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The Design of TwoJayer Flexible Hinge Milli-gripperHuibao Chen Ping SunDept of Precision Mech.Eng. Shanghai UniveisityJiading. shanghai 201800,P.R.CAbstract The paper introduces the design, manufacture and experiment of the two-layer flexible lunge milli-gripper in details. After the stmeture and the characters of piezoelectric element and two-lay er flexible hinge are analyzed, the amplification of di splacement is calculated . The gripper has high sensitivity, simple structure, a wide rang of usage and a great future such as milli-robot for medical treatment, inspecting and mini tube as well as military usage. It is a new type of millimachine.1. INTRODUCTIONEver since micio-clectyonic teclinology found its vzay in vanous fields of mechanical enguiecnng. Mechafaonic has become tlie most nnpoitant feature of modem mechanical science and teclinology. This,at the same time, greatly stnnulated tlie precision machine to develop towards micio and nitelligent.Iinitiated berween mid and late 80s,tlie study of micromachnie intensively has reflected.Micromachine can be divided into micromachine and millimachine Millnnachine vzhich has the character of millimeter size and micromachine vzhich has the character of micron size have become the focus of up-to-date research in mteinational science and technology society The satus in medical treatment,mecroelectronic precision mantfacture,micro assembly and acmautic can not be replaced by otlier size limited ma chines,hence it reasonably fast is developed.Milli-gnppei is a veiy nnpoitant part both in tlie development of micro-robot and in the practice of millimachine.Therefore,we is novz heading for the study and the development of mollogripper.The milli-gnppei is made up of two parts dnving part and operating part There are two linds of common driving parts. One is called PZT(piezoelectric) actuator, vzhich has the characters of sunple pinciple, steady action and high response In present,tliere are many types of Operating mechanism,but mainly tliere are three fomis in our research and use gear and rack mechanism, oblique vzedge mechanism and flexible hinge mechansm The flexible hinge mechanism which can amplify the micio displacement in tnnes is better tlian the other two Utilizing tins advantage,it is made up the milli-gtippei using PZT actuator with PZT element togetlier Figure gives a model we made Figure la is a single stage flexible hinge gripper and figure lb is two-stage flexible hinge gripper.卜 single Ktruclureb (uo-jeta nictu neFig 1 : Flexible lunge milli-grippersHowever, the defonnation of the PZT clement is quite limited (about6.5/mi/lOOvAlthough we have succeeded in amplify!ng to mechanism:the output of greppers is still not large.(normally about 90/mi/10(X )fthe amplification ofmechanism is in creased,the gripper*s size will be bigger and its tsing field will be limited. In orser to in crease the amplification and not to en large the grippers size.another type og milli-gripper is designedt is the weo-layer flexible hinge milligripper using PZT actuator.2. STRUCTURE AND ACTION PRINCIPLEThe tvzo-layer flexile hinge milligiipper is an integrated module which tuens electric power into mechanic power to realize the gnpping function.Its driving source is tlie PZT elementwhich is embedded in tlie centei of the first layer flexible hinge, Wlien the relevant voltage applied to tlie PZT element, the PZT element products defonnation, tliereby making the first layer flexible hinge realize the first stage amplification of displacement and then, it will make the second layei realize the second stage amplification, at the same time, realize the gnpping function, figure 2si tlie simple graph of tlie gnppeiFig.2:Two-layer flexible hinge milli-gnpperThe actuator is the stacked PZT element, which is made up of many layers of PZT element lamellae(figure 3 shows this)Theie are electiodes along tlie polarization direction of the both sodes of the element,the polarization direction of adjacent lamellae is opposite directions of elechic field.so they hace tlie same directions of displacement. When many PZT element lamellae are connected eceiyone is under the same voltage, and tlie whole displaecment is the sum of all the lamellaes diplacement.Fig. 3 The stmcture graph of stacked PZT elementThe hinge of flexible hinge mechanism is a type of elastic deformation atound axis, v/hich is receisible There are two kinds of hinges,tlie single axis and the double axis, and v/e choose tlie single axis one.(figure4)Fig.4:Hinge dunensions and various forces and moments which cause deflectionsIn design nig, the input and output axis must be nnnble, and hinges should have rigidity enough.In practice, tlie gnpping ends of the gripper should have greater output,so we use the tvzo single-stage flexible hinge mechanisms, as each one can amplify one stage, The two mechanisms are connected together with hinge,inthis way, tliey form the operating part of the twoJayer flexible hinge milli-giippeiThis design can not only decrease tlie structure size of gripper in X and Y directions butcan also increase tlie amplification of the displacement. In working condition,tliere are relative rotation at the connection of the loint tods So ,tlie connection style should keep the relative position betvzeen tlie two joint eods, and be able to rotate freely at the same time .And it also should prevent the clearance fit between tlie shaft and the hole fbom losing too much displacement, so we use the rotation fit with small clearance e c.o.5H8/f7.3 CALCULATING AND ANALYZING OF THE GRIPPERS STRUCTUREThe operating mechanism of two-layer flexible hinge mille-gripper is made up of two parts, and also made up of tvzo parts, figure 5 shows tlie loadnig condition of the tvzo single-stage flexible hinge mechanismFig5 The loading condition of flexible hingesFigure 5a is the first layer single-grade flexible hinge mechanism, and stmcture is symmetric. To make brief merely one side of the output is calculated The simple loading graph is shown as figure 6. According to the theory of the equilibrium of static force and moment the following equations is calculated:lbFig6: The simple drawing of structure口 AL. A . n Az-AX-AXr Fx一 F lhFx = Mkci = Ax2.kc3,0 =-=-1心+心If A=杀+雄円=心+心Then there will bezl A F-L8= 51 + ABecause y = lb &Then1? +Ak& 12 +Ak,Furtherm ore,if a given object is gripped, the following relations will exist.Ay = B Az-C (4)Where1+ A- Kq13 +Ak,When B is a constant, express the amplification of the above flexible lunge, C is function of Fy,but if grips a given object, Fy will be a constant and C will be a constant too.Az is subject to the stnicture of the stacked PZT element. A force Fz only charges on the stacked PZT element in Z & recti on when working, the simple graph of force is shown as figure7.Fig 7 The loading condition of the sracked PZT elementThe thickness of a lamella of the PZT element is t, and the displacement of the element lamella isAt.At can be calculated with following equationAt = (f E t + Ez t(5)岛,d33 are tlie peifonnance parameter of tlie PZT elementelastic constant, 2.52x1011d33piczoclectiic constant, 9.95xl0-1If the section area of the element is S, the charging voltage is V, tlienThe total displacement if n layers of PZT element is AzAz = n At =為 Fz n 七 + d33 V n(6)sAs the displacement of the PZT element is different from each other, the elastic counter force from the lunge is different either, Az is calculated by following equations, and the simple graph of londing condition is shown as figure 6.(replacing Fz by FA)Az=2A+d33.v.n,For this reasonAz =d33V-n 此i】tk&(1? +AkS哇 Fy % nt l(12 +Aks+城k&nt1If D =.li為i】tk& (1，+AkSThen there will be此片入ntl(P+Ak&)s +務 监ntAz= D Vn-P (9)From the above formula (9) we know tliat the PZT element that drives the flexible hinge mechainsm can not work under the maximum deformation because of the counter forces.Substitute Az into equation(4). The following equation can be got:y=M d33Vn-N (10)In the above equationM=ED=1 + A k&1爲 mtk。Q? +AksN = P B + C 二1%| A.%.F + Ak l2 +4k&涯 Fy jb ll t l(1，+ Ak&)s +醴k&iitWhen working, the output Ay is a linear function of the exerted voltage. Through the voltagecharged on the PZT element ,the output of the gripper is controlled and adjusted. M is the amplification of the mechainsm without charge/The amplification of the first layer of the flexible hinge can be got:(Fy=0 ,D=1)11b仁(ID1 + A k&The loading condition of the second layer of flexible hinge is shown as figure 5b. It is a amplifying mechainsm that makes use of the change of arms length, so larger amplificatuion can be got ,and the amplification can be calculated as below:K3 = -(12)In our design ,the struclure size of the flexible hinge is decided on the basis of tensile rigidity .shearing rigidity and torsional rigidity enough ,the parameter to be designed is presented in the table belowTable 1Hinge N210Radius R (mm)0.300.30Hinge width t (mm)0 300.30Link width h (mm)1 701.30Hinge thickness b (mm)2 002.00Distance between hinges 1 (mm)2 352.35= 14911%mY-P= -2 tail1 /= + Jl-(l + 0-汙 J20 + 0Y-Py p Jl-(l+0fIn which F=0, and the parameter of the material are tensle rigidity:In whichth0 = = 2R 2REelastic modulusY-PIn which0 = -2t an1 7i-(i+z?-z)2Gshearing elastic modulusTorsional rigidity:k厂牛“6484%z1+03 + 20+0，7(20 + 内6(1+0)_(20 +俨於_ tan10 Jl(1+0一汀Then the amplification of the two-layer flexible hinge nulli-gripper is:12+小 I3= 35.11Figure 8 shows the size of the structure for main parts. Figure 9 shows the model of two-layer flexible hinge nulli-gripper we made.Fig. 8The structure size of main partsFig 9 Two-layer flexible hinge milli-gripper modelWhen the input of driving element is Az .the output of the gripper(Fy = 0)is calculated as following relation2Ay = k-Az-2 (13)In whichthe most fit clearance,0 030mmWhen 0.5So, the output of the gripper is2Ay = 35.11Az-0.060(14)1. EXPERIMENT AND ANALYZING4.1 MaterialThe gripper consist of two mechanisms, but the stnicture forms are all flexible lunge Obviously,the flexible hinge take advantage of the characters of oneself elastic. So, when selecting material, theelastic function,thesensitiveness as well as high diamagnetic should be prefered. Such asQBc2 , QS116.5 0.1 as the material of flexible hinge4.2 AssemblingWe get the stacked PZT eleme nt from TOKIN compa ny .It has the characters of small size, high response and low driving voltage .The parameter of NLA-2x3x9 stacked PZT element used during the experiment are listed as the following from:Table 2Size axbxc (mm)2x3x9Maximum voltage V100Maximum load kg/lOOV21 20%Deformation /Z1I1/10(V6.5 10%Stacks65Distance between poles /fill115Insulation resistance MG500Capacitance 11F197The manufacture of the flexible hinge use the electric machining technology . The machiningaccueacy of the technology can be /Zill grade. The need of design can be satisfied basically.Assembly sequence are:Two flexible lunge mechanisms are drilled hole together一connecting twpmechanisms一assembling the driving element.In which.assembling PZT element is extremely difficult. As a result of the output of PZT element is very small /All and the output of (ZT element can be reduced in assembling process. The assembling accuracy must be high For this reason ,we desogned following structure:Size of mounting PZT element length size 1 =of PZT elementThrough grinding the size 1 of the mechanism , we can achieve a micro interference tolz ,thatsthe fit between gripping mechanism and PZT element is small interference fit.4.1 MeasuringA DWY-2 type vibrating displacement instrument was used to detected the displacement of the ZT element .Measuring range is 0.05100 /ZillThe measuing value of the PZT element: when lOOv is applied the output of PZT element is 6.4 “11Hence then，&om formula (14),the most output of the gripper can be calculated:T = 2Ay = 3 5.11x0.0064- 0.060= 0.1647nniThe measurement of actual output go on the experimental facility in figure 10. and the output of the gripper we detected is in table 3.Measuring number N1o345Output of displacement2 Ay0.1210.1190.1350.1240.130Measuring number N678910Output of displacement2 Ay0.1180.1310.1220.1260.131The actual output of the gripper can be gotyieal = 2Ay= 0.125nmOverall size of two-layer flexible lunge nulli -gnpper is 15.2x9.8x3 . Total weight is 3g After the experiment result be compared to theory calculating value , we can got : yreal ytfeoiy ,and &sparity is 23.6%5. CONCLUSION(l) Two-layer flexible hinge nulli-gripper is a new integrated nulli-gripper using PZT element driving . From the exploratory development we made ,we found a way for micro miniaturization of the mechanism of flexible lunge, that is , a certain number of flexible hinge mechanisms are stacked in tluckiiess direction for increasing amplification This way is feasible ,for stack tliree layer and or four layer further, same reference data are provided.(2) the integrated nulli-gripper composed of the PZT element and flexible hinge has the advantage of the PZT element and flexible hinge has the advantage of controllable output ,no fmtion ,no clearanve and easy making(3) The size of the integratede type nulli-gripper is closely telated with the size and the piezoelectric character of the PZT element In order to minimize the size of the gripper further, smaller size PZT element is needed.(4) The output and deformation of flexible hinge is calculated in this paper.The theory value of the gripper output is calculated, the actual output of the gripper is detecred and both is compared But the result exist disparity and needs working in the futureREFERENCES1 Linzhi.Sun/The Design of Milli-gripper Using Piezoelectric Actuator as well as Electrmagnetic Force for the 1995 IEEE Sixth Liternational Symposium on 14icro Machine and Human ScienceParos and L Weisbord/How to Design Flexure Hinges、Machine Design, Nov.25 1965. 3List of TOKIN Company Piezoelectric Element Product.
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