Simulation study on evolutionary cycle to cycle time control of exposure controlled projection lithography

Abstract

Purpose

Exposure controlled projection lithography (ECPL) is an additive manufacturing process based on controlled UV photopolymerization. This paper aims to explore an advanced closed-loop control methodology to ECPL.

Design/methodology/approach

This paper proposes an evolutionary cycle to cycle (EC2C) control method, and started with a reduced order EC2C time control to control only the exposure time for given DMD bitmaps, which correspond to target 3D part cross-sections. A preliminary EC2C time control scheme was developed and followed by two types of EC2C time controllers based on two different parameter estimation methods, recursive least squares and L₁ norm minimization (L₁Min). Both algorithms were in an exponential weighted form, resulting in EWRLS and EWL₁Min, to weight more on recent data to reflect the current process dynamics.

Findings

EWRLS was found to outperform EWL₁Min in terms of computation speed and stability. The simulation study demonstrated that the proposed EC2C time control method was capable of adaptively tracking the ECPL process dynamics and updating online the model parameters with real-time measurements. It could control perfectly the exposure time for each bitmap, achieving the desired height for each layer and resulting in a total cured height conforming to the target 3D part height.

Research limitations/implications

The accuracy of EC2C time control method relies heavily on fast and accurate measurement, and this research assumes availability of an adequate real-time metrology. Measurement errors are not considered in this paper and will be explored in future. Only simulation study was performed without physical experiments to verify the EC2C controller.

Practical implications

For implementation, a real-time measurement system needs to be developed and the EC2C control software needs to be programmed and interfaced with the physical system.

Originality/value

It concludes that EC2C control method is very promising for a physical implementation, and could be extended for the development of a more comprehensive closed-loop controller for both exposure time and intensity to improve the ECPL process precision and robustness.

Citation

Zhao, X. and Rosen, D.W. (2016), "Simulation study on evolutionary cycle to cycle time control of exposure controlled projection lithography", Rapid Prototyping Journal, Vol. 22 No. 3, pp. 456-464. https://doi.org/10.1108/RPJ-01-2015-0008