The first hypothesis proposed in this study, that in addition to volume and structural effects a third and not well studied contributor (pressure effects) exists, was supported by the data collected. Pressure effects were documented in the closed volume (arm only) test condition, and contributed upwards of 8% of additional torque at high flexion angles.
The second hypothesis proposed in this study, that pressure effects are a statistically significant contributor to total suit rigidity, was not supported by the data collected.
While it is true that capped and uncapped tests of the EMU arm (arm volume only) produced statistically significant results for angles equal to or greater than 90˚ (or
approximately 60˚ degrees from neutral position), these findings were not replicated in the tests conducted with a representative EMU volume. No statistically significant increases in torque were measured for any angle for this test condition.
These findings help identify the relative influence of each of the three contributing factors to total space suit rigidity for the well-optimized elbow joint, and also help to map specific rigidity mechanisms to different operating regimes. They confirm prior modeling that identified volume effects as the dominant contributor to joint rigidity.
They also shed new light on the non-zero contribution of structural effects to total joint torque, and in doing so expose a seemingly counter-intuitive negative torque contribution at high flexion angles. And as a result, these findings demonstrate that rigidity modeling based solely on volume effects contributions do not fully represent the real life condition.
Perhaps most importantly, though, these findings speak to a potentially unrecognized source to suit rigidity that to this point had not been given serious consideration: pressure effects. While pressure effects were found to be statistically insignificant for the elbow joint when attached to the full EMU, their presence in tests conducted with smaller operating volumes has implications for other joints in the suit. Since the EMU elbow joint does a relatively good job maintaining constant volume, and is a relatively small and simple joint when compared to others in the EMU, it is likely that less-optimized and larger diameter joints will be prone to significant torque increases due to pressure effects stemming from volume changes during movement. Thus, these effects must be considered when modeling joint mobility, designing future suit joint prototypes, and also when crafting suit operating pressure requirements and designing internal pressure regulation systems. And, if pressure effects can be successfully mitigated, suit mobility could be increased in spite of volume changes caused by joint movement.
Incidentally, these findings also demonstrate that any hypobaric chamber joint testing conducted with an open internal volume will not be fully representative of the true suit condition, as these tests would necessarily ignore the contribution of pressure effects.
In summary, these findings expose both the strengths and shortcomings of current joint torque modeling efforts, and provide new information relevant to suit mobility and design. Ultimately, these contributions may lead to more mobile gas-pressurized suit concepts.
2.5.2 - Limitations
A small leak was detected early on between the EMU arm housing and the hypobaric chamber, causing the “closed-volume” gas contained inside the arm when capped to slowly leak into the vacuum environment over time. While several attempts were made to eliminate this leak, we were unable to completely mitigate the problem throughout the testing process. The consequences of this leak are not fully understood, though it stands to reason that it may have artificially weakened the pressure effects seen in this study since the arm was not perfectly capable of sustaining pressure spikes stemming from changes in volume (meaning the pressure effects values presented here may have actually underestimated the true effect). Proper countermeasures were put in place in the test method to prevent this leak from affecting the data (such as installing a valve in the arm cap which allowed the internal arm environment to be “reset” before each test).
However, the full effect of this leak on the pressure effects measured cannot be fully characterized (it should be noted that the significance of detecting pressure effects in the smaller volume elbow joint tests necessarily increases given the existence of this leak).
Hysteresis effects related to joint movement, which have been well documented in space suit mobility studies (Schmidt et al., 2001), were largely ignored in this study. With the exception of the volume change tests, all tests were conducted in the flexion direction only, and the arm was reset to its original neutral position between each flexion. It was assumed that the volume change tests, which were conducted starting at maximum flexion and moving through extension to the neutral resting position of the arm, were not affected by hysteresis effects. The tests were done in this manner for practicality purposes, because extension of the arm induced increases in volume (thus causing the water level to drop), making it possible for testers to add water to characterize the change
in volume. Conducting these tests in the flexion direction (which would have raised the water level at each increment due to progressively decreasing volume and required testers to remove water to measure the changes), would have vastly increased the difficulty of accurately measuring the change in volume.
As previously discussed, the measurements taken towards the end of this study were affected by a data drift issue. It is believed the effects of this drift were minimized by repeating earlier tests at the end of the study, enabling “apples-to-apples” comparisons.
However, because the source of this data drift was not identified with complete certainty, it is possible that it stemmed from physical changes in the test specimen over time, which may confound the analysis presented herein.
Finally, the PVC pipe used as a dummy volume for the EMU may not have been a perfect substitution for attaching the arm to a complete suit. While the pipe was sized to match the free volume of a pressurized, occupied suit, it was relatively independent of the arm volume (it was connected using rubber tubing) and was considerably different in shape than the free volume inside a suit (the pipe was cylindrical with no internal objects or corners to disrupt gas movement). This may have introduced errors into the transient response of the working gas during flexion.