An obstacle avoidance control scheme for the “Moray arm” on the basis of posture space analysis

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Abstract

“Moray arm” is a slider-installed manipulator that has large degrees of kinematic redundancy, thus possesses unconventional features such as the ability to enter a narrow space while avoiding obstacles. In this study, an obstacle avoidance control scheme for the Moray arm is proposed, which is based on the 2-DOF Moray drive control method and the posture space analysis. The 2-DOF Moray drive control derogates the control problem of the hyper-redundant degrees of freedom (DOFs) to the simple one of 2 DOFs: one of the control variables signifies the linear combination of the objective initial and final postures of the arm, while another one expresses the amount of the pull-out-displacement when pulling the arm out of the housing slider. The obstacle collision-free trajectory is generated for the Moray arm by analyzing a defined “posture space” that is determined by the above-stated two variables. Simulations are executed to verify the proposed scheme, and show that the scheme works well in the case of the static environment.

Introduction

In this paper, we propose a new obstacle avoidance control scheme for the “Moray arm”. The Moray arm inspired from the motion of a fish “Moray” living in the ocean bottom, is realized by installing a hyper-redundant manipulator arm on a powerful slider.

Hyper-redundant manipulators have very large degrees of freedom of kinematic redundancy, thus possess unconventional features such as the ability to enter a narrow space while avoiding obstacles. These robot manipulators, thus, are suitable for applications where a conventional industrial robot could never be introduced. There have been several attempts to construct a hyper-redundant manipulator in consideration of reducing the manipulator arm weight, because the weight is almost a crucial factor in practical realization of the hyper-redundant manipulator. One design tried to minimize the mass of each joint by the mechanism in which the actuators were mounted on the arm base and their power was transmitted by tendons and/or special transmission mechanism to each joint [1], [2], [3], [4], [5], [6]. Another idea is to use the special light-weight and powerful rotating joints [7], [8], [9]. Nevertheless, those contrivances were not yet fully satisfying the specifications required in practical applications. Recently, some hyper-redundant manipulators were developed [10], [11], [12], [13], and lots of control methods were also proposed, for example, in [14], [15], [16], [17]. However, the discussion was limited on the fixed-base hyper-redundant manipulator but not the moved-base one, and no discussion on how to effectively use the driven base to reduce the arm weight itself was performed. In order to reduce further the weight of the arm itself, the authors have proposed a mechanism of the hyper-redundant manipulator consisting of the arm itself and a powerful driving system (slider) installed at the base [18], and developed a mechanical model of the arm [19]. This hyper-redundant manipulator is analogous in design and operation to the fish “Moray”, thus was named as “Moray arm”. The weight consideration places severe limitations on the actuators which may be installed on the arm. However, the slider, most of which is mounted on the ground, is free from this restriction. Therefore, it seems reasonable to attempt powerful motions of the hyper-redundant manipulator by subrogating most of the driving power to powerful actuator installed on the slider. Another virtue of the slider concept is that it can simultaneously act as a housing for the arm; if a given task requires only a short arm, for example, the rest of the arm remains clean and safe from the damages in the slider/housing. This kind of the hyper-redundant manipulator, adding to the feature of the ability to enter a narrow space, also has the feature of little fluid resistance for the operation in a fluid environment.

In this paper, we first introduce the “Moray arm”, and the basic control methods specific for the Moray arm. Next, we propose a new obstacle avoidance control scheme for the Moray arm, based on the 2-DOF Moray drive control and the analysis in the defined posture space.

Section snippets

Moray arm and its control methods

The “Moray arm” was proposed to perform the motion of the fish “Moray” [18], and its mechanical model was developed too [19]. In this section, we introduce the mechanism of the Moray arm, and its basic control methods [18], [20].

Obstacle avoidance control algorithm for Moray arm

Many obstacle avoidance control techniques for manipulator have been proposed, however, they cannot be directly extended to the obstacle avoidance control of the Moray arm because of the complexity. In this section, we propose a novel obstacle avoidance control scheme which is more suitable for the Moray arm.

The motion of the Moray arm generated by Moray drive, as stated-above, is along the given trajectory, thus is ideal for avoidance of obstacles since the arm passes through very narrow

Simulations and experiments

We shall use a Moray arm shown in Fig. 4 to verify the proposed obstacle avoidance trajectory generation algorithm. The length of each link of the arm is 0.08 m, and the obstacles are simply given by two circles with radius 0.05 m centered at (0.49 m, 0.03 m) and (0.61 m, −0.11 m), as shown in Fig. 8(a). The following Clothoid curve is selected as the initial and final postures of the Moray arm, since the Clothoid curve is optimal for minimum time operation in case of the Moray drive [18].ρarm=κs,0⩽s⩽

Conclusions

In this paper, we proposed a novel obstacle avoidance control scheme for the Moray arm to perform a payload-location task from point to point while avoiding the existing static obstacles in environment. The new obstacle avoidance control scheme is based on the 2-DOF Moray drive and the analysis in the posture space, where two parameters were used to determine the Moray arm posture. The simulations through the computer and a mechanical model were also executed, and show that the proposed scheme

Shugen Ma was born in China in 1963. He received his B.E. degree (first class honors) in Mechanical Engineering from Hebei University of Technology, Tianjin, China, in 1984. He received the M.E. and Dr. Eng. degrees in Mechanical Engineering Science from Tokyo Institute of Technology, Tokyo, Japan, in 1988 and 1991, respectively. From 1991 to 1992, he worked for Komatsu Ltd. as a Research Engineer, and from 1992 to 1993 he was at University of California, Riverside, California, as a Visiting

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    Shugen Ma was born in China in 1963. He received his B.E. degree (first class honors) in Mechanical Engineering from Hebei University of Technology, Tianjin, China, in 1984. He received the M.E. and Dr. Eng. degrees in Mechanical Engineering Science from Tokyo Institute of Technology, Tokyo, Japan, in 1988 and 1991, respectively. From 1991 to 1992, he worked for Komatsu Ltd. as a Research Engineer, and from 1992 to 1993 he was at University of California, Riverside, California, as a Visiting Scholar. Since July 1993 he has been with Ibaraki University, Japan, Faculty of Engineering. Right now he is an Associate Professor in the Department of Systems Engineering. His research interest is in the design and control theory of new types of robots, the mechanism and control of redundant manipulators, and Bio-mechanics. He was awarded the Outstanding Paper Prize from SICE in 1992. He is a member of the IEEE, JSME, SICE, and the Robotics Society of Japan.

    Isao Kobayashi was born in Japan in 1974. He received his B.E. and M.E. degrees in Systems Engineering from Ibaraki University, Japan, in 1997 and 1999, respectively. From 1999, he has joined the Brother Industries Ltd. His research interest is in the mechanism and control of redundant manipulators. He is a member of the Robotics Society of Japan.

    This work was presented in part at 1998 Symposium on Image, Speech, Signal Processing, and Robotics (ISSPR’98), Hong Kong, 1998.

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