Akira Mohri

Trajectory control of incompletely restrained parallel-wire-suspended mechanism based on inverse dynamics

This paper discusses parallel wire mechanisms where an end-effector of the mechanism is suspended by multiple wires. The mechanisms enable not only three-dimensional (3-D) positioning but also 3-D orienting of the end-effector, unlike typical wire suspension-type mechanisms such as overhead crane. To discuss the parallel-wire-suspended mechanisms generally, two forms of basic dynamic equations are presented. Then the parallel wire mechanisms are classified into two types based on the basic equations. Dynamical properties of the two types of wire-suspended positioning mechanism are discussed. In this paper, one of the wire-suspended mechanism, incompletely restrained-type parallel wire mechanism, is mainly discussed on its inverse dynamics problem and its trajectory control problem. The inverse dynamics problem for the incompletely restrained-type mechanism plays an important role on its control problem, because the mechanism has low stiffness based on incomplete constraints on the suspended object which is governed by its dynamics. The paper proposes an antisway control method for the suspended object. In the method, the inverse dynamics calculation is used for nonlinear dynamics compensation to control the suspended object of the incompletely restrained parallel wire mechanism.

Trajectory planning of mobile manipulator with stability considerations

This paper presents methods of trajectory planning for a mobile manipulator with stability considerations. The proposed trajectory planning method is to generate a trajectory for the mobile manipulator from a given path of the end-effector considering stability. Then, we derive a dynamics model of the mobile manipulator considering it as the combined system of the manipulator and the mobile platform. ZMP criterion is used as an index for the system stability. The trajectory planning problem is formulated as an optimal control problem with some constraints. To solve the problem, we use a hierarchical gradient method which synthesizes the gradient function in a hierarchical manner based on the order of priority. The simulation results of the 2-link planar nonholonomic mobile manipulator are given to show the effectiveness of the proposed algorithm.

Quasi-time-optimal motion planning of mobile platforms in the presence of obstacles

This paper addresses a problem of optimal motion planning of mobile platforms amidst obstacles, considering the mobile platform dynamics. Due to nonholonomic constraints, actuator constraints, and state constraints by obstacle avoidance, the planning problem of mobile platform with two independently driven wheels is a complicated one. In this study, a dynamical model for the mobile platform is presented, including nonholonomic kinematic constraints. The idea of a path parameter is introduced to simplify the planning problem by considering the dynamics and nonholonomic constraints. Using the path parameter, the optimal motion planning problem is divided into two sub-problems: 1) time-optimization of trajectory along specified path, and 2) search for optimal path. Then two methods are proposed the solve the problems using the path parameter and parametrization by B-spline function. Finally, quasi-time-optimal solution for the original problem are planned by combining the two methods. Numerical examples show effectiveness of the motion planner.

Inverse dynamics and control of crane-type manipulator

This paper discusses an inverse dynamics problem for a parallel wire mechanism with multi-degrees of freedom. Firstly, a crane-type parallel wires mechanism which has three trolleys and three wires is proposed for handling heavy objects. This handling mechanism enables not only three dimensional positioning but also orientating of objects, unlike typical crane mechanism. A dynamical model using wire force vector is derived. Based on the dynamical model with wire force vector, inverse dynamics problem is analytically solved by a linear equation in terms of the wire force vector. The trajectory of end-effector of the crane-type manipulator can be transformed into the trajectory of wire length and trolley position by the calculation method. Using the trajectory of wire length and trolley position, the end-effector of the crane-type manipulator is controlled. Two dimensional experimental crane-type manipulator is also developed to verify an effectiveness of the inverse dynamics calculation and the trajectory control method.

Trajectory planning of mobile manipulator with end-effector's specified path

In this paper, a trajectory planning method of a mobile manipulator with the end-effectors specified path is presented. We derive the dynamics of the mobile manipulator considering it as the combined system of the manipulator and the mobile platform. The planning problem is formulated as an optimal control problem. To solve the problem, we use the concept of the order of priority. A gradient-based iterative algorithm which synthesize the gradient function in a hierarchical manner based on the order of priority is used. The simulation results of the 2-link planar nonholonomic mobile manipulator are given to show the effectiveness of the proposed algorithm.

Pattern Recognition of a Grasped Object by Unit-Vector Distribution

A pattern recognition method using an artificial hand to identify the pattern of a grasped object and to locate its position relative to the hand coordinates is discussed. In the proposed recognition method, an object is recognized as a pile of two-dimensional slices, namely, a pile of plane closed curves. Each curve (i.e., contour) is described as the distribution pattern of unit-vectors, called the unit-vector distribution (UVD). The proposed recognition method using UVD functions is as follows. 1) Given the a priori information about the contours of objects to be grasped, the UVDs of their contours are calculated and memorized. These are called the original patterns (OPs). 2) Then a sampled UVD is formed from real tactile data and called the data pattern (DP). 3) The DP is compared with the memorized patterns (MPs), which are obtained by blurring the corresponding OP, so as to make it easier to single out the best fitted OP. 4) Finally, the contour is regenerated in the hand coordinate system by using the UVD of the singled out pattern. An experiment using a trial sensor and a model hand has shown the effectiveness of the proposed method in the recognition of several kinds of objects having cylindrical and prismatic contours.

Sub-optimal trajectory planning of mobile manipulator

A trajectory planning method of a mobile manipulator is presented. We derive the dynamics of the mobile manipulator considering it as the combined system of the manipulator and the mobile platform. The planning problem is formulated as an optimal control problem. To solve the problem, we use the concept of the order of priority. A gradient-based iterative algorithm which synthesize the gradient function in a hierarchical manner based on the order of priority is used. The simulation results of the 2-link planar nonholonomic mobile manipulator are given to show the effectiveness of the proposed algorithm.

Anti-sway control for wire-suspended mechanism based on dynamics compensation

Proposes a feedback control method for wire-suspended mechanisms. The wire-suspended mechanisms are classified into two types, which are completely restrained type mechanisms and incompletely restrained type mechanisms. The paper discusses mainly incompletely restrained type mechanisms form the viewpoint of actual control. The incompletely restrained type wire-suspended mechanism has the merit that it enables three dimensional positioning and orientation of suspended object with simple mechanism using a small number of wires. However it also has the drawback that the manipulated object is easy to swing as seen in overhead crane which is a simple incompletely restrained type mechanism. For this reason, an anti-sway control method for incompletely restrained type mechanisms is needed. To resolve the problem of swing, inverse dynamics for general incompletely restrained type wire-suspended mechanism is discussed, then a feedback control method based on dynamics compensation is presented.

Online navigation of mobile robot under the existence of dynamically moving multiple obstacles

Proposes an online navigation method of mobile robots for collision avoidance with dynamically moving obstacles, where the multiple obstacles always change their velocities. For more applications of mobile robots in the real world, the robots are expected to work well in everyday space where people are walking around. The robot must reach the desired point even in a dynamic environment where many people (obstacles) are moving. In such a situation where the environmental objects are dynamically changing their position, it is not realistic to have all information of the dynamical environment such as trajectory of a human beings motion. To cope with the situation, the robots usually have a sensor system to obtain real time environmental information, and robots should have an ability of sensor-based online motion planning for the dynamical environment. The paper considers the online motion planning problem where the workspace has multiple moving obstacles. The proposed motion planner basically uses an idea of velocity obstacle. After discussing some problems when applying the idea of velocity obstacle to online motion planning problem for dynamical environment, a modified method using the idea is presented. The main point of the modification is for coping with velocity changes of moving obstacles during the sensor cycle. The paper also discusses the efficiency of the resultant trajectory produced by the motion planner. Some simulations of online motion planning problems where multiple obstacles are changing their velocities are shown.

Two wheels caster type odometer for omni-directional vehicles

This paper proposes a new type of two wheels caster type odometer to estimate the current position and orientation of omni-directional vehicles. The proposed odometer is composed of two passive wheels and one passive rotational axis. The two wheels rotate independently, and the rotational axis changes the odometers orientation as the omni-directional vehicle moves. Three rotary encoders using a gear train measure angular velocities of the two wheels and rotational axis. A mechanism of the gear train is proposed to prevent the encoder from being entangled with it. Using measured values of the three encoders, the current velocity vector of an omni-directional vehicle is calculated by kinematics of the odometer. Using the velocity vector, the current position and orientation of the omni-directional vehicle are estimated by dead reckoning. Results of localization experiments by the developed odometer are shown.