Evaluation Performance of the Magnetic Rotational Field for Magnetically Actuated Microrobot System

Qiang Fu*1, Zhuocong Cai*1, Jian Guo*1, Shuxiang Guo*1*2

1Tianjin Key Laboratory for Control Theory & Application in Complicated Systems and Biomedical Robot Laboratory Tianjin University of Technology, Binshui Xidao 391, Tianjin, China

2Faculty of Engineering and Design, Kagawa University, Takamatsu, Kagawa 760-0396, Japan

guoshuxiang@hotmail.com Fuqiang6369@hotmail.com

*Corresponding author Abstract - At present, the capsule robot for clinical examination has been widely used, which can complete the tasks that ordinary cable gastrointestinal endoscopy cannot achieve, especially the magnetic driven capsule robot has become a research trend because of its active control. However, the active control accuracy of the magnetic driven capsule robot is not high, so it is not enough to reach the level of clinical examination. In this study, in order to deal with the difficult operation of the medical magnetic driven capsule robot, a method to drive the micro robot more accurately by controlling the magnetic field is proposed. The electromagnetic simulation is consistent with the parameters of the actual platform. Moreover, its rationality can be verified by mathematical model. The magnetic field platform for medical examination can be improved through this method to achieve a more accurate control level of the capsule robot.

Index Terms - Capsule robot. Magnetic driven microrobot. Electromagnetic simulation.

I. INTRODUCTION

In medical clinical examination, colonoscopy with long cable cannot complete all intestinal examination, and will bring great discomfort to patients. Therefore, in the past research, due to its small size and comprehensive examination, including the small intestine, capsule robot has been widely used in the medical field, and with the development of science and technology, a variety of intestinal robots with less discomfort to patients were designed, and various driving methods were designed to drive the active movement of micro robots [1]-[9]. Particularly, microrobot driven by external magnetic field has great potential because it can realize the active motion of robot through external control, so it has been widely studied.

The magnetic capsule micro robot system is realized by doctors through the main equipment remote control robot to achieve various medical tasks [10]-[11]. And Fig. 1 shows the overall control system of the medical examination micro robot driven by external magnetic field. At the main end, the doctor controls the software platform through external control. Once the slave end receives the command from the main end, the

platform of the three-axis Helmholtz coil generates a rotating magnetic field. Then the doctor operates the wireless micro robot through the image monitor to control the speed and direction of the robot, so as to detect or treat diseases in the human intestinal environment. The sensor feedback can be realized by the O-type magnet inside the capsule micro robot, and the calculated data can be displayed in the external monitor. The current position and posture of the robot can be obtained by using the least square method. Therefore, doctors can accurately obtain the position and posture of the wireless micro robot in the human body, and accurately control the magnetic micro robot to complete the medical task.

This paper is organized as follows. Firstly, the micro robot model and dynamics principle are introduced. Secondly, the driving principle of the external magnetic field is introduced. Then, the electromagnetics simulation model which is the same as the experimental platform is established, the correctness of the simulation is verified by the mathematical model, and a method to measure and control the torque of the permanent magnet in the robot is proposed. The last part of this paper puts forward the conclusion of this study.

II. MOTION MODEL OF CAPSULE ROBOT

Most of the traditional capsule robots rely on the natural peristalsis of human intestine, which makes the inspection incomplete and the disease cannot be treated. Based on the previous research, a capsule robot with embedded O-ring permanent magnet as the driver is proposed [12]-[15]. The robot can realize active motion through spiral jet motion, which has the advantages of flexible motion, simple structure and good stability, as shown in Fig.2.

The robot is equipped with a shell to prevent friction damage to the intestinal tract. The O-type permanent magnet embedded in the spiral structure is driven to rotate by the external drive, and the spiral structure of the robot rotates together. As the fluid enters and flows into the shell, the reaction force formed by the fluid movement is pushed the robot forward when the spiral structure rotates to push the

fluid backward. The motion pattern of the capsule robot in the intestine is shown in Fig. 3, and the motion characteristics can be calculated by equation (1) [10].

sin sinP R B Gvm F F F Ft

θ θ∂= − ±

∂ (1)

where, FP is the propulsive force, FR is the resistance, FB and FG is buoyancy and gravity respectively.

III. PRINCIPLE OF MAGNETIC CONTROL

In order to realize that the capsule robot can be accurately controlled by the external power supply and realize the active motion of the robot, it can be used the external rotating magnetic field to drive the spiral jet robot to rotate and move forward. In previous studies, a three-axis Helmholtz coils which can produce a rotating magnetic field in any direction was proposed [17]-[27]. The experimental platform of three-axis Helmholtz coil is established, and the parameters are shown in Table Ⅰ. According to Biot-Savart law, the magnetic flux density of any point on the axis of each pair of Helmholtz coils can be calculated by equations (2) [16].

2

0 03 3

2 22 22 2

1 1=2

2 2

i NRB

L LR x R x

µ

+ + − + +

(2)

where, B is the magnetic flux density. i0 is the current flowing into the coil, μ0 is the vacuum permeability, d is the distance between a pair of coils. And x is an position from the center of a pair of coils.

The external magnetic field generated by the three-axis Helmholtz coil acts on the O-type permanent magnet embedded in the microrobot, which exerts magnetic torque to generate propulsion force. The magnetic force and magnetic moment are given by equations (3) and (4) [16]. ( )0F V M Bµ= ∇ (3) 0T VM Bµ= × (4) where M is the magnetization of the permanent magnet. V is the volume of the permanent magnet. B is the magnetic flux density.

Fig. 2 Prototype of the capsule robot

Fig. 3 Movement mode of the capsule robot

Fig. 4 Model of the three-axis Helmholtz coil

TABLE I PARAMETERS OF THREE-AXIS HELMHOLTZ COIL

X-axis Y-axis Z-axis

Inner diameter (mm) 294.3 211.4 157.4

Winding diameter (mm) 1.8 1.8 1.8

Turns per coil 216 174 126 Distance of coil center

(mm) 147.15 105.7 78.7

Resistance (Ω) 5.52 3.29 1.86

Winding material Cu Cu Cu

Fig. 1 Inspection method of robot system.

IV. SIMULATION RESULTS

A. Magnetic driven method According to the characteristics of three-axis Helmholtz coil, the same coil model as the actual experimental platform is established by the software of Ansoft Maxwell, and the electromagnetic simulation of the model is carried out. By applying the same current to each pair of coils, the established coil model is shown in Fig. 4, and the simulation results of magnetic induction intensity at a certain time after applying current are shown in Fig. 5.

Fig. 4 shows the model of the three-axis Helmholtz coil, and Fig. 5 (a), (b) and (c) are the results of the magnetic flux density of the cross-section in the three axis directions, respectively. It can be observed that a certain range of uniform magnetic field is generated in the central region by applying the same current (1A) to each pair of coils, and through measurement and calculation, the generated uniform region is consistent with the ideal uniform region of the experimental equipment. The capsule robot can maintain a highly controllable state in the uniform area. And due to the effect of magnetic dipole moment, the O-magnet embedded in the robot will follow the magnetic pole of the magnetic field. When the magnetic field rotates, the magnet will almost keep consistent with the rotation of the magnetic field, and with the increase of magnetic induction strength, the rotation speed of the microrobot will also be higher.

Since the three-axis Helmholtz coil model is established in software under the same specifications as the actual experimental platform, the comparison results of the magnetic flux density data collected at the central point and the formula calculation under ideal conditions are shown in Fig. 6. The simulation results are almost consistent with the ideal results. It shows that with the increase of input current, the magnetic flux density also increases in proportion. That is to say, if a greater magnetic flux density is needed to drive the robot, a greater current needed to be pumped into the coils.

(a) Magnetic flux density of YZ-plane

(b) Magnetic flux density of XY-plane

(c) Magnetic flux density of XZ-plane

Fig. 5 Electromagnetic simulation of three-axis Helmholtz coil.

Fig. 6 Relationship between the electric current and magnetic flux density.

B. Magnetic torque For the precise measurement of the magnetic moment of the O-ring permanent magnet embedded in the capsule robot, the simulation design is adopted according to the actual size of the experimental platform and the same input signal under ideal conditions. As a result of only the O-ring magnet embedded in the robot participates in the electromagnetic analysis, the other parameters are not considered, so the O-type permanent magnet model is only established in the three-axis Helmholtz coil model to measure the magnetic torque. And simulation results are shown in Fig. 7. Fig. 7 (a) shows the torque of the permanent magnet in the positive Y-axis direction in a uniform magnetic field by controlling the direction of the magnetic field, and the magnetic induction intensity cloud diagram of the O-type magnet in the current coil environment. Fig. 7 (b) shows that a vector diagram of the magnetic flux density of a permanent magnet under a uniform magnetic field generated by the three-axis Helmholtz coil. The permanent magnet will be subjected to torque in the direction of the magnetic pole and rotate in the direction of the magnetic field. Through electromagnetic simulation, it can be collected that the magnetic torque of the O-type magnet changes with the change of magnetic induction intensity, and the results are shown in Fig. 8.

In theory, the stronger the magnetic induction intensity of the magnetic field generated by the three-axis Helmholtz coil, the greater the magnetic torque received by the O-ring magnet, so the rotation speed of the robot will be greater. Moreover, the rotation speed and axial speed of the robot can be set through the relationship between the magnetic torque and the parameters of robot model, so as to achieve the goal of precise control of the robot.

V. CONCLUSION

This paper proposes a method to measure magnetic flux density and torque of the magnetic-controlled capsule robot in the magnetic field platform. This method can realize the design process before medical clinical examination and the design of control algorithms for each type of magnetic-controlled robot. This method can obtain the relationship between magnetic flux density and external magnetic field as well as the relationship between magnetic field and the torque of the internal O-ring magnet of the robot.

ACKNOWLEDGMENT

This research is supported by Natural Science Foundation of Tianjin (18JCYBJC43200) and Tianjin Key Laboratory for Control Theory and Application in Complicated Systems (TJKL-CTACS-201903) and Innovative Cooperation Project of Tianjin Scientific and Technological Support (18PTZWHZ00090).

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