Youngil Youm

Real-time ZMP compensation method using null motion for mobile manipulators

The dynamic stability of a mobile manipulator using ZMP compensation is considered. A unified approach for the two subsystems is formulated using a redundant scheme. First, to conserve the dynamic stability of the system, we define the performance index for the redundant system using the ZMP (zero moment point). This performance index represents the stability of the whole mobile manipulator system. Then, the redundancy resolution problem for optimizing the given performance index is solved using the null motion. Finally, the performance of this method is demonstrated by simulation study.

Stiffness control of a coupled tendon-driven robot hand

This paper presents the methods of design and control for a coupled tendon-driven robot hand with stiffness control capability. By using the tendon as a force transmission mechanism, compact design of the robot hand is achieved with the developed controller. Problems specific to the tendon characteristics are considered, such as the slacking problem of tendons when the joint is disturbed, and the tendon elongation problem when the collocated position sensing method is used for compact design. To cope with these problems, two fundamental algorithms are developed and implemented on a laboratory apparatus, the POSTECH Hand II. First, a position estimation algorithm is developed to evaluate the accurate position of the hand, leading to an antagonistic tendon controller. Secondly, an active stiffness control algorithm is developed to control the fingertip force. It is shown that the finger produces excellent linear stiffness characteristics which justifies the effectiveness of the proposed algorithm. The object stiffness control is also implemented to exert desired force to the environment when the hand grasps an object, and is evaluated via experiments. >

On the coarse/fine dual-stage manipulators with robust perturbation compensator

A dual-stage, fast and fine robotic manipulator is presented. By adopting merits of both coarse and fine actuator, a desirable system having the capacity of large workspace with high resolution of motion is enabled. We constructed an ultra precision XY-manipulator with dual-stage structure where the PZT driven fine stage is mounted on the motor driven XY positioner and applied it to fine tracking controls and micro-teleoperations as a slave manipulator. We describe essential merits of the compound actuation mechanism and the control strategy to successfully utilize it with a proper servo system design. Through experimental results, the effectiveness of the coarse/fine manipulation by the dual stage manipulator is shown.

Geometrical approach for the workspace of 6-DOF parallel manipulators

Since parallel manipulators have relatively small workspace when compared to serial manipulators, it is important to determine the workspace in the design stage. However, due to the complexity of the closed-loop chain mechanism, the determination of the workspace is difficult. In this paper, a fully geometrical method for the determination of the workspace of 6-DOF parallel manipulators is presented using the concept of a 4-bar linkage. The reachable and dexterous workspace can be determined from the proposed algorithm. In order to evaluate the workspace, each leg is considered as an open chain, and two kinematic constraints-position and mechanism constraints-are developed. The proposed method is verified by simulation.

Task based design of modular robot manipulator using efficient genetic algorithm

A modular robot manipulator is a robotic system assembled from discrete joints and links into one of many possible manipulator configurations. This paper describes a task based design method of modular robot manipulators. A locking mechanism which provides quick coupling and decoupling is developed and a parallel connection method is devised reducing the number of components on each module. To automatically determine the optimal link lengths of a modular manipulator for a given, task, the algorithm is two step: determine the necessary configuration of the robot using kinematic equations and then determine the optimal link length using the proposed efficient genetic algorithm. Some of design examples are shown.

New Resolution Scheme of the Forward Kinematics of Parallel Manipulators Using Extra Sensors

This paper presents a new closed-form resolution scheme of the forward kinematics of parallel manipulators based on two concepts, local structurization and mechanism partition. This scheme is applied to 6-DOF Stewart platform manipulators and the effectiveness of this scheme is verified through numerical examples. It is shown that one extra sensor is sufficient for both 3-3 SPM and 6-3 SPM to exactly resolve the forward kinematic problem (FKP) in closed form and two sensors for 6-6 SPM. In previous research, at least three extra sensors were needed for closed-form resolution of the FKP for 6-6 SPM. Consequently, the new resolution scheme is efficient to implement and easy for real-time applications for the control of parallel manipulators.

Control of a car-like mobile robot for parking problem

Deals with the parking problem of a car-like mobile robot. The parking problem corresponds to the point-stabilization problem of nonholonomic systems. We propose a simple and efficient algorithm for the problem with stability analysis. Using this algorithm, a car-like mobile robot could be controlled to move to a desired posture within a prescribed boundary. The performance is verified through simulations and experiments using a car-like mobile robot.

General ZMP Preview Control for Bipedal Walking

An online pattern generator for bipedal walking control is designed based on the notion of ZMP preview control using the full dynamics model. The method is called the general ZMP preview control. Conventional implementation by Kajita is based on an approximated table-cart model, which neglects the actual system dynamics. This leads to erroneous ZMP tracking for real robots, even if the system tracks the generated COM pattern exactly. Numerical simulations are provided to show the advantage and character of the method

Robust controller design for PTP motion of vertical XY positioning systems with a flexible beam

This paper presents a point-to-point (PTP) motion control method for accurate positioning and vibration suppression of a vertical XY positioning system with a flexible beam. The proposed method is composed of a feedforward and feedback controller. The input preshaping based on the analytic modeling and frequency equation of the system is proposed as a feedforward controller to produce the desired responses. The feedback controller based on a robust internal-loop compensator is designed to meet the specified performance and to stabilize the whole system in the presence of uncertainties and disturbances. By integrating the input preshaping controller and feedback controller, it is shown that the system is stable and the vibration of the flexible beam is suppressed. The proposed algorithm is demonstrated experimentally on an XY positioning system which consists of a base cart, elastic beam and moving mass.

Multiple tasks kinematics using weighted pseudo-inverse for kinematically redundant manipulators

This paper proposes a method to accommodate multiple tasks for redundancy utilization, which is based on a specific weighted pseudo-inverse. The proposed method also has task priority imposition property same as those conventional task priority based methods. In order to deal with general situations of task specification, the so-called semi-definitely weighted pseudo-inverse is devised.