- Email: [email protected]
SMA microgripper with integrated antagonism
Sensors and Actuators 83 Ž2000. 208–213 www.elsevier.nlrlocatersna
SMA microgripper with integrated antagonism M. Kohl a,) , E. Just a , W. Pfleging a , S. Miyazaki b b
a Forschungszentrum Karlsruhe, IMT, Postfach 3640, 76021 Karlsruhe, Germany Institute of Materials Science, UniÕersity of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Accepted 17 December 1999
Abstract A microgripper of 2 = 3.9 = 0.1 mm3 size is presented consisting of a single device microfabricated from a shape memory alloy ŽSMA. thin sheet. The device consists of two integrated actuation units of a stress-optimized shape, which actuate in opposite directions and thus form an antagonistic pair. The fabrication procedure is reduced to one micromachining step of a rolled SMA sheet and subsequent bonding onto a substrate. The maximum displacement of the gripping jaws is 180 mm, the maximum gripping force 17 mN. For an electrical power of 22 mW, a response time of 32 ms is observed. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Microgripper; Shape memory actuator; Monolithic microdevice; Smart materials; TiNi cold-rolled sheets
1. Introduction For microrobotic applications, compact actuator designs with high work outputs are required. Attractive solutions have been achieved by using shape memory alloys ŽSMAs. w1,2x. Besides high work outputs, further important features of SMA microdevices are compatibility to microelectronics, a high reliability due to an intrinsic actuation mechanism and a frictionless actuation. Recently, a fabrication concept based on the lateral micromachining of SMA sheets or films has been developed w3x, which allows integration of several functional units in a single SMA piece and thus, a high flexibility in device design. This concept enables the realization of microactuators with complex functionality and a large degree of miniaturization w4,5x. The paper presents a microgripper of TiNi consisting of two integrated actuation units with opposite moving directions. This allows separate control of the opening and closing motion of integrated gripping jaws in an antagonistic way. After presentation of the design of the
)
Corresponding author. E-mail address: [email protected] ŽM. Kohl..
actuation units, experimental details and actuation properties of the used SMA materials are discussed. Design simulation and optimization have been performed by finite element calculations. Later on, the mechanical and electrical properties of the microgrippers are presented and solutions for positioning of the gripping jaws are discussed.
2. Gripper design Fig. 1 shows a scheme of the SMA microgripper in open and closed condition. The gripper mainly consists of two actuation units with bond pads for mechanical and electrical connection, a link between both units and two gripping jaws. Actuation unit I comprises a folded-beam structure and unit II comprises two circular beams. Due to a heat treatment, the memory shape of the units corresponds to the undeflected beam shape. The gripper is mounted on a substrate in prestrained condition. Thus, deformation is created in the beam structures, which can be controlled by electrical heating. By selective heating of actuation unit I above the phase transformation temperature, the folded beams recover their memory shape, which leads to a linear motion of the link w4x. Consequently, the circular beams are deformed and the gripping jaws are closed. This condition can be reset by selective heating of
0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 3 8 5 - 4
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209
electrical resistance of the double-beam devices has been determined by the four-point method. The heat-treated sheets of TiNi are the basic material for realization of the microgripper. For a detailed understanding of the mechanical and electrical properties of the gripper, the actuation units were fabricated and investigated separately. Force-displacement characteristics were determined by mounting the actuation units in vertical orientation and attaching calibrated microweights close to the link as sketched in Fig. 2. Heating of the actuation units was performed directly by applying an electrical current.
Fig. 1. Operation principle of the SMA microgripper.
the circular beams. The link between the actuation units is designed to provide sufficient thermal isolation.
3. Experimental Cold-rolled sheets of an equiatomic TiNi alloy have been heat treated in vacuum at 5308C for 10 min to adjust the one-way shape memory effect. Phase transformation temperatures were determined by differential scanning calorimetry ŽDSC. and electrical resistance measurements. For characterization of the mechanical material properties, double-beam test devices were fabricated by laser cutting, which were investigated by beam-bending experiments as a function of temperature and load. The test devices were loaded by a microweight at the beam end and the corresponding vertical displacements of the beam ends were determined optically by a video microscope. The experiments were performed in a thermostat, which allowed adjustment of the temperature by ambient heating or cooling. Stationary equilibrium conditions have been maintained by cycling the temperature stepwise with sufficient waiting time in every data point. Simultaneously, the
Fig. 2. Scheme for measurement of the mechanical and electrical characteristics of actuation units I Ža. and II Žb..
4. Material properties The transformation behavior of the SMA material has been investigated by DSC, electrical resistance, and mechanical bending experiments of double-beam test devices. Fig. 3 shows a DSC measurement of the cold-rolled TiNi sheet performed at the heat flow of 10 Krs. The temperatures of the austenitic, martensitic and rhombohedral ŽR. phase transformations have been determined to A s s 568C, A f s 668C, Ms s 208C, Mf s 128C, R s s 448C, and R f s 408C. The indices s and f denote start and finish temperatures, respectively. Since the martensitic transformation occurs below room temperature, only the intermediate R-phase transformation can be induced by electrical heating. This material behavior is of special interest, since R-phase transformations exhibit a narrow hysteresis width of about 1 K and negligible fatigue effects w6x. Fig. 4 shows temperature displacement and electrical resistance characteristics of a double-beam test device for two different loads of 16 and 42 mN in the transformation temperature regime. Upon cooling, the R-phase transformation and martensitic phase transformation are observed. Upon heating, the martensite directly transforms to the
Fig. 3. Differential scanning calorimetry measurement of a cold-rolled TiNi sheet. The transformation temperatures of martensitic, R-, and austenitic phase are indicated. The indices s, p and f denote start, peak and finish temperatures.
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M. Kohl et al.r Sensors and Actuators 83 (2000) 208–213
lateral width profile in open and closed condition. White regions indicate zero stress; maximum stress values are
Fig. 4. Beam-bending and electrical resistance measurements of a double-beam test device for two different loads. The start transformation temperatures of martensitic, R-, and austenitic phase are indicated.
austenitic parent phase. Compared to the DSC measurements, the phase transformation temperatures are slightly shifted to higher values due to load-induced stress. At room temperature Ž238C., a full R-phase transformation is observed upon cooling, while the martensitic phase transformation just starts at Ms .
5. Design simulation and optimization In order to avoid local stress concentrations in the actuation units upon loading, the lateral widths of the actuation units have been optimized. Stress and strain profiles were calculated by finite element simulations ŽFEM.. The required material parameters on stress and strain were determined before from beam-bending experiments. Based on the FEM model, the geometry of the actuation units was optimized to obtain a homogeneous stress profile. In this way, fatigue effects due to stress peaks are avoided and a maximum fraction of material contributes to the shape memory effect w4x. The maximum stress has to be sufficiently high to obtain high work outputs. However, it should not exceed a material-dependent critical limit, above which plastic deformation occurs. Stress optimization has been performed by using the computer aided optimization ŽCAO. method w7x, which is based on iterative FEM. Fig. 5 shows calculated von Mises stress profiles in actuation units of TiNi with optimized
Fig. 5. Von Mises stress profiles in open Ža. and closed Žb. condition. Stress intensities are indicated by different gray levels between black Žmaximum stress. and white Žno stress..
M. Kohl et al.r Sensors and Actuators 83 (2000) 208–213
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Fig. 6. SMA microgripper.
indicated in black. Homogeneous stress profiles occur along the beam surfaces in both actuation units. The maximum stress is adjusted by the degree of prestrain. In open gripper condition, actuation unit II is in the austenitic state and unit I is in R-phase condition. From Fig. 5a, a maximum stress in actuation unit II of about 90 MPa is determined. A closed gripper condition is reached, if actuation unit I is in the austenitic state and unit II is in R-phase condition. In this case, a maximum stress of about 60 MPa is determined in unit II as shown in Fig. 5b. Since the maximum stress is kept in all cases below 100 MPa, fatigue effects play a minor role and, thus, a large number of reversible actuation cycles is possible w8x.
7. Results and discussion 7.1. Stationary performance Fig. 7 shows typical displacement and electrical resistance characteristics of the actuation unit I. The resistance
6. Fabrication Since the microgrippers consist of a single piece of material, which combines all functional units, the fabrication procedure is reduced to one micromachining step and subsequent bonding onto a substrate. Micromachining of SMA grippers with optimized design was performed by laser cutting of a cold-rolled TiNi sheet of 100 mm in thickness. After the microfabrication step, the microgrippers were prestrained and then mounted on a ceramic substrate by adhesive bonding. Fig. 6 shows a microgripper of 100 mm in thickness. The lateral size is 2 = 3.9 mm2 . The minimum beam width is 0.05 mm.
Fig. 7. Resistance and displacement characteristics of actuation unit I for three different loads.
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Fig. 8. Displacement of the gripping jaws.
characteristics show a narrow hysteresis for all loads reflecting R-phase transformation. In contrast, a pronounced hysteresis of the corresponding displacement characteristics S is observed, which tends to increase with increasing load. The hysteresis broadening is due to stress-induced martensite ŽSIM., which is formed locally in the regions of maximum stress along the beam surfaces. These regions mainly determine the observed displacements. However, the resistance characteristics mainly reflect the average resistance of the whole SMA device rather than local resistance changes. Thus, they are almost unaffected by SIM. The maximum displacements increase with increasing load. For a load of 42 mN, a maximum displacement of 28 mm is observed in austenitic condition above 25 mW, which corresponds to a spring constant of unit I of 1500 Nrm. At zero electrical power, unit I is much softer due to shape accommodation in the R-phase and SIM condition. For a load of 42 mN, the maximum displacement is 110 mm. The corresponding maximum strain has been calculated by FEM simulation to 0.6%. From the observed displacements, the maximum stroke of unit I is determined to about 80 mm. Typical displacement characteristics of the gripping jaws attached to the actuation unit II S2 are shown in Fig. 8 for the same loads as in Fig. 7. Again, the hysteresis is broadened due to SIM formation. Zero displacements S2 correspond to completely closed gripping jaws. This condition is reached at zero electrical power for external loads above about 40 mN. For 40 mN, the stroke of the gripping jaws D S2 reaches an optimum value of about 180 mm. The corresponding gear ratio SrDS2 is 2.3. At smaller loads, the gripping jaws cannot be closed completely. At higher loads, the maximum displacement S2 decreases as a function of the load. In the microgripper, a load of 40 mN is realized by prestraining actuation unit I by about 100 mm. In this case, the gripping force is estimated to 17 mN.
Fig. 9. Response times in actuation unit II. The inset shows a time-resolved electrical resistance measurement for an electrical power Pel of 17 mW.
tance measurements. Upon heating, the electrical resistance decreases and finally reaches a minimum, which coincides with the end position of the corresponding displacement, as can be seen in Fig. 7. Fig. 9 shows results of the response times determined from the minima of time-resolved electrical resistance curves as shown in the inset. With increasing electrical power Pel , the response times decrease inversely proportional to Pel as expected for adiabatic heating processes. For an electrical power of 22 mW, the response time decreases to 32 ms. The cooling times are considerably longer, in the order of 300 ms for 22 mW. However, the cooling performance has no influence on the response times. Due to the used antagonism, the response times for opening and closing are determined by the heating performance of the corresponding actuation unit. The cooling performance determines the maximum frequency of complete actuation cycles. 7.3. Position control Due to the rather broad hysteresis width of the displacements, open loop control of the actuation units only gives rise to a positioning accuracy of about 25 mm. In the present case, the hysteresis in displacement cannot be compensated by the corresponding electrical resistance
7.2. Dynamic performance The response times of the actuation units upon heating have been investigated by time-resolved electrical resis-
Fig. 10. Optical transmission scheme for closed-loop position control.
M. Kohl et al.r Sensors and Actuators 83 (2000) 208–213
signal, as has been demonstrated for the case of straight wires of certain TiNi alloys w9x. Therefore, a setup for closed loop control has been developed, which makes use of an optical transmission scheme as shown schematically in Fig. 10. Between the actuation units, a slit is integrated, which moves into the transmission path between a light emission diode and an optical detector, if the actuation unit I is heated. The detected change in optical transmission is used to control the motion of the actuation units. The transmission signals have been calibrated with respect to the actuator displacements and used as a reference. In this way, a maximum spatial resolution of 1 mm is obtained for a signal change of 0.33%. By this setup, preliminary tests show a positioning accuracy of better than 3 mm.
8. Conclusions and outlook A SMA microgripper was developed, which consists of a single microdevice with integrated actuation units for antagonistic control of integrated gripping jaws. The stress-optimized design of the actuation units allows optimum use of the shape memory effect and minimization of fatigue. The presented concept provides a cheap and compact solution for microrobotic tasks with high demands on size and work output. Possible applications are the handling of microparts of complex shape in cleanroom or vacuum environments, where adhesive or vacuum gripping is not suitable, or in restricted environments with difficult access. For a microgripper prototype of TiNi with 2 = 3.9 = 0.1 mm3 in size, the gripping jaws allow a maximum stroke of 180 mm and a maximum gripping force of 17 mN. Due to the used antagonistic mechanism, opening and closing motion of the gripper is determined by the heating performance of the actuation units. A typical response time is 32 ms for 22 mW electrical driving power. Future developments will concentrate on the integration of the optical transmission setup for position control and on aspects of inexpensive and repeatable mass-production. Further improvements of materials will be necessary to allow a broad use of the microgripper, in particular, the increase of transformation temperatures by development of SMA sheets made of a suitable ternary alloy such as TiNiPd.
Acknowledgements The authors would like to thank H. Besser for laser cutting.
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References w1x K. Kuribayashi, Microrobot using reversible SMA actuator, Journal of Robotics and Mechatronics 3 Ž1. Ž1990. 52–56. w2x A.P. Lee, D.R. Ciarlo, P.A. Krulevitch, S. Lehew, J. Trevino, M.A. Northrup, A practical microgripper by fine alignment, eutectic bonding and SMA actuation, Sensors and Actuators, A 54 Ž1996. 755–759. w3x M. Kohl, E. Quandt, A. Schußler, R. Trapp, D.M. Allen, Characteri¨ zation of NiTi shape memory microdevices produced by microstructuring of etched sheets or sputter deposited film, in: H. Borgmann, K. Lenz ŽEds.., Proceedings of Actuator 94, Bremen, Germany, 1994, pp. 317–320. w4x M. Kohl, K.D. Skrobanek, Linear actuators based on the shape memory effect, Sensors and Actuators, A 70 Ž1998. 104–111. w5x Y. Bellouard, R. Clavel, R. Gotthardt, T. Sidler, J.E. Bideaux, A new concept of monolithic shape memory alloy microdevices used in microrobotics, in: H. Borgmann ŽEd.., Proceedings of Actuator 98, Bremen, Germany, 1998, pp. 499–502. w6x S. Miyazaki, K. Otsuka, Deformation and transition behavior associated with the R-phase in TiNi alloys, Metallurgical Transactions A 17A Ž1986. 53–63. w7x C. Mattheck, S. Burkhardt, A new method of structural shape optimization based on biological growth, International Journal of Fatigue 12 Ž1990. 185–190. w8x J.V. Humbeeck, D. Reynaerts, R. Stalmans, Shape memory alloys: functional and smart, in: H. Borgmann, K. Lenz ŽEds.., Proceedings of Actuator 94, Bremen, Germany, 1994, pp. 312–316. w9x J. Hesselbach, R. Pittschellis, H. Stork, E. Hornbogen, M. Mertmann, Optimization and control of electrically heated shape memory actuators, in: H. Borgmann, K. Lenz ŽEds.., Proceedings of Actuator 94, Bremen, Germany, 1994, pp. 337–340.
Biographies M. Kohl received his PhD in Physics in 1989 from the University of Stuttgart. In the time-period 1990–1991 he was working as an IBM post-doctoral fellow at the T.J. Watson Research Center in Yorktown Heights ŽUSA. and joined later on the Forschungszentrum Karlsruhe in Germany. Since 1993, he is head of a research group at the Institut fur ¨ Mikrostrukturtechnik working on the development of microactuators. E. Just received his diploma in Mechanical Engineering from the University of Karlsruhe in 1997 with an emphasis on fluidic microcomponents for medical applications. He is now working on his thesis at the Institut fur ¨ Mikrostukturtechnik of the Forschungszentrum Karlsruhe in the field of design, fabrication, and characterization of shape memory microgrippers. W. Pfleging received his PhD in 1997 from the Institut fur ¨ Lasertechnik at the Frauenhofer Gesellschaft in Aachen. He is at the Forschungszentrum Karlsruhe since 1997 as head of a research group at the Institut fur ¨ Materialforschung I working on laser technologies. S. Miyazaki received the PhD in 1979 from the Department of Materials Science and Engineering, Osaka University, Osaka, Japan. Since then, he has been working on a variety of topics relating the materials science and applications of shape memory alloys, and is now a Professor at the University of Tsukuba. Recently, he has been developing sputter-deposited TiNi-based shape memory alloy thin films and their applications for microactuators.
SMA microgripper with integrated antagonism M. Kohl a,) , E. Just a , W. Pfleging a , S. Miyazaki b b
a Forschungszentrum Karlsruhe, IMT, Postfach 3640, 76021 Karlsruhe, Germany Institute of Materials Science, UniÕersity of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Accepted 17 December 1999
Abstract A microgripper of 2 = 3.9 = 0.1 mm3 size is presented consisting of a single device microfabricated from a shape memory alloy ŽSMA. thin sheet. The device consists of two integrated actuation units of a stress-optimized shape, which actuate in opposite directions and thus form an antagonistic pair. The fabrication procedure is reduced to one micromachining step of a rolled SMA sheet and subsequent bonding onto a substrate. The maximum displacement of the gripping jaws is 180 mm, the maximum gripping force 17 mN. For an electrical power of 22 mW, a response time of 32 ms is observed. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Microgripper; Shape memory actuator; Monolithic microdevice; Smart materials; TiNi cold-rolled sheets
1. Introduction For microrobotic applications, compact actuator designs with high work outputs are required. Attractive solutions have been achieved by using shape memory alloys ŽSMAs. w1,2x. Besides high work outputs, further important features of SMA microdevices are compatibility to microelectronics, a high reliability due to an intrinsic actuation mechanism and a frictionless actuation. Recently, a fabrication concept based on the lateral micromachining of SMA sheets or films has been developed w3x, which allows integration of several functional units in a single SMA piece and thus, a high flexibility in device design. This concept enables the realization of microactuators with complex functionality and a large degree of miniaturization w4,5x. The paper presents a microgripper of TiNi consisting of two integrated actuation units with opposite moving directions. This allows separate control of the opening and closing motion of integrated gripping jaws in an antagonistic way. After presentation of the design of the
)
Corresponding author. E-mail address: [email protected] ŽM. Kohl..
actuation units, experimental details and actuation properties of the used SMA materials are discussed. Design simulation and optimization have been performed by finite element calculations. Later on, the mechanical and electrical properties of the microgrippers are presented and solutions for positioning of the gripping jaws are discussed.
2. Gripper design Fig. 1 shows a scheme of the SMA microgripper in open and closed condition. The gripper mainly consists of two actuation units with bond pads for mechanical and electrical connection, a link between both units and two gripping jaws. Actuation unit I comprises a folded-beam structure and unit II comprises two circular beams. Due to a heat treatment, the memory shape of the units corresponds to the undeflected beam shape. The gripper is mounted on a substrate in prestrained condition. Thus, deformation is created in the beam structures, which can be controlled by electrical heating. By selective heating of actuation unit I above the phase transformation temperature, the folded beams recover their memory shape, which leads to a linear motion of the link w4x. Consequently, the circular beams are deformed and the gripping jaws are closed. This condition can be reset by selective heating of
0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 3 8 5 - 4
M. Kohl et al.r Sensors and Actuators 83 (2000) 208–213
209
electrical resistance of the double-beam devices has been determined by the four-point method. The heat-treated sheets of TiNi are the basic material for realization of the microgripper. For a detailed understanding of the mechanical and electrical properties of the gripper, the actuation units were fabricated and investigated separately. Force-displacement characteristics were determined by mounting the actuation units in vertical orientation and attaching calibrated microweights close to the link as sketched in Fig. 2. Heating of the actuation units was performed directly by applying an electrical current.
Fig. 1. Operation principle of the SMA microgripper.
the circular beams. The link between the actuation units is designed to provide sufficient thermal isolation.
3. Experimental Cold-rolled sheets of an equiatomic TiNi alloy have been heat treated in vacuum at 5308C for 10 min to adjust the one-way shape memory effect. Phase transformation temperatures were determined by differential scanning calorimetry ŽDSC. and electrical resistance measurements. For characterization of the mechanical material properties, double-beam test devices were fabricated by laser cutting, which were investigated by beam-bending experiments as a function of temperature and load. The test devices were loaded by a microweight at the beam end and the corresponding vertical displacements of the beam ends were determined optically by a video microscope. The experiments were performed in a thermostat, which allowed adjustment of the temperature by ambient heating or cooling. Stationary equilibrium conditions have been maintained by cycling the temperature stepwise with sufficient waiting time in every data point. Simultaneously, the
Fig. 2. Scheme for measurement of the mechanical and electrical characteristics of actuation units I Ža. and II Žb..
4. Material properties The transformation behavior of the SMA material has been investigated by DSC, electrical resistance, and mechanical bending experiments of double-beam test devices. Fig. 3 shows a DSC measurement of the cold-rolled TiNi sheet performed at the heat flow of 10 Krs. The temperatures of the austenitic, martensitic and rhombohedral ŽR. phase transformations have been determined to A s s 568C, A f s 668C, Ms s 208C, Mf s 128C, R s s 448C, and R f s 408C. The indices s and f denote start and finish temperatures, respectively. Since the martensitic transformation occurs below room temperature, only the intermediate R-phase transformation can be induced by electrical heating. This material behavior is of special interest, since R-phase transformations exhibit a narrow hysteresis width of about 1 K and negligible fatigue effects w6x. Fig. 4 shows temperature displacement and electrical resistance characteristics of a double-beam test device for two different loads of 16 and 42 mN in the transformation temperature regime. Upon cooling, the R-phase transformation and martensitic phase transformation are observed. Upon heating, the martensite directly transforms to the
Fig. 3. Differential scanning calorimetry measurement of a cold-rolled TiNi sheet. The transformation temperatures of martensitic, R-, and austenitic phase are indicated. The indices s, p and f denote start, peak and finish temperatures.
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lateral width profile in open and closed condition. White regions indicate zero stress; maximum stress values are
Fig. 4. Beam-bending and electrical resistance measurements of a double-beam test device for two different loads. The start transformation temperatures of martensitic, R-, and austenitic phase are indicated.
austenitic parent phase. Compared to the DSC measurements, the phase transformation temperatures are slightly shifted to higher values due to load-induced stress. At room temperature Ž238C., a full R-phase transformation is observed upon cooling, while the martensitic phase transformation just starts at Ms .
5. Design simulation and optimization In order to avoid local stress concentrations in the actuation units upon loading, the lateral widths of the actuation units have been optimized. Stress and strain profiles were calculated by finite element simulations ŽFEM.. The required material parameters on stress and strain were determined before from beam-bending experiments. Based on the FEM model, the geometry of the actuation units was optimized to obtain a homogeneous stress profile. In this way, fatigue effects due to stress peaks are avoided and a maximum fraction of material contributes to the shape memory effect w4x. The maximum stress has to be sufficiently high to obtain high work outputs. However, it should not exceed a material-dependent critical limit, above which plastic deformation occurs. Stress optimization has been performed by using the computer aided optimization ŽCAO. method w7x, which is based on iterative FEM. Fig. 5 shows calculated von Mises stress profiles in actuation units of TiNi with optimized
Fig. 5. Von Mises stress profiles in open Ža. and closed Žb. condition. Stress intensities are indicated by different gray levels between black Žmaximum stress. and white Žno stress..
M. Kohl et al.r Sensors and Actuators 83 (2000) 208–213
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Fig. 6. SMA microgripper.
indicated in black. Homogeneous stress profiles occur along the beam surfaces in both actuation units. The maximum stress is adjusted by the degree of prestrain. In open gripper condition, actuation unit II is in the austenitic state and unit I is in R-phase condition. From Fig. 5a, a maximum stress in actuation unit II of about 90 MPa is determined. A closed gripper condition is reached, if actuation unit I is in the austenitic state and unit II is in R-phase condition. In this case, a maximum stress of about 60 MPa is determined in unit II as shown in Fig. 5b. Since the maximum stress is kept in all cases below 100 MPa, fatigue effects play a minor role and, thus, a large number of reversible actuation cycles is possible w8x.
7. Results and discussion 7.1. Stationary performance Fig. 7 shows typical displacement and electrical resistance characteristics of the actuation unit I. The resistance
6. Fabrication Since the microgrippers consist of a single piece of material, which combines all functional units, the fabrication procedure is reduced to one micromachining step and subsequent bonding onto a substrate. Micromachining of SMA grippers with optimized design was performed by laser cutting of a cold-rolled TiNi sheet of 100 mm in thickness. After the microfabrication step, the microgrippers were prestrained and then mounted on a ceramic substrate by adhesive bonding. Fig. 6 shows a microgripper of 100 mm in thickness. The lateral size is 2 = 3.9 mm2 . The minimum beam width is 0.05 mm.
Fig. 7. Resistance and displacement characteristics of actuation unit I for three different loads.
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Fig. 8. Displacement of the gripping jaws.
characteristics show a narrow hysteresis for all loads reflecting R-phase transformation. In contrast, a pronounced hysteresis of the corresponding displacement characteristics S is observed, which tends to increase with increasing load. The hysteresis broadening is due to stress-induced martensite ŽSIM., which is formed locally in the regions of maximum stress along the beam surfaces. These regions mainly determine the observed displacements. However, the resistance characteristics mainly reflect the average resistance of the whole SMA device rather than local resistance changes. Thus, they are almost unaffected by SIM. The maximum displacements increase with increasing load. For a load of 42 mN, a maximum displacement of 28 mm is observed in austenitic condition above 25 mW, which corresponds to a spring constant of unit I of 1500 Nrm. At zero electrical power, unit I is much softer due to shape accommodation in the R-phase and SIM condition. For a load of 42 mN, the maximum displacement is 110 mm. The corresponding maximum strain has been calculated by FEM simulation to 0.6%. From the observed displacements, the maximum stroke of unit I is determined to about 80 mm. Typical displacement characteristics of the gripping jaws attached to the actuation unit II S2 are shown in Fig. 8 for the same loads as in Fig. 7. Again, the hysteresis is broadened due to SIM formation. Zero displacements S2 correspond to completely closed gripping jaws. This condition is reached at zero electrical power for external loads above about 40 mN. For 40 mN, the stroke of the gripping jaws D S2 reaches an optimum value of about 180 mm. The corresponding gear ratio SrDS2 is 2.3. At smaller loads, the gripping jaws cannot be closed completely. At higher loads, the maximum displacement S2 decreases as a function of the load. In the microgripper, a load of 40 mN is realized by prestraining actuation unit I by about 100 mm. In this case, the gripping force is estimated to 17 mN.
Fig. 9. Response times in actuation unit II. The inset shows a time-resolved electrical resistance measurement for an electrical power Pel of 17 mW.
tance measurements. Upon heating, the electrical resistance decreases and finally reaches a minimum, which coincides with the end position of the corresponding displacement, as can be seen in Fig. 7. Fig. 9 shows results of the response times determined from the minima of time-resolved electrical resistance curves as shown in the inset. With increasing electrical power Pel , the response times decrease inversely proportional to Pel as expected for adiabatic heating processes. For an electrical power of 22 mW, the response time decreases to 32 ms. The cooling times are considerably longer, in the order of 300 ms for 22 mW. However, the cooling performance has no influence on the response times. Due to the used antagonism, the response times for opening and closing are determined by the heating performance of the corresponding actuation unit. The cooling performance determines the maximum frequency of complete actuation cycles. 7.3. Position control Due to the rather broad hysteresis width of the displacements, open loop control of the actuation units only gives rise to a positioning accuracy of about 25 mm. In the present case, the hysteresis in displacement cannot be compensated by the corresponding electrical resistance
7.2. Dynamic performance The response times of the actuation units upon heating have been investigated by time-resolved electrical resis-
Fig. 10. Optical transmission scheme for closed-loop position control.
M. Kohl et al.r Sensors and Actuators 83 (2000) 208–213
signal, as has been demonstrated for the case of straight wires of certain TiNi alloys w9x. Therefore, a setup for closed loop control has been developed, which makes use of an optical transmission scheme as shown schematically in Fig. 10. Between the actuation units, a slit is integrated, which moves into the transmission path between a light emission diode and an optical detector, if the actuation unit I is heated. The detected change in optical transmission is used to control the motion of the actuation units. The transmission signals have been calibrated with respect to the actuator displacements and used as a reference. In this way, a maximum spatial resolution of 1 mm is obtained for a signal change of 0.33%. By this setup, preliminary tests show a positioning accuracy of better than 3 mm.
8. Conclusions and outlook A SMA microgripper was developed, which consists of a single microdevice with integrated actuation units for antagonistic control of integrated gripping jaws. The stress-optimized design of the actuation units allows optimum use of the shape memory effect and minimization of fatigue. The presented concept provides a cheap and compact solution for microrobotic tasks with high demands on size and work output. Possible applications are the handling of microparts of complex shape in cleanroom or vacuum environments, where adhesive or vacuum gripping is not suitable, or in restricted environments with difficult access. For a microgripper prototype of TiNi with 2 = 3.9 = 0.1 mm3 in size, the gripping jaws allow a maximum stroke of 180 mm and a maximum gripping force of 17 mN. Due to the used antagonistic mechanism, opening and closing motion of the gripper is determined by the heating performance of the actuation units. A typical response time is 32 ms for 22 mW electrical driving power. Future developments will concentrate on the integration of the optical transmission setup for position control and on aspects of inexpensive and repeatable mass-production. Further improvements of materials will be necessary to allow a broad use of the microgripper, in particular, the increase of transformation temperatures by development of SMA sheets made of a suitable ternary alloy such as TiNiPd.
Acknowledgements The authors would like to thank H. Besser for laser cutting.
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References w1x K. Kuribayashi, Microrobot using reversible SMA actuator, Journal of Robotics and Mechatronics 3 Ž1. Ž1990. 52–56. w2x A.P. Lee, D.R. Ciarlo, P.A. Krulevitch, S. Lehew, J. Trevino, M.A. Northrup, A practical microgripper by fine alignment, eutectic bonding and SMA actuation, Sensors and Actuators, A 54 Ž1996. 755–759. w3x M. Kohl, E. Quandt, A. Schußler, R. Trapp, D.M. Allen, Characteri¨ zation of NiTi shape memory microdevices produced by microstructuring of etched sheets or sputter deposited film, in: H. Borgmann, K. Lenz ŽEds.., Proceedings of Actuator 94, Bremen, Germany, 1994, pp. 317–320. w4x M. Kohl, K.D. Skrobanek, Linear actuators based on the shape memory effect, Sensors and Actuators, A 70 Ž1998. 104–111. w5x Y. Bellouard, R. Clavel, R. Gotthardt, T. Sidler, J.E. Bideaux, A new concept of monolithic shape memory alloy microdevices used in microrobotics, in: H. Borgmann ŽEd.., Proceedings of Actuator 98, Bremen, Germany, 1998, pp. 499–502. w6x S. Miyazaki, K. Otsuka, Deformation and transition behavior associated with the R-phase in TiNi alloys, Metallurgical Transactions A 17A Ž1986. 53–63. w7x C. Mattheck, S. Burkhardt, A new method of structural shape optimization based on biological growth, International Journal of Fatigue 12 Ž1990. 185–190. w8x J.V. Humbeeck, D. Reynaerts, R. Stalmans, Shape memory alloys: functional and smart, in: H. Borgmann, K. Lenz ŽEds.., Proceedings of Actuator 94, Bremen, Germany, 1994, pp. 312–316. w9x J. Hesselbach, R. Pittschellis, H. Stork, E. Hornbogen, M. Mertmann, Optimization and control of electrically heated shape memory actuators, in: H. Borgmann, K. Lenz ŽEds.., Proceedings of Actuator 94, Bremen, Germany, 1994, pp. 337–340.
Biographies M. Kohl received his PhD in Physics in 1989 from the University of Stuttgart. In the time-period 1990–1991 he was working as an IBM post-doctoral fellow at the T.J. Watson Research Center in Yorktown Heights ŽUSA. and joined later on the Forschungszentrum Karlsruhe in Germany. Since 1993, he is head of a research group at the Institut fur ¨ Mikrostrukturtechnik working on the development of microactuators. E. Just received his diploma in Mechanical Engineering from the University of Karlsruhe in 1997 with an emphasis on fluidic microcomponents for medical applications. He is now working on his thesis at the Institut fur ¨ Mikrostukturtechnik of the Forschungszentrum Karlsruhe in the field of design, fabrication, and characterization of shape memory microgrippers. W. Pfleging received his PhD in 1997 from the Institut fur ¨ Lasertechnik at the Frauenhofer Gesellschaft in Aachen. He is at the Forschungszentrum Karlsruhe since 1997 as head of a research group at the Institut fur ¨ Materialforschung I working on laser technologies. S. Miyazaki received the PhD in 1979 from the Department of Materials Science and Engineering, Osaka University, Osaka, Japan. Since then, he has been working on a variety of topics relating the materials science and applications of shape memory alloys, and is now a Professor at the University of Tsukuba. Recently, he has been developing sputter-deposited TiNi-based shape memory alloy thin films and their applications for microactuators.