A leakage compensated charge control driving circuit with sensor feedback for a comb drive actuator

Abstract

We present the design of a novel driver for a comb drive actuator. This actuator is required to have both a large displacement of 200 μm and a high resolution of 50 nm. Traditionally, electromechanical MEMS actuation is done under voltage control. In this paper, we want to examine a charge controlled driver, as it might provide some advantages. A comb drive under charge control exhibits a longitudinal displacement proportional to Q2/3, rather than V2 in case of voltage control. This more linear behavior might help to increase the achievable displacement resolution. It can also be shown that the stability range of an actuator under voltage control, for any type of actuator, will never be larger than the range of that same actuator under charge control. However, for a well-designed and well-fabricated comb drive actuator, where this instability is expressed in lateral pull-in, this effect is minimal. In this paper, we first create an electromechanical model of a comb drive, to be used in simulation of our driver. The created model is based on a previously designed comb drive with a displacement of 200 μm at 91.6 V, or 138.1 pC. We then focus on the driver itself, minimizing the effects of leakage currents, as these are detrimental for a good charge controlled actuation. As the actuator will be driven dynamically, we cannot rely on the estimated charge on the comb drive to determine the immediate displacement. A sensor is needed that measures the position of the comb drive. Four different topologies of sensors are described, each of which uses the comb drive itself to estimate its position. A full electromechanical simulation is performed and a comparison is made between the different sensor topologies, evaluating their ability to determine the exact displacement. One of the proposed sensor topologies achieved the 50 nm resolution.