Remotely controllable mobile microrobots acting as nano positioners and intelligent tweezers in scanning electron microscopes (SEMs)

Transcription

1 Proceedings of the 2001 IEEE International Conference on Robotics & Automation Seoul, Korea May 21-26, 2001 Remotely controllable mobile microrobots acting as nano positioners and intelligent tweezers in scanning electron microscopes (SEMs) Ferdinand SCHMOECKEL, Heinz WÖRN Institute for Process Control and Robotics (IPR), Computer Science Department, Universität Karlsruhe (TH), Kaiserstraße 12, D Karlsruhe, Germany, Abstract In the scanning electron microscope, many studies require micromanipulation tools that can be flexibly and comfortably used for the handling and positioning of objects ranging from few millimeters down to fractions of microns. In this paper, some aspects of the control of small mobile microrobots by teleoperation are addressed facing the special behavior of their piezoelectric actuators and the challenge of handling tasks in ranges of very different orders of magnitude (100 nm 100 mm). In order to give the teleoperated tweezers a kind of low-level intelligence, a control loop using the SEM as a sensor system is being developed. In different open-loop handling experiments, the employed 6D-mouse proves satisfactory as an intuitive user interface. Keywords: microrobots, micromanipulation, scanning electron microscopy 1. Introduction Existing facilities for manipulation tasks in the micron range are rather large, very expensive, and usually tailored to a specific assignment. Flexible robot systems for various applications requiring precise handling of microscopic objects in a scanning electron microscope (SEM) are completely missing. Direct-driven few cm³ small robots are likely to help solving this problem. In contrast to stationary robot systems as employed in the Nano Manipulation World developed by Sato et al. (1995) [1], at our Institute, directdriven few cm³ small mobile robots have been developed. These piezoelectric microrobots can perform highprecision manipulations with a precision up to 20 nm and transport the gripped objects at speeds up to 3 cm/s employing a slip-stick principle [2]. They work within the Flexible Microrobot-based Micromanipulation Station (FMMS) [3] on the stage of a light microscope or inside the vacuum chamber of an SEM. Automatically controlled with the help of visual sensors, these robots can carry out micromanipulation tasks and may free humans from having to handle tediously very small objects manually. Munassypov et al [4] introduced the microrobots motion control approach basing on vision sensors similar to Allegro and Jacot, 1997 [5]. One can distinguish between coarse motion (i. e. navigation of the robot over a long distance) and fine motion (i. e. manipulation of parts under a microscope). The microscope (here: an SEM) can act as the local sensor system monitoring the robots fine motion. For being employed inside the vacuum chamber of a conventional SEM, a special prototype, MINIMAN III, has been implemented (Figure 1). Like all mobile microrobots developed for the FMMS so far, it consists of a mobile positioning unit that can move across a smooth surface in three degrees of freedom (DOF) and a manipulation unit here, a steel ball carrying the endeffector that is mounted on the platform. Both the positioning unit and the manipulation unit are each driven by three tube-shaped piezoelectric legs. By bending these legs coordinately, the robot and the manipulator ball respectively can be moved in any direction. Microgripper Piezoelectric leg Figure 1: MINIMAN III prototype Manipulation unit Mobile positioning unit /01/$ IEEE 3909

2 2. Intelligent tweezers 2.1. Teleoperation Numerous experiments and studies in various research fields going on at present require the manipulation of objects smaller than 100 µm. These experiments often have to be performed in the SEM due to the tiny dimensions of those objects, which are usually also fragile and hard to handle. An example can be taken from the investigation of environmental pollution. Figure 2, left, shows 10 µm dried down acid rain crystals on a damaged leaf. It would be very interesting for biologists to grasp these particles and carry them to a defined surface that is suitable for examining them by x-ray analysis. Further demands for micromanipulation within the SEM exist for instance in the research fields of fossils (Figure 2, right), radioactive particles and pest control. Figure 2: Acid crystals dried-down on a leaf (left) and fossil micro creature (right). Source: 2nd intermediate report of the ESPRIT project No MINIMAN Flexible, mobile microrobots are helpful tools in such situations. They can act as a pair of tweezers that the operator can control by teleoperation. When necessary, the robot s endeffector can be moved automatically from the waiting position into the microscope s field of view using a global sensor system. Because of their small size, two or more mobile robots can be employed when complex handling tasks are required. For example just the task of laying down a grasped microobject often requires a helping hand when the object remains sticking to the gripper. It is desirable that the tweezers have a kind of intelligence to be able to avoid obstacles and perform difficult tasks as the releasing of objects automatically. This can be achieved by using the sensor system described in chapter 2.2. The flexibility of the microrobots is also advantageous in the assembly of hybrid microsystems, especially at development stages when single parts have to be handled and the final tolerances are not yet determined. As an example, Figure 3 shows the teleoperated microassembly of a micro gear with the help of a 6D-mouse (Figure 4) and an additional lateral miniature camera. 500 µm Figure 3: Mounting a 500 µm wheel of a planetary micro gear by teleoperation with a 6D-mouse: lateral camera images and an SEM image (lower left) The 6D-mouse is a very intuitive and sensitive user interface. Besides the transition data, it directly provides the desired turning angles of the manipulator. To move the manipulation unit accordingly, the bend direction of each piezoleg must be calculated from these turning angles. This is shown in Figure 4 (right) for the rear leg and the yaw axis (b), exemplarily. b l m ey ex ez Figure 4: 6D-mouse [6] (left) and sketch of the piezolegs and the manipulator ball (right) The required bend direction m of each piezoleg is calculated by transferring the coordinates of all three ball axes into the coordinate system of the piezoleg (ex,ey,ez). The absolute values of these vectors are the desired turning velocities around these axes. The resulting bending vectors of the leg can be summed up, since the rotation axes are perpendicular to each other [7]. 3910

3 Compensation of the anisotropic bending behavior of the piezolegs The legs are tube-shaped piezoelectric actuators. The outer electrode of these tubes is divided into four parts (see Figure 5). Thus, the legs can be bent in all directions by applying voltages (±150 V) between one or more of the outer electrodes and the inner one. In Figure 5, an example for such a bending situation is shown in the cross section of a leg. For a desired bending direction the necessary voltage of the electrodes can easily be calculated from the corresponding components of this vector. However, in diagonal directions, four electrodes (two contracting and two expanding) cause the bending while in horizontal and vertical direction only two electrodes are involved. Therefore, the diagonal component of the calculated vector must be weakened by the factor Otherwise, the bending is distorted towards the diagonal causing e.g. a tilting of the turning axes of the manipulator ball. The vector addition for this compensation is shown in Figure 5 resulting in a corrected vector. Using its components for driving the electrodes results in a leg bending in the desired direction. Desired direction Corrected vector Electrode Diagonal component: weakened by 0.5 Figure 5: Photo of a piezoleg and sketch of its cross section Gear change Some manipulating experiments as shown in Figure 3 with relatively large microobjects have been performed with open-loop control. In this object range above 100 µm, the fast slip-stick moving principle is employed that is also used for driving the robot over long distances across the microscope stage. In this mode, the force (and torque, respectively) applied on the knob of the 6Dmouse, which is a measure for the desired speed of the robot s movement in the corresponding axes, determines the frequency of microsteps. By an intuitive gear setting, the frequency range can be selected by the operator who thus can choose the fineness of the movement or, conversely, the sensitivity of the mouse. The resolution of this movement is approximately the 6-µm step width of the piezolegs. In order to proceed towards smaller and smaller objects to be handled or to nano positioning tasks as described in chapter 3, a next lower gear can be chosen. In this sneak mode, the robot does not perform one microstep after the other any more, but shifts on the spot only by slowly bending its legs within their 6x6-µm² range. The achievable resolution of this motion is ca. 20 nm resulting from the resolution of the D/A converters. Again, the user controls the robot s speed with the 6D-mouse determining how fast the voltages of the piezos electrodes are changed. If the robot reaches the boundary of this sneak area, it stops and the operator is informed by an acoustic signal. If he keeps on pushing the mouse into the same direction, the robot performs half a microstep after a short waiting period starting again in the middle of its new sneak area. By this motion and teleoperation principle, the robot system covers an operating range of several orders of magnitude while the sensitive and intuitive user interaction is always maintained supported by the quickly adjustable magnification of the SEM Sensor-based teleoperation As shown above, remotely controlled by the 6D-mouse in open loop, the described robot is already fully employable as a pair of micro tweezers that can be used for simple pick and place operations. This is because in many cases the human operator is able to compensate small inaccuracies due to the slip-stick principle at once. However, tasks like turning a micro object around its vertical or horizontal axis are rather difficult because of the long distances the robot has to cover for this movements in relation to the small field of view of the microscope image. Hence, a closed-loop control is desirable to assist the user in teleoperation or make automation possible including path planning and collision avoidance. As already mentioned and described in [7], the sensor system proposed for closed-loop control consists of a global and a local part. The global sensor system uses the image of a CCD camera supervising the microrobot s workspace, i. e. the microscope s XY-stage. The real-time image processing software detects the global position and orientation of the robot and its manipulator resulting in a positioning accuracy of the tool center point better than 0.5 mm. This is sufficient to navigate a robot into the field of view of the microscope. 3911

4 SEM stage pos. ( scanning area) Position of ROI SEM Input 6D-mouse Global control Yaw angle between manipulator and platform Line scan position Nominal gripper position Angle around roll axis Disturbances Gripper position in ROI Measured position in scanning area Nom./act. comparison Moving vectors Actual gripper position Electron beam triangulation line Closed-loop control Figure 6: Control loop of the intelligent tweezers The actual manipulations with microobjects are monitored by the local sensor system that again consists of two parts. The real-time processing of the SEM image can provide accurate two-dimensional position information within a region of interest (ROI) a small part of the 4096²-pixel area that can be scanned by the electron beam. Additionally, by so-called electron beam triangulation information about the gripper s height can be obtained with a line scan on a cathodoluminescent coating of the gripper that is being observed by a lateral optical miniature microscope (cf. [7]). Miniature light microscope ROI Luminescent lines on the gripper for height measurement Line scan Figure 7: Gripper, ROI and triangulation scan in the scanning area of the electron beam Exemplarily, a configuration of the scene in the scanning area of the electron beam is shown in Figure 7. Figure 6 shows the proposed control loop of the SEM robot system. It is divided into four parts. The input determines the nominal gripper position, which is given by the 6D-mouse in teleoperation. The actual control loop compares the nominal position with the measured values. They are provided by the fusion of sensor data of the triangulation module (height), the image recognition module (2D information from the SEM image ROI) and the global module. The latter provides the turning angle of the microgripper around its roll axis. Furthermore, it confirms that the yaw angle between the robot platform and its manipulator does not change due to inaccuracies of the slip-stick motion. This is taken into account when calculating the robot s new moving vectors. Finally, the measured position of the gripper is also used for readjusting the parameters of the SEM when necessary, such as the triangulation line scan, the position of the ROI and the position of the SEM stage which corresponds to the area that can be scanned by the electron beam. 3. Positioning of SEM samples by mobile micro robots Simply by exchanging the manipulation unit, the microrobot MINIMAN III can act as a small positioning stage with five DOF. In Figure 8, the microgripper was replaced by a hemisphere with a standard sample holder in its center. 3912

5 Especially in the sneak mode, the robot represents a stable high resolution positioning device. Because of the direct transmission of the 6D-mouse s axes to the robot axes this setup allows a quick and convenient approaching of the desired sample position and orientation. Figure 8: Miniman III as sample positioning unit Sample Simply by using two Miniman III robots one equipped with the hemisphere and one with a pair of tweezers complex handling tasks with altogether ten DOF are possible. microrobotics ; Proc. SPIE 2906, Boston, 1996, pp [3] Fatikow, S.; Seyfried J.; Fahlbusch St.; Buerkle, A.; Schmoeckel, F.: A Flexible Microrobot-Based Microassembly Station, Journal of Intelligent & Robotic Systems 27, pp , 2000 [4] Munassypov, R.; Grossmann, B.; Magnussen, B.; Fatikow, S.: Development and Control of Piezoelectric Actuators for the Mobile Micromanipulation System, Proc. ACTUATOR, pp , Bremen, 1996 [5] Allegro, S.; Jacot, J.: Automated Microassembly by Means of a Micromanipulator and External Sensors, Proc. of the Int. Conf. Microrobotics and Micromanipulation, SPIE 97, pp , Vol. 3202, Pittsburgh, USA, 1997 [6] LogiCad3D GmbH, Gilching, Germany, Driver CD, 1999 [7] Schmoeckel, F.; Fatikow, S.:"Smart flexible microrobots for SEM applications, Journal of Intelligent Material Systems and Structures, accepted 4. Conclusion and future work In this paper, the development of a microrobot acting as a pair of tweezers for handling tiny objects in the SEM has been presented. For the enhancement of the control system a closed-loop control has been proposed basing on a sensor system that mainly consists of a standard SEM. The implementation of this control strategy can allow the accomplishing of more complex tasks by the robot in the near future followed by the expansion to a two-robot system and automation steps later on. Acknowledgements This research work has been performed at the Institute for Process Control and Robotics (Head - Prof. H. Wörn), Computer Science Department, Universität Karlsruhe (TH). It is being supported by the European Union (ES- PRIT Project MINIMAN, Grant No ). Reference [1] Sato Tomomasa et al.: Hand-eye system in nano manipulation world ; IEEE Int. Conf. on Robotics, and Automation, 1995, pp [2] Breguet, J.-M.; Pernette, E.; Clavel, R.: Stick and slip actuators and parallel architectures dedicated to 3913