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ECE5320 Mechatronics Assignment#01: Literature Survey on Sensors and Actuators Comb-Drive Actuators (Electrostatic Micro-actuators) Prepared by: Amin Hernandez Dept. of Electrical and Computer Engineering Utah State University T: (435)797-; F: (435)797-3054 (ECE Dept.) [email protected] Outline Reference List To Explore Further Major Applications Basic Working Principle Fabrication Sample Configurations Major Specifications Limitations Construction ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Referent Lists http://www.iop.org/EJ/abstract/0960-1317/6/3/004 http://www.ee.ucla.edu/~wu/ee250b/Electrostatic%20Actuators-2.pdf http://en.wikipedia.org/wiki/Nanoelectromechanical_systems http://www.ansys.com/industries/mems/mems-what-is.asp http://www.ansys.com/industries/mems/mems-downloads/moussa_ieee.pdf http://www.imechanica.org/files/GroupD_final_paper.pdf http://ieeexplore.ieee.org/iel4/23/15882/00736529.pdf?tp=&isnumber=&arnumber=736529 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators To Explore Further To see more information concerning applications, and technical information should visit these wide sites: http://www.ansys.com/industries/mems/mems-downloads/moussa_ieee.pdf http://www.freepatentsonline.com/7085122.html http://www.iop.org/EJ/article/0960-1317/6/3/004/jm6304.pdf?request-id=MvdZrXzs3BGjLXu2wi7Kghttp://ieeexplore.ieee.org/iel3/1052/7345/00296932.pdf?tp=&isnumber=&arnumber=296932 http://www.springerlink.com/content/7272718738835774/ ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Major Applications Restoring springs, levers, and crankshafts MEMS tunable capacitors Vision science applications Telecommunications networks Micro-accelerometers Ranges of movements Stable and reliable operations Design flexibility ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Basic Working Principle The driving principle of electrostatic Comb-Drive actuator is based on electrostatic characteristics. As shown in the figure, when voltage is applied to the fixed electrode while the movable electrodes are potential grounded, a potential difference results across the electrodes and they become electrically charged. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Basic Working Principle This action will induce a certain capacitance in the charged electrodes. In addition, an electrostatic force is generated causing a displacement in the x direction. The driving force F created in the Comb-Drive can be expressed as: Where U is the energy associated with the applied electric potential V, is the relative permittivity of the dielectric material between the two electrodes, is the permittivity in the free space and equal to 8.85 pF/m, n is the number of the pairs of electrodes, t is the thickness of the electrodes. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Basic Working Principle If the movable electrode is displaced following Hooke’s law, the reaction force Fs Induced in the suspension beam holding the movable electrodes, which represents a spring suspension system, will be related to the stiffness of the beam and the displacement and can be calculated from the equation: Where Kx is the spring stiffness in the x-direction and X is the displacement. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Basic Working Principle The variation of the Comb-Drive displacement versus the number of moving fingers, (n) ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Basic Working Principle The total displacement is rather shorter than what is possible in straight actuation, but the force is constant, meaning an easier control of the displacement. This kind of system offers a very high precision level, and a simpler electronic control. In a comb-drive, there is no pull-in effect, unless a design error make electrodes reach each others. The displacement is linear, contrary to straight actuation. But the design is limited by the needs to place electrodes beams opposite to each other. Designer must also take care of the end stop to avoid contact between mobile part and fixed part, and to keep forces quite symmetric so that the direction of the displacement is kept as planned. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Fabrication ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Sample Configurations While lateral comb drives do not suffer from pull-in, the capacitance tuning relies on the lateral motion of the movable fingers Hence, the tuning ratio is limited by the maximum separation of these fingers and their overall lengths ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Sample Configurations An electrostatic comb drive actuator, consisting of inter-digitated capacitors, is one of the most important of MEMS devices. The use of comb drive actuators has made it possible to achieve a very large tuning ratio. In a typical comb drive, the gap between the fixed and moving fingers is uniform, resulting in an electrostatic driving force that is independent of the position of the moving fingers except at the ends of the range of travel. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Sample Configurations When a voltage is applied between electrodes V1 and V2, the imbalance of the electric field distribution will result in a vertical induced force. Consequently, comb fingers will move in an upward motion. On the other hand, when the voltage is applied between electrodes V3 and V4, the comb fingers will move in a downward motion. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Sample Configurations The vertical electrostatic comb drive actuator has the advantages of low driving voltage with large displacement, high motion speed and absence of pull-in phenomenon. It also allows the use of stiffer springs for higher resonant frequencies without excessively high operating voltages. The principle of vertical comb drive actuation is illustrated in the following figure. Each comb finger is composed of an upper electrode layer, an insulation layer and a lower electrode layer. The fixed and moving comb fingers are separated upper and lower electrodes insulated by an insulation layer. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Major Specifications Dimensions of a Comb-Driver actuator: ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Limitations Electrostatic force depends largely on the size of the structures and the distance between electrode. For large electrode surface compared to distance to travel, electrostatic actuation has a large advantage. But the equation of the force gives a dependency to the square of the distance. This means that the longer the distance is, the higher the actuation voltage is. This is one of the main problem with this physics principle: actuation voltage are often quite high, easily reaching tens, and even hundreds of volts to be used. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Limitations Another consideration to take care of is the electric field itself: the nature of the material between electrodes: water, for example, is conductive at low frequencies, so electrostatic actuation cannot be used in these conditions. Void and neutral gases are the best environments. Finally, the hysteresis behavior of straight actuators can be as well an advantage and a problem, depending on the application. It reduces sensitivity of devices to electrical noise, but it also means larger voltage variation for a complete actuation cycle when pullin/pull-out is desired. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Construction The figure shows schematically how, in our process, selective stiction is applied to construct selfaligned vertical comb-sets. The mechanical springs attached to the stictionplate could allow only vertical motion, and the original locations of the stiction-plates are held to maintain self-alignment between the rotor and the stator comb sets The stiction-plate springs must be flexible in the out-ofs ubstrate direction to allow stiction to occur but rigid in the in-plane direction without affecting the self-alignment of stator and rotor comb sets ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Construction Selective stiction has been successfully applied to the batch fabrication of angular vertical comb actuators made of singlecrystal silicon with self-aligned comb sets. The fabrication process with unique designs of mechanical springs enables the stiction of microstructures in a controlled manner and significantly reduces unwanted compliances on the actuators, preventing unwarranted motion and providing stable operations. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators