Nano-Positioning of a Electromagnetic Scanner with a MEMS Capacitive Sensor Xinghui Huang, Ju-Il Lee, Narayanan Ramakrishnan, Mark Bedillion, Patrick Chu Seagate Technology, 1251 Waterfront Place, PA 15222, USA (e-mail: xinghui.huang@seagate.com; juil.lee@seagate.com) Abstract: This paper presents the control design and experimentation of a prototype electromagnetic scanner with an integrated capacitive linear and rotational position sensor for small form factor probe storage. An array of probe heads is to be precisely positioned in X/Y linear and rotation directions so that high areal density (>1 terabit/in 2 ) and high data throughput can be achieved. The scanner has X/Y motion capabilities with a linear stroke of about 300 µm. It can also generate rotational motion with offset actuators to compensate for disturbances, mechanical tolerance and nonlinearities. System characterization, modeling, MIMO control design and simulation, and preliminary experimental results are presented. The feasibility of rotation control with the developed capacitive sensor and offset actuators is experimentally confirmed. 1. INTRODUCTION Compared to flash storage, probe storage technology is ex- pected to offer ultra-high capacities and competitive data rates at reduced costs. Substantial research effort has been made to realize probe storage technology (Mamin et al. [1999], Sebas- tian et al. [2005]). The basic idea behind probe storage is to form bits using very small tips analogous to punch card technol- ogy but with densities in excess of 1 Tb/in 2 . An array of probe tips can be employed in order to achieve high data throughputs. At a storage density of 2.4 Tb/in 2 , the corresponding track pitch is about 30 nm, which requires a positioning error of less than 1.5 nm at each tip. These specifications place stringent requirements on the servo mechanical system. The lithographic challenges to make ultra-small memory cells in flash memory have now been translated to the more tractable challenges in positioning accuracy in probe devices. This paper describes implementation of probe storage based on conventional manufacturing and assembly processes. In this implementation, a sled scanner is driven by electromagnetic ac- tuators consisting of coil-magnet pairs, which generates linear forces in the X and Y directions and torque in the Z direction. In addition to servo marks on the media, capacitive sensors are Yaw X-scanning Y-tracking Pitch Roll Magnetic actuator Sled Spring Gimbal (a) Magnetic actuators Reflective bars for measurement Stacked media and sled (b) Fig. 1. Sled assembly schematic and the testing fixture Electromagnetic actuator Plastic sled Package frame Capacitive sensor R/W heads Media Fig. 2. Sled assembly cross-section view formed between the static head substrate and the moving media for linear and rotational position feedback. At any time instant, a row is chosen for simultaneous reading or writing in order to meet the data rate requirement. The head array has three in-plane degrees of freedom (DOF) including yaw, which will be referred to as rotation in what follows. When a row of heads are position-controlled simultaneously, this undesirable rotation has to be suppressed in order to achieve a small positioning error at all heads. The rotation control capability is generated by offsetting the four magnetic actuators counterclockwise from their nominal symmetric position by a small amount, so that a torque is formed in the Z direction when the actuator pair in one direction is driven differentially. 2. SYSTEM CONFIGURATION AND MODELING The schematic view of the sled assembly is shown in Fig. 1. It has three in-plane DOFs: X-scanning, Y-tracking, and Z-yaw. Inevitably, it also has some out-of-plane modes: roll, pitch and bounce. Higher order modes are not considered here since their resonance frequencies are sufficiently high with respect to the operating frequency. 2.1 Frequency Responses The frequency responses of the sled assembly are determined by a sweepsine measurement and are shown in Fig. 3. There are four major in-plane modes: X spring mode (X1), X gimbal mode (X2), Y spring mode (Y), and Z rotation mode (R). Proceedings of the 17th World Congress The International Federation of Automatic Control Seoul, Korea, July 6-11, 2008 978-1-1234-7890-2/08/$20.00 © 2008 IFAC 3130 10.3182/20080706-5-KR-1001.0582