1. A computer system for controlling a non-linear physical process, the computer system comprising: a linear controller connected to receive a command signal for control of the non-linear physical process and an output signal from the non-linear physical process, the linear controller generating a control signal based on the command signal and the output signal, the linear controller designed using a fixed linear model for the process; and a neural network connected to receive a combined control signal and the output signal from the non-linear physical process, the neural network using at least one of the combined control signal and the output signal from the non-linear physical process to modify the connection weights of the neural network on-line as the neural network and linear controller are used to control the non-linear physical process, the neural network generating a modified control signal based on the modified connection weights and at least one of the combined control signal and the output signal from the non-linear physical process, the modified control signal combining with the control signal to form the combined control signal used to control the non-linear physical process and to correct for errors inherent in controlling the physical process using the linear controller designed using the fixed linear model.