Design and Implementation of a Control System for a Counter Gravity Casting Machine

In counter-gravity casting, pressure is used to draw molten metal, against gravity, into the flask in order to replace the foam pattern and form the final product. The performance of the current pressure controller needs improvement to increase the quality and repeatability of the final product. There are two major hardware difficulties that needed to be compensated for. The first is the nonlinear behavior of valves (hysteresis nonlinearity), which is addressed using a gain scheduling approach by decomposing the nonlinear system into approximate linear subsystems. Consequently, each linear sub-system is controlled using a PID controller. The second difficulty is the decrease of the plenum pressure responsible for creating the vacuum, which requires a new control approach. A PID controller with a gain scheduling scheme and a feed forward term was developed to overcome the nonlinear characteristic of the valves and to compensate for variation of the pressure inside the plenum. The new controller showed considerable performance improvements. Furthermore, the experimental results show how the control is robust in the presence of significant dynamic variations in the input profile.