Thermo-bimorph Microcilia Arrays for Small Spacecraft Docking

Microelectromechanical system (MEMS) technology promises to improve performance of future spacecraft components while reducing mass, cost, and manufacture time. Arrays of microcilia actuators offer a lightweight alternative to conventional docking systems for miniature satellites. Instead of mechanical guiding structures, such a system uses a surface tiled with MEMS actuators to guide the satellite to its docking site. This paper describes an experimental setup for precision docking of a “picosatellite” with the help of MEMS cilia arrays. Microgravity is simulated with an aluminum puck on an airtable. A series of experiments is performed to characterize the cilia, with the goal to understand the influence of normal force, picosat mass, docking velocity, cilia frequency, interface material, and actuation strategy (“gait”) on the performance of the MEMS docking system. We demonstrate a 4 cm cilia array capable of docking a 45 gram picosat with a 2 mm contact area at micrometer precision. It is concluded that current MEMS cilia arrays are useful to position and align miniature satellites with up to several kg of mass. INTRODUCTION A number of MEMS cilia systems have been developed with the common goal of moving and positioning small objects, so far always under the force of gravity [1-3]. Similar to biological cilia, all of these systems rely on many actuators working in concert to accomplish a common goal. Recent techniques range from single crystal silicon arrays [4,5] actuated using electrostatic force to arrays constructed with polyimide [6,7], and relying on the wide range of coefficients of thermal expansion (CTE) inherent in these materials. The goal of this project is to investigate the feasibility of a MEMS-based space docking system. For such a system, the docking approach is divided into two phases: (1) free flight and rendezvous, with the goal to achieve physical contact between the two satellites, and (2) precision docking with the goal to reach accurate alignment between the satellites (e.g., to align electrical or optical interconnects). Phase 1 constitutes unconstrained motion with 6 degrees of freedom and lower accuracy; phase 2 constitutes planar motion with 3 degrees of freedom and high accuracy. This paper focuses on phase 2 and investigates MEMS cilia as a means to achieve precise alignment between two satellites. During this project thermally actuated polyimide based microcilia, as seen in Figure 1 and identical to those published in [6], are extensively characterized to ascertain their practicality for docking miniature spacecraft. To this end, experiments were performed using an airtable, seen in Figure 2, which was designed to support the microcilia in a vertical configuration. The airtable can be tilted towards the microcilia producing a known normal force against the faces of the chips. This force can then be adjusted independently from the mass of the picosatellite puck. To increase the realism of the experiment and to ease data collection, position sensing and position feedback are incorporated and computer controlled. Two position sensing systems are used: an array of Hall effect sensors and a video capture based system. These are strictly non-contact techniques compatible with a space environment. The purpose of this paper is to describe the experiments that were performed with the microcilia and to evaluate the appropriateness of microcilia to spacecraft docking applications. Through the course of this study microcilia are able to provide the speed, robustness, reliability and strength for use in miniature spacecraft applications. The microcilia successfully moved blocks of aluminum in excess of 40g of mass and calculations indicate that a patch 25cm in radius is sufficient to position a 40kg satellite. MICROSATELLITE DOCKING Figure 3 describes a large, broad purpose satellite, surrounded by a constellation of smaller, mission specific satellites. The miniature satellites provide inspection,