Surface Temperature Control in Plasma Deposition of Metal Matrix Composites

Abstract

This paper describes control of target surface temperature in the inductively coupled plasma deposition (ICPD) process used in manufacture of metal matrix composites. Temperature feedback is provided by an infrared camera; the DC plate current input to an RF power oscillator circuit is adjusted to maintain the desired surface temperature. The spray target is a cylindrical mandrel which translates and rotates beneath a stationary spray gun. Because of the periodic translational and rotational motions of the mandrel, the linearized model relating the torch power to observed temperature is periodic time-varying. By holding the input constant over half the translation period and averaging the output over that time period, a nearly time-invariantlinear model of the process is obtained. A logic-based signal processing algorithm removes the spurious and irrelevant measurements and averages valid measurements over a single deposition pass. System identification on the processed input-output data produced models in innovations form. The identified models and model uncertainty estimates were used to design a controller for spatially averaged substrate or deposit temperature. A frequency-weighted LQG controller design augments the plant with two integrators, in order to ft, How the controller to track a ramp reference command with zero steady-state error. Worst-case analysis of tracking performance based on the modeuncertainty bounds gave a robust controller as well as insight into limitations due to model uncertainty. A workstation-hased rapid prototyping system that included an i386 real-time controller was used to acquire data, identifymodels, and perform analysis and design. Real-time implementation of the algorithms was facilitated by a C language source code generator and interactive real-time displays on the workstation monitor. The resulting controllercommunicated with the facility's conventional programmable controller and included an interactive operator interface taking input from both the mouse and keyboard. Using scripts to perform analysis and design functions, thecomplete development cycle, including system identification, control design, and robustness analysis, and re-implementation takes on the order of several hours to repeat in the event of changes in process equipment or configuration.