Qiuling Zou and James Yang


Pregnant woman, multibody system, motion synthesis, optimization


Pregnant women have to live as a regular human in a daily life to accomplish different tasks such as standing, walking, sit-to-stand, and climbing stairs. However, during the course of pregnancy, the size, shape, and weight of pregnant women change dramatically. These changes can cause safety issues to pregnant women. This paper presents an optimization-based method to synthesize motion and investigate the effect of size, shape, and weight changes of pregnant women on dynamic motion. The pregnant woman model is considered as a multibody system that includes rigid bodies (links) and revolute or prismatic joints. This multibody system consists of 55 degrees of freedom (DOFs), where six DOFs are used to define the global translational and rotational motion. In this paper, motion synthesis includes pseudo standing, falling, and pulling tasks. Motion synthesis for this multibody system can be formulated as an optimization-based problem where equations of motion are considered as constraints instead of direct integration to synthesize motion efficiently. The summation of all joint actuator torque squares works as the objective function. The common constraints for standing, falling, and pushing tasks include joint angle limits, joint torque limits, stability constraints, feet position, and ground penetration constraints. In addition, each task has its own special constraints. To study the effect of the size, shape, and weight changes on the motion, three different cases (non-pregnancy, 6-month and 9-month pregnancy) are carried out for comparison. For the pulling task, three different pulling forces (2, 100, and 200 N) are applied at the hands, respectively. The simulation results show that pregnant woman motion varies with different month of pregnancy in general. Ground reaction forces, joint angle profiles, and joint torques are also different for different months of pregnancy. Visual validation method is used to check the feasibility of simulation results.

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