- Email: [email protected]
Real-time gripping detection for a mechanically actuated microgripper
Available online at www.sciencedirect.com
Microelectronic Engineering 85 (2008) 1022–1026 www.elsevier.com/locate/mee
Real-time gripping detection for a mechanically actuated microgripper M.M. Blideran a,*, M. Fleischer a, F. Grauvogel b, K. Lo¨ffler b, M.G. Langer b,1, D.P. Kern a a
Institute for Applied Physics, University of Tu¨bingen, 72076 Tu¨bingen, Germany b Department of Applied Physiology, University of Ulm, 89081 Ulm, Germany
Received 6 October 2007; received in revised form 14 December 2007; accepted 10 January 2008 Available online 26 January 2008
Abstract The possibility of controlled gripping during micromanipulation procedures is widely desired in the fields of microbiology and microassembly. For achieving it, measurement or calculation of the forces exerted by the end segment of the manipulator are required. This work presents a method for detecting the gripping moment for a mechanically actuated silicon microgripper. A procedure that combines measurements with simulation results was developed for calculating the forces exerted by the tips of the gripper. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Micromanipulation; Microgripper; Lateral force microscopy; Laser beam deflection
1. Introduction Understanding matter, whether of biological or nonbiological nature, at the research stage often requires handling and manipulation of objects with sizes down to few micrometers or even below. Depending on the nature of the objects and their surrounding medium, when using a gripper, an appropriate mode of actuation has to be chosen: electrical, magnetic, optical, thermal or mechanical, each mode presenting its advantages and disadvantages with regard to the application for which the gripper is intended. Besides choosing the driving force, the control over the object to be manipulated is another challenge that arises: to know exactly when the object under investigation is grabbed and what pressures are applied to it. So far, several microgripper concepts have been reported [1]. These grippers however find their use in the field of microassembly, meaning that they address objects *
Corresponding author. E-mail address: [email protected] (M.M. Blideran). 1 Present address: Carl Zeiss NTS GmbH, 73447 Oberkochen, Germany. 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.029
with sizes of several tens of micrometers but they also need relatively large tips to be able to grab millimeter–diameter objects. This means relatively wide surfaces for the gripper and the tips and relatively large spaces between different components of the gripper that allow the implementation of various sensors directly on the gripper. Microgrippers with integrated electromagnetic actuators and piezoelectric force sensors [2], with integrated self-sensing SPM cantilevers that measure the stress-induced electrical resistance changes [3], and control of the gripping with a haptic device and an optical microscope [4,5] have been reported. Further challenges arise when the objects to be manipulated are only several micrometers or less in size. Precise gripping requires tips with dimensions of the same order as the objects, thus drastically limiting the areas where detectors could be implemented. Moreover, when biological structures are investigated in a life sustaining, thus electrically conductive environment, electrical fields and potential differences may cause problems, therewith limiting even more the detection methods that one could implement. Our work focused on the development of a method that will help detecting, in real-time, the moment of gripping for our mechanically actuated silicon microgripper, whose fabrication, performance and advantages have been presented
M.M. Blideran et al. / Microelectronic Engineering 85 (2008) 1022–1026
previously [6,7]. The method is based on the laser deflection detection used in atomic force microscopy (AFM)[8]. The laser beam is reflected from the back-side of a cantilever and the position change of the laser spot on a position-sensitive detector (PSD) is measured. It has been demonstrated in the context of photothermal spectroscopy, that ˚ can be accurately detected displacements of less than 1 A with this simple optical scheme [9,10]. A similar approach has also been reported[11] for a microgripper suitable for handling objects with sizes of hundreds of micrometers with a cantilever as ‘‘one finger” and a thicker second ‘‘finger” and a laser beam positioned on the reflecting back of the cantilever detecting its deflection. In addition, our study has been extended to estimate the forces applied by the tips of the microgripper to the handled object by means of a finite element simulation program. 2. The gripper Our mechanically actuated silicon microgripper is illustrated in the scanning electron micrograph in Fig. 1. Briefly, the gripper consists of three sections of different thicknesses: 452 lm for the bulk support, 52 lm for the thick arms and the contact segment and 11 lm for the actuated segment. These three sections increase its stability during operation and most important during assembly. The force required to close the tips is applied to the 400 400 lm2 contact area situated between the arms (Fig. 1a) and transferred to the actuated segment (Fig. 1b) along the pullers. The thin hinges (Fig. 1b) bend and close the gap between the tips.
1023
Fig. 2. Gripper assembly mounted to a Kleindiek manipulator.
The mechanical force is provided by a two piezo-bar assembly, arranged in such a way that the action of the two piezo-bars is added and at the same time thermal drift is compensated. The whole assembly is mounted on the end of a Kleindiek manipulator [12] for precise handling (Fig. 2). 3. Experiments 3.1. The principle of the real-time detection method Since our gripper is aimed at the handling of microobjects of inorganic as well as organic nature, only its small features close to the tips and their minute movements are available for measurements. The detection method we developed takes advantage of the 110 lm2 triangular flat area adjacent to the 1 lm thin and 3 lm long parallel tips. The principle of the method is the following: when the tips are being closed, starting from the moment of contact, the resistance of an off-centered object within the gap (the contact shall occur below the mid-height line of the tips), e.g. a microsphere, acting against the closing motion generates a torque as illustrated in Fig. 3a. This will tilt the tips and their top surface (Fig. 3b), thus deflecting a laser beam pointing at one of them. The position of the laser spot on a position sensitive detector (PSD) will change, thus indicating the moment of gripping in real-time. Since the tips are 11 lm high, the situation depicted here will apply to most objects that fit into the 8 lm gap between the tips. 3.2. Experimental set-up
Fig. 1. (a) The functional part of the gripper; (b) the actuated segment of the gripper: thin hinges along the arms ensure an increased flexibility of the structure and the closing under the action of an external force.
The set-up we used to prove the principle of the method described in the previous section is illustrated in the schematic drawing in Fig. 4a. It consists of two major parts: the optical microscope and the microgripper. All components of the system are mounted on an electrically controlled vibration isolation table. A pigtailed laser diode system is used as a light source with a wave length of 635 nm for the optical detection. A mirror (M1) reflects the collimated and linearly polarized laser light through a polarizing beam splitter cube. A convex lens (FL; f = 300 mm) focuses the laser beam into the back focal plane of the objective. A k/4-plate, located between the lens FL and mirror M2, converts the linear into circular polarized
1024
M.M. Blideran et al. / Microelectronic Engineering 85 (2008) 1022–1026
Fig. 3. The real-time detection scheme: (a) torsion of the tips during gripping; (b) schematics of the laser beam deflection set-up, similar to optical detection of cantilever deflection.
Fig. 4. Schematic drawing of the experimental set-up including the microscope, the detection system and the microgripper: (a) schematic representation of the system; (b) optical picture of the system; (c) and (d) manipulator positioned under the objective.
light. The optical notch filter mirror (M2) reflects the laser light by 90° to the objective. Laser light is collimated by the objective and directed to the microgripper. To improve its reflectivity, the silicon gripper was sputtered with 20 nm of gold. The light is reflected the same way back to the beam splitter cube. Since the k/4-plate shifts the polarization of the laser by 90°, it is now reflected by the polarizing beam splitter cube onto the position-sensitive photodiode (PSD). The eyepiece and the camera were removed from the microscope and mounted 15 cm above on two separate aluminum columns. Between the dichroic mirror and the eyepiece, a laser-blocking filter was added to protect the eyes of the experimenter. A picture of the complete set-up is presented in Fig. 4b. The microgripper attached to the Kleindiek nanomanipulator driven under the objective is illustrated in Fig. 4c and a closer view of the gripper during operation in Fig. 4d. 3.3. Results and discussion Under an optical microscope a glass plate was attached to a piezoelectric scanning stage. The microgripper was
positioned such that during a scan the stage would slightly touch the bottom of one tip at the end of the 35 lm stroke. We positioned the laser spot with a diameter of less than 5 lm on the 110 lm2 flat area between the tip and the hinge, such that it completely remained on this surface during the entire experiment. This way we ensured that the changes in signal on the PSD were only due to changes in the orientation of the reflecting surface. The stage was repeatedly driven against the arm of the gripper. Fig. 5a shows an optical micrograph illustrating the set-up. In Fig. 5b, the detector signal for one linear stage motion towards the tip over a distance of 35 lm is shown. Towards the end of the scan, just before the stage touches the tip, a short drop in the PSD signal below the 0 mV level is observed, followed by a steep linear rise. The tip first snaps to the glass edge due to adhesion forces, since the measurement was performed in air and under ambient conditions, where a water meniscus is always present. The increase in detector signal is then caused by the changing angle of the tip’s top surface due to the force exerted on its bottom by the glass plate. In order to test whether the signal really measures a change in orientation of the
M.M. Blideran et al. / Microelectronic Engineering 85 (2008) 1022–1026
1025
Fig. 5. (a) An optical micrograph of the set-up with the glass edge pushing against the tip of the gripper; (b) signal detected on the position-sensitive detector during a 35 lm scan.
gripper’s top plane, we lowered the position of the gripper by about 5 lm, thus centering it relative to the glass edge. In this case, the PSD did not record any signal since the scanning stage was hitting the tip at middle height, indicating that our assumptions regarding the detection method are correct.
The bottom of the arm is fixed in all three directions and no motion is allowed; this reflects the fact that the arms do not end at that point but are attached to the massive support structure. The bottom of the puller is fixed in x and y directions, reflecting the motion along the z axis of the pullers during operation.
4. Estimation of the gripping force By simple geometrical considerations, the signal recorded on the PSD can be translated into a tilting angle of the gripper surface. This tilt is associated with a certain torque which is generated by the force applied to the bottom of the tip. Thus for a given PSD signal, the force acting on the bottom of the tip can be determined by simulation. For symmetry and simplicity reasons, only the actuated segment of one arm was considered for finite element simulations. In accordance with the experimental situation the following constraints were applied to the model:
In Fig. 6 a simulation result is presented where the boundary constraints are indicated. The force was applied along an edge of the tip, namely its 3 lm inner length (Fig. 6a). The distribution of the ‘‘von Mises stress” [13] is depicted on the body of the model. The maximum value is almost three orders of magnitude below the critical value of 7 GPa for silicon (Si) when a force of 1 lN is applied. For the same force the model’s displacement out of the z–x plane – the displacement in y direction – is illustrated in Fig. 6b. This corresponds to a tilt of the area between the tips and the hinges on top of which the laser spot was positioned during the
Fig. 6. Simulation results: (a) distribution of the von Mises stress on the model surface; (b) out of plane displacement (tilting) along the top surface of the model.
1026
M.M. Blideran et al. / Microelectronic Engineering 85 (2008) 1022–1026
experiments. For the applied force of 1 lN, the maximum displacement along the y direction is 4 nm, corresponding to a tilt angle of 0.02°. The detection method we have implemented for our microgripper uses basically the same optical beam deflection (OBD) as in lateral scanning force microscopy [14]. ˚ deflection of cantilevers [15] in lateral Detection of sub-A scanning and friction traced to the single atom level [16] has been reported. This means that applying this method to our microgripper could easily resolve forces of 1 lN. 5. Conclusions & Outlook We have shown that the moment of gripping can be detected in real-time by using the OBD method, a very suitable method for investigations in air as well as in liquid since it does not involve electrical potentials or thermal gradients. Furthermore we developed a procedure involving gripping detection and finite element simulations which leads to an estimation of the forces exerted on the object subjected to manipulation. The sensitivity lies below the micronewtons level. Extension of the experiments and simulations to objects, e.g. spheres, of different sizes are required together with calibration of the microgripper and validation of the simulation and experimental method by using a piezoresistive cantilever instead of a sphere and performing the closing experiment.
References [1] A. Albut, Q. Zhou, C.d. Corral, H.N. Koivo, in: Proc. Second VDE World Microtechnologies Congress, MICRO.tec 2003, 2003, p. 507. [2] D.-H. Kim, M.G. Lee, B. Kim, Y. Sun, Smart Mater. Struct. 14 (2005) 1265. [3] S. Fahlbusch, S. Fatikow, Ultramicroscopy 86 (2001) 181. [4] A. Ferreira, C. Cassier, Y. Haddab, P. Rougeot, N. Chaillet, Proc. SPIE 4568 (2001) 112. [5] D.H.B. Wicaksono, A. Andreykiv, K. Kwok, H.-C. Wu, M. Tichem, DIMES & DCMM Summer School 2005 Report,. [6] M.M. Blideran, G. Bertsche, W. Henschel, D.P. Kern, Microelectron. Eng. 83 (2006) 1382. [7] M.M. Blideran, M. Fleischer, W. Henschel, D.P. Kern, J. Sterr, K. Schock, S. Kleindiek, M.G. Langer, K. Lo¨ffler, F. Grauvogel, J. Vac. Sci. Technol. B 24 (2006) 3239. [8] G. Meyer, N.M. Amer, Appl. Phys. Lett. 53 (1988) 2400. [9] N.M. Amer, M.A. Olmstead, Surf. Sci. 132 (1983) 68. [10] N.M. Amer, A. Skumanich, D. Ripple, Appl. Phys. Lett. 49 (1986) 137. [11] Y. Zhou, B.J. Nelson, Proc. SPIE 3834 (1999) 211. [12] Kleindiek Micromanipulator, . [13] R. von Mises, Angew. Math. Mech. 8 (1928) 161. [14] C.M. Mate, G.M. McClelland, R. Erlandsson, S. Chiang, Phys. Rev. Lett. 59 (1987) 1942. [15] G. Meyer, N.M. Amer, Appl. Phys. Lett. 57 (1990) 2089. [16] F.J. Giessibl, M. Herz, J. Mannhart, PNAS 99 (2002) 12006.
Microelectronic Engineering 85 (2008) 1022–1026 www.elsevier.com/locate/mee
Real-time gripping detection for a mechanically actuated microgripper M.M. Blideran a,*, M. Fleischer a, F. Grauvogel b, K. Lo¨ffler b, M.G. Langer b,1, D.P. Kern a a
Institute for Applied Physics, University of Tu¨bingen, 72076 Tu¨bingen, Germany b Department of Applied Physiology, University of Ulm, 89081 Ulm, Germany
Received 6 October 2007; received in revised form 14 December 2007; accepted 10 January 2008 Available online 26 January 2008
Abstract The possibility of controlled gripping during micromanipulation procedures is widely desired in the fields of microbiology and microassembly. For achieving it, measurement or calculation of the forces exerted by the end segment of the manipulator are required. This work presents a method for detecting the gripping moment for a mechanically actuated silicon microgripper. A procedure that combines measurements with simulation results was developed for calculating the forces exerted by the tips of the gripper. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Micromanipulation; Microgripper; Lateral force microscopy; Laser beam deflection
1. Introduction Understanding matter, whether of biological or nonbiological nature, at the research stage often requires handling and manipulation of objects with sizes down to few micrometers or even below. Depending on the nature of the objects and their surrounding medium, when using a gripper, an appropriate mode of actuation has to be chosen: electrical, magnetic, optical, thermal or mechanical, each mode presenting its advantages and disadvantages with regard to the application for which the gripper is intended. Besides choosing the driving force, the control over the object to be manipulated is another challenge that arises: to know exactly when the object under investigation is grabbed and what pressures are applied to it. So far, several microgripper concepts have been reported [1]. These grippers however find their use in the field of microassembly, meaning that they address objects *
Corresponding author. E-mail address: [email protected] (M.M. Blideran). 1 Present address: Carl Zeiss NTS GmbH, 73447 Oberkochen, Germany. 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.029
with sizes of several tens of micrometers but they also need relatively large tips to be able to grab millimeter–diameter objects. This means relatively wide surfaces for the gripper and the tips and relatively large spaces between different components of the gripper that allow the implementation of various sensors directly on the gripper. Microgrippers with integrated electromagnetic actuators and piezoelectric force sensors [2], with integrated self-sensing SPM cantilevers that measure the stress-induced electrical resistance changes [3], and control of the gripping with a haptic device and an optical microscope [4,5] have been reported. Further challenges arise when the objects to be manipulated are only several micrometers or less in size. Precise gripping requires tips with dimensions of the same order as the objects, thus drastically limiting the areas where detectors could be implemented. Moreover, when biological structures are investigated in a life sustaining, thus electrically conductive environment, electrical fields and potential differences may cause problems, therewith limiting even more the detection methods that one could implement. Our work focused on the development of a method that will help detecting, in real-time, the moment of gripping for our mechanically actuated silicon microgripper, whose fabrication, performance and advantages have been presented
M.M. Blideran et al. / Microelectronic Engineering 85 (2008) 1022–1026
previously [6,7]. The method is based on the laser deflection detection used in atomic force microscopy (AFM)[8]. The laser beam is reflected from the back-side of a cantilever and the position change of the laser spot on a position-sensitive detector (PSD) is measured. It has been demonstrated in the context of photothermal spectroscopy, that ˚ can be accurately detected displacements of less than 1 A with this simple optical scheme [9,10]. A similar approach has also been reported[11] for a microgripper suitable for handling objects with sizes of hundreds of micrometers with a cantilever as ‘‘one finger” and a thicker second ‘‘finger” and a laser beam positioned on the reflecting back of the cantilever detecting its deflection. In addition, our study has been extended to estimate the forces applied by the tips of the microgripper to the handled object by means of a finite element simulation program. 2. The gripper Our mechanically actuated silicon microgripper is illustrated in the scanning electron micrograph in Fig. 1. Briefly, the gripper consists of three sections of different thicknesses: 452 lm for the bulk support, 52 lm for the thick arms and the contact segment and 11 lm for the actuated segment. These three sections increase its stability during operation and most important during assembly. The force required to close the tips is applied to the 400 400 lm2 contact area situated between the arms (Fig. 1a) and transferred to the actuated segment (Fig. 1b) along the pullers. The thin hinges (Fig. 1b) bend and close the gap between the tips.
1023
Fig. 2. Gripper assembly mounted to a Kleindiek manipulator.
The mechanical force is provided by a two piezo-bar assembly, arranged in such a way that the action of the two piezo-bars is added and at the same time thermal drift is compensated. The whole assembly is mounted on the end of a Kleindiek manipulator [12] for precise handling (Fig. 2). 3. Experiments 3.1. The principle of the real-time detection method Since our gripper is aimed at the handling of microobjects of inorganic as well as organic nature, only its small features close to the tips and their minute movements are available for measurements. The detection method we developed takes advantage of the 110 lm2 triangular flat area adjacent to the 1 lm thin and 3 lm long parallel tips. The principle of the method is the following: when the tips are being closed, starting from the moment of contact, the resistance of an off-centered object within the gap (the contact shall occur below the mid-height line of the tips), e.g. a microsphere, acting against the closing motion generates a torque as illustrated in Fig. 3a. This will tilt the tips and their top surface (Fig. 3b), thus deflecting a laser beam pointing at one of them. The position of the laser spot on a position sensitive detector (PSD) will change, thus indicating the moment of gripping in real-time. Since the tips are 11 lm high, the situation depicted here will apply to most objects that fit into the 8 lm gap between the tips. 3.2. Experimental set-up
Fig. 1. (a) The functional part of the gripper; (b) the actuated segment of the gripper: thin hinges along the arms ensure an increased flexibility of the structure and the closing under the action of an external force.
The set-up we used to prove the principle of the method described in the previous section is illustrated in the schematic drawing in Fig. 4a. It consists of two major parts: the optical microscope and the microgripper. All components of the system are mounted on an electrically controlled vibration isolation table. A pigtailed laser diode system is used as a light source with a wave length of 635 nm for the optical detection. A mirror (M1) reflects the collimated and linearly polarized laser light through a polarizing beam splitter cube. A convex lens (FL; f = 300 mm) focuses the laser beam into the back focal plane of the objective. A k/4-plate, located between the lens FL and mirror M2, converts the linear into circular polarized
1024
M.M. Blideran et al. / Microelectronic Engineering 85 (2008) 1022–1026
Fig. 3. The real-time detection scheme: (a) torsion of the tips during gripping; (b) schematics of the laser beam deflection set-up, similar to optical detection of cantilever deflection.
Fig. 4. Schematic drawing of the experimental set-up including the microscope, the detection system and the microgripper: (a) schematic representation of the system; (b) optical picture of the system; (c) and (d) manipulator positioned under the objective.
light. The optical notch filter mirror (M2) reflects the laser light by 90° to the objective. Laser light is collimated by the objective and directed to the microgripper. To improve its reflectivity, the silicon gripper was sputtered with 20 nm of gold. The light is reflected the same way back to the beam splitter cube. Since the k/4-plate shifts the polarization of the laser by 90°, it is now reflected by the polarizing beam splitter cube onto the position-sensitive photodiode (PSD). The eyepiece and the camera were removed from the microscope and mounted 15 cm above on two separate aluminum columns. Between the dichroic mirror and the eyepiece, a laser-blocking filter was added to protect the eyes of the experimenter. A picture of the complete set-up is presented in Fig. 4b. The microgripper attached to the Kleindiek nanomanipulator driven under the objective is illustrated in Fig. 4c and a closer view of the gripper during operation in Fig. 4d. 3.3. Results and discussion Under an optical microscope a glass plate was attached to a piezoelectric scanning stage. The microgripper was
positioned such that during a scan the stage would slightly touch the bottom of one tip at the end of the 35 lm stroke. We positioned the laser spot with a diameter of less than 5 lm on the 110 lm2 flat area between the tip and the hinge, such that it completely remained on this surface during the entire experiment. This way we ensured that the changes in signal on the PSD were only due to changes in the orientation of the reflecting surface. The stage was repeatedly driven against the arm of the gripper. Fig. 5a shows an optical micrograph illustrating the set-up. In Fig. 5b, the detector signal for one linear stage motion towards the tip over a distance of 35 lm is shown. Towards the end of the scan, just before the stage touches the tip, a short drop in the PSD signal below the 0 mV level is observed, followed by a steep linear rise. The tip first snaps to the glass edge due to adhesion forces, since the measurement was performed in air and under ambient conditions, where a water meniscus is always present. The increase in detector signal is then caused by the changing angle of the tip’s top surface due to the force exerted on its bottom by the glass plate. In order to test whether the signal really measures a change in orientation of the
M.M. Blideran et al. / Microelectronic Engineering 85 (2008) 1022–1026
1025
Fig. 5. (a) An optical micrograph of the set-up with the glass edge pushing against the tip of the gripper; (b) signal detected on the position-sensitive detector during a 35 lm scan.
gripper’s top plane, we lowered the position of the gripper by about 5 lm, thus centering it relative to the glass edge. In this case, the PSD did not record any signal since the scanning stage was hitting the tip at middle height, indicating that our assumptions regarding the detection method are correct.
The bottom of the arm is fixed in all three directions and no motion is allowed; this reflects the fact that the arms do not end at that point but are attached to the massive support structure. The bottom of the puller is fixed in x and y directions, reflecting the motion along the z axis of the pullers during operation.
4. Estimation of the gripping force By simple geometrical considerations, the signal recorded on the PSD can be translated into a tilting angle of the gripper surface. This tilt is associated with a certain torque which is generated by the force applied to the bottom of the tip. Thus for a given PSD signal, the force acting on the bottom of the tip can be determined by simulation. For symmetry and simplicity reasons, only the actuated segment of one arm was considered for finite element simulations. In accordance with the experimental situation the following constraints were applied to the model:
In Fig. 6 a simulation result is presented where the boundary constraints are indicated. The force was applied along an edge of the tip, namely its 3 lm inner length (Fig. 6a). The distribution of the ‘‘von Mises stress” [13] is depicted on the body of the model. The maximum value is almost three orders of magnitude below the critical value of 7 GPa for silicon (Si) when a force of 1 lN is applied. For the same force the model’s displacement out of the z–x plane – the displacement in y direction – is illustrated in Fig. 6b. This corresponds to a tilt of the area between the tips and the hinges on top of which the laser spot was positioned during the
Fig. 6. Simulation results: (a) distribution of the von Mises stress on the model surface; (b) out of plane displacement (tilting) along the top surface of the model.
1026
M.M. Blideran et al. / Microelectronic Engineering 85 (2008) 1022–1026
experiments. For the applied force of 1 lN, the maximum displacement along the y direction is 4 nm, corresponding to a tilt angle of 0.02°. The detection method we have implemented for our microgripper uses basically the same optical beam deflection (OBD) as in lateral scanning force microscopy [14]. ˚ deflection of cantilevers [15] in lateral Detection of sub-A scanning and friction traced to the single atom level [16] has been reported. This means that applying this method to our microgripper could easily resolve forces of 1 lN. 5. Conclusions & Outlook We have shown that the moment of gripping can be detected in real-time by using the OBD method, a very suitable method for investigations in air as well as in liquid since it does not involve electrical potentials or thermal gradients. Furthermore we developed a procedure involving gripping detection and finite element simulations which leads to an estimation of the forces exerted on the object subjected to manipulation. The sensitivity lies below the micronewtons level. Extension of the experiments and simulations to objects, e.g. spheres, of different sizes are required together with calibration of the microgripper and validation of the simulation and experimental method by using a piezoresistive cantilever instead of a sphere and performing the closing experiment.
References [1] A. Albut, Q. Zhou, C.d. Corral, H.N. Koivo, in: Proc. Second VDE World Microtechnologies Congress, MICRO.tec 2003, 2003, p. 507. [2] D.-H. Kim, M.G. Lee, B. Kim, Y. Sun, Smart Mater. Struct. 14 (2005) 1265. [3] S. Fahlbusch, S. Fatikow, Ultramicroscopy 86 (2001) 181. [4] A. Ferreira, C. Cassier, Y. Haddab, P. Rougeot, N. Chaillet, Proc. SPIE 4568 (2001) 112. [5] D.H.B. Wicaksono, A. Andreykiv, K. Kwok, H.-C. Wu, M. Tichem, DIMES & DCMM Summer School 2005 Report,