A novel hybrid microrobot using rotational magnetic field ... · A novel hybrid microrobot using...

A novel hybrid microrobot using rotational magnetic fieldfor medical applications

Qiang Fu & Shuxiang Guo & Yasuhiro Yamauchi &Hideyuki Hirata & Hidenori Ishihara

# Springer Science+Business Media New York 2015

Abstract Magnetically actuated microrobots for such toolshave potential accomplish procedures in biological and med-ical applications. In this paper, a novel magnetically actuatedhybrid microrobot with hybrid motion driven by an electro-magnetic actuation system has been proposed. An o-ring typepermanent magnet is embedded in the hybrid microrobot as anactuator driven by rotational magnetic field which is generatedby a 3 axes Helmholtz coils. It is composed by two motionmechanisms. One is the spiral jet motionmoved by rotating itsbody. The other one is fin motionmoved by vibrating its body.Because only one permanent magnet is used inside the hybridmicrorobot, two motions can be controlled separately withoutany interference. The hybrid microrobot can change its twomotions to realize multi-DOFs movement and flexibility mo-tion. The verified experiments are conducted in the pipe. Theexperimental results indicate that the moving speed can becontrolled by adjusting the magnetic field changing frequencyand the direction of motion can be controlled by changing themagnetic field direction.

Keywords Multi-DOFsmovement . Magnetically actuatedhybridmicrorobot . Rotational magnetic field .

Electromagnetic actuation system . Hybridmotion

1 Introduction

The endoscope as a helpful tool for medical applications typ-ically refers to inspect the interior of the body in medicalprocedures (Guo et al. 2002 and 2003; Behkam and Sitti2005; Nacci et al. 2008; Yu et al. 2010; Mir-Nasiri andSiswoyo Jo 2011; Yim and Sitti 2012; Guo et al. 2014). Thefirst endoscope was developed by Philipp Bozzini (1806). Theendoscope was made by Lichtleiter (light conductor) for ex-amination of the interior of a hollow organ in the human body.In the 1960s, the first fiberoptic endoscope consisting of abundle of flexible glass fibres was invented by BasilHirschowitz and Larry Curtiss. It has the ability to coherentlytransmit an image and becomes a vital tool for diagnosinggastrointestinal (GI) diseases. There were physical limits tothe image quality of a fibroscope. The electronic endoscopebecame more popular with the development of the electronictechnology. Doctors can monitor intestinal image via the dis-play and perform examination, rather than staring at eyepiece.Generally, these endoscopes are used to examine inside a largeintestine. However, they brought much painful and traumaticdue to being driven by a cable or tube. Especially, they arehard to carry deeply to some small or narrow areas within thetissue of living in human body. To solve these problems, dif-ferent kinds of wireless capsule endoscopes (WCEs) havebeen developed (Moglia et al. 2007; Wang and Meng 2011;Basar et al. 2012). These WCEs are mainly consisted by anembedded camera, image sensor and battery for power supply.They are swallowed by the patient and travel through thegastrointestinal tract. Then it transmits images towards a mon-itor outside the body. In general, the time for medical

Electronic supplementary material The online version of this article(doi:10.1007/s10544-015-9942-0) contains supplementary material,which is available to authorized users.

Q. Fu (*) :Y. YamauchiGraduate School of Engineering, Kagawa University, 2217-20,Hayashi-cho, Takamatsu 761-0396, Japane-mail: [email protected]

S. Guo :H. Hirata :H. IshiharaFaculty of Engineering, Kagawa University, 2217-20, Hayashi-cho,Takamatsu 761-0396, Japan

S. Guoe-mail: [email protected]

S. GuoSchool of Life Science, Beijing Institute of Technology, 5 SouthZhongguancun Street, Haidian District, Beijing 100081, China

Biomed Microdevices (2015) 17:31 DOI 10.1007/s10544-015-9942-0

http://dx.doi.org/10.1007/s10544-015-9942-0

procedure is about 8 h. After examination, the capsule is ex-creted naturally from the anus. In addition, several companiesalso developed some capsule endoscopes which are used fordiagnosis of different GI parts, such as Norika, Olympus andso on. Conversely, compared with traditional endoscopes, the-se capsule endoscopes cannot allow the operator to realizeflexible control in the GI tract, due to their movement dependon the peristalsis of intestines.

At present, researchers have proposed different kindsof control method to address this problem. One is inter-nal control method and the other is external control (re-mote motion control). Generally, the motors (Wang et al.2005), shape memory alloy SMA (Kim et al. 2005) ac-tuators or ICPF actuators (Gao et al. 2011) are utilized torealize internal control for the capsule endoscope. Theyrequire inner power or a line from an external powersource. But it is still difficult to develop a microrobotwith compact structure, flexible locomotion and long-distance movement by power supply, because the threecharacteristics conflicts each other. A number of re-searchers have proposed numerous microrobotic controlmethods utilizing the magnetic field to realize remotemotion control (Honda et al. 2001; Pan and Guo 2007;Zhang et al. 2009; Jeong et al. 2012; Guo et al. 2013; Fuet al. 2014). The reason is that it can integrate the com-ponents for motion control and wirelessly power supplyinside microrobot. In particular, the use of magnetic fieldfor microrobot control is increasing. The mechanismsand structures of the microrobot are different due tousing the different type of magnetic field. Yesin et al.(2006a and b) proposed a kind of magnetic microrobot.This kind of microrobot is driven by a gradient magneticfield generating the pushing and pulling force to themicrorobot. However, the magnetic field strength forlocomotion depends on the microrobot materials,volume and environment. Pan et al. (2011) developed atype of wireless microrobot utilizing an elastic tail whichis driven by an alternating magnetic field. However, be-cause of the structural limitation of the control system,the microrobot is hard to realize changing its motiondirection. Another control method is using the rotationalmagnetic field which generates a rotational motion (Panand Guo 2009; Okada et al. 2012a). For example, Choiet al. (2009) developed an EMA system which consistsof Helmholtz and Maxwell coils. The microrobot canmove in the 2-D surface in this system. However, mostof microrobots based on a rotational magnetic field canonly realize a single motion. Therefore, locomotion ofthe microrobots is limited by changes in the environmentdue to their structure. In this paper, a novel type ofmagnetically actuated hybrid microrobot (MAHM) basedon a rotational magnetic field is proposed. The hybridmicrorobot with spiral jet motion and fin motion has a

small scale with a wireless power supply, can be pro-pelled by low voltage, and has a quick response. Andalso the hybrid microrobot can work for a long time inhuman. It can convert its two motions (spiral jet motionand fin motion) through the proposed structure of themicrorobot with rotational magnetic field, so that it real-izes the movement in the different environment. Thebody of the microrobot with a spiral jet motion can ob-tain a stable motion and high propulsive force than thespiral motion. The fin motion can improve the dynamiccharacteristic and reduce the shake which caused by theaxial propulsive force of the spiral jet motion. Spiralmotion and fin motion can be controlled separated with-out any interference, due to the hybrid microrobot hasonly use one actuator to realize the spiral jet motionand fin motion. The 3 axes Helmholtz coils is used togenerate the rotational magnetic field in any direction,which is used to provide a magnetic torque for control-ling the microrobot at the workspace. The experimentalresults indicated the hybrid microrobot can change itstwo motions to realize multi-DOFs movement and flexi-bility motion in the pipe. In addition, the moving speedis controlled by adjusting the magnetic field changingfrequency and the direction of motion is controlled bychanging the magnetic field direction.

A whole microrobot control system is illustrated inSection 2. A novel magnetically actuated hybridmicrorobot with hybrid locomotion is developed inSection 3. Electromagnetic actuated methods for the hy-brid microrobot is discussed in Section 4. Section 5describes the experimental assessment to confirm perfor-mance of hybrid motion and flexibility. Finally, conclu-sions and future works are illustrated.

2 Conceptual design of the whole control system

Generally, the patients are placed on a liquid dietstarting after lunch the day before the examination.And then, the patients should be drink the polytetramethylene ether glycol 2000 ml and water 500 ml,2~4 h before examination. After all preparations, themicrorobot is swallowed inside the digestive organsand can be manipulated (Moglia et al. 2007; Kelleret al. 2012). Conceptual diagram of the wholemicrorobotic control system is shown in Fig. 1. Thecontrol system consists of a magnetically actuated hy-brid microrobot, magnetic sensor array, three axes-Helmholtz coils, CT-Scan, monitor, and operation device(Phantom Omni device). Firstly, an intestinal image isgenerated by the CT scanner and the doctor views theintestinal image to confirm the area of a target in amonitor. Secondly, the doctor operates the Phantom

31 of 12 Biomed Microdevices (2015) 17:31

Omni device to control the magnetically actuated hybridmicrorobot to move to the target by using an externalmagnetic field. A 3 axes Helmholtz coils is used togenerate the external rotational magnetic field. Whenthe hybrid microrobot arrives at the target area, the op-erator controls the position and posture of the hybridmicrorobot to detect or treat disease in an intestinaltract. Meanwhile, the monitor shows the real time posi-tion and position of the hybrid microrobot calculatedfrom the measurement data of magnetic sensor array.The magnetic sensor array is used to realize the close-loop control and ensure the hybrid microrobot to reachthe target location.

3 Magnetically actuated hybrid microrobot design

3.1 Concept of the magnetically actuated hybrid microrobot

A novel type of magnetically actuated hybrid microrobot(MAHM) with compact and efficient structure is proposed,as shown in Fig. 2. The prototype of magnetically actuatedhybrid microrobot is illustrated in Fig. 3. The specification is

given in Table 1. The spiral propeller is connected to a tube.An o-ring type permanent magnet is assembled at the frontend of the tube and a bearing is assembled at the back end ofthe tube. Inner ring of bearing connects to the tube and outerring of bearing is fastened inside the body. The fin is connect-ed to the back end of the propeller with a bearing and a shaft.When the microrobot moves with the spiral jet motion, theouter shell of the microrobot is not rotates, it can be reducedamage for the intestinal wall and the fin can improve thedynamic characteristic and reduce the shake caused by theaxial traction force at the spiral jet motion.

3.2 Motion mechanism of the magnetically actuated hybridmicrorobot

The MAHM has two motion mechanisms. One is thespiral jet motion which can move by rotating the spiralpropeller. The other is fin motion which can move byvibrating the fin. The MAHM can switch between thetwo motions to realize movement in various workingenvironments. The spiral jet motion is used when themicrorobot needs precision operation and stable move-ment. The fin motion is used when the high propulsive

Fig. 1 Conceptual diagram of thewhole control system

(a) Overall view of the robot (b) Cross section

Fig. 2 Conceptual design ofmagnetically actuated hybridmicrorobot (MAHM)

Biomed Microdevices (2015) 17:31 of 12 31

force is needed. Due to just only use one magnet insidethe MAHM, two motions can be controlled separatelywithout any interference. It can obtain rapid responsivethan the robot (Okada et al. 2012b).

Based on the magnetic theory, rotation of themicrorobot in an external magnetic field requires at leasta pair of forces in opposite directions, and a torsionalmoment should be generated. The microrobot is rotateddue to an embedded inner o-ring type magnetic withradial magnetization, and the axial propulsive force isgenerated by pushing the fluid backward to obtain theforward motion due to the reaction force. Fig. 4a showsthe principle of bi-directional movement with spiral jetmotion according to changes in rotational direction ofthe magnetic field. When the rotational direction of themagnetic field is clockwise (CW), the microrobot gen-erates a forward propulsive force and the microrobotmoves forwardly. When the rotational direction of themagnetic field is counter-clockwise (CCW), themicrorobot generates a backward propulsive force andthe microrobot moves backwardly. In addition, whenan alternate magnetic field parallel to the direction ofadvance is applied, movement due to an impelling forcearising from a permanent magnet rotates and vibratesthe connected fin as shown in Fig. 4b. The rotationalmovement is shown in Fig. 5. When the plane of therotational magnetic field is CCW, the microrobot canturn left at a branch point. When the plane of the rota-tional magnetic field is CW, the microrobot can turnright at a branch point. Therefore, the velocity of thehybrid microrobot can be controlled by adjusting therotational magnetic field changing frequency. And the

direction of motion can be controlled by changing therotational magnetic field direction.

3.3 The principle of spiral Jet motion

The propulsive forcemodel of the spiral jet motion is shown inFig. 6. According to the fluid mechanics, spiral jet propulsivemechanism is analyzed. The velocity of the outflow is givenby Eq. (1).

v2 ¼ v1 þ A1⋅v1A2

¼ v1 þ Q

A2ð1Þ

Flow for the spiral jet motion is calculated by Eq. (2).

Q ¼ a⋅b⋅ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffip2 þ 2πrð Þ2

q⋅Ω ð2Þ

The propulsive force of the spiral jet motion is obtainedindicated in Eq. (3):

Fspiral ¼ ρ⋅A2⋅v22−1

2⋅CD⋅ρ⋅A⋅v22 ð3Þ

Where, Fspiral is the propulsive force of the microrobot. v1is velocity of the inflow. v2 is velocity of the outflow. Q is theflow for the spiral jet motion. a is the height of the spiral ditchand b is the width of the spiral ditch. p is the pitch of the spiral,and r is the radius of the robot’s width. Ω is the speed ofrevolution. A is the cross section of the body. A1 is the crosssection of the inflow. A2 is the cross section of the outflow.The simulation results are shown in Fig. 7.

The propulsive forces for various frequencies were mea-sured using a measurement setup which mainly consists of alaser displacement sensor and a copper beam and electric bal-ance (Pan 2008), as shown in Fig. 8. The copper beam wassoft enough to bend under the propulsive force and the pro-pulsive force is evaluated by the electric balance. The dis-placement of copper beam and propulsive force are able tobe considered as right triangle. According to the measurementof displacement (x) and the force (F), a calibration calculationof relationship between the propulsive force the bending dis-placement of the copper beam is obtained as shown in Eq. (4).

F ¼ 1:354� x ð4Þ

where, x is the bending displacement of the copper beammeasured by a laser sensor and F is the propulsive force.

Fig. 3 Prototype of magnetically actuated hybrid microrobot (MAHM)

Table 1 Specification of the magnetically actuated hybrid microrobot

Magnetically actuated hybrid microrobot

Length Diameter Weight Material

51 mm 14 mm 3.56 g Polystyrene

O-ring type magnet

Outer diameter Internal diameter Height Material

9.5 mm 5.1 mm 1.5 mm Neodymium


During the testing experiment of the propulsive force, themicrorobot was placed in the pipe which was filled the water.And then, by adjusting the frequency of input electric currentsto control the rotating speed of the body, the displacement ofthe copper beam was obtained by the laser sensor. At last, thecalibration calculation equation is used to calculate the pro-pulsive force. Measurement results of mean propulsive forceare shown in Fig. 9a. In experiment condition, the experimen-tal results are smaller than the simulation results due to drag

force of the water. This tendency is similar to the simulationresults. The Fig. 9b shows the measurement results of speed.The experimental result indicates speed of the microrobot be-came more faster with the frequency increasing.

3.4 The principle of Fin motion

According to Vortex Peg Hypothesis, the fish uses tail or fin topropel itself forward by thrusting its body against the

(a)) Spiral jet mot

(b) Fin motion

ion

n

Fig. 4 Bi-directional movement(a) Spiral jet motion (b) Finmotion

Fig. 5 Rotational movement


seemingly fixed vortices and utilizing their rotational energy.The placement and direction of these vortices indicate that thewake structure generated by the fin takes the form of a seriesof parallel waves or a reverse Karman vortex street which isindicative of thrust generation. The velocity field induced bythis wake structure expels a jet of water in the downstreamdirection, propelling the fin forward to conserve momentum.Then, thrust of fish-like robot generated by the added massand vorticity methods combine to yield the total propulsiveforce. In previous study, we have discussed that the undulatorymotion of the tail which can obtain good performances isbetter than the oscillatory motion. Therefore, the undulatorymotion is used to generate the thrust for the microrobot. Amodel of the tail with fin motion is shown in Fig. 10. The totalpropulsive force (Ffin) of fin motion and normal force (N) areindicated in Eqs. (5) (6) (7).

FFin ¼Z b fð Þ

2

−b fð Þ2

tdy ¼Z b fð Þ

2

−b fð Þ2

Lsinα−Dcosαð Þdy ð5Þ

N ¼Z b fð Þ

2

−b fð Þ2

ndy ¼Z b fð Þ

2

−b fð Þ2

Lconαþ Dsinαð Þdy ð6Þ

The induced flow angle (α) is

α ¼ tan−1Vx

V z

� �ð7Þ

where, t is propulsive force. n is normal force. Vx is velocity ofthe x direction and Vz is velocity of the z direction. The dis-placement b is a function of the frequency (f).

Using Eqs. (5) (6) (7), the power is calculated, as followsEq. (8):

P ¼Z b

2

−b2

nh0−mk

0� �dy ¼

Z b2

−b2

h0Lcosαþ Dsinαð Þ−mk 0

h idy ð8Þ

where, h is the heaving motion of the fin. k is the featheringmovement of the fin.

The mechanical efficiency η is calculated by Eq. (9):

η ¼ FFinV

P¼ CT

CPð9Þ

where, CT is propulsive coefficient and CP is powercoefficient.

The resonance frequency of the fin motion is confirmed byEqs. (10) (11) and (12). Assuming the beam deflection for thetransverse vibration, the transverse vibration of fin is calculat-ed by Eq. (10). The conditional expression by using fixed endis shown in Eq. (11).

EI∂4y∂x4

þ ρA∂2y∂t2

¼ 0 ð10Þ

1þ cosβ⋅coshβ ¼ 0 ð11Þ

The resonance frequency (f) of the fin can be calculated asfollows (12):

f ¼ β2π

1

L2

ffiffiffiffiffiffiEI

ρA

sð12Þ

where, β is characteristic value, E is young’s modulusof the fin, I is second moment of area, L is length ofthe fin, ρ is density, A is cross section of the fin. Therelationship between resonance frequency and displace-ment of fin is measured with measurement system.Figure 11 shows the experimental results that the

Fig. 6 Propulsive force model of the spiral jet motion

Fig. 7 Simulation results of propulsive force

Laser Di

Copper Be

A

X

isplacement Se

M

eam

C

ensor

Microrobot

A/D

Computer

Water

Fig. 8 Measurement system of the propulsive force


maximum displacement of the fin with the same widthand the different length is measured. The resonance

frequency is from 0 to 20 Hz. And maximum displace-ment was obtained at 7 Hz.Figure 12 shows the exper-imental results that the performance of the fin motionwith different materials. One is the fin which is made ofpolyethylene terephthalate (PET). The other is the finwhich is made of ethylene-vinyl acetate (EVA) andP E T . T h e s i z e i s c o n s t a n t w h i c h i s20 mm*10 mm*0.1 mm. It indicated the fin which iscomposed by PET and EVA obtained higher movingspeed and wider frequency than the fin which is madeby PET. Because the former is softer than the latter, theformer one can realize the higher moving speed withthe undulatory motion better. Therefore, the former isutilized for the microrobot in our research.

4 Force analysis of microrobot

When the microrobot moves in the fluid, the force consists ofpropulsive force, hydraulic resistance buoyancy and gravity.Motion equations of the microrobot are given in Eqs. (13) and(14).

Ma ¼ Fspiral þ FFin−FD þ Fρcosγ−Mgcosγ ð13Þ

FD ¼ 1

2� CD � ρ� A� v2 þ 2� C � πrωL ð14Þ

(a) Measurement results of propulsive force

(b) Measurement results of speed

0 5 10 15 20 25 30 35 400

0.5

1

1.5

2

2.5

3

3.5

Frequency (Hz)

Spe

ed (

mm

/s)

Experimental resultsCurve fitting data

Fig. 9 Measurement results (a) Measurement results of propulsive force(b) Measurement results of speed

(a)Force analysis in X-Z plane (b)Sectional view in X-Y plane

tN

X

Z

b/2

-b/2

Moving direction

θ

X

Y Moving direction

L

D

Fig. 10 Themodel of tail with finmotion

Fig. 11 Relationship between frequency and displacement of fin withdifferent length


where, M is the mass of the microrobot. CD is the coefficientof resistance. ρ is fluid density. a is acceleration. FD is totalresistance. A is projected area. γ is angle with horizontalplane. L is the length of the microrobot.

The magnetic field applies a magnetic torque for themicrorobot to produce the propulsive force. The magneticforce FM and magnetic torque TM acting on the o-ring magnetin the external magnetic field of the 3 axes Helmholtz coils isgiven by the Eqs. (15) and (16):

FM ¼ μ0V M ˙∇ð ÞH � sinα ð15Þ

TM ¼ μ0VM � H � sinα ð16Þ

where, M is the average magnetization of the internal magnetand V is the volume of the internal magnet. H is magnetic fieldintensity. α is the angle of between M and H.

The rotating torque TR is calculated by Eq. (17),

TR ¼ I � θ″ þ C � θ0 ð17Þ

where, I is moment of inertia, C is coefficient of viscosityfriction, θ is rotational angle. During the measurement exper-iment, we adjusted the frequency of input electric currents tocontrol the rotating speed of the microrobot. The torque wasmeasured by the torque sensor at the same time. The relation-ship between rotating speed and rotation torque is measured,as shown in Fig. 13.

5 Experimental results

The experimental setup and the schematic diagram areshown in Fig. 14. The hybrid microrobot is placed in atransparent pipe with diameter 26 mm. The transparentis filled with water (water density: 998.203 kg/m3) attemperature 22 °C and inserted 3 axes Helmholtz coils.The web camera will monitor inside of pipe and gener-ate a visual image on the display. Meanwhile, the oper-ator operates the Phantom Omni device to control theposition and posture of the hybrid microrobot whileviewing the display. The display shows the data to ob-tain the real-time position of the robot. The drive sig-nals are generated by Phantom Omni device and sent tothe 3 axes Helmholtz coils to control the hybridmicrorobot. The magnetic flux density changing fre-quency is shown by oscilloscope.

5.1 Multi-DOF locomotion of the hybrid microrobot

Using the experimental setup, the multi-DOF locomo-tion of the hybrid microrobot is to be tested in thepipe. The microrobot realized multi-DOFs locomotionby this spiral jet motion and fin motion. Themicrorobot moves forward-backward and stops bychanging the rotational uniform magnetic field.Additionally, it is confirmed the microrobot has equiv-alent moving speed even backward by experiment.Experimental results of the hybrid motion in a horizon-tal direction are shown in Fig. 15. Experimental resultsof the moving state of the microrobot in vertical direc-tion are shown in Fig. 16. Based on experimental re-sults, the hybrid microrobot realized the movement inthe horizontal plane and vertical plane by adjustingrotational magnetic field.

Experiment of forward-turning-forward locomotionis realized and evaluated, as shown in Fig. 17.Experimental states of the microrobot which is turningleft are shown in Fig. 17a. The microrobot moves for-ward along X axis toward the center by rotating mo-tion or fin motion, as shown in Fig. 17b. Secondly, theplane of rotational magnetic field is rotated by

Fig. 12 Relationship between frequency and moving speed withdifferent materials

Fig. 13 Relationship between rotating speed and torque


counter-clockwise (CCW), the microrobot turns left atbranch point, as shown in Fig. 17c. Finally, themicrorobot moves forward along Y axis as shown inFig. 17d. In the same way, the plane of rotationalmagnetic field is rotated by clockwise (CW), themicrorobot turns right at branch point. Experiment ofbackward-turning-forward locomotion is also conduct-ed and evaluated.

5.2 Experimental results and discussions

The relationships between the propulsive force, movingspeed and frequency are indicated in Fig. 18. The ex-perimental results implied that when the hybridmicrorobot with spiral jet motion move from 10 to40Hz, the moving speed is increasing with the propul-sive force increasing. The maximum propulsive forceis 5 .4mN. The performance of hybr id mot ionmicrorobot in water is evaluated in water. The experi-mental results show the fin motion and the spiral jet

motion work in different frequency, as shown inFig. 19. The hybrid microrobot can move using thefin motion below 10Hz. The maximum moving speedis 4.74 mm/s. And it can move using the fin motion orspiral jet motion from 10 to 25 Hz by controlling therotational magnetic field. The maximum speed of thefin motion is 7 mm/s at 14 Hz. While the frequency isabove 25Hz, the hybrid microrobot moves using thespiral jet motion. Additionally, the performance is eval-uated in oil to prove the microrobot is able to move ina high viscosity liquid, and the result is shown inFig. 20. The maximum moving speed is 0.22 mm/sin oil. According to these results, we did an experi-ment of hybrid motion by adjusting the rotational mag-netic field. The experimental results of forward withfin motion (FF), stop motion (S), forward with spiraljet motion (FS) and backward with spiral jet motion(BS) are shown in Fig. 21.

In this experimental result of spiral jet motion, thereare two phenomena of interest. One is the initial

((a) Experimentaal setup (b) Schematic diaggram of the conntrol system

Fig. 14 Whole control system

(

(

(a) t=0s

(e) t=23s

(b) t=5s

(f) t=26

(c

6s (

c) t=13s

(g) t=35s

(d) t=20s

(h) t=45s

s

s

Fig. 15 Moving state inhorizontal direction. (a) and (b)are forward motion with finmotion. (c) and (f) are stopmotion. (d) and (e) are forwardmotion with spiral jet motion. (g)and (h) are backward motion withspiral jet motion


frequency (10Hz). It means that the hybrid microrobotgenerates enough propulsion to overcome resistance offluid at this frequency. The other one is step out fre-quency (40Hz) that means the frequency above whichthe microrobot can no longer rotate continuously in syn-chronous with the rotational magnetic fields. Step-outfrequency is well understood for magnetic helicalmicrorobots in uniform fields, it is discussed in ourprevious researches. The experimental results of the spi-ral jet motion indicated the linearity between magneticfield changing frequency and the moving speed from 10to 40Hz. The approximate numerical formula of spiraljet motion is shown in Eq. (18). Because the correlationcoefficient is 0.9693, it has a high correlation. The

approximate numerical formula of fin motion is shownin Eq. (19).

vspiral fð Þ ¼ 0:08921 f −0:4737 ð18Þ

vfin fð Þ ¼ −0:03279 f 2 þ 0:8229 f ð19Þ

6 Conclusions

In this paper, to increase the dynamic efficiency andadapt this EMA system to use in various working envi-ronments, a novel magnetically actuated hybrid

(a) tt=0s (b) t=4s (c) t=10s

Fig. 16 Moving state in verticaldirection

Fig. 17 Forward-turning-forward locomotion


microrobot with spiral jet motion and fin motion is pro-posed. The structure and mechanisms of hybrid motion(spiral jet motion and fin motion) are analyzed. Andthen major part of EMA system which is 3 axes Helmholtzcoils is analyzed, designed and evaluated performance, in or-der to realize flexible control the hybrid microrobot. Finally,the motion characteristics of hybrid microrobot are evaluatedand the hybrid realized the multi-DOFs locomotion in pipe.The experimental results indicated a number of advantages ofthe hybrid microrobot as follows:

1. The hybrid microrobot has a compact structure and wire-less power supply and a small size.

2. The hybrid microrobot with one actuator can bedriven to realize hybrid motion by a rotational mag-netic field.

3. The hybrid motion can be controlled separated with-out any interference. In addition, the fin can im-prove the dynamic characteristic and reduce theshake which caused by the axial propulsive forcefor at the spiral jet motion.

4. The hybrid microrobot can move in different envi-ronment by adjusting the hybrid motion or fin mo-tion at different frequency. By adjusting the magnet-ic field changing frequency, the moving speed iscontrolled. The maximum speed of spiral jet motionis 4.74 mm/s and the maximum speed of fin motionis 7 mm/s.

5. Exper imen ta l r e su l t s ind ica ted the hybr idmicrorobot realize flexible motion in pipe byadjusting the magnetic field changing direction.Such as, horizontal motion, vertical motion, turn-ing motion and so on.

Future work of this research will focus on optimizingthe structure to increase propulsive force. In addition,the propulsive force can be limited at high frequencydue to the dynamics of the 3 axes Helmholtz coils sys-tem and natural frequency, the actuation frequency alsoshould be improved. And also, the real time imageshould be feedback to operators for an accurate controlabout moving speed and direction of hybrid microrobot.It has a potential capability to satisfy the requirementsfor medical applications.

Spe

ed (

mm

/s)

Fig. 18 Relationships between propulsive force, moving speed andfrequency

Fig. 19 Hybrid motion in water

Fig. 20 Hybrid motion in oil

Fig. 21 Forward-stop-backward motion


Acknowledgment This research was supported by Kagawa UniversityCharacteristic Prior Research Fund 2013.

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