Digital PID process control apparatus

A digital PID process control apparatus comprising:

• digital PID controlling means for calculating a control signal as a process input signal in accordance with PID control parameters and a difference between a set-point signal and a process signal as a process output signal so as to supply the control signal to a process;
• identifying means for identifying a pulse transfer function of the process in accordance with the process input signal and the process output signal while superposing a persistently exciting signal as an identification signal on the control signal and supplying a superposed signal to the process, and for calculating an S-transfer function of the process in accordance with the identified pulse transfer function;
• tuning means for tuning the PID control parameters of said digital PID controlling means in accordance with the S-transfer function;
• identification-complete detecting means for detecting a convergence of the S-transfer function so as to stop generating the persistently exciting signal by said identifying means;
• characteristic change detecting means for detecting a model error of the process in accordance with the process input signal, the process output signal and the pulse transfer function, and for changing the set point signal in a pulse form;
• and
• restarting means for detecting a change in the model error when the set point signal is changed in the pulse form and for restarting generation of the persistently exciting signal by said identifying means.