In this paper, we present an on-line task modification method (OTMM) to realize singularity avoidance for nonredundant and redundant manipulators at the velocity level. The method introduces a correction vector, constructed from the task velocity and the singular vector corresponding to the minimum singular value, into the task velocity. The performance is simply affected by the choice of the lower limit of the minimum singular value and a scalar adjusting function, which is monotone with respect to the minimum singular value. The method makes unnecessary avoiding the singularity point by off-line path planning for nonredundant or redundant manipulators, and the effort to check whether the singularity is escapable for redundant manipulators. The simulation results show the effectiveness of the OTMM for on-line singularity avoidance in manipulator motion control.
PurposeThe purpose of this paper is to present a two‐wheeled inverted pendulum with self‐tilt‐up motion ability. With this ability, the two‐wheeled inverted pendulum can erect without assistance, and then the vehicle can be autonomously deployed. The paper proposes an approach to achieve this self‐tilt‐up motion, which involves precessional motion.Design/methodology/approachA flywheel is mounted inside the vehicle to perform high‐speed spinning. The flywheel and body of the vehicle are forced to move around a fixed point and precessional motion occurs. As a result of the precessional motion, a moment is synchronously generated to tilt the body up to the upright position. Since no external force is applied on this two‐wheeled inverted pendulum, it is called self‐tilt‐up motion. A 3D model and a prototype are built to validate this approach.FindingsThe simulation and experimental results show that the self‐tilting‐up motion is successful.Research limitations/implicationsThis paper presents a self‐tilt‐up motion for a two‐wheeled inverted pendulum. With the analysis of the dynamics, simulation demonstrations and prototype development, the results show that the vehicle could perform self‐tilt‐up motion without any assistance. The principle of this self‐tilt‐up motion involves processional motion of rigid body. We also pointed out the factors that play important roles in influencing the performance of self‐tilt‐up motion and then define the switching time for the motion to switch to dynamic balance movement.Originality/valueTraditional multi‐wheel robots cannot work when they overturn. However, the two‐wheeled inverted pendulums with self‐tilt‐up ability do not have this shortcoming. They can stand up to keep working even if they fall down. A two‐wheeled inverted pendulum with self‐tilt‐up ability can be applied to many places. Equipped with solar battery, it can be used as an independent explorer. This type of vehicle can be deployed in swarms for planetary detection. For example, many small two‐wheeled inverted pendulums assist a lunar rover for exploration, samples gathering, etc.
This paper presents a quadratic programming (QP) form algorithm to realize on-line planning of mobile manipulators with consideration for improving the stability level. With Lie group and screw tools, the general tree topology structure mobile robot dynamics and dynamic stability attributes were analysed. The stable support condition for a mobile robot is constructed not only in a polygonal support region, but also in a polyhedral support region. For a planar supporting region, the tip-over avoiding requirement is formulated as the tip-over prevent constraints with the reciprocal products of the resultant support wrench and the imaginary tip-over twists, which are constructed with the boundaries of the convex support polygon. At velocity level, the optimized resolution algorithm with standard QP form is designed to resolve the inverse redundant kinematics of the Omnidirectional Mobile ManipulatorS (OMMS) with stability considerations. Numerical simulation results show that the presented methods successfully improve the stability level of the robot within an on-line planning process.
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