Design, fabrication, and testing of a moving magnet actuator for large range nanopositioning (2011)

Moving magnet actuators are promising candidates for achieving nanometric motion quality over a large (10mm) motion range. Large range nanopositioning capability addresses an important instrumentation need in several areas of nanotechnology. In pursuit of this goal, we present an optimal moving magnet actuator design integrated with a double parallelogram flexure bearing and thermal management system. The proposed design is systematically optimized in the context of fundamental actuationand system-level performance tradeoffs that we have identified. The overall system is custom-fabricated and tested to validate the desired actuation performance. Preliminary closed-loop results highlight the proposed actuator’s potential for large range nanopositioning. INTRODUCTION AND MOTIVATION Large range (~ 10 mm) nanopositioning systems are becoming increasingly desirable in applications such as scanning probe microscopy, nanometrology, nanolithography, hard-drive and semiconductor inspection, and memory storage [1-2]. However, most existing nanopositioning systems are generally limited to a motion range of a few hundred microns [2-3], in part due to actuation challenges. The voice coil actuator (VCA), which is a single-phase electromagnetic actuator, has stood out among other options such as piezoelectric, inchwormtype, and multi-phase electromagnetic actuators, for its non-contact, frictionless and cog-free motion characteristics and sufficient motion range suitable for large range nanopositioning [4-5]. However, in a VCA, the non-deterministic disturbance due to the moving connecting wires and heat dissipation from the coil connected to the motion stage degrade the overall motion quality (resolution, precision and accuracy). While the operation of a moving magnet actuator (MMA) is similar to that of a VCA, its construction is different in that it employs nonmoving coils and a moving magnet. FIGURE 1 illustrates a traditional MMA configuration, featuring an axially-oriented cylindrical magnet and iron pole pieces that are connected to the motion stage of the bearing, which serve as the “mover”. The “stator” consists of a tubular backiron, along with two oppositely wound coils that are connected in series. This configuration gives MMAs two distinct advantages over the VCA: I. The non-deterministic disturbance due to the moving cables is eliminated, making MMAs truly non-contact actuators. II. Since the coils are connected to the static back-iron as opposed to the mover, the heat generated due to resistive losses in the coils remains physically separated from the motion stage. Given these attributes, MMAs are promising candidates for large range nanopositioning. The objective of this research is to systematically develop and demonstrate an optimal MMA design that takes into account I. Physical performance trade-offs, as described further below, and II. MMA-specific challenges including greater non-uniformity in force over stroke and off-axis instability of the mover [6]. PERFORMANCE TRADE-OFFS An MMA, double parallelogram flexure bearing, and thermal management system were concurrently designed to meet the following system-level performance criteria: I. Maximize the first natural frequency (or open-loop bandwidth) since this is closely related to the achievable speed and disturbance rejection of the motion system in closed-loop operation. II. Minimize the power consumption of the actuator N S