A PCB BASED CAPACITIVE SHEAR FORCE SENSOR DEVELOPED FOR ROBOTIC GRIPPER APPLICATION

Transcription

1 Measuremet of Mass, Force ad Torque (APMF 23) Iteratioal Joural of Moder Physics: Coferece Series Vol. 24 (23) 3627 ( pages) The Authors DOI:.42/S A PCB BASED CAPACITIVE SHEAR FORCE SENSOR DEVELOPED FOR ROBOTIC GRIPPER APPLICATION JIAN-LIN HUANG Ceter for Measuremet Stadards, Idustrial Techology Research Istitute, Hsichu 3, Taiwa, R.O.C. LogaHuag@itri.org.tw SHENG-JUI CHEN * Ceter for Measuremet Stadards, Idustrial Techology Research Istitute, Hsichu 3, Taiwa, R.O.C. SJ.Che@itri.org.tw GWO-JEN WU Ceter for Measuremet Stadards, Idustrial Techology Research Istitute, Hsichu 3, Taiwa, R.O.C. GJWu@itri.org.tw CHUNG-LIN WU Ceter for Measuremet Stadards, Idustrial Techology Research Istitute, Hsichu 3, Taiwa, R.O.C. CLWu@itri.org.tw SHEAU-SHI PAN Ceter for Measuremet Stadards, Idustrial Techology Research Istitute, Hsichu 3, Taiwa, R.O.C. Sheau.shi.Pa@itri.org.tw The shear force sesor is oe of the key elemets i future robotic idustry, it is of great importace i applicatios where robotic arms are required to delicately iteract with objects to be hadled. I this paper, we preset the developmet of a capacitive shear force capable of sesig shear forces i two degrees of freedom. The fabricatio of the sesor is based o the prited circuit board (PCB) fabricatio process, a well-kow ad mature techology. We adopt the capacitace sesig scheme for its high sesitivity ad easy implemetatio. For sesor characterizatio, we used a force gauge ad a optical iterferometer to measure sesor s parameters icludig its sesig coefficiet ad resolutio. The dimesio of our prototype shear This is a Ope Access article published by World Scietific Publishig Compay. It is distributed uder the terms of the Creative Commos Attributio 3. (CC-BY) Licese. Further distributio of this work is permitted, provided the origial work is properly cited. * Correspodig author 3627-

2 J.-L. Huag et al. force sesor icludig the metal housig is 26 mm 3 mm 58 mm suitable for the itegratio with commercial robotic grippers. For sesor performace, we achieved a shear force sesig coefficiet of 23.3 ff/n ad a resolutio of smaller tha 5mN. Keywords: Force sesor; tactile sesig; capacitive sesig; robot gripper.. Itroductio Recetly, the robot egieerig has bee developig rapidly ad evolvig ito humaoid implemetatios. The itelliget sesig ability such as visio [], auditio [2] ad tactile sese [3-4] plays a importat role i applicatio where the humaoid robots are required to delicately iteract with eviromets. Graspig ad movig a object with a robotic gripper ad without huma itervetio is oe of the major research topics i such kid of robotic applicatio. To meet this eed, the developmet of such tactile sesor is of great iterest. The piezoresistive tactile sesor [5-6] ad capacitive tactile sesor are two most commo types used i robotic grippers. Capacitive sesig techique is kow to be very sesitive ad widely used i applicatios where high resolutio sesig are required, such as closed-loop liear stages, micro/ao-force sesors etc. I this research for shear force sesig, we adopt the differetial capacitive sesig techique for further reducig the sigal drift due to the ambiet temperature variatio. Although the emergig micro electro mechaical systems (MEMS) based capacitive force sesors [7-8] have a tiy size ad a high sesitivity, however the force measuremet rage is typically ot sufficiet for idustrial graspig applicatios. Ad the array type sesig elemets eed more complex electroic circuit desig. Cosiderig the force rage ad the robustess of the device, we use the well-kow ad mature techique, the PCB fabricatio process, for the fabricatio of the sesor. 2. Desig ad Fabricatio 2.. Desig ad Workig Priciple The basic elemet for a force sesig device is a sprig which is used to covert the force uder measuremet ito a measurable quatity: a deformatio. I our shear force sesor, we use a square block of silicoe rubber as the sprig elemet. For deformatio measuremet, we adopt the capacitive sesig techique. The schematic of the shear force sesor is show i Fig., where the red trapezoid, blue rectagles, yellow rectagles ad rectagle of light gray with slash represet a load butto (or the bump), substrates, metal electrodes ad a silicoe rubber respectively. For simplicity, the schematic shows oly oe upper electrode ad two bottom electrodes for illustratig the shear force sesig i directio of x-axis. The deformatio ca be detected by measurig the differetial capacitace defied as C = C 2 C. () Whe the sesor experieces oly the ormal force, i.e. the coditio of Fig. (a), by usig the model of parallel-plate capacitor the differetial capacitace C ca be writte

3 A PCB based Capacitive Shear Force Sesor Developed for Robotic Gripper Applicatio as ε rε A C = d z 2 ε rε A d z =, (2) where ε r is the dielectric costat, ε is the vacuum permittivity, A is the area of the capacitor plate ad d i is the gap distace betwee two plates of the ith capacitor. For the coditio of Fig. (b) where the sesor experieces both the ormal ad shear forces, the differetial capacitace C becomes εrε( A+ δ A) εrε( A δ A) C = d z d z εrεδ A = 2 d z ε ε z + Lx d d r 2 s. The differetial capacitace C is i proportio to the shear deformatio x s. The term L represets the legth/width of the capacitor plate. (3) Figure. Schematic of the shear force sesor ad illustratio of exteral forces ad ideal deformatios. I practical coditio, as illustrated i Fig. 2, a shear force usually results ot oly a liear deformatio but also a rotatio due to a torque created by the shear force. Takig the rotatio ito accout, the differetial capacitace is writte as ε rε( A+ Lxs ) ε rε( A Lxs ) C =. (4) d + z d + z

4 J.-L. Huag et al. Because the substrates for attachig the metal electrodes are much stiffer tha the silicoe rubber, the deformatio of the silicoe rubber is costraied to move i a way show i Fig. 2. Therefore, we ca assume the vertical deformatio is of the followig form z = z z 2 = z + z s zs, (5) where z s represets the deformatio i z-axis caused by shear force. Replacig z ad z 2 i (3) with (4), ad rearragig the equatio by Taylor series. We obtai the approximate equatio of the differetial capacitace: z s ε rε z ε rε A zs C 2 + Lxs + 2. (6) d d d d f N C C 2 x s f s Figure 2. Liear displacemet ad rotatio resulted by the shear force. Ideally, the deformatios x s ad z s are both liearly proportioal to the shear force f s resultig also a liear relatioship betwee the shear force f s ad the differetial capacitace C. The effect from the ormal force ca be miimized by makig z /d. The deformatio patter has bee verified by the fiite elemet aalysis (FEA). A fiite elemet model with a similar shape show i Fig. 2 was costructed. The model was applied by both ormal ad shear forces ad correspodig deformatios were calculated. With the deformatio obtaied from the FEA, we ca estimate the differetial capacitace C. Fig. 3 shows the differetial capacitace C with differet iput forces i both shear ad ormal directios. The correspodig parameters used i the FEA are A = 4.25 mm 2, L = mm, d =. mm ad ε r = 3.2. The upper part of Fig. 3 idicates a good liear relatioship betwee f s ad C. The lower part of Fig. 3 shows the relative stadard deviatio that is defied as the stadard deviatio of a set of C divided by the mea of that set. I order to check the effect from the ormal force, the set of C cotais differetial capacitace values at the same shear force but differet ormal forces. The relative deviatio is well below % idicatig the sesor s sesitivity to shear force is ot altered by the presece of a ormal force. z -z s z x

5 A PCB based Capacitive Shear Force Sesor Developed for Robotic Gripper Applicatio 3 Figure 3. Differetial capacitace C with differet iput forces i both shear ad ormal directios estimated from the fiite elemet aalysis results Fabricatio C (ff) Relative deviatio (%) Shear force (N) Shear force (N) f N = N f N = N f N = 2 N f N = 3 N The prototype of the shear force sesor was fabricated with the prited circuit board (PCB) fabricatio process. The prototype cosists of two PCBs, a sheet of silicoe rubber ad a metal housig. The electrodes were directly pattered o the PCBs. The other side of each PCB was fully covered by a copper layer. These layers serve as groud plaes i the sesig circuit which icrease the stability reduce the iterferece from exteral electric/magetic fields. The material for the PCB s substrate was the FR4 (glass-reiforced epoxy). The thickess of the copper layer o the PCB was about 35 µm. To have a good liear respose to the shear force, the shape of the bottom electrode was chose to be a triagular combied with a rectagular ad the shape of the upper electrode was a square. The cofiguratio of the electrodes is show i Fig. 4. The sheet of silicoe rubber was sadwiched ad betwee the two PCBs by ijectio moldig techique. The metal housig was used to cotai the PCB sesor ad provide a rigid base for attachig the sesor to a exteral object such as a robotic gripper. I additio, the metal housig also acts as a electrical shield prevetig the electric field iside the sesor from beig altered by the outside objects

6 J.-L. Huag et al Capacitace Readout Electroics Figure 4. Cofiguratio of the electrodes. Uit is i mm. The measuremet of the differetial capacitace C was accomplished by a commercial capacitace sesig IC (AD7746) from Aalog Device, Ic. The AD7746 is a twochael, 24-bit resolutio Σ capacitace-to-digital coverter. Accordig to the specificatio, the AD7746 has a resolutio dow to 4 af, a accuracy of 4 ff ad a fullscale capacitace rage of ±4 pf. To commuicate with ad cotrol the AD7746 over a I 2 C-compatible two-wire serial iterface, we used a FPGA based embedded cotroller from Natioal Istrumets as the master cotroller. The schematic of the capacitace readout circuit is show i Fig. 5. A program for cotrollig the AD7746 over the I 2 C was developed usig the LabVIEW FPGA module. The program was desiged to cotiuously acquire the differetial capacitive variatios correspodig to two shear sesig directios, ad to output the sigals via the aalog output ports for further feedback cotrol usage. The performace of the AD7746 is show i Fig. 6. The data i Fig. 6 were acquired uder all iput pis of the AD7746 were ope (ot coected to the PCB sesor). The measured stadard deviatio was about.6 ff ad peak-to-peak oise was about.3 ff. PCB C C 2 AD7746 GND EXC CIN(+) SCL SDA FPGA based Embedded cotroller Aalog Output CIN(-) Figure 5. Schematic of the capacitace readout circuit

7 A PCB based Capacitive Shear Force Sesor Developed for Robotic Gripper Applicatio C (ff) time (s) C (ff) Sesor Characterizatio Figure 6. Capacitace sesig performace of the AD7746. Several sample sesors were fabricated ad their mechaical ad electrical properties were tested to cofirm the desig ad performace. 3.. Experimetal apparatus ad setup time (s) I this experimet, we costructed a test platform which cosists of a force measuremet system, a motio cotrol system, a capacitace measuremet system ad a displacemet measuremet system. All the measuremet systems were cotrolled by LabVIEW program for applyig force o sesor ad acquirig the sesor s respose. Fig. 7 shows theschematic of the test platform, ad each measuremet block is described i followig sub-sectios Force measuremet ad motio cotrol systems To apply a test force o sesor s bump, the sesor was moved by a precisio motorized traslatio stage (967-XYZ-PNP from New Focus), ad brought to cotact with a Polyoxymethylee (POM) probe mouted o a force gauge (see Fig. 8). The magitude of the test force was cotrolled by the movemet of the traslatio stage. The miimum icremetal displacemet of the traslatio stage was less tha 3 m. The maximum

8 J.-L. Huag et al. load ad sesig resolutio of the digital force gauge (HF- from Algol istrumet co., LTD) are N ad. N, respectively. Shear force data were recorded ad moitored i host computer via a RS-232 iterface. Figure 7. Schematic of the test platform. Figure 8. Photograph of shear force sesor characterizatio setup

9 A PCB based Capacitive Shear Force Sesor Developed for Robotic Gripper Applicatio Displacemet measuremet system We set up a laser iterferometer displacemet measurig system for trackig the displacemet of the motorized liear stage. This system cosists of a Laser head (557B, Hewlett-Packard), a high-resolutio iterferometer (HP 76A) ad a axis board (32B, Excel). The liear resolutio of the displacemet measurig system is 2.5 m. With this displacemet measuremet system, we are able to estimate the shear deformatio of the silicoe rubber betwee floatig PCB plate ad base PCB plate Experimetal results I the experimet of sesor characterizatio, a shear force i a rage from N to 3 N with a icremet of about 5 mn was applied o the PCB shear force sesor. As metioed i the previous sectio, the magitude of the shear force was cotrolled by the movemet of the traslatio stage, which was stepped with a icremet (ad a decremet) of about 2 µm. The correspodig force icremet/decremet was about 5 mn. Fig. 9 shows the data acquired i oe cycle of the characterizatio measuremet. Both the shear force (blue lie) ad the capacitace variatio (red lie) were recorded simultaeously. Figure shows the plot of the shear force versus the capacitace sigal with liear ad quadratic fittig curves. It ca be see that the shear force sesor shows a slightly o-liear respose to the applied shear force. The sesig coefficiet of the PCB shear force sesor was foud to be 23.3 ff/n, which was obtaied by the slope of the liear fittig curve. Capacitace (ff) Force (N) Displacemet (µm) Figure 9. Output sigals of the PCB shear force sesor ad the referece force gauge

10 J.-L. Huag et al. 8 Capacitace (ff) Force (N) data liear quadratic Residual error (ff) Figure. Determiatio of the sesig coefficiet of the PCB shear force sesor. 4. Discussio ad Outlook Force (N) I this work, we have developed ad fabricated ashear force sesor based o the PCB fabricatio process ad differetial capacitive sesig techique. The sesor is capable of measurig shear forces i two degrees of freedom. The sesor was characterized by a test platform which was assembled by a force gauge, a motorized liear stage ad a optical iterferometer. The sesig coefficiet of the sesor was measured to be 23.3 ff/n. The capacitace oise of the shear force was foud to be about. ff equivalet to a force sesig oise of below 5 mn. The dyamic respose is curretly limited by the capacitace sesig IC. For sesig oly oe degree of freedom, a sigal update rate of 9 Hz (or poit per secod) ca be achieved. For sesig two degrees of freedom simultaeously, the update rate was slowed dow to 33 Hz oly. The couplig betwee axes was ot sigificat. For hysteresis effect, the mechaical property of silicoe rubber will be further studied ad refied by tuig the compositio of the silicoe rubber. We are also desigig a ew metal sprig elemet which is expected to have a better hysteresis performace. I the future works, we will improve the fabricatio process for better performace ad evaluate the robustess of the sesors. Refereces. A. Morales, P. J. Saz, A. P. del Pobil ad A. H. Fagg, Visio-based three-figer grasp sythesis costraied by had geometry, Robotics ad Autoomous Systems, vol. 54, p (26). 2. K. Nakadai, Real-timesoud source localizatio ad separatio forrobot auditio. I Proc. IEEE IteratioalCoferece o Spoke Laguage Processig, (22), p

11 A PCB based Capacitive Shear Force Sesor Developed for Robotic Gripper Applicatio 3. KittiSuwaratchatamaee, Robotic Tactile Sesor System ad Applicatios, IEEE Trasactios o Idustrial Electroics, vol. 57, o. 3, (2). 4. Alexader Schmitz, Methods ad Techologies for the Implemetatio of Large-Scale Robot Tactile Sesors,IEEE Trasactios o Robotics, vol. 27, o. 3, (2). 5. J.H. Sha, The desig ad fabricatio of a flexible three-dimesioal force sesor ski,i IEEE/RSJ cof. Itelliget Robots ad Systems, (25), p K. Noda, A shear stress sesor for tactile sesig with the piezoresistive catilever stadig i elastic material,sesors ad Actuators A: Physical, vol. 27,( 26), p M.Y. Cheg, Tactile ad shear stress sesig array usig capacitive mechaisms with floatig electrodes,ieee Cof. Micro Electro Mechaical Systems,(2), P H. K. Lee, Normal ad shear force measuremet usig a flexible polymer tactile sesor with embedded multiple capacitors, Joural of Microelectromechaical Systems, vol. 7,(28), p