Byung-Ju Yi

Design and experiment of a 3 DOF parallel micro-mechanism utilizing flexure hinges

A planar 3 DOF parallel-type micro-positioning mechanism is designed with the intention of accurate flexure hinge modeling. For this, a preliminary kinematic analysis that includes inverse kinematics, internal kinematics, and analytic stiffness modeling referenced to the task coordinate is presented. First, the revolute type of a 1 DOF flexure hinge is considered. The simulation result based on FEM, however, is not coincident to the analytic result. This is due to the minor axial elongation along the link direction that keeps the mechanism from precise positioning. To cope with this problem, a 2 DOF flexure hinge model that includes this additional motion degree as a prismatic joint is employed in part. On the basis of this model, the positional accuracy is ensured. The effectiveness of this accurate model is shown through both simulation and experimentation This work emphasizes that the precise modeling of a flexure hinge is significant to guarantee the positional accuracy of parallel micro-mechanisms using flexure hinge.

Geometric analysis of antagonistic stiffness in redundantly actuated parallel mechanisms

Parallel closed-chain mechanical architectures allow for redundant actuation in the force domain. Antagonistic actuation, afforded by this input force redundancy, in conjunction with nonlinear linkage geometry creates an effective stiffness directly analogous to that of a wound metal spring. A general stiffness model for such systems is derived and it is shown that the constitutive relationship between actuation effort and active stiffness is the second-order kinematic constraint set relating the actuation sites. The extent of stiffness modulation possible is then evaluated and necessary conditions for full stiffness modulation are obtained. Configuration-dependent, second-order, geometric singularities affecting stiffness generation are illustrated in terms of a three-degree-of-freedom parallel spherical mechanism example and discussed in relation to their more commonly investigated first-order counterparts that affect force and velocity transmission. Finally, a load distribution methodology for simultaneous motion and stiffness generation is introduced, and it is shown that with hyperredundant actuation the internal load state of the mechanism can be controlled independent of its motion and effective stiffness.

The kinematics for redundantly actuated omnidirectional mobile robots

Omnidirectional mobile robots have been popularly employed in several application areas. However, the kinematics and singularity analysis for these systems have not been clearly identified, especially for the redundantly actuated case, which is common in current omnidirectional mobile robots. In light of this fact, this article introduces two different kinematic approaches for a typical omnidirectional mobile robot having three caster wheels, and examines singularity configurations of such systems. Then, a singularity-free load-distribution scheme for a redundantly actuated three-wheeled omnidirectional mobile robot is proposed. Through simulation, several advantages of the redundantly actuated mobile robot (singularity avoidance, input-load saving, and exploiting several subtasks) are presented.

Modeling and analysis on the internal impact of a Stewart platform utilized for spacecraft docking

In this paper, impacts at the joints of a closed-chain mechanism are modeled and analyzed. It is assumed that a Stewart platform is attached to one end of a spacecraft to be utilized as a docking platform and the spacecraft docks with another spacecraft. When the spacecrafts dock with each other, impact will rise at the docking point and the impact is absorbed by the docking platform. Although the impact at the contact point is very large, most of the impact is absorbed by the docking platform and negligible impact is transmitted to the spacecraft. The docking platform can absorb the shock by inertial effect, not by damping or friction. To show this, modeling of the internal impulses at the joints of a multi-body system is introduced and the internal impulses of the docking platform are analyzed by the modeling method.

Design and Motion Planning of a Two-Module Collaborative Indoor Pipeline Inspection Robot

This paper deals with a design and motion planning algorithm of a caterpillar-based pipeline robot that can be used for inspection of 80-100-mm pipelines in an indoor pipeline environment. The robot system uses a differential drive to steer the robot and spring loaded four-bar mechanisms to assure that the robot expands to grip the pipe walls. Unique features of this robot are the caterpillar wheels, the analysis of the four-bar mechanism supporting the treads, a closed-form kinematic approach, and an intuitive user interface. In addition, a new motion planning approach is proposed, which uses springs to interconnect two robot modules and allows the modules to cooperatively navigate through difficult segments of the pipes. Furthermore, an analysis method of selecting optimal compliance to assure functionality and cooperation is suggested. Simulation and experimental results are used throughout the paper to highlight algorithms and approaches.

Open-loop stiffness control of overconstrained mechanisms/robotic linkage systems

A novel approach for the control of task-space stiffness characteristics in systems consisting of a superabundance of kinematically dependent inputs is proposed. When there are more input actuations than operational degrees of freedom, internal preloads can be generated that produce effective restoring forces in the face of displacement or disturbances imposed on the system. Examples of this excessive actuation can be found in certain modes of structurally overconstrained parallel manipulators and in antagonistically structured serial manipulators. The input loads are synthesized offline (prior to operation) and entered as a feedforward component so that the desired effective loads on objects are obtained, and (simultaneously) significant disturbances at the task level can be largely rejected in an open-loop fashion. This reduces the burden and shortcomings of standard feedback schemes. Moreover, a layered feedback scheme is used to compensate for small perturbations and unmodeled dynamics. The open-loop task-based stiffness control scheme as applied to structurally parallel mechanism/robotic linkage systems is investigated. The schemes applicability as a programmable active compliance device is also discussed.<<ETX>>

Non-dimensionalized performance indices based optimal grasping for multi-fingered hands

Abstract When a multi-fingered hand grasps an object, the ways to grasp it stably are infinite, and thus an optimal grasp planning is necessary to find the relatively optimized grasp points on object for achieving the objective of the given grasping and manipulating task. For this, we first define several grasp indices to evaluate the quality of each feasible grasp. Since the physical meanings of the defined grasp indices are different from each other, it is not easy to combine those indices to identify the optimal grasping. Thus, we propose a new generalized grasping performance index to represent all of the grasp indices as one measure based on a non-dimensionalizing technique. Next, by using the proposed grasping performance index, we try to determine the optimal grasp points for multi-fingered hands performing contact tasks. Through task-based simulation studies, we discuss the feasibility of each grasp index as the grasp polygons and then, we show that the trend of the proposed optimal grasp planning is coincident to the physical sense of human grasping. Furthermore, some experimental results showing the task specific performances are incorporated to corroborate the effectiveness of the proposed optimal grasp planning algorithms.

Optimal grasping based on non-dimensionalized performance indices

For a multi-fingered hand to grasp an object, there are numerous ways to grasp it stably, and thus an optimal grasp planning is necessary to find the optimal grasp point for achieving the objective of the given task. First, we define several grasp indices to evaluate the quality of each feasible grasp. Since the physical meanings of the defined grasp induces are different from each other, it is not easy to combine those indices to identify the optimal grasping. In this paper, we propose a new generalized grasping performance index to represent all of the grasp indices as one measure based on a non-dimensional technique. Through simulations, we show that the proposed optimal grasp planning is resemblant to the physical sense of human grasping.

A five-bar finger mechanism involving redundant actuators: analysis and its applications

A five-bar finger mechanism driven by redundant actuators is given as an illustrative example. It is shown that judicial choice of the location of one redundant actuator greatly enhances the load handling capacity of the system, when compared to those of minimum actuation and more than two redundant actuators. Also, methodologies for stiffness and motion frequency modulations via redundant actuation are investigated. Internal load distribution associated with the stiffness and motion frequency modulations is further discussed. Specifically, the motion frequency of the system is modulated by employing inertial and spring-like impedance properties created by internal loading. The motion frequency as well as the amplitude of oscillation can be actively adjusted during the motion, and the equilibrium position about which the vibration occurs can also be arbitrarily changed during the motion. Furthermore, using the stiffness modulation capability, a point-to-point motion can be accomplished by a progressive movement of equilibrium posture, which is called a virtual trajectory. To show the effectiveness of the proposed algorithms, several simulation results are illustrated.

The dynamic modeling and analysis for an omnidirectional mobile robot with three caster wheels

Recently quite a few applications of an omnidirectional mobile robot have been reported. However, understanding some fundamental issues still remains as further study. One of the issues is the exact dynamic model. Previous studies very often ignore the wheel dynamics of the mobile robot and suffer from algorithmic singularity. Thus, actuator sizing or control algorithms based on the incomplete plant model does not guarantee the control performance of the system. This paper deals with the singularity-free, exact dynamic modeling and analysis of an omnidirectional mobile robot with three caster wheels. Initially, the exact dynamic model of the mobile robot including the wheel dynamics is introduced. A natural orthogonal complement approach is also introduced. The joint-space and operational-space dynamic models are derived as analytical forms. Through simulation, the discrepancy of the incomplete dynamic model is shown by comparison with the exact dynamic model. Furthermore, the useful aspect of operational dynamics in terms of impact geometry is also discussed.