Header for SPIE use
Mechanical gripper system for handling and assembly in MEMS Petrovic D.*a, Popovic G.a, Chatzitheodoridis E.a, Del Medico O.b, Almansa Aa, Detter H.a, Brenner Wa a Institute for Micro Technique and Precision Engineering (IMFT) - Vienna University of Technology, bINOCON Technologie GmbH.
ABSTRACT Nowadays the application of specially designed grippers in micro technology is an important topic and a necessity for the industrialisation. In order to transfer the manual assembly of micro electro mechanical system (MEMS) to an automatic assembly process, specially designed handling tools with sensing capabilities are required. Keeping the dimensions of the microparts in mind the handling and assembly process requires supervision with microscopes, positioning with high precision and application of specially designed tools. This paper describes a miniaturised mechanical gripper system with specially designed grippers and with implemented force-feedback for general microassembly purpose. The described grippers are fabricated from spring steel by wire electro-discharge-machining (EDM). The design of the microgripping system allows handling of pieces with sizes from 10µm up to 2 mm. Keywords: handling, assembly, gripper, MST, MEMS, microparts, electro-discharge-machining, EDM
1. INTRODUCTION Handling and assembly of microparts are the most important and challenging steps in the progression from simple microstructures to a promising microsystem. These steps of the fabrication sequence are of essential importance for the final production costs, the quality and the process reliability, but today they are predominantly done manually by skilled human operators. In order to transfer MEMS from laboratory scale to real industrial mass production, it is necessary to construct, produce and assemble reliable and cheap products. With the word “handling” we define the process of safely (without damages) picking-up of microparts of any shape or any kind of material, transporting or moving them to the desired position and then precisely positioning them on or connecting them with other microstructures. To bring the production in MEMS on an industrial level, special methods, tools and systems for automation in microhandling and assembly have to be developed. The semi-automatic systems can assist the human operator in picking-up microscopically small structures, holding them and placing on the right position. This will not only improve the operators working conditions but will also decrease the production costs and will increase both, the process reliability and the product quality after assembly [1,2]. Assembly of multiple microstructures into a microsystem requires: ¾
extremely precise positioning systems,
¾
special small grippers and micro-manipulators for assembly,
¾
pattern recognition system,
¾
simulation of interactions between the handled microstructure and the assembly tools (e.g. microgrippers or tweezers),
*
[email protected]; phone **43 1 58801 758 31; fax **43 1 58801 358 99; http://www.ifwt.tuwien.ac.at; Institute for Micro Technique and Precision Engineering (IMFT), Vienna University of Technology, Floragasse 7/2, 1040 Vienna, Austria
¾
joining techniques for assembly (e.g. gluing, thermal or ultrasonic bonding, laser welding, press and snap-in, etc.),
¾
characterization methods for quality improvement after the assembly process.
2. DESIGN OF THE GRIPPER AND THE GRIPPING SYSTEM There are numerous functional principles that can be applied for micromanipulation: vacuum principle, mechanical grippers, electrostatically or electromagnetically driven grippers. The design of the micro-gripper system described in this paper was defined by the following characteristics: ¾
miniaturized gripper for general microassembly purposes,
¾
the 'gripping field' on the tip of the gripper should have dimensions of 100x150µm and a thickness of 50 µm,
¾
opening or closing of both gripper arms should be parallel,
¾
linear movements should be performed by mechanical actuation,
¾
the design of the microgripper should allow handling of pieces with sizes from 10 µm up to 2 mm,
¾
possibility for easy and fast replacement of the gripper with another set of grippers using the same holder should be enabled,
¾
design of the gripper has to include force feedback (control of the gripping force),
¾
the gripper shall be produced with µ-EDM technology,
¾
used material for fabrication of the grippers should be steel (respecting all demands on the mechanical stability and elasticity, as well as on conductivity for the EDM process).
Respecting all mentioned requirements on the mechanical gripper, the following solution with simultaneous opening of the left and the right gripper arm is shown in Fig. 1. The actuation is performed by linear translation of an arrow-shaped tool using a stepper motor, which allows opening and closing of the two arms of the gripper. Each gripper arm is supported by 100 μm leaf springs. It is separated from the supporting structure by a narrow gap where the optical sensor is placed.
Fig. 1: Micro-gripper system with simultaneous opening of the left and the right gripper arm (actuation with one arrow)
The gripper arms can be opened up to 4 mm distance from tip to tip. In the whole range of the movement the opening or closing cinematic is linear and the gripping surfaces of both gripper arms are always parallel to each other. This is because of the special construction and design of the spring elements. They have the function of 'carrying' the gripper arms and at the same to prevent their torsion.
Fig. 2: SEM graphs of the gripper (left) and the spring elements (right) after fabrication by wire EDM
The second gripper type (Fig. 3 and Fig. 4) is based on the similar principle, but the double arrows and the two actuating drives can be utilized for independent opening/closing of each gripper arm. This can additionally enable a slight rotation of the handled micropart relative to its axis, what is sometimes required for correct positioning. Another difference in the design of the second gripper type is the number of the spring elements. After the first experimental experience with the first gripper type (Fig. 1), the number of the spring elements was reduced to two per gripper arm as shown in Fig. 3 and Fig. 4 (no necessity for more spring elements).
Fig. 3: Planar view of the design of the microgripper system with double arrows
Fig. 4: Side view of the microgripper system with double arrows The gripper-holder is designed as universal holder for all types of grippers produced at the Institute for Micro Technique and Precision Engineering (IMFT). The gripping system is embedded in the holder under angles of 30° respectively to the working field. This angle is necessary, because the gripper-tip itself is designed with 30° angle. In this way the lower edge of the gripper is parallel to the working field. Such non-planar design of the gripper (30° angle) is optimal for an easy approach to microparts that have to be handled and assembled, avoiding unwanted contacts to other objects in the surrounding on the working platform.
Arrow actuation
Arm, leaf spring and gap Optical Sensor
3 mm
Fig. 5: Gripper system with one arrow actuaion and with force sensor (left) and gripping of metallic micro-gears (right)
Precise control of the gripping force is required in order to avoid damages on the handled microstructures but also to prevent from damages on the gripper itself. Therefore optical sensors are fixed on both arms of the microgripper, giving a signal in volts that is proportional to the force exerted in each arm (Fig. 5, left picture). The assembly of the gripper to the whole gripper-system needs precise integration of the optical sensors on each of the gripper arms. Glue is used to join the sensors to the gripper taking care that the light beam is centred in the V-shaped gap of the gripper arm. Two gripper arms are then positioned on the holder. Teflon tape and thin metal spacers are used to align the tips of the gripper, allowing at the same time the free movement of the arrow actuator.
3. WORKING PRINCIPLE The developed gripper is incorporated in an assembly station developed at IMFT. The station is assisted by a vision system integrating a stereoscopic optical microscope connected to a digital camera and a frame grabber, as well as using a special software developed at IMFT (based on commercially available image processing libraries). In the first development the gripping system is controlled by a simple open loop electronic position control. The computer calculates the target position of the arrow-shaped part to get the closing value for the gripper or target distance between the two gripper arms. After obtaining the target value, it is forwarded by the computer to a memory buffer in the electronic controller. The position of the arrow is permanently driven by electronic controller. The controller is constantly comparing the present position value with the last target value received from the computer. In case of differences the controller generates the pulses needed for positioning the system in the new target position with a given speed. The integrated optical sensor for force feedback includes a LED for light emission, a photosensitive element as light receptor and the electronics for signal conditioning and transmission. By increasing the force in the gripper arm, the light beam from the LED to the photosensitive element is gradually intercepted. In this way, after calibration of the device, the relation between the voltage output of the optical sensor and the force exerted in the gripper arm can be precisely given. The force feedback information sets-on an alarm whenever the measured force exceeds the threshold value. In this way collisions or disproportionate forces that could cause damages to the gripped microstructure or to the gripper itself can be avoided. If the evaluated force is higher than the threshold force, the alarm-bit is set to 1 and the electronic controller stops the action of the motor.
4. GRIPPER CHARACTERISATION To characterize the force sensing of the two microgripper prototypes, for each gripper arm basic characterization tests have been performed. The working behaviour of each gripper arm is characterized by the elastic performance and the relation between the voltage output of the photodiode and the force exerted by the gripper. The testing system includes a highly precise balance (SARTORIUS R200D), accurate manual XYZ positioning table and a Dewetron PC-based measurement system. The step resolution of the cinematic (moving up and down) of the gripper arm with the manual positioning system is up to 1 μm and the chosen steps are 10 μm. Considering the negligibility of the displacement of the balance plate, the position of the arm can be related to the exerted force through the elastic characteristic of the gripper arm. The detected light is converted into voltage by a photodiode, stabilized by a preamplifier and then measured by the Dewetron PC-based measurement system. With statistical analysis of the data from the figure 6 (force versus distance) important parameters for the characterisation of the gripper can be obtained, like the elastic coefficient and the value of the force limit (in order to prevent disproportional forces that could lead to damages to the gripper itself). For simultaneous gripping of both gripping arms (gripping symmetry) the elastic coefficient should be similar in both arms. Figure 7 presents the dependence of the sensor output signal (in volts) from the exerted force. For conversion of the voltage output of the sensor to the force value, approximation of this curve through an interpolated polynomial function is required. The sensing element has been designed to provide the most linear behaviour in the central measuring range.
The reliability of the results is high. The divergence in the value of the elastic constant is only ±0,3%. Force(mN) - Output(V)
Force (mN) - Distance (mm)
14
12
10
12
3
2
y = -1,2958x + 4,5644x - 8,6936x + 11,212 2
Force (mN)
Force [mN]
10
8
6
R = 0,9994 8
6
4
4
2 2
0
0 0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
0
0,5
1
1,5
2
2,5
Output [V]
X (mm)
Figure 6. : Force vs distance
Figure 7. : Force vs output
5. CONCLUSION The described mechanical micro-gripper types and the gripping system with force sensing have been incorporated into an assembly station developed at IMFT (TU Wien), assisted by a vision system, which is used for position measurement. The design with open loop position control and alarm in case of too high applied forces or even in case of collisions were developed to contribute to the automation in handling and assembly in MEMS. Additionaly, the gripping system is not only aimed for technical purposes, but has also other application fields like in cell-manipulations and micro-biology.
ACKNOWLEDGMENTS This paper and the described work have been made possible with financial support provided by the European Commission within the TMR-Programme (Training and Mobility of Researchers, HAFAM-Network) and the Austrian Ministry for Education, Science and Culture (BM:BWK).
REFERENCES
1. S. Fatikow and U.Rembold, Microsystems Technology and Micro-robotics, Springer-Verlag, Berlin, 1997. 2. M. Cohn, K. Böhringer, A. Singh, J. Noworolsky, C. Keller, K. Goklberg, R. Howe, “Microassembly technologies for MEMS”, in Proc. SPIE Micromachining and Microfabrication, Santa Clara, CA, USA, Sept. 20-22, pp. 216., 1998.
