1 Design of a Micromachined Geneva Wheel as a mechanism to obtain intermittent motion from a constantly rotating source Varadarajan Vidya Palani Kumaresan Department of Electrical Engineering Department of Mechanical Engineering University of California, Berkeley University of California, Berkeley ABSTRACT Converting constant rotary motion into intermittent rotary motion gives rise to a range of useful applications in silicon micromachining. This paper discusses the design and fabrication of one such mechanism called the Geneva Wheel mechanism. The standard SUMMiT process has been made use of in developing this. All the related mathematics of the Geneva wheel was developed and the system was analysed. Keywords: Geneva Mechanism, Microengines, Sandia SUMMiT Process 1. INTRODUCTION With the introduction of 4 and 5 level polysilicon surface micromachining technology by the Sandia National Laboratories, a whole new range of mechanisms have been made possible at the micro level. Primary amongst these are the microengines that have, in turn, been used in a variety of applications. Since the output of a microengine is some sort of a continuous motion, most of the applications so far using microengines have been restricted to similar kinds of motion. In this paper, we discuss the details of design and fabrication of a mechanism called the Geneva Wheel mechanism, using which, continuous motion can be converted into intermittent motion. A conversion technique like this can be made use of in a variety of applications such as discretized positioning of micromirrors in optical switching applications. 2. GENEVA WHEEL MECHANISM The basic structure of a four slot Geneva wheel is shown in Fig.1. The system consists of a constantly rotating disk coupled with a slotted disk, which gives rise to the desired discrete motion. A rotation of 2p radians of the former causes 2p/N radians of rotation of the latter, where N is the number of slots available on the slotted disk. Thus, one complete rotation of the slotted wheel requires N complete rotations of the other disk, thereby also increasing the total time period. The conversion mechanism of this disk system is as follows. Referring to Fig.1, pinwheel W rotates constantly about axis A and as shown below, has a pin ‘a’ attached to it. This pin ‘a’ engages into the slots ‘s’ of the Geneva Wheel G (a basic 4-slot Geneva mechanism is shown here) and rotates it as long as it is engaged with the slot. While the wheel W rotates continuously, the Geneva wheel G has a discrete rotation about axis ‘b’. Wheel G has a rotation time period of tr when it is moving along with disk W and an idling time period ti, when the pin ‘a’ is not inside one of the slots ‘s’ and is moving freely. The three quarter wheel ‘L’ is placed in order to prevent any unintentional rotation of wheel G while it is idling. For a four slot Geneva mechanism, the rotation time period tr is one third the idling time period ti [1]. By varying the number of slots on G, one can vary the time period and the angular displacement of the same. If this system is now coupled with some optical system like a micromirror (through a rack and pinion kind of arrangement), then it can be used to deflect light rays in different directions (by discretely positioning the moving mirror by using the discrete angular positions of the Geneva wheel) thereby giving rise to an optical switching technique. a A Wb L s G Fig.1. Mechanism of Geneva Wheel In the following sections, four slot and six slot Geneva wheels have been analysed and a design layout has been provided. Along the same lines, multiple slot wheels can be designed. The basic criterion that has to be maintained in designing any number of slotted Geneva wheel is that, the pin has to enter and leave the slots radially. This will again be discussed in detail in the following sections. 3. DESIGN Design of the Geneva wheel has been done using the 4-level polysilicon surface micromachining technology by Sandia National Laboratories. All the four levels of polysilicon are required for designing this mechanism. The SUMMiT process [2,3] and the layout design have been discussed in detail in this section.23.1 The SUMMiT Process The Sandia Ultra-planar, Multi-level MEMS Technology (SUMMiT) process[2,3,4] is a standard process developed by the Sandia National Laboratories. A cross-section of the main layers in the process is shown in Fig.2. In this process four layers of polysilicon alternated by sacrificial silicon dioxide layers are laid down. The first level is a silicon dioxide and nitride stack layer. The oxide layer in this level is used as an insulating layer. The nitride layer acts as an etch stop and protects this oxide layer when the sacrificial oxide etch is carried out. The four polysilicon layers function as the structural layers used for developing various micromachined structures. The oxide layers alternating between the polysilicon layers are used to physically isolate the polysilicon layers. Once the whole structure has been developed, these oxide layers are etched away and polysilicon structures are released. The thickness of the various layers is given in Fig.2. Fig.2. Layers of the SUMMiT process Courtesy: Sandia National Laboratories The two things that are unique to this process and which make a variety of designs possible at the micro level are the conformal SACOX2 layer and planarized SACOX3 layers. 3.2 Wheel design Two types of slot designs for the Geneva wheel were considered. The designs were laid out using the Cadence software for MEMS layouts. Since the technology file available with the software allowed for only three levels of polysilicon, the structural poly0 level was not laid down. For any N slotted wheel, the angle by which the slotted wheel rotates for a given rotation of the constantly rotating wheel is 2p/N. The slots are thus placed at 2p/N radians intervals. An important requirement is that during every rotation, the pin should enter and leave the slots in such a way that the tangent to the
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