IROS2019国际学术会议论文集 2205

Magnetic Needle Assisted Micromanipulation of Dynamically Self Assembled Magnetic Droplets for Cargo Transportation Qianqian Wang1 Student Member IEEE Xingzhou Du1 2 Student Member IEEE Fengtong Ji1 and Li Zhang1 3 4 Senior Member IEEE Abstract Dynamic self assembly is treated as a promising approach for generating a robotic swarm to perform coor dinated tasks and the assembled pattern can be tuned by regulating the energy input However location of a dynamically assembled pattern is hard to be determined especially under global fi elds such as magnetic fi eld In this paper we report the formation and manipulation of dynamic self assembled droplets at the air liquid interface with the assistance of a magnetic needle Affected by the locally induced fi eld gradient near the needle reconfi gurable assembled droplets are obtained with higher time effi ciency and the location of the pattern can be determined The pattern is reversibly tuned to exhibit expansion and shrinkage by adjusting the height of the needle Assembled droplets are able to be steered via following the needle in a controlled manner Moreover cargo is trapped by exploiting the induced rotational fl ow around the droplets and it can also be caged into the central area of the pattern and transported to the desired location The proposed method opens new prospects of using energy dissipative pattern as an untethered end effector for microrobotic manipulation Index Terms Dynamic self assembly Small scale robots Mag netic actuation Collective behavior Micromanipulation I INTRODUCTION Untethered robots at small scales driven by external power sources have been drawing attention for decades due to their abilities to perform robotic manipulation task at small scales 1 2 Various design of small scale robots actuated by magnetic fi elds have been presented and different manipula tion tasks are reported A soft millimeter scale robot can per form pick and place and cargo release tasks through various actuation modes 3 A miniature magnetic robot is able to be actuated in two vibration modes which is exploited for effectively pushing and releasing a millimeter scale object 4 Jing et al recently reported that a microrobot with a sensing end effector is capable of manipulating cells with force sensing ability 5 Other manipulation strategies using small scale robots are reported ranging from millimeter The research work is fi nancially support by the General Research Fund GRF with Project No 14218516 from the Research Grants Council RGC of Hong Kong the ITF projects with Project No s ITS 440 17FP and MRP 036 18X funded by the HKSAR Innovation and Technology Commission ITC 1 Q Wang X Du F Ji and L Zhang are with the Depart mentofMechanicalandAutomationEngineeringofTheChinese University of Hong Kong CUHK Shatin NT Hong Kong China lizhang mae cuhk edu hk 2 Department of Biomedical Engineering The Chinese University of Hong Kong Shatin NT Hong Kong China 3Chow Yuk Ho Technology Center for Innovative Medicine The Chinese University of Hong Kong Shatin NT Hong Kong China 4 CUHK T Stone Robotics Institute The Chinese University of Hong Kong Shatin NT Hong Kong China Cage B Trap Transport Transport Magnetic needle ShrinkingExpanding Self assembly a b c Magnetic needle Fig 1 Dynamic self assembly of magnetic droplets at the air liquid interface a The assembled pattern exhibits expansion and shrinkage by adjusting the height of the iron needle and it is further steered in a controlled manner Taking advantage of the reversible pattern and the induced fl uid fl ow cargo can be trapped caged and transported to the desired location b Dynamic self assembly of droplets under a precessing magnetic fi eld c The reversible pattern under different heights of the iron needle Scale bars are 4 mm scale to microscale and even nanoscale 6 8 However the amount and size of cargo that a single robot can manipulate may meet a limitation and repeated transportation of cargo has to be applied 9 To solve these issues control of a swarm of robots is worth to be investigated especially at small scales A robotic swarm can be tuned by regulating the external energy input which is applied to perform cooperative tasks that is hard to achieve using a single agent 10 12 Compared to the usage of a single robot a larger amount of objects are delivered in a batch using a robotic swarm 13 14 Interactions among individual agents are essential for the formation of a swarm and complex interactions have been induced by various energy sources e g external fi elds 15 17 light 18 and chemical reactions 19 To obtain a stable swarm in a practical timescale self assembly offers a variety of choices and yields two major systems static and dynamic A static self assembly system remains stable once reaching equilibrium and usually hard to be tuned A swarm formed in a dynamic self assembly system dissipates energy and the interactions among building blocks dominate 2019 IEEE RSJ International Conference on Intelligent Robots and Systems IROS Macau China November 4 8 2019 978 1 7281 4003 2 19 31 00 2019 IEEE1595 20 15 10 50 Vertical distance from the needle mm 8 8 5 9 9 5 10 10 5 11 11 5 12 Total magnetic field strength mT a b c 0 mm 20 mm Y Z 0246810 Distance from the needle mm 0 018 0 016 0 014 0 012 0 01 0 008 0 006 0 004 0 002 0 Magnetic field gradient T m 5 mm 10 mm 15 mm Needle Needle Fig 2 Simulation results of the induced magnetic fi eld near the iron needle a White arrows indicate the fi eld direction color refers to the magnitude of the fi eld strength marked by the color legend on its right side The yellow arrow indicates the applied magnetic fi eld b Magnetic fi eld strength along the axial direction Z axis of the needle The inset schematically illustrates the data acquisition direction c Magnetic fi eld gradient along the Y axis at vertical distances of 5 mm 10 mm and 15 mm from the needle The curves are fi tted based on the data points from the simulation 0510152025 d 2 r3 rad s 2 mm 3 0 10 20 30 40 50 Hydrodynamic repulsion 10 8 N X Y ab Fr Fig 3 a Simulated fl uid fl ow induced by fi ve rotating droplets of 1 mm radius beneath the air liquid interface Angular velocity of droplets is set as 4 rad s Yellow arrows indicate the fl uid direction and color contour shows the magnitude of the fl ow velocity mm s b Simulated hydrodynamic repulsive forces Fr acting on a droplet inset The separation distances are set as 2 5 5 mm and angular velocities are set as 2 4 and 6 rad s Points are data from simulations and the line is the linear fi t of the data the formation of the pattern Regulating the energy input can tune the interactions and the resulting structures are usually not accessible under equilibrium status 20 21 Among these building blocks magnetic colloidal suspensions are promising candidates for designing various swarm patterns due to their high controllability fast response and diverse interactions By tuning the applied magnetic fi eld the swarm pattern can be tuned to perform coordinated tasks that would be impossible using a single manipulator 22 23 However the location of a dynamic self assembled pattern is hard to be determined especially in a system energized by a global magnetic fi eld The time of forming and tuning a swarm pattern is essential to the effi ciency of performing manipulation tasks Quickly form and steer a robotic swarm at small scale still remain a challenge and need investigation In this work we propose the magnetic needle assisted formation and manipulation of dynamically self assembled magnetic droplets at the air liquid interface Fig 1 A precessing magnetic fi eld is applied as the energy input and a locally induced inhomogeneous magnetic fi eld and gradient are generated near the magnetized iron needle Various ordered patterns of assembled droplets are obtained using a different number of droplets The interactions be tween droplets are modeled and the induced fi eld gradient is studied based on simulations Taking advantage of the induced fi eld gradient the required time for forming an ordered pattern decreases signifi cantly and the center of the assembled pattern is determined Moreover the assembled pattern is able to expand and shrink in a controlled manner by adjusting the induced fi eld gradient near the assembled pat tern Following the magnetic needle assembled droplets are able to be steered along a preplanned path We demonstrate that a cargo can be trapped by the induced rotational fl ow around the assembled pattern and a non contact manipulation is achieved Furthermore we show that the cargo can also be caged into the central area of the pattern and transported by exploiting the reversibility of the pattern at the air liquid interface Compared to the swarm that actuated by a coil system 24 our method effectively improves the effi ciency in assembly navigation and cargo manipulation process This strategy has a great potential for microrobotic application that requires fast gathering and high effi ciency and provides support to fundamentally understand collective behaviors at small scales II MATHEMATICALMODELING ANDSIMULATION One drop 5 L of carbonyl iron microparticle water suspension is added into dibenzyl ether and a droplet with a diameter around 2 mm is beneath the air liquid interface The pattern of a self assembled droplets is mainly affected by the magnetic hydrodynamic and capillary interactions among droplets All droplets are attracted towards the magnetized needle due to the induced magnetic gradient A Induced magnetic fi eld gradient To investigate the interactions between the magnetized needle and droplets a simulation of induced magnetic fi eld distribution is established using COMSOL Multiphysics The iron needle has a diameter of 1 mm and a length of 60 mm During experiments we use a precessing magnetic fi eld to actuate droplets and magnetize the iron needle After turning off the magnetic fi eld there is no detectable magnetic fi eld near the iron needle measured by LakeShore Gaussmeter model 410 The applied magnetic fi eld in simulation has an angle of 30 to the Z axis and a strength of 8 mT Fig 2 a shows the simulated magnetic fi eld distribution near the tip of the needle and the relationship between the total fi eld strength and the distance below the tip is plotted in Fig 2 b The fi eld strength decays with an increasing distance to the 1596 Fig 4 Experimental setup The Helmholtz coil system is controlled by the controller box and fi eld parameters are adjusted through the control PC The iron needle is fi xed on a copper holder A robot arm is placed at the right side to grip the holder A camera is mounted for video recording needle and approaches the strength of the applied magnetic fi eld Magnetized by the applied magnetic fi eld fi eld gradient is also induced near the needle The relationship of fi eld gradient along the Y axis at different vertical distances from the needle is plotted in Fig 2 c The fi eld gradients show the maximum value of 0 017 T m and the minus sign indicates the fi eld gradient has a direction opposite to the Y axis B Interactions between droplets The magnetic interactions between droplets are dominated by the dipole dipole interaction Here we only consider the force of interaction averaged over a fi eld cycle 2 and the interaction potential is expressed as 25 U r om 2 d 4 3cos 2 1 r3 3cos 2 1 2 1 where mdis the magnetic dipole moment of a droplet and is aligned with the external fi eld is the angle between the Z axis and the center vector r of two neighbouring droplets During experiments all droplets are beneath the air liquid interface and the centers of all droplets are coplanar 2 Therefore the magnetic force between droplets is obtained by derivation as Fm U r 2 3 om 2 d 4 r4 3cos 2 1 2 2 The above analysis indicates the magnetic interaction can be attractive or repulsive by changing the precession angle of the external fi eld i e attractive if 54 74 90 and repulsive if 0 54 74 At a particular angle of 54 74 the dipolar interaction averages to zero The adjustable mag netic interaction shows a difference from assembly systems that driven by a permanent magnet where the magnetic interactions between building blocks are attraction 16 26 During actuation a rotating droplet creates surrounding rotational fl ow and moves in a fl ow created by the remaining droplets Simulation results of induced fl uid fl ow are shown in Fig 3 a Two rotational fl ows are induced both inside the pattern central fl ow and outside the pattern surrounding fl ow The hydrodynamic interactions between droplets are governed by Magnus effect i e the generated lift force This is different from rotating objects in 2D where hydrodynamic 0 s30 s140 s 0 s15 s30 s Magnetic needle B a b c 0246810 Trials No 50 100 150 Time s 5 N 7 N 10 N 5 7 10 B Fig 5 Dynamic self assembly process of magnetic droplets a without and b with the magnetized needle at the air liquid interface Black and blue arrows illustrate the rotation direction of droplets and the precession of the assembled pattern Scale bars are 4 mm c Time requirement for a self assembly process with and without the magnetic needle Legend 5 N and 5 refer to the process of fi ve droplets with and without the help of the magnetic needle respectively Each line refers to the mean value of the corresponding ten trials 29 7 s 5 N 29 3 s 7 N 33 2 s 10 N 130 8 s 5 135 3 s 7 143 5 s 10 attraction overwhelms the Magnus repulsion and yields a co rotating system 27 In our system each droplet experiences a repulsion perpendicular to the local direction of fl ow Numerical simulation results by Climent et al show that no repulsion between two rotating spheres exists and their distance remains the same at zero Reynolds number 28 Therefore the repulsive interaction between droplets is clear an effect of fl uid inertia The velocity fi eld of fl uid generated by the jth droplet is written as uj x d rx ad rx 3 3 where rx is the position vector defi ned from the center of the droplet rx rx The hydrodynamic force acting on the ith droplets by the generated fl uids of the jth droplets can be estimated as 29 Fr cf 2 d a7 d ri rj ri rj 4 4 where cfis a constant of proportionality and ri rj is the distance between the two droplets The direction of the vector ri rj is pointing from the jth to the ith droplets indicating that this hydrodynamic interaction is repulsion between droplets Fig 3 b shows the simulated hydrody namic repulsive forces at different angular velocities d and separation distances r Based on Eq 4 the repulsive forces show the relationship of Fr 2 d r 3 and the simula tion results show good agreement with our analysis In Eq 1597 4 we only consider the interactions between two rotating droplets while in the simulation the forces are calculated in a pattern of fi ve droplets Because Fr r 3 the infl uence from non neighbouring droplets becomes negligible which explains why the simulation result of hydrodynamic forces in Fig 3 b still shows good agreement with our mathematical model The capillary attraction between millimeter scale objects beneath the air liquid interfaces exists and is expressed as 30 31 Fc r Lc 1 2 2 B2C2e r Lc 5 where 1 17 10 2N m is the surface tension of the surrounding liquid dibenzyl ether Lc p g is the capillary length and has a value of 1 04 mm B and C are two dimensionless parameters and are calculated to be 0 85 and 3 respectively 30 31 Unlike the previous work reported by Wang et al 16 here the capillary interactions become attractive rather than repulsive Additionally a fi eld gradient induced magnetic force is exerted on droplets pointing to the center of the pattern as F B md B 6 where the values of the fi eld gradient are obtained from simulation results Fig 2 Therefore the separation distance between droplets in an ordered pattern is able to be estimated by force balance between attractive forces Eq 5 6 and repulsive forces Eq 2 4 III EXPERIMENTALSETUP ANDMETHODS Magneticactuationisachievedusingathree axis Helmholtz coil system Fig 4 The three pair of coils are ac tuated by servo amplifi ers ADS 50 5 4 Q DC Maxon Inc and controlled by the control PC The input fi eld parameters are controlled by an I O card Model 826 Sensoray Inc through the controller box A camera is mounted at a tilting angle of 30 for video recording A fi ve degree of freedom robot arm is placed at one side of the coils and the iron needle is gripped by the gripper of the arm The iron needle has a length of 60 mm and a diameter of 1 mm The robot arm is actuated by Arduino Mega and controlled using a LabVIEW program A glass Petri dish fi lled with dibenzyl ether is put at the center of the coil system Carbonyl iron microparticles BASF China are microspheres with an average diameter of 3 m based on scanning electron microscope and these microparticles have ferromagnetic properties based on the magnetic hysteresis loop IV EXPERIMENTALRESULTS ANDDISCUSSION A Dynamic self assembly of magnetic droplets The magnetic microparticles sink to the bottom of the droplets because of gravity t 0 s Fig 5 a After turning on the precessing magnetic fi eld particle chains are formed fi rst due to the magnetic attractive forces be tween microparticles During actuation these chains are aligned and rotating with the external fi eld synchronously a b B ShrinkShrinkShrink 0 s20 s35 s 45 s60 s70 s ShrinkExpandExpand 468101214 Height of magnetic needle mm 1 2 3 4 5 6 Mean separation distance mm f 3 Hz f 4 Hz Fig 6 Reversible pattern tuned by adjusting the distance between the interface and the magnetized needle a Red arrows indicate the movement direction of the needle The magnetic fi eld has a frequency of 3 Hz and a 40 precession angle Scale bar is 4 mm b The relationship between the mean separation distance r and the height of the magnetized needle H at input frequencies of 3 Hz and 4 Hz The height is defi ned as the distance between the interface and the tip of the needle All error bars deno