The Design of a Robotic Arm to Measure Elbow Torque and Contact Pressures in an EVA Suit Arm

NASA awarded the Exploration Extravehicular Activity Services (xEVAS) contract to develop the next generation of EVA suits to be used in Low Earth Orbit (LEO) and on the Artemis Lunar missions. These new EVA suits must protect the astronauts in the extreme environments of LEO and the Moon, while also providing the mobility to support EVA operations, such as surface exploration or spacecraft maintenance. Current testing of spacesuits to ensure sufficient mobility and comfort for the astronauts during EVA is performed with the crewmember in the suit, either in 1 g or in a neutral buoyancy environment. An alternative method of evaluating the mobility and comfort of EVA suits would be to use robotic systems to measure various fit and performance metrics. Robotic testing of spacesuits has historically been limited to either measuring the torque about a joint or measuring the contact pressure between a limb and the suit, but never both simultaneously. Merging the two measurements into a single test would provide essential data for predicting the performance of spacesuits. In this research, the Robotic Arm for Evaluating Spacesuit Torque and Contact (RAESTAC) was developed to simultaneously capture joint torque at the elbow and contact pressure at the anterior forearm, anterior bicep, and olecranon of the ulna. The system incorporates a 3D printed arm into an inflated lower arm pressure garment assembly, modeled after the Extravehicular Mobility Unit (EMU) used on the International Space Station (ISS). Driven by cables attached to a servo motor and gear train assembly, the RAESTAC system rotates the 3D printed arm at the elbow through a 120-degree arc, simulating the elbow angle that an astronaut might require to reach their Display and Control Module (DCM). To evaluate the effect of individual arm anthropometrics on “performance”, a 3dMD photogrammetric scanner was used to capture a digital scan of a subject's arm from the acromioclavicular joint to the tip of the distal phalanx of the third digit. This scan was then separated at the elbow joint and manipulated to incorporate a one degree of freedom (DOF) pin joint with ball bearings. Steel wire cables were routed through the arm and connected to S-type load cells and the servo motor/gear train. The torque about the elbow joint was calculated using the moment arm and the tension in the cable. Three Tekscan I-scan pressure mapping sensors were used to measure contact pressure between the 3D printed arm and the pressurized garment at the three locations. The RAESTAC system was also used to test how torque and contact pressure were affected by varying the arm geometries in a single suit arm design. Two subject's arms were scanned and tested using the same simulated EMU lower arm. It is concluded that RAESTAC may be used to evaluate the effects of arm suit design on specific subjects and can therefore be used to iterate and inform the design of future EVA suits.