The push-pull principle: an electrostatic actuator concept for low distortion acoustic transducers

Abstract

Electrostatic actuators are of particular interest for microsystems (MEMS), and in particular for MEMS audio transducers for use in advanced true wireless applications. They are attractive because of their typically low electrical capacitance and because they can be fabricated from materials that are compatible with standard complementary metal-oxide semiconductor (CMOS) technology. For high audio performance and in particular low harmonic distortion (THD) the implementation of the push-pull principle provides strong benefits. With an arrangement of three electrodes in a conjunct moving configuration on a beam, we demonstrate here for the first time a balanced bending actuator incarnating the push-pull principle operating at low voltages. Our first design already exhibits a harmonic distortion as low as 1.2% at 79 dB using a signal voltage of only 6 Vp and a constant voltage of only ±10 Vdc in a standard acoustic measurement setup. Thus, exceeding our previously reported approach in all three key performance indications at the same time. We expect that our novel electrode configurations will stimulate innovative electrostatic actuator developments for a broad range of applications. In this paper we report the basic theory, the fabrication and the performance of our novel actuator design acting as an audio transducer.

A non-destructive electro-acoustic method to characterize the pull-in voltage of electrostatic actuators

“…Together this underlines the high compatibility between the 1-DOF model and the experimental data. In summary, the 1-DOF model in equation 2 reproduces not only the sound pressure level [10] but simultaneously also the harmonic distortions for a symmetric as well as asymmetric driving within a wide voltage range also for experimental measurements with an actuator ensemble.…”

“…The real pull-in voltage is estimated by destructive measurements at different chips from the same wafer. Optical measurements with a digital holographic microscope (DHM) as in [10] are performed without the cover waver. At stable positions quasi-static oscillations with 50 Hz are measured.…”

“…al. inspires a similar approach for a balanced NED actuator [10,22]. Figure 1 illustrates the basic appearance of the actuator.…”

“…Bond wires connect the chip to a printed circuit board (PCB). A detailed description of the actuator and the fabrication process can be found in [10].…”

“…Motivated by electrostatic balanced systems, Schenk et al proposed a NED equivalent [19], suppressing the otherwise dominant even harmonic distortions [8]. A detailed description of this loudspeaker can be found in [10]. Since reaching the pull-in often causes terminal device failure, a non-destructive prediction of the pull-in voltage is highly desirable, enabling e.g.…”