Journal of Applied Physics., 15 May 2002, Volume 91, № 10.

ELUCIDATION OF PROPULSIVE FORCE OF MICROROBOT

USING MAGNETIC FLUID.

Norihiko Saga ¹ and Taro Nakamura ² .

Faculty of systems science and Technology, Akita University,

Japan, 015 – 0055, Honjyo, 84 – 4, Tsuchiya.

E – mail: Saga@akita-pu.ac.jp, Taro@akita-pu.ac.jp

Abstracts

Using the pattern of the peristaltic movement of an earthworm, a microrobot was developed for traveling in a tube using a magnetic fluid. In this microrobot, a cell equivalent to a segment of the earthworm is composed of a natural rubber tube into which a water-based magnetic fluid is sealed up. The cells are connected with rod-like elastic bodies of natural rubber. It was confirmed that this robot can travel in an acrylic tube (inner diameter: 12 mm, outer diameter: 14 mm) by providing a shifting magnetic field from the exterior. This article will describe how our microrobot is propelled, the mechanism of its propulsion, and the analytical results of the propulsive force. 2002 American Institute of Physics [DOI: 10.1063/1.1452197]

I. Introduction

Our interests are fixed upon the peristaltic movement of earthworm as a locomotive mechanism that would replace the wheels or gait (walking). The earthworm runs the peristaltic movement that propagates the stretch wave from forward to backward by changing the length and width of its body [1]. Figure 1 illustrates this locomotion of the earthworm.

Figure 1. Locomotion pattern of an earthworm.

The driving method of the previous earthworm-type microrobot that runs the peristaltic movement used the evacuation pressure [2], shape memory alloy [3] etc. The demerits of these robots, however, include complicated structures, difficult miniaturization of driving actuator, and laborious directional control. Therefore, we propose an earthworm-type microrobot that employs a magnetic fluid in order to resolve these disadvantageous problematic points. In this configuration, a cell equivalent to a segment of the earthworm is composed of a natural rubber tube (thickness: 0,5 mm) in which a water-based magnetic fluid (W-35) is sealed up, and the cells are connected with rod-like elastic bodies of natural rubber. The total number of cells in this robot of our trial production was eight. Moreover, a shifting magnetic field is assumed to be electromagnet, but a permanent magnet is used in this experiment. Figure 2 shows the configuration of the microrobot.

Figure 2. Configuration of the microrobot.

II. Experiment and analysis

Figure 3 shows a diagrammed outline of an experimental device. First, the microrobot from our trial production was inserted into an acrylic tube (inner diameter: 12 mm; outer diameter: 14 mm). A pair of permanent magnets (Nd-Fe-B,  25X15 mm) provided on the right and left sides toward the advancing direction was made to travel at 40 mm/s. The clearance between the magnets was set at 16 mm. The displacement of the microrobot was photographed with a video camera and the image was analysed.

Figure 3. Outline of an experimental device.

Furthermore, the magnetic force exerted on a cell was analyzed by the finite element model (FEM). I estimed the travel speed of the microrobot from view data. 10 cc of magnetic fluid was sealed up into one cell and the contact area of this cell with the tube was assumed to be 60 mm2. The analytical model of the microrobot was simplified to the cube, and the microrobot was analyzed as a two-dimensional plane model. The values of physical properties used for the analysis were: 1 for relative magnetic permeability and 8,0 X 10⁵ A / m for coercive force of the permanent magnet, and 1 for the relative magnetic permeability of the air and 2,2 for that of the magnetic fluid.

III. Results and discussion

Figure 4 represents the movement image of the microrobot. With the state of the magnet which is situated in the top position toward the advancing direction as the reference (elapsed time t = 0), operational conditions were shown at stages of (a) – (e) elapsed time. From these, it is confirmed that, as the magnet displaces itself from (b) to (c), the cells approached by the magnet contract, thicker and shorter, and are drawn forward. At the stage of (d), the contraction of the body of the microrobot as a whole is elicited with the backward displacement of the magnet. At the stage of (e) where the magnet recovers its top position, the front side is drawn toward the magnet, while the tail is at a standstill due to the friction with the acrylic tube. Repeated movements of (a) – (e) allowed us to make sure of the advancement by peristaltic movement.

Figure 4. Movement image of the microrobot.

Figure 5. Measured locomotion of the microrobot.	Figure 6. Static magnetic analysis of the microrobot.

Furthermore, Figure 5 shows the quantitative results of the movement of a microrobot, which were obtained from the images. Based on the tail of a microrobot, there is a protrusion time showing a locomotion and a stance time showing the condition of standing still like an earthworm, and it turned out that this microrobot can go about 20 nm by one pass (3,6 s) of the magnet. It then was elucidated from the analysis of the magnetic field shown in Figure 6 that 2,9 N of propulsive force is produced on the cell when the center of the magnet is 10 mm ahead of the cell.

IV. Summary

A microrobot is tentatively produced consisting of a natural rubber tube and magnetic fluid. Providing, from the exterior, a moving magnetic field (magnetic force: 2,9 N) made it clear that the can move forward by earthwormlike peristaltic movement. For future studies, we to develop robots which can move inside the tube of nuclear power plants, inside the human body like an active catheter, and robots, which can move in rubble as a rescue robot.

Acknowledgments

This research was partially carried out with a Grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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