Underwater Assessments of Space Suit Reach Envelopes
3.3 PLSS/HUT Orientation Ranges
The excursion ranges of PLSS/HUT orientations are listed in Table 3.1, as measured by the 97.5th and 2.5th percentile ranges. The sagittal flexion/extension range is larger for ELTA than ZLTA. However, ZLTA shows over two-fold larger ranges for the horizontal rotation and lateral bending angles.
Table 3.1 PLSS orientation excursion range. Data from one subject.
Sagittal Flexion/Extension Horizontal Rotation Lateral Bending
ELTA 167 ° 75 ° 53 °
ZLTA 147 ° 151 ° 25 °
4 Discussion
The goal of this study was to develop a hardware and software system to evaluate the kinematic mobility of suited crewmembers, by measuring the 3-D whole-body reach envelopes in an underwater condition. Previously, a computer model generated simulated unsuited microgravity reach envelopes using the anthropometry samples of 192 male astronaut candidates measured in 1979 and 1980 (NASA, 1995). In a different study, underwater suit mobility assessments were performed by manually marking the hand traces on a 2-D grid board placed at intervals (KLAUS et al., 1989). However, with this new system, more naturalistic 3-D motions can be captured at reduced time and cost. Using this new motion capture method and assessment technique, the maximum reach capacity and reach envelopes of the space suit can be evaluated in a simulated microgravity environment.
An optical triangulation technique has been used for underwater archaeology (WEHKAMP et al., 2014) and marine biology survey (SHORTIS, 2015). Unlike common “dry land” motion capture systems, infrared lighting or structured-light based depth camera system may not be appropriate due to absorption and scattering in water (BIANCO et al., 2013; CHAPLIN, 2016). Thus, this system used an active LED marker system to improve detectability in the underwater environment, improving marker-tracking accuracy significantly. The system provides a level of accuracy adequate for the context of suit performance testing measuring “gross” motion trajectories, although different applications require a higher precision level. A separate study reported the measurement error of this system as 1.90 cm on average (BERNAL et al., 2017).
Reach envelope estimation in this study was based on a novel technique to permute the hand reach traces by the PLSS/HUT motions and suppress the sub-maximal reach points using a polar-histogram and cross-section outlining technique. The reach envelope estimated by a parametrically deformed icosphere provided a reasonably accurate representation of reach point cloud. However, the permuted point clouds (Figure 2.3) do not provide the information about kinematic characteristics inside the maximum reach volume. However, the “raw” reach traces (Figure 3.1) provide complementary information, including the reach span angle of the right hand in the horizontal plane.
The raw hand reach traces and estimated reach envelopes revealed the mobility differences between the suit conditions. Although the analysis is based on a single test subject, Z-2 with the ELTA configuration shows larger reach spans in the fore-aft direction, while the ZLTA configuration shows larger span distance in the lateral (left-right) direction and horizontal rotation angle. This observation is in agreement with the PLSS/HUT excursion ranges, which show the larger horizontal rotation and lateral bending angles in ZLTA. PLSS/HUT motions are largely determined by lower torso assembly flexibility and mobility, which potentially suggest more naturalistic kinematic patterns in torso and lower extremity motions for suited crewmembers in ZLTA.
Overall, the new method reported in this study can be used in different ergonomic environments using simple off-the-shelf equipment that are ready to use. The
validation of this new motion capture system proved it to be a usably accurate source for motion capture data collection and analysis.
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