Past Projects in the MIT Precision Motion Control Laboratory

Wafer steppers are used in photolithography to position the uncut silicon wafer underneath a lens assembly in order to expose each of the future microchips. As such, the machine is required to have long travel, sub-micron resolution, and a settling time that is as fast as possible. The faster the machine settles, the more microchips can be produced. Typically, steppers are approached by stacking a short-travel, high resolution stage on top of a long-travel, low resolution design. This can lead to difficulties in achieving fast settling times. Our design uses magnetic bearings in combination with a six phase linear motor to combine the actions of the coarse and fine stages into a single moving element, thus allowing the long travel, high resolution, and fast settling times required. [More Info]

This project involves the design and implementation of various linear and nonlinear control schemes for a single DOF magnetic suspension. Plant uncertainty is introduced on purpose and countered using Robust and Adaptive techniques. Robust adaptive control is found to give the best results. [More Info]

A magnetically-levitated stage for photolithography in the semiconductor manufacturing industry is under development. A single moving part generates all six-degree-of-freedom motions required for focusing, and large planar motions for positioning. The stage includes four linear permanent-magnet motors as key actuators to produce the levitation force, to cancel the weight of the moving part, as well as the driving force. The position stability of the stage is aimed at tens of nanometers so that the stage can be applicable as a high-precision planar positioner, such as a wafer stepper in the current deep-submicron technology. [More Info]

Capacitance sensors can be used for noncontact measuring of an airgap. Typically, the probe and drive electronics are separated by a length of cable, and disturbances to this cable lead to noise in the position reading. This project involves integrating the capacitance probe electronics into the probe head itself, transforming the displacement measurement directly into a digital signal.

We have designed a compact, simple magnet suspension for use in an artificial heart pump. Our design unifies the magnetic bearings and motor. Our motor spins the impeller and also can regulate the other five degrees of freedom. This results in a simpler, more compact design. Since each segment of the motor can provide drive and suspension forces, it is easier to design for redundancy and robustness which are essential in this application. [More Info]

We analyze and design a high force per unit volume linear motor for use in machine tools. The motor is the first to incorporate coils wound with separated end-turns so that each layer of the coil can be directly cooled. Oil flows through the gaps in the end-turns on both sides of a coil to remove heat. A current of 1.6 A causes a 100°C temperature rise in a free convection-cooled coil; it takes a current of 9.0 A to cause the same temperature rise with our cooling technique. Thus our design allows nearly 6 times higher force in steady state and dissipates 32 times as much heat. We also investigate a second cooling scheme where we insert a comb-shaped piece of copper into the separated end-turn coil. Thermal analyses corroborated by experimental results are presented for both techniques.

Design of a 6 D.O.F. actuator to position a scanning mirror in a Fourier Transform Infrared (FTIR) interferometer. The scanning mirror is used on NASA's Geosynchronous Operational Environmental Satellites (GEOS) used for severe weather observation and anomalous atmospheric behavior.

This device uses active magnetic bearings in combination with squeeze film dampers to form a very stable, vibration resistant motion control stage. Total travel is within a cube of 100 microns, and resolution in the linear axes is better than 0.5 nm. One possible application involves providing the sample motions required in scanned probe microscopy. [More Info]

A common problem associated with controlling magnetic suspensions is the lack of an accurate model for the nonlinear relationship between the actuator force, coil current, and gap. We use this calibration fixture to cycle the current to an actuator while capacitance probes and load cells measure the gap and force. In this way, each actuator's behavior (including saturation and hysteresis) can be documented for use in later control system design.