Comfort Analysis in EVA Reachable Envelope Based on Human-Spacesuit Integrated Biomechanical Modeling
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We proposed a biomechanical framework for modeling human-spacesuit arm interaction while carrying out EVAs. In the model, there is detailed definition of spacesuit joint rotations, included spacesuit joint stiffness model and a delicate human arm musculoskeletal model in the Anybody Modeling System. The framework is able to predict human joint torque, muscle forces and joint reactions in various positions and postures while wearing spacesuit. Based on the predicted maximum muscle force, we made an evaluation of the comfort scale in various positions in the reach envelope. The predicted most comfortable area was compared to measured most comfortable area for model prediction validation.
KeywordsEVA, spacesuit Reach envelope Comfort Biomechanical modeling
Humans have explored the space for decades since Gagarin’s first spacewalk and Armstrong’s first step on the moon. During the exploration, many missions are conducted through EVAs, such as space station construction and maintenance, scientific experiments and sample collection. As astronauts are faced with extreme environment in space, appropriate protection is necessary by wearing spacesuit. However, the heavy and pressured spacesuit design also restricts the mobility of astronaut, making astronaut working within a smaller envelope and conquering additional resistance to keep posture [1, 2]. Till now, most studies adopt the experimental method of kinematic measure for determining EVA reach envelope and most comfortable area. Only few studies tried using model-based methods to handle work envelope issues. A kinematic model using D-H parameters in robotics and Monte Carlo method is introduced by researchers in ACC (China Astronaut Research and Training Center) to predict EVA work envelope . And the comfort is first evaluated by Schmidt using dynamic modeling method and relative joint torque criterion .
Biomechanical modeling method offers a quantified solution to ergonomic assessment of human joint and muscle workload by calculating joint torques and muscle activation . Since there are few studies on comfort analysis in EVA reach envelope using biomechanical modeling method, we propose a human-spacesuit integrated modeling method for handling this issue. The predicted reach envelope and its comfort are compared with former experimental results for validation.
2.1 Kinematic Model of Spacesuit Arm
2.2 Joint Stiffness Measurement and Modeling
2.3 Human Musculoskeletal System Modeling
We modeled the human arm musculoskeletal system in AnyBody using available bone, joint, muscle and joint reaction definitions . These definitions were based on hundreds of anthropometric and anatomic researches of human musculoskeletal system. We utilized Hill type three-element muscle model to describe the feature that muscle capability changes with muscle length and contracting velocity. As human musculoskeletal system is a redundant system that is believed to contract muscles in an optimal way, we chose the criterion that minimizes that maximum muscle activation as the optimization goal for the inverse dynamic analysis. Considering that the muscle with the maximum activation is most inclined to be fatigued, the criterion postpones muscle fatigue at most. Finally, the inverse dynamic analysis solved all the muscle forces and joint reactions using optimization methods.
2.4 Human Spacesuit Arm Integration
We implemented human spacesuit arm integration using the methods widely used in interface modeling between human and exoskeleton . The integration can be divided into two parts: kinematic and dynamic integration.
In kinematic integration, human hand and spacesuit glove were connected by fixed soft joint which constrains six degrees of freedoms. Human and spacesuit elbow position were also constrained to make sure that human elbow and spacesuit elbow center stay close. Soft joint was included because it allows minimal error which simulates the contact between spacesuit and human skin. By kinematic integration, spacesuit arm moves consistently with human arm.
Astronaut moves spacesuit arm with his own arm by reaction forces between each other, and this is what dynamic integration does. Virtual muscles were introduced to form the reaction element, which works like reaction forces. This method was recently used for predicting reaction forces between foot and ground in gait analysis . Virtual muscles were also included in the inverse dynamic analysis as unknown forces like other muscles. A main difference between virtual muscles and other muscles is that their force capabilities are large so that the optimization algorithm handles them inferior to other muscles. Generally speaking, the algorithm can be divided into two steps for understanding: firstly, find the reaction combination that will produce the minimal activation; secondly, find the muscle activation combination that is minimal.
2.5 Comfort Criteria
In our study, we chose muscle activation and relative joint torque as our comfort criteria. It has been proved that endurance time inclines as muscle load declines. And endurance time changes little after arriving at a typical load between ten and twenty percent of muscle strength, which can be seen as a threshold for comfort. And this percent definition is exactly muscle activation. As maximum muscle activation is the bottleneck and it determines comfort, we chose it as our criterion.
2.6 Reach Envelope and Comfort Prediction
Based on the integrated biomechanical model, we traversed all the arm postures according to the kinematic model of spacesuit arm and its joint RoM. In the simulation, average parameters of subjects such as muscle strength scale, height, weight, upper arm and lower arm length were adopted. The joints included in traverse included φF, φA, φR and φEF as these joints determine the position of hand while other joints only change hand orientation and position little. The resolution is set to 10° for all the joint angles. A total of over 10000 postures were evaluated using the integrated model, returning the hand position, maximum muscle activation and joint torques of shoulder, elbow and wrist joint. With these data, we determined which space was reachable and then determined the minimal muscle activation index and minimal composite index based on relative joint torque. Reach envelope was then modified by collision detection and elimination between suit glove and spacesuit trunk. Minimal index in a space was chosen because there are several postures for a given space or even give point, among these astronauts will choose the most comfortable one automatically. Finally, comfort was evaluated with the minimal criterion index.
Nine subjects participated in the reach envelope and comfort evaluation experiment with height of 172±7 cm and weight of 70±12 kg
3.2 Data Collection
Reflective markers were placed on spacesuit glove for hand position capture with NDI Optotrak when subjects moved horizontally and vertically layer by layer in the reachable area. Reach envelope was then determined with the captured positions.
When assessing comfort, subjects were firstly required to give three heights that were comfortable: lowest, medium and highest. For every height, a horizontal supporting bar was placed at a comfortable distance from subjects to relieve the effect of gravity. Subjects were required to move his hand along the bar, finding the most comfortable left and right boundaries.
4 Result and Validation
4.1 Comfort Analysis Using Different Criteria
Two different criteria were proposed for comfort assessment, one based on maximum muscle activation and the other based on relative joint torque of the total arm. We compared the result on the horizontal plane where z = 212 mm, as is shown in the following picture. The region in the middle of every figure is the predicted reach envelope. The area with the smaller index is more comfortable compared to those with larger index. It is shown that the comfortable areas using the two different criteria are very close, except that their scales are different.
4.2 Most Comfortable Area Validation
In the experiment, subjects were required to find the most comfortable area on the lowest, medium and highest horizontal plane. After taking an average of the measured areas, we determined the plane where z axis is respectively −82 mm, 34 mm and 200 mm as the lowest, medium and highest horizontal plane. We showed the measured most comfortable area in the comfort contour map predicted using max muscle activation criterion as follows. The measured most comfortable area (white area) was within the predicted most comfortable area which validated the model.
Human-spacesuit integrated biomechanical modeling is an effective tool for determining reach envelope and comfort in it. Generally speaking, the contributions of our work to ergonomic assessment of spacesuit are summarized as following: firstly, we proposed and realized a complete biomechanical modeling framework for human-spacesuit integration; Secondly, we proposed predicting comfort scale in the reachable envelope using quantified method based on predicted muscle activations instead of subjective ratings alone. The comfort-scale assessment in the reach envelope is just a case illustration of the applicability of human-spacesuit arm integration model in an inverse biomechanical framework. The model can be used for modeling realistic operational tasks when motion data is collected, making work envelope analysis of astronauts in EVAs related to operation. In future, the model is supposed to include detailed surface definition of spacesuit and human skin for more delicate simulation.
This work was supported by National Basic Research Program of China (NO. 2011CB711000), and advanced space medico-engineering research project of China (No: 2011SY5405002).
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