1. An adaptive controller for use in controlling a process, comprising: a controller input to receive a process variable input signal from the process; a controller output to provide a process control signal for use in controlling the process; a control block coupled between the controller input and the controller output that determines the control signal by implementing process control calculations using the input signal and a set of controller tuning parameters; and a tuning block that determines new values for the set of controller tuning parameters during operation of the process using a set of stored process model parameter values and a process state variable, wherein the set of stored process model parameter values includes a process model parameter value for a particular process model parameter for each of a plurality of different process operating points defined by the process state variable; the tuning block including; a model adaptation routine that determines the stored process model parameter value for the process model parameter at each of the plurality of different process operating points; a model parameter determination routine that stores an interpolation function parameter for determining a particular process model parameter value for a particular process operating point using an interpolation technique based on the process state variable value and two or more of the process model parameter values of the set of stored process model parameter values; and a controller tuning parameter routine that determines the set of controller tuning parameters for the particular process operating point from the determined process model parameter value and a stored tuning rule.