Stiffness modeling of parallel mechanisms at limb and joint/link levels

Drawing on screw theory and the virtual joint method, this paper presents a general and hierarchical approach for semianalytical stiffness modeling of parallel mechanisms. The stiffness model is built by two essential steps: 1) formulating the map between the stiffness matrices of platform and limbs using the duality of wrench and twist of the platform; and 2) formulating the map between stiffness matrices of a limb and a number of elastic elements in that limb using the duality of the wrench attributed to the limb and the twist of the endlink of that limb. By merging these two threads, the Cartesian stiffness matrix can be explicitly expressed in terms of the compliance matrices of joints and links. The proposed approach bridges the gap between two currently available approaches and is thereby very useful for evaluating stiffness over the entire workspace and investigating the influences of joint/link compliances on those of the platform in a quick and precise manner. A stiffness analysis for a 3-PRS parallel mechanism is presented as an example to illustrate the effectiveness of the proposed approach

Design of Wheelchair Robot for Active Postural Support (WRAPS) for Users with Trunk Impairments

People with severe trunk impairments cannot maintain or control upright posture during sitting or reaching out with the upper body. Passive orthoses are clinically available to support the trunk and promote the use of upper extremities in this population. However, these orthoses only rigidly position the torso on a wheelchair but do not facilitate movement of the trunk. In this dissertation, we introduce a novel active-assistive torso brace system for upperbody movements by a subject while seated. We have named this system as Wheelchair Robot for Active Postural Support (WRAPS). We propose designs of two robots, one for the pelvis and the other for the trunk. Each of the two devices has a parallel chain architecture to accommodate the range of motion (ROM), respectively for the pelvic and thoracic segments. The first thoracic robot was designed for the upper trunk motion relative to the pelvis. It has a 2[RP]S-2UPS architecture which provides four degrees-of-freedom (DOFs) to the end-effector placed on the upper trunk. The second is a pelvic robot which is designed to orient the pelvic segment relative to the seat. It has a 3-DOF [RRR]U-2[RR]S architecture, coupled with translation to accommodate pelvic movements relative to the seat. These robot architectures are synthesized based on human movement data. WRAPS can modulate the displacement of both the pelvic and the thoracic segments. Additionally, the forces can be applied on the torso through the end-effectors of these robots. Each of the robot prototypes was evaluated with able-bodied subjects to assess the device wearability, kinematic performances, and control system