. In fact, NASA did not successfully measure quasi-steady acceleration until STS-40. The difficulty is caused, in part, by various mechanical and crew operations that excite the normal vibrational modes of the spacecraft and produce a wide spectrum of periodic accelerations (g-jitter) with amplitudes on the order of milli-g's that ride on top of the micro-g quasi-steady accelerations. Since the g-jitter arises from internal forces and, therefore, must time-average to zero, it should be possible to time-average out the oscillating component of acceleration and recover the quasi-steady component. However, a simple calculation will show that, if the oscillating component has amplitude A and period &dgr;t, the uncertainty in the time average taken over time P will be ±A &dgr;t /2&pgr; P. Thus, if one expects to extract a quasi-steady component, when there is an oscillating component that is 3 orders of magnitude larger, the integration time will have to be ~1000 times longer than the period of the oscillating component in order to obtain any reasonable accuracy. In other words, the data collection that is required to accurately separate the oscillating component from the quasi-steady component is unreasonably burdensome.
Instrument bias is another problem that must be overcome to effectively measure microgravity acceleration. Simple mass-spring accelerometers, which have been commonly used as the basis for many of the accelerometer systems flown in support of microgravity experiments, do not always return to the same null position when acceleration is removed, resulting in an instrument offset that affects the accuracy of the measurement. This instrument offset is sensitive to temperature as well as previous acceleration history. Therefore, no matter how carefully such an instrument is calibrated on the ground prior to flight, the instrument offset will be difference once in space, and can still change with time. This instrument bias is typically on the order of 100-200 micro-g. Attempts have been made to calibrate out this bias by inverting the accelerometer periodically under the assumption that the quasi-steady acceleration does not change during this interval. However, given the limited accuracy that one can obtain from taking a time average over a small interval of time and the fact that one is trying to accurately measure a fraction of a micro-g by subtracting two numbers that are two orders of magnitude larger, such a procedure is problematic at best.
The Orbital Acceleration Research Equipment (OARE) accelerometer, which is based on an electrostatic suspension system, has also been used as a microgravity accelerometer on space flights. The OARE system comprises a charged proof mass suspended electrostatically within a chamber and held in place by an electric field. Voltage is applied to plates surrounding the proof mass in order to maintain the proof mass in a central location within the chamber. The acceleration acting on the proof mass is related to the voltage necessary to keep the proof mass centered. While the OARE instrument is far more sensitive than the mechanical accelerometers, it is also considerably more complex and expensive. Further, it also requires in-flight calibration as the charge on the proof mass may vary. For this purpose, it is mounted on a turntable to either invert it or to apply a known amount of centripetal acceleration to the sensor. The OARE instrument is also sensitive to g-jitter, which must be filtered out electronically or through software.
There remains a need in the art for a microgravity acceleration measurement method and apparatus that requires little or no in-flight calibration and exhibits no loss of accuracy due to g-jitter or instrument bias.
SUMMARY OF THE INVENTION
The invention provides an accelerometer capable of measuring microgravity quasi-steady components of acceleration due to drag and gravity gradient effects. The accelerometer of the invention requires no in-flight calibration and suffers no performance or accuracy problems arising from g-jitter or instrument bias. The invention involves use of a differentially heated flow chamber to measure the temperature difference between two temperature sensors spaced apart along a line normal to the thermal axis of the chamber. The measured temperature difference can be used to calculate a quasi-steady component of acceleration.
One embodiment of the apparatus of the present invention comprises an elongate flow chamber having a first end and a second end. The chamber contains a liquid. A first plug member engages the first end and is operatively positioned to block flow of the liquid through the first end. A second plug member engages the second end and is operatively positioned to block flow of the liquid through the second end. Each plug member is capable of being maintained at a different known temperature, such that a temperature gradient exists across the length of the flow chamber. At least one pair of temperature sensors is immersed in the liquid. The temperature sensors are spaced apart along a line intersecting the axis of the flow chamber and normal thereto, one temperature sensor positioned on either side of the point of intersection. The apparatus can include two pairs of temperature sensors such that each pairs of temperature sensors is spaced along a line intersecting the thermal axis of the flow chamber and normal thereto. The lines defined by each pair of temperature sensors are also normal to each other. Preferably, the two pairs of temperature sensors are located in the same plane and at the approximate midpoint of the flow chamber.
The temperature sensors can be spaced at various positions between the wall of the flow chamber and the axis of the flow chamber. In one embodiment, the temperature sensors are positioned adjacent to the inner surface of the wall of the flow chamber. In another embodiment, the temperature sensors are positioned at the approximate midpoint between the axis of the flow chamber and the wall.
In a preferred embodiment, the apparatus includes an expansion chamber in fluid communication with the flow chamber. A piston is positioned for axial movement within the expansion chamber for accommodating thermal expansion of the liquid into the expansion chamber. A spring is operatively connected to the piston for biasing the piston towards the flow chamber. The expansion chamber can be located in either plug member.
The invention also provides a method of measuring microgravity acceleration wherein an elongate flow chamber defined by at least one wall and having a first end and a second is provided. The flo
Alston & Bird LLP
Moller Richard A.
University of Alabama in Huntsville
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