Ultra-compact, zero-power magnetic latching piezoelectric inchworm motor with integrated position sensor

Abstract

This paper reports the design, fabrication and characterization of a large-stroke, piezoelectrically actuated linear motor with zero-power latching for portable electronics. The motor consists of a stator having a micromachined piezoelectric actuator and a high-permeability metal guide, a permanent magnet rotor, and an integrated position sensor. The rotor is tethered to the metal guide by magnetostatic attraction due to its permanent magnets. The resulting frictional forces are sufficient to immobilize the rotor to the stator. This does not require any power, and the combined bearing-braking feature is thus termed zero-power latching. The integrated position sensor demonstrates the feasibility of precisely monitoring the rotor position. The completed actuator has a volume of 1 mm(w) × 3 mm(t) × 5 mm(l). With a 50 kHz driving frequency, precision of 5 μm at a scalable full-stroke of 5 mm and a speed of 10 mm/s at 50 kHz was achieved. A capacitive position sensor was also designed into the motor, and was able to determine the rotor position with a sensitivity of 1 fF/μm.

Introduction

Micro-linear motors have been widely researched for applications that require ultra-compact, high-precision positioning, such as microsystems assembly and robotics [1], [2], [3], [4], [5]. Recently, these needs have expanded to encompass large-stroke and low-power consumption, especially for portable applications. For example, decades of image sensor research have enabled camera modules that occupy less than 10 mm³, but contain over 10 million pixels. But, fully realizing these modules’ potential for delivering high-quality pictures requires auto-focus and zoom functionalities, both of which in turn require compact, high-precision linear stages. Alternatively, ultra-compact linear actuators could be used to actuate micro-valves for miniaturized fuel cells, which have medical, military, space, industrial, and even consumer applications. All these applications call for small size, large displacement and force, fast response, low-power consumption, and, most importantly, cost effectiveness considering mass-manufacturability.

Linear motors based on microelectromechanical systems (MEMS) technologies have been extensively researched, with the most focus having been placed on electrostatic, electromagnetic, thermal and piezoelectric actuation [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. An electrostatic actuator is typically composed of two electrodes facing each other, separated by a small gap. The application of voltage across the electrodes generates force, proportional to the voltage and inversely proportional to the gap size. The electrostatic actuator consumes low power and is typically CMOS compatible, both in terms of drive circuitry requirements and in terms of fabrication [6], [7]. However, the sharp nonlinearity of the gap-force relationship limits electrostatic actuators to small displacements [1], [5], [6], [7]. Electromagnetic actuators are attractive since the generated forces are not only generally larger than in electrostatic actuators, but also capable of operating across larger gaps [8]. In order to achieve practical coupling efficiency and force magnitudes, large currents as well as expansive, complicated structures such as high-turn-count windings, high-permeability magnetic cores, and/or permanent magnets are usually needed. Thus, the benefits of electromagnetic actuation at the microscale can be easily outweighed by the increased space consumption and structural complexity. Thermal actuators are found to be suitable for applications requiring slow response, large forces and large displacements at low drive voltages [3], [4], [11]. In contrast, piezoelectric materials are capable of producing very large forces with low power and very fast response times, but offer only extremely small displacements. This set of characteristics makes piezoelectric actuation an excellent fit for inchworm-type linear motors. These devices directly overcome the small displacement limitation, and have become a widely adopted approach to large-stroke, low-power actuators [9], [10], [11].

While seeking to utilize the aforementioned MEMS actuation technologies for linear motion in portable systems, the dominant qualitative requirements are small form factor, minimal power consumption, and a practical latching mechanism. Ding and Ziaie reported a remarkably simple structure, inspired by the motion of insects, based on polydimethylsiloxane (PDMS) rods that have sawtooth-shaped legs [12]. In the study, they provided the fundamental idea and the experimental results of a bidirectional device motion using a shaker table. Although their work establishes substantial promise for biomimetic linear motion using polymer rods, they demonstrated only preliminary results, using the off-the-shelf shaker table at a low frequency, and did not provide for latching mechanisms of any kind. Piezoelectric Technology Co., Ltd. demonstrated a 4-mm diameter piezoelectric linear motor for cellular phone and office machinery applications [13]. This motor implemented a frictional driving scheme consisting of a rubber band and a quartz guide on the actuator. It achieved 10 mm of travel, a top speed of 10 mm/s and 98 mN of force, all in a very small package. However, the actuator, lacking position feedback, cannot provide precise positioning. In addition, its inherent coupling of both driving and latching forces through the rubber-quartz friction interface implies severe tribological constraints on both repeatability and device lifetime.

In this paper, which extends our previous work presented in [14], we report the design, fabrication, and characterization of the 1st and 2nd generations of an ultra-compact, large-travel, high-precision, high-speed, piezoelectrically actuated linear motor with zero-power magnetic latching, as shown in Fig. 1(a). Since the proposed linear motor is targeted for inclusion in cell phone camera modules, cost effectiveness and long-term reliability are pivotal, but secondary, concerns. Fig. 1(b) shows a conceptual drawing of a typical camera module adopting the proposed inchworm motor in a compact camera module application. The camera module consists of two lens groups, each affixed to a piezoelectric linear motor.