Modeling the Extravehicular Mobility Unit (EMU) Space Suit: Physiological Implications for Extravehicular Activity (EVA)

Extravehicular activity (EVA) is investigated through experiments testing an actual extravehicular mobility unit (EMU) performing several EVA tasks in the laboratory, and a dynamic model of the EMU space suit is developed. Building directly on earlier work in EVA simulation, the space suit model was created from mass, inertia, and performance data to augment the unsuited 12-segment human model used in previous studies. A modified Preisach model was used to mathematically describe the hysteretic torque characteristics of joints in a pressurized space suit, and implemented numerically based on observed suit parameters. Computational simulations, based loosely on a 1995 EVA involving manipulation of the Spartan astrophysics payload, were performed to observe the effect of suit constraints on simulated astronaut performance. Results show that the shoulder joint work required for a suited EVA crewmember to move the payload while in an inefficient posture was an order of magnitude greater than it was in the unsuited condition. Moving to a posture more accommodating to the suit's neutral position, the simulated astronaut completed the task using only 23% of the work required in an inefficient posture. However, the ankle joint was forced to use its long lever arm to manipulate the payload, resulting in ankle work 3 times greater than in the unsuited condition. These results agree with anecdotal evidence of post-EVA ankle fatigue, and suggest promise for both the space suit model and the simulation technique. Current experimental research that complements the analytical EMU dynamic modeling is targeted towards gathering simultaneous joint angle and torque data from actual space suit tests. Since it is not possible to measure joint torques in human subjects, NASA's robotic space suit tester (RSST) is used for torque measurements. The database of joint angles and torques caused by the space suit provides a verification and enhancement to the space suit dynamic model, including more joints with higher fidelity for complex motions.