Comb-drive actuators

Microactuators, Fig. 1 Diagrams of (a) in-plane electrostatic comb-drive actuator and actuation force and (b) out-ofplane parallel plate electrostatic actuator... 

Electrostatic actuators are commonly used in MEMS devices because they scale well in the micro domain, use very little power, and are straightforward to fabricate in a number of different processes. Two common forms are parallel plate actuators and comb-drive actuators. The parallel plate actuator is a parallel plate capacitor with one of the plates released so that it is able to move, as shown in Figure 3.1. The relationship between the capacitance C, voltage V, and charge Q for a parallel plate capacitor is given by... 

Figure 3.2 Comb-drive actuator. One set of comb teeth is anchored to the substrate and the other set of teeth is released. The N teeth are w wide and have a separation of g. The initial overlap between the teeth is given by t, and the initial gap between the anchored and released teeth is given by g,. When a voltage is applied between the two sets of teeth there is an attractive force that draws the teeth together. The deflection of the teeth is given by x.

In contrast to the parallel plate actuator, the comb-drive actuator varies capacitance through a change in the overlap area between a set of interpenetrating comb fingers, as shown in Figure 3.2 [1]. 

The force that can be developed by the comb-drive actuators is given by... 

Show that the electrostatic force Fe of a pull-pull comb-drive actuator, as shown schematically in Figure 3.11, can be linearized in voltage by applying Vl= Vi,ias - v to the left set of fixed comb-drive fingers and = V ias + v to the right set of fixed comb-drive fingers. 

R. R. A. Syms, B. M. Hardcastle, and R. A. Lawes, Bulk micromachined silicon comb-drive electrostatic actuators with diode isolation, Sensors Actuators A 63, 61, 1997. 

Motion is generated by electrostatic attraction force between electrically charged surfaces. Examples of electrostatic MEMS actuators are in-plane comb drives (Fig. la) and out-of-plane parallel plate actuators (Fig. lb) [4]. Both types of actuators require large driving voltages... 

The comb-drive resonator shown in Figure 3.7 is a common device in surface micromachining processes [4]. We will estimate the resonance frequency by calculating the spring constant of the folded spring and the mass of the released elements that are driven into resonance by electrostatic actuation. 

W.C. Tang, T.-C.H. Nguyen, M.W. Judy, and R.T. Howe, Electrostatic-comb drive of lateral polysilicon resonators, Sensors and Actuators A21-23,... 

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