IROS2019国际学术会议论文集 1347

Abstract This paper proposes a new object handling device for a robot in which a conventional vacuum suction cup is provided with force sensing ability by coating a piezoelectric thin film polymer i e PolyVinylidene DiFluoride PVDF on its surface It can detect the applied force by piezoelectric effect of coated thin film The feature of our study is that the suction cup can work not only as a grasping tool but also as a force sensor PVDF thin film was prepared by dip coating method which can coat thin film directly on a three dimensional 3D surface Although the film with sufficient piezoelectric d33 constant e g 34 pC N was achieved after polarization process in case of flat surface it was not achieved however on 3D bellows surface To address this problem as an alternative method the feasibility of lamination of already polarized film was investigated I INTRODUCTION Bellows shaped vacuum suction cups have been preferred as robot end effectors in logistics and transportation fields 1 5 In those fields a robot is required to manipulate various kinds of objects however it is difficult to grasp an object whose shape is complex or unknown only by using a conventional force sensor attached to the wrist of robotic arm since the contact force between the object and the end effector cannot be measured accurately not to mention the relative contact positon between them The reasons for inadequate contact force detection are that the sensor is placed considerably away from the contact position and that the resolution of commercial multi axes force sensor is rather low at the present As for measuring the contact position methods using camera or laser range sensor have been developed 6 10 however there is still a problem that the object is optically occluded while the end effector is approaching it To get the information of contact force accurately measuring the force applied to the suction cup which directly contacts with the object is effective To get the information of contact positon even in the optically occluded conditions using multiple suction cups and measuring the force applied to each suction cup are effective since the approximate contact positon can be obtained by knowing the contact non contact of each suction cup against the object which is usable for detecting the misalignment of end effector from the object center We propose a new object handling device for a robot arm Seiji Aoyagi Tatsuki Morita Takuto Shintani Tomokazu Takahashi and Masato Suzuki are with the Faculty of Engineering Science Department of Mechanical Engineering Kansai University Osaka 564 8680 JAPAN corresponding author to provide e mail aoyagi kansai u ac jp in which a conventional vacuum suction cup is provided with force sensing ability by directly coating a piezoelectric thin film polymer i e PolyVinylidene DiFluoride PVDF on its surface PVDF has been studied for use as various kinds of sensors and energy harvesters due to its flexibility and high piezoelectricity as polymer material 14 17 Proposed sensable suction cups can detect the applied force by piezoelectric effect of coated PVDF film The first feature of our study is that the suction cup can work not only as a grasping tool but also as a force sensor as schematically shown in Fig 1 Compared with a usual handling system using both conventional force sensor and suction cup they are united to one device in our study making the system simple and inexpensive The idea comes from that the suction cup itself can be used as a strain generating body since it is easily deformed due to its bellows shape and its flexibility thanks to constituent material e g silicone rubber or poly urethane The second feature is that the thin film formation on a complex three dimensional 3D surface like bellows suction cup can be achieved by lamination method after feasibility study of dip coating method and lamination one Dip coating method is used to form nano or micro order uniform thin films In previous reports however targets of the dip coating method were limited to flat surfaces 15 16 and there have not reported that achieved the coating thin film on a complex 3D surface Laminating method is to adhere already prepared film i e given electrodes and polarized on 3D surface making the fabrication process simple by omitting the electrode formation and polarization process Using one sensable cup contact between a suction cup and an object can be detected followed by vacuum sucking the object for picking it up as shown in Fig 1 The third feature is that by using multiple sensable cups the device can cope with inclined surface misalignment from object center undulated 3D surface etc as shown in Fig 2 For example tactics of automatically grasping an unknown object having inclined surface is as follows one sensable suction cup detects the contact then the robot rotates the end effecter so as that another cup contacts with the surface for preventing vacuum leakage followed by the sucking and picking the object see Fig 2 a Hiroki Takise is with the Engineering Science Major Mechanical Engineering Kansai University Osaka 564 8680 JAPAN Formation of PVDF Piezoelectric Film on 3D Bellows Surface of Robotic Suction Cup for Providing Force Sensing Ability Feasibility Study on Two Methods of Dip coating and Lamination Seiji Aoyagi Member IEEE Tatsuki Morita Takuto Shintani Hiroki Takise Tomokazu Takahashi Member IEEE and Masato Suzuki Member IEEE 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 IEEE7128 The organization of this article is as follows the dip coating of PVDF thin film was conducted and the relationship between drawing speed and film thickness was investigated Piezoelectricity of dip coated film was evaluated by measuring piezoelectric constant d33 PVDF film was formed on a 3D bellows surface using dip coating method Finally lamination method was conducted and PVDF film was directly adhered on a bellows suction cup II PVDF FILM FORMATION BY DIP COATING METHOD A Searching of Optimal Conditions of Dip coating PVDF powder Kureha Corp Type KF Piezo W 2200 was used in this study For the use of coating method PVDF powder was dissolved with organic solvent i e methyl ethyl ketone MEK and N N Dimethyl Acetamide DMAc by stirring at 60 degree The component ratio of formulated solution is shown in Table 1 A dip coater machine SDI Company type MD 0408 S6 was used to form PVDF thin film The photograph and schematic equipment composition are shown in Fig 3 The process of dip coating method is divided into three stages as follows at first sample is dipped into solution at a constant speed Then it is drawn up at a constant speed and thin liquid layer of solution is deposited on the sample surface Finally sample is heated to evaporate solvent i e MEK and DMAc In dip coating method the drawing speed affects the film thickness In order to investigate the relationship between the drawing speed and the thickness of PEDOT thin film a film was formed on a silicon wafer and its thickness was measured using a stylus step profiler Bruker Corp Type Dektak XT Thickness measurement result is shown in Fig 4 It was confirmed that a thicker film was formed as the drawing speed of dip coating became faster A thin film with 2 m in thickness was deposited at the drawing speed of 0 25 mm sec condition of which is used hereinafter in this paper Fig 2 Schematic of automatic grasping methods using multiple sensable suction cups Rotate and push Vacuum and pick up Approach and sense a Inclined object Vacuum leakage Using multiple suction cups a robot can cope with object surface Multiple suction cups b Misalignment from the object center Approach and sense Correct position All suction cups can be pressed adequately c Undulated 3D surface Proposed method Vacuum suction cup having force sensing ability Object can be detected followed by sucking it Conventional force sensor Vacuum suction cup Conventional object handling United Single suction cup Force sensor utilizes easy deformation of suction cup due to bellows shape Sensor and suction cup are united Merit Fig 1 Concept of robotic arm using sensable suction cup for grasping an unknown object a Fig 3 a Photograph and b schematic of dip coating equipment b Controller Motor Liner slider Sample Clamp Solution 30 cm Fig 4 Relationship between drawing speed and thickness of PVDF film Table 1 the mixing ratio of PVDF solution C Componentomponent R Ratio atio wt wt PVDF 12 Methyl Ethyl Ketone 38 N N Dimethylacetamide 50 Measured value First order approximation line 0 25 7129 B Evaluation of Piezoelectricity of Dip coated PVDF film To evaluate piezoelectricity of dip coated PVDF thin film piezoelectric constant d33 of it was measured The fabrication process of the test sample is as follows PVDF film was formed on a low resistance silicon wafer size 30 mm square resistivity 0 2 cm or less by dip coating method A lead wire was connected to the low resistance silicon followed by corona polarization of PVDF film Schematic illustration of setup of the corona polarization is shown in Fig 5 Procedures of corona polarization are as follows a needle electrode was located above the PVDF film followed by applying voltage of 10 0 kV for 10 min Distance between the electrode and the test sample was set to 20 mm After polarization Al aluminum electrode was formed on the film by sputtering and lead wire was connected to it Piezoelectric constant d33 of PVDF film was measured by the d33 measurement system which is developed in our laboratory The schematic composition of the system is shown in Fig 6 PVDF is pressurized by compressed air At the same time as applying pressure the output voltage generated at the resistance and pressure inside the chamber were measured Calculations were carried out as follow 17 Output charge Q was calculated using piezoelectric constant d33 33 1 where A is area of electrode is applied pressure Current I was obtained by differentiating equation 1 as follows 33 2 where V is output voltage generated at the load resistance R Figure 7 shows the obtained waveforms of output voltage and applied pressure By changing the pressure value it is possible to change d dt The result of relationship between I V R and A d dt is shown in Fig 8 As the inclination of the approximate straight line d33 can be obtained As a result d33 was estimated as 34 pC N Considering that the value of d33 reported previously were from 10 to 30 pC N 18 resultant piezoelectricity of PVDF film which is dip coated on flat surface was sufficient C Dip coating PVDF to 3D bellows suction cup A PVDF solution was dip coated onto a commercially available bellows type suction cup Schmaltz Corp Type FSG 12 SI 55 M5 AG After coating the suction cup was heated at 100 to form a PVDF thin film The film thickness of the PVDF thin film formed on the bellows suction cup was measured The measurement method is as follows a bellows suction cup coated with a PVDF thin film was partially cut out using an ultrasonic cutter and fixed on a flat plate so that it can be measured with a surface profiling equipment Fig 9 In this experiment in order to verify the difference in film thickness between the convex portion and concave portion of the bellows samples were cut from both parts and the film thickness of each PVDF thin film was measured The measurement results are shown in Fig 10 Fig 5 Schematic illustration of corona polarization Sample 10 0 kV Needle electrode 20 mm Fig 6 Schematic of the setup for measure d33 of PVDF Sample Pressure sensor Oscilloscope Resistance 100 k Compressor Chamber Valve 16000 12000 8000 4000 0 0100200300400500 V R pC s A d dt N s Fig 7 Obtained waveforms of output voltage and applied pressure 4 3 2 1 0 1 0 0 1 0 2 0 3 0 4 0 5 02004006008001000 Voltage mV Pressure MPa Measurment time ms Pressure Voltage d dt Measurement time ms The voltage swings to the negative side is due to the polarity of the piezoelectric sheet Fig 8 Obtained data and approximate line to estimate d33 of PVDF film 7130 The film thickness of the bellows convex part was 5 3 m and that of concave part was 14 1 m It was possible to coat the PVDF thin film on the surface of the bellows suction cup having 3D surface by dip coating method The reason why the film thickness is different between concave and convex part of bellows is that the PVDF solution is accumulated in the concave part due to its high surface tension III FABRICATION OF PIEZOELECTRIC FORCE SENSOR BY DIP COATING METHOD We tried to fabricate sensable suction cup equipped with piezoelectric force sensing function using dip coating method Poly 3 4 ethylenedioxythiophene PEDOT was used as the sensor electrode In the preliminary experiments Al electrodes formed by sputtering were used but it was confirmed that they were broken when applied large deformation of bellows resulting in not functioning as electrodes Instead of Al PEDOT was adopted as a flexible electrode that can follow the deformation of bellows PEDOT can be dispersed in water and can be used as a solution That is PEDOT can also be formed into a thin film by the dip coating method A method for preparing the sample is described below In order to coat the PEDOT water dispersion onto the silicone rubber suction cup its surface was made hydrophilic by O2 plasma treatment As the lower electrode PEDOT thin film was formed by dip coating method followed by connecting a conductive wire to it PVDF thin film was formed thereon by dip coating method followed by corona polarization treatment to impart piezoelectricity to the PVDF thin film After polarization PEDOT film was formed on it as the upper electrode in the same manner as the lower electrode followed by connecting the conducting wire The composition of films on bellows cup is schematically shown in Fig 11 A photograph of the prepared sample is shown in Fig 12 The output voltage from the sample was measured when compressing the bellows cup but the output due to the piezoelectric effect of PVDF could not be observed Since the PVDF film was formed on 3D surface it is considered that the film cannot properly accumulate charges on its surface making the polarization failed IV FABRICATION OF PIEZOELECTRIC FORCE SENSOR BY LAMINATION METHOD Even if it was attempted to apply high electric field on the PVDF thin film formed on a complicated 3D structure such as bellows cup it was impossible to uniformly apply the electric field resulting in failure polarization of PVDF To address this problem we changed the method from dip coating followed by polarization to lamination using the already polarized film As an already polarized PZT film commercial one PIEZO FILM SHEET TOKYO SENSOR Corp 28 m PVDF thickness was employed which was also already equipped with both of upper and lower electrode films made of NiCu A Fabrication of mold for lamination process The method of affixing the PVDF thin film to 3D surface is shown below The liquid adhesive is applied to the surface of the bellows suction cup by dip coating method and the polarized PVDF thin film is pressed against the bellows cup surface to stick it This method is called as lamination method in this paper In the lamination method a mold for pressing the PVDF film against the bellows cup was prepared by using a 3D printing machine SSS Corp Type Link Box4 as shown in Fig 13 In the subsequent experiments the lamination method is carried out using this mold Fig 10 Result of film thickness at concave and convex parts on bellows suction cup 14 1 m 5 3 m PVDF Suction cup Fig 9 Schematic of measuring the thickness of PVDF Cut a part of sample and attached to the plate Ultra sonic cutter PVDF Stylus step profiler Scan line Obtain surface profile by stylus scanning Fig 12 Photograph of fabricated sensable suction cup 5 mm Fig 11 Schematic composition of films on the fabricated sensable suction cup PEDOT Silicone rubber PVDF Lead wire 7131 B Lamination of PVDF thin film on 3D surface Procedure of lamination method is schematically shown in Fig 14 Adhesive solution diluted from adhesive material was coated on the surface of bellows suction cup by dip coating method Immediately after this PVDF thin film was applied on it In order to adhere the film evenly to the irregular surface of the bellows the mold produced in the previous section was pressed against the cup and it was left for about 6 hours until the adhesive liquid was dried After releasing the product from the mold a lead wire was connected to the upper electrode of PVDF film The state of fabricated sample by lamination method is shown in Fig 15 As a result the PVDF film are overlapped by itself which generates wrinkles mainly at concaved parts of the bellows making it impossible to laminate the film uniformly C Improvement by cutting PVDF film In order to solve the problem of the previous section unnecessary PVDF thin film was cut before lamination Figure 16 a shows the outline of PVDF thin film with the parts to be cut The PVDF film affixed to the concave part of bellows was cut in advance because it is liable to be overlapped The fabricated result by lamination method using the cut film with incisions is shown in Fig 17 By cutting the PVDF sheet and using it the lamination was successfully achieved with much reducing the wrinkles Compared with using multiple long PVDF films cutting one large film with incision has advantage in good area efficiency of PVDF which would increase the sensitivity D Measurement of output voltage by pressurization Output voltage from the fabricated PVDF film on the bellows suction cup by lamination method was measured by pressurizing it Experimental setup is shows in Fig 18 Test sample was connected to the load resistance of 100 k and the voltage generated at it was measured The measurement Fig 14 Procedure of lamination method PVDF film Mold Press Lead wire Suction cup coated adhesive a Suction cup was coated adhesive and PVDF film wa