Değişken kapasiteli yandan tahrikli elektrostatik mikromotor tasarımı
Başlık çevirisi mevcut değil.
- Tez No: 75587
- Danışmanlar: PROF. DR. R. NEJAT TUNÇAY
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1998
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Elektrik Ana Bilim Dalı
- Bilim Dalı: Elektrik Bilim Dalı
- Sayfa Sayısı: 67
Özet
Bu çalışmada 12:8 Elektrostatik Yandan Tahrikli Değişken Kapasiteli Mikromotor'a ilişkin bir tasarım yapılmıştır. Tasarımda öncelikle motora ait büyüklükler belirlenmiştir. Değişken kapasiteli mikromotor, mikromekanikte kullanılan ilk mikromotordur. Tasarımda magnetik relüktans motoruyla benzerlik kurularak parametreler belirlenmiştir. Endüktansın yerini ( L ) kapasite ( C ), akımın yerini (I) gerilim ( V ) almıştır. Bu motor tasarım ve üretim basitliği nedeniyle tercih edilir. Değişken kapasiteli mikromotor, birbiri üzerinde hareket edebilen iki plaka arasındaki çekim kuvveti nedeniyle yaratılan kuvvete dayan ır. Stator kutupları kondansatörün üst plakası ve rotorun metal kısmı kondansatörün diğer plakasıdır. Çıkık bir rotoru vardır. Hava aralığı mümkün olduğunca dar olmalıdır. Rotor çıkıklığı ve dar hava aralığı rotor -stator kapasitesini, sonuçta da momenti en büyük hale getirebilmek için istenir. Rotor elektriksel olarak topraklanmış olmalıdır. Stator elektrotları iletkendir. Elektriksel olarak uyarılmışlardır ve kontrollüdürler. Çalışmada öncelikle stator-rotor kapasitesinin rotor açısına bağlı değişimi verilmiştir. Düzeltme faktörünün değişimi ve hesaplanma yöntemi gösterilmiştir. Kx ve C( 0 ) kullanılarak makinadan elde edilebilecek maksimum moment hesaplanmıştır. Sonuç olarak 1 mm den ufak motorların elektrostatik olarak imal edilmesinin avantajları görülmüştür. Moment ifadesi rotor çapıyla doğru orantılı, hava aralığı ile ters orantılıdır. Bu çalışma diğer benzer tip cihazlara da genişletilebilir.
Özet (Çeviri)
The surface micromachining of silicon-related materials has recently been applied to the fabrication of very small mechanical structures. An application of surface micromachining techniques to fabricate a specific motor, electrostatic micromotor. [1] In general electrostatic micromotors are preferred over magnetostatic motors in the microworld because electrostatic forces scale favorably as dimensions shrink and because dielectric materials are more easily patterned and processed than magnetic materials especially in the realm of silicon processing. The three- dimensional windings required for magnetostatic motors would be very hard to fabricate in silicon, but the small gap sizes that allow electrostatic micromotors to take advantage of the ability to withstand increased electric fields before breakdown are easily fabricated using photolithograohic techniques..[2] The torque than can be generated by an electric field motor is limited at least by the electrical field breakdown strength of its gap, while the torque that can be generated by a magnetic motor is limited at least by the saturation of its magnetic circuit [3]. For macro-scale motors, the magnetic limitation permits torque generation which is typically four orders of magnitude greater than that permitted by the electric limitation. This explains why most macro-scale motors are magnetic. However, given micron- scale gaps, the electrical field breakdown strength rises by two orders of magnitude, hence the torque limitation rise by four orders of magnitude. Under this conditions the two limitation are nearly identical. Therefore given the difficulty associated with including magnetically permeable materials within the material set from which the micromotors could be fabricated and and given the difficulty associated with fabricating windings, the decision to design electric micromotors are easily justified. Consequently, electrostatic micromotors can be advantageous over the normal electromagnetic motors if its diameter is smaller than 1 mm, because the electric charge stored on the surface of a particle is large compared with the particle volume if the particle is large compared with the particle volume if the particle is small. In figure I is shown a micromotor. [4]Fig. 1 Electrostatic Micromotor The Micromotors Types 1 ) Electrostatic variable - capacitance micromotors a) Side - drive micromotors (Ordinary motors ) b) Harmonic side -drive micromotors ( Wobble motors) c) Top drive micromotors 2) Piezoelectric micromotors 3) Induction micromotors 4) Permanent magnet micromotors In this work,12:8 variable - capacitance side -drive motors design procedurs, operational principles and fabrication process are invastigated. A variable - capacitance micromotor requires a salient rotor, that is, one with a conducting layer patterned to make salient electrodes around the perimeter..[5]The rotor rotates around central bearing. The stator consist of two sets of drive electrodes around the perimeter, one set patterned on the silicon substrate and on a rigid overhang. Stator electrodes encircle the rotor in its plane of rotation. A bushing is included in the prototype to offset the rotor from the substrate. The bushing limits the contact area between the rotor and substrate, which minimizes friction and adhesion forces. The bearing overhangs the rotor thereby securingthem to their respective substrates. The micromotor is fabricated above a silicon substrate which is covered with an electricaly insulating layer. Micromotor is radially directed gap. The prototype variable - capacitance micromotor has a composite rotor fabricated from CVD silicon nitride and heavily doped polysilicon. It requires only conducting and insulating films, are more efficient and has useful energy densities due to the higher electric fields sustainable in micron-sized air gaps. Fabrication processses based on a standart polysilicon surface micromachining proces a variation which uses a local oxidation of silicon ( LOCOS ) step. Undoped low tempatature LPCVD silicon dioxide is used for the sacrifical layers LPCVD silicon nitride is used for electric isolation and heavily phosphorous - doped LPCVD polycrystalline silicon is used for the structural components. Since the variable - capacitance micromotor is a synchronous motor, the drive electronics must be capable of providing uniform voltage pulses at the synchronous rate. For the prototype design, the electronics must drive the capasitive load represented by the phase capacitance. The design of electric micromotors can benefit from duality relationships with the well understood magnetic motor types. A knowledge of the physics of electrostatic breakdown in planar micron and sub-micron air gaps is essential for designing electric micromotors. The excitation of a stator phase induces a charge of opposite polarity on the rotor poles nearest to those of excited phase. This results in a torque which acts on the rotor so as to align its charged poles with the excited stator poles. The properly timed sequential excitation of the phases then causes the rotor to turn in synchronism with the excitation. If micromotor operation did not require air levitation, this operation was limited to under one minute, at which time the motors ceased to operate. [6]. Motor operation overcome frictional forces associated with the clamping of the rotor to the electric shield beneath it. This rotor clamping was attributed to electric fields between the rotor and the shield caused by a lack of proper electrical contact between these respective parts. The rotor - shield elecrical contact during motor operation is intended to come from mechanical contact of the rotor to the shield or the bearing. The bearing is fabricated in electrical contact with the shield. The bearing clearance allows radial displacement of the rotor when the rotor is electrically grounded and is symmetrically centered in between a set of excited stator poles the resultant of the radial forces on the rotor due to each excited stator poles is zero. However, when the rotor displaces radially,a net radial force develops which pushes the rotor into the bearing post. Bearing frictional forces at this contact may be significant.The purpose of the shield in a micromotor is to shield the rotor from vertically directed electric fields which could exist between the substrate and the rotor in the absance of the shield. Such fields would cause an attraction of the rotor from, to the substrate and thereby increase the friction torque acting on the rotor..[3] Assuming that the rotor is in electric contact with the shield through either the bushings or bearings, there will no electric fields between the shield and rotor, and hence no attraction of the rotor to the shield. This micromotors are the dual of the magnetic doubly salient variable reluctance motor. In this work, the 3:2 design is used because of provide superior torque coverage with higher minimum torque values as compared to the 3:2 and 2:1 designs.[7]. For typical micromotors, the rotor-stator capacitance is more directly a function of the rotor-stator thickness and not of the vertical rotor -stator pole face overlap. The capasitive phases are electrically linear. The rotor -stator capasitance varies with rotor position and this variation gives rise to an avarage torque at synchronous speed. Potential advantages of the variable - capacitance micromotor include its physical simplicity, the lack of rotor excitation and the high efficiency (Superconductors are required to build a magnetic motor as efficient as a variable - capacitance micromotor ).[8] Micromotors are to prove useful beyond self-contained application, such as contrallable mirror mounts, cotrallable shutters or gyroscopes, a means of hamesssing. These micromotors may one day enable the development of very small high bandwith high accuracy motion control systems. The simplerst control of a micromotor is open loop control. Other applications are microsurgical instruments, micromachining devices for processsing electronic components and micropumps for biological and biochemical processes. High-precision positioners such as those needs for various scanning microscope technologies and optical alignment systems could also benefit from miniature motion. New materials and designs may provide for better bushings and bearings. Motion sensors should be developed and integrated into the micromotors. Motion sensors could also facilitate dynamometry studies. Mechanical power takeoff should be studied detail. A judicious level of electronic circuitry should be develop and integrated into the micromotors and their motion sensors. Finally, motion sensing and control algorithms must be developed, perhaps with the initial goal of accomodating friction. A preliminary understanding of the performance characteristics of these micromotors has been established through the development of experimental techiques for obtaining quantative performance data for micromotor operation. However, an important prerequisite for advancing our current understanding and models of the micromotors is accurate analysis of electric fields in these devices.All motive torque and the axial force values calculated below are based on an excitation voltage of 100 V. For the motive torque and axial force calculations, it is analyzed the case in which the rotor radial position is concentric with the bearing and the stator, resulting in equal gaps for all rotor/stator pole pairs. Due to the clearance in the bearing, the rotor radial instability may displace radially during motor operation. All other parameters constant, as the rotor/stator gap size becomes larger than the stator/shield seperation, an increasing number of the electric field lines originating on a stator pole will terminate on the shield rather than the rotor. As a result, the actual rotor/stator capacitance and the motive torque are increasingly smaller than those predicted by in plane 2-B FEA. Another important parameter that influences the micromotor performance is the axial force on the rotor due to the stator electric excitation. All practial rotor/stator gap sizes, the axial force on the rotor is zero at a positive axial position. As a result, the rotor must be raised high enough to balance out the number of the stator electric field lines terminating on the top and bottom surfaces of the rotor. It is found that a large reduction in maximum motive torque results when the rotor/stator gap size is larger than the stator/shield seperation. When the rotor/stator gap size is larger than the stator/shield seperation, more of the stator fields lines terminate on the shield rather than the rotor, resulting in a reduction in motive torque. This problem can be avoided by ensuring that the stator/shield seperation is larger than the rotor/stator gap size. Finally, axial position of the rotor does not significantly effect the motive torque with the geometrical constraints of our micromotor.
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