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This paper presents a numerical and experimental investigation of ceramic shell cracking during the burnout process in investment casting with internally webbed laser stereolithography patterns. Considered are the cracking temperature of the ceramic shell, the buckling temperature of the web link, and the glass transition temperature of the epoxy resin. Our hypothesis is that shell cracking will occur if the ceramic rupture temperature is lower than the temperature of glass transition and the temperature of web buckling. This hypothesis is validated by a good agreement we obtained between experimental observations and numerical simulations. It is found that the shell cracking and web link buckling are strongly related to the cross-sectional dimensions and span length of the web structure and the shell thickness, and that shell cracking can be prevented by buckling of the epoxy webbed pattern in early stages of the burnout process.

This paper presents some practical design criteria for a plug-in type repetitive controller to achieve good tracking performance within a specified frequency range. Upper and lower bounds of the repetitive controller parameters that ensure the stability and the desired performance are derived. It has been found that the decay rate of the tracking error due to periodic inputs is related to the peak value of a defined regeneration spectrum function. The control performance of the present method is evaluated in an experimental software cam system, which needs periodic linear motions, where the real-time control algorithms are implemented using a floating-point digital signal processor (DSP). Both computer simulation and experimental results are presented to illustrate the effectiveness of the proposed repetitive controller design.

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Repetitive controllers have been shown to be effective for tracking periodic reference commands with known period. For a reference with fixed period, its implementation can be done with time fixed sampling where an integer number of samples in each period can be required. With variable periodic signal tracking with real-time realization, the number of samples per period may be a noninteger with a fixed sample rate. This paper presents an adaptive repetitive control scheme for reducing tracking errors due to variable periodic reference signals with two nonsynchronized sampling. Aside from time sampling, a position pulse signal generated by optical encoder is used to produce another fixed sampling. This technique can accommodate the variable periodic signal and variable samples per period for synchronization can be achieved by the position sampling. Experimental studies of a twin linear motor control system which comes up with variable periodic references are given. The experimental results illustrate the validity of the proposed implementation method in that adaptive repetitive control with two nonsynchronized sampling can effectively eliminate steady-state errors within a few cycles that are caused by variable periodic references.

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