IROS2019国际学术会议论文集 2169

Abstract In this paper we describe the newly proposed capillary force gripper that combines fast water refilling via the capillary phenomenon and fast droplet forming via the on off control of a piston slider The capillary force is suitable for capturing and releasing heterogeneous and complex shaped micro objects because it is one of the most dominant forces in the microscopic world and acts on any shaped objects by the water flexible deformation However a water droplet easily evaporates and loses during every pick and place operations To solve this problem we developed a gripper that can quickly form droplets with simple control In the experiments we confirm that the gripper can generate a sufficient capillary force to grip a 1 mm3 micro object for over 100 s Furthermore we realize the automatic pick and place of cube cone and semicylinder samples We compare the positioning errors among them to discuss the feasibility future prospects and applications in electronics and MEMS fields I INTRODUCTION Recently the miniaturization and sophistication of products are advancing in the automobile optics electronics and medical fields with the development of micro manufacturing technologies such as the micro stereolithography MSL process 1 2 3 or micro laser sintering MLS process 4 5 Therefore there is growing demand for assembling miniscule and complicated parts that are less than 1 mm3 in size In the conventional surface mounting technology SMT the suction force generated by compressed air is mainly used to pick up flat electronic components This method has various advantages such as cleanliness high speed and high load capacity However it cannot pick up complex shapes due to the air leakage it can only hold flat shaped objects Various methods have been proposed to manipulate micro complex shapes The following methods are available the use of frictional forces 6 7 electrical polarization 8 9 van der Waals forces 10 capillary force by the surface tension of water 11 12 13 14 ultrasonic wave and laser trapping force 15 16 Although the capillary force cannot adapt to operations that require strong forces such as pushing pulling and cutting it can pick and place heterogeneous and complex shaped micro objects The reason is that the capillary force is one of the most dominant forces in the microscopic world and acts on any shape via the flexible deformation of water it is suitable for manipulating flexible fragile and complex shaped objects In particular this method is feasible for manipulating wires and helixes as actual applications Wires The authors are with the Department of Mechanical Engineering Yokohama National University 79 5 Tokiwadai Hodogaya ku Yokohama Kanagawa Japan phone 81 45 339 3693 e mail ohmif ynu ac jp are an important basic component in electronic circuits and MEMS parts 17 Helixes made from shape memory alloy SMA are one of the feasible MEMS actuators because of their large generative force and displacement 18 In the previous work we proposed the capillary force gripper using a pair of wetting rods to decrease the positioning errors in pick and place operations 19 However water droplets at the tip of the rod evaporate in a few seconds The active hydrophilicity controlling method was developed to control the capillary force by extending and contracting hydrophobic and hydrophilic elastic rings 20 The combination of the capillary force and an air nozzle was also developed with additional pressure sensor and air compressor to control the volume of the droplet 21 Those methods require one to supply the water droplet from the outer water tank to the nozzle because a water droplet easily evaporates and loses during every pick and place operations thus a fast and simple droplet forming method is required In this paper we describe a newly designed gripper with a fast droplet forming mechanism and simple control We describe the dynamical analysis in chapter II the design in chapter III the evaluation of the capillary force in chapter IV and the experimental setup for the automatic pick and place as shown in Fig 1 in chapter V In chapter VI we evaluate the positioning errors for cubic conical and semicylindrical shaped objects as an initial investigation of the feasibility of the proposed method for complicated shaped micro objects Capillary Force Gripper for Complex Shaped Micro Objects with Fast Droplet Forming by On Off Control of a Piston Slider Wataru Hagiwara Takatoshi Ito Kenta Tanaka Ryota Tokui and Ohmi Fuchiwaki Member IEEE Fig 1 Working area of the pick and place of cubes cones and semi cylinders with 1 mm length by the newly proposed capillary force gripper 1mm IEEE Robotics and Automation Letters RAL paper presented at the 2019 IEEE RSJ International Conference on Intelligent Robots and Systems IROS Macau China November 4 8 2019 Copyright 2019 IEEE II APPROXIMATION OF THE CAPILLARY FORCE The capillary force is geometrically determined if we approximate that the section curves of the meniscus are parts of the arc 11 22 In this chapter we approximate the capillary forces using geometrical parameters when the objects to pick up are a plane a cone and a cylinder A Plane Here we consider the capillary force that acts between two parallel planes as shown in Fig 2 Fig 2 Capillary bridge between two parallel planes Here nd nre etermide by the nrc npproximntiod ns follows 1 2 is efide ns the rn ius of the nrc npproximnte mediscus is the istndce betweed the two plndes nd nre the codtnct ndgles of the mediscus to the top nd bottom plndes respectively is gived by the followidg 3 The cedter of curvnture of the mediscus is gived by the followidg 4 5 Here we npproximnte thnt volume of the liqui bri ge is equnl to thnt of the columd ns follows 6 where is the midimum rn ius of the sectiod of the liqui bri ge is represedte by nd from 6 ns follows 7 The mend curvnture C of the mediscus is ns follows 8 where is n positive The secod term id right hnd si e of 8 becomes degntive becnuse codtributes to the codcnve surfnce The ifferedce id pressure ncross the idterfnce is gived by Lnplnce s equntiod ns follows 9 Here nd nre efide ns the liqui nd ntmospheric pressures respectively is the surfnce tedsiod is gived by the followidg 10 We etermide the verticnl gederntive force usidg Lnplnce pressure ns nd usidg the surfnce tedsiod ns From 9 is represedte ns follows 11 is cnlculnte ns the pro uct of the surfnce tedsiod nd the ledgth of the idterfnce circle ns follows 12 The require cnpillnry force is the sum of nd ns follows 13 From 3 7 11 12 and 14 we approximate the capillary force as follows 14 B Cone Here we consider the capillary force that acts between a plane and a cone as shown in Fig 3 11 Fig 3 Capillary bridge between a plane and a cone From n similnr geometricnl cnlculntiod the cnpillnry force is ns follows 15 Where 16 17 C Cylinder Finally we consider the capillary force that acts between a plane and a cylinder as shown in Fig 4 11 Fig 4 Capillary bridge between a plane and a cylinder Similnrly the cnpillnry force is gived by the followidg 18 III DESIGN The gripper consists of the flow channel tank piston valve stem and valve membrane For the fabrication the flow channel devices tank and valve stem were made of acrylic The piston was made of PTFE The valve membrane was made of silicon rubber We used a glass tube for the tip of the channel which we call the nozzle In this study we used water as the liquid whose surface tension is 52 mN m as measured by the ring method This gripper is designed to grasp objects with the maximum volume of 1 mm3 In the experiments we used acrylic samples with a density of 1 2 g cm3 so we require more than 12 N of the capillary force Both contact angles and to the acrylic samples and glass tube are estimated to be 60 degrees The outer radius of the glass tube is 0 5 mm If we estimate that the height of the capillary bridge is 0 3 mm the liquid volume is 0 2 mm3 and the radius of the bottom section is equal to the capillary forces between two flat planes are estimated to be approximately 189 N from 15 The estimated capillary force is sufficient because it is over 10 times larger than the gravity force The capillary forces are also estimated to be 114 and 119 N if the objects are a cone and a cylinder respectively as listed in Table I This gripper is also designed to apply the droplet on a substrate We use the droplet on the substrate to release the picking up object in experiments in chapter VI Fig 5 shows the principle of water filling by pulling back the valve stem and piston Fig 6 shows the principle of forming a droplet by pushing the piston and the valve stem The piston and valve stem are connected to a piston holder not shown in Fig 5 and 6 As shown in the left panel of Fig 5 the piston and valve stem are pushed up by the shrinking force of a spring As shown in the right panel of Fig 5 when the flow channel is raised the piston and valve are relatively moved downwards the channel is automatically filled with water because of the capillary phenomenon As shown in Fig 6 when the channel is moved down again the valve closes the channel to prevent the backflow the piston pushes water the droplet is formed at the tip of the channel This mechanism also enables one to automatically refill water to the tip of the gripper even if the water evaporates from the tip of the gripper Therefore regardless of the amount of water evaporation the positions with water refill are always constant The volume of the droplet is changeable according to the relative displacement and speed between the flow channel and the piston The speed affects the volume because there is gap between the valve membrane and the channel the valve cannot perfectly close the channel until the valve stem is pushed completely TABLE I SPECIFICATION OF MICRO OBJECTS Nnme Cube Cone Semi Cylinder Imnge Geometric pnrnmeters Depth 1 mm Wi th 1 mm Height 1 mm Rn ius 0 5 mm Height 0 8 mm Rn ius 0 5 mm Wi th 1 mm Estimnte Cnpillnry force Approximntiod 189 N 15 114 N 16 119 N 19 Pnrnmeters 1 60 eg 32 eg b 0 3 mm 1 20 eg 45 eg 1 30 eg h 0 3 mm 2 60 eg R1 0 29 mm R2 0 46 mm r1 0 5 mm r2 0 5 mm 52 mN m Mnterinl Acrylic Fig 5 Principle of water filling to the tip of the flow channel Fig 6 Principle of droplet formation IV VERIFICATION OF THE CAPILLARY FORCE We used the precision electronic balance AUW220D SHIMADZU Corp to measure the tensile capillary force Fig 7 shows the experimental setup To measure the static capillary force we used an acrylic cylinder with a diameter of 1 mm for the bottom of the liquid bridge The surface property is hydrophilic and the contact angle after the drop adhesion is approximately 67 7 degrees average and SD of 10 times The temperature was 21 25 degrees and the humidity was 20 23 The liquid bridge height h was adjusted by a linear motion stage with a positioning resolution of 1 m nd a repeatability of 1 m MMU 40X Chuo Precision Industrial Co Ltd We determined h to be 0 3 mm 0 4 mm and 0 5 mm The amount of water at the tip was 0 72 0 03 mm3 Fig 8 shows the plots of the capillary force vs time when we fixed h until the liquid bridge was broken At any h the liquid bridge broke between 150 and 200 s We consider 150 s to be a sufficient duration for the pick and place in electronics and MEMS fields although the required hold time due to the applications Fig 9 shows the hysteresis curves between the capillary force and the height h of the liquid bridge The broken line indicates the time of descent and the solid line indicates the time of ascent First the gripper was slowly lowered from the initial h 540 m then it was slowly increased from the turnaround points of h 500 400 300 200 and 100 m udtil the liquid bridge was broken We measured the time changes of the capillary force from the beginning to the broken points for all turnaround points The descending speed of the gripper was 10 m s and the ascending speed was 30 m s From Fig 9 we check that the capillary force is positive when h is approximately less than 300 m in the descending It becomes negative again at approximately 400 m in the ascending and finally all 5 hysteresis curves match from approximately 1200 m to each broken point Thus the generated capillary force is more than 130 N id ndy changing pattern of h This gripper can grasp 10 times heavier samples than the 1 mm3 acrylic although we must consider the effect of the hydrophilic and geometrical properties Fig 7 Experimental setup to measure the tensile capillary force V EXPERIMENTAL SETUP To investigate the basic property of the fabricated capillary force gripper we have built a system to pick and place micro objects The organization of the entire experimental setup is shown in Fig 10 In this study the work surface was positioned in the X and Y axes with a positioning resolution of 1 m nd a repeatability of 0 5 m YA10A L1 Kohzu Precision Co Ltd The gripper was operated in a vertical direction by the linear stage MMU 40X Chuo Precision Industrial Co Ltd Therefore a micro object moves in the xy directions when it is on the substrate and in the z direction when it is grasped by the gripper We used two CCD cameras with 24322050 pixels CV H500M KEYENCE Corp attached to magnification lens LA LM510 KEYENCE Corp on the top and side of the workbench to measure the position and posture of the samples with the image analyzer CV 5700 KEYENCE Corp The measuring resolution of this experimental setup is approximately 3 m We used LabVIEW 2018 National Instruments Corp as the programing language The pick and place process is described as follows 1 Place the micro objects within the view of the top CCD camera on the substrate to measure the xy coordinates Fig 8 Plots of the capillary force vs time for V 0 72 0 03 mm3 250 200 150 100 50 0 50 050100150200 Capillary force N Time s h 300 m h 400 m h 500 m 250 200 150 100 50 0 50 100 150 200 0500100015002000 Capillary force N Gripper height m h 100 m h 200 m h 300 m h 400 m h 500 m Start Turnaround Break 540 100 1600 540 200 1615 540 300 1690 540 400 1420 540 500 1225 Start point 540 0 Fig 9 Hysteresis curves of the capillary force and the height for V 0 72 0 03 mm3 and the start point of h 540 m 2 Move the XY stage and locate the targeted placing point on the work surface directly below the nozzle 3 Move the gripper up and down by the Z stage and apply a droplet onto the work surface 4 Move the XY stage and locate the targeted object directly below the nozzle 5 Move the gripper up and down to pick up the object 6 Move the XY stage to locate the placing point directly below the nozzle 7 Move the gripper down so that the picked object is placed on the preapplied droplet of 3 8 Move the XY stage so that the pick and placed object is within the view of the CCD to measure the xy coordinates 10 samples were placed in the view of the CCD camera and steps 2 7 were repeated 10 times The micro objects were spaced at 2 5 mm apart from one another The success rates of the pick and place and positioning errors in the XY positions were quantitatively evaluated for 3 types of objects cube cone and semicylinder as listed in Table I We also changed the volumes of the droplet applied to the surface and the liquid bridge In this paper we do not evaluate the posture errors to focus on the positioning property in the XY plane although an additional rotating stage is required to control the posture of the samples Fig 10 Configuration of the experimental setup for the automatic pick and place organized by the gripper XY stage Z stage a pair of CCD cameras and the image analyzer VI EXPERIMENTS A Positioning property for cubic samples It is known that when a micro object is dropped onto a droplet it moves toward the center of the droplet due to surface tension This phenomenon is called self alignment and has been put into practical use in SMT such as mounting electronic components using molten solder 23 24 It is also known that when micro objects are placed on a droplet with tweezers the placement error varies depending on the volume of the droplets 25 Therefore we evaluated whether there was a change in positioning error depending on the droplet volume with using capillary gripper We adjusted the discharge amount of the droplet by changing the sliding speed of the piston We set the sliding speed of the piston at 0 5 1 2 and 3 mm s The conditions were named A1 A2 A3 and A4 respectively The applied droplet volume was measured from the image taken with a CCD camera The results were 0 12 0 05 mm3 0 16 0 06 mm3 0 23 0 06 mm3 and 0 28 0 10 mm3 average SD for A1 A4 The factors of variations are mainly due to the nonuniformity of the surface roughness of the substrate and nozzle The substrate of the polyacetal block was spray coated with fluorocarbon resin Fig 11 shows the sequential photograph of the fifth pick and place operation under condition A4 for both preapplied droplet and pick and place liquid bridge We confirmed that the gripper could pick and place the cubic samples without solid contact Fig 12 shows images of the distribution of ten cubic samples before and after the pick and place for A4 We confirmed that the ten samples could be arranged across the alignment mark Table II shows the success rates of the pick and place in each experimental condition Fig 13 shows a comparison image after the fifth placing of each condition Fig 14 shows the comparison of the distributions of positioning errors for conditions A1 A2 A3 and A4 with the cube shaped sample Table III lists the positioning error under these conditions As shown in Table II the success rate of all items was 100 under all conditions except A1 A1 failed once for pick up and twice for placing We presume that the cause is an adhesion force from the substrate such as the electrostatic force Since Nozzle Fig 11 Sequential photograph of the pick and place experiment under condition A4 a Droplet discharge from the nozzle b Application of the droplet by stamping the nozzle to the surface c Fo