- Email: [email protected]
SMA microgripper system
Sensors and Actuators A 97±98 (2002) 646±652
SMA microgripper system M. Kohla,*, B. Kreveta, E. Justb a
b
Forschungszentrum Karlsruhe GmbH, IMT, Postfach 3640, 76021 Karlsruhe, Germany Albert-Ludwigs-UniversitaÈt Freiburg, IMTEK, Georges-KoÈhler-Allee 103, 79110 Freiburg, Germany Received 12 June 2001; received in revised form 4 October 2001; accepted 9 October 2001
Abstract A microgripper system is presented, which consists of a monolithic shape memory alloy (SMA) device of 2 mm 5:8 mm 0:23 mm size and an integrated optical position sensor. Gripper closing and opening is performed by two integrated actuators, which form an antagonistic pair. Investigations of temperature pro®les by coupled ®nite-element simulations and infrared microscopy demonstrate a suf®cient thermal insulation of the actuators for their selective control. The motion of gripping jaws is transmitted by an integrated gearing mechanism into a linear motion of an integrated optical slit, which is detected by change of optical transmission. The maximum stroke and force of the gripping jaws are 300 mm and 35 mN, respectively. In the range between 10 and 90% of the maximum stroke, positioning is achieved within 140 ms with an accuracy of about 2 mm. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microgripper system; Shape memory actuator; Monolithic microdevice; Position sensor
1. Introduction
2. Gripper design
For manipulation of microobjects during fabrication and assembly, microgrippers are demanded, which are compact in size, but still generate large work outputs [1±6]. Mechanical grippers based on two or more ®ngers provide the highest ¯exibility with respect to shape and material of the gripping object. Attractive solutions have been achieved by using shape memory alloy (SMA) microactuators [4,6,7]. Recently, a mechanical SMA microgripper with integrated antagonism has been developed [6], which consists of a monolithic SMA gripping device with two integrated actuators with opposite actuation directions. This allows on/ off operation between open and closed position, which is suitable for manipulation of objects with simple shape and suf®cient rigidity. In the case of complex object geometries and sensitive object materials, however, active control of gripping motion and force is advantageous. This paper presents an extended design of the SMA microgripper with integrated actuation and position sensing. After discussion of the gripper design, a simulation model for determination of the thermal properties of the gripping device is presented. Then, the multifunctional properties of the gripping device and the positioning performance of the microgripper system will be presented.
Mechanical grippers usually consist of an actuator and a gearing mechanism, which is connected to gripping jaws. By the gearing mechanism, the motion of the actuator is transferred to a gripping motion. Fig. 1a shows a scheme of a mechanical gripper, which comprises a linear actuator and a gearing mechanism with ®ve joints. This scheme is implemented in a completely monolithic SMA gripping device. The structure and operation principle of the gripping device are shown in Figs. 1b and c for open and closed condition, respectively. For linear actuation a SMA actuator is used, which consists of a folded-beam structurewith stress-optimized geometry [8]. The joints of the gearing mechanism are realized as ¯exible hinges. Since the gearing mechanism consists of SMA material, it also works as an actuator. The design of the actuators allows frictionless motion and easy miniaturization. Both, linear and gear actuator are prestrained with respect to each other before mounting on a substrate. The resulting deformation of the beams and hinges can be controlled by electrical heating. By selective heating of the gear actuator above the phase transformation temperature, the hinges recover their undeformed shape due to the one-way shape memory effect. In this case, only the linear actuator is deformed and the gripping jaws are opened. Selective heating of the linear actuator causes deformation of the hinges and, thus, restores the closed condition. Details on the stressoptimized design of the gear actuator can be found in [6].
*
Corresponding author. Tel.: 49-7247-822798; fax: 49-7247-824331. E-mail address: [email protected] (M. Kohl).
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 8 0 3 - 2
M. Kohl et al. / Sensors and Actuators A 97±98 (2002) 646±652
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Fig. 1. Mechanical gripper: (a) schematic of the gripping device; (b and c) monolithic SMA gripping device in open and closed condition, respectively.
Between the actuators, an optical slit is integrated, which performes the same motion as the linear actuator. Due to a ®xed gear-ratio, the position of the slit S1 directly re¯ects the separation of the gripping jaws S2. Thus, control of the slit position allows positioning of the gripping jaws. This concept results in a compact microgripper system, which is sketched in Fig. 2. The SMA gripping device is mounted between two substrates with integrated light emitting diode (LED) and photosensor, which are aligned with respect to the optical slit. In this set-up, the light intensity transmitted through the optical slit yields the actual slit position.
generated in the gripping device, which varies in space and time. Calculation of the temperature-distribution involves an electrical simulation step for determination of the distribution of current density and of electrical heating power and a thermal simulation step. Both simulations have been performed using the ®nite-element method (FEM). As the electrical and thermal properties of the gripping device depend on each other, a special FEM coupling routine has been used [9]. The electrical potential U is calculated from the differential equation
3. Simulation
~ fj0 g s rU
The performance of the gripping device is determined by the heating and cooling processes during actuation. By application of an electrical current, a temperature-distribution is
The parameter s denotes the electrical conductivity. In the electrical FEM simulation step, the non-linear change and hysteresis of the electrical conductivity in the phase transformation regime is neglected, which gives rise to an estimated relative error of about 10%. From the current density {j} the heating power Q_ is calculated by volume integration of the product sfjgT fjg. The heat transfer within the gripping device is determined by the differential equation
s DU 0
(1)
and the Neumann boundary condition at the electrical connections (2)
m cp h TT_ Q_
l DT
(3)
which takes into account the sensible heat, the heat of phase transformation h(T), the heating power generated by electrical current Q_ and losses due to heat conduction. The parameters m, cp and l denote the mass, speci®c heat capacity and thermal conductivity, respectively. At the device surface, the following boundary condition is considered: ~ K T l rT
Fig. 2. Schematic of the SMA microgripper system.
TE
(4)
which takes into account the convective heat exchange with the environment of temperature TE. The parameter K denotes the heat transfer coef®cient. Eqs. (3) and (4) are solved by a thermal FEM simulation step.
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M. Kohl et al. / Sensors and Actuators A 97±98 (2002) 646±652
Table 1 Simulation parameters for determination of the temperature-distribution Thermal conductivity l (W m 1 K 1) Electrical conductivity s (O 1 m 1) Specific heat cp (J kg 1 K 1) Mass density (kg m 3) Thermal expansion coefficient a (K 1) Heat-transfer coefficient K (W m 2 K 1) Integral latent heat (J kg 1) Environment temperature TE (8C)
18 1.2 106 450 6.5 103 10 5 70 4500 25
The heat-transfer coef®cient K has been determined empirically for different cases of free convection without external air ¯ow and forced convection by ®tting simulated temperature pro®les of a test device to experimentally determined pro®les from infrared microscopy. The used simulation parameters are summarized in Table 1. Within the temperature range of 25±60 8C, they have been treated as constants.
is 0.05 mm. Before integration on a substrate, the SMA gripping device is prestrained to adjust the maximum stress and strain level for actuation. Below the optical slit, an opening of 1:8 mm 0:4 mm is cut into the substrate by a laser, which serves as an aperture for an LED attached to the substrate from the bottom side. Above the optical slit, a photosensor with sensing area of 2 mm 2 mm is mounted. Integration of the SMA gripping device and the optical components is performed by adhesive bonding. Electrical contact are made by wedge±wedge bonding. The overall size of the microgripper system including the housing is 2:5 mm 8 mm 9 mm. Fig. 3 shows the microgripper system holding a piece of optical ®ber. In this case, a constant gripping force is maintained by electrical heating of the linear actuator. The upper substrate containing the photosensor has been removed to allow a complete view on the SMA gripping device. 5. Experimental
4. Fabrication Cold-rolled sheets of 230 mm thickness of an equiatomic TiNi alloy are heat-treated in vacuum at 530 8C for 10 min to adjust the one-way shape memory effect. The gripping devices are micromachined by laser cutting. The lateral size of the devices is 2 mm 5:8 mm. The minimum beam width
Phase transformation temperatures of the SMA sheets have been investigated by differential scanning calorimetry and electrical resistance measurements. The temperatures of the rhombohedral (R) and corresponding reverse phase transformation to austenite (A) have been determined to Rs 44 8C, Rf 40 8C, As 41 8C and Af 45 8C. The
Fig. 3. SMA microgripper system holding a piece of optical fiber of 140 mm diameter.
M. Kohl et al. / Sensors and Actuators A 97±98 (2002) 646±652
indices s and f denote start and ®nish temperatures, respectively. The martensitic transformation occurs below room temperature. Therefore, only the R-phase transformation is used for actuation, which is of special interest, since R-phase transformations exhibit a narrow hysteresis width of about 1 K and negligible fatigue effects [10]. The temperature-distribution along the surface of the gripping device has been investigated as a function of electrical heating power by infrared microscopy. In order to obtain a homogeneous coef®cient of thermal emission equal to 1, device and substrate have been coated by a thin layer of graphite. The maximum spatial resolution has been 25 mm and the maximum time resolution 100 ms. Force±displacement characteristics of individual actuators have been determined in vertical orientation. The actuators have been loaded by calibrated weights and the corresponding vertical displacements have been determined optically by a video microscope. The experiments have been performed in a thermostat, which allows adjustment of the temperature by ambient heating or cooling. Stationary equilibrium conditions have been maintained by cycling the temperature step-wise with suf®cient waiting time in every data point. Simultaneously, the electrical resistance has been determined by the four-point method. 6. SMA gripping device 6.1. Thermal properties Simulated temperature pro®les in the gripping device are shown in Fig. 4 for a heating power of 100 mW after stationary equilibrium is reached. Due to symmetry, only
649
one half of the device has been taken into account. Electrical heating of the gear actuator (Fig. 4a) causes transformation of the ¯exible hinges to the austenitic phase except small transition regions close to the electrical bond pads. The maximum temperature achieved in open condition is determined to about 53 8C. Due to heat conduction, also the gripping jaws and the optical slit are heated to about 50 and 40 8C, respectively. The linear actuator, however, remains below the start temperature of austenitic phase transformation As. By electrically heating of the linear actuator (Fig. 4b), the folded-beam structure transforms to austenite, while the gear actuator remains in R-phase condition. The simulations demonstrate, that the actuators will be suf®ciently thermally insulated by the optical slit as long as the heating power is kept below about 100 mW. It is important to note, that the gripping jaws remain almost at room temperature in closed condition and, thus, do not thermally affect the gripping object. Fig. 5 shows temperature pro®les along the surface of the gripping device determined by infrared microscopy. After heating the gear actuator with an electrical heating power of 80 mW for 150 ms (Fig. 5a), the gear actuator displays a maximum temperature of about 50 8C, while the linear actuator remains below 33 8C. Thus, the shape memory effect is selectively induced in the gear actuator. Fig. 5b shows the temperature-distribution after a cooling period of 150 ms and subsequent heating the linear actuator with an electrical heating power of 80 mW for 150 ms. In this case, a maximum temperature of about 49 8C is reached in the linear actuator, while the gear actuator is cooled to temperatures below about 37 8C. Consequently, only the linear actuator displays the shape memory effect. The measured
Fig. 4. Simulated temperature profiles in the gripping device in open (a) and closed (b) condition in stationary equilibrium. The heating power is 100 mW.
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Fig. 5. Temperature profiles along the surface of the gripping device in open (a) and closed (b) condition determined by infrared microscopy. The heating time and power are 150 ms and 80 mW, respectively. Numerical values are indicated for five different locations on the gripping device.
temperature-distributions agree very well with the simulations. The results demonstrate, that the actuators are suf®ciently thermally separated by the optical slit. Therefore, it is possible to control the two actuators of the gripping device selectively in an antagonistic way. Due to the limited time resolution, the infrared experiments only allow a rough estimate of the cooling time, which the average temperature requires to reach the Rf-temperature. This time is about 200 ms for the gear actuator and about 300 ms for the linear actuator. The heating times of the actuators, given by the time interval of the average temperature to reach the Af-temperature, have been determined by time-resolved electrical resistance measurements. For a heating power of 80 mW, this time is about 40 ms for the gear actuator and about 50 ms for the linear actuator. Similar experiments on gripping devices of 100 mm thickness revealed typical heating times in the order of 30±35 ms for 22 mW heating power [6].
force of 88 mN, the stroke of the gripping jaws S2 reaches a maximum value of about 300 mm. The corresponding vertical displacement of the weight S1 is about 130 mm, which corresponds to a gear-ratio S2/S1 of 2.3. The maximum gripping force depends on the degree of prestrain before mounting the gripper. By prestraining the gear actuator by about 130 mm, the gripping force is estimated to 35 mN. Despite the use of a R-phase transformation, a narrow hysteresis width is only present in the electrical resistance characteristic for a heating power above 15 mW. The decrease of resistance at small heating power indicates the onset of a thermally-induced martensitic phase
6.2. Mechanical properties For a detailed understanding of the mechanical and electrical properties of the SMA gripping device, linear and gear actuator have been fabricated and characterized separately. Fig. 6 shows typical displacement and electrical resistance characteristics of the gear actuator. Zero displacements S2 correspond to completely closed gripping jaws. This condition is reached at zero electrical power for external loads above 80 mN. At smaller loads, the gripping jaws cannot be closed completely. The displacement of the gripping jaws increases for increasing electrical power. For a
Fig. 6. Displacement and electrical resistance characteristic of the gear actuator for a load of 88 mN. The inset illustrates the measurement set-up.
M. Kohl et al. / Sensors and Actuators A 97±98 (2002) 646±652
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Fig. 7. Calibration characteristic of the optical transmission signal.
transformation, which, however, has no effect on the mechanical performance of the gear actuator. The broad hysteresis in the displacement characteristic is due to stressinduced martensite, which is formed locally in the regions of maximum stress along the beam surfaces, and due to the inhomogeneous temperature-distribution. These effects mainly determine the observed displacements, but have almost no in¯uence on the electrical resistance. Similar hysteresis effects are present in the linear actuator. 6.3. Optical properties The optical transmission through the optical slit has been investigated as a function of slit position. Fig. 7 shows a calibration characteristic of the optical transmission signal as a function of slit displacement. Within the range of gripper displacements, an almost linear behavior is present. In this range, a spatial resolution of 1 mm corresponds to a signal change of 0.7%. For an accuracy of the optical measurement below 0.5% and a gear-ratio of 2.3, the separation of the gripping jaws can be determined with an accuracy below 2 mm. 7. SMA microgripper system The hysteresis effects observed in the mechanical and electrical resistance characteristics raise the problem of a suitable control method for positioning. Recent investigations of certain TiNi wires showed an almost linear relationship between length change and electrical resistance, which enables smart actuation by using the sensing capability of the wire itself [11]. However, in the gripping device, the nonuniform stress- and temperature-distribution is associated with a non-uniform distribution of different phases, which affect the electrical and mechanical characteristics in a different way. Therefore, an external displacement sensor is used. Control tests of the microgripper system have been performed with a PC using a data acquisition board. The optical transmission intensity detected by the photosensor has been processed by a proportional-integral (PI) control algorithm using LabVIEW. The PI-control parameters have been
Fig. 8. Step-response of the PI control and sensor signal.
determined by an empirical routine in order to obtain an optimum control velocity and accuracy. The algorithm generates negative and positive control voltages in order to discriminate between the two antagonistic actuators. The maximum power to drive the actuators has been 80 mW, which corresponds to a maximum control signal of 10 V. The control frequency has been 125 Hz. A typical step-response of the control and sensor signal is shown in Fig. 8. The initial position is maintained by an average control signal of 2.5 V. After change of the set value for the transmission intensity from 0.65 to 0.5, the change of the actual displacement takes about 140 ms. The achieved positioning accuracy is about 0.7%, which corresponds to a separation of the gripping jaws of about 2 mm. This accuracy is considered to be suf®cient for object dimensions between 10 and 300 mm. Even sensitive objects may be handled without problem, since the gear actuator is in R-phase condition during closing of the gripping jaws and, thus, displays suf®cient compliance. Fig. 9 shows the response of the microgripper system to an arbitrary sequence of set values. Within 10±90% of the gripping range, the positioning time and accuracy are similar
Fig. 9. Response of the sensor signal to a positioning sequence.
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M. Kohl et al. / Sensors and Actuators A 97±98 (2002) 646±652
to Fig. 8. However, close to the maximum and minimum gripper separations, longer time constants occur, which depend on the maximum control power used for gripper control. 8. Conclusions and outlook The presented microgripper system consists of a monolithic SMA gripping device, which comprises mechanical, electrical and thermal functions, and an optical transmission scheme for position sensing. The design concept of monolithic SMA microdevices is demonstrated to be a powerful means for the realization of compact multifunctional microsystems with only a few fabrication steps. The SMA microgripper system is suitable for microrobotic tasks with high demands on smallness and work output. Possible applications are the handling of microparts of complex shape in restricted environments with dif®cult access, in vacuum environments or clean rooms, where adhesive or vacuum gripping is not suitable. For a microgripper prototype of TiNi with 2 mm 5:8 mm 0:23 mm in size, a maximum stroke of 300 mm and a maximum gripping force of 35 mN are achieved. The positioning accuracy and time are about 2 mm and 140 ms, respectively. The positioning dynamics is attractive for microassembly as typical time constants for position control by digital imaging are in the same order of magnitude [12]. Acknowledgements The authors would like to thank Gamer Lasertechnik GmbH for fast processing and S. Hoffmann for technical assistance. References [1] Y. Tatsue, T. Kitahara, Microgrip system, J. Robotics Mech. 3 (1) (1990) 57±59. [2] G. Greitmann, R.A. Buser, Tactile microgripper for automated handling of microparts, Sens. Actuators A 53 (1996) 410±415.
[3] G. Thornell, M. Bexell, J.A. Schweitz, S. Johansson, Design and fabrication of a gripping tool for micromanipulation, Sens. Actuators A 53 (1996) 428±433. [4] 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, Sens. Actuators A 54 (1996) 755±759. [5] R. Salim, H. Wurmus, Multi-gearing compliant mechanisms for piezoelectric actuated microgrippers, in: H. Borgmann (Ed.), Proceedings of the Actuator'98, Bremen, Germany, 1998, pp. 186± 188. [6] M. Kohl, E. Just, W. Pfleging, S. Miyazaki, SMA microgripper with integrated antagonism, Sens. Actuators 83 (2000) 208±213. [7] M. Kohl, Shape memory actuators in MEMS, in: Proceedings of the SMST'99, Antwerp Zoo, Belgium, 1999, pp. 267±279. [8] M. Kohl, K.D. Skrobanek, Linear actuators based on the shape memory effect, Sens. Actuators A 70 (1998) 104±111. [9] B. Krevet, W. Kaboth, Coupling of FEM programs for simulation of complex systems, in: Proceedings of the MSM'98, Santa Clara, 1998, pp. 320±324. [10] S. Miyazaki, K. Otsuka, Deformation and transition behavior associated with the R-phase in TiNi alloys, Metall. Trans. A 17A (1986) 53±63. [11] J. Hesselbach, R. Pittschellis, H. Stork, E. Hornbogen, M. Mertmann, Optimization and control of electrically heated shape memory actuators, in: H. Borgmann, K. Lenz (Eds.), Proceedings of the Actuator'94, Bremen, Germany, 1994, pp. 337±340. [12] B. KoÈhler, F. Eberle, A digital image processing system to automate the production of microstructures with hot stamping technology, in: Proceedings of the Microsystems Technologies'98, Potsdam, Germany, 1998.
Biographies M. Kohl received his PhD in physics in 1989 from the University of Stuttgart. In 1990±1991 he was working as an IBM post-doctoral fellow at the T.J. Watson Research Center in Yorktown Heights (USA) and joined later on the Forschungszentrum Karlsruhe in Germany. Since 1993, he is head of a research group at the Institut fuÈr Mikrostrukturtechnik working on the development of microactuators. B. Krevet received his PhD in physics in 1980 from the University of Karlsruhe. Since then, he has been working on a variety of topics related to materials science. In recent years, he is mainly working on the development of software tools for coupled finite-element simulation. E. Just has been working at the Forschungszentrum Karlsruhe in the field of design, fabrication and characterization of shape memory microgrippers. He received his PhD in mechanical engineering from the University of Karlsruhe in 2000. Since 2001, he is working at the University of Freiburg on simulation of microsystems devices.
SMA microgripper system M. Kohla,*, B. Kreveta, E. Justb a
b
Forschungszentrum Karlsruhe GmbH, IMT, Postfach 3640, 76021 Karlsruhe, Germany Albert-Ludwigs-UniversitaÈt Freiburg, IMTEK, Georges-KoÈhler-Allee 103, 79110 Freiburg, Germany Received 12 June 2001; received in revised form 4 October 2001; accepted 9 October 2001
Abstract A microgripper system is presented, which consists of a monolithic shape memory alloy (SMA) device of 2 mm 5:8 mm 0:23 mm size and an integrated optical position sensor. Gripper closing and opening is performed by two integrated actuators, which form an antagonistic pair. Investigations of temperature pro®les by coupled ®nite-element simulations and infrared microscopy demonstrate a suf®cient thermal insulation of the actuators for their selective control. The motion of gripping jaws is transmitted by an integrated gearing mechanism into a linear motion of an integrated optical slit, which is detected by change of optical transmission. The maximum stroke and force of the gripping jaws are 300 mm and 35 mN, respectively. In the range between 10 and 90% of the maximum stroke, positioning is achieved within 140 ms with an accuracy of about 2 mm. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microgripper system; Shape memory actuator; Monolithic microdevice; Position sensor
1. Introduction
2. Gripper design
For manipulation of microobjects during fabrication and assembly, microgrippers are demanded, which are compact in size, but still generate large work outputs [1±6]. Mechanical grippers based on two or more ®ngers provide the highest ¯exibility with respect to shape and material of the gripping object. Attractive solutions have been achieved by using shape memory alloy (SMA) microactuators [4,6,7]. Recently, a mechanical SMA microgripper with integrated antagonism has been developed [6], which consists of a monolithic SMA gripping device with two integrated actuators with opposite actuation directions. This allows on/ off operation between open and closed position, which is suitable for manipulation of objects with simple shape and suf®cient rigidity. In the case of complex object geometries and sensitive object materials, however, active control of gripping motion and force is advantageous. This paper presents an extended design of the SMA microgripper with integrated actuation and position sensing. After discussion of the gripper design, a simulation model for determination of the thermal properties of the gripping device is presented. Then, the multifunctional properties of the gripping device and the positioning performance of the microgripper system will be presented.
Mechanical grippers usually consist of an actuator and a gearing mechanism, which is connected to gripping jaws. By the gearing mechanism, the motion of the actuator is transferred to a gripping motion. Fig. 1a shows a scheme of a mechanical gripper, which comprises a linear actuator and a gearing mechanism with ®ve joints. This scheme is implemented in a completely monolithic SMA gripping device. The structure and operation principle of the gripping device are shown in Figs. 1b and c for open and closed condition, respectively. For linear actuation a SMA actuator is used, which consists of a folded-beam structurewith stress-optimized geometry [8]. The joints of the gearing mechanism are realized as ¯exible hinges. Since the gearing mechanism consists of SMA material, it also works as an actuator. The design of the actuators allows frictionless motion and easy miniaturization. Both, linear and gear actuator are prestrained with respect to each other before mounting on a substrate. The resulting deformation of the beams and hinges can be controlled by electrical heating. By selective heating of the gear actuator above the phase transformation temperature, the hinges recover their undeformed shape due to the one-way shape memory effect. In this case, only the linear actuator is deformed and the gripping jaws are opened. Selective heating of the linear actuator causes deformation of the hinges and, thus, restores the closed condition. Details on the stressoptimized design of the gear actuator can be found in [6].
*
Corresponding author. Tel.: 49-7247-822798; fax: 49-7247-824331. E-mail address: [email protected] (M. Kohl).
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 8 0 3 - 2
M. Kohl et al. / Sensors and Actuators A 97±98 (2002) 646±652
647
Fig. 1. Mechanical gripper: (a) schematic of the gripping device; (b and c) monolithic SMA gripping device in open and closed condition, respectively.
Between the actuators, an optical slit is integrated, which performes the same motion as the linear actuator. Due to a ®xed gear-ratio, the position of the slit S1 directly re¯ects the separation of the gripping jaws S2. Thus, control of the slit position allows positioning of the gripping jaws. This concept results in a compact microgripper system, which is sketched in Fig. 2. The SMA gripping device is mounted between two substrates with integrated light emitting diode (LED) and photosensor, which are aligned with respect to the optical slit. In this set-up, the light intensity transmitted through the optical slit yields the actual slit position.
generated in the gripping device, which varies in space and time. Calculation of the temperature-distribution involves an electrical simulation step for determination of the distribution of current density and of electrical heating power and a thermal simulation step. Both simulations have been performed using the ®nite-element method (FEM). As the electrical and thermal properties of the gripping device depend on each other, a special FEM coupling routine has been used [9]. The electrical potential U is calculated from the differential equation
3. Simulation
~ fj0 g s rU
The performance of the gripping device is determined by the heating and cooling processes during actuation. By application of an electrical current, a temperature-distribution is
The parameter s denotes the electrical conductivity. In the electrical FEM simulation step, the non-linear change and hysteresis of the electrical conductivity in the phase transformation regime is neglected, which gives rise to an estimated relative error of about 10%. From the current density {j} the heating power Q_ is calculated by volume integration of the product sfjgT fjg. The heat transfer within the gripping device is determined by the differential equation
s DU 0
(1)
and the Neumann boundary condition at the electrical connections (2)
m cp h TT_ Q_
l DT
(3)
which takes into account the sensible heat, the heat of phase transformation h(T), the heating power generated by electrical current Q_ and losses due to heat conduction. The parameters m, cp and l denote the mass, speci®c heat capacity and thermal conductivity, respectively. At the device surface, the following boundary condition is considered: ~ K T l rT
Fig. 2. Schematic of the SMA microgripper system.
TE
(4)
which takes into account the convective heat exchange with the environment of temperature TE. The parameter K denotes the heat transfer coef®cient. Eqs. (3) and (4) are solved by a thermal FEM simulation step.
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M. Kohl et al. / Sensors and Actuators A 97±98 (2002) 646±652
Table 1 Simulation parameters for determination of the temperature-distribution Thermal conductivity l (W m 1 K 1) Electrical conductivity s (O 1 m 1) Specific heat cp (J kg 1 K 1) Mass density (kg m 3) Thermal expansion coefficient a (K 1) Heat-transfer coefficient K (W m 2 K 1) Integral latent heat (J kg 1) Environment temperature TE (8C)
18 1.2 106 450 6.5 103 10 5 70 4500 25
The heat-transfer coef®cient K has been determined empirically for different cases of free convection without external air ¯ow and forced convection by ®tting simulated temperature pro®les of a test device to experimentally determined pro®les from infrared microscopy. The used simulation parameters are summarized in Table 1. Within the temperature range of 25±60 8C, they have been treated as constants.
is 0.05 mm. Before integration on a substrate, the SMA gripping device is prestrained to adjust the maximum stress and strain level for actuation. Below the optical slit, an opening of 1:8 mm 0:4 mm is cut into the substrate by a laser, which serves as an aperture for an LED attached to the substrate from the bottom side. Above the optical slit, a photosensor with sensing area of 2 mm 2 mm is mounted. Integration of the SMA gripping device and the optical components is performed by adhesive bonding. Electrical contact are made by wedge±wedge bonding. The overall size of the microgripper system including the housing is 2:5 mm 8 mm 9 mm. Fig. 3 shows the microgripper system holding a piece of optical ®ber. In this case, a constant gripping force is maintained by electrical heating of the linear actuator. The upper substrate containing the photosensor has been removed to allow a complete view on the SMA gripping device. 5. Experimental
4. Fabrication Cold-rolled sheets of 230 mm thickness of an equiatomic TiNi alloy are heat-treated in vacuum at 530 8C for 10 min to adjust the one-way shape memory effect. The gripping devices are micromachined by laser cutting. The lateral size of the devices is 2 mm 5:8 mm. The minimum beam width
Phase transformation temperatures of the SMA sheets have been investigated by differential scanning calorimetry and electrical resistance measurements. The temperatures of the rhombohedral (R) and corresponding reverse phase transformation to austenite (A) have been determined to Rs 44 8C, Rf 40 8C, As 41 8C and Af 45 8C. The
Fig. 3. SMA microgripper system holding a piece of optical fiber of 140 mm diameter.
M. Kohl et al. / Sensors and Actuators A 97±98 (2002) 646±652
indices s and f denote start and ®nish temperatures, respectively. The martensitic transformation occurs below room temperature. Therefore, only the R-phase transformation is used for actuation, which is of special interest, since R-phase transformations exhibit a narrow hysteresis width of about 1 K and negligible fatigue effects [10]. The temperature-distribution along the surface of the gripping device has been investigated as a function of electrical heating power by infrared microscopy. In order to obtain a homogeneous coef®cient of thermal emission equal to 1, device and substrate have been coated by a thin layer of graphite. The maximum spatial resolution has been 25 mm and the maximum time resolution 100 ms. Force±displacement characteristics of individual actuators have been determined in vertical orientation. The actuators have been loaded by calibrated weights and the corresponding vertical displacements have been determined optically by a video microscope. The experiments have been performed in a thermostat, which allows adjustment of the temperature by ambient heating or cooling. Stationary equilibrium conditions have been maintained by cycling the temperature step-wise with suf®cient waiting time in every data point. Simultaneously, the electrical resistance has been determined by the four-point method. 6. SMA gripping device 6.1. Thermal properties Simulated temperature pro®les in the gripping device are shown in Fig. 4 for a heating power of 100 mW after stationary equilibrium is reached. Due to symmetry, only
649
one half of the device has been taken into account. Electrical heating of the gear actuator (Fig. 4a) causes transformation of the ¯exible hinges to the austenitic phase except small transition regions close to the electrical bond pads. The maximum temperature achieved in open condition is determined to about 53 8C. Due to heat conduction, also the gripping jaws and the optical slit are heated to about 50 and 40 8C, respectively. The linear actuator, however, remains below the start temperature of austenitic phase transformation As. By electrically heating of the linear actuator (Fig. 4b), the folded-beam structure transforms to austenite, while the gear actuator remains in R-phase condition. The simulations demonstrate, that the actuators will be suf®ciently thermally insulated by the optical slit as long as the heating power is kept below about 100 mW. It is important to note, that the gripping jaws remain almost at room temperature in closed condition and, thus, do not thermally affect the gripping object. Fig. 5 shows temperature pro®les along the surface of the gripping device determined by infrared microscopy. After heating the gear actuator with an electrical heating power of 80 mW for 150 ms (Fig. 5a), the gear actuator displays a maximum temperature of about 50 8C, while the linear actuator remains below 33 8C. Thus, the shape memory effect is selectively induced in the gear actuator. Fig. 5b shows the temperature-distribution after a cooling period of 150 ms and subsequent heating the linear actuator with an electrical heating power of 80 mW for 150 ms. In this case, a maximum temperature of about 49 8C is reached in the linear actuator, while the gear actuator is cooled to temperatures below about 37 8C. Consequently, only the linear actuator displays the shape memory effect. The measured
Fig. 4. Simulated temperature profiles in the gripping device in open (a) and closed (b) condition in stationary equilibrium. The heating power is 100 mW.
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Fig. 5. Temperature profiles along the surface of the gripping device in open (a) and closed (b) condition determined by infrared microscopy. The heating time and power are 150 ms and 80 mW, respectively. Numerical values are indicated for five different locations on the gripping device.
temperature-distributions agree very well with the simulations. The results demonstrate, that the actuators are suf®ciently thermally separated by the optical slit. Therefore, it is possible to control the two actuators of the gripping device selectively in an antagonistic way. Due to the limited time resolution, the infrared experiments only allow a rough estimate of the cooling time, which the average temperature requires to reach the Rf-temperature. This time is about 200 ms for the gear actuator and about 300 ms for the linear actuator. The heating times of the actuators, given by the time interval of the average temperature to reach the Af-temperature, have been determined by time-resolved electrical resistance measurements. For a heating power of 80 mW, this time is about 40 ms for the gear actuator and about 50 ms for the linear actuator. Similar experiments on gripping devices of 100 mm thickness revealed typical heating times in the order of 30±35 ms for 22 mW heating power [6].
force of 88 mN, the stroke of the gripping jaws S2 reaches a maximum value of about 300 mm. The corresponding vertical displacement of the weight S1 is about 130 mm, which corresponds to a gear-ratio S2/S1 of 2.3. The maximum gripping force depends on the degree of prestrain before mounting the gripper. By prestraining the gear actuator by about 130 mm, the gripping force is estimated to 35 mN. Despite the use of a R-phase transformation, a narrow hysteresis width is only present in the electrical resistance characteristic for a heating power above 15 mW. The decrease of resistance at small heating power indicates the onset of a thermally-induced martensitic phase
6.2. Mechanical properties For a detailed understanding of the mechanical and electrical properties of the SMA gripping device, linear and gear actuator have been fabricated and characterized separately. Fig. 6 shows typical displacement and electrical resistance characteristics of the gear actuator. Zero displacements S2 correspond to completely closed gripping jaws. This condition is reached at zero electrical power for external loads above 80 mN. At smaller loads, the gripping jaws cannot be closed completely. The displacement of the gripping jaws increases for increasing electrical power. For a
Fig. 6. Displacement and electrical resistance characteristic of the gear actuator for a load of 88 mN. The inset illustrates the measurement set-up.
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Fig. 7. Calibration characteristic of the optical transmission signal.
transformation, which, however, has no effect on the mechanical performance of the gear actuator. The broad hysteresis in the displacement characteristic is due to stressinduced martensite, which is formed locally in the regions of maximum stress along the beam surfaces, and due to the inhomogeneous temperature-distribution. These effects mainly determine the observed displacements, but have almost no in¯uence on the electrical resistance. Similar hysteresis effects are present in the linear actuator. 6.3. Optical properties The optical transmission through the optical slit has been investigated as a function of slit position. Fig. 7 shows a calibration characteristic of the optical transmission signal as a function of slit displacement. Within the range of gripper displacements, an almost linear behavior is present. In this range, a spatial resolution of 1 mm corresponds to a signal change of 0.7%. For an accuracy of the optical measurement below 0.5% and a gear-ratio of 2.3, the separation of the gripping jaws can be determined with an accuracy below 2 mm. 7. SMA microgripper system The hysteresis effects observed in the mechanical and electrical resistance characteristics raise the problem of a suitable control method for positioning. Recent investigations of certain TiNi wires showed an almost linear relationship between length change and electrical resistance, which enables smart actuation by using the sensing capability of the wire itself [11]. However, in the gripping device, the nonuniform stress- and temperature-distribution is associated with a non-uniform distribution of different phases, which affect the electrical and mechanical characteristics in a different way. Therefore, an external displacement sensor is used. Control tests of the microgripper system have been performed with a PC using a data acquisition board. The optical transmission intensity detected by the photosensor has been processed by a proportional-integral (PI) control algorithm using LabVIEW. The PI-control parameters have been
Fig. 8. Step-response of the PI control and sensor signal.
determined by an empirical routine in order to obtain an optimum control velocity and accuracy. The algorithm generates negative and positive control voltages in order to discriminate between the two antagonistic actuators. The maximum power to drive the actuators has been 80 mW, which corresponds to a maximum control signal of 10 V. The control frequency has been 125 Hz. A typical step-response of the control and sensor signal is shown in Fig. 8. The initial position is maintained by an average control signal of 2.5 V. After change of the set value for the transmission intensity from 0.65 to 0.5, the change of the actual displacement takes about 140 ms. The achieved positioning accuracy is about 0.7%, which corresponds to a separation of the gripping jaws of about 2 mm. This accuracy is considered to be suf®cient for object dimensions between 10 and 300 mm. Even sensitive objects may be handled without problem, since the gear actuator is in R-phase condition during closing of the gripping jaws and, thus, displays suf®cient compliance. Fig. 9 shows the response of the microgripper system to an arbitrary sequence of set values. Within 10±90% of the gripping range, the positioning time and accuracy are similar
Fig. 9. Response of the sensor signal to a positioning sequence.
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to Fig. 8. However, close to the maximum and minimum gripper separations, longer time constants occur, which depend on the maximum control power used for gripper control. 8. Conclusions and outlook The presented microgripper system consists of a monolithic SMA gripping device, which comprises mechanical, electrical and thermal functions, and an optical transmission scheme for position sensing. The design concept of monolithic SMA microdevices is demonstrated to be a powerful means for the realization of compact multifunctional microsystems with only a few fabrication steps. The SMA microgripper system is suitable for microrobotic tasks with high demands on smallness and work output. Possible applications are the handling of microparts of complex shape in restricted environments with dif®cult access, in vacuum environments or clean rooms, where adhesive or vacuum gripping is not suitable. For a microgripper prototype of TiNi with 2 mm 5:8 mm 0:23 mm in size, a maximum stroke of 300 mm and a maximum gripping force of 35 mN are achieved. The positioning accuracy and time are about 2 mm and 140 ms, respectively. The positioning dynamics is attractive for microassembly as typical time constants for position control by digital imaging are in the same order of magnitude [12]. Acknowledgements The authors would like to thank Gamer Lasertechnik GmbH for fast processing and S. Hoffmann for technical assistance. References [1] Y. Tatsue, T. Kitahara, Microgrip system, J. Robotics Mech. 3 (1) (1990) 57±59. [2] G. Greitmann, R.A. Buser, Tactile microgripper for automated handling of microparts, Sens. Actuators A 53 (1996) 410±415.
[3] G. Thornell, M. Bexell, J.A. Schweitz, S. Johansson, Design and fabrication of a gripping tool for micromanipulation, Sens. Actuators A 53 (1996) 428±433. [4] 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, Sens. Actuators A 54 (1996) 755±759. [5] R. Salim, H. Wurmus, Multi-gearing compliant mechanisms for piezoelectric actuated microgrippers, in: H. Borgmann (Ed.), Proceedings of the Actuator'98, Bremen, Germany, 1998, pp. 186± 188. [6] M. Kohl, E. Just, W. Pfleging, S. Miyazaki, SMA microgripper with integrated antagonism, Sens. Actuators 83 (2000) 208±213. [7] M. Kohl, Shape memory actuators in MEMS, in: Proceedings of the SMST'99, Antwerp Zoo, Belgium, 1999, pp. 267±279. [8] M. Kohl, K.D. Skrobanek, Linear actuators based on the shape memory effect, Sens. Actuators A 70 (1998) 104±111. [9] B. Krevet, W. Kaboth, Coupling of FEM programs for simulation of complex systems, in: Proceedings of the MSM'98, Santa Clara, 1998, pp. 320±324. [10] S. Miyazaki, K. Otsuka, Deformation and transition behavior associated with the R-phase in TiNi alloys, Metall. Trans. A 17A (1986) 53±63. [11] J. Hesselbach, R. Pittschellis, H. Stork, E. Hornbogen, M. Mertmann, Optimization and control of electrically heated shape memory actuators, in: H. Borgmann, K. Lenz (Eds.), Proceedings of the Actuator'94, Bremen, Germany, 1994, pp. 337±340. [12] B. KoÈhler, F. Eberle, A digital image processing system to automate the production of microstructures with hot stamping technology, in: Proceedings of the Microsystems Technologies'98, Potsdam, Germany, 1998.
Biographies M. Kohl received his PhD in physics in 1989 from the University of Stuttgart. In 1990±1991 he was working as an IBM post-doctoral fellow at the T.J. Watson Research Center in Yorktown Heights (USA) and joined later on the Forschungszentrum Karlsruhe in Germany. Since 1993, he is head of a research group at the Institut fuÈr Mikrostrukturtechnik working on the development of microactuators. B. Krevet received his PhD in physics in 1980 from the University of Karlsruhe. Since then, he has been working on a variety of topics related to materials science. In recent years, he is mainly working on the development of software tools for coupled finite-element simulation. E. Just has been working at the Forschungszentrum Karlsruhe in the field of design, fabrication and characterization of shape memory microgrippers. He received his PhD in mechanical engineering from the University of Karlsruhe in 2000. Since 2001, he is working at the University of Freiburg on simulation of microsystems devices.