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Design and fabrication of a gripping tool for micromanipulation
s ORs AWORS A
ELSEVIER
Sensors and Actuators A 53 (1996) 428-433
PHYSICAL
Design and fabrication of a gripping tool for micromanipulation Greger Thornell, Mats Bexell, Jan-.~ke Schweitz, Stefan Johansson Uppsala University, Materials Science, Box 534, S-751 21 Uppsala, Sweden
Abstract A tool for rnicromanipulation is presented in this paper. A titanium gripper is fabricated by a combination of electro-discharge machining and etching, the latter of which is to provide a material less prone to cracking by removing the heat-affected surface. Evaluation of the design has been carried out by finite-element analysis and the performance of the gripper has been qualitatively as well as quantitatively established. The manipulation system to which the tool belongs is also briefly described. Keywords: Wire electro-discharge machining; Gripping tool; Micromanipulation
1. Introduction One way to build three-dimensional highly sophisticated structures is to assemble small and relatively simple parts. This is possible by high-precision manipulation and an adequate bonding method under simultaneous supervision [ 1 ]. A scanning electron microscope (SEM), with its excellent depth of field and resolution, is most suitable for this purpose. No commercial micromanipulating equipment is available and units are therefore, for the time being, constructed by the user. Our present version is designed for two independent manipulators, one of which is already installed, both with five degrees of freedom (DOF) and a specimen table with four DOF [2]. All tables are capable of movements with submicrometre resolution. Mounted in a cradle, with two additional DOF, together with facilities such as a heating table and force cells, they provide a 'micromechanical laboratory' inside the SEM. Of crucial importance in this laboratory is a tool for gripping the elements. In the size range for common micromechanical details gravitational forces are still significant and cannot be neglected, and mechanical gripping as in the real world seems to be the easiest solution for handling microelements [ 3]. The frictional forces, which in the case of small gripping forces are due mainly to attraction on an atomic or molecular level, could even constitute a problem for very small elements if they adhere too well. Other handling techniques might be necessary in this case. This article deals with the design, fabrication and evaluation of a pincerlike tool intended for manipulation of objects in the range 50/xm-1 mm. A number of properties should be considered. For instance, since this kind of tool is easy to damage and subject to 0924-4247/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved PII S0924-4247(96) 01146-6
mechanical, chemical and thermal ordeals, it must be easy to replace and cheap enough to discard. A high-strength material with high melting point ( > 700 °C) is necessary. Good biocompatibility is desirable, since instruments of this kind could also find applications in bio-medicine. Furthermore, the structure must be as rigid as possible except in the direction intended for gripping. Unwanted modes of deformation, due to misalignment in mounting or poor definition of the loading point, must not seriously lower the performance. Tactility would be an asset, but due to the additional processing neeessary, this has been omitted. Besides, other phenomena provide other means (e.g., charging effects) to determine the point of contact.
2. Design and performance simulation Taking into account the not yet mature microactuator technology, and recollecting the replacement demand, its seems too much of a luxury with an active gripper. Some sort of external actuation seems far more practical. A commercial piezo driver forms a simple and common solution. The dimensions of our experimental set-up, governed by the size of the vacuum chamber of the SEM, limit the size of the piezo driver to = 60 mm. The motion range ofa piezo driver implies quite a high displacement magnification factor. With a typical displacement of 45 /zm and a desirable pinch of 1 mm, it must be at least 22 or even higher to allow for some gripping force in the lower range of the gap width. Sliding contacts are unfavourable in miniature constructions, especially in vacuum, because of friction and wear, and a combination of levers and flexible joints is more suitable.
429
G. Thornell et al. / Sensors and Actuators A 53 (1996) 428~133
Table 1 Results from differentloading cases shown in Fig. 1 and combinationsthereof. The stress, Oeq, refers to the highest equivalent stress observed anywherein the structure and the overlap is the projected fraction, in the z-direction, of the very tip of one jaw on the other in a complete gripping action. In the last loading situation the misalignment is raised from 5 to 15% in the most critical direction, i.e.~ along the z-axis Load situation
Force or displacement case (cf. Fig. 1) 1 (mN)
I 11 III IV v vI vii viii IX X X1 XII XI|I XIV
2 (mN)
3 (mN)
4 (/xm)
5 (/xm)
6 (/xm)
o. 1 o. 1 0.4 2 2 2 o. 1 o. 1 o. 1 O.1 O.1 O.1 O.1 0.1
o. 1 o. 1 0.1 O.1 O.1 O.1 O.1 O.1
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
2 2 2 2 2 - 2 6
The material requirements are readily fulfilled by titanium, a high-temperature material with acknowledged biocompatibility [4]. Alloyed with 6% aluminium and 4% vanadium, its mechanical characteristics are excellent (yield strength --- 850-900 MPa, Young's modulus = 110 GPa). Of course the thickness of the structure is important for the torsional stability, but too thick a structure would not only make the gripper clumsy, but also prevent access in small spaces. A thickness of about 100/xm seems reasonable to allow picking and placing of 50/xm elements without imbalancing the jaws. A symmetric closing of the tool jaws, e.g., Ref. [5], is here given lower priority than replaceability. A rough estimate makes clear, however, that rotational stability is impossible to attain if the entire deformation is to be maintained by just one pliable element. Instead both jaws have to be actuated, but still via one single point of force applied by the piezo driver. Based on these conditions, the design in Fig. 1 is proposed. Further calculations gave the length of necessary levers and flexible elements shown in Fig. 1. Finite-element analysis (FEA) was used for verification and improvements of the calculated design, as well as for finding the critical
2 2
Maximum equivalent stress, tr~ (MPa)
Jaw overlap (%)
775 779 787 773 775 785 784 776 777 792 780 783 793 788
94 98 100 91 100 100 95 85 95 95 98 98 97 68
point for actuation. If chosen to be 2 8 0 / x m from the upper two flexible elements, the necessary displacement of this point in the negative x-direction for a complete closing of the gap was found to be 40/xm. The numbers in Fig. 1 refer to different loading cases, all of which were treated in advance by FLEA.The force direction arrows 1 and 2 represent the case with a loading of each jaw by 0.1 mN in a direction perpendicular to both the direction of gripping and external actuation. This equals the weight of a metallic cube (say gold, p-- 20 g c m - 3) of edge 1 ram. To keep hold of such an item, assuming a coefficient of friction of about 0.5, a minimum gripping force of about 0.4 mN is necessary (case (or arrow) 3). If the tool is mounted in a non-parallel fashion to the driver, no matter how slightly in terms of geometry, it may significally decrease the performance. In order to simulate this, 5% of the maximum necessary actuation displacement (i.e., 2 txm), was added to the point of application, but in three perpendicular directions (cases 4, 5 and 6, Fig. 1). The result expressed in maximum equivalent stress and jaw overlap of these cases, and combinations thereof, is presented in Table 1. The calculated equivalent stress, ~r~q, defined through Eq. ( 1 ), is generally used to predict the onset of yield:
2mm 1/f
5 actu
overlap
Y
J
x
\
Fig. 1. The proposed design for the gripping tool. The different cases of loading simulated by the finite-element method (FEM) are indicated by arrows. Insert: definition of the jaw overlap.
O'eq~---(1 [ ( O r l - O'2)2"~- (Or2--O'3)2"~ (O'3-- O'1)2]} 1/2
(1)
where cr~-~r 3 are the principal stresses in a coordinate system oriented so that shear stresses vanish [6]. The necessary actuation displacement for completely closing the gap (i.e., 4 0 / z m ) is superimposed on all the situations shown in Table 1. This procedure actually results in rather pessimistic estimates, since in reality an item loading the jaws cannot demand a full closing of the jaws, due to its size. On the other hand, small objects could be stuck or in need of extra force during
430
G. Thornell et al. / Sensors and Actuators A 53 (1996) 428-433
mounting, for instance. In a complete gripping action the lower jaw is to move = 15% of the gap, according to the simulations. The maximum stresses presented in Table 1 are found at the endpoints of the horizontal flexible element. However, being close to the yield strength of the material, these values show little scatter (within a few percent), indicating satisfactory dimensioning. The jaw overlap ranges from 68 to 100%, but is in all situations, except the worst one, far above the 50% necessary.
3. Fabrication Owing to the material and size, wire electro-discharge machining (WEDM) [7] seems most appropriate for the realization. One drawback of the method is the formation of a heat-affected layer on the surface, called hard skin, in which crack initiation is likely to appear. This could, however, be eliminated by subsequent etching. With some modification a conventional WEDM equipment is able to cut 40/xm thick elements of considerable length. In this case straight polarity was used and the dielectric was water (conductivity < 10 #S), which is frequently used in high-precision EDM [8].
A 100/xm diameter brass wire and the finest setting on the Charmilles F40 was used. The result is shown in Fig. 2. Closer investigation of the machined surface revealed relatively large resolidified spheres, especially in the concave corners (Fig. 2(a)), and in general a rather rough surface (Fig. 2 (c)). The most slender parts were therefore somewhat overdimensioned to allow for subsequent etching. Prior to the two-step etching cycle the material was degreased and cleaned in acetone. The structure was then pickled for 5 min in an aqueous caustic solution (43% NaOH) close to boiling and then etched for 1 min in an aqueous nitric-hydrofluoric acid solution (35% HNO3, 2% HF) at room temperature. After the first cycle the resolidified spheres shown in Fig. 2(a) were almost completely removed. The procedure was then repeated twice to smooth the surface further and to reach the critical width of the pliable elements i.e., 40/xm. In each cycle = 8-9 p,m were removed. During the first two cycles, the structure changed its form a little, probably because of the removal of the residual stresses induced in the surface in the EDM process. A comparison between the unetched and etched surface is made in Fig. 2. In Fig. 3 selected parts of the structure are shown inclined. Noticeable is the retention of edges and corners and also the similarity in roughness between the machined surfaces and those not machined.
Fig. 2. A comparisonbetween an unetchedand etched concavecomer (a) and (b), and a pliable element (c) and (d).
431
G. Thornell et al. / Sensors and Actuators A 53 (1996) 428-433
Fig. 3. SeJected parts of the structure, after etching, chosen to show the retention of edges and comers and the similarity in roughness of machined surfaces and those not machined.
4. Evaluation of the finished tool In Fig. 4 the complete gripper is shown, undeformed and fully deformed. The appearance of the latter state is very similar to that foreseen by the FEA. No permanent deformation was obtained on closing and opening the gap repeatedly. The fraction of the total gap covered by the lower jaw was also in agreement with that found by simulation, i.e., about 15%, as shown by Fig. 5, where, in addition, the satisfactory linearity of gap size versus elongation of the piezo driver is demonstrated. A few experiments have been performed in order to establish the gripping force of the tool. The force necessary for extraction of an object from the jaws during different displacements of the boss (i.e., piezo elongation) has been recorded. As seen from Fig. 6, it of course depends
1000,
800 .~ 600
:4
400
200 0
0
20 40 60 80 100 fraction of maximum piezo elongation (%)
Fig. 5. Position of upper and lower jaw as a function of activation of piezo driver. 20 f[ nylon wire O 0.22 mm [ _ _ _ . . _ ~ / 15 I[tungsten wire ¢ 0'10 mm ~
0
!
25
'
'
'
"~'!
'
'
-~¢"
I
,
~
,
,
i
. . . .
30 35 40 elongation ofpiezo driver (p.m)
45
Fig. 6. Force necessary to liberate wires of two different dimensions and materials from the jaws as a function of activation of piezo driver.
on both the material (coefficient of friction) and size of the object.
5. Description of the tool system
Fig. 4. The gripper in an undeformed (a) and in a fully deformed state (b). The total length of the tool is 13 mm.
The coupling between the piezotube and the gripper is shown in Fig. 7. The piezotube itself is not visible, but a second tube on its outside to which the tweezer tool is glued can be seen. The boss, barely seen here, but shown in Fig. 3(right), is reached by a rod fixed to the end of the piezo driver. The time for replacement of the gripper could be shortened further by clamping instead of gluing. The unit is mounted on a flexible arm on a manipulator station, with five degrees of freedom (DOF), capable of
432
G. Thornell et al. /Sensors and Actuators A 53 (1996) 428-433
Fig. 7. The complete tool with the gripper mounted on a tube with the piezo driver inside (length 80 mm).
Fig. 8. The manipulator station allowing for precise positioning of the gripping tool. The gripper is located slightly to the left of the picture's centre, above a heating table. To the right can be seen one side of the cradle, dimensioned for in situ experiments inside an SEM. large-stroke high-resolution ( = 10 nm), movements. In Fig. 8 this station is shown together with a heating table (on the left-hand side) with four additional DOF. The construction allows for manipulation around, and simultaneous supervision of, an eucentric position. In the future there will also be a second station left for temperature or force probing, or of course a second gripper. The whole equipment is designed for in situ use in a scanning electron microscope and is therefore vacuum compatible.
6. Discussion and conclusions The hard skin formation usually encountered in wire electro-discharge machining could, for a material such as the Ti alloy chosen here, perhaps cause fatigue cracking, and thus have an injurious effect on the otherwise high strength. In
this particular case the heat-affected layer is removed by the etchant, but the surface thus obtained is far from smooth and stress concentrations could lead to local plastic deformation. If occurring during normal actuation it would result in perhaps just a lesser pitch, but when the tool is misaligned or otherwise incorrectly loaded the structure could become permanently skewed and useless. So, the alignment may in fact be more critical than shown by the simulations in which perfectly flat surfaces were assumed. However, during testing of the finished tool, no such problems have occurred. There are ways to improve the smoothness of the machined surface, for instance, by reversing the polarity and using the electron current or lowering the discharge energy. Using reversed polarity and very short pulses, the wear of the tool electrode could be reduced due to the low mobility of the ions, which otherwise would seriously lower the performance of EDM. In W E D M this is less of a problem since the tool electrode is continuously replenished during machining. A low discharge energy could be achieved by lowering the capacitance of the equipment. For the same reason highprecision machines tend to be small in order to minimize the stray capacitance [9]. It has been shown in this paper that small mechanical elements can be fabricated from a polycrystalline metallic material by a combination of two conventional processes, W E D M and etching, the first of which offers an aspect ratio as high as 100 or more and an estimated minimum element thickness of 10/xm, or even less with improved equipment. Considering the fabrication cost, it could be argued that the method lacks the advantage of batch manufacturing offered by traditional lithography-based micromachining. Dealing with prototypes or small series, W E D M offers a faster and more economic alternative, however. In fact, for fiat designs, batch processing can be achieved simply by electro-discharge machining of a stack of foils glued together with a conducting adhesive.
References [ 1] A.-L. Tiensuu, M. Bexell, J.-,~. Schweitz, L. Smith and S. Johansson, Assembling three-dimensional microstructures using gold-silicon eutectic bonding, Sensors and Actuators A, 45 (1994) 227-236. [2] S. Johansson, Micromanipulationfor micro- and nano-manufacturing, Proc. Emerging Technologies and Factory Automation, Paris, 1995,
pp. 3-8. 13] F.W. West, The size of man, American Scientist, 56 (1968) 400-413. [4] P. Tengvall and I. LundstriSm, Physico-chemical considerations of titanium as a biomaterial, Clin. Mater., 9 (1992) 115-134. [5] H. Morishita and Y. Hatamura, Development of ultra precise manipulator system for future nanotechnology, Proc. 1st Workshop Micro Robotics and Systems, Karlsruhe, Germany, 1993, pp. 34-42. [6l A.P. Boresi and O.M. Sidebottom,Advanced Mechanics of Materials, Wiley, New York, 4th edn., 1985. [7] R. Snoeys, F. Staelens and W. Dekeyser, Current trends in nonconventional material removal processes, Ann. CIRP, 35 (1986) 467480. [8] C. van Osenbruggen, High-precision spark machining, Philips Tech. Rev., 30 (1969) 195-208.
G. Thornellet al. /Sensors and Actuators A 53 (1996) 428-433
[9] T. Masaki, K. Kawata and T. Masuzawa, Micro electro-discharge machining and its applications,Proc. IEEE Micro Electro Mechanical Systems, Napa Valley, CA, USA, 1990, pp. 21-26.
Biographies Greger Thornell graduated as an M.Sc. in materials science from Uppsala University in 1994. He has now joined the Micromcchanics Programme at the Division of Materials Science, Uppsala University, as a Ph.D. student. His research interests include three-dimensional microprocessing and actuation principles for microrobotic purposes. Mats Bexel! received his M.Sc. degrec in materials sciencc from Uppsala University in 1992. Currently he is a Ph.D. student within the Micromechanics Programme at the Division of Materials Science, Uppsala University. His research interests are materials for microactuation intended fro micromotors and microrobotics. Jan-Ake Schweitz received his B.Sc. degree in 1967 from the University of Gothenburg, and an M.Sc. degree in engi-
433
neering physics from Uppsala University in 1969. He joined the Solid State Physics Department at the same university as a graduate student in 1970, and received his Dr.Sc. degree in 1975. In 1977 he became an associate professor, and is now acting as a full professor of material science at Uppsala University. In 1984 he was one of the initiators of the Micromechanics Programme in Uppsala, which is presently headed jointly by the two initiators. Stefan Johansson received his M.Sc. degree in engineering physics from Uppsala University in 1982. In 1983 he joined the Division of Material Science, Uppsala University, where he was concerned with development methods for evaluation of the micromechanical properties of silicon. In 1988 he received his Dr.Sc. degree in materials science at Uppsala University. Since 1988 he has been working with different materials-science aspects of micromechanics, and in particular with the development of three-dimensional microprocessing and design of microrobotic components. He holds an associate professor position within the Micromechanics Programme at Uppsala University.
ELSEVIER
Sensors and Actuators A 53 (1996) 428-433
PHYSICAL
Design and fabrication of a gripping tool for micromanipulation Greger Thornell, Mats Bexell, Jan-.~ke Schweitz, Stefan Johansson Uppsala University, Materials Science, Box 534, S-751 21 Uppsala, Sweden
Abstract A tool for rnicromanipulation is presented in this paper. A titanium gripper is fabricated by a combination of electro-discharge machining and etching, the latter of which is to provide a material less prone to cracking by removing the heat-affected surface. Evaluation of the design has been carried out by finite-element analysis and the performance of the gripper has been qualitatively as well as quantitatively established. The manipulation system to which the tool belongs is also briefly described. Keywords: Wire electro-discharge machining; Gripping tool; Micromanipulation
1. Introduction One way to build three-dimensional highly sophisticated structures is to assemble small and relatively simple parts. This is possible by high-precision manipulation and an adequate bonding method under simultaneous supervision [ 1 ]. A scanning electron microscope (SEM), with its excellent depth of field and resolution, is most suitable for this purpose. No commercial micromanipulating equipment is available and units are therefore, for the time being, constructed by the user. Our present version is designed for two independent manipulators, one of which is already installed, both with five degrees of freedom (DOF) and a specimen table with four DOF [2]. All tables are capable of movements with submicrometre resolution. Mounted in a cradle, with two additional DOF, together with facilities such as a heating table and force cells, they provide a 'micromechanical laboratory' inside the SEM. Of crucial importance in this laboratory is a tool for gripping the elements. In the size range for common micromechanical details gravitational forces are still significant and cannot be neglected, and mechanical gripping as in the real world seems to be the easiest solution for handling microelements [ 3]. The frictional forces, which in the case of small gripping forces are due mainly to attraction on an atomic or molecular level, could even constitute a problem for very small elements if they adhere too well. Other handling techniques might be necessary in this case. This article deals with the design, fabrication and evaluation of a pincerlike tool intended for manipulation of objects in the range 50/xm-1 mm. A number of properties should be considered. For instance, since this kind of tool is easy to damage and subject to 0924-4247/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved PII S0924-4247(96) 01146-6
mechanical, chemical and thermal ordeals, it must be easy to replace and cheap enough to discard. A high-strength material with high melting point ( > 700 °C) is necessary. Good biocompatibility is desirable, since instruments of this kind could also find applications in bio-medicine. Furthermore, the structure must be as rigid as possible except in the direction intended for gripping. Unwanted modes of deformation, due to misalignment in mounting or poor definition of the loading point, must not seriously lower the performance. Tactility would be an asset, but due to the additional processing neeessary, this has been omitted. Besides, other phenomena provide other means (e.g., charging effects) to determine the point of contact.
2. Design and performance simulation Taking into account the not yet mature microactuator technology, and recollecting the replacement demand, its seems too much of a luxury with an active gripper. Some sort of external actuation seems far more practical. A commercial piezo driver forms a simple and common solution. The dimensions of our experimental set-up, governed by the size of the vacuum chamber of the SEM, limit the size of the piezo driver to = 60 mm. The motion range ofa piezo driver implies quite a high displacement magnification factor. With a typical displacement of 45 /zm and a desirable pinch of 1 mm, it must be at least 22 or even higher to allow for some gripping force in the lower range of the gap width. Sliding contacts are unfavourable in miniature constructions, especially in vacuum, because of friction and wear, and a combination of levers and flexible joints is more suitable.
429
G. Thornell et al. / Sensors and Actuators A 53 (1996) 428~133
Table 1 Results from differentloading cases shown in Fig. 1 and combinationsthereof. The stress, Oeq, refers to the highest equivalent stress observed anywherein the structure and the overlap is the projected fraction, in the z-direction, of the very tip of one jaw on the other in a complete gripping action. In the last loading situation the misalignment is raised from 5 to 15% in the most critical direction, i.e.~ along the z-axis Load situation
Force or displacement case (cf. Fig. 1) 1 (mN)
I 11 III IV v vI vii viii IX X X1 XII XI|I XIV
2 (mN)
3 (mN)
4 (/xm)
5 (/xm)
6 (/xm)
o. 1 o. 1 0.4 2 2 2 o. 1 o. 1 o. 1 O.1 O.1 O.1 O.1 0.1
o. 1 o. 1 0.1 O.1 O.1 O.1 O.1 O.1
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
2 2 2 2 2 - 2 6
The material requirements are readily fulfilled by titanium, a high-temperature material with acknowledged biocompatibility [4]. Alloyed with 6% aluminium and 4% vanadium, its mechanical characteristics are excellent (yield strength --- 850-900 MPa, Young's modulus = 110 GPa). Of course the thickness of the structure is important for the torsional stability, but too thick a structure would not only make the gripper clumsy, but also prevent access in small spaces. A thickness of about 100/xm seems reasonable to allow picking and placing of 50/xm elements without imbalancing the jaws. A symmetric closing of the tool jaws, e.g., Ref. [5], is here given lower priority than replaceability. A rough estimate makes clear, however, that rotational stability is impossible to attain if the entire deformation is to be maintained by just one pliable element. Instead both jaws have to be actuated, but still via one single point of force applied by the piezo driver. Based on these conditions, the design in Fig. 1 is proposed. Further calculations gave the length of necessary levers and flexible elements shown in Fig. 1. Finite-element analysis (FEA) was used for verification and improvements of the calculated design, as well as for finding the critical
2 2
Maximum equivalent stress, tr~ (MPa)
Jaw overlap (%)
775 779 787 773 775 785 784 776 777 792 780 783 793 788
94 98 100 91 100 100 95 85 95 95 98 98 97 68
point for actuation. If chosen to be 2 8 0 / x m from the upper two flexible elements, the necessary displacement of this point in the negative x-direction for a complete closing of the gap was found to be 40/xm. The numbers in Fig. 1 refer to different loading cases, all of which were treated in advance by FLEA.The force direction arrows 1 and 2 represent the case with a loading of each jaw by 0.1 mN in a direction perpendicular to both the direction of gripping and external actuation. This equals the weight of a metallic cube (say gold, p-- 20 g c m - 3) of edge 1 ram. To keep hold of such an item, assuming a coefficient of friction of about 0.5, a minimum gripping force of about 0.4 mN is necessary (case (or arrow) 3). If the tool is mounted in a non-parallel fashion to the driver, no matter how slightly in terms of geometry, it may significally decrease the performance. In order to simulate this, 5% of the maximum necessary actuation displacement (i.e., 2 txm), was added to the point of application, but in three perpendicular directions (cases 4, 5 and 6, Fig. 1). The result expressed in maximum equivalent stress and jaw overlap of these cases, and combinations thereof, is presented in Table 1. The calculated equivalent stress, ~r~q, defined through Eq. ( 1 ), is generally used to predict the onset of yield:
2mm 1/f
5 actu
overlap
Y
J
x
\
Fig. 1. The proposed design for the gripping tool. The different cases of loading simulated by the finite-element method (FEM) are indicated by arrows. Insert: definition of the jaw overlap.
O'eq~---(1 [ ( O r l - O'2)2"~- (Or2--O'3)2"~ (O'3-- O'1)2]} 1/2
(1)
where cr~-~r 3 are the principal stresses in a coordinate system oriented so that shear stresses vanish [6]. The necessary actuation displacement for completely closing the gap (i.e., 4 0 / z m ) is superimposed on all the situations shown in Table 1. This procedure actually results in rather pessimistic estimates, since in reality an item loading the jaws cannot demand a full closing of the jaws, due to its size. On the other hand, small objects could be stuck or in need of extra force during
430
G. Thornell et al. / Sensors and Actuators A 53 (1996) 428-433
mounting, for instance. In a complete gripping action the lower jaw is to move = 15% of the gap, according to the simulations. The maximum stresses presented in Table 1 are found at the endpoints of the horizontal flexible element. However, being close to the yield strength of the material, these values show little scatter (within a few percent), indicating satisfactory dimensioning. The jaw overlap ranges from 68 to 100%, but is in all situations, except the worst one, far above the 50% necessary.
3. Fabrication Owing to the material and size, wire electro-discharge machining (WEDM) [7] seems most appropriate for the realization. One drawback of the method is the formation of a heat-affected layer on the surface, called hard skin, in which crack initiation is likely to appear. This could, however, be eliminated by subsequent etching. With some modification a conventional WEDM equipment is able to cut 40/xm thick elements of considerable length. In this case straight polarity was used and the dielectric was water (conductivity < 10 #S), which is frequently used in high-precision EDM [8].
A 100/xm diameter brass wire and the finest setting on the Charmilles F40 was used. The result is shown in Fig. 2. Closer investigation of the machined surface revealed relatively large resolidified spheres, especially in the concave corners (Fig. 2(a)), and in general a rather rough surface (Fig. 2 (c)). The most slender parts were therefore somewhat overdimensioned to allow for subsequent etching. Prior to the two-step etching cycle the material was degreased and cleaned in acetone. The structure was then pickled for 5 min in an aqueous caustic solution (43% NaOH) close to boiling and then etched for 1 min in an aqueous nitric-hydrofluoric acid solution (35% HNO3, 2% HF) at room temperature. After the first cycle the resolidified spheres shown in Fig. 2(a) were almost completely removed. The procedure was then repeated twice to smooth the surface further and to reach the critical width of the pliable elements i.e., 40/xm. In each cycle = 8-9 p,m were removed. During the first two cycles, the structure changed its form a little, probably because of the removal of the residual stresses induced in the surface in the EDM process. A comparison between the unetched and etched surface is made in Fig. 2. In Fig. 3 selected parts of the structure are shown inclined. Noticeable is the retention of edges and corners and also the similarity in roughness between the machined surfaces and those not machined.
Fig. 2. A comparisonbetween an unetchedand etched concavecomer (a) and (b), and a pliable element (c) and (d).
431
G. Thornell et al. / Sensors and Actuators A 53 (1996) 428-433
Fig. 3. SeJected parts of the structure, after etching, chosen to show the retention of edges and comers and the similarity in roughness of machined surfaces and those not machined.
4. Evaluation of the finished tool In Fig. 4 the complete gripper is shown, undeformed and fully deformed. The appearance of the latter state is very similar to that foreseen by the FEA. No permanent deformation was obtained on closing and opening the gap repeatedly. The fraction of the total gap covered by the lower jaw was also in agreement with that found by simulation, i.e., about 15%, as shown by Fig. 5, where, in addition, the satisfactory linearity of gap size versus elongation of the piezo driver is demonstrated. A few experiments have been performed in order to establish the gripping force of the tool. The force necessary for extraction of an object from the jaws during different displacements of the boss (i.e., piezo elongation) has been recorded. As seen from Fig. 6, it of course depends
1000,
800 .~ 600
:4
400
200 0
0
20 40 60 80 100 fraction of maximum piezo elongation (%)
Fig. 5. Position of upper and lower jaw as a function of activation of piezo driver. 20 f[ nylon wire O 0.22 mm [ _ _ _ . . _ ~ / 15 I[tungsten wire ¢ 0'10 mm ~
0
!
25
'
'
'
"~'!
'
'
-~¢"
I
,
~
,
,
i
. . . .
30 35 40 elongation ofpiezo driver (p.m)
45
Fig. 6. Force necessary to liberate wires of two different dimensions and materials from the jaws as a function of activation of piezo driver.
on both the material (coefficient of friction) and size of the object.
5. Description of the tool system
Fig. 4. The gripper in an undeformed (a) and in a fully deformed state (b). The total length of the tool is 13 mm.
The coupling between the piezotube and the gripper is shown in Fig. 7. The piezotube itself is not visible, but a second tube on its outside to which the tweezer tool is glued can be seen. The boss, barely seen here, but shown in Fig. 3(right), is reached by a rod fixed to the end of the piezo driver. The time for replacement of the gripper could be shortened further by clamping instead of gluing. The unit is mounted on a flexible arm on a manipulator station, with five degrees of freedom (DOF), capable of
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Fig. 7. The complete tool with the gripper mounted on a tube with the piezo driver inside (length 80 mm).
Fig. 8. The manipulator station allowing for precise positioning of the gripping tool. The gripper is located slightly to the left of the picture's centre, above a heating table. To the right can be seen one side of the cradle, dimensioned for in situ experiments inside an SEM. large-stroke high-resolution ( = 10 nm), movements. In Fig. 8 this station is shown together with a heating table (on the left-hand side) with four additional DOF. The construction allows for manipulation around, and simultaneous supervision of, an eucentric position. In the future there will also be a second station left for temperature or force probing, or of course a second gripper. The whole equipment is designed for in situ use in a scanning electron microscope and is therefore vacuum compatible.
6. Discussion and conclusions The hard skin formation usually encountered in wire electro-discharge machining could, for a material such as the Ti alloy chosen here, perhaps cause fatigue cracking, and thus have an injurious effect on the otherwise high strength. In
this particular case the heat-affected layer is removed by the etchant, but the surface thus obtained is far from smooth and stress concentrations could lead to local plastic deformation. If occurring during normal actuation it would result in perhaps just a lesser pitch, but when the tool is misaligned or otherwise incorrectly loaded the structure could become permanently skewed and useless. So, the alignment may in fact be more critical than shown by the simulations in which perfectly flat surfaces were assumed. However, during testing of the finished tool, no such problems have occurred. There are ways to improve the smoothness of the machined surface, for instance, by reversing the polarity and using the electron current or lowering the discharge energy. Using reversed polarity and very short pulses, the wear of the tool electrode could be reduced due to the low mobility of the ions, which otherwise would seriously lower the performance of EDM. In W E D M this is less of a problem since the tool electrode is continuously replenished during machining. A low discharge energy could be achieved by lowering the capacitance of the equipment. For the same reason highprecision machines tend to be small in order to minimize the stray capacitance [9]. It has been shown in this paper that small mechanical elements can be fabricated from a polycrystalline metallic material by a combination of two conventional processes, W E D M and etching, the first of which offers an aspect ratio as high as 100 or more and an estimated minimum element thickness of 10/xm, or even less with improved equipment. Considering the fabrication cost, it could be argued that the method lacks the advantage of batch manufacturing offered by traditional lithography-based micromachining. Dealing with prototypes or small series, W E D M offers a faster and more economic alternative, however. In fact, for fiat designs, batch processing can be achieved simply by electro-discharge machining of a stack of foils glued together with a conducting adhesive.
References [ 1] A.-L. Tiensuu, M. Bexell, J.-,~. Schweitz, L. Smith and S. Johansson, Assembling three-dimensional microstructures using gold-silicon eutectic bonding, Sensors and Actuators A, 45 (1994) 227-236. [2] S. Johansson, Micromanipulationfor micro- and nano-manufacturing, Proc. Emerging Technologies and Factory Automation, Paris, 1995,
pp. 3-8. 13] F.W. West, The size of man, American Scientist, 56 (1968) 400-413. [4] P. Tengvall and I. LundstriSm, Physico-chemical considerations of titanium as a biomaterial, Clin. Mater., 9 (1992) 115-134. [5] H. Morishita and Y. Hatamura, Development of ultra precise manipulator system for future nanotechnology, Proc. 1st Workshop Micro Robotics and Systems, Karlsruhe, Germany, 1993, pp. 34-42. [6l A.P. Boresi and O.M. Sidebottom,Advanced Mechanics of Materials, Wiley, New York, 4th edn., 1985. [7] R. Snoeys, F. Staelens and W. Dekeyser, Current trends in nonconventional material removal processes, Ann. CIRP, 35 (1986) 467480. [8] C. van Osenbruggen, High-precision spark machining, Philips Tech. Rev., 30 (1969) 195-208.
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[9] T. Masaki, K. Kawata and T. Masuzawa, Micro electro-discharge machining and its applications,Proc. IEEE Micro Electro Mechanical Systems, Napa Valley, CA, USA, 1990, pp. 21-26.
Biographies Greger Thornell graduated as an M.Sc. in materials science from Uppsala University in 1994. He has now joined the Micromcchanics Programme at the Division of Materials Science, Uppsala University, as a Ph.D. student. His research interests include three-dimensional microprocessing and actuation principles for microrobotic purposes. Mats Bexel! received his M.Sc. degrec in materials sciencc from Uppsala University in 1992. Currently he is a Ph.D. student within the Micromechanics Programme at the Division of Materials Science, Uppsala University. His research interests are materials for microactuation intended fro micromotors and microrobotics. Jan-Ake Schweitz received his B.Sc. degree in 1967 from the University of Gothenburg, and an M.Sc. degree in engi-
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neering physics from Uppsala University in 1969. He joined the Solid State Physics Department at the same university as a graduate student in 1970, and received his Dr.Sc. degree in 1975. In 1977 he became an associate professor, and is now acting as a full professor of material science at Uppsala University. In 1984 he was one of the initiators of the Micromechanics Programme in Uppsala, which is presently headed jointly by the two initiators. Stefan Johansson received his M.Sc. degree in engineering physics from Uppsala University in 1982. In 1983 he joined the Division of Material Science, Uppsala University, where he was concerned with development methods for evaluation of the micromechanical properties of silicon. In 1988 he received his Dr.Sc. degree in materials science at Uppsala University. Since 1988 he has been working with different materials-science aspects of micromechanics, and in particular with the development of three-dimensional microprocessing and design of microrobotic components. He holds an associate professor position within the Micromechanics Programme at Uppsala University.