Underwater space suit performance assessments part 1: Motion capture system development and validation

Abstract This study aimed to develop and deploy a novel motion capture system capable of measuring space suit kinematics in an underwater test environment. The system was built using off-the-shelf, dive-rated hardware and open source software tools. The new system performance was validated by comparing the measurement outcome to a reference motion capture system in a dry-land condition. Measurement errors, defined as the linear distances of the marker position measurements between the developed and reference system, were 1.9 cm root-mean-square error (RMSE) with a 50-percentile error of 1.3 cm and a 95-percentile error of 3.6 cm. Measurement error tended to increase with motion speed. Similarly, the error showed a slight tendency of increasing with the distance from the center of the calibrated capture volume. However, the trend was not clearly identifiable. A second metric of system accuracy performance was calculated by assessing the wand length. The system was deployed underwater and tested for space suit kinematic assessments. Given the speed and range of space suit motions underwater, the measurement error of the developed system underwater was estimated to be approximately 1.39 cm and the wand length estimation error had a RMSE of 0.67 cm with a 50-percentile error of 0.51 cm and a 95-percentile error of 0.92 cm. Overall, the new system showed reliable and acceptably accurate kinematic measurements comparable to a common dry land motion capture system and can provide usable suit performance metrics in a simulated microgravity environment. Relevance to industry An underwater motion capture system was developed using off-the-shelf equipment. The new system was deployed to assess the kinematic mobility of space suits. The system can offer an inexpensive solution where traditional motion capture system may not be applicable.