1MICRO PLANETARY REDUCTION GEAR USING SURFACE-MICROMACHINING Woo Chul Kim and Michael Chapp Nanoengineering laboratory Department of Mechanical Engineering University of California at Berkeley Berkeley, CA, U. S. A. Abstract A micro planetary gear mechanism featuring a high gear reduction ratio with compactness in size is presented in this paper. SUMMiT V is employed for the fabrication method so that the redundancy of assembling parts is eliminated. The design rules of which has also been checked. To make full use of the benefits of the surface-micromachining, the planetary reduction gear is designed toward using the on-chip micro-engine. The expected gear reduction ratio is calculated and compared with the conventional chain gear mechanism. The micro-planetary gear mechanism presented in this paper is expected to have 162:1 reduction ratio utilizing less space consumption. This is an order of magnitude higher than the previously reported design in a single reduction gear train. Keywords: MEMS, planetary gear, reduction gear, surface-micromachining, SUMMiT V process Nomenclature a sun gear b planet gears c internal gear (fixed) d internal gear (rotary) n the number of units of gear train D diameter of the pitch circle N number of teeth P number of planets ω angular velocity Introduction The gear mechanisms in microelectromechanical systems (MEMS) are commonly expected to generate high torque in the confined micro-size systems. However, it is generally difficult for the micro-scale systems to have such a high torque without having multiple reduction systems. The design of the reduction gear drive based on a planetary paradox gear mechanism can increase the torque within a compact area, since the micro planetary gear system has an advantage of high reduction ratio per unit volume [1]. However its mechanism is so complicated that relatively few attempts have been made to miniaturize the gear systems [2-3]. Suzumori et al. [2] used the mechanical paradox planetary gear mechanism to drive a robot for 1-in pipes forward or backward. They employed a single motor to drive the gear mechanisms with high reduction ratio. Precise gear fabrication was enabled by micro wire electrical discharge machining (micro-EDM). These parts, however, should be assembled before the drive motor is attached to the gearbox. Takeuchi et. al. [3] also used micro-EDM to fabricate the micro planetary gears. They suggested special cermets or High Carbon Steel for possible materials. While the design can achieve a reduction ratio of 200, the gears should also be assembled and motor driven. To enable the driving of the planetary gear by on-chip means, Sandia Ultra-planar Multi-level MEMS Technology (SUMMiT-V) process [4] for planetary gear fabrication is adopted in this study. The SUMMiT-V process is the only foundry process available which utilizes four layers of releasable2polysilicon, for a total of five layers (including a ground plane) [5]. Due to this fact, it is frequently used in complicated gear mechanisms being driven by on-chip electrostatic actuators [5]. However, in many cases, the microengines may not produce enough torque to drive the desired mechanical load, since their electrostatic comb drives typically only generate a few tens of micronewtons of force. Fortunately, these engines can easily be driven at tens of thousands of revolutions per minutes. This makes it very feasible to trade speed for torque [7]. Rodgers et al. [7] proposed two dual level gears with an overall gear reduction ratio of 12:1. Thus six of these modular transmission assemblies can have a 2,985,984:1 reduction ratio at the cost of the huge space. With the desire for size compactness and at the same time, high reduction ratios, the planetary gear system is presented in this paper. It will be the first planetary gear mechanism using surface-micromachining, to the authors knowledge. The principles of operations of the planetary gear mechanism, fabrication, and the expected performance of the planetary gear systems are described in this paper. Principles of operation An alternative way of using gears to transmit torque is to make one or more gears, i.e., planetary gears, rotate outside of one gear, i.e. sun gear. Most planetary reduction gears, at conventional size, are used as well-known compact mechanical power transmission systems [1]. The schematic of the planetary gear system employed is shown in Figure 1. Since SUMMiT V designs are laid out using AutoCAD 2000, the Figure 1 is generated automatically from the lay out masks (Appendix [1]). One unit of the planetary gear system is composed of six gears: one sun gear, a, three planetary gears, b, one fixed ring gear, c, one rotating ring gear, d, and one output gear. The number of teeth for each gear is different from one another except among the planetary gears. An input gear is the sun gear, a, driven by the arm connected to the micro-engine. The rotating ring gear, d, is served as an output gear. For example, if the arm drives the sun gear in the clockwise direction, the planetary gears, b, will rotate counter-clockwise at their own axis and at the same time, those will rotate about the sun gear in clockwise direction resulting in planetary motion. Due to the relative motion between the planetary gears, b, and the fixed ring gear, c, the rotating ring gear, d, will rotate counter-clockwise direction. This is so called a 3K mechanical paradox planetary gear [1]. Fabrication procedure and test structures The features of the SUMMiT V process offer four levels of structural polysilicon layers and an electrical poly level, and also employ traditional integrated circuit processing techniques [4]. The SUMMiT V technology is especially suitable for the gear mechanism. The planetary gear mechanism can be driven by the on-chip engine and thus is another reason of using the SUMMiT V process. Since the Sandia process is such a well-known procedure [5-7], only brief explanation is presented. Figure 2 represents the cross-sectional view of Figure 1, and also was generated from the AutoCAD layout masks (Appendix [1]). The discontinuity in the cross-section is for the etch holes. The poly1 (gray) is used for the hubs and also patterned to make the fixed ring gear, i.e., c, the sun gear, i.e., a, the rotating ring gear, i.e., c, and the output gear is patterned in the poly2. Since the Figure 1. The schematic of the planetarygear
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