Mark A. Unrath

High Performance Motion Tracking Control for Electronic Manufacturing

Motion control requirements in electronic manufacturing demand both higher speeds and greater precision to accommodate continuously shrinking part/feature sizes and higher densities. However, improving both performance criteria simultaneously is difficult because of resonances that are inherent to the underlying positioning systems. This paper presents an experimental study of a feedforward controller that was designed for a point-to-point motion control system on a modern and state of the art laser processing system for electronics manufacturing. We systematically apply model identification, inverse dynamics control, iterative refinement (to address modeling inaccuracies), and adaptive least mean square to achieve high speed trajectory tracking. The key innovations lie in using the identified model to generate the gradient descent used in the iterative learning control, encoding the result from the learning control in a finite impulse response filter and adapting the finite impulse response coefficients during operation using the least-mean-square update based on position, velocity, and acceleration feedforward signals. Experimental results are provided to show the efficacy of the proposed approach, a variation of which has been implemented on the production machine.

Active Thermal Management for Precision Positioning

Precision positioning systems are inevitably subject to various thermal disturbances including heating from motors, friction between components, and ambient temperature fluctuations. Thermal disturbances cause unwanted thermal expansion and contraction; the resulting distortion of the components in the positioning system could lead to degraded positioning accuracy. This paper explores the application of estimation and control techniques to address thermally-induced positioning error. A simplified planar system, motivated by linear stages used in semiconductor manufacturing, is considered as a case study. The goal is to estimate the displacements of the locations of interest at the top of the stage and control their positions within an acceptable error bound from the preset target positions. A linear time-invariant model is first identified from the numerical simulation of the system model. Kalman filter is applied for state estimator design and the optimal error covariance is used to guide the temperature sensor placement. It is found that the estimator performance does not significantly improve beyond a small number of well chosen temperature sensors. For the active heating control, an offset is introduced to allow for two-way (both positive and negative) control action. The linear-quadratic-Gaussian design is used for both actuator location selection as well as closed loop active heating control. It is found that the assumed boundary condition between different portions of the stage drastically affect the controllability of the locations of interest. Locations of interest far from the constrained points are well regulated, while those near the constrained points show limited improvement.

Robust Control for Linear Stages in Electronic Manufacturing

This paper considers the control of an x-y linear stage commonly used in electronic manufacturing. The challenge of the control problem includes coupled and nonlinear dynamics, flexibility due to the air bearing dynamics, geometric nonlinearity due to the interferometer position measurement, and tight cross-axis positioning requirements. We evaluate two control design approaches: robust control and gain scheduling. For the robust controller, a nominal linear time invariant design model is chosen with nonlinear dynamics considered as uncertainties. The choice of uncertainty characterization and weighting functions significantly affect the performance of the controller. For the gain scheduling controller design, robust controllers based on linearized models at several operating points within the workspace are combined. From the nonlinear simulation, it is shown that with suitable tuning, good performance is obtained throughout the work volume.