THE advancements in microelectromechanical systems

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1 434 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 14, NO. 4, AUGUST 2009 Design and Implementation of a Micromanipulation System Using a Magnetically Levitated MEMS Robot Caglar Elbuken, Mir Behrad Khamesee, Member, IEEE, and Mustafa Yavuz Abstract Magnetic levitation of microrobots is presented as a new technology for micromanipulation tasks. The microrobots were fabricated based on microelectromechanical systems technology and weigh less than 1 g. The robots can be positioned in 3-D using magnetic field. It is shown that microrobots can be produced using commercially available magnets or electrodeposited magnetic films. A photothermal microgripper is integrated to the microrobots to perform micromanipulation operations. The microgrippers can be actuated remotely by laser focusing that makes the microrobot free of any wiring. This leads to increased motion range with more functionality in addition to dust-free motion and ability to work in closed environments. The 3-D motion capability of the microrobots is verified experimentally and it was demonstrated that the microgrippers can be operated in a vertical range of 4 mm and a horizonal range of 4 mm 5 mm. Micromanipulation experiments such as pick-and-place, pushing, and pulling were demonstrated using objects with 100 µm and1mmdiameter. Index Terms Magnetic levitation, microelectromechanical systems (MEMS), microgripper, micromanipulation, microrobot. I. INTRODUCTION THE advancements in microelectromechanical systems (MEMS) in the past few years have opened new avenues in many different disciplines. Mechatronics is one of the fields that is greatly affected from this trend. Researchers have been working on methods or systems to have more control on the microdomain. Micromanipulation is such a technique that enables precise positioning of microobjects. Applications of micromanipulation are numerous such as microassembly of mechanical components, handling of biological samples, microsurgery or tribology. Mostly these applications require custom-designed microrobotic stations for ultraprecise manipulation [1], [2]. High accuracy, repeatability, gentle handling, and high maneuverability are some of the most critical design requirements for these systems. The literature reports many microrobotic stations designed for specific applications [3]. Usually these systems have a microscale end-effector that is attached to a large-scale controller. Although the end-effector is in MEMS scale, the connection arms and other moving mechanical assembly put restrictions Manuscript received October 22, 2008; revised March 19, First published June 19, 2009; current version published August 14, Recommended by Guest Editor A. Ferreira. The authors are with the Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L3G1, Canada ( khamesee@uwaterloo.ca). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TMECH on the working range. One possible solution to this problem is having untethered microrobots that have advanced sensing and actuation capabilities [4] [6]. A walking or crawling microrobot with an on-board power supply and controller can move freely to perform the designated task [7]. Although there have been significant advancements in the design and implementation of such microrobots, there are still many problems to be addressed. The main challenge for untethered microrobots is the integration of an on-board power supply [8]. Using batteries significantly increase the size of the robot. On the other hand, other alternative power sources are either not efficient enough or are still in infancy to be used in such microrobots. Using external actuation mechanisms such as magnetic driving forces resolves these problems [9]. The latest research shows that untethered microrobots are also good candidates for in vivo drug delivery applications [10] [12]. The second challenge is the modeling of complex surface forces. Due to scaling laws, as dimensions are scaled down, surface forces become more important than volume forces. Therefore, for successful operation of microrobots, the surface forces should be well-defined and modeled accurately [13]. However, these forces are very complicated since they are greatly affected by the material used, surface properties, and operation conditions such as humidity and temperature. Flying microrobots are good candidates to avoid problems associated with complex surface forces. A flying microrobot hovering in the working domain can provide versatile solutions for many operations. However, the implementation of flying microrobots is much more challenging than walking, crawling, or swimming robots, due to the inherent instability of flying. Designing an efficient and reliable flapping mechanism in microscale is very demanding. The studies about flying microrobots is mostly performed under the title of micro-air-vehicle (MAV) [14], [15]. Today the smallest MAV reported in the literature [16] is far from being used in any microscale applications. MAVs are mostly developed for surveillance and reconnaissance applications. This study presents a new technology for manipulation using flying microrobots, that is a passive, magnetically levitated microrobot. The system has multiple degrees of freedom of control (3-DOF plus gripper) of an untethered MEMS robot in air, and not restricted to a planar surface which most Maglev systems are. The novelty is using a flying MEMS robot for precise micromanipulation tasks. Conventional MEMS fabrication techniques are used to fabricate the microrobot (MEMS robot). Magnetic levitation is used to position the MEMS robot in /$ IEEE

2 ELBUKEN et al.: DESIGN AND IMPLEMENTATION OF A MICROMANIPULATION SYSTEM D using an external magnetic drive unit. Since the power is supplied externally, the robot does not carry a power source or a controller. Due to magnetic levitation, friction and adhesion forces are completely eliminated during the positioning of the MEMS robot. The elimination of any mechanical components such as connection arms or wires increases maneuverability and also yields higher repeatability. Dust-free motion and operability in closed environments are the other key features of the magnetically levitated microrobot. The microrobot has a magnetic section to interact with external magnetic field to generate the levitation force. This magnetic piece is made of either commercial NdFeB magnets or electrodeposited Co Ni Mn P films. Therefore, two versions of microrobots are implemented. As an end-effector, a photothermally actuated polymer (SU-8) microgripper is designed and fabricated. The microgripper can be remotely actuated by laser focusing. The microgripper is a very dexterous tool that can not only be used for simple pushing, pulling, or lifting operations but also can handle relatively complicated pick-and-place tasks. This paper is organized as follows. Section II explains the principle of magnetic levitation and describes the magnetic levitation setup. Section III discusses the design of MEMS robot to be levitated. Section IV demonstrates the magnetic levitation experiments using the microrobot with commercial NdFeB magnets. The MEMS robot is tested for vertical and horizontal trajectories to verify 3-D motion. Similarly, Section V shows levitation results for the microrobot with electrodeposited Co Ni Mn P films. A comparison of the levitation performance of robots using NdFeB and Co Ni Mn P magnets is given. Section VI demonstrates some micromanipulation experiments performed by the levitated MEMS robot. Pick-and-place, pulling, and pushing of microobjects are demonstrated. II. MAGNETIC LEVITATION SYSTEM A. Principle of Magnetic Levitation Magnetic levitation is achieved by compensating the weight of the objects with an equivalent and opposite levitation force. This levitation force originates from the interaction of a magnetic material with the external magnetic field. Earnshaw s theorem forbids levitation using a static configuration of permanent magnets [17]. Therefore, to achieve successful levitation of permanent magnets, the external magnetic field should be dynamically controlled. The magnetic levitation system described in this study achieves it by generating the external field using electromagnets and by continually tuning the magnetic field. When a permanent magnet with a volume magnetization of M and a volume of V is placed in an external field H, the Zeeman energy E z of the system can be calculated as E z = µ 0 (M H) dv (1) V where µ 0 is the absolute permeability. If the object is uniformly magnetized, magnetization can be written as, M = m 0 /V, where m 0 is the magnetic dipole moment. Also for millimetersize objects, it can be assumed that the external magnetic field intensity, H, is constant throughout the volume of the object. This simplifies (1) as ( m0 ) E z = µ 0 V H V = µ 0 m 0 H = m 0 B (2) where B is the external magnetic flux density. Virtual displacement method allows calculation of force if the corresponding energy is known. Using Zeeman energy, the magnetic levitation force F lev can be found as F lev = E z = ( m 0 B) = (m 0 B). (3) In the Cartesian coordinate system, the levitation force can be written as F lev = x x (m 0 B)+ŷ y (m 0 B)+ẑ z (m 0 B). (4) Equation (4) states that to have a force in a certain direction, the magnetic flux density B should be nonuniform along that direction. To compensate the weight of the levitated object, a nonuniform magnetic flux density should be formed along the z-direction so that ẑ z (m 0 B) =mg (5) where m is the mass of the object and g is the gravitation constant. In order to adjust the height of the levitated object, the external magnetic field should be precisely controlled so that (5) is satisfied at that height. Additionally, to achieve equilibrium at a specific angle, the torque on the object should be zero. Therefore, the magnetic torque, τ = m 0 B, should be balanced with a mechanical torque. It should also be kept in mind that any nonuniformity in the magnetic field on the horizontal plane causes a force with x y components. At the height that it is levitated, the object moves along the increasing field direction and comes to an equilibrium at the maximum field point B max. This basic understanding sets the criteria for the design the experimental setup and controller that are explained in Section II-B. B. Experimental Setup This section gives a description of the experimental setup implemented for magnetic levitation with micrometer resolution. A schematic of the setup is shown in Fig. 1. The setup consists of a soft-iron yoke that carries seven electromagnets. The yoke preserves the generated magnetic flux in a loop and helps to achieve higher magnetic fields in the air gap region. The yoke has a square cross section and its height can be adjusted between 65 and 80 cm. These dimensions ensure that the system has enough structural stiffness as a stand-alone setup and also that it can carry the magnetic flux generated by the electromagnets without saturation. The electromagnets generate the magnetic field in the air gap. Each electromagnet has 750 turns and they are connected to each other with a circular disc (pole piece) from the bottom. Since each electromagnet generates a separate B max point along its own axis, this additional piece is required to have a smoother magnetic field and to obtain a single B max point on the horizontal plane. The current applied to the electromagnets is determined by a real-time controller (National

3 436 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 14, NO. 4, AUGUST 2009 Fig. 1. Schematic of the magnetic levitation system. Fig. 2. Picture of the magnetic levitation system with a closeup view to the working domain. Instruments PXI-8186). The power is provided by 40-V voltage supplies (Sorensen DCS40-30E). For continuous 3-D position feedback to the controller, three sets of charge-coupled device (CCD) laser line displacement sensors are used to measure position in each axis. A picture of the setup is shown in Fig. 2 with a closeup view to the sensors that define the working range. A host computer with a remote controller is used for operator inputs. A 5-megapixel camera (Prosilica GC2450) was used for visual feedback to the operator during the micromanipulation experiments. A 635-nm, 200-mW laser diode was used for the noncontact actuation of the microgrippers that is explained in Section III. It is intuitive that the electromagnets positioned above the air gap, as shown in Fig. 2, generate a magnetic field gradient along the vertical axis. However, the magnetic field should be controlled precisely on the horizontal plane for 3-D positioning of the levitated microrobots. The 3-D positioning capability of the system was first verified using finite-element simulations before proceeding with micromanipulation experiments. A solid body of the levitation setup was drawn by ANSYS to simulate the magnetic field in the gap region for various current configurations. Fig. 3 demonstrates the simulated magnetic flux density as surface plots (left) and contour plots (right) for different settings. When currents of equal magnitude, 1.5 A, were applied to the electromagnets, a symmetric magnetic flux density was obtained on the plane at a distance of 80 mm below the electromagnets. The B max point lies on the symmetry axis. To move the B max point in the x-direction, the current applied to the electromagnets along that direction was increased to 2 A, while the current applied to the electromagnets on the other side was reduced to 1 A. Fig. 3(b) shows that B max point shifted in the x-direction by 2.1 cm. Similarly, B max point can be moved in the y-direction by changing the symmetry of the currents applied to the electromagnets.

4 ELBUKEN et al.: DESIGN AND IMPLEMENTATION OF A MICROMANIPULATION SYSTEM 437 electromagnet. Depending on the measurement range of the laser sensors, the range of x ctrl and y ctrl was determined as following using ANSYS simulations 1.8 x ctrl 1.8 (7) 1.8 y ctrl 1.8. (8) Fig. 3. Simulated magnetic flux density on horizontal plane. (a) Same current is applied to all electromagnets. (b) More current is applied to electromagnets in the x-direction. The shifting of B max point can be seen. The levitation system uses a PID closed-loop controller for the vertical levitation and an open-loop controller for horizontal position control. For vertical position control, the position of the levitated object is continuously detected by the laser sensor and is sampled by the A/D converter. During micromanipulation operations it is crucial to minimize the over/undershoots for safer operation. The potential over/undershoots should be avoided when the microrobot is moved around delicate objects. Since gentle handling is more important than speedy motion, a set point ramping controller was implemented. More information about the controller design of the magnetic levitation system can be found in [18] and [19]. If the object is moved vertically along the symmetry axis of the air gap, the same control current is applied to all of the electromagnets. When horizontal motion is required, the amount of current is increased for the electromagnets in the desired direction of motion, while keeping the summation of currents constant. The governing equations for the horizontal motion were derived in [20] as I 1 = I(1 + x ctrl )(1 + y ctrl ) I 3 = I(1 x ctrl )(1 + y ctrl ) I 4 = I(1 + x ctrl )(1 y ctrl ) I 6 = I(1 x ctrl )(1 y ctrl ) I 2 = (6I I 1 I 3 I 4 I 6 )( y ctrl ) 2 I 5 = (6I I 1 I 3 I 4 I 6 )( y ctrl ) 2 I 7 = I (6) where x ctrl and y ctrl are the current ratio factors changed by the operator commands and I i is the current applied to the ith It can be seen that when x ctrl = y ctrl =0, the same current is applied to the electromagnets and the robot is aligned with the central axis of air gap. In short, the z position of the object is adjusted by the vertical levitation force that is controlled by changing the total current applied to the electromagnets. On the other hand, to adjust the horizontal position (x, y) of the object, the B max point is moved by changing the current ratio of the electromagnets. In a previous study of the authors [21], it was demonstrated that a damping mechanism is highly required to improve the positioning accuracy during levitation of millimeter scale objects. The eddy current damping mechanism was preferred because of its ease of employment and noncontact operation. In addition, eddy current damping does not require a change in the controller algorithm or does not increase the cost or complexity of the system. Eddy-current damping is applied to the system by placing nonferromagnetic, conductor (aluminum) plates underneath the levitated object. During the oscillations of the levitated object, a changing magnetic field is generated in the gap region. The time-varying magnetic field has two sources: 1) the change of the field generated by the electromagnets (when position of the object changes, the controller adjusts the currents supplied to the electromagnets) and 2) the self magnetic field of the moving microrobot. If a conductor is placed in the varying field, circulating eddy currents are formed. The direction of the current is such that, magnetic field generated by this eddy current opposes the change in the field itself. Consequently, the conductor plate serves as a damper to the levitating magnet. A detailed derivation of damping coefficient in the case of the levitation of circular permanent magnets is given in [21]. For the experiments in this study, a 6061-Al disc of 3 cm radius and 4 mm thickness is used as a stage that also servers as the eddy current damper. III. MICROROBOT DESIGN The magnetic levitation setup explained in Section II-B generates a vertical magnetic field gradient. Any magnetized object placed in this field aligns itself with this gradient and determines the type of levitation: vertical or horizontal, as shown in Fig. 4. For the microrobots used in this study, both in-plane and out-of-plane samples are used when fabricating the magnetic section of the robot. Out of many possible configurations, the one shown in Fig. 5 is selected because of the line displacement laser sensors. The laser sensors used for position detection requires line-of-sight. Therefore, if the microrobot has a very thin cross section along any of the dimensions, it loses the sight of the laser very easily during the 3-D motion. The microgripper is attached to the bottom sample, as shown in Fig. 5. In this way,

5 438 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 14, NO. 4, AUGUST 2009 Fig. 4. Vertical and horizontal levitation. stiffness method. The relationship between finger deflection and gripping force is derived for the microgripper shown in Fig. 5. The procedure described below can be applied to microgrippers with different geometries. The direct stiffness method divides the given structure into beam elements with two nodal points at each ends. For the microgripper shown in Fig. 5, the structure is simplified using a three-element model, as shown in Fig. 6. Only half of the gripper is analyzed because of the symmetry. For a single element under transverse displacement, axial deformation, and rotation, the relationship between the displacement and force can be written as F x1 u 1 F y 1 υ 1 M F = KU, F = 1 ; U = φ 1 (9) F x2 F y 2 M 2 u 2 υ 2 φ 2 Fig. 5. Schematic of the microrobot with in-plane and out-of-plane magnetized samples. the microgripper is protected from potential damages that can occur during launching and landing. The microgripper attached to the microrobot is a photothermally actuated SU-8 microgripper [22]. When a laser beam is focused on the circular region of the microgripper, the heat is absorbed by the device. The flow of heat along its arms results in thermal expansion that, in turn, opens the gripper fingers. The amount of finger opening can be controlled by changing the power of the laser beam. A 635-nm, 200-mW laser diode was used as the laser source. The use of SU-8 allows higher deflections with relatively lower temperatures (<480 K) compared to metal or silicon microgrippers. The benefit of photothermal actuation is the noncontact operation of the fingers and the ease of implementation for the passive microrobot. Since the gripper actuation power as well as the levitation power are provided externally, depending on the application, the microrobot itself can be minimized in the limit of the available microfabrication techniques. The objects manipulated in this study are cylindrical samples with a diameter of 100 µm and 1 mm. Therefore, various microgrippers are designed with different initial finger openings [22]. The microgrippers are 4 mm in length and 3 mm in width. The fabrication and characterization of photothermal microgrippers is demonstrated in [22]. In general, finger deflection, gripping force, response time, and repeatability are the main criteria for microgripper design. For the magnetically levitated robots with microgrippers, finger deflection, and gripping force are the most important parameters. The gripping force is a function of the finger deflection. Therefore, an analytical model governing these two parameters is critical for the design of microgrippers. Such a model can be developed using the direct where F ki is the force applied along the k-direction at node i, M i is the moment at node i, and u i, υ i, and φ i are the displacement along x, displacement along y, and rotation around z, respectively. K is a standard 6 6 matrix representing the equilibrium compatibility of the beam element as a function of geometrical parameters [23]. The three beams in Fig. 6 are tilted, therefore K matrix should be transformed to local coordinates using the standard transformation matrix R as K global = R T K local R. (10) The beam elements are connected to each other at node 2. In addition, nodes 1 and 4 are fixed, and thus, have zero DOF. Then, the force deflection equation for the frame can be written as F x2 u 2 F y 2 ( ) υ 2 M 2 K1d + K = 2a + K 3a K 2b φ 2 (11) F x3 F y 3 M 3 K 2c K 2d u 3 υ 3 φ 3 where K ia,b,c,d are 3 3 submatrices of K i given by ( ) Kia K K i = ib. (12) K ic K id In (9), F matrix shows the external loads applied on the structure, K matrix gives the stiffness of the frame, and U matrix gives the corresponding deflections. In order to find the relationship at the tip, a gripping force (F grip ) is applied at the tip (node 3) in the x-direction as shown in Fig. 6. Then, F x3 = F grip whereas all other elements of F matrix is zero. Solving (11) for u 3 gives the deflection of the tip in the x-direction. By using a range of F grip values, the dependence of F grip is determined for the deflection range of the microgripper used in this study. The results are plotted in Fig. 7. The microgripper fingers can achieve deflections up to 100 µm. The gripping force is linear with the increasing finger deflection. For a maximum deflection of 100 µm per finger, the gripping force is determined as 23 µn for the dimensions specified in Fig. 6.

6 ELBUKEN et al.: DESIGN AND IMPLEMENTATION OF A MICROMANIPULATION SYSTEM 439 Fig. 6. Finger deflection force model of the microgripper. (a) Frame representation of the microgripper. (b) Schematic drawing of the frame with numbered nodes and elements. Fig. 9. Microrobot with electrodeposited Co Ni Mn P films. Fig. 7. Fig. 6. Fig. 8. Finger deflection gripping force model for the microgripper shown in Microrobot with commercial NdFeB permanent magnets. For the magnetized samples used for the microrobot, two different materials are selected: commercial NdFeB permanent magnets and electrodeposited Co Ni Mn P films. The two microrobots using these magnets are shown in Figs. 8 and 9, respectively. They are placed on a penny to give a sense of the size to the reader. NdFeB hard magnets are one of the most powerful magnets commercially available. They have very high magnetization values, and hence, can be operated with low external magnetic field (5). However, it is hard to produce these magnets in any given geometry. On the other hand, electrodeposition of hard magnets allows the fabrication of microrobots in a certain geometry that depends on the shape of the substrate. The magnets shown in Fig. 9 are electrodeposited on silicon substrate at room temperature [24]. The main limitation of the electrodeposited Co Ni Mn P magnets is lower magnetization values that require a stronger external flux density. The levitation performance of microrobots using NdFeB magnets and Co Ni Mn P magnets is discussed in Sections IV and V, respectively. When an in-plane and out-of-plane magnetized sample were used together, as shown in Fig. 5, the mass of the magnetic portion of the microrobot is increased. Then, if the total weight of the microrobot is considered, the mass of the microgripper and the levitated object can be neglected since the microrobot has a massive magnetized section. Therefore, this configuration resolves the problems associated with the weight change during manipulation experiments. Otherwise, an adaptive controller is required to account for the change in the total weight of the levitated object. IV. LEVITATION OF MICROROBOT WITH COMMERCIAL NDFEB MAGNETS The levitation experiments of the microrobot with NdFeB magnets were performed using the robot shown in Fig. 8. This

7 440 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 14, NO. 4, AUGUST 2009 Fig. 10. Vertical motion of the microrobot with NdFeB magnets. Fig. 11. Horizontal motion trajectory of the robots. robot has a total weight of 0.62 g. The bottom permanent magnet is a 4 4 1mm 3 in-plane magnetized magnet, whereas the top one is a mm 3 out-of-plane magnetized magnet. The direction of the magnetization snaps these magnets together and holds them stable, as shown in Fig. 8. The microgripper was attached using an epoxy glue. The gripper has an initial finger opening of 80 µm. When 3 V is applied to the laser diode, an incident laser beam of 65 mw is focused on the gripper. This gives a finger opening of 150 µm, which ensures that this microrobot can be used for manipulation of objects with sizes between 80 and 150 µm. The levitation performance was tested for both vertical and horizontal motion. First, a vertical trajectory composed of step and ramp inputs was applied. The microgripper was moved in a vertical range of 4 mm. The input trajectory and the measured position of the microrobot is shown in Fig. 10. The position data shown on y-axis are measured from the bottom of the electromagnets. It is seen that the levitated microgripper can successfully follow the given trajectory with an rms positioning error of 13.2 µm. The over/undershoots are suppressed for both 2 and 4 mm step commands. Second, the horizontal motion performance of the robot was tested by moving the microrobot along a predetermined trajectory with a range of 4 mm 5 mm. The path for horizontal motion experiments is illustrated in Fig. 11. The robot is moved from Home position to point D through points A, B, and C. The robot paused for 10 s at each checkpoint. The x ctrl and y ctrl parameters were changed from 0.03 to 0.03 to achieve this motion range. The recorded experimental position data are Fig. 12. Horizontal motion of microrobot with NdFeB magnets. (a) Measured x position of microrobot with commercial magnets. (b) Measured y position of microrobot with commercial magnets. (c) Measured x y trajectory of microrobot with commercial magnets. shown in Fig. 12. The first two plots demonstrate the individual measurements from x-axis laser and y-axis laser as a function of time, whereas the third plot shows the recorded position of the microrobot on the horizontal plane. It can be observed that the microrobot can move along the horizontal path. The rms positioning error was measured as 38.1 µm. However, Fig. 12(c) also shows that when waiting at a checkpoint, the microrobot experiences horizontal wobbling. In fact, this behavior originates from the slight rotation of the microrobot around itself. The microrobot has a square-shaped body. Therefore, any spin of the microrobot is interpreted as a position change by the horizontal laser sensors. If the body of the microrobot were made using cylindrical magnets, the misreading of the lasers could be avoided. It is also worth mentioning that since an open-loop controller was used for horizontal position, this laser misreading does not affect the levitation performance. V. LEVITATION OF MICROROBOT WITH ELECTRODEPOSITED CO NI MN P FILMS The same levitation experiments presented in Section IV were repeated for the robots with electrodeposited Co Ni Mn P film. The robot has two mm 3 silicon substrate coated with

8 ELBUKEN et al.: DESIGN AND IMPLEMENTATION OF A MICROMANIPULATION SYSTEM 441 Fig. 14. Vertical motion of the microrobot with Co Ni Mn P magnets. Fig. 13. Magnetic flux density measurements performed on the modified levitation setup. Co Ni Mn P magnetic films (see Fig. 9). The same microgripper explained in Section III was used again. The total weight of the robot was measured as 0.09 g. The initial experiments revealed that the magnetic field gradient generated by the magnetic levitation setup was not high enough to levitate the electrodeposited magnets. It is mainly because of the lower magnetization value of Co Ni Mn P films (0.26e6 A/m) compared to NdFeB magnets (1e6 A/m). In addition, the Co Ni Mn P magnetic films have to carry the nonmagnetic silicon sample; however, the NdFeB magnets are only carrying their own weight. The levitation equation along the vertical direction is given in (5). Since magnetic dipole moment is constant, the levitation force is proportional to the gradient of the external magnetic flux density. Therefore, to levitate the electrodeposited films, the external flux density should be increased. Detailed explanation about the calculation of the external flux density is given in [25]. In order to increase the gradient of the flux density, two modifications were made to the magnetic levitation system: 1) one more electromagnet was integrated at the bottom of the pole piece and 2) a large permanent magnet was placed in the magnetic yoke assembly. To verify the improvement in the magnetic flux density, the magnetic field in the gap region was measured using a Lakeshore 421 Gaussmeter. The gaussmeter probe was scanned through the motion range of the microrobot using a five-joint robot arm, as shown in Fig. 13. The additional electromagnet and permanent magnet can also be seen in this figure. The magnetic field measurements revealed that these two changes increased the magnetic flux density more than ten times (from 0.9 to 11 G/mm) [25]. The experimental results of vertical and horizontal positioning using the modified setup is shown in Figs. 14 and 15, respectively. The rms positioning error for vertical motion was measured as 34.3 µm, although the robot can follow the given path. On the other hand, for the horizontal position control an rms position error of 212 µm was measured from the recorded experimental data shown in Fig. 15(a). Similar to the horizontal levitation results shown in Section IV, the self-spin of the microrobot causes this error. It can be seen that in the case of electrodeposited magnetic films, the spinning is much greater Fig. 15. Horizontal motion of microrobot with Co Ni Mn P magnets. (a) Measured x position of microrobot with commercial magnets. (b) Measured y position of microrobot with commercial magnets. (c) Measured x y trajectory of microrobot with commercial magnets. than the commercial magnets. This can be explained with the nonuniform magnetization of the deposited films compared to the commercial magnets. Demonstrating the levitation experiments for the microrobots composed of electrodeposited magnets and commercial magnets, it is crucial to make a comparison between the two systems. The microrobots using the electrodeposited thin films are much more cost effective. In a single batch using a 4-in silicon wafer, tens of robots can be produced. In addition, the use of a MEMS-compatible deposition process allows the design of

9 442 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 14, NO. 4, AUGUST 2009 TABLE 1 COMPARISON OF MICROROBOTS USING DIFFERENT MAGNETIC SAMPLES various levitating MEMS robots using different end-effector tools. After the MEMS robot is manufactured, it can be coated with the magnetic film using the electrodeposition bath given in [24] and can be levitated. On the other hand, the electrodeposited films have inferior magnetic properties compared to the commercial magnets. Stronger magnetic field should be generated by the magnetic drive unit, which requires higher currents applied to the electromagnets. This generates excessive heating of the electromagnet assembly and affects the system properties. Also, the positioning error is more than two times higher for the microrobots using electrodeposited thin films than the ones with commercial magnets. The comparison between the microrobots using electrodeposited thin films and commercial magnets is summarized in Table I. When the levitation performance of microrobots with commercial magnets is considered, it is first observed that precision is greatly improved. This can be explained by the uniform magnetization of the commercial permanent magnets. The use of powerful commercial magnets also requires less current and reveals the problems with excessive heating. When the additional eighth electromagnet is removed, a larger air gap is obtained for the levitating microrobot. Another advantage of using commercial magnets is that the microrobot can be used for the manipulation of magnetic samples as well. The levitation force is a function of the gradient of magnetic flux density, as derived in Section II-A. When powerful magnets are used, the required magnetic field gradient is smaller. Therefore, objects that have smaller magnetic dipole moments will not be levitated by the drive unit. In this way, the microrobot can be operated close to magnetic objects, as well. The downsides of the microrobot with commercial magnets is that the dimensions of the robot is determined by the sizes of the permanent magnets available in the market. Also, the flexibility in terms of the shape of the robot is very limited. Although custom-designed magnets can be used, it substantially increases the cost of the robot. The attachment of the microrobot with the commercial magnet can also be an issue to be addressed if the size of the microrobot is further reduced. VI. MICROMANIPULATION EXPERIMENTS In this section, some micromanipulation experiments are demonstrated. It has been shown that the microrobots with NdFeB magnets can achieve a higher positioning resolution, therefore these robots are used for the manipulation tasks that require high precision. The position of the levitated microrobot was controlled by the operator through visual feedback. The high-magnification camera was used for real-time imaging of the robot and its surrounding. Movies were recorded during the operation of the microrobot. Sequential snapshots were taken from the recorded movies to demonstrate the gripping, lifting, and pulling of some microobjects. All operations were operated on a surface with 1 cm 1 cm grid lines for comparison of the size. First, the actuation of the microgripper fingers is shown in Fig. 16. In Fig. 16(a), the fingers are in closed position. When the laser is focused on the circular spot, the fingers are opened, as shown in Fig. 16(b). The laser spot is aligned with the microrobot using x y microstages. The z-axis adjustment is done manually. Fig. 17 shows the pick-and-place of a cable strip with a diameter of 1 mm. Since the object is large, microgrippers with long arms and large finger opening was used for this experiment. The fingers are opened during the grasping and releasing of the cable strip. One end of the cable was always left on the stage, because it is hard to balance the weight of a large object equally during the operation. First, the microgripper grasps the cable standing on the edge of a stage, as shown in Fig. 17(a). Then, the microgripper is moved upward and toward left [see Fig. 17(b) and (c)] while holding the object. When it is required to release the cable, the finger arms are opened with laser actuation, and finally, the microrobot is moved away from the object [see Fig. 17(d)]. The microrobot with the microgripper can be used for various other types of manipulation tasks. Different kind of manipulation schemes such as lifting and pushing are also tested. An electrical wire with a diameter of 100 µm is positioned at the edge of a stage. Fig. 18 shows the images during the lifting of the wire. The microrobot first approaches to the wire from

10 ELBUKEN et al.: DESIGN AND IMPLEMENTATION OF A MICROMANIPULATION SYSTEM 443 Fig. 16. Sequence of images demonstrating the operation of the levitating microgripper. Fig. 17. Sequence of images demonstrating the manipulation of cable strip. Fig. 18. Sequence of images demonstrating the microgripper lifting a piece of wire.

11 444 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 14, NO. 4, AUGUST 2009 Fig. 19. Image demonstrating the microgripper pulling a piece of wire. underneath and aligns its tip with the wire [Fig. 18(a)]. Then, the wire is carried by the side of one of the fingers [Fig. 18(b)]. In Fig. 19, it has been shown that the same wire is pulled by the finger tips without using the laser. This appears to be the easiest type of manipulation. The wire is moved using the inside of the fingers. Larger objects can be manipulated by pulling them; however, gripping can only be used for certain sizes of objects. The motion range of the fingers and the contact area of the finger tips put a limit on the size of the objects to be gripped. Therefore, the microgripper should be replaced with a different one, when objects with various dimensions need to be moved. It has been realized that, for some objects, lifting can be more convenient rather than gripping. The attempts to grip the electrical wire failed because of the small contact area of the microgripper finger tips. Another challenge was the adhesion of the gripper fingers with the gripped object. This was addressed by releasing the objects at an angle as suggested by Arai et al. [26]. It is also worth mentioning that when the size of the object is reduced, the adhesion becomes more problematic. The adhesion is mainly caused by van der Waals and electrostatic forces [27]. Due to the scaling laws, weight of the object becomes negligible and the adhesion forces dominate as the gripped objects become smaller. Surface treatment of the gripper fingers for hydrophobic coating and fabricating serrated finger tips can overcome the adhesion problem. The serrated finger tips will reduce the contact area between the gripped object. In this study, the minimum feature size was 10 µm because of the resolution of the transparency mask used during photolithography [22]. Chromium masks can be used to achieve the serrated structures that require higher lithography resolutions. Finally, the levitation of the microgrippers was tested inside closed environments. The microrobot was placed inside a sealed pyrex chamber, as demonstrated in Fig. 20. The 3-D trajectories discussed in Section IV was applied and the same position recordings were observed. These experiments showed that the system is successful for operation in closed environments. It is worth mentioning that the chamber itself should not be a conductor for the magnetic field to penetrate inside it. In addition, for the current system, because the position measurements are performed using line-of-sight laser sensors, the chamber should be transparent. However, if another measurement tech- Fig. 20. Levitation of the microrobot in a closed transparent chamber. nique without line-of-sight requirement is used, the system can also be applied for manipulation in opaque chambers or closed channels, as well. The current magnetic levitation system only has 3 DOF; therefore, the microgripper should be aligned with the object to be manipulated so that no rotational motion is required. This can be overcome if a rotational control can be achieved on the microrobot by using additional electromagnets. Also the positioning precision of the setup can be enhanced by more precise laser measurements. However, the inherent time delay caused by the actuation of the electromagnets put a limit on the controller speed. VII. CONCLUSION This paper has shown micromanipulation of objects using a magnetically levitated microrobot. The microrobot was positioned in 3-D using magnetic fields. A magnetic levitation setup was implemented to generate the magnetic field that forms the levitation force. The magnetic field was precisely controlled to enable accurate positioning of the microrobot. The microrobot was made of either NdFeB magnets or Co Ni Mn P magnetic films. It was shown that the microrobots with NdFeB magnets lead to a higher positioning accuracy, and therefore, used for the micromanipulation experiments. A photothermal polymer microgripper was attached to the microrobot to perform various operations. The microgripper can be actuated by laser focusing, so the microrobot can operate in a noncontact manner. A wire of 100 µm diameter and a cable strip of 1 mm diameter were repositioned successfully using the microrobot. The operation of the microrobot in a closed chamber was also demonstrated that can open the path for micromanipulation of biological samples and hazardous items in enclosed environments. ACKNOWLEDGMENT The authors would like to thank Dr. C. L. Ren and Dr. L. Gui from the Waterloo Microfluidics Laboratory at the University of Waterloo for their kind assistance during the fabrication of microgrippers.

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Caglar Elbuken received the B.S. degree in electrical and electronics engineering from Bilkent University, Ankara, Turkey, in 2004, and the Ph.D. degree in mechanical engineering from the University of Waterloo, Waterloo, ON, Canada, in He is currently a Research Associate in the Waterloo Microfluidics Laboratory, Department of Mechanical and Mechatronics Engineering, University of Waterloo. His current research interests include microfluidics and microrobotics with a focus on biological manipulation and detection Mir Behrad Khamesee (M 04) received the M.S. and Ph.D. degrees from Mie University, Tsu City, Japan, in 1996 and 1999, respectively, both in mechanical engineering (mechatronics). From 1999 to 2002, he worked in industry in Japan. From 2002 to 2003, he was a Postdoctoral Researcher at the University of Alberta, Edmonton, AB, Canada. In March 2004, he was appointed as an Assistant Professor at the University of Waterloo, Waterloo, ON, Canada, where he will become an Associate Professor in July He is currently the Director of the MagLev Microrobotics Laboratory at the University of Waterloo. His current research interests include design, modeling, and control of advanced mechatronics systems, particularly microrobotic magnetic levitation and electromagnetic dampers. There have been several news releases on his research outcomes in magazines and newspapers such as The Economist and The Record. He has received a scholarship from the Japanese Government. Dr. Khamesee is a member of the ASME. He is involved in conferences program committees, has organized several sessions at international conferences, and is a Technical Reviewer for several IEEE journals. Mustafa Yavuz received two Ph.D. degrees, one in materials engineering in 1996, and one in applied physics in 1995, from the University of Wollongong, Wollongong, Australia. He was a Postdoctoral Fellow at Tohoku University, Sendai, Japan, where he was appointed as an Assistant Professor in September Between 1998 and 1999, he was a Principal Project Engineer at Accelerator Technology Corporation, Bryan, TX. Between 1999 and 2004, he was a Texas Engineering Experiment Station (TEES) Research Assistant and an Associate Professor at Texas A&M University. He is currently the Director of the Nano/Micro-Systems Research Laboratory, University of Waterloo. His current research interests include design, fabrication, and characterization of nanoscale materials (thin films, nanowires, and nanotubes) for micro- and nanoelectromechanical systems (N/MEMS). Dr. Yavuz is a Fellow of the American Society of Materials and the American Physical Society. He received the U.S. National Mechanical Engineering Honor Society (Pi Tau Sigma) Award in 2003 and the Japan Society for the Promotion of Science (JSPS) Fellowship in 2006, 2007, and 2008.