This paper presents a rigorous mathematical formulation for modelling the upper extremity that is capable of considering a relatively large number of degrees of freedom, thus yielding a realistic model and associated envelope. Kinematic models are used to determine the reach envelope in closed form and to better understand human motion. Joint ranges of motion are taken into account by transforming unilateral inequality constraints into equalities that are included in the formulation. Methods from geometry are implemented to analyze the motion and delineate barriers within the workspace. These barriers are, in fact, observed to be surfaces where one or more joints of the limb are at their limits, but also where the hand's motion has encountered a kinematic singular configuration. Such a configuration is mathematically defined, and is physically associated with two links being parallel at an instant in time or where two joints axes are parallel (e.g., a fully extended arm yields a singular configuration). Barriers to motion can now be characterized in terms of different human performance measures, thus leading to a better understanding of the path trajectories assumed by humans as they execute tasks.
This paper presents studies of the coordination of human upper body voluntary movement. A minimum-jerk 3D model is used to obtain the desired path in Cartesian space, which is widely used in the prediction of human reach movement. Instead of inverse kinematics, a direct optimization approach is used to predict each joint's profile (a spline curve). This optimization problem has four cost function terms: (1) Joint displacement function that evaluates displacement of each joint away from its neutral position; (2) Inconsistency function, which is the joint rate change (first derivative) and predicted overall trend from the initial target point to the final target point; (3) The non-smoothness function of the trajectory, which is the second derivative of the joint trajectory; (4) The non-continuity function, which consists of the amplitudes of joint angle rates at the initial and final target points, in order to emphasize smooth starting and ending conditions. This direct optimization technique can be used for potentially any number of degrees of freedom (DOF) system and it reduces the cost associated with certain inverse kinematics approaches for resolving joint profiles. This paper presents a high redundant upper-body modeling with 15 DOFs. Illustrative examples are presented and an interface is set up to visualize the results.
Vehicle motions can adversely affect the ability of a driver or occupant to quickly and accurately push control buttons located in many advanced vehicle control, navigation and communications systems. A pilot study was conducted using the U.S. Army Tank Automotive and Armaments Command (TACOM) Ride Motion Simulator (RMS) to assess the effects of vertical ride motion on the kinematics of reaching. The RMS was programmed to produce 0.5 g and 0.8 g peak-to-peak sinusoidal inputs at the seat-sitter interface over a range of frequencies. Two participants performed seated reaching tasks to locations typical of in-vehicle controls under static conditions and with single-frequency inputs between 0 and 10 Hz. The participants also held terminal reach postures during 0.5 to 32 Hz sine sweeps. Reach kinematics were recorded using a 10camera VICON motion capture system. The effects of vertical ride motion on movement time, accuracy, and subjective responses were assessed. Performance decrements associated with vertical ride motion were found to depend strongly on reach direction and frequency.
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